When we look at the sky, we see different types of objects. Some are man-made (like the International Space Station), some are from our solar system (like Venus or Saturn), but many are twinkling, shiny objects — of course, stars from outside our solar system.
Stars have fascinated humans since time immemorial, especially because sometimes, they seem to twinkle. Stars don’t actually twinkle per se — the twinkling we observe here has more to do with the atmosphere on Earth rather than the stars themselves. There are three main factors that influence how stars “twinkle”, and to truly understand them, we need to take a short dive into some atmospheric physics.
The first physical phenomenon that makes stars appear to twinkle is turbulence.
We observe stars that are far away because the light that they emit reaches our eyes (or telescopes). But in order to do that, it must first pass through the atmosphere. That means that light is indirectly subjected to phenomena that affect the Earth’s atmosphere.
Turbulence is a phenomenon that often happens on smaller scales. In the atmosphere, we have large-scale phenomena like cold fronts or hurricanes happening every day, but inside these events, turbulence is significant on a small scale. So cold fronts bring large thunderstorms, the clouds within the front can make the sky turbulent, and that’s when the airplane pilot tells you “Ladies and gentlemen, we’re experiencing some turbulence.”
There are several types of turbulence, including one called thermal turbulence — which happens when there is a mix between hotter and colder air. This could happen whether the sky is cloudy or not. When a mass of air in the atmosphere is hotter than its surroundings, it starts to rise, creating convective currents. Basically, you end up with moving columns or pockets of heated air that arise from warmer surfaces of the earth.
These moving pockets of air can create turbulence, and in the process, they also distort light that passes through them.
When it comes to stars, twinkling is caused by the passing of light through different layers of the turbulent atmosphere. This is more pronounced near the horizon than directly overhead since light rays near the horizon pass through denser layers of the atmosphere, but twinkling (technically called scintillation) can be observed on all parts of the sky.
But there’s more to this story.
When light passes through any medium (including the Earth’s atmosphere), some of it is reflected back, while some passes through the atmosphere, but at a different angle — something called refraction. When the atmosphere is turbulent in a region, the refraction angle is not constant, so light can change path quickly.
Altering the refractive index changes the apparent position of objects, just like the straw in a glass of water experiment, it looks bent. So the turbulent sky, constantly changing the refractive index makes stars appear to be moving, so they twinkle, or scintillate.
Due to scale differences, if an astronomical object is large enough compared to the turbulence, it won’t affect the way we see it. But the light of a smaller object (or one that’s farther away) will be affected as it crosses the turbulent air. That’s the reason why planets twinkle less (or almost don’t twinkle at all) — they are closer and it makes them ‘bigger’ compared to the turbulence.
Fortunately, atmospheric scientists developed a way to monitor changes in the refractive index of the atmosphere due to turbulence. They use instruments to measure the turbulence and use it to try to estimate a future outcome.
For astronomers, twinkling can be quite problematic. So they look for the “best sky” to avoid the phenomenon. Usually, this means an environment whose climate is very dry. When that’s not possible, they try to find the dryness by placing the instruments at a high altitude. Whenever is possible to combine altitude and mostly dry weather, they have a good spot for a telescope.
In the images above we see the difference very clearly: both skies were clear when the images were taken, but one (on the left) was less turbulent than the other (on the right). On the left, we see a video of a star recorded on Mount Fuji in Japan — the star appears to be bouncing chaotically due to a turbulent sky. On the right, we see a recording of the same star taken on the Andes Mountains in Chile, a very dry, high-altitude area; the star bounces, but much less than in the Japanese images.
So stars don’t exactly twinkle, but they do appear to twinkle from here on Earth. For astronomers, though, making sure they eliminate the “twinkling” is important.
Of course, if you set your telescopes in space, you don’t have these problems because your observation point is above the atmosphere. But even here on Earth, astronomers are careful to pick the best locations for placing large optical telescopes. They typically look for the driest areas, at the highest altitude possible, without any light pollution. There’s another consideration: because the air is usually flowing from west to east because of Earth’s rotation, a way to avoid pollution is placing telescopes on west coasts or in ilands in the middle of the ocean. This rules out the vast majority of places on Earth, which is why astronomers are so particular about where they place their telescopes.
An international team of astronomers reports on a new sighting of fluorine in another galaxy. This is the farthest the element has ever been detected and will help us better understand the stellar processes that lead to its creation.
Fluorine is the lightest chemical element in the halogen group, which it shares with other gases such as chlorine. It’s a very reactive element, and in our bodies, it helps give our bones and teeth mechanical strength as fluoride.
New research is helping us understand how this element is formed inside stellar bodies. The study also marks the farthest this element has ever been detected from our galaxy.
From stars to pearly whites
“We all know about fluorine because the toothpaste we use every day contains it in the form of fluoride,” says Maximilien Franco from the University of Hertfordshire in the UK, who led the new study.
“We have shown that Wolf–Rayet stars, which are among the most massive stars known and can explode violently as they reach the end of their lives, help us, in a way, to maintain good dental health!” he adds, jokingly.
The findings were made possible by a joint effort between the Atacama Large Millimeter/submillimeter Array (ALMA) and the European Southern Observatory (ESO), and pertain to a galaxy that’s 12 billion light-years away. The team identified fluorine in the form of hydrogen fluoride as large clouds of gas in the galaxy NGP-190387.
Due to the distance between Earth and NGP-190387, we still see it as it was at only 1.4 billion years old, around one-tenth of the estimated age of the Universe.
Like most of the chemical elements known to us, fluoride forms inside active stars. However, until now, we didn’t know the details of this process, or which stars produced the majority of the fluorine in the Universe.
This discovery helps us better understand how fluorine forms because stars expel chemical elements from their core near to or during the end of their lives. Due to the young age we perceive this galaxy as having from Earth, we can infer that the stars which formed the clouds of hydrogen fluoride must have appeared and died quickly in the grand scheme of things.
Wolf-Rayet stars, very large stellar bodies that only live for a few million years, are the main candidate that the team is considering. They fit the criteria of having short lives, and their size would allow for the huge quantities of hydrogen gas spotted in NGP-190387. Plus, it fits with our previous theories — Wolf-Rayet stars have been suggested as an important source of fluorine in the past, but we didn’t have enough data to confirm this theory, nor did we know how important they were for this process.
Although other processes have been suggested as likely sources of cosmic fluorine, the team believes that they couldn’t account for the time frame involved, nor for the sheer quantity of the element in NGP-190387.
“For this galaxy, it took just tens or hundreds of millions of years to have fluorine levels comparable to those found in stars in the Milky Way, which is 13.5 billion years old. This was a totally unexpected result,” says Chiaki Kobayashi, a professor at the University of Hertfordshire and co-author of the paper. “Our measurement adds a completely new constraint on the origin of fluorine, which has been studied for two decades.”
This is also the first time fluoride has been identified in such a far-away, star-forming galaxy. Since the distances involved in studying the Universe also mean that the further you look, the further back in time you see, it’s also the youngest star-forming galaxy we’ve ever detected fluoride in.
The paper “The ramp-up of interstellar medium enrichment at z > 4” has been published in the journal Nature Astronomy.
Researchers report finding the smallest white dwarf — and likely, smallest star in general — we’ve ever seen. And it’s just a tad smaller than our Moon.
White dwarves are dead stars, the leftover cores of stars which reached the red giant stage but petered out. They’re extremely dense things, usually composed mainly of carbon and oxygen. This particular one, named ZTF J1901+1458, has a radius of approximately 1,700 kilometers — just shy of the Moon’s radius of 1,737 — and sits some 130 light-years away from us.
Despite its size, however, the dwarf has around 1.3 times the mass of the Sun.
Small but mighty
“That’s not the only very amazing characteristic of this white dwarf,” astrophysicist Ilaria Caiazzo of Caltech said June 28 in an online news conference. “It is also rapidly rotating.”
White dwarfs are typically similar in size to the Earth, which has a radius of around 6,300 kilometers. But one of their interesting properties is that they tend to be smaller the more mass they contain. This has to do with how they maintain stability. White dwarfs can’t generate the same physical processes that keep other stars from collapse, as they have no fuel to ‘burn’. Instead, their shape is maintained by the electrons in their atoms being physically pushed into one another to their limit. The tighter the squeeze, the more these electrons push back through quantum processes (electrons hate being near other electrons). So higher mass white dwarves, which have a stronger gravitational pull trying to make them collapse, need to become smaller in order to squeeze their electrons that much harder and counteract the pull.
Given its small size, then, ZTF J1901+1458 is one of the most dense objects of its kind.
It’s also quite restless, making a full spin once every seven minutes or so. The Earth makes a full rotation once every day. All this motion means that ZTF J1901+1458 produces quite the impressive magnetic field, estimated to be at least a billion times stronger than our planet’s. Needless to say, this is not a peaceful place to visit.
The stellar remnant was discovered using the Zwicky Transient Facility at Palomar Observatory in California, which scours the sky for objects with variable brightness. Given that they’re basically stellar corpses with no internal source of energy, white dwarves start out bright and incandescent but slowly cool and dim over time, eventually becoming an extinguished black dwarf.
As for how it came to be, we’re still unsure — but its mass provides a solid hint. The team’s working hypothesis is that ZTF J1901+1458 was born from the merger of two white dwarves that orbited one another and eventually merged into a single, extra-chunky, dwarfier white dwarf. This would also explain why it’s spinning so fast and why its magnetic field is so powerful.
All things considered, this merging could have easily ended badly. If ZTF J1901+1458 was more massive, it wouldn’t have been able to support its own weight and would have exploded. Finding a body so close to the edge of what’s possible will help us better understand what we’re going to run into once we eventually start trekking through space.
The paper “A highly magnetized and rapidly rotating white dwarf as small as the Moon” has been published in the journal Nature.
The Sun is a ball of nuclear plasma so large its own weight keeps it from exploding. Very cool, but also quite hot. However, the Sun has a complicated interior structure, and surprisingly large temperature variations, both on and under its surface. Today, we’re going to look at why this is, and how we know.
A lot of the things happening on Earth are, ultimately, fueled by energy from the Sun. We see that energy as sunshine, feel it hot on our skin on a clear day. It drives winds, and it powers rain cycles. Almost all life on Earth is fed by plants capturing sunlight. It’s very fortunate for us, then, that the Sun produces a monumental amount of energy. Just a fraction of its output reaches our planet, since a lot of it is lost in transit, reflected, or radiates away from Earth and into the void. Even so, it’s much more than we’d know what to do with, and only about 1% of it is enough to keep all the plants on Earth alive.
Energy is never lost or created, but transformed from one ‘flavor’ into another. Still, not all of them are equal, and heat seems to be the baseline that all others eventually degrade into. The Sun, therefore, gets quite hot.
Just how hot?
It depends on a lot of factors — mostly on exactly where you’re taking the measurement. There’s a lot of variation here.
First off is the Sun’s core. Here is where the fusion reaction that drives the star actually takes place. Due to the sheer mass of gas pressing down on the core, ambient pressure here is immense. Temperatures, too, are extremely high, due to how compressed everything gets. This is ideal, because such extreme conditions are needed for fusion to take place. To the best of our knowledge, temperatures at the core of the Sun can reach in excess of 15 million °C (27 million °F), which is a lot.
The next layer of the star is its ‘radiative zone’. Energy from the core moves out to this area, carried by bodies of superheated, ionized atoms, where it becomes trapped. It spends up to 1 million years here, before finally managing to escape the strong gravity and electromagnetic fields and reach the convective zone. This zone represents the upper layer of the Sun’s core, and temperatures here are believed to be around 2 million °C (3.5 million °F).
Hot plasma from this convective zone can bubble up towards the surface of the Sun. The next layer it encounters is the photosphere, which is about 5,500 °C (10,000 °F), significantly cooler compared to the previous layer. It is here that the radiation produced inside the star can first be perceived as light by an outside observer. A photosphere (‘sphere of light’ in ancient Greek) is defined as the deepest region of a light-emitting body that is still transparent to photons of certain wavelengths. In other words, the photosphere starts where the Sun’s plasma becomes transparent enough for light to be able to escape it.
However, the photosphere is not uniform. Areas of intense electromagnetic activity produce sunspots, which are darker and cooler than their surroundings; temperatures in the center of a sunspot can drop to lows of 4,000 °C (7,300 °F).
The next layer, the chromosphere (‘sphere of colors’ in ancient Greek), is a tad cooler, at about 4,320 °C (7,800 °F) on average. Light from this layer is thus dimmer, and we don’t usually see it. But, during a solar eclipse (when the moon covers the sun), this is the really fancy bit you see around its outline, the red rim surrounding the Sun. This color is emitted by the high content of hydrogen gas in the chromosphere.
In relative terms, temperatures in the photo- and chromatosphere aren’t that high — a candle, for example, burns at around 1,000 °C (1,800 °F). We know these two layers exist because their relatively mellow conditions allow for simple molecules such as water and carbon monoxide to survive, and we’ve picked up on their spectral emissions.
Lastly, there is the corona — the Sun’s crown. A bit unexpectedly, temperatures shoot back up in this layer, despite it being the farthest away from the core. In fact, it has average temperatures at the same order of magnitude as the core, although they are still lower. These range between 1 million °C and 10 million °C (roughly 1.7 – 17 million °F), according to the National Solar Observatory (NSO). The corona and chromatosphere are kept separated from this layer by a transition zone of highly-ionized helium atoms. This is less of a hard boundary and more of a chaotic, ever-churning sea of clouds. The corona might be so hot due to ‘nanoflares‘, but we’re still unsure.
Beyond the corona lies the Sun’s extended atmosphere, the heliosphere, which is less of a layer per se and more of an area of influence that the Sun exerts. While emissions such as solar winds or flares can send super-heated, charged particles flying off from a star into its heliosphere, this is much cooler than the layers we’ve discussed previously. The main component of the heliosphere is magnetic, and it has a key part to play in forming the Sun’s shield around our solar system.
How do we know how hot it is?
Sticking a thermometer in the Sun is, understandably, a bit tricky. So we’ve had to rely on indirect methods of measurement to tell exactly how hot it can become.
If you put a kettle on to boil, you’ll be able to feel heat coming from the water even after you take it off the stove. How much of it you will perceive depends a lot on how close you hold your hand to the water. This process relies on the radiative properties of electromagnetic energy. To keep it simple, particles start moving when they heat up, and this motion generates thermal radiation; infrared cameras pick up on this kind of radiation, for example. Our senses perceive it as heat.
One of the simplest and also least accurate ways of assessing the Sun’s temperature is our own senses. Sunlight can be really hot on a clear day. Considering the star is around 149 million kilometers away, it must be outputting a lot of thermal energy for it to reach all the way here. Still, researchers like numbers, especially accurate ones, so they did develop several other means of gauging the star’s temperature.
One approach uses the link between heat and the light coming from a body — because in physics, unlike life, being hotter automatically makes you brighter, too.
Thermal radiation is a kind of electromagnetic radiation, but so is light. In very broad terms, as long as the cause is temperature, the brighter an object glows, the hotter it is. Irons heating in a forge are a good example. The hue can help us determine this temperature exactly. Red light is cooler than yellow light which is cooler than blue, and so on. A yellow-flamed candle burns colder than the blue flames on your stove.
That’s the working principle. In practice it becomes more complicated since the Sun doesn’t output a single type (wavelength) of light, but a whole cocktail of wavelengths that mix and interact to create the final, white light we perceive. We use a device called a spectrograph to tease these colors apart into individual wavelengths. They work much like raindrops do when creating rainbows.
Once we break down sunlight like this, we can look at each individual color (wavelength) of light and determine what temperature is associated with it. Every wavelength carries different amounts of energy, so the final step is to average out their temperatures to determine the final ‘product’. Think of it like determining the energy level of the middle-most color by looking at each individual color present in sunlight.
Sunlight can also be used to determine the chemical composition of our star. Every stellar body emits light across multiple wavelengths. But these ’emission spectra’ also show very small, generally well-defined gaps, tight wavelength intervals where no light is emitted (or where light is being absorbed). This comes down to how atoms interact with radiation but, suffice to say, these gaps are extremely reliable signatures of certain elements. As long as you understand the trace each of them leaves on the emission spectrum, you can determine a body’s composition from the light it emits. We call these traces Fraunhofer lines.
Which extinction lines are present, as well as how well-defined they are, are influenced by temperature. As such, this step can help determine both the composition and temperature of a star.
Still, we said earlier that the photosphere is a hard boundary in regards to our perception of light — we can’t see below this limit. Spectroscopy, then, as well as other optical methods, can only help us determine temperatures down to this layer. At the same time, even if the corona is much hotter, it’s also significantly less bright than the photosphere, so it has a very small contribution to this type of measurement.
As for anything deeper than the photosphere? That’s more theoretical. It’s not pulled out of our assumptions, but it’s still a theoretical estimate. Temperature conditions inside the Sun are based on the idea that it is in a state of hydrostatic equilibrium. That is, that its gravity (inward pressure) and expansive (outward pressure) generated by nuclear fusion at the core cancel each other out. If they wouldn’t, the star would blow up or turn black holey, so it’s a solid starting point.
If you combine this with chemical readings from spectroscopy, and estimates of the star’s mass (also calculated or obtained indirectly), you can determine what temperatures should be at the core to keep it all stable. We also know from our efforts at making fusion happen on Earth that humongous pressures and temperatures are needed to convince hydrogen atoms to merge; the Sun does that on a monumental scale, every second.
This is all based on tried-tested-and-true methods and theorems regarding natural processes, so they are reliable, but they’re still just estimates. If you’re the kind of person that needs exact measurements and figures, talking about the Sun might not be the best hobby for you. But, if it makes you find a way to go there and actually stick a thermometer in the thing so we can all find out, I won’t complain.
New images captured by the GREGOR telescope in Tenerife, Spain, are giving us an unique view of the surface of the sun.
These are the highest-resolution images of our host star ever taken by a European telescope, according to the authors. The results definitely support that claim — they give us a stunning look at the shapes and movements of solar plasma and the eerie dark voids of sunspots.
Although GREGOR has been in operation since 2012, it underwent a major redesign this year and also suffered a temporary pause in activity due to the pandemic. Now it’s up and running again, and its new, improved systems allow it to spot details as small as 50 kilometers in size on the solar surface. It might not sound like much, but this is the highest resolution of any European telescope (and, relative to the sun’s diameter of 1.4 million kilometers, quite good).
“This was a very exciting but also extremely challenging project,” said Lucia Kleint, who led the upgrade efforts on GREGOR. “In only one year we completely redesigned the optics, mechanics, and electronics to achieve the best possible image quality.”
To give you an idea of the telescope’s new abilities, she describes the images it captured as “if one saw a needle on a soccer field perfectly sharp from a distance of one kilometer”.
The sun isn’t a solid object with a static, solid contour. Rather, its surface is always roiling and churning with super-heated plasma. The new images from GREGOR show the twisting structures created on the star’s surface and the contrasting darkness of sunspots. Sunspots are areas on the solar surface where magnetic fields are extremely strong, generating a spike in local pressure which darkens the area.
Researchers at the University of Warwick report finding a white dwarf barreling through space at great speed. The source, they say, was likey a “partial supernova” which ejected the core of the star.
Sitting at about 40% the mass of our sun, white dwarf SDSS J1240+6710 is much smaller, and denser. Data from the Hubble telescope allowed researchers to confirm that its atmosphere is an unusual mix of gases.
A peculiar star
“There is a clear absence of what is known as the ‘iron group’ of elements, iron, nickel, chromium and manganese,” explains a statement from the University of Warwick.
“These heavier elements are normally cooked up from the lighter ones and make up the defining features of thermonuclear supernovae.”
White dwarfs are born when a star completely consumes its fuel. Most of its mass blows away to form a nebula, leaving behind a white-hot core. They usually have atmospheres consisting of hydrogen or helium, the researchers add, with traces of carbon and oxygen produced as the star grew old.
This particular one, however, was made from oxygen, neon, magnesium and silicon. Furthermore, the lack of elements in the iron group points to it undergoing a “partial supernova” before it died. Heavier elements are formed by light atoms being pushed together in stars as they explode.
Its speed — this solar remnant is travelling at around 559,234 mph — would indicate that it was thrown out in the event.
“This star is unique because it has all the key features of a white dwarf but it has this very high velocity and unusual abundances that make no sense when combined with its low mass,” says Professor Boris Gaensicke from the Department of Physics at the University of Warwick, lead author of the paper.
“It has a chemical composition which is the fingerprint of nuclear burning, a low mass and a very high velocity: all of these facts imply that it must have come from some kind of close binary system and it must have undergone thermonuclear ignition.”
The paper “SDSS J124043.01+671034.68: the partially burned remnant of a low-mass white dwarf that underwent thermonuclear ignition?” has been published in the journal Monthly Notices of the Royal Astronomical Society.
The observable universe contains an estimated 1×1024 stars. They come in very different sizes, and their masses and brightness can vary dramatically. Even so, researchers have developed a system that managed to look at them all together.
The most common classification system is called the Morgan–Keenan (MK), which classifies stars based on temperature and luminosity. The MK system essentially merges two different systems:
the Harvard classification, where stars are classified on their surface temperature using a single letter of the alphabet: O (the coldest), B, A, F, G, and M (the hottest). These are the so-called main-sequence stars, but as we’ll see, there are also more extreme types of stars (like giant stars, supergiant stars, and white dwarfs). These broad types are also split, so you can have G1 stars, G2 stars, and so on.
the Yerkes spectral classification (sometimes used synonymously with the MK system), which is luminosity, where stars are named using Roman numerals: 0, I (Ia, Iab, Ib), II, III, IV, V, VI, and VII.
In the MK system, stars are defined with a letter from the Harvard classification and a Roman numeral from the Yerkes one (the Sun is a G2-V star).
Over the years, astronomers have created different catalogs where stars are classified in somewhat different ways, and it’s not always a straightforward process, nor is the classification set in stone — it has changed over the years, as our understanding of stars has improved. So if anything, the stellar classification discussed is a general indication more than anything else.
Now, let’s look at things in a bit more detail and see why this matters.
A star is born
The universe is big, dark, and cold — the vast majority of it, at least.
But if you were to wander this universe, you’re bound to come across something that’s hot and bright. Stars, as these objects are called, are giant thermonuclear reactors. They’re responsible for heating up some planets to a livable temperature, they produce the vast majority of the chemical elements we know of, and they’re basically the reason why the universe is not a completely frozen and barren place.
A star’s life begins when a gaseous nebula of material starts to collapse on its own gravity.
Just like planets, all stars are round, because of this gravitational collapse. When the stellar core gathers sufficient mass, it starts to develop a significant gravitational attraction, which packs the particles even more tightly, increasing the density. Then, once a crucial point is reached and the stellar core becomes sufficiently dense, hydrogen starts to be converted into helium through nuclear fusion, releasing energy in the process — and thus, a star is born.
This hydrogen-helium fusion is the fuel of the star, and the heat it produces also creates a pressure that prevents stars from collapsing on themselves. Depending on how massive they get, stars will also end up on a different course.
In general, larger stars have a shorter lives, although we’re still usually talking about billions of years. However, their lifecycle greatly depends on their mass.
For instance, a star with a mass greater than 0.4 times the Sun will expand and turn into a red giant. Then, as it ejects much of its mass violently across the galaxy, the star would become a white dwarf. Based on its mass, the star might also become a brown dwarf, a neutron star, or, if it is sufficiently massive, a black hole. So already, we’re seeing that different types of stars can turn into different things after they use up their fuel.
Average stars become white dwarfs. They eject their outer layers but still maintain a hot core comparable in size to the Earth. Counterintuitively, the smaller the white dwarf, the larger its mass — this happens because it’s the pressure from fast-moving electrons that keeps these stars from collapsing, so the more massive the star, the more it is able to overcome this pressure, and thus, massive white dwarfs tend to become smaller.
White dwarfs can become active stars once again. If a white dwarf is close enough to a companion star, it can start sucking material from the second star, and if it acquires enough, it can produce a burst of nuclear fusion, causing it to brighten and turn into a nova. This typically lasts a few days, after which the process starts again. If enough material is gathered in one go, the white dwarf can erupt into a supernova.
Larger stars become supernovae. A supernova is not only a big nova. In a nova, the star’s surface explodes but in a supernova, the core also collapses and explodes. The supernova-generating process is cataclysmic — in mere seconds, the core shrinks from thousands of miles to only a few dozen, and the temperature can spike by over 100 billion degrees. A supernova eruption can outshine an entire galaxy. Every year, we discover 20-40 supernovae in other galaxies.
Supernovae can become neutron stars . After this ungodly process, the core collapses. If the collapsing stellar core has a mass of 1.4-3 solar masses, what’s left behind is a neutron star. Neutron stars are the smallest and densest stars, if we exclude black holes or hypotheticals such as white holes. Neutron stars have a radius on the order of 10 kilometers, and the ones we’ve observed so far seem very hot, although they don’t actually produce heat anymore and slowly cool down over time.
Large supernovae turn into black holes. If the stellar core remaining after the supernova explosion is larger than three solar masses, it will collapse into a black hole. Black holes are the most massive objects we know of in the universe, so massive that even light cannot escape their grasp. Black holes are so exceptional that they start to break our current understanding of physics. Black holes are thought to be extremely cold on the inside, but incredibly hot just outside.
Stellar debris often forms new stars. The dust and debris ejected by novae and supernovae are excellent seeds for new stars, recycling the material and potentially forming new generations of stars.
We’ve already seen some types of stars but before we dig into the classification per se, let’s take a brief look at what we know and don’t know about stars.
Outside of the sun, the closest star to Earth is one of the three stars in the Alpha Centauri system, some 4.3 light-years from Earth. That’s very, very far. To get an idea how far it is, the distance from Earth to Mars is around 0.00001 light-years away and, with our best available space ships, it would still take around 7 months for the trip. To get to Alpha Centauri at that speed, it would take over 100,000 years. So while mankind has looked at the stars since forever, we’ve only recently started to look at them closely.
Of course, we’re not physically close to the stars, but with the aid of telescopes, we can get a closer look at them. Ground-based telescopes enable scientists to study stars in different wavelengths of light, including the visible spectrum, radio waves, and even infrared light.
It’s not just observation — researchers also conduct experiments in the lab to infer the properties of stars and investigate the processes that fuel stars. Ultimately, computer modeling has also helped improve our understanding of stars.
With computer models, we can approximate different properties of stars (such as density, pressure, velocity, composition) and see how they influence observations.
Enough chit-chat. Let’s look at some stars.
Main sequence stars — and beyond
In astronomy, the ‘main sequence‘ is a term used to denote stars that fit in the common group of stars on the color versus luminosity charts. These plots, also called Hertzsprung–Russell diagrams after their co-developers, are the common classification tool for stars.
The Hertzsprung–Russell diagram is essentially a square where stars are plotted with color on the horizontal and luminosity on the vertical. Color is dependent on temperature, and brightness depends on size so you can also see it as a temperature-size chart.
The main sequence of stars is exactly what the name says: the main sequence in which these stars are grouped, from top left (bright and blue/hotter) to bottom right (less bright and less hot).
It’s hard to say why stars are grouped along this chart, but about 90% of all the known stars in the universe (including the sun) are main-sequence stars.
That doesn’t mean that main sequence stars are monotonous — quite the opposite. There can be huge variation, ranging from a tenth of the mass of the sun to up to 200 times as massive.
There are also other clusters of stars — to the left we see the less bright white dwarfs, which we’ve discussed already, and above the main sequence, we have the giants and supergiants. Here’s what a real diagram of stars looks like, from 22,000 stars plotted from the Hipparcos Catalogue and 1,000 from the Gliese Catalogue of nearby stars, with the most prominent being the main-sequence diagonal.
So the first stellar classification is into main-sequence stars and non-main-sequence stars. Main sequence are stars are the norm, other types of stars can exist scattered around the diagram (though they also tend to group into clusters).
Types of main sequence stars
As we’ve already mentioned, main sequence stars are also extremely varied, so there is also a separate classification for them.
The traditional classification here is called the Harvard classification, a one-dimensional classification where stars are grouped by their temperature, with a letter representing each category. However, this classification does not distinguish between stars with the same temperature but different luminosities. So a new classification system was devised — the Morgan–Keenan (MK) classification, which also includes the luminosity.
The modern classification system for them is called the Morgan–Keenan (MK) classification and also includes the luminosity.
Each star is assigned a spectral class from the older Harvard spectral classification, as well as a luminosity class using Roman numerals as explained below, forming the star’s classification. So you’d have, for instance, B0 stars, BI, stars, BII, and so on.
So M stars are the smallest, O stars are the biggest, and the luminosity ranges from I (brightest) to VI (less bright) and D (white dwarfs). As mentioned, the Sun is a type G2V star. Alpha Centauri A is also a G2V star, while Proxima Centauri is a type M5.5-V star.
The classification can also be expanded to stars outside of the main sequence. A plot of the stars would look like this:
Why we classify stars
Well, for starters, researchers love to classify all things — that’s how we better understand trends and patterns. That’s how we know, for instance, that M type stars are by far the most abundant in the known universe. The Sun belongs to a relatively rare group of stars.
Astronomers also calculate the habitable zone around different types of stars — the distance at which a planet can exist around the star so that it hosts liquid water, and therefore, could potentially host life.
NASA’s Kepler mission is searching for habitable planets around nearby main-sequence stars less massive than type A stars but more massive than type M — so mostly around K, G, and F types of stars.
Ultimately, we class stars because we want to better understand and define them. We’re still only scratching the surface of a very vast universe of knowledge, and it helps to group things in an orderly, systematic fashion.
More exotic stars
It’s not the only stellar classification, and it’s not entirely fixed and set-in-stone. There are other classifications and talk about changing classifications is not unheard of in the astronomical world, as our understanding progresses and improves.
There are also more exotic types of stars. For instance, recent research has suggested that Class L (small, dark, reddish) stars and Class T (methane dwarf) stars could be more common than all the other classes combined, but they are more difficult to discover.
There is yet another class — Class Y of brown dwarf stars, cooler than those of spectral class T and with different spectral signatures. So far, less than two dozen of these stars have been confirmed. Class C of carbon stars is also discussed, and Class D stars is typically used to denote any stars that are not currently undergoing fusion. It’s estimated that there are 300 billion stars in our galaxy alone, and we have very little idea how many galaxies there are in the universe, but estimates range in the trillions. I’ll let you do the math.
As our detection capacity and image processing power advances, we are likely to keep discovering new stars that improve and challenge our current understanding. There is much, much more to discover.
In the move Interstellar, former NASA pilot Joseph Cooper (Matthew McConaughey) is sent hurtling through time and space via a wormhole in order to save humanity. Essentially, humans have screwed up this planet to the extent that in order to survive, we all have to move to a less-screwed-up planet. The quickest way to this new future home is through a wormhole near Saturn. This opening is the entrance to a distant galaxy located near a black hole named Gargantua. And, of course, if anyone can do it, it’s the bad-ass McConaughey.
Wormholes have always been the fascinating stuff of sci-fi, because come on, intertwining dimensions by bending space and time is pretty cool. Unfortunately, they’ve never been proven to actually exist. Luckily, however, that hasn’t put off scientists from trying. Now a new study out of the University of Buffalo (UB) has attempted to take a look at what they would look like if they were real.
Obviously, the first place you would look for a wormhole would be a black hole or a binary black hole system, which involves two black holes circling one another. Theoretically, the insane amount of gravity would pull them together and create a tunnel.
For their study, the researchers focused on Sagittarius A*, an object that’s thought to be a supermassive black hole at the heart of the Milky Way galaxy. While there’s no evidence of a wormhole there, it’s a good place to look for one because wormholes are expected to require extreme gravitational conditions, such as those present at supermassive black holes.
Black holes are massive pits of gravity that will bend space-time due to their incredibly dense centers, or singularities. When a massive star dies, it collapses inward, and as it does so, the star explodes into a supernova — a catastrophic expulsion of its outer material. This dying star will continue to collapse until it becomes either a neutron star or a singularity — something consisting of zero volume and infinite density. This seemingly impossible contradiction is what causes a black hole to form.
The UB scientists believed that if a wormhole does exist at Sagittarius A, nearby stars would be influenced by the gravity of stars at the other end of the passage. As a result, it would be possible to detect the presence of a wormhole by searching for small deviations in the expected orbit of stars near Sagittarius A.
“If you have two stars, one on each side of the wormhole, the star on our side should feel the gravitational influence of the star that’s on the other side. The gravitational flux will go through the wormhole,” says Dejan Stojkovic, PhD, cosmologist and professor of physics in UB’s College of Arts and Sciences. “So if you map the expected orbit of a star around Sagittarius A*, you should see deviations from that orbit if there is a wormhole there with a star on the other side.”
The research, which was published in Physical Review D, focuses on how scientists could hunt for a wormhole by looking for perturbations in the path of S2, a star that astronomers have observed orbiting Sagittarius A*.
While current surveying techniques are not yet precise enough to reveal the presence of a wormhole, Stojkovic says that collecting data on S2 over a longer period of time or developing techniques to track its movement more precisely would make such a determination possible. These advancements aren’t too far off, he says, and could happen within one or two decades.
The good doctor cautions, however, that although this new method might be used to detect a wormhole if one is there, it will not strictly prove that a wormhole is present.
“When we reach the precision needed in our observations, we may be able to say that a wormhole is the most likely explanation if we detect perturbations in the orbit of S2,” he says. “But we cannot say that, ‘Yes, this is definitely a wormhole.’ There could be some other explanation, something else on our side perturbing the motion of this star.”
Wormholes were originally conceived in 1916 by Ludwig Flamm. The Austrian physicist was reviewing another physicist’s solution to the equations in Albert Einstein’s theory of general relativity when he believed another solution might, in fact, be possible. His “white hole” was a theoretical time reversal of a black hole. Entrances to both black and white holes could be connected by a space-time conduit.
Though, if we ever do find one, it probably won’t be the one that science fiction has shown us.
“Even if a wormhole is traversable, people and spaceships most likely aren’t going to be passing through,” says Stojkovic . “Realistically, you would need a source of negative energy to keep the wormhole open, and we don’t know how to do that. To create a huge wormhole that’s stable, you need some magic.”
A cleverly designed experiment takes us one step closer to a fundamental truth — but there’s still a long way to go.
When something is called “dark energy”, it’s bound to be mysterious and weird, but dark energy is really weird. For starters, we don’t even know what is.
It seems counterintuitive, but our universe is expanding. Not only is it expanding, but this expansion is also accelerating — which seems really bizarre, as you’d expect gravity to slowly clump things closer together. Dark energy is believed to be the reason behind this acceleration.
It seems to permeate all the space in the universe and it’s very homogenous, but it only interacts with the gravitational force and is extremely rarefied, which makes it extremely difficult to study and analyze. This leaves the question “so what is it” very much on the table, with no satisfying answer.
Some physicists have proposed that dark energy is a fifth fundamental force — adding to the well-known gravity, electromagnetic, weak nuclear and strong nuclear forces. This hypothesis has been put to the test by researchers at Imperial College London and the University of Nottingham.
If this were the case and dark energy was a force, you’d expect it to be some sort of repulsive force, something that makes the universe “larger“. To test this, the experiment worked on single atoms, using a device called an atom interferometer. This detects any extra force which might be acting on the atom. The experimental setup featured a small metal sphere placed in a vacuum chamber, with atoms freefalling through the chamber.
In theory, if dark energy was a fifth force, it would be weaker when there is more matter around. So in this design, the freefalling atoms would change paths ever so slightly as they passed by the sphere. However, this turned out to not be the case. The atoms continued unabated as they passed the sphere, essentially ruling out the idea that dark energy is a fundamental force.
This does more than just rule out one possibility — it helps constrain the cosmological models attempting to describe dark energy. Professor Ed Copeland, from the Centre for Astronomy & Particle Physics at the University of Nottingham, explains:
“This experiment, connecting atomic physics and cosmology, has allowed us to rule out a wide class of models that have been proposed to explain the nature of dark energy, and will enable us to constrain many more dark energy models.”
The fact that this experiment is relatively simple but helps to reveal one of the fundamental truths of the universe makes it all the more remarkable, researchers say.
“It is very exciting to be able to discover something about the evolution of the universe using a table-top experiment in a London basement,” said Professor Ed Hinds of the Department of Physics at Imperial.
Only 35,000 light-years away from Earth, astronomers have spotted a red giant star that was forged just a couple hundred million years after the big bang.
Artist impression of the formation of the very first stars. Credit: WISE, ABEL, KAEHLER.
The recent discovery was made by astronomers led by Dr. Thomas Nordlander of the ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), who found a record-low amount of iron in a star located at the edge of the Milky Way’s halo. The red giant, unceremoniously called SMSS J160540.18–144323.1, has an iron content of just one part per 50 billion or 1.5 million times less than the sun.
“That’s like one drop of water in an Olympic swimming pool,” Dr. Nordlander said in a statement.
Why is this star so significant? After the early universe started to cool off, the only available elements were hydrogen, helium, and trace amounts of lithium. The earliest stars — let’s call them 1st generation — fused these light-weight elements inside their very massive and very hot cores. However, these stellar pioneers were very short-lived, quickly running out of fuel before going out with a bang, turning supernova. The massive explosion that signals the end of a star spews its forged elements across the universe, where they can be incorporated by new stars. Over the course of generations, increasingly heavy elements can be forged such as silicon or iron.
None of the first stars have survived, so a lot of what we suppose about them cannot be verified. However, there’s still a lot to learn from their surviving cosmic relatives. If a star has a lot of iron, scientists can infer that this star must have formed after a predictable number of stellar generations. For instance, based on its metal content, astronomers believe that the sun is about 100,000 generations away from the big bang.
“The good news is that we can study the first stars through their children – the stars that came after them like the one we’ve discovered,” said study co-author Professor Martin Asplund, a chief investigator of ASTRO 3D at the Australian National University.
Considering the record-low amount of iron found in SMSS J160540.18–144323.1, astronomers believe that it was formed after one of the first stars exploded, just a couple hundred million years after the big bang. Dr. Norlander and colleagues believe that the exploding star that seeded SMSS J160540.18–144323.1’s iron was only ten times more massive than the sun. It must have also exploded rather feebly so most of its iron and other heavy elements were pulled back into the core by the gravity of the remnant neutron star.
It’s remarkable to learn that our galactic backyard still houses stars from the earliest generations — although they might not last for long. The newly found star is a red giant, which means its at the very end of its life cycle before exploding in a supernova. In the future, astronomers hope to find more such second-generation stars that might tell us more about what the early universe looked like.
The findings include two mini-Neptunes and a rocky super-Earth.
Artistic depiction of the newly-discovered planets around their star, with the Earth for reference. Image credits: NASA’s Goddard Space Flight Center/Scott Wiessinger.
It seems hard to believe that we only discovered our first exoplanet — a planet outside of our solar system — in the 1990s. We now know thousands of exoplanets, and astronomers are verifying even more potential candidates. Much of what we know about exoplanets comes from the Kepler telescope, which was retired recently after 9 years of service.
But Kepler’s successor, TESS, is already bringing in results.
Researchers working with NASA’s Transiting Exoplanet Survey Satellite (TESS) have discovered three new worlds in our cosmic neighborhood, a mere 73 light-years away. The planets are all in a solar system that seems very different from our own.
For starters, the planets in our solar system are extremes — we have everything from the very large Jupiter and Neptune down to Earth and Mars, and to smaller rocky planets like Mercury. This new planetary system, which has been dubbed TOI-270, seems to have planets much closer in size to each other. All three planets are intermediate planets — something which is lacking from our solar system.
The two mini-Neptunes are exciting for astronomers because they represent a “missing link” in planetary formation. Mini-Neptunes are, as the name implies, Neptune-like planets with deep layers of ice and liquid (not necessarily from water, though). However, unlike Neptune, whose mass is 17 times larger than that of the Earth, mini-Neptunes are at most 10 times more massive.
“There are a lot of little pieces of the puzzle that we can solve with this system,” says Maximilian Günther, a postdoc in MIT’s Kavli Institute for Astrophysics and Space Research and lead author of a study published in Nature Astronomy that details the discovery. “You can really do all the things you want to do in exoplanet science, with this system.”
There’s another interesting peculiarity of the system: the planets line up in what astronomers call a resonant chain.
Example of resonant chain from our own solar system (via Wikipedia).
In other words, the planets’ orbits are aligned in a way that is very close to whole integers. In this case, it’s 2:1 for the outer pair, and 3:5 for the inner pair. In our solar system, the moons of Jupiter are lined up in such a way (and researchers have found evidence of other exoplanets arranged in a similar way).
“For TOI-270, these planets line up like pearls on a string,” Günther says. “That’s a very interesting thing, because it lets us study their dynamical behavior. And you can almost expect, if there are more planets, the next one would be somewhere further out, at another integer ratio.”
As for habitability, TOI-270 also raises some interesting questions. The rocky super-Earth and one of the mini-Neptunes are too close to their star. However, the other mini-Neptune, called TOI-270-d appears to lie in the habitable zone, where temperatures might be sufficient to host liquid water and possibly life. However, although the planet lies in the right area, this is still quite unlikely, the new study reveals.
TOI-270-d most likely has a thick atmosphere which produces an intense greenhouse effect, causing the planet’s surface to be too hot for habitation. But the system could still hold other planets — if these planets have a similar structure but lie farther away from the star, the temperature might be just right.
Thankfully, the host star, TOI-270, is remarkably well-suited for habitability searches, “as it is particularly quiet”, researchers write. The team now wants to focus other instruments, especially the upcoming James Webb Space Telescope on the star and its solar system to see if there are indeed other planets and to assess their physical parameters. It’s safe to say that we will probably be hearing about the system in the not-too-distant future.
“TOI-270 is a true Disneyland for exoplanet science, and one of the prime systems TESS was set out to discover,” Günther says. “It is an exceptional laboratory for not one, but many reasons — it really ticks all the boxes.”
The study “A super-Earth and two sub-Neptunes transiting the bright, nearby and quiet M-dwarf TOI-270” has been published in Nature Astronomy.
Artist rendering of NASA’s Parker Solar Probe observing the sun. Credit: NASA/Wikimedia Commons.
One of the most logically-baffling solar mysteries is the fact that the sun’s surface is close to 10,000 degrees Fahrenheit while its outer atmosphere is several million degrees hotter. The body of the heat’s source itself is cooler than the atmosphere surrounding the fireball — and that’s simply against the common sense of physics.
Some physicists think that the terrific, intense heat displayed in the outer limits of the sun’s atmosphere may be explained by magnetic waves traveling to and from the solar surface, bouncing off the upper atmosphere (otherwise known as the corona) of the star. Recent studies have suggested that this activity could be tied to the sun’s zone of preferential ion heating. In this zone, ions reach scorching temperatures exceeding those at the very core of the sun.
Another element which has a role to play in this outlying solar vortex are Alfven waves. These waves are low-frequency oscillations traveling through a plasma in a magnetic field. Scientists think that these waves are making solar wind particles to collide and ricochet off one another. But once it hits the outskirts of the zone of preferential heating, the solar wind sweeps by at an extremely fast pace. Thus, it manages to evade the Alfven waves from there on out.
Researchers at trying to definitively mark the extent to which the superheating effect reaches beyond the sun. Recent research has brought light to a connection between the Alfven point (the point of altitude beyond the solar surface that permits solar wind particles to break free of the sun) and the outskirts of the zone of preferential heating. These two fields have fluctuated in unison. They shall continue their dance, and in 2021, NASA’s Parker Solar Probe, christened in honor of physicist Eugene Parker, should come in contact with the two boundaries.
The spacecraft includes instruments capable of recording a number of significant data pertaining to those solar fields. The information it would collect in some two years to come would be invaluable in this particular study.
The Parker Solar Probe was launched in August 2018. It made its second successful fly-by of our sun in early April with the follow-up perihelion (the point at which it gets closest to the sun) scheduled to occur on September 1. Visit NASA’s page on the Parker Solar Probe to learn more about it and its mission. To learn of interesting updates, check out the website of Parker Solar Probe Science Gateway.
An international team of researchers has managed to identify the first coronal mass ejection, or CME, in a star other than our Sun.
Image credits NASA / GSFC.
An intense flash of X-rays, followed by the bursting on an immense bubble of plasma — that’s what researchers led by Costanza Argiroffi, a researcher at the University of Palermo and associate researcher at the National Institute for Astrophysics in Italy, have seen in the corona of HR 9024, an active star about 450 light-years away from us. This is the first CME ever spotted in a star outside our solar system.
The findings help us better understand how CME fits into the lives of active stars across the Universe and will help us systematically study such dramatic events in the future.
“The technique we used is based on monitoring the velocity of plasmas during a stellar flare,” said Costanza Argiroffi. “This is because, in analogy with the solar environment, it is expected that, during a flare, the plasma confined in the coronal loop where the flare takes place moves first upward, and then downwards reaching the lower layers of the stellar atmosphere.”
“Moreover, there is also expected to be an additional motion, always directed upwards, due to the CME associated with the flare.”
The team used data collected by NASA’s Chandra X-ray Observatory to analyze a “particularly-favorable” flare, according to a Chandra Observatory press release. Solar flares are sudden, quite violent events, during which a star’s brightness increases substantially. Flares are sometimes, but not always, associated with CMEs.
The High-Energy Transmission Grating Spectrometer (HETGS) device aboard Chandra is the only instrument we have at our disposal so far that can be used to measure the movement of matter involved in CMEs. CMEs involve the expulsion of plasma — very hot, electrically-charged gas — in a star’s corona (atmosphere), at speeds of up to tens of thousands of miles per hour.
CMEs are only produced in magnetically-active stars, the results confirm. The findings also support the validity of what we know about CMEs so far, for example, that material involved in a flare is very, very hot (from 18 to 45 million degrees Fahrenheit), and that it first rises and then drops with speeds between 225,000 to 900,000 miles per hour.
“This result, never achieved before, confirms that our understanding of the main phenomena that occur in flares is solid,” said Argiroffi. “We were not so confident that our predictions could match in such a way with observations, because our understanding of flares is based almost completely on observations of the solar environment, where the most extreme flares are even a hundred thousand times less intense in the X-radiation emitted.”
The “most important” discovery, however, is that after the flare a body of much cooler plasma (of around 7 million degrees Fahrenheit) rises from the star’s body with “a constant speed of about 185,000 miles per hour,” adds Argiroffi. Such a result is “exactly what one would have expected for the CME associated with the flare.”
The team adds that, based on Chandra’s readings, the mass of the CME in questions was roughly two billion pounds. This would make it about ten thousand times as massive as the largest CMEs put out by the Sun. This last tidbit reinforces the idea that more magnetically active stars generate larger-scale versions of solar CMEs.
“The observed speed of the CME, however, is significantly lower than expected. This suggests that the magnetic field in the active stars is probably less efficient in accelerating CMEs than the solar magnetic field,” Argiroffi concludes.
The paper “A stellar flare−coronal mass ejection event revealed by X-ray plasma motions” has been published in the journal Nature.
Artist impression of the disc of dust and gas surrounding the massive protostar MM 1a, with its companion MM 1b forming in the outer regions. Credit: J. D. Ilee / University of Leeds.
When astronomers cast their telescope towards an infant star, they were surprised to find that it was nursing a smaller stellar companion within its massive stellar disk. The amazing discovery marks the first time scientists have observed a star forming out of the fragmented disk of another.
Mega star and mini star
Some solar systems have two stars — they generally have a common center of gravity around which planets, asteroids, and other celestial bodies orbit. Such binary systems are quite common in the universe and astronomers believe that they usually form from the same molecular cloud.
Once a star or binary system settles, it starts forming planets out of the dense disk of gas and dust that surrounds them. Imagine the surprise of astronomers from the University of Leeds when they zoomed in on a young star called MM 1a and found a much smaller star, MM1b, lurking in the outer accretion disk where planets should normally form.
“Stars form within large clouds of gas and dust in interstellar space,” said Dr. Ilee, from the School of Physics and Astronomy at Leeds, said in a statement.
“When these clouds collapse under gravity, they begin to rotate faster, forming a disc around them. In low mass stars like our Sun, it is in these discs that planets can form.”
“In this case, the star and disc we have observed is so massive that, rather than witnessing a planet forming in the disc, we are seeing another star being born.”
The astronomers used turned to the Atacama Large Millimetre/submillimetre Array (ALMA), nested atop the Chilean desert, to spot the unusual stellar pairing. This unique instrument exploits a phenomenon called interferometry which enables 66 individual dishes to mimic the power of a single telescope with a theoretical diameter of 4 kilometers.
Credit: J. D. Ilee / University of Leeds.
The researchers were able to calculate the mass of both stars by measuring the amount of radiation emitted by the fragmented disk, as well as the very subtle shifts in the frequency of light emitted by disk’s gas. They found that MM 1a is 40 times more massive than the Sun while its companion star, MM 1b, weighs less than half the mass of the Sun.
Binary stars are often very similar in mass, meaning they likely formed as siblings. In this particular case, the mass ratio of the two stars is 80 to 1, clearly suggesting an entirely different process of formation for the two cosmic objects.
The British researchers came to the conclusion that the most favorable formation process for MM1b is in the outer regions of the massive accretion disk. In this outmost region, the disk can be gravitationally unstable, thereby collapsing under its own weight, forming a new star.
What’s more, the researchers believe that the small, young star could be surrounded by an accretion disk, which could lead to the formation of planets of its own. That’s a hypothesis that will need to be verified by subsequent observations. But even if such planets are in the process of forming or already exist, they’ll be shortlived.
“Stars as massive as MM 1a only live for around a million years before exploding as powerful supernovae, so while MM 1b may have the potential to form its own planetary system in the future, it won’t be around for long,” Dr. Ilee said.
A new tantalizing study suggests that the Milky Way galaxy died over 7 billion years ago — only to come back to life in a different epoch.
As far as galaxies go, the Milky Way seems pretty average: it’s a barred spiral galaxy with a diameter between 150,000 and 200,000 light years, containing somewhere between 100 and 400 billion stars. Shockingly high numbers but again, pretty average for a galaxy.
But what makes the Milky Way special, at least as far as we’re concerned, is that somewhere, on one of its spiral arms, there’s a solar system orbiting around a dwarf star; and in that solar system, there’s a planet mostly covered by water, where intelligent life has evolved in the form of primates. Now, some of these primates have learned that the Milky Way itself might not be all that average: it seems to have been born twice.
The key of the new study lies in the chemical make-up of Milky Way stars. Their chemical compositions can reveal information about the gasses from which they formed, providing important clues regarding the history of their galactic neighborhood. Masafumi Noguchi of Tohoku University in Sendai, Japan, proposes that stars in our galaxy were formed in two distinct epochs. He analyzed so-called alpha process elements (or α-elements) such as oxygen, magnesium, and silicon, thanks to a process called cold flow accretion. Some 10 billion years ago, when the universe was still in its early stages, stars contained significant amounts of these gases — and researchers can now analyze them to date cosmic objects, somewhat like tree rings record the age of a tree.
These early stars tended to end in massive but short-lived supernova explosions. These supernovae explosions were also rich with these α-elements but after a while, but they were so hot that they prevented cold flow accretion throughout the galaxy, stopping the new gas from flowing into the galaxy and forming new stars. This hiatus for about 3 billion years, when a new generation of stars began to form — but unlike the old one, this one was rich in iron. So the Milky Way entered a state of dormancy — essentially, it died, only to be reborn once again some 5 billion years later.
Credits: M. Noguchi / Nature.
The existence of two distinct groups of stars in the solar neighborhood, one with high [α/Fe] and the other with low [α/Fe], suggests two different origins
According to Benjamin Williams from the University of Washington, who wasn’t involved in this study, our neighbor galaxy, Andromeda, also formed stars in two separate epochs. Noguchi proposes a model that can explain this phenomenon and predicts that massive spiral galaxies like the Milky Way and Andromeda experience a gap in star formation, whereas smaller galaxies made stars continuously.
However, the exact mechanism underlying this phenomenon isn’t well understood, and Noguchi calls for future observations of nearby galaxies, which he says “may revolutionize our view about galaxy formation.”
A team has now found evidence of iron and titanium vapors in the atmosphere of the hottest planet ever discovered.
Artistic depiction of KELT-9b orbiting its host star, KELT-9. Image credits: NASA/JPL-Caltech.
Planets don’t really get much hotter than KELT-9b. With a surface temperature well over 4,000 ℃, the planet was detected in 2017 using the Kilodegree Extremely Little Telescope, and the unusual finding raised quite a few eyebrows at the time.
KELT-9, the star around which the planet revolves, is located some 650 light years from Earth, in the constellation Cygnus (the Swan). It’s twice as hot as the Sun, but that’s not the reason why planet KELT-9b is so hot.
The reason is the orbiting distance: located 30 times closer than the Earth’s distance from the Sun, the planet revolves around its star in only 36 hours. As a result, temperatures reach 4,600° Kelvin (4,300 °C / 7,800 °F). That’s not quite as hot as the Sun, but it’s definitely hotter than many stars.
Artistic depiction of KELT-9b. Image credits: Denis Bajram.
Astronomers aren’t really sure what the planet’s atmosphere might look like. Initial observations revealed that the atmosphere mostly comprises of hydrogen, but now, a new study has revealed some of its less abundant elements. Theoretical models suggested that metallic elements might be present in its atmosphere.
“The results of these simulations show that most of the molecules found there should be in atomic form because the bonds that hold them together are broken by collisions between particles that occur at these extremely high temperatures,” explains Kevin Heng, professor at the University of Bern.
Now, using the HARPS-North spectrograph, astronomers discovered a strong signal corresponding to iron vapor in the planet’s spectrum. They also found evidence of titanium in vapor form.
“With the theoretical predictions in hand, it was like following a treasure map,” says Jens Hoeijmakers, a researcher at the Universities of Geneva and Bern and lead author of the study, “and when we dug deeper into the data, we found even more,” he adds with a smile. Indeed, the team also detected the signature of another metal in vapur form: titanium.
This newly discovered planet might force scientists to open up a new planetary category: so-called “ultra-hot Jupiters.” Hot Jupiters are a class of gas giant exoplanets that are generally similar to Jupiter, but orbit much closer to their star — and are therefore much hotter. Ultra-hot Jupiters are even closer to their star
KELT-9b might not be the only planet to boast such an atmospheric composition. However, astronomers believe that planets like it, orbiting so close to their star, may be obliterated by the hellish environment in the star’s proximity. KELT-9b seems massive enough to take the pummeling, but others may have not been so fortunate.
Journal Reference: Hoeijmakers et al. Atomic iron and titanium in the atmosphere of the exoplanet KELT-9b. Nature, 2018; DOI: 10.1038/s41586-018-0401-y
Some 450 light years away from Earth, the young star RW Aur A just finished chowing down on a planet — probably.
RW Aur A has captured astronomers’ attention ever since 1937. Nestled in the Taurus-Auriga Dark Clouds, which host stellar nurseries containing thousands of infant stars, its light tends to dim “every few decades for about a month,” according to NASA. Needless to say, this has made researchers very curious ever since we realized it. But then, back in 2011, something happened to throw all this interest into high gear: the star became dimmer far more often, and for longer periods of time.
A groundbreaking feast
To get to the bottom of things, a team of researchers pointed the Chandra X-ray Observatory towards RW Aur A over a five-year period. Chandra is a space telescope first launched in 1999, but which still boasts extremely sensitive X-ray sensors that can make sense of the radiation emitted even by young stars such as RW Aur A.
While young stars can be just as perky as any other, they’re typically shrouded in thick disks of gas, dust, and larger debris — which filter their radiation output and alter their intensity. While this makes less-sensitive instruments practically blind to the shrouded stars, instruments like Chandra can use the ‘filtered’ radiation to estimate what the disks are made of.
And that’s exactly what the team did in this case. According to the paper reporting the findings, Chandra detected surprisingly high levels of iron around RW Aur A. Since previous measurements didn’t record the same concentrations of iron (rather they picked up on much lower levels), the only possible explanation is that an event ejected a huge quantity of the element around the star.
They believe that all this iron came from a planet — or a few planetesimals — colliding with one another around the star. If any one of these bodies was rich in iron, it would explain the high levels seen in the disks around RW Aur A. Chandra recordings in 2017 revealed strong emission from iron atoms, indicating that the disk contained at least 10 times more iron than recordings captured in 2013 during a bright period.
The team speculates that this iron excess comes from a collision of two infant planetary bodies — including at least one object large enough to be a planet — in the space surrounding RW Aur A. Such an event would vaporize a large amount of material from the stars, including some iron. Furthermore, as the larger chunks of debris fall towards the star under its gravitational tug, they would release even more iron as the intense heat breaks them apart and solar winds batter them. Taken together, it would explain the high levels of iron observed in the star’s corona.
Better yet, it would also explain the dimming we see. As this debris falls into the star, it could be physically obscuring its light.
“If our interpretation of the data is correct, this would be the first time that we directly observe a young star devouring a planet or planets,” says Hans Guenther, who led the study out of MIT’s Kavli Institute for Astrophysics and Space Research.
With this in mind, an alternative explanation is also possible — if far less epic. RW Aur A is part of a binary star system, the sister of (you’ll never guess it) RW Aur B. If small grains of iron-rich particles can become trapped in certain parts of a star’s disk, and if that disk is perturbed by something massive (say, another star) the resulting interplay of tidal forces could stir the iron-rich particles — and make the disk seem richer in iron as all this dust falls into RW Aur A and obscures its light.
The team plans to continue their observations of the star over the next couple of years to see if iron levels stay constant. If they do, it would point to a massive source of iron (i.e. in favor of the collision scenario); if not, the tidal interaction between the two stars would seem like the more likely choice.
“Much effort currently goes into learning about exoplanets and how they form, so it is obviously very important to see how young planets could be destroyed in interactions with their host stars and other young planets, and what factors determine if they survive,” Guenther says.
Needless to say, I’m rooting for the collision scenario.
The paper “Optical Dimming of RW Aur Associated with an Iron-rich Corona and Exceptionally High Absorbing Column Density” has been published in the journal The Astronomical Journal.
Ever had a moment when you feel like you’re important and what you do matters? Here’s the antidote.
Infrared view of a section within the North Galactic Pole, a region near the constellation Coma Berenices. Every point of light in this image represents an entire galaxy. Image: ESA/Herschel/SPIRE; M. W. L. Smith et al 2017.
At a first glance, not much is going on in this image — just some yellowish noise on a blue-green background. But this photo from ESA’s Herschel Space Observatory shows much more than you’d think: every yellowish speck is a galaxy.
This is the North Galactic Pole, an area which covers some 180 square degrees of the sky and features a galaxy-rich cluster known as the Coma Cluster, which contains at least 1,000 points of light (read: galaxies).
[panel style=”panel-default” title=”Spherical coordinates” footer=””]Just like on Earth, astronomers define observations using a coordinate system — but unlike the XYZ coordinate systems you might be more familiar with, they use a spherical coordinate system. In the former, a point is described by its X, Y, and Z coordinates.
A visual depiction of the spherical coordinate system for a point P. The polar angle is in blue, the azimuthal angle in red.
In a spherical system, a point is also described by three coordinates but, in this case, it’s the radial distance of that point from a fixed origin, the polar angle, and the azimuth angle. It can be a bit weird to wrap your head around, but it can be much easier to navigate astronomical observations. [/panel]
So here, we have the North Galactic Pole, which lies far from the cluttered disc of the Milky Way and offers a good view of the distant Universe beyond our home galaxy.
Zoomed-in view showing about 8 percent of the entire photo width. How many galaxies can you count? Image: ESA/Herschel/SPIRE; M. W. L. Smith et al 2017. via Gizmodo.
The image above was taken at a wavelength of 250 μm, in the infrared range (the human visible range is generally within 0.4 – 0.7 μm). It was taken using the Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS). Unfortunately, Herschel isn’t active anymore — it functioned from 2009 to 2013, using its instruments to study the sky in the far infrared range.
Aside from making us feel incredibly small and showing us just how puny our struggles really are, these pictures also help astronomers to estimate how many galaxies there are in the Universe. Recent surveys have estimated that number to be around 20 trillion, which is 20 times more than previous estimates gathered using the Hubble telescope. All these galaxies are packed with billions of stars, which can also host planets just like Earth.
The European Southern Observatory just published a breathtaking image of a nearby star nursery.
Star cluster RCW 38. Image credits ESO / K. Muzic.
Earlier today, we’ve talked about the first colors complex-ish life created — it was a story of algae, fossils, and pink. Moving on from this daring display by early life, however, I thought we’d seize the occasion to look at what colors accompany birth in the other direction — up in space.
Our eyes can’t peer that far out, but, luckily for us, the European Southern Observatory (ESO) can. Using the HWAK-I (High Acuity Wide field K-band Imager) infrared imager mounted on the Very Large Telescope (VTL) in Chile, the ESO captured some spectacular shots of stars being born in the Vela constellation.
Ashes to ashes, dust to stars
RCW 38 in the constellation of Vela (The Sails). The map shows most of the stars visible to the unaided eye under good conditions. Image credits ESO / IAU / Sky & Telescope.
The image depicts the star cluster RCW 38 as seen in infrared. ESO chose this bit of the electromagnetic spectrum for their observations since infrared can see ‘through’ the clouds shrowding star nurseries such as RCW 38. The cluster itself contains hundreds of young, brightly hot, and quite massive stars. Even at the relatively short distance of 5500 light-years away, however, their (visible) light can’t peer through the vast bodies of dust surrounding the cluster.
The central area, seen as a bright blue region, houses numerous very young stars as well as a few protostars — ‘stars’ that are still forming. Observations by the Chandra X-ray Observatory revealed the presence of over 800 X-ray emitting young stellar objects in the cluster. You won’t be surprised to hear, then, that the area is drenched in radiation, making local gas clouds glow vividly. Cooler bodies of dust languishing in front of the cluster carry more subdued, darker hues of red and orange. The end result, a ‘colorful celestial landscape’ as ESO puts it, is quite the striking interplay of color and light.
This image was captured as part of a series of tests — a process known as science verification — for HAWK-I and GRAAL (the ground layer adaptive optics module of the VLT). These tests are performed to ensure newly-commisioned instruments work as intended and include a set of test observations that verify and demonstrate the capabilities of the new instrument.
Star cluster RCW 38 in the visible spectrum. Image credits ESO – Digitized Sky Survey 2 / Davide De Martin.
Previous images of this region — snapped in the visible spectrum — show a very different sight. Optical images appear almost devoid of stars in comparison with those taken in the infrared spectrum due to dust obscuring the view.
Peering through dense bodies such as dust clouds or nebulae is actually one of the HWAK-I’s main roles. The device also projects four laser beams out into the night sky to use as artificial reference stars — used to correct for any atmospheric turbulence, which can bend incoming light — to increase the quality of the final image.
Astronomers have snapped the first pictures of a planet forming.
This is the first clear image of a planet forming around the dwarf star PDS 70. Image credits ESO / Müller et al., 2018, A&A.
Researchers led by a group at the Max Plank Institute for Astronomy in Heidelberg, Germany, are spying on a baby planet. The object of their attention is a still-forming planet that orbits around PDS 70, a young dwarf star. This is the first time we’ve captured clear images of a forming planet and its travels through the dust cloud surrounding young stars.
The images were captured using the SPHERE instrument installed on Unit Telescope 3 of the European Southern Observatory (ESO’s) Very Large Telescope (VTL) array in Chile. SPHERE, the Spectro-Polarimetric High-contrast Exoplanet REsearch instrument, is one of the most powerful planet-finding tools astronomers have at their disposal today. What makes SPHERE stand out in the field of exoplanet exploration is that, unlike the majority of its contenders, it relies on direct imaging — SPHERE takes actual photographs of planets millions or billions of kilometers away.
SPHERE relies on a technique known as high-contrast imaging to produce such amazing shots. The device uses complex observation techniques and powerful data processing algorithms to tease out the faint traces of light incoming from planets around bright stars. Astronomers draw on the Earth’s rotation to help them better observe such planets — SPHERE continuously takes images of the star over a period of several hours, while keeping the instrument as stable as possible. This creates images of a certain planet taken from slightly different angles and at different points in the stellar halo, giving the impression that it’s slowly rotating or moving about. The stellar halo, meanwhile, appears immobile. The last step is to combine all the images and filter out all the parts that do not appear to move — blocking out signals that don’t originate from the planet itself.
The new planet, christened PDS 70b, stands out very clearly in the images SPHERE recorded. It appears as a bright point to the right of that blackened blob in the middle of the image. That blob is a coronagraph — a mask that researchers apply directly onto the star, lest its light blocks out everything else in the image.
Some examples of how such images from different angles helps astronomers tease out the light incoming from exoplanets. Image credits Müller et al., 2018, A&A.
PDS 70b is a gas giant with a mass several times that of Jupiter. It’s about as far from its host star as Uranus is to the Sun. Currently, PDS 70d is busy carving a path through the planet-forming material surrounding the young star, the researchers note, making it instantly stand out.
“These discs around young stars are the birthplaces of planets, but so far only a handful of observations have detected hints of baby planets in them,” explains Miriam Keppler, who lead the team behind the discovery of PDS 70’s still-forming planet. “The problem is that until now, most of these planet candidates could just have been features in the disc.”
PDS 70d is already drawing a lot of attention from astronomers. A second paper, which Keppler also co-authored, has followed-up on the initial observations with a few months of study. The data from SPHERE also allowed the team to measure the planet’s brightness over different wavelengths — based on which they estimated the properties of its atmosphere. The planet is blanketed in thick clouds, the team explained, and its surface is currently revolving around a crisp 1000°C (1832°F), which is much hotter than any planet in the Solar System.
The findings also helped researchers make heads and tails of a structure known as a transition disc. This is a ring-like protoplanetary (meaning it is involved in early planetary formation) structure. Transitional disks roughly resemble a stadium, with a clean area in the middle (from which planets drew their matter), surrounded by a ring of dust and gas. While these gaps have been known for several decades now and speculated to be produced by the interaction between forming planets and its host star’s disk, this is the first time we’ve actually seen them.
“These objects represent […] disks whose inner regions are relatively devoid of distributed matter, although the outer regions still contain substantial amounts of dust,” explains a paper published by Strom et al. in 1989.
All this data helps flesh out our understanding of the early stages of planetary evolution — which are quite complex and, up to now, “poorly-understood”, according to André Müller, leader of the second team to investigate the young planet.
“We needed to observe a planet in a young star’s disc to really understand the processes behind planet formation,” he explains.
The findings further help improve our overall knowledge of how planets form. By determining PDS 70d’s atmospheric and physical properties, astronomers now have a reliable data point from which to extrapolate — which will help improve the accuracy of our planetary formation models.
Not bad for a bunch of photographs.
The first paper, “Discovery of a planetary-mass companion within the gap of the transition disk around PDS 70” has been published in the journal Astronomy & Astrophysics.
The second paper, “Orbital and atmospheric characterization of the planet within the gap of the PDS 70 transition disk,” has also been published in the journal Astronomy & Astrophysics