Tag Archives: white dwarf

We’ve found a dead star that’s about as large as the Moon, but weighs more than the Sun

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.

Artist’s rendering of a white dwarf. Image via Pixabay.

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.

Gallery of stellar winds around cool ageing stars, showing a variety of morphologies, including disks, cones, and spirals. The blue colour represents material that is coming towards you, red is material that is moving away from you. (L. Decin, ESO/ALMA)

How Stellar Winds of Dying Stars Are Shaped

New observations have revealed that stellar winds are not spherical as previously believed, but instead come in a variety of shapes that resemble those of planetary nebulae — created when a dying star explosively sheds its outer layers, which by a weird naming quirk actually have nothing to do with planets.  In fact, those winds could mark out the ‘molds’ by which planetary nebulae are shaped.

The discovery comes as a result of research conducted by a team of astronomers including Leen Decin, from the Institute of Astronomy, KU Leuven, and is detailed in a paper published today in the journal Science. “We noticed these winds are anything but symmetrical or round,” Decin says. “Some of them are actually quite similar in shape to planetary nebulae.”

Gallery of stellar winds around cool ageing stars, showing a variety of morphologies, including disks, cones, and spirals. The blue colour represents material that is coming towards you, red is material that is moving away from you. (L. Decin, ESO/ALMA)
Winds of red giant stars observed around Gamma Aquilae
[Credit: Decin et al., Science (2020)] (L. Decin, ESO/ALMA)

The team believes that this variety in stellar winds and planetary nebulae shape around dying stars are connected and a result of interactions with companion stars in binary pairings, or even from exoplanets in orbit around the stars. “The Sun — which will ultimately become a red giant — is as round as a billiard ball,” Decin explains. “So we wondered; how can such a star produce all these different shapes?”

The findings collected by the team could explain a long-standing mystery of planetary nebulae around stellar remnants like red dwarfs come in a variety of close-but-not-quite-spherical shapes. 

Planetary nebulae display such a wide range of complex shapes and structures that although the influence of binary companions has been suggested as a possible cause of this diverse range of asymmetric forms, the fact they can arise around stars with spherically symmetric stellar winds has, until now, remained unexplained.

The answer found by the team is that these winds aren’t symmetric at all and that the shape of the winds directly informs the shape of planetary nebulae. 

Dying Stars’ Companions are a Bad Influence

The observations of the stellar winds of 14 AGB stars using the Atacama Large Millimeter/submillimeter Array made by the team were so-detailed that they actually allowed the team to categorize the shapes of the stellar winds and planetary nebula. Some were disc-shaped, some contained spirals, and some were conical — a good indication that the shapes were not created randomly — but, none had spherical symmetry.

Gallery of stellar winds around cool ageing stars, showing a variety of morphologies,
including disks, cones, and spirals. The blue colour
represents
material that is coming towards you, red
is material that is moving away from you. (L. Decin, ESO/ALMA)

In fact, the team realized it was the presence of other low-mass stars or exoplanets in the vicinity of the primary star that was shaping the stellar wind and planetary nebula. Professor Decin is on hand to provide a useful and colorful analogy: “Just like how a spoon that you stir in a cup of coffee with some milk can create a spiral pattern, the companion sucks material towards it as it revolves around the star and shapes the stellar wind.”

Stellar winds are important to astronomers as they account for one of the main mechanisms by which stars lose mass. This mechanism becomes even more critical when attempting to understand the death throes of stars of similar sizes to the Sun and as their cores contract and the outer layers swell creating planetary nebulae — the other major contributor to mass-loss in aging stars. Discovering the role played by stellar companions in such a process is a surprise, to say the least. 

The stellar wind of R Aquilae resembles the structure of rose petals. (L. Decin, ESO/ALMA)

“All our observations can be explained by the fact that the stars have a companion,” says Decin. “Our findings change a lot. Since the complexity of stellar winds was not accounted for in the past, any previous mass-loss rate estimate of old stars could be wrong by up to a factor of 10.”

Following this discovery, the team will now research how it impacts other crucial characteristics involved in the life and death stars like the Sun. In the process, the team believes that their research will add more depth to our view of stars.

The Stellar winds around R Hydrae take a more conical shape (L. Decin, ESO/ALMA)

“We were very excited when we explored the first images,” adds co-author Miguel Montargès, also from KU Leuven. “Each star, which was only a number before, became an individual by itself. Now, to us, they have their own identity. “This is the magic of having high-precision observations: stars are no longer just points anymore.”

But, whilst we are on the subject of the future, the team says their findings have particular ramifications for the end of our own star.

Death Spiral: How the Sun Dies and What it Leaves Behind

The Sun is roughly halfway through its lifetime, with half its core hydrogen exhausted, meaning that in approximately 5 billion years it will start to die. For a star of the Sun’s mass, this means undergoing the transformation into a red giant.

For stars with masses greater than the Sun, the collapse of their core will spark a new lease of life, with the fusion of helium into heavier elements being kick-started by tremendous gravitational pressure, providing an outward force that halts the collapse.

The Sun, in contrast, will fade as its core cools, the planetary nebula will continue to expand outwards, ultimately resulting in a white dwarf surrounded by diffuse material that was once its outer layers. 

The team’s research gives us an idea of just what shape this planetary nebula will take, and how it will be crafted by the solar system’s largest planets. “Jupiter or even Saturn — because they have such a big mass — are going to influence whether the Sun spends its last millennia at the heart of a spiral, a butterfly, or any of the other entrancing shapes we see in planetary nebulae today,” Decin notes. 

“Our calculations now indicate that a weak spiral will form in the stellar wind of the old dying Sun.”

Artist impression of planetary fragment orbiting a white dwarf. Credit: University of Warwick/Mark Garlick.

Planetary fragments orbiting dead star hints at what Earth’s final days might look like

Artist impression of planetary fragment orbiting a white dwarf. Credit: University of Warwick/Mark Garlick.

Artist impression of planetary fragment orbiting a white dwarf. Credit: University of Warwick/Mark Garlick.

Astronomers have discovered a planetary body orbiting a white dwarf — the remaining compact core of a deaf low-mass star. This discovery hints at what conditions Earth might encounter when the Sun begins to die, billions of years from now.

Planetary leftovers

The observable universe is littered with white dwarf stars, however, this was one of the few rare occasions that scientists have discovered orbiting debris around such a star. The planetesimal, which lies 410 light-years from Earth in the constellation Virgo, is believed to be no larger than a couple of hundreds of miles in diameter.

When a star similar in size to the Sun runs out of fuel, it starts expanding greatly in size into a red giant. As it does so, its intense gravity is capable of ripping apart any closely orbiting planets. Astronomers think that this is what happened to the small rocky body that they’ve observed, which probably used to be a dense planet.

When our sun will go through the same process in about 5 billion years, it will obliterate everything inside Mars’ orbit and disrupt the orbit of planets further out. The survival of life on Earth under these conditions is out of the question and scientists are still debating whether our planet will physically survive or be devoured by the sun. These latest findings suggest a bleak outcome is very likely.

“The star would have originally been about two solar masses, but now the white dwarf is only 70% of the mass of our Sun. It is also very small – roughly the size of the Earth – and this makes the star, and in general all white dwarfs, extremely dense,” Manser said in a statement.

“The white dwarf’s gravity is so strong – about 100,000 times that of the Earth’s – that a typical asteroid will be ripped apart by gravitational forces if it passes too close to the white dwarf.”

In order to find the planetesimal, researchers led by University of Warwick astrophysicist Christopher Manser employed a method called spectroscopy, which involves analyzing the different wavelengths of light emitted by an object. With the help of the Gran Telescopio Canarias in La Palma, Spain, the team of astronomers detected changes in the color of light emitted by a disc around the white dwarf known as SDSS J122859.93+10432.9, orbiting with a period of two to three minutes. The disc has a comet-like tail and is mostly made of iron, nickel, and other metals. It is the second solid remnant of a planet to have ever been discovered orbiting a white dwarf.

“The general consensus is that 5-6 billion years from now, our Solar System will be a white dwarf in place of the Sun, orbited by Mars, Jupiter, Saturn, the outer planets, as well as asteroids and comets. Gravitational interactions are likely to happen in such remnants of planetary systems, meaning the bigger planets can easily nudge the smaller bodies onto an orbit that takes them close to the white dwarf, where they get shredded by its enormous gravity,” Manser said.

Other objects might still orbit the dying distant star. However, the white dwarf is so faint that astronomers are unable to see anything orbiting farther out with their current tools. In the future, Manswer and colleagues plan on using spectroscopy to discover other planetary fragments orbiting white dwarfs.

“Learning about the masses of asteroids, or planetary fragments that can reach a white dwarf can tell us something about the planets that we know must be further out in this system, but we currently have no way to detect,” Manser concluded.

The findings appeared in the journal Science.

White dwarf goes nova after a long slumber

Years of observation have finally paid off as a team of Polish astronomers have captured an incredibly rare event from start to finish: a white dwarf going nova.

Artistic depiction of a white dwarf sucking up hydrogen from its companion star. K. Ulaczyk / Warsaw University Observatory

White dwarfs are very dense stellar remnants, without any fusion taking place inside of them. They are thought to be the final evolutionary state of stars whose mass is not high enough to become a neutron star, and it’s estimated that 97% of all stars in the Milky Way will eventually become such a star.

In order for a white dwarf to go nova, you usually need a dual system – a regular star and a white dwarf sucking its hydrogen until it eventually goes ‘boom’ in a huge white flash. In the case of V1213 Cen (aka Nova Centauri 2009), astronomers got lucky. They observed the system from 2009 when the explosion happened, being able to study the entire nova process from start to finish. The rarity lies herein, as we usually miss the start of such explosions.

“When novae or supernovae go off, they are usually followed up with many telescopes, and therefore we know a great deal about the ‘after’ of these explosions,” Carles Badenes, an astronomer at the University of Pittsburgh, who was not involved in the study, told the Verge. “But it is of course very hard to know…which star is going to do something interesting, so the ‘before’ is very much a mystery.”

Unlike a supernova, both stars can survive a nova. However, the process takes a very long time to shape up.

“From time to time such systems undergo large-amplitude brightenings. The most spectacular eruptions, with a ten-thousandfold increase in brightness, occur in classical novae and are caused by a thermonuclear runaway on the surface of the white dwarf,” the authors wrote in the study. “Such eruptions are thought to recur on timescales of ten thousand to a million years.”

But in the meantime, the data is being used to refine the so-called hibernation model – the theory that nova explosions are intertwined with periods of hibernation, in which the dual star system lies dormant. The white dwarf is now in a hibernation stage, but it’s considerably brighter than it was before the explosion. This would suggest that there will be another explosion, though sometime in the next couple of million years.

“Within the six years before the explosion, the system revealed dwarf nova outbursts indicative of a low mass-transfer rate. The post-nova is two orders of magnitude brighter than the pre-nova at minimum light with no trace of dwarf nova behaviour, implying that the mass-transfer rate increased considerably as a result of the nova explosion.”

Journal Reference: The awakening of a classical nova from hibernation.

Scientists discover first pulsing white dwarf binary star

Scientists from the University of Warwick have discovered a new type of binary star – a pulsing white dwarf. This rapidly-spinning, burnt-out star sweeps beams of particles and radiation over its companion red dwarf, a behavior that has never been observed in this type of star.

Illustration of the pulsing white dwarf lashing particles and radiation onto its companion red dwarf. Credit: University of Warwick

Illustration of the pulsing white dwarf lashing particles and radiation onto its companion red dwarf. Credit: University of Warwick

The unique star was initially discovered by a group of amateur astronomers back in May of 2015. After the initial discovery, the University of Warwick spearheaded a combined effort between amateur and professional astronomers to get a better look at the star system, which is named AR Scorpii or AR Sco for short.

“AR Sco was discovered over 40 years ago, but its true nature was unsuspected until we observed it last May with a high-speed astronomical camera called ULTRACAM on the William Herschel Telescope,” said Tom Marsh of the University of Warwick and lead author of the study. “We realized we were seeing something extraordinary within minutes of starting to observe it.”

The pulsing white dwarf is found in the constellation Scorpius approximately 380 light-years from Earth. It is 200,000 times more massive than the Earth and is in a 3.6-hour orbit with its cool red dwarf star companion, which is around one-third the mass of the Sun.

AR Sco creates beams of radiation and particles that lash its red dwarf star, causing the entire system to light up and fade away twice every two minutes. This unique process accelerates electrons in the red dwarf’s atmosphere to close to the speed of light, which has never been observed in similar types of stars. The rapidly-spinning magnetic field of the white dwarf accelerates these electrons, although their exact location in the red dwarf’s atmosphere is still not known.

“We’ve known pulsing neutron stars for nearly fifty years, and some theories predicted white dwarfs could show similar behavior,” said Boris Gänsicke of the University of Warwick and co-author of the study. “It’s very exciting that we have discovered such a system, and it has been a fantastic example of amateur astronomers and academics working together.”

Journal Reference: A radio-pulsing white dwarf binary star. 27 July 2016. 10.1038/nature18620

Hubble captures the death of a star, offering a glimpse of our sun’s final days

A spectacular image captured by the Hubble Space Telescope’s Wide Field Planetary Camera 2 (WFPC2) gives us a glimpse into how the Sun will look at its death.

Launched in 1990, the Hubble Space Telescope is among the most powerful and versatile tools astronomers have at their disposal even to this day. On Monday, the European Space Agency released a photo taken bu Hubble’s WFPC2 of the planetary nebula Kohoutek 4-55 that reminds us that nothing under the sun lasts forever — but the star itself also abides by that saying.

Five billion years from now, this is most likely how the sun will look. By then, the star is anticipated to be on the throes of death.
(Photo : NASA, ESA and the Hubble Heritage Team (STScI/AURA). Acknowledgment: R. Sahai and J. Trauger (JPL))

This photo is a composite image of three individual shots taken at specific wavelengths, to allow researchers to distinguish light from particular gas atoms. The red wavelength corresponds to nitrogen gas, blue to oxygen and green signifies hydrogen.

At the center of the colorful swirl of gas is a star, about the same size as the sun, on the throes of death. The star is about as massive as the sun. As stars age and consume their fuel, the nuclear reactions that produces their light and warmth start to slow down; The irregular energy patterns of energy production causes aging stars to pulsate irregularly making them eject their outer layers.

As the outer layers of gases are released the star’s core is revealed, giving of massive amounts of UV light. That radiation is responsible for the glow of the gas and the nebula’s beauty.

The sun is anticipated to behave in a similar manner to the Kohoutek 4-55 star,ejecting its outer layers to reveal its core — until it gradually cools down into a white dwarf. The image allows scientists a glimpse the distant future of our sun, expected to die off 5 billion years from now.

“By that time, Earth will be long gone, burnt to a crisp as the Sun dies,” ESA wrote. “But the beauty of our star’s passing will shine across the Universe.”

Pulsars with black holes could hold the ‘holy grail’ of gravity

Pulsars and black holes, two of the most enigmatic celestial bodies in the Universe may actually hold the key to understanding how Einstein’s theory of relativity and gravity interact.

Artistic depiction of a pulsar and the emitted radiation. Image via National Radio Astronomy Observatory.

A pulsar is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. Pulsars are from when a star that turns becomes a supernova and then collapses into a neutron star; the neutron star maintains its angular momentum, but because it has lost most of its mass, it starts to spin incredibly fast –  usually between a 2 and 50 times per second! The longest known spin period is just over 8 seconds. Due to this spin, pulsars are also excellent time keepers, as they emit intermittent light at regular intervals. Now, researchers believe that pulsars could be used to put Einstein’s theory of relativity to the test, especially if a pulsar would be found in the vicinity of a black hole. The only problem is that so far, this scenario has never been encountered.

“Pulsars act as very precise timekeepers, such that any deviation in their pulses can be detected,” Diego F. Torres, ICREA researcher from the Institute of Space Sciences (IEEC-CSIC), explains. “If we compare the actual measurements with the corrections to the model that we have to use in order for the predictions to be correct, we can set limits or directly detect the deviation from the base theory.”

Deviations mentioned by Torres occur when there is an object with significant mass close to the pulsar; in the lack of a black hole, that’s usually a white dwarf or another neutron star. By analyzing the interactions between pulsar-white dwarf or pulsar-neutron star interactions, astrophysicists can put not only the theory of gravity, but also Einstein’s relativity to the test. In the theory of relativity, the gravitational movement of a body results from the accelerating force exerted by the gravitational fields and nothing else. It is relatively constant in direction and magnitude. In other words, if you set up a free-fall experiment in a laboratory, the results will be independent on where the laboratory is in space and time and will depend only on the gravitational force(s).

This has been confirmed by previous observations, but in a new study, Torres and his colleague Manjari Bagchi argue that if you really want to put this idea to the test, you need to find a pulsar-black hole system; all that’s left now… is to actually find one.

: This artist's conception shows the suspected progenitor of a new kind of supernova called Type Iax. Material from a hot, blue helium star at right is funneling toward a carbon/oxygen white dwarf star at left, which is embedded in an accretion disk. In many cases the white dwarf survives the subsequent explosion. Image is provided courtesy of Christine Pulliam (CfA).

New type of supernova discovered. Hint: it’s tiny and faint

Supernovae are highly energetic events caused by the explosion of stars that are at times so bright they can outshine whole galaxies. These are thought to occur in two varieties, but a recently published paper has a described a third type of supernova, one that’s fainter than the other two and distinguishes itself by the fact that its parent star isn’t necessarily obliterated in the supernova event.

The two main types of supernovae discovered thus far are core-collapse and Type Ia supernovae. The first is the brightest and most energetic typically occurring in the wake of the explosion of a star 10 to 100 times as massive as our sun. Type Ia supernovae on the other hand surface when white dwarf stars are destroyed – faint star remnants that have passed their lifetime and are out of fuel.

: This artist's conception shows the suspected progenitor of a new kind of supernova called Type Iax. Material from a hot, blue helium star at right is funneling toward a carbon/oxygen white dwarf star at left, which is embedded in an accretion disk. In many cases the white dwarf survives the subsequent explosion. Image is provided courtesy of Christine Pulliam (CfA).

: This artist’s conception shows the suspected progenitor of a new kind of supernova called Type Iax. Material from a hot, blue helium star at right is funneling toward a carbon/oxygen white dwarf star at left, which is embedded in an accretion disk. In many cases the white dwarf survives the subsequent explosion. Image is provided courtesy of Christine Pulliam (CfA).

The newly discovered category of supernovae is called a Type Iax and essentially encompasses tiny supernovae that are fainter than Type Ia supernovae and which, as the latter, come from exploding white dwarfs. The main difference between the two lies in the fact that while a Type Ia will completely obliterate the generating white dwarf, a Type Iax won’t necessarily cause this.

The team of astronomers at Carnegie Institute for Science, led by Max Stritzinger, has identified so far 25 examples of the new type of supernova, none of which having been found in elliptical galaxies, typically filled with older stars, suggesting Type Iax supernovae are generated by young star systems. The reason they haven’t been identified until now is because they’re very faint and only recently after a technological barrier was breached could astronomers study them.

Based on their collection of astronomical data, the researchers claim Type Iax supernovae come from binary systems formed out of a white dwarf and a companion  star that has lost its outer hydrogen, leaving it helium dominated. The latter becomes thus exposed to the hungry for fuel white dwarf that will feed helium off the normal star.

The exact mechanisms that trigger Type Iax haven’t been identified yet, but the researchers believe it’s possible  the outer helium layer ignites first, sending a shock wave into the white dwarf. Just as well,  the white dwarf might ignite first due to the influence of the overlying helium shell.

Oddly enough, though newly discovered, it’s believed Type Iax are about a third as common as Type Ia supernovae. “The closer we look, the more ways we find for stars to explode,” the authors note.

The Type Iax supernovae have been reported in a paper published in The Astrophysical Journal

Starlight shining through the atmosphere of an exoplanet can reveal its chemical composition. (c) ESA

Alien life hunters hold white dwarf stars as safest bet

Though hundreds of potentially life harboring exoplanets have been discovered thus far, until the James Webb Space Telescope becomes operational, sometime around 2018, scientists today lack the resources to peer into the guts of these planet and  determine a realistic chance of hosting life. Even when the JWT goes live, however, it will take hundreds of hours of observations to come up with solid data for planets orbiting stars similar to the Sun.

Artist impression of a possible Earth-like planet capable of harboring life orbiting around its parent white dwarf star. (c) David A. Aguilar (CfA)

Artist impression of a possible Earth-like planet capable of harboring life orbiting around its parent white dwarf star. (c) David A. Aguilar (CfA)

Scientists at the Harvard-Smithsonian Center for Astrophysics have found that, given out current technological limitations, we should concentrate our efforts of finding life harboring planets to those orbiting white dwarf stars. At the end of its evolution, if it isn’t massive enough, a dying star will turn into a white dwarf – a very dense star no longer capable of sustaining  nuclear fusion.

Nuclear fusion is what powers most star, including our Sun, allowing energy to spread through sunlight. However, even without fusion a white dwarf can still emanate a considerable amount of residual thermal energy, enough to keep it warm for billions of years until it slowly fades away. Some white dwarfs actually have been found to continue to spread heat even though they’ve been dated from the dawn of the Universe.

What makes the James Web Telescope so important for alien life research is its cutting edge instruments, capable of inspecting the spectral fingerprint of the planet’s atmosphere. With today’s tech, astronomers can establish the orbit, size or mass of a planet based on its transient motion around its parent star. With the James Web Telescope, scientists will also be able to determine key chemical composition elements found in the atmosphere’s of alien worlds.

Starlight shining through the atmosphere of an exoplanet can reveal its chemical composition. (c) ESA

Starlight shining through the atmosphere of an exoplanet can reveal its chemical composition. (c) ESA

This can be achieved using a technique astronomers call transmission spectroscopy. Chemical elements in a planet’s atmosphere absorb some of the starlight, meaning more light than normal will be blocked at that particular wavelength associated with the element, thus offering a spectrum of the planet.

“Detecting any of these biomarkers in the atmosphere of an Earth-copy planet around a nearby normal star, using JWST, will be extremely challenging, if not impossible,” said Dan Maoz from Tel-Aviv University in Israel. “The difficulty lies in the extreme faintness of the signal, which is hidden in the glare of the ‘parent’ star. The novelty of our idea is that, if the parent star is a white dwarf, that glare is greatly reduced, and one can now realistically contemplate seeing the O2 biomarker. Detecting other biomarkers will require future space telescopes that are even more ambitious than JWST.”

Looking for life around dying stars

The technique seems wonderful, in theory, however in practice it’s a whole lot more difficult to apply. Most parent stars are very bright, causing noise that blocks planetary signals. A white dwarf, though very dim and difficult to detect, offers less resistance to spectroscopy surveys, thus offering the best chance at detecting a possibly life harboring planet. For planets around other types of stars, such as red dwarfs, observation might take hundreds of hours, compared to mere hours in the case of white dwarf stars.

“In the quest for extraterrestrial biological signatures, the first stars we study should be white dwarfs,” said theorist Avi Loeb in a CfA press release.

What are the signals astronomers are looking for? Well, the most important biomarkers are oxygen and methane, since these chemical elements are typically generated by life, on Earth at least, and would quickly degrade were it not for their constant regeneration.

No planets have yet been detected orbiting a white dwarf, due to the difficulty in observing these faint stars, however there is some evidence to suggest that such planets might exist. In the paper published in the journal MNRAS, the researchers believe that in a survey of 500 dwarf stars –  if a third of all white dwarfs host an Earth-mass planet within their habitable zones (where liquid water is supported) – one such planet might be found.

Multiwavelength X-ray / infrared image of SN 1572 or Tycho's Nova, the remnant of a Type Ia supernova (NASA/CXC/JPL-Caltech/Calar Alto O. Krause et al.)

Astronomers paint a clearer picture of how supernovae are born

Supernovae are one of the most energetic and brightest events in the cosmos, often so powerful they outshine whole galaxies. They’re considered  to play a major role in our understanding of the Universe, which is why scientists have invested so much time and effort into studying them. A recent study of X-ray and ultraviolet observations from NASA’s Swift satellite has helped astronomers understand better how Type Ia supernovae come to be.

Multiwavelength X-ray / infrared image of SN 1572 or Tycho's Nova, the remnant of a Type Ia supernova (NASA/CXC/JPL-Caltech/Calar Alto O. Krause et al.)

Multiwavelength X-ray / infrared image of SN 1572 or Tycho's Nova, the remnant of a Type Ia supernova (NASA/CXC/JPL-Caltech/Calar Alto O. Krause et al.)

A Type Ia supernova forms when a white dwarf, the remnant of a star that has completed its normal life cycle and has ceased nuclear fusion,  reaches a critical mass and detonates. This certain supernova family has been found to be extremely useful to astronomers’ studies, who have used their intense brightness as beacons or candle lights to determine vast distances in space. Also, studies of Type Ia supernovae led to the discovery of dark energy, which garnered the 2011 Nobel Prize in Physics.

Despite the fact astronomers have known for decades how Type Ia supernovae form, the exact mechanisms that lead to their formation are currently yet obscured.

“For all their importance, it’s a bit embarrassing for astronomers that we don’t know fundamental facts about the environs of these supernovae,” says Stefan Immler, an astrophysicist at NASA’s Goddard Space Flight Center.

“Now, thanks to unprecedented X-ray and ultraviolet data from Swift, we have a clearer picture of what’s required to blow up these stars.”

What sets off a supernova

The main model of formation for a Type Ia supernova involves a close binary star system. There are two dominant theories regarding this. The first and most popular theory currently suggests a white dwarf orbits a normal star and pulls a stream of matter from it, feeding from it until it reaches the necessary mass and explodes into a supernova. A second possible mechanism for triggering a Type Ia supernova is the merger of two white dwarfs, which collide like vast hypermassive billiard balls leading to a cataclysmic blast.

NASA’s Swift satellite, which orbits the Earth and is primarily used to sniff out gamma-ray bursts emitted from far away black holes, is also used from time to time to study supernovae. Its latest find came after it was directed towards the closest Type Ia supernova, called SN 2011fe, offering scientists data that suggest the white dwarf from which it sprang was a particularly picky eater.

“It’s hard to understand how a white dwarf could eat itself to death while showing such good table manners,” said Alicia Soderberg of the Harvard-Smithsonian Center for Astrophysics (CfA).

Namely, the astronomers couldn’t find any signs or traces left behind from a possible star explosion, the supernova exploded perfectly clean. Additional studies using NASA’s Swift satellite, which examined a large number of more distant Type Ia supernovae, appear to rule out giant stars as companions for the white-dwarf progenitors. When X-ray data was studied, scientists couldn’t find any X-ray point source, indicating that supergiant stars, and even sun-like stars in a later red giant phase, likely aren’t present in the host binaries. Swift’s X-ray Telescope (XRT) has studied more than 200 supernovae to date, of which about 30 percent are Type Ia.

Also, Swift’s Ultraviolet/Optical Telescope (UVOT) looked at 12 Type Ia supernova events within 10 days since their explosion. If the supernova would’ve been triggered by the interaction with larger, brighter stars, then its shock wave should have produced an enhanced ultraviolet light. Nothing of the kind was detected, which combined with other studies findings and X-ray evidence suggests Type Ia supernovae likely originate from a more exotic scenario, possibly the explosive merger of two white dwarfs.

“This is an exciting time in Type Ia supernova research since it brings us closer to solving one of the longest-standing mysteries in the life cycles of stars,” said Raffaella Margutti of the CfA, lead author of the second paper.

The researchers’ findings are set for publishing in April in the journals The Astrophysical Journal Letters and The Astrophysical Journal.

Coldest star so far found – not hotter than a cup of coffee

An artist's impression of the coldest brown dwarf found so far, CFBDSIR 1458 10b captioned on the right. Image (c) L. Calçada, ESO.

Astronomers usually classify stellar objects by a spectra going from hotter to cooler, using the letters O, B, A, F, G, K, and M. As observational technology progressed and a myriad of new astronomical findings were made, in the last 15 years alone two new classes  L and T emerged designed to describe ultracool brown dwarfs. A recent scientific finding suggests that yet another spectra might need to be added to accommodate the coldest star discovered so far.

Dubbed CFBDSIR 1458 10b, the brown dwarf has a remarkably low surface temperature of  97 degrees C (206 degrees F) –  just about as hot as a freshly made morning cup of coffee.

Over the years there has been steady but slow progress in pushing the boundaries of finding the coldest stars,” said study leader Michael Liu, an astronomer at the University of Hawaii were a team of researchers studied and published a paper about CFBDSIR 1458 10b.

“But with this latest discovery we have made a big leap forward—besting the previous record holder by at least 150 Kelvin [270 degrees F, or 150 degrees C],” he said.

Astronomers using the Keck II telescope recorded this composite infrared image of the brown dwarf binary designated CFBDSIR J1458+10. The fainter component is at present date considered the coldest star discovered so far. (c) Michael Liu - Univ. of Hawaii

CFBDSIR J1458+1013B did not cool down after starting out hot like our Sun, instead “it never became very hot in the first place because it developed from a fairly small cloud of gas,” said Duane Pontius, professor of physics at Birmingham-Southern College (BSC) in Alabama. “Gravity pulled the cloud together and compressed the gas, which heats it up just as a bicycle pump heats up when you compress air into a tire. But relative to brighter stars, there wasn’t as much gas, so this star never heated up much.”

This means that because it has such a low gravitational energy and mass, the dwarf was never able to sustain hydrogen fusion reactions in its core, which creates scorching high surface temperatures like 5,500 degrees Celsius on our own sun (a fairly low temperature compared to other stellar bodies). Astronomers at Keck II telescope, Canada-France-Hawaii telescope, and European Southern Observatory Very Large Telescop managed to describe CFBDSIR J1458+1013B after tracing it’s very dim infrared signature. The dwarf is paired in orbit with yet another dwarf star, the later a lot brighter though.

The study seems to gray the line between scientists decide what can be considered a planet and what can be considered a star, since CFBDSIR J1458+1013B is estimated to have a mass only 6 to 15 the mass of Jupiter, which has a surface temperature of -149 degrees C (-236 degrees F ).

“…this new object is so much colder than anything else seen that it now enters the regime where it may actually have an atmosphere with water clouds,” Liu said.

“The most exciting aspect of this finding is that we might be on the threshold of finding a new class of objects that blurs the line between gas-giant exoplanets and brown dwarf stars previously seen—something I think that is really surprising the astronomical community.”

You think CFBDSIR J1458+1013B is pretty cold for a star? Well, NASA scientists are trying to determine the exact temperature of a newly discovered brown dwarf, called WD 0806-661b, which is believed to have a temperature of roughly ~30 degrees Celsius and a mass 7 times that of Jupiter.

“I think it’s pretty neat to find a ‘star’ that could have a temperature similar to that of Earth,” says Kevin Luhman (Penn State University), who led one of the observing teams. Luhman and two colleagues used NASA’s infrared Spitzer Space Telescope to study WD 0806-661b, the companion of a faint white-dwarf star 63 light-years distant in the southern constellation Volans.

Story via skyandtelescope.com

Physicists create a supernova in a jar

A supernova is a stellar explosion of cosmic proportions, that often can outshine the entire galaxy it is located in, before fading away in a matter of weeks or months. During this short period however, supernovae emit as much energy as the Sun emits during its entire life span – it’s the same kind of phenomena that researchers from the university of Toronto and Rutgers managed to mimic at scale.

In a certain (quite common) type of supernova the detonation starts with a flame ball that is buried deep in a white dwarf; the flame ball is much hotter and brighter than the environment surrounding it, so it rises rapidly making a plume topped with an accelerating smoke ring.

The autocatalytic reactions mainly do two things: they release heat and change the chemical composition of the liquid, which causes some forces that stir it, further progressively amplifying the effects.

“A supernova is a dramatic example of this kind of self-sustaining explosion in which gravity and buoyancy forces are important effects. We wanted to see what the liquid motion would look like in such a self-stirred chemical reaction,” says Michael Rogers, who led the experiment as part of his PhD research, under the supervision of Morris.

“It is extremely difficult to observe the inside of a real exploding star light years away so this experiment is an important window into the complex fluid motions that accompany such an event,” Morris explains. “The study of such explosions in stars is crucial to understanding the size and evolution of the universe.”

“We created a smaller version of this process by triggering a special chemical reaction in a closed container that generates similar plumes and vortex rings,” says Stephen Morris, a University of Toronto physics professor.

White dwarf considered the hottest ever discovered

Astronomy & Astrophysics has published spectroscopic observations of one of the hottest stars discovered so far – a white dwarf called KPD 0005+5106. The observations were made using the space-based Far-Ultraviolet Spectroscopic Explorer (FUSE), showing that the temperature can reach 200 000 K at the surface of the star.

Temperatures can get so high that the emission lines are exhibited by the photosphere in the ultraviolet spectrum, this being the first time when the phenomenon was ever observed by specialists. This happens because of the extremely ionized calcium which holds the record for the highest ionization stage of a chemical element ever to be discovered in a photospheric stellar spectrum.

Usually, stars which have intermediate masses (no lighter than the sun and no heavier than 8 solar masses) end up as white dwarves of the size of the Earth, once their nuclear fuel has been terminated. During this process, stars become very hot, scientists knowing them to reach 100 000 K. Even though it was thought that they can reach much higher temperatures, finding supporting evidence was hard because the process is much shorter in these cases.

Since it was discovered in 1985, KPD 0005+5106 attracted specialist’s attention because its atmosphere was dominated by helium, thus reaching higher temperatures than other similar stars. Observations made with the Hubble Telescope showed that the star had reached 120 000 K, which was a record at the time.

Observations were continued by a team including K. Werner, T. Rauch, and J.W. Kruk, more and more data being gathered until finally the star reached 200 000 K.

Scientists believe that similar stars may be discovered in the future. They raised a lot of interest as their composition, different in many ways from the Sun – concerning helium and calcium values and properties-, brings a challenge to the concepts of stellar evolution because these stars were not predicted by its models.

Thank you

Astronomy & Astrophysics

for providing the data and raw material.

Astronomers Discover Stars With Carbon Atmospheres

stars with carbon
We have another piece of evidence which goes to show that we fail to understand numerous things about our universe. Astronomers have discovered white dwarf stars with pure carbon atmospheres. It is something that probably nobody would have believed.

The exact way these stars evolved is still pretty much a mystery for astrophysicists. They believe that this kind of stars evolved from stars which are not quite massive enough to explode as supernovae but are just on the borderline. Here is how the sequence is. Think of a star as a fire. It burns out helium and leaves “ashes” of carbon and oxygen. So when the fuel is no more the star dies as a white dwarf. They are very dense as they have the mass of about a star and the volume of about our planet.

The scientists believe that a white dwarf has a core made of carbon and oxygen which is hidden from view by a surrounding atmosphere of hydrogen or helium. But they were baffled to find out that these stars have carbon atmospheres.

“We’ve found stars with no detectable traces of helium and hydrogen in their atmospheres,” said University of Arizona Steward Observatory astronomer Patrick Dufour. “We might actually be observing directly a bare stellar core. We possibly have a window on what used to be the star’s nuclear furnace and are seeing the ashes of the nuclear reaction that once took place.”. “When I first started modeling the atmospheres of these hotter DQ stars, my first thought was that these are helium-rich stars with traces of carbon, just like the cooler ones,” Dufour said. “But as I started analyzing the stars with the higher temperature model, I realized that even if I increased the carbon abundance, the model still didn’t agree with the SDSS data,” he added.

Here is something interesing to ponder. It was discovered somewhat random. In May 2007, “out of pure desperation, I decided to try modeling a pure-carbon atmosphere. It worked,” Dufour said. “I found that if I calculated a pure carbon atmosphere model, it reproduces the spectra exactly as observed. No one had calculated a pure carbon atmosphere model before. No one believed that it existed. We were surprised and excited. We don’t know if these carbon atmosphere stars are the result of nine-or-10 solar mass star evolution, which is a key question,” Liebert said. But this discovery is very important and it sheds a bit of light in what concerns the life of a star.