Tag Archives: planetary nebula

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.”

The Hubble Telescope Captures Image of Rare “Cosmic Butterfly”

Hubble has recently captured a dazzling image of a “cosmic butterfly” – the planetary nebula (PN) M2-9. The star has not only ejected its outer layers, but exposed its inner core, which is now illuminating the layers in a spectacular and violent display.

Minkowski's butterfly - Hubble captures spectacular photo of a planetary nebula.

Minkowski’s butterfly – Hubble captures spectacular photo of a planetary nebula.

The M in this name refers to Rudolph Minkowski, the German-American astronomer who discovered this particular nebula in 1947. “Planetary nebula” is technically a misnomer, because the term denotes an expanding glowing shell of ionized gas ejected from an old red giant star (or several). Just 20% of all observed nebulas are spherically symmetric, and a wide variety of shapes exist with some very complex forms seen; this particular one is bipolar – it involves two stars.

The two stars are about the same mass and size as the Sun, ranging from 0.6 to 1.0 solar masses for the smaller star, and from 1.0 to 1.4 solar masses for the other one. The larger star has ejected most of its outer material, while the smaller one has already ejected everything and has evolved into a white dwarf – a stellar remnant composed mostly of very dense electron-degenerate matter. The “wings” are still growing, and it’s estimated that this nebular ejection started about 1,200 years ago – extremely recent in astronomical terms.

Astronomers still aren’t sure if bipolar nebulas emerge from bipolar star systems, or if the two stars somehow got tangled together afterwards. The two seem to circle themselves every 100 years (approximately), and their rotation creates the butterfly wings – actually, very violent jets stripped by the white star from its companion. Recently, the nebula has inflated dramatically due to a fast stellar wind which blew out the surrounding disk and inflated the large, hourglass-shaped wings perpendicular to the disk.

 

The NGC 1846 globular cluster captured by the Hubble telescope. A hidden green gem lies hidden. (c) NASA

Spectacular spherical star cluster imaged by Hubble

The ever faithful Hubble telescope has offered us yet another fantastic glimpse into the Universe’s hidden gems. Captioned below is the NGC 1846 globular cluster, which lies 160,000 light-years away in the constellation Doradus. The cluster is actually the product of the Large Magellanic Cloud, which neighbors our own Milky Way.

The NGC 1846 globular cluster captured by the Hubble telescope. A hidden green gem lies hidden. (c) NASA

The NGC 1846 globular cluster captured by the Hubble telescope. A hidden green gem lies hidden. (c) NASA

Check out this very large, high resolution photo from NASA for a more detailed peak.

Red and blue dots represent aging stars, while middle-aged stars, which are younger than the sun, can be seen in their whitish color. Now, what’s most intriguing about this photo is that green dot. Haven’t seen it yet? Well, check out the high res photo again, head for the center of the cluster and scroll downwards. There you have it. Now, stars aren’t green of course. That’s actually what scientists describe as a planetary nebula –  the remnant of a succumbed star, stripped to its bare dense core after it puffed out its gaseous layers. These are extremely rare, and so far only a handful of planetery nebulas have been spotted. To make it increasingly difficult to observe, these last only a few thousand of years before they dim out completely.

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