Astronomers Found

Astronomers have found the source of life in the universe – Inverse

Every second, a star dies in the universe. But these stellar beings don’t just completely vanish, stars always leave something behind.

Some stars explode in a supernova, turning into a black hole or a neutron star, while the majority of stars become white dwarfs, a core of the star it once used to be. However, a new study reveals that these white dwarfs contribute more to life in the cosmos than previously believed.

The study, published Monday in the journal Nature Astronomy, suggests that white dwarf stars are the main source of carbon atoms in the Milky Way, a chemical element known to be crucial to all life.

White dwarf stars are a primary source for one of the building blocks of life. NASA and H. Richer (University of British Columbia)

When stars like our own Sun, a yellow dwarf star, run out of fuel, they turn into a white dwarf. In fact, 90 percent of all stars in the universe end up as white dwarf stars.

White dwarfs are hot, dense stellar remains with temperatures that reach 100,000 Kelvin. Over time, billions of years, these stars cool and eventually dim as they shed their outer material. However, right before they collapse, their remains are transported through space by winds that originate from their bodies.

These stellar ashes contain chemical elements such as carbon.

Carbon is the fourth most abundant chemical in the universe and is a key element in the formation of life as it is the basic building block to most cells.

All of the carbon in the universe originated from stars, therefore the phrase that we are made of stars is not only poetic but rather accurate. However, astronomers could not agree on which type of star is responsible for spreading the most amount of carbon across the cosmos.

The scientists behind the new study used observations of white dwarfs in open star clusters, groups of a few thousand stars formed around the same time, in the Milky Way by the W. M. Keck Observatory in Hawaii in 2018.

They measured the stars’ initial-final mass relation, which is the relationship between the stars’ masses when they first formed and their masses as white dwarfs.

Usually, the larger the star was, the more massive a white dwarf will be. However, the study found that the stars’ masses as white dwarfs were larger than the scientists had anticipated considering their initial mass when they first formed.

“Our study interprets this kink in the initial-final mass relationship as the signature of the synthesis of carbon made by low-mass stars in the Milky Way,” Paola Marigo, a researcher at the University of Padua in Italy, and lead author of the study, said in a statement.

The team of scientists concluded that stars bigger than 2 solar masses also contributed to the galactic enrichment of carbon, while stars of less than 1.5 solar masses did not.

“Now we know that the carbon came from stars with a birth mass of not less than roughly 1.5 solar masses,” Marigo said.

The new study suggests that carbon was essentially trapped in the raw material that formed the Solar System 4.6 billion years ago.

Abstract: The initial–final mass relation (IFMR) links the birth mass of a star to the mass of the compact remnant left at its death. While the relevance of the IFMR across astrophysics is universally acknowledged, not all of its fine details have yet been resolved. A new analysis of a few carbon–oxygen white dwarfs in old open clusters of the Milky Way led us to identify a kink in the IFMR, located over a range of initial masses, 1.65 ≲Mi/M≲ 2.10. The kink’s peak in white dwarf mass of about 0.70−0.75 M is produced by stars with Mi ≈ 1.8−1.9 M, corresponding to ages of about 1.8−1.7 Gyr. Interestingly, this peak coincides with the initial mass limit between low-mass stars that develop a degenerate helium core after central hydrogen exhaustion, and intermediate-mass stars that avoid electron degeneracy. We interpret the IFMR kink as the signature of carbon star formation in the Milky Way. This finding is critical to constraining the evolution and chemical enrichment of low-mass stars, and their impact on the spectrophotometric properties of galaxies.

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Astronomers First

Astronomers see first light flare from two distant black holes colliding – The Verge

A whopping 7.5 billion light-years from Earth, two black holes, each about the size of Long Island, rapidly spun around each other several times per second before smashing together in a cataclysmic explosion that sent shockwaves through the Universe. Normally, violent unions like this are dark events, but astronomers think they saw a flare of light emerge from this celestial dance — potentially the first time light has ever been seen from black holes merging.

It’s a unique discovery since black holes are notorious for not producing any light at all. These super dense objects are so massive that nothing can escape their gravitational pull — not even light. So how exactly did researchers see a flare from two black holes that aren’t supposed to flare?

Well, the black holes may have just been in the right place at the right time, according to a new study published in the journal Physical Review Letters. When they spun together, they were located inside a giant disc of gas and dust. This disc of material spans light-years and actually surrounds a third black hole — a supermassive one at the center of a galaxy. Since the dueling black holes were inside this dusty environment, their spinning and eventual merger created something like a shock wave that slammed into the surrounding dirt and gas. That heated up the nearby material, causing it to glow brighter than normal — and allowing researchers from Earth to spot it.

“If it’s two black holes merging, you don’t expect to see anything,” Matt Graham, a research professor of astronomy at Caltech and lead author of the study, tells The Verge. “But because the black holes are surrounded by this stuff, by this accretion disc, that’s different.”

The researchers pinpointed this oddball event with the help of the LIGO-Virgo collaboration, an international scientific partnership that’s become increasingly skilled at detecting cataclysmic events like black holes merging. More specifically, LIGO and Virgo seek out tiny ripples in the fabric of the Universe, known as gravitational waves, that stem from distant celestial events. Whenever two massive objects in the faraway Universe merge, they create undulating waves in the fabric of space and time that travel outward at the speed of light. When they reach Earth, such ripples are very tiny, but LIGO’s two observatories in the US and Virgo’s observatory in Italy are just sensitive enough to pick them up.

LIGO made history in 2015 when the collaboration detected gravitational waves for the first time from two black holes merging. Since then, LIGO and now Virgo, which came online in 2017, have been beefing up their resumes, detecting a whole slew of mergers throughout the Universe, including those of black holes, neutron stars, and maybe even a black hole colliding with a neutron star. When neutron stars collide, the mergers can sometimes be picked up by observatories that measure their light, even though the objects are really faint. When black holes collide, it’s not something we can see — until perhaps now. “It’s a weird and wonderful event, and in fact we don’t know how rare they are,” Chiara Mingarelli, an assistant professor at the University of Connecticut studying gravitational waves, who was not involved in the study, tells The Verge.

One of LIGO’s observatories in Livingston, Louisiana.
Image: LIGO

To find this flare, Graham and his colleagues capitalized on LIGO’s triumph at finding mergers throughout space to help them solve a puzzle. Graham and his team study really active supermassive black holes in galaxies — known as quasars — and they’d been noticing a weird trend. Sometimes these quasars would flare unexpectedly, glowing super bright without warning, and they wanted to know why. “And we sort of said, ‘Well I wonder what happens if you had black holes in that environment?’” says Graham.

Two of Graham’s colleagues, Saavik Ford and Barry McKernan, put out a paper theorizing that black holes merging in these gaseous discs could cause the mysterious flare-ups. “The idea that there might be black holes in the centers of galaxies, very nearby a supermassive black hole, is actually pretty uncontroversial,” Ford tells The Verge, adding, “[We] sat down to think about what the consequences of that might be, and we started to flesh out a theory that we’ve been pursuing for the last decade.”

They then decided to put that theory to the test. In 2019, LIGO did a third observational run, scanning for a new crop of mergers in space. Meanwhile, Graham and colleagues were working at Caltech’s Zwicky Transient Facility, which performs a survey of the entire night sky, looking for odd behavior — like flares in distant galaxies. The astronomers decided to wait about six months after LIGO’s observations had ended to see how many mergers the collaboration detected. They then tried to match up those mergers with the flares they had detected with ZTF, to see if any of them corresponded.

Once they got all the potential mergers from LIGO and Virgo, it was just a matter of narrowing everything down. They matched up all the flares they had seen with ZTF to the mergers LIGO had spotted, making sure they matched the right part of the sky, at the right distance from Earth. The team also looked at timing; they predicted that a flare caused by a merger would occur about 60 to 100 days after the collision took place, as it would take time for things to heat up and cause that glow. They then made sure the flares they found matched the right profile they expected, and it didn’t look like they’d been caused by an exploding star or some other explanation.

That ultimately led Graham and his team to the black hole merger they found. And actually finding something they’d theorized about was pretty exciting. “It’s the sort of thing that you dream about as a scientist,” says Ford, “to say, ‘I think the universe is going to do that. I’m going to call my shot.’ And have the Universe go, ‘Yeah, here you go!’”

Though, things still aren’t totally confirmed just yet. The black hole merger detected by LIGO-Virgo is still just a candidate; it hasn’t been officially named as a merger, and LIGO hasn’t released detailed data about the detection. But the good news is Graham’s team might get extra verification in the future that the flare they recorded did indeed come from swirling black holes. When the black holes merged, it’s likely the resulting black hole that was formed got kicked out of the surrounding dusty disc. However, that hole is still orbiting around the supermassive black hole at the center of the galaxy, and it’s probably going to cross paths with the hot disc of gas in a year or two, heating up the material and causing another bright flare. So if the team sees another brightening in the same galaxy, they’ll be pretty certain their findings were correct.

When that happens, the measurement of the flare could help the team learn more about this galaxy and better constrain just how massive the supermassive black hole is at the center. “It will actually allow us to directly probe these disks around supermassive black holes in ways that we that we couldn’t do before,” says Mingarelli.

This discovery also gives astronomers another clue about how some faraway galaxies form. It tells them that there may be strange objects doing strange things in the discs that surround supermassive black holes. “It’s not just a large gas disc falling into a supermassive black hole,” says Graham. “You’ve got stars and black holes in there doing things as well.”

Plus, this bizarre dance of black holes inside a giant gaseous disc may be the only way we can actually “see” black holes merging in deep space. And that’s even more information that researchers can use to study the cosmos. “We actually now have this probe, both from the electromagnetic signature, and the gravitational wave — both of which provide information,” says Ford. “It’s a brand new, totally different tool for studying how galaxies got to be the way they are.”

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Astronomers strange

Astronomers spot a strange, first-of-its-kind asteroid near Jupiter – Engadget

There’s still plenty of opportunity for astronomers to discover strange new objects. Researchers using the University of Hawaii’s Asteroid Terrestrial-impact Last Alert System (ATLAS) have found (via Gizmodo) a Trojan asteroid, 2019 LD2, that not only follows an odd orbit ahead of Jupiter but also sports an icy tail — it’s a unique “crossover” between asteroid and comet. It only appears to have been active for less than a year, too, which is unusual when Jupiter Trojans are often billions of years old and should have lost their ice a long time ago.

More data will be needed to determine just what led to this one-of-a-kind finding, but the university’s Institute for Astronomy suggested that Jupiter might have recently captured the asteroid from a distant (and thus colder) orbit, or that a collision with another space rock might have exposed ice that was previously ‘safe’ from the Sun’s heat. Either way, this revelation could offer more insights about the nature of the Solar System and its history.

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Astronomers discover

Astronomers discover regular rhythms in mysterious pulsating stars – CNET


A screenshot of a simulation showing the pulsations of star HD 31901

Chris Boshuizen/Simon Murphy/Tim Bedding

The stars speak! Understanding stars, including our own sun, has largely revolved around examining their outsides: the surfaces and surrounding atmospheres we can see. Although we can’t look inside a star, we can listen to the rumbling it makes based on pulsations and oscillations that occur in the interior. Studying the pulses, astronomers are able to decipher what’s happening in the heart of a star.

For a particular class of stars, known as delta Scuti stars, it has been difficult to nail down the rhythm. But now, thanks to NASA’s latest planet-hunting space telescope, astronomers have pulled back the insanely hot curtain on this class of stars to get a sense of what happens inside. 

A new study, published in the journal Nature on Wednesday, details the rhythm of dozens of delta Scuti stars, which are about two times as massive as our sun, finding they exhibit clear, obvious rhythms. The discovery provides a new way for astronomers to understand the unusual physics occurring within the hearts of these stars, which are relatively common across the galaxy.

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To tune into the insides of the stars, astronomers at the University of Sydney turned to NASA’s Transiting Exoplanet Survey Satellite, which has been performing an extensive survey of the cosmos since its launch in 2018. The satellite is designed to hunt for planets around other stars by dividing the sky into sectors and taking snapshots of all the blazing furnaces in its field of view. Every two minutes, TESS grabs a quick image of the sky and measures the brightness of thousands of stars to determine how that changes over time. A dip in brightness might correspond to a planet passing in front of the star.

But the research team wasn’t looking for distant planets. Instead, it used the extremely precise changes in brightness detected by TESS to look at the stars themselves.The incredibly tiny changes in brightness correspond to pulsations and oscillations in the heart of the star. Because TESS can resolve the brightness of stars so exquisitely, it creates a great data set for trying to listen to a star’s heartbeat. 

The team focused in on a batch of TESS data containing a sample of 92,000 stars and, with some clever coding, was able to develop a tool to quickly sift through the huge data set. A chance finding in the TESS data led to a list of around 1,000 stars with similar rhythms. Eventually, the researchers were able to nail down the list to 57 delta Scuti stars with discernible rhythms.

The stars are all relatively close to us, in galactic terms, lying between 60 and 1,400 light-years away. For reference, the Milky Way Galaxy is over 100,000 light-years wide. 

Tim Bedding, an astronomer at the University of Sydney and first author on the paper, said the new data allowed his team to “cut through the noise.”

“Previously we were finding too many jumbled up notes to understand these pulsating stars properly,” he said in a release. “It was a mess, like listening to a cat walking on a piano.” Using the TESS data, things became a lot clearer. Bedding says now it’s more like “listening to nice chords being played.”

The rhythms of many other types of stars have been discovered in the last few decades, including those of huge, red giants like Betelgeuse, which has allowed astronomers to determine what’s happening inside the blazing hot balls of gas. Although delta Scuti stars are widely distributed across the universe, previous research had failed to find a regular rhythm. 

“We think this is because they rotate rapidly, which makes the patterns less regular,” said Bedding.  

With a pattern of pulses now understood, future research will be able to more accurately determine the age of stars and help astronomers pick apart how galaxies and star systems might evolve. 

“We are now in a position to start to probe these stars, and to use them as benchmarks to help us interpret the huge numbers of other stars in the group that present more complicated pulsation spectra,” said Bill Chaplin, an astronomer at the University of Birmingham and co-author on the study. 

NASA’s TESS is still surveying the sky and sending mountains of data back to Earth each month. Bedding says it’s sometimes likened to “drinking from a fire hose,” and there are many more sectors to look through. His team will now examine other, more complex delta Scuti stars to see if they can identify patterns. 

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Astronomers Discovered

Astronomers have discovered closest black hole yet in trinary star system – Ars Technica

And a black hole makes three —

Just 1,000 light years from Earth, its two companion stars are visible to the naked eye.

Artist’s impression showing orbits of the objects in the HR 6819 triple system. There is an inner binary with one star (orbit in blue) and a newly discovered black hole (orbit in red), as well as a third star in a wider orbit (also in blue).

Enlarge / Artist’s impression showing orbits of the objects in the HR 6819 triple system. There is an inner binary with one star (orbit in blue) and a newly discovered black hole (orbit in red), as well as a third star in a wider orbit (also in blue).

ESO/L. Calçada

Astronomers with the European Southern Observatory (ESO) have discovered a black hole that is the nearest such object yet found, just 1,000 light years away—close enough to be seen with the unaided eye. It is part of a triple star system, dubbed HR 6819, and the ESO scientists believe other members of this class of systems may also harbor black holes that previously were not a high priority for black hole searches. They announced their discovery in a new paper published in the journal Astronomy and Astrophysics.

Scientists think there are far more black holes in the Universe than we have discovered to date—probably hundreds of millions of them, given the age of our Universe—because we can’t observe them directly; we can merely infer their presence by their effect on surrounding matter. A black hole’s gravitational effects can influence the orbits of nearby stars, for example, or infalling matter can form an accretion disk of hot gas rapidly orbiting the black hole, emitting powerful X-rays. Or an unfortunate star will get too close to a black hole and be torn apart for its trouble, with the infalling remnants also accelerating and heating up to emit X-rays into space.

But the majority of black holes are actually quiet and hence very difficult to detect. This latest discovery offers useful clues about where at least some of the truly dark black holes might be hiding. “One will never get enough telescope time to do a thorough search like that on all objects,” ESO scientist Thomas Rivinius, a co-author on the paper, told Ars. “What you need to do is a staged approach to help you identify candidates, then thin out the candidates list, and only then have a close and detailed look at the remaining ones. Knowing what to look for should put us in a better position to find them.”

The ESO team had been conducting a study of double-star systems, and HR 6819 was included as part of their observational data-gathering since it appeared to be just such a system. But while reviewing their data, the astronomers found clear evidence of an unexpected third object in the system: a black hole that had previously eluded detection.

In a trinary star system, two of the stars orbit each other as a binary pair, while the third star orbits the pair at a greater distance. This ensures that the system is stable, since if the inner and outer orbits were the same size, one of the stars would eventually be ejected from the system. In the case of HR 6819, one of the two visible stars orbits an invisible object every 40 days, while the other visible star orbits farther away. By studying the orbit of the star in the inner pair, the team was able to infer the black hole’s presence and also calculate its mass. “An invisible object with a mass at least four times that of the Sun can only be a black hole,” said Rivinius.

  • Wide field view. While the black hole is invisible, the two stars in HR 6819 can be viewed from the southern hemisphere on a dark, clear night without binoculars or a telescope.

    ESO/Digitized Sky Survey 2, with Davide De Martin

  • Chart showing the location of the HR 6819 triple system in the constellation of Telescopium.

    ESO, IAU and Sky & Telescope

  • Southern sky over La Silla, ESO’s first observatory site, just after sunset.

    ESO/José Francisco Salgado

“We used to believe that single stars are the most usual ones,” said Rivinius. “In fact, at least for the really massive ones, single stars are probably the rarest.” That’s because the greater a star’s mass, the less likely it is to be alone, and Rivinius points out that even single massive stars could, in fact, be the survivors of multiple star systems that were “disrupted,” or have fainter companion stars we just can’t detect. Trinary systems like HR 6819 are less common, but nor are they extremely rare. Physicists currently believe that the supernovae that give rise to black holes would actually disrupt the structure of multiples. “If a significant number of multiples, however, survive the supernovae, this changes the statistics,” said Rivinius.

“If such a system happens to be in the immediate neighborhood, it is likely common in other regions of the galaxy as well,” said Rivinius. His back-of-the-envelope calculation suggests that there could be 2,500 such systems. That’s not going to clear up the large discrepancy between the black holes we’ve discovered and the number astronomers believe could be out there. “But considering so far we were not aware any such triple could exist, it is quite a step,” he added. The ESO team has already identified a second star system that might also be a trinary with a black hole, although more observational data is needed to confirm this.

The discovery that a black hole can be part of a trinary star system is also relevant because astronomers have suggested that such triple systems could be progenitors of binary systems with two black holes, or a black hole/neutron star pairing. When the partners in those binary systems merge, the violent event emits gravitational waves that can be detected by the LIGO collaboration.

“The problem with LIGO detections is that for two black holes in a normal, lonely binary, it takes a very long time to close in to each other, until they finally merge,” said Rivinius. “In fact, it takes longer than the current age of the Universe, and we really shouldn’t see as many mergers as we do, if that was the only mechanism. But the closer they are already, the (much) faster it goes.”

It’s known as the Lidov-Kozai mechanism. It occurs when, for example, a close binary inner pair has a circular orbit, but not in the same plane as the outer orbit. This causes the inner orbit to become more “eccentric,” according to Rivinius. “In short, it means the third body can help the two inner ones get close to each other, at least at times,” he said.

However, that is not going to be the case with the HR 6819 trinary system. “The two stars in HR 6819 are not massive enough to explode as a supernova and form a black hole,” said ESO’s Dietrich Baade, another co-author. “Therefore, HR 6819 will never harbor two black holes, and it will never be a full equivalent of the progenitors of gravitational wave events. But it is a useful nearby proxy to investigate.”

DOI: Astronomy and Astrophysics, 2020. 10.1051/0004-6361/202038020  (About DOIs).

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Astronomers changing

Astronomers are changing the way we think of ‘potentially habitable’ planets – CNN

(CNN)Astronomers are seeking to understand the types of exoplanets, and their atmospheric and environmental conditions, that could potentially host life outside of our solar system. But since we can’t visit exoplanets for an up-close look, data from telescopes and modeling of atmospheres, climates and environments provide the picture we can’t see.

The models, hypotheses and lab experiments of exoplanet researchers are a test bed to question what’s possible.
Among the search for weird exoplanets — some so alien when compared with our own solar system — many researchers are focused on a key phrase: “potentially habitable.”
Exoplanets that are referred to as potentially habitable don’t show signs of life. It means that the planet is at the right distance from its star so that it’s in the so-called Goldilocks zone, or habitable zone — not too hot, not too cold and just right, within a possible surface temperature range where liquid water could exist on the planet’s surface. We equate liquid water with life on Earth, and it has informed our search for life beyond it as well.
But more research in recent years is suggesting that we broaden our understanding of life and the conditions under which it could form.
Two new studies broaden the range of conditions and locations of exoplanets that could be potentially habitable. Having that research in hand as new, advanced telescopes come online in the next few years and decades could aid in the search for exoplanets that may not have been of interest before and provide targets for future study.
Now, they just have to find these exoplanets.

Exoplanet atmospheres

The Hubble Space Telescope became the first to make direct detections of exoplanet atmospheres and study their composition. Future telescopes, like NASA’s James Webb Space Telescope that’s scheduled to launch next year, will be able to characterize exoplanet atmospheres in greater detail.
If we were to study Earth’s atmosphere that way, we’d find that it’s 78% nitrogen, 21% oxygen, 0.9% argon and 0.03% carbon dioxide with trace elements of other gases.
Astrophysicist and planetary scientist Sara Seager and her colleagues this week released a new study examining how E. coli bacteria and yeast, representatives of single-celled prokaryotes and eukaryotes, would react to a hydrogen atmosphere.
The study published Monday in the journal Nature Astronomy.
The lab-based experiments relied on growing E. coli and yeast in a 100% hydrogen atmosphere. Both reproduced normally, but at lower and slower rates than in oxygenated air. For example, E. coli reproduced two times slower and yeast was 2.5 times slower. The researchers believed this is due to a lack of oxygen.
But Seager and her colleagues wanted to see if the microorganisms would survive and reproduce, which they did. The researchers weren’t surprised because hydrogen is nontoxic, but their study acts as proof-of-concept research for exoplanet scientists, Seager said.
Rocky exoplanets that are larger than Earth can maintain a lot of hydrogen in their atmospheres. These atmospheres would likely be larger and more extended because hydrogen is a lightweight gas. And an extended atmosphere would be easier for future telescopes, like Webb and others, to detect, Seager said.
“We hope to shift astronomers’ perspective that life in general can survive in hydrogen-dominated or even a wide variety of atmosphere types,” Seager said in an email to CNN. “It’s going to be tough to find signs of life on rocky planets with nitrogen-dominated or carbon dioxide-dominated atmospheres. So I want astronomers and the telescope allocation committees to be aware that there are other options to consider.”
While there aren’t any known rocky exoplanets with a hydrogen-dominated atmosphere yet, according to Seager, she has a proposal for the Webb telescope to identify rocky planets with hydrogen atmospheres.
When looking at other research concerning E. coli and yeast, Seager found something promising.
“We learned that the simple E. coli [bacteria] produces quite a range of gases, including many that were already studied in computer simulations as potential biosignature gases,” Seager said. “That such a simple life form has a diverse metabolic machinery to produce the diverse set of gases is encouraging — simple life elsewhere may do the same.”

Planets orbiting dead stars

Exoplanets have been found orbiting stars similar to our sun, or those lower in mass, like small red dwarf stars. They haven’t been found around white dwarfs, or the remaining core of a star after it explodes — yet.
But in a study published last week, Carl Sagan Institute at Cornell University doctoral student Thea Kozakis and Director Lisa Kaltenegger assembled and released a guide for detecting biosignatures within the atmospheres of exoplanets that could be orbiting white dwarfs.
The study published last week in The Astrophysical Journal Letters.
“The key question we asked is, if life existed on such a planet, could we even spot it because it orbits around a long-dead star? The answer is yes, if it is there, we could spot it,” Kaltenegger said in an email to CNN. She is also an associate professor of astronomy at Cornell.
So how does a planet exist around a white dwarf star? Kozakis explained that the planets could already exist in the star system before the star died.
“Either the planets would have to be from the original star system, although much farther away from their star — anything close to a star would be destroyed during the red giant phase of stellar evolution,” Kozakis said in an email to CNN.
“But some studies have shown that planets or moons initially far from their host stars could migrate inwards due to gravitational interactions with other planets in the system after the white dwarf forms. This would mean that these planets would have been very cold before their host became a white dwarf, although it is in our outer solar system that over 99% of water resides, so they could be very interesting objects.”
When our sun becomes a red giant in 5 billion years, the expansion of the sun during this last phase before dying will destroy Mercury, Venus and Earth.
“The other possibility is that the planets could have formed after the white dwarf,” Kozakis said. “White dwarfs form after a star casts off its outer layers as a ‘planetary nebula,’ and some white dwarfs have been observed to have disks of material around them possibly from these events. Similarly to how planets form around new stars, perhaps planets could form out of these new disks.”
So they could be first generation planets on the outer part of the solar system, or second generation planets that form from the debris disk of the star after it dies, Kaltenegger said.
“In both scenarios the really interesting question would be where the water would come from,” she said. “If it is a planet that initially was on the outer parts of the solar system, then it should have a lot of ice, which would melt if it gets moved closer to the warm white dwarf.”
White dwarfs, which are slightly larger than Earth, begin very hot but cool over time. This means that the habitable zone shrinks around the star as it cools.
If an Earth-size planet passed in front of the white dwarf, it would cause a 50% drop in brightness, as opposed to the .01% drop in brightness if Earth passed in front of the sun.
“This much more similar star-planet size significantly improves the quality of the data we could get from a white dwarf planetary system, and would greatly decrease the necessary observation time to detect the composition of the atmosphere,” Kozakis said.
When the planet passes in front of the star, starlight illuminates the atmosphere.
“We are hoping for, and looking for, that kind of transit,” Kozakis said. “If we observe a transit of that kind of planet, scientists can find out what is in its atmosphere, refer back to this paper, match it to spectral fingerprints and look for signs of life. Publishing this kind of guide allows observers to know what to look for.”
Based on their models, Kozakis and Kaltenegger believed that “spectral fingerprints” like ozone, methane and water could be seen in the atmosphere of planets orbiting white dwarfs by telescopes like Webb or the upcoming Extremely Large Telescope being built in Chile’s Atacama Desert.
These “fingerprints” can be indicators of life on a planet, so it is important that we understand how they are affected by the white dwarf host, Kozakis said.
This is one aspect of finding so-called fingerprints for potentially habitable planets that Kaltenegger’s team is working on.
“Think of it like a large fingerprint database to be able to identify a fingerprint on a crime scene,” she said.
The “fingerprints” can be used to identify what is observed using Webb and other future telescopes.
“Once we observe the spectrum of such rocky worlds in the habitable zone of their stars, we want to figure out if what we are seeing are signs of life … to answer the fascinating question of whether we are alone in the universe,” Kaltenegger said.

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Astronomers Found

Astronomers Have Found a Star That Survived Being Swallowed by a Black Hole – ScienceAlert

(Chandra X-Ray Observatory)


27 APRIL 2020

When black holes swallow down massive amounts of matter from the space around them, they’re not exactly subtle about it. They belch out tremendous flares of X-rays, generated by the material heating to intense temperatures as it’s sucked towards the black hole, so bright we can detect them from Earth.

This is normal black hole behaviour. What isn’t normal is for those X-ray flares to spew forth with clockwork regularity, a puzzling behaviour reported last year from a supermassive black hole at the centre of a galaxy 250 million light-years away. Every nine hours, boom – X-ray flare.

After careful study, astronomer Andrew King of the University of Leicester in the UK believes he has identified the cause – a dead star that’s endured its brush with a black hole, trapped on a nine-hour, elliptical orbit around it. Every close pass, or periastron, the black hole slurps up more of the star’s material.

“This white dwarf is locked into an elliptical orbit close to the black hole, orbiting every nine hours,” King explained.

“At its closest approach, about 15 times the radius of the black hole’s event horizon, gas is pulled off the star into an accretion disk around the black hole, releasing X-rays, which the two spacecraft are detecting.”

The black hole is the nucleus of a galaxy called GSN 069, and it’s pretty lightweight as far as supermassive black holes go – only 400,000 times the mass of the Sun. Even so, it’s active, surrounded by a hot disc of accretion material, feeding into and growing the black hole.

According to King’s model, this black hole was just hanging out, doing its active accretion thing, when a red giant star – the final evolutionary stages of a Sun-like star – happened to wander a little too close. The black hole promptly divested the star of its outer layers, speeding its evolution into a white dwarf, the dead core that remains once the star has exhausted its nuclear fuel (white dwarfs shine with residual heat, not the fusion processes of living stars).

But rather than continuing on its journey, the white dwarf was captured in orbit around the black hole, and continued to feed into it.

Based on the magnitude of the X-ray flares, and our understanding of the flares that are produced by black hole mass transfer, and the star’s orbit, King was able to constrain the mass of the star, too. He calculated that the white dwarf is around 0.21 times the mass of the Sun.

While on the lighter end of the scale, that’s a pretty standard mass for a white dwarf. And if we assume the star is a white dwarf, we can also infer – based on our understanding of other white dwarfs and stellar evolution – that the star is rich in helium, having long ago run out of hydrogen.

“It’s remarkable to think that the orbit, mass and composition of a tiny star 250 million light years away could be inferred,” King said.

Based on these parameters, he also predicted that the star’s orbit wobbles slightly, like a spinning top losing speed. This wobble should repeat every two days or so, and we may even be able to detect it, if we observe the system for long enough.

This could be one mechanism whereby black holes grow more and more massive over time. But we’ll need to study more such systems to confirm it, and they may not be easy to detect.

For one, GSN 069’s black hole is lower mass, which means that the star can travel on a closer orbit. To survive a more massive black hole, a star would have to be on a much larger orbit, which means any periodicity in the feeding would be easier to miss. And if the star were to stray too close, the black hole would destroy it.

But the fact that one has been identified offers hope that it’s not the only such system out there.

“In astronomical terms, this event is only visible to our current telescopes for a short time – about 2,000 years, so unless we were extraordinarily lucky to have caught this one, there may be many more that we are missing elsewhere in the Universe,” King said.

As for the star’s future, well, if nothing else is to change, the star will stay right where it is, orbiting the black hole, and continuing to be slowly stripped for billions of years. This will cause it to grow in size and decrease in density – white dwarfs are only a little bigger than Earth – until it’s down to a planetary mass, maybe even eventually turning into a gas giant.

“It will try hard to get away, but there is no escape,” King said. “The black hole will eat it more and more slowly, but never stop.”

The research has been published in the Monthly Notices of the Royal Astronomical Society.

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