Listen to a star 'twinkle'

Many people know that stars appear to twinkle because our atmosphere bends starlight as it travels to Earth. But stars also have an innate "twinkle" -- caused by rippling waves of gas on their surfaces -- that is imperceptible to current Earth-bound telescopes.

In a new study, a Northwestern University-led team of researchers developed the first 3D simulations of energy rippling from a massive star's core to its outer surface. Using these new models, the researchers determined, for the first time, how much stars should innately twinkle.

And, in yet another first, the team also converted these rippling waves of gas into sound waves, enabling listeners to hear both what the insides of stars and the "twinkling" should sound like. And it is eerily fascinating.

The study will be published on July 27, in the journal Nature Astronomy.

"Motions in the cores of stars launch waves like those on the ocean," said Northwestern's Evan Anders, who led the study. "When the waves arrive at the star's surface, they make it twinkle in a way that astronomers may be able to observe. For the first time, we have developed computer models which allow us to determine how much a star should twinkle as a result of these waves. This work allows future space telescopes to probe the central regions where stars forge the elements we depend upon to live and breathe."

Anders is a postdoctoral fellow in Northwestern's Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). He is advised by study coauthor Daniel Lecoanet, an assistant professor of engineering sciences and applied mathematics in Northwestern's McCormick School of Engineering and member of CIERA.

Chaotic convection

All stars have a convection zone, a wild and disorderly place where gases churn to push heat outward. For massive stars (stars at least about 1.2 times the mass of our sun), this convection zone resides at their cores.

"Convection within stars is similar to the process that fuels thunderstorms," Anders said. "Cooled air drops, warms and rises again. It's a turbulent process that transports heat."

It also makes waves -- small rivulets that cause starlight to dim and brighten, producing a subtle twinkle. Because the cores of massive stars are shrouded from view, Anders and his team sought to model their hidden convection. Building upon studies that examined properties of turbulent core convection, characteristics of waves and possible observational features of those waves, the team's new simulations include all relevant physics to accurately predict how a star's brightness changes depending upon convection-generated waves.

'Soundproofing' stars

After convection generates waves, those waves bounce around inside of the simulated star. While some waves eventually emerge to the star's surface to produce a twinkling effect, other waves become trapped and continue to bounce around. To isolate the waves that launch to the surface and create twinkling, Anders and his team built a filter that describes how waves bounce around inside of the simulations.

"We first put a damping layer around the star -- like the padded walls you would have in a recording studio -- so we could measure exactly how the core convection makes waves," Anders explained.

Anders compares it to a music studio, which leverages soundproof padded walls to minimize the acoustics of an environment so musicians can extract the "pure sound" of the music. Musicians then apply filters and engineer those recordings to produce the song how they want.

Similarly, Anders and his collaborators applied their filter to the pure waves they measured coming out of the convective core. They then followed waves bouncing around in a model star, ultimately finding that their filter accurately described how the star changed the waves coming from the core. The researchers then developed a different filter for how waves should bounce around inside of a real star. With this filter applied, the resulting simulation shows how astronomers expect waves to appear if viewed through a powerful telescope.

"Stars get a little brighter or a little dimmer depending on various things happening dynamically inside the star," Anders said. "The twinkling that these waves cause is extremely subtle, and our eyes are not sensitive enough to see it. But powerful future telescopes may be able to detect it."

Music in the stars

Taking the recording studio analogy one step further, Anders and his collaborators next used their simulations to generate sound. Because these waves are outside the range of human hearing, the researchers uniformly increased the frequencies of the waves to make them audible.

Depending on how large or bright a massive star is, the convection produces waves corresponding to different sounds. Waves emerging from the core of a large star, for example, make sounds like a warped ray gun, blasting through an alien landscape. But the star alters these sounds as the waves reach the star's surface. For a large star, the ray gun-like pulses shift into a low echo reverberating through an empty room. Waves at the surface of a medium-sized star, on the other hand, conjure images of a persistent hum through a windswept terrain. And surface waves on a small star sound like a plaintive alert from a weather siren.

Next, Anders and his team passed songs through different stars to listen to how the stars change the songs. They passed a short audio clip from "Jupiter"(a movement from "The Planets" orchestral suite by composer Gustav Holst) and from "Twinkle, Twinkle, Little Star" through three sizes (large, medium and small) of massive stars. When propagated through stars, all songs sound distant and haunting -- like something from "Alice in Wonderland."

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Starlight and the first black holes: researchers detect the host galaxies of quasars in the early universe

New images from the James Webb Space Telescope have revealed, for the first time, starlight from two massive galaxies hosting actively growing black holes -- quasars -- seen less than a billion years after the Big Bang. A new study in Nature this week finds the black holes have masses close to a billion times that of the Sun, and the host galaxy masses are almost one hundred times larger, a ratio similar to what is found in the more recent universe. A powerful combination of the Subaru Telescope and the JWST has paved a new path to study the distant universe.

The existence of such massive black holes in the distant universe has created more questions than answers for astrophysicists. How could these black holes grow to be so large when the universe was so young? Even more puzzling, observations in the local universe show a clear relation between the mass of supermassive black holes and the much larger galaxies in which they reside. The galaxies and the black holes have completely different sizes, so which came first: the black holes or the galaxies? This is a "chicken-or-egg" problem on a cosmic scale.

An international team of researchers, led by Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) Project Researcher Xuheng Ding and Professor John Silverman, and Peking University Kavli Institute for Astronomy and Astrophysics (PKU-KIAA) Kavli Astrophysics Fellow Masafusa Onoue have started to answer this question with the James Webb Space Telescope (JWST), launched in December 2021. Studying the relation between host galaxies and black holes in the early universe allows scientists to watch their formation, and see how they are related to one another.

Quasars are luminous, while their host galaxies are faint, which has made it challenging for researchers to detect the dim light of the galaxy in the glare of the quasar, especially at great distances. Before the JWST, the Hubble Space Telescope was able to detect host galaxies of luminous quasars when the universe was just under 3 billion years old, but no younger.

The superb sensitivity and the ultra-sharp images of the JWST at infrared wavelengths finally allowed researchers to push these studies to the time when the quasars and galaxies first formed. Just a few months after JWST started regular operations, the team observed two quasars, HSC J2236+0032 and HSC J2255+0251, at redshifts 6.40 and 6.34 when the universe was approximately 860 million years old. These two quasars were discovered in a deep survey program of the 8.2m-Subaru Telescope on the summit of Maunakea in Hawai'i. The relatively low luminosities of these quasars made them prime targets for measurement of the host galaxy properties, and the successful detection of the hosts represents the earliest epoch to date at which starlight has been detected in a quasar.

The images of the two quasars were taken at infrared wavelengths of 3.56 and 1.50 micron with JWST's NIRCam instrument, and the host galaxies became apparent after carefully modeling and subtracting glare from the accreting black holes. The stellar signature of the host galaxy was also seen in a spectrum taken by JWST's NIRSPEC for J2236+0032, further supporting the detection of the host galaxy.

Analyses of the host galaxy photometry found that these two quasar host galaxies are massive, measuring 130 and 34 billion times the mass of the Sun, respectively. Measuring the speed of the turbulent gas in the vicinity of the quasars from the NIRSPEC spectra suggests that the black holes that power them are also massive, measuring 1.4 and 0.2 billion times the mass of the Sun. The ratio of the black hole mass to host galaxy mass is similar to those of galaxies in the more recent past, suggesting that the relationship between black holes and their hosts was already in place 860 million years after the Big Bang.

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Astronomers discover last three planets Kepler telescope observed before going dark

More than 5,000 planets are confirmed to exist beyond our solar system. Over half were discovered by NASA's Kepler Space Telescope, a resilient observatory that far outlasted its original planned mission. Over nine and a half years, the spacecraft trailed the Earth, scanning the skies for periodic dips in starlight that could signal the presence of a planet crossing in front of its star.

In its last days, the telescope kept recording the brightness of stars as it was running out of fuel. On Oct. 30, 2018, its fuel tanks depleted, the spacecraft was officially retired.

Now, astronomers at MIT and the University of Wisconsin at Madison, with the help of citizen scientists, have discovered what may be the last planets that Kepler gazed upon before going dark.

The team combed through the telescope's last week of high-quality data and spotted three stars, in the same part of the sky, that appeared to dim briefly. The scientists determined that two of the stars each host a planet, while the third hosts a planet "candidate" that has yet to be verified.

The two validated planets are K2-416 b, a planet that is about 2.6 times the size of the Earth and that orbits its star about every 13 days, and K2-417 b, a slightly larger planet that is just over three times Earth's size and that circles its star every 6.5 days. For their size and proximity to their stars, both planets are considered "hot mini-Neptunes ." They are located about 400 light years from Earth.

The planet candidate is EPIC 246251988 b -- the largest of the three worlds at almost four times the size of the Earth. This Neptune-sized candidate orbits its star in around 10 days, and is slightly farther away, 1,200 light years from Earth.

"We have found what are probably the last planets ever discovered by Kepler, in data taken while the spacecraft was literally running on fumes," says Andrew Vanderburg, assistant professor of physics in MIT's Kavli Institute for Astrophysics and Space Research. "The planets themselves are not particularly unusual, but their atypical discovery and historical importance makes them interesting."

The team has published their discovery today in the journal Monthly Notices of the Royal Astronomical Society. Vanderburg's co-authors are lead author Elyse Incha, at the University of Wisconsin at Madison, and amateur astronomers Tom Jacobs and Daryll LaCourse, along with scientists at NASA, the Center for Astrophysics of Harvard and the Smithsonian, and the University of North Carolina at Chapel Hill.

Data squeeze

In 2009, NASA launched the Kepler telescope into space, where it followed the Earth's orbit and continuously monitored millions of stars in a patch of the northern sky. Over four years, the telescope recorded the brightness of over 150,000 stars, which astronomers used to discover thousands of possible planets beyond our solar system.

Kepler kept observing beyond its original three-and-a-half-year mission, until May 2013, when the second of four reaction wheels failed. The wheels served as the spacecraft's gyroscopes, helping to keep the telescope pointed at a particular point in the sky. Kepler's observations were put on pause while scientists searched for a fix.

One year later, Kepler restarted as "K2," a reworked mission that used the sun's wind to balance the unsteady spacecraftin a way that kept the telescope relatively stable for a few months at a time -- a period called a campaign. K2 went on for another four years, observing over half a million more stars before the spacecraft finally ran out of fuel during its 19th campaign. The data from this last campaign comprised only a week of high-quality observations and another 10 days of noisier measurements as the spacecraft rapidly lost fuel.

"We were curious to see whether we could get anything useful out of this short dataset," Vanderburg says. "We tried to see what last information we could squeeze out of it."

By eye

Vanderburg and Incha presented the challenge to the Visual Survey Group, a team of amateur and professional astronomers who hunt for exoplanets in satellite data. They search by eye through thousands of recorded light curves of each star, looking for characteristic dips in brightness that signal a "transit," or the possible crossing of a planet in front of its star.

The citizen scientists are especially suited to combing through short datasets such as K2's very last campaign.

"They can distinguish transits from other wacky things like a glitch in the instrument," Vanderburg says. "That's helpful especially when your data quality begins to suffer, like it did in K2's last bit of data."

The astronomers spent a few days efficiently looking through the light curves that Kepler recorded from about 33,000 stars. The team worked with only a week's worth of high-quality data from the telescope before it began to lose fuel and focus. Even in this short window of data, the team was able to spot a single transit in three different stars.

Incha and Vanderburg then looked at the telescope's very last, lower-quality observations, taken in its last 11 days of operation, to see if they could spot any additional transits in the same three stars -- evidence that a planet was periodically circling its star.

During this 11-day period, as the spacecraft was losing fuel, its thrusters fired more erratically, causing the telescope's view to drift. In their analysis, the team focused on the region of each star's light curves between thruster activity, to see if they could spot any additional transits in these less data-noisy moments.

This search revealed a second transit for K2-416 b and K2-417 b, validating that they each host a planet. The team also detected a similar dip in brightness for K2-417 b in data taken of the same star by NASA's Transiting Exoplanet Survey Satellite (TESS), a mission that is led and operated by MIT. Data from TESS helped to confirm the planet candidate around this star.

"Those two are pretty much, without a doubt, planets," Incha says. "We also followed up with ground-based observations to rule out all kinds of false positive scenarios for them, including background star interference, and close-in stellar binaries."

"These are the last chronologically observed planets by Kepler, but every bit of the telescope's data is incredibly useful," Incha says. "We want to make sure none of that data goes to waste, because there are still a lot of discoveries to be made."

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Dust plumes observed being 'pushed' into interstellar space by intense starlight

The results, made using infrared images of the binary star system WR140 taken over 16 years, are reported in the journal Nature.

In a complementary study of WR140, published in Nature Astronomy, NASA's James Webb Space Telescope (JWST) was able to see much deeper to snap an image of not just a single accelerating dust plume, but almost 20 of them, nested inside each other like a giant set of onion skins.

WR140 is comprised of a huge Wolf-Rayet star and an even bigger blue supergiant star, gravitationally bound in an eight-year orbit. This binary star, in the Cygnus constellation, has been monitored for two decades with one of the world's largest optical telescopes at the Keck Observatory in Hawaii.

WR140 episodically puffs out plumes of dust stretching thousands of times the distance from the Earth to the Sun. These dust plumes, produced every eight years, give astronomers a unique opportunity to observe how starlight can affect matter.

It's known that light carries momentum, exerting a push on matter known as radiation pressure. Astronomers often witness the outcome of this phenomenon in the form of matter coasting at high speed around the cosmos, but it's been a difficult process to catch in the act. Direct recording of acceleration due to forces other than gravity is rarely witnessed, and never in a stellar environment like this.

"It's hard to see starlight causing acceleration because the force fades with distance, and other forces quickly take over," said Yinuo Han from Cambridge's Institute of Astronomy, first author of the Nature paper. "To witness acceleration at the level that it becomes measurable, the material needs to be reasonably close to the star or the source of the radiation pressure needs to be extra strong. WR140 is a binary star whose ferocious radiation field supercharges these effects, placing them within reach of our high-precision data."

All stars generate stellar winds, but those from Wolf-Rayet stars can be more like a stellar hurricane. Elements such as carbon in the wind condense out as soot, which remains hot enough to glow bright in the infrared. Like smoke in the wind, this gives telescopes something that can be observed.

The team used an imaging technology known as interferometry which was able to act like a zoom lens for the 10-metre Keck telescope mirror, enabling the researchers to recover sufficiently sharp images of WR140 for the study.

Han and his team found that the dust does not stream out from the star with the wind in a hazy ball. Instead, the dust forms where the winds from the two stars collide, on the surface of a cone-shaped shock front between them.

Because the orbiting binary star is in constant motion, the shock front also rotates. The sooty plume gets wrapped into a spiral, in the same way that droplets form a spiral in a garden sprinkler.

The researchers found that WR140 has other tricks up its sleeve. The two stars are not on circular but rather elliptical orbits, and dust production turns on and off as the binary nears and departs the point of closest approach. By modelling these effects into the three-dimensional geometry of the dust plume, the astronomers were able to measure to location of dust features in three-dimensional space.

"Like clockwork, this star puffs out sculpted smoke rings every eight years, with all this wonderful physics written then inflated in the wind like a banner for us to read," said co-author Professor Peter Tuthill from the University of Sydney. "Eight years later as the binary returns in its orbit, another appears the same as the one before, streaming out into space inside the bubble of the previous one, like a set of giant nested Russian dolls."

Because the dust produced by this Wolf-Rayet is so predictable and expands to such large distances, it offered the astronomers a unique laboratory to examine the acceleration zone.

"In the absence of external forces, each dust spiral should expand at a constant speed," said Han, who is also a co-author on the JWST paper. "We were puzzled at first because we could not get our model to fit the observations, until we finally realised that we were seeing something new. The data did not fit because the expansion speed wasn't constant, but rather that it was accelerating. We'd caught that for the first time on camera."

"In one sense, we always knew this must be the reason for the outflow, but I never dreamed we'd be able to see the physics at work like this," said Tuthill. "When I look at the data now, I see WR140's plume unfurling a like giant sail made of dust. When it catches the photon wind streaming from the star, like a yacht catching a gust, it makes a sudden leap forward."

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Sculpted by starlight: A meteorite witness to the solar system's birth

In 2011, scientists confirmed a suspicion: There was a split in the local cosmos. Samples of the solar wind brought back to Earth by the Genesis mission definitively determined oxygen isotopes in the sun differ from those found on Earth, the moon and the other planets and satellites in the solar system.

Early in the solar system's history, material that would later coalesce into planets had been hit with a hefty dose of ultraviolet light, which can explain this difference. Where did it come from? Two theories emerged: Either the ultraviolet light came from our then-young sun, or it came from a large nearby star in the sun's stellar nursery.

Now, researchers from the lab of Ryan Ogliore, assistant professor of physics in Arts & Sciences at Washington University in St. Louis, have determined which was responsible for the split. It was most likely light from a long-dead massive star that left this impression on the rocky bodies of the solar system. The study was led by Lionel Vacher, a postdoctoral research associate in the physics department's Laboratory for Space Sciences.

Their results are published in the journal Geochimica et Cosmochimica Acta.

"We knew that we were born of stardust: that is, dust created by other stars in our galactic neighborhood were part of the building blocks of the solar system," Ogliore said.

"But this study showed that starlight had a profound effect on our origins as well."

*Tiny time capsule

All of that profundity was packed into a mere 85 grams of rock, a piece of an asteroid found as a meteorite in Algeria in 1990, named Acfer 094. Asteroids and planets formed from the same presolar material, but they've been influenced by different natural processes. The rocky building blocks that coalesced to form asteroids and planets were broken up and battered; vaporized and recombined; and compressed and heated. But the asteroid that Acfer 094 came from managed to survive for 4.6 billion years mostly unscathed.

"This is one of the most primitive meteorites in our collection," Vacher said. "It was not heated significantly. It contains porous regions and tiny grains that formed around other stars. It is a reliable witness to the solar system's formation."

Acfer 094 is also the only meteorite that contains cosmic symplectite, an intergrowth of iron-oxide and iron-sulfide with extremely heavy oxygen isotopes -- a significant finding.

The sun contains about 6% more of the lightest oxygen isotope compared with the rest of the solar system. That can be explained by ultraviolet light shining on the solar system's building blocks, selectively breaking apart carbon monoxide gas into its constituent atoms. That process also creates a reservoir of much heavier oxygen isotopes. Until cosmic symplectite, however, no one had found this heavy isotope signature in samples of solar system materials.

With only three isotopes, however, simply finding the heavy oxygen isotopes wasn't enough to answer the question of the origin of the light. Different ultraviolet spectra could have created the same result.

"That's when Ryan came up with the idea of sulfur isotopes," Vacher said.

Sulfur's four isotopes would leave their marks in different ratios depending on the spectrum of ultraviolet light that irradiated hydrogen sulfide gas in the proto-solar system. A massive star and a young sun-like star have different ultraviolet spectra.

Cosmic symplectite formed when ices on the asteroid melted and reacted with small pieces of iron-nickel metal. In addition to oxygen, cosmic symplectite contains sulfur in iron sulfide. If its oxygen witnessed this ancient astrophysical process -- which led to the heavy oxygen isotopes -- perhaps its sulfur did, too.

"We developed a model," Ogliore said. "If I had a massive star, what isotope anomalies would be created? What about for a young, sun-like star? The precision of the model depends on the experimental data. Fortunately, other scientists have done great experiments on what happens to isotope ratios when hydrogen sulfide is irradiated by ultraviolet light."

Sulfur and oxygen isotope measurements of cosmic symplectite in Acfer 094 proved another challenge. The grains, tens of micrometers in size and a mixture of minerals, required new techniques on two different in-situ secondary-ion mass spectrometers: the NanoSIMS in the physics department (with assistance from Nan Liu, research assistant professor in physics) and the 7f-GEO in the Department of Earth and Planetary Sciences, also in Arts & Sciences.

*Putting the puzzle together

It helped to have friends in earth and planetary sciences, particularly David Fike, professor of earth and planetary sciences and director of Environmental Studies in Arts & Sciences, and Clive Jones, research scientist in earth and planetary sciences.

"They are experts in high-precision in-situ sulfur isotope measurements for biogeochemistry," Ogliore said. "Without this collaboration, we would not have achieved the precision we needed to differentiate between the young sun and massive star scenarios."

The sulfur isotope measurements of cosmic symplectite were consistent with ultraviolet irradiation from a massive star, but did not fit the UV spectrum from the young sun. The results give a unique perspective on the astrophysical environment of the sun's birth 4.6 billion years ago. Neighboring massive stars were likely close enough that their light affected the solar system's formation. Such a nearby massive star in the night sky would appear brighter than the full moon.

Today, we can look to the skies and see a similar origin story play out elsewhere in the galaxy.

"We see nascent planetary systems, called proplyds, in the Orion nebula that are being photoevaporated by ultraviolet light from nearby massive O and B stars," Vacher said.

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