Astronomy

Can a black hole turn back into a star?

Can a black hole turn back into a star?


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I understand that a black hole happens after a star exhaustions elements capable of undergoing fusion. Without the energy produced by fusion the remaining mass of the star collapses on itself and forms a black hole. I conclude black holes must be deficient in hydrogen, helium and elements that undergo fusion in stars. I understand that hydrogen and the like undergoes fusion in stars in part (or completely?) because of the gravitational pressure which occurs in stars.

Suppose a young star replete with hydrogen merged with a black hole. My question: would the result be a larger black hole, or a radiant star with a very massive heart? Could a massive infusion of (fusible?) elements reboot the black hole back into a star?


No.

A black hole is characterised by the formation of an event horizon. Matter which travels past an event horizon cannot return to the space outside the event horizon, to do so would require the matter to travel faster than light, and so would require infinite energy.

There is nothing like a regular star "inside" a black hole. The general relativity model of a black hole contains a singularity. Matter which enters a black hole is certain to reach this singularity in a (usually short) amount of time. Adding hydrogen to a black hole would just make the black hole more massive. There is a theorem in mathematical physics that the only properties that a black hole can have are mass, angular momentum and charge. In other words, a black hole made of collapsed hydrogen would be identical to one made of collapsed iron. There is no matter in a black hole, only mass.

The formation of a black hole normally occurs when a core in a massive star has a certain amount of iron, which saps the core of energy causing collapse. If the resulting object has more than about 3 solar masses, then nothing in the universe can stop it from collapsing all the way to a singularity.

As with much about general relativity, your intuition is a very bad guide to how black holes work. However General relativity is a very good model for how black holes should form and behave


Black Holes Help With Star Birth


Virtual Milky Way: gas density around a massive central galaxy in a group in the virtual universe of the TNG50 simulation. Gas inside the galaxy corresponds to the bright vertical structure: a gaseous disk. To the left and right of that structure are bubbles - regions that look like circles in this image, with markedly reduced gas density inside. This geometry of the gas is due to the action of the super massive black hole that hides at the center of the galaxy and that pushes out gas preferably in directions perpendicular to the galaxy gaseous disk, carving regions of lower density. CREDIT TNG Collaboration/Dylan Nelson

Research combining systematic observations with cosmological simulations has found that, surprisingly, black holes can help certain galaxies form new stars.

On scales of galaxies, the role of supermassive black holes for star formation had previously been seen as destructive - active black holes can strip galaxies of the gas that galaxies need to form new stars. The new results, published in the journal Nature, showcase situations where active black holes can, instead, "clear the way" for galaxies that orbit inside galaxy groups or clusters, keeping those galaxies from having their star formation disrupted as they fly through the surrounding intergalactic gas.

Active black holes are primarily thought to have a destructive influence on their surroundings. As they blast energy into their host galaxy, they heat up and eject that galaxy's gas, making it more difficult for the galaxy to produce new stars. But now, researchers have found that the same activity can actually help with star formation - at least for the satellite galaxies that orbit the host galaxy.

The counter-intuitive result came out of a collaboration sparked by a lunchtime conversation between astronomers specializing in large-scale computer simulations and observers. As such, it is a good example for the kind of informal interaction that has become more difficult under pandemic conditions.

Astronomical observations that include taking a distant galaxy's spectrum - the rainbow-like separation of a galaxy's light into different wavelengths - allow for fairly direct measurements of the rate at which that galaxy is forming new stars.

Going by such measurements, some galaxies are forming stars at rather sedate rates. In our own Milky Way galaxy, only one or two new stars are born each year. Others undergo brief bursts of excessive star formation activity, called "star bursts", with hundreds of stars born per year. In yet other galaxies, star formation appears to be suppressed, or "quenched," as astronomers say: Such galaxies have virtually stopped forming new stars.

A special kind of galaxy, specimens of which are frequently - almost half of the time - found to be in such a quenched state, are so-called satellite galaxies. These are part of a group or cluster of galaxies, their mass is comparatively low, and they orbit a much more massive central galaxy similar to the way satellites orbit the Earth.

Such galaxies typically form very few new stars, if at all, and since the 1970s, astronomers have suspected that something very much akin to headwind might be to blame: Groups and clusters of galaxies not only contain galaxies, but also rather hot thin gas filling the intergalactic space.

As a satellite galaxy orbits through the cluster at a speed of hundreds of kilometers per second, the thin gas would make it feel the same kind of "headwind" that someone riding a fast bike, or motor-bike, will feel. The satellite galaxy's stars are much too compact to be affected by the steady stream of oncoming intergalactic gas.

But the satellite galaxy's own gas is not: It would be stripped away by the oncoming hot gas in a process known as "ram pressure stripping". On the other hand, a fast-moving galaxy has no chance of pulling in a sufficient amount of intergalactic gas, to replenish its gas reservoir. The upshot is that such satellite galaxies lose their gas almost completely - and with it the raw material needed for star formation. As a result, star-formation activity would be quenched.

The processes in question take place over millions or even billions of years, so we cannot watch them happening directly. But even so, there are ways for astronomers to learn more. They can utilize computer simulations of virtual universes, programmed so as to follow the relevant laws of physics - and compare the results with what we actually observe. And they can look for tell-tale clues in the comprehensive "snapshot" of cosmic evolution that is provided by astronomical observations.

Annalisa Pillepich, a group leader at the Max Planck Institute for Astronomy (MPIA), specializes in simulations of this kind. The IllustrisTNG suite of simulations, which Pillepich has co-led, provides the most detailed virtual universes to date - universes in which researchers can follow the movement of gas around on comparatively small scales.

IllustrisTNG provides some extreme examples of satellite galaxies that have freshly been stripped by ram pressure: so-called "jellyfish galaxies," that are trailing the remnants of their gas like jellyfish are trailing their tentacles. In fact, identifying all the jellyfish in the simulations is a recently launched citizen science project on the Zooniverse platform, where volunteers can help with the research into that kind of freshly quenched galaxy.

But, while jellyfish galaxies are relevant, they are not where the present research project started. Over lunch in November 2019, Pillepich recounted a different one of her IllustrisTNG results to Ignacio Martín-Navarro, an astronomer specializing in observations, who was at MPIA on a Marie Curie fellowship. A result about the influence of supermassive black holes that reached beyond the host galaxy, into intergalactic space.

Such supermassive black holes can be found in the center of all galaxies. Matter falling onto such a black hole typically becomes part of a rotating so-called accretion disk surrounding the black hole, before falling into the black hole itself. This fall onto the accretion disk liberates an enormous amount of energy in the form of radiation, and oftentimes also in the form of two jets of quickly moving particles, which accelerate away from the black hole at right angles to the accretion disk. A supermassive black hole that is emitting energy in this way is called an Active Galactic Nucleus, AGN for short.

While IllustrisTNG is not detailed enough to include black hole jets, it does contain physical terms that simulate how an AGN is adding energy to the surrounding gas. And as the simulation showed, that energy injection will lead to gas outflows, which in turn will orient themselves along a path of least resistance: in the case of disk galaxies similar to our own Milky Way, perpendicular to the stellar disk for so-called elliptical galaxies, perpendicular to a suitable preferred plane defined by the arrangement of the galaxy's stars.

Over time, the bipolar gas outflows, perpendicular to the disk or preferred plane, will go so far as to affect the intergalactic environment - the thin gas surrounding the galaxy. They will push the intergalactic gas away, each outflow creating a gigantic bubble. It was this account that got Pillepich and Martín-Navarro thinking: If a satellite galaxy were to pass through that bubble - would it be affected by the outflow, and would its star formation activity be quenched even further?

Martín-Navarro took up this question within his own domain. He had extensive experience in working with data from one of the largest systematic surveys to date: the Sloan Digital Sky Survey (SDSS), which provides high-quality images of a large part of the Northern hemisphere. In the publicly available data from that survey's 10th data, he examined 30,000 galaxy groups and clusters, each containing a central galaxy and on average 4 satellite galaxies.

In a statistical analysis of those thousands of systems, he found a small, but marked difference between satellite galaxies that were close to the central galaxy's preferred plane and satellites that were markedly above and below. But the difference was in the opposite direction the researchers had expected: Satellites above and below the plane, within the thinner bubbles, were on average not more likely, but about 5% less likely to have had their star formation activity quenched.

With that surprising result, Martín-Navarro went back to Annalisa Pillepich, and the two performed the same kind of statistical analysis in the virtual universe of the IllustrisTNG simulations. In simulations of that kind, after all, cosmic evolution is not put in "by hand" by the researchers. Instead, the software includes rules that model the rules of physics for that virtual universe as naturally as possible, and which also include suitable initial conditions that correspond to the state of our own universe shortly after the Big Bang.

That is why simulations like that leave room for the unexpected - in this particular case, for re-discovering the on-plane, off-plane distribution of quenched satellite galaxies: The virtual universe showed the same 5% deviation for the quenching of satellite galaxies! Evidently, the researchers were on to something.

In time, Pillepich, Martín-Navarro and their colleagues came up with a hypothesis for the physical mechanism behind the quenching variation. Consider a satellite galaxy travelling through one of the thinned-out bubbles the central black hole has blown into the surrounding intergalactic medium. Due to the lower density, that satellite galaxy experiences less headwind, less ram pressure, and is thus less likely to have its gas stripped away.

Then, it is down to statistics. For satellite galaxies that have orbited the same central galaxies several times already, traversing bubbles but also the higher-density regions in between, the effect will not be noticeable. Such galaxies will have lost their gas long ago.

But for satellite galaxies that have joined the group, or cluster, rather recently, location will make a difference: If those satellites happen to land in a bubble first, they are less likely to lose their gas then if they happen to land outside a bubble. This effect could account for the statistical difference for the quenched satellite galaxies.

With the excellent agreement between the statistical analyses of both the SDSS observations and the IllustrisTNG simulations, and with a plausible hypothesis for a mechanism, this is a highly promising result. In the context of galaxy evolution, it is particularly interesting because it confirms, indirectly, the role of active galactic nuclei not only heating intergalactic gas up, but actively "pushing it away", to create lower-density regions. And as with all promising results, there are now a number of natural directions that either Martín-Navarro, Pillepich and their colleagues or other scientists can take in order to explore further.

I. Martín-Navarro et al Anisotropic satellite galaxy quenching modulated by supermassive black hole activity Nature, June 10, 2021


This black hole glitched like a cosmic computer…then restarted itself out of nowhere

Can a black hole really flip a switch on itself? At least one did, since it was caught doing just that in real time.

Phenomena like this just don’t happen. That explains the shock a team of MIT astronomers got when they observed a flash of brightness from a supermassive black hole that then dimmed as if someone pulled the plug on it. Then, as if out of nowhere, it restarted like all your electrical appliances after a thunderstorm blackout. What actually went down was that the black hole’s corona (a blindingly bright ring of electrons and other high-energy charged particles right on the edge of its event horizon) vanished into darkness. Then it reignited.

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“We expect that luminosity changes this big should vary on timescales of many thousands to millions of years,” said Erin Kara, assistant professor of physics at MIT, in a press release. “But in this object, we saw it change by [a factor of] 10,000 over a year, and it even changed by a factor of 100 in eight hours, which is just totally unheard of and really mind-boggling.”

Kara and her team, who recently published a study in The Astrophysical Journal Letters, had seen something extraordinary — and baffling — through ASSASN (All-Sky Automated Survey for Super-Novae). Black holes devour everything that crosses the point of no return otherwise known as the event horizon, but why would one start to self-cannibalize by eating its own corona? That’s not how it works. That’s not how any of this works. Something else would have to have caused chaos within the black hole, and Kara’s team believed they knew what it was. Sometimes, even supermassive black holes can bite off way more than they can proverbially chew.

Black hole blackout. Credit: NASA

The astronomers became convinced that the hungry black hole’s immense gravitational pull snagged a star that was too much for it to handle. That star is thought to have then run rogue through the black hole’s accretion disc, the ever-swirling disc of light particles and other cosmic debris that gets caught by a black hole’s gravity and are destined to end up in its gaping maw. It must have disrupted the accretion disc to the point that the corona and everything else were suddenly caught in the inescapable gravity of the black hole. Then it was lights out, at least for a while, but the team kept an eye on the black hole with NASA’s X-ray telescope NICER.

“Our observations could be explained by the interaction between the accretion flow and debris from a tidally disrupted star,” Kara and her colleagues said in the study.

Celestial bodies are pulled towards and away from each other by tidal forces depending on which body has stronger gravitational forces. When the star came too close to the black hole, tidal disruption occurred, and the gravitational forces of the much more massive black hole pulled at the star so violently that they tore it apart. That sent both the corona and most of the material accreted by the black hole hurtling past the event horizon. The black hole then seemed dead. What the astronomers didn’t expect was that the corona would start rebuilding itself at unbelievable speed. The black hole started to rebuild its accretion disc and corona by pulling in material from its outer reaches.

Because this black hole is still acting temperamental, Kara and her team are going to keep an eye on it to watch out for anything else bizarre. Any unlucky stars crossing its path in the future could cause an epic outburst.


Supermassive Black Holes can Turn Star Formation On and Off in a Large Galaxy

In the 1970s, astronomers discovered that a particularly large black hole (Sagittarius A*) existed at the center of our galaxy. In time, they came to understand that similar Supermassive Black Holes (SMBHs) existed in the center of most massive galaxies. The presence of these black holes was also what differentiated galaxies that had particularly luminous cores – aka. Active Galactic Nuclei (AGN) – from those that didn’t.

Since that time, astronomers and cosmologists have pondered what role SMBHs have on galactic evolution, with some venturing that they have a profound impact on star formation. And thanks to a recent study by an international team of astronomers, there is now direct evidence for a correlation between and SMBH and a galaxy’s star formation. In fact, the team demonstrated that a black hole’s mass could determine when star formation in a galaxy will end.

The study, titled “Black-Hole-Regulated Star Formation in Massive Galaxies“, recently appeared in the scientific journal Nature. Led by Ignacio Martín-Navarro, a Marie Curie Fellow at the University of California Observatories , the study team also consisted of members from the Max-Planck Institute for Astronomy and the Instituto de Astrofísica de Canarias.

The primary mirror of the Hobby-Eberly Telescope (HET) at McDonald Observatory. The mirror is made up of 91 segments, and has an effective aperture of 9.2 meters. Credit: Marty Harris/McDonald Observatory

For the sake of their study, the team relied on data gathered the Hobby-Eberle Telescope Massive Galaxy Survey in 2015. This systematic survey used the 10m Hobby-Eberly Telescope (HET) at the McDonald Observatory to conduct an optical long-slit spectroscopic survey of over 1000 galaxies. This survey not only provided spectra for these galaxies, but also produced direct mass measurements of the central black holes for 74 of these galaxies.

Using this data, Martín-Navarro and his colleagues found the first observational evidence for a direct correlation between the mass of a galaxy’s central black hole and its history of star formation. While astrophysicists have been operating under this assumption for decades, the proof was missing until now. As Jean Brodie, professor of astronomy and astrophysics at UC Santa Cruz and a coauthor of the paper, said in a UCSC press release:

“We’ve been dialing in the feedback to make the simulations work out, without really knowing how it happens. This is the first direct observational evidence where we can see the effect of the black hole on the star formation history of the galaxy.”

Roughly 15 years ago, the correlation between a SMBHs mass and the total mass of a galaxy’s stars was discovered, which led to a major unresolved question in astrophysical circles. While this correlation appeared to be a central feature of galaxies, it was unclear as to what could have caused it. How could the mass of a comparatively small and central black hole be related to the mass of billions of stars distributed throughout a galaxy?

The galaxy NGC 660 – in this and other galaxies, the rate at which new stars are formed appears to be linked to the evolution of the galaxy’s central black hole. Credit: ESA/Hubble/NASA

One possible explanation was that more massive galaxies collected larger amounts of gas, thus resulting in more stars and a more massive central black hole. However, astrophysicists also believed their was a feedback mechanism at work, where growing black holes inhibited the formation of stars in their vicinity. In short, when matter accretes on a central black hole, it sends out a tremendous amount of energy in the form of radiation and particle jets.

If this energy is transferred to gas and dust surrounding the core of the galaxy, stars will be less likely to form in this region since gas and dust need to be cold in order to undergo areas of collapse. For years, feedback of this kind has been included in cosmological simulations to explain the observed star-formation rates in galaxies. According to these same simulations, minus this mechanism, galaxies would form far more stars than have been observed.

However, no direct evidence of this phenomena had previously been available. The first step to obtaining some was to reproduce the stellar formation histories of the 74 target galaxies used for the study. Martín-Navarro and his colleagues did this by subjecting spectra obtained from each of these galaxies to computational techniques that looked for the best combination of stellar populations to fit the data.

In so doing, the team was able to reconstruct the history of star formation within the target galaxies for the past 12.5 billion years. After examining these histories, they noticed some predictable results, but also some rather significant differences. For starters, as predicted, the regions of around the galaxies’ central black holes demonstrated a clear dampening influence on the rate of star formation.

Artist’s concept of the most distant supermassive black hole ever discovered. It is part of a quasar from just 690 million years after the Big Bang. Credit: Robin Dienel/Carnegie Institution for Science

As predicted, there was also a clear correlation between the mass of the central black holes and stellar mass in these galaxies. However, the team also noted that in cases where stellar mass was slightly smaller than expected (relative to the mass of their central black holes), star formation rates were lower. In some other cases, galaxies had larger-than-expected stellar masses (again, relative to their black holes) and their star formation rates were higher.

This correlation was not only more consistent than that observed between black hole mass and stellar mass, it occurred independently of other factors (such as shape or density). As Martín-Navaro explained:

“For galaxies with the same mass of stars but different black hole mass in the center, those galaxies with bigger black holes were quenched earlier and faster than those with smaller black holes. So star formation lasted longer in those galaxies with smaller central black holes.”

They also noted that this correlation extends into the deep past, where the galaxies with supermassive central black holes have been consistently producing a comparatively low rate of stars for the past 12.5 billion years. This constitutes the first strong evidence for a direct, long-term connection between star formation and the existence of a central black hole in a galaxy.

Close-up of star near a supermassive black hole (artist’s impression). Credit: ESA/Hubble, ESO, M. Kornmesser

Another interesting takeaway from the study was the way it addressed possible correlations between AGN luminosity and star formation. In the past, other researchers have sought to find evidence of a link between the two, but without success. According to Martín-Navarro and his team, this may be because the time scales are incredibly different. Whereas star formation occurs over the course of eons, outbursts from AGNs occur over shorter intervals.

What’s more, AGNs are highly variable and their properties are dependent on a number of factors relating to their black holes – i.e. size, mass, rate of accretion, etc. We used black hole mass as a proxy for the energy put into the galaxy by the AGN, because accretion onto more massive black holes leads to more energetic feedback from active galactic nuclei, which would quench star formation faster,” said Martin-Navarro.

Looking ahead, the team hopes to conduct further research and determine exactly how central black holes arrest star formation. At present, the possibility that it could be due to radiation or jets of gas heating up surrounding matter are not definitive. As Aaron Romanowsky, an astronomer at San Jose State University and UC Observatories, indicated:

“There are different ways a black hole can put energy out into the galaxy, and theorists have all kinds of ideas about how quenching happens, but there’s more work to be done to fit these new observations into the models.”

Part of determining how the Universe came to be is knowing what mechanisms were at play and the extent of their roles. With this latest study, astrophysicists and cosmologists can take comfort in the knowledge that they’ve been getting it right – at least in this case!


Can a black hole ever fill up?

Is there a limit to how much these exotic objects can consume?

Despite their insatiable appetite, a black hole can never be filled up. Image Credit: NASA

No, in fact they are a result of something being filled too much. Black holes form when a colossal amount of material gets crammed into a tiny space that’s much too small for it all to exist at once. When this happens, it collapses into something called a singularity. This infinitely small point contains all the mass and the subsequent gravity that results from it. Because of the huge amount of mass, close to the black hole the gravity is so strong that nothing can escape. This strong gravity pulls material into the black hole and it will consume until there is nothing left around it. Once it has cleared out the area around it, it may eventually evaporate away in a process taking billions of years.

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In a first, astronomers watch a black hole's corona disappear, then reappear

This diagram shows how a shifting feature, called a corona, can create a flare of X-rays around a black hole. The corona (feature represented in purplish colors) gathers inward (left), becoming brighter, before shooting away from the black hole (middle and right). Astronomers don't know why the coronas shift, but they have learned that this process leads to a brightening of X-ray light that can be observed by telescopes. Credit: NASA/JPL-Caltech

It seems the universe has an odd sense of humor. While a crown-encrusted virus has run roughshod over the world, another entirely different corona about 100 million light years from Earth has mysteriously disappeared.

For the first time, astronomers at MIT and elsewhere have watched as a supermassive black hole's own corona, the ultrabright, billion-degree ring of high-energy particles that encircles a black hole's event horizon, was abruptly destroyed.

The cause of this dramatic transformation is unclear, though the researchers guess that the source of the calamity may have been a star caught in the black hole's gravitational pull. Like a pebble tossed into a gearbox, the star may have ricocheted through the black hole's disk of swirling material, causing everything in the vicinity, including the corona's high-energy particles, to suddenly plummet into the black hole.

The result, as the astronomers observed, was a precipitous and surprising drop in the black hole's brightness, by a factor of 10,000, in under just one year.

"We expect that luminosity changes this big should vary on timescales of many thousands to millions of years," says Erin Kara, assistant professor of physics at MIT. "But in this object, we saw it change by 10,000 over a year, and it even changed by a factor of 100 in eight hours, which is just totally unheard of and really mind-boggling."

Following the corona's disappearance, astronomers continued to watch as the black hole began to slowly pull together material from its outer edges to reform its swirling accretion disk, which in turn began to spin up high-energy X-rays close to the black hole's event horizon. In this way, in just a few months, the black hole was able to generate a new corona, almost back to its original luminosity.

"This seems to be the first time we've ever seen a corona first of all disappear, but then also rebuild itself, and we're watching this in real-time," Kara says. "This will be really important to understanding how a black hole's corona is heated and powered in the first place."

Kara and her co-authors, including lead author Claudio Ricci of Universidad Diego Portales in Santiago, Chile, have published their findings today in Astrophysical Journal Letters. Co-authors from MIT include Ron Remillard, and Dheeraj Pasham.

A nimble washing machine

In March 2018, an unexpected burst lit up the view of ASSASN, the All-Sky Automated Survey for Super-Novae, that surveys the entire night sky for supernova activity. The survey recorded a flash from 1ES 1927+654, an active galactic nucleus, or AGN, that is a type of supermassive black hole with higher-than-normal brightness at the center of a galaxy. ASSASN observed that the object's brightness jumped to about 40 times its normal luminosity.

"This was an AGN that we sort of knew about, but it wasn't very special," Kara says. "Then they noticed that this run-of-the-mill AGN became suddenly bright, which got our attention, and we started pointing lots of other telescopes in lots of other wavelengths to look at it."

The team used multiple telescopes to observe the black hole in the X-ray, optical, and ultraviolet wave bands. Most of these telescopes were pointed at the the black hole periodically, for example recording observations for an entire day, every six months. The team also watched the black hole daily with NASA's NICER, a much smaller X-ray telescope, that is installed aboard the International Space Station, with detectors developed and built by researchers at MIT.

"NICER is great because it's so nimble," Kara says. "It's this little washing machine bouncing around the ISS, and it can collect a ton of X-ray photons. Every day, NICER could take a quick little look at this AGN, then go off and do something else."

With frequent observations, the researchers were able to catch the black hole as it precipitously dropped in brightness, in virtually all the wave bands they measured, and especially in the high-energy X-ray band—an observation that signaled that the black hole's corona had completely and suddenly vaporized.

"After ASSASN saw it go through this huge crazy outburst, we watched as the corona disappeared," Kara recalls. "It became undetectable, which we have never seen before."

Physicists are unsure exactly what causes a corona to form, but they believe it has something to do with the configuration of magnetic field lines that run through a black hole's accretion disk. At the outer regions of a black hole's swirling disk of material, magnetic field lines are more or less in a straightforward configuration. Closer in, and especially near the event horizon, material circles with more energy, in a way that may cause magnetic field lines to twist and break, then reconnect. This tangle of magnetic energy could spin up particles swirling close to the black hole, to the level of high-energy X-rays, forming the crown-like corona that encircles the black hole.

Kara and her colleagues believe that if a wayward star was indeed the culprit in the corona's disappearance, it would have first been shredded apart by the black hole's gravitational pull, scattering stellar debris across the accretion disk. This may have caused the temporary flash in brightness that ASSASN captured. This "tidal disruption," as astronomers call such a jolting event, would have triggered much of the material in the disk to suddenly fall into the black hole. It also might have thrown the disk's magnetic field lines out of whack in a way that it could no longer generate and support a high-energy corona.

This last point is a potentially important one for understanding how coronas first form. Depending on the mass of a black hole, there is a certain radius within which a star will most certainly be pulled in by a black hole's gravity.

"What that tells us is that, if all the action is happening within that tidal disruption radius, that means the magnetic field configuration that's supporting the corona must be within that radius," Kara says. "Which means that, for any normal corona, the magnetic fields within that radius are what's responsible for creating a corona."

The researchers calculated that if a star indeed was the cause of the black hole's missing corona, and if a corona were to form in a supermassive black hole of similar size, it would do so within a radius of about four light minutes—a distance that roughly translates to about 75 million kilometers from the black hole's center.

"With the caveat that this event happened from a stellar tidal disruption, this would be some of the strictest constraints we have on where the corona must exist," Kara says.

The corona has since reformed, lighting up in high-energy X-rays which the team was also able to observe. It's not as bright as it once was, but the researchers are continuing to monitor it, though less frequently, to see what more this system has in store.

"We want to keep an eye on it," Kara says. "It's still in this unusual high-flux state, and maybe it'll do something crazy again, so we don't want to miss that."


It looks like a star 750 million light years away *was* torn apart by a mid-sized black hole

In 2006, the light from a ridiculously violent event reached Earth. Hugely diminished by the time it reached us, it started out as an extremely powerful blast of X-rays… but unlike events such as a typical supernova or other high-energy processes which come and go rather quickly, this one faded slowly, visibly dropping in brightness over the course of more than ten years.

Not too many things can behave this way, and the most common is also one of the scariest: An entire star being ripped to shreds by the gravity of a black hole.

More Bad Astronomy

When a team of astronomers published their observations of this event — which is called 3XMM J215022.4−055108 (the name is due to it being observed by the XMM-Newton observatory together with its coordinates on the sky) — they concluded the most likely culprit was an intermediate mass black hole, a theorized but still unproven class of such bottomless pits.

If a star gets too close to a black hole, very bad things happen. The black hole has incredibly strong gravity, but what can be more important is the change in gravity the star feels across its width. Because black holes are small, stars can get close to them, and because the force of gravity gets stronger the closer you get to the source, the side of the star closer to the black hole can feel a lot more gravity than the opposite side of the star. The outcome is the star gets stretched.

If the change in gravity is enough, the star can be stretched so much it is literally torn apart! The change in gravity you feel with distance is called tides, so this event is called a tidal disruption event, or TDE.

Pretty prosaic for a star being pulled apart like taffy by a black hole.

An animation depicting what happens to a star that gets too close to a black hole, based on observations of such an event seen in 2010.

In this case, the star ripped apart generated a vast amount of energy, emitting more than a billion times the entire energy output of the Sun in just X-rays! The galaxy is about 750 million light years from Earth, and the event occurred about 40,000 light years from the center of the galaxy at least, well outside the main body. Sitting right at that spot is a dot of light… and in the original observations it wasn’t possible to say what that dot was.

A distant galaxy hosts a star cluster (circled) that in turn may host an intermediate-mass black hole, revealed when it tore apart a star and ate the remnants. Credit: NASA, ESA, and D. Lin (University of New Hampshire)

Now though it is. The new Hubble observations clearly show the dot of light is extended in size, resolved as a fuzzy circle rather than a single dot. This object is either a very large globular cluster, or, more likely, an ultra-compact dwarf galaxy, the remains of a small dwarf galaxy after encounters with a bigger galaxy have torn away everything but the galaxy’s core.

Either way, this is the right environment to find big black holes. The crowded environment there means lots of food for black holes — stars can collide, or fall into black holes, or even black holes formed from stars there can eat other and grow — so it’s a prime spot to find them.

Even better, the way a star gets torn apart changes with the mass of the black hole, which changes the light we see from such an event. This is why this event is so exciting: Physical models of the light from 3XMM J215022.4−055108 indicate the black hole had a mass of 50,000 times the mass of the Sun.

Artist’s conception of a star getting eaten by a black hole, with material ripped from the falling down into the black hole and forming a hot disk. Credit: ESA, NASA and Felix Mirabel

Why is that a big deal? Stellar mass black holes (ones formed when massive stars explode) are probably the most common kind, and have masses from roughly 5–50 times the mass of the Sun. Supermassive black holes, found at the centers of most galaxies, have millions or billions of times the Sun’s mass. We have lots of examples of both of these kinds of black holes, and no astronomer seriously doubts they exist.

But that huge mass gap in the middle is weird. Black holes of this mass are the intermediate mass black holes, and the problem is we’ve never definitively seen one. We have lots of candidates (heck, there are five candidates orbiting close in to the supermassive black hole in the center of the Milky Way), but measuring their mass is difficult, so we can't be certain.

That makes this one a pretty dang good bet. The X-rays, the location… both point to this being a middle-weight black hole.

And that's great news! There’s some thought that supermassive black holes grow from intermediate mass ones, so the more of these we find the better we’ll understand them, and the more we learn about black holes in general, including their (very very very) big brothers.


Conclusion

We have discussed the idea that black holes are matter generators and have taken a look at scientific evidence that points the validity of this idea. In the next article we will continue to investigate the idea of continual creation in a Dynamic Steady State Universe.

Throughout the Science section of Cosmic Core you will notice that the processes on each scale will mirror one another. That is, we conclude that Nature uses the same processes to create life and matter but just on different scales. Nature is consistent in her laws. It would make common sense that Nature would work this way and there is much evidence that points in this direction. This suggests that there is a profound order in the Universe and that not only is matter structured off a fractal-holographic matrix, but processes in time are also fractal-holographic. The more things naturally fit together the more it points to an actual working model that does not require ad hoc hypotheses to force it to work.

The point is there was no Big Bang with a singular creation event that happened one time 13.8 billion years ago. There are creation events happening all the time. This lends much credibility to the esoteric idea that creation is happening all the time, the universe is in a constant state of becoming and consciousness is the continual shaper of reality.

In order for black holes to be matter generators, there must be an Aether and there must be two realms that pour into one another. We call these metaphysical time/space and physical space/time. These two concepts are absolutely crucial to understand the new scientific paradigm.


Astronomers Map Surroundings of Supermassive Black Hole

As material spirals towards a black hole, it is heated up and emits X-rays that, in turn, echo and reverberate as they interact with nearby gas. These regions of space are highly distorted and warped due to the extreme nature and crushingly strong gravity of the black hole. Now, a team of astronomers and astrophysicists has used ESA’s XMM-Newton X-ray observatory to track these light echoes and map the surroundings of the black hole in the center of the highly variable active galaxy IRAS 13224-3809.

These illustrations show the surroundings of a black hole feeding on ambient gas as mapped using ESA’s XMM-Newton X-ray observatory. As the material falls into the black hole, it spirals around to form a flattened disk, as shown here, heating up as it does so. At the very center of the disk, close to the black hole, a region of very hot electrons — with temperatures of around a billion degrees — known as the corona produced high-energy X-rays that stream out into space. Alston et al used the reverberating echoes of this radiation, as observed by XMM-Newton, to map the surroundings of a black hole. They focussed on the black hole at the core of the active galaxy IRAS 13224-3809, which is one of the most variable X-ray sources in the sky, undergoing very large and rapid fluctuations in brightness of a factor of 50 in mere hours. By tracking the X-ray echoes, it was possible to trace the dynamic behaviour of the corona itself, where the intense X-ray emission originates from. The corona is shown here as the bright region hovering over the black hole, changing in size and brightness. The researchers found that the corona of the black hole within IRAS 13224-3809 changed in size incredibly quickly, over a matter of days. Image credit: ESA.

IRAS 13224-3809, also known as LEDA 88835 and 2MASX J13251937-3824524, is located approximately one billion light-years away in the constellation of Centaurus.

The galaxy hosts a relatively low-mass (about one million solar masses) supermassive black hole in its center.

It is one of the most variable X-ray sources in the sky, undergoing very large and rapid fluctuations in brightness of a factor of 50 in mere hours.

“Everyone is familiar with how the echo of their voice sounds different when speaking in a classroom compared to a cathedral — this is simply due to the geometry and materials of the rooms, which causes sound to behave and bounce around differently,” said Dr. William Alston, an astrophysicist at the University of Cambridge.

“In a similar manner, we can watch how echoes of X-ray radiation propagate in the vicinity of a black hole in order to map out the geometry of a region and the state of a clump of matter before it disappears into the singularity. It’s a bit like cosmic echo-location.”

As the dynamics of infalling gas are strongly linked to the properties of the consuming black hole, Dr. William and colleagues were also able to determine the mass and spin of the supermassive black hole in IRAS 13224-3809 by observing the properties of matter as it spiralled inwards.

The inspiralling material forms a disk as it falls into the black hole. Above this disk lies a region of very hot electrons — with temperatures of around a billion degrees — called the corona.

While the researchers expected to see the reverberation echoes they used to map the region’s geometry, they also spotted something unexpected: the corona itself changed in size incredibly quickly, over a matter of days.

“As the corona’s size changes, so does the light echo — a bit like if the cathedral ceiling is moving up and down, changing how the echo of your voice sounds,” Dr. William said.

“By tracking the light echoes, we were able to track this changing corona, and — what’s even more exciting — get much better values for the black hole’s mass and spin than we could have determined if the corona was not changing in size.”

“We know the black hole’s mass cannot be fluctuating, so any changes in the echo must be down to the gaseous environment.”

The scientists used the longest observation of an accreting black hole ever taken with XMM-Newton, collected over 16 spacecraft orbits in 2011 and 2016 and totaling 2 million seconds — just over 23 days.

This, combined with the strong and short-term variability of the black hole itself, allowed the team to model the echoes comprehensively over day-long timescales.

Their results appear in the journal Nature Astronomy.

W.N. Alston et al. A dynamic black hole corona in an active galaxy through X-ray reverberation mapping. Nat Astron, published online January 20, 2020 doi: 10.1038/s41550-019-1002-x

This article is based on text provided by the European Space Agency.


Can a black hole turn back into a star? - Astronomy

I have read a lot about black holes and have gained wisdom of the subject. My question: are there such things as white holes - and if so, what do they do?

Before I answer this question I would like to point you to Kate Becker's discussion of why it is so hard to understand the expansion of the universe. The exact same argument applies here. White holes are not something that it is possible to understand using physical intuition. White holes pop up in general relativity (which also explains the expansion of the universe) and that theory as a whole is not easy to understand physically. The only way most people can understand general relativity is through mathematics, which, like Kate said, it not the way that most people are used to understanding things.

Hopefully that will give you some idea of why it is so hard to explain some of these concepts without resorting to mathematics. This does not mean that we shouldn't try, but it does mean that we might not succeed at the first try. Many smart people try very hard to get ideas from General Relativity across without using the Mathematics which the theory is based on, but that is something that is very hard to do. It probably requires a level of understanding of the theory which I would absolutely NOT claim to have.

The short answer is that a white hole is something which probably cannot exist in the real universe. A white hole will turn up in your mathematics if you explore the space-time around a black hole without including the star which made the black hole (i.e. there is absolutely no matter in the solution). Once you add any matter to the space-time, the part which included a white hole disappears.

How can you have a black hole with no mass?

  1. Mathematically this is actually the simplest kind of black hole. Once the singularity is set up it will hold itself together, so the tricky part is setting up the singularity.
  2. The only way to set up the singularity in the real universe is to start with it being there. Somehow the universe has to form with ready made singularities.

Why can't white holes exist in nature?

  1. There is no reason to suggest that the universe started out with ready made singularities. It would actually be quite odd if it did.
  2. Once even the tiniest speck of dust enters the part of space-time which includes the black hole, the part which includes the white hole disappears. The universe has been around for a long time and so even if it did start with white holes, they all would have disappeared by now.

Why bother with the solution if it isn't realistic?

  1. It's easier than realistic solutions. (!)
  2. Part of the solution is close to being realistic. This part does not include the white hole and describes the space-time outside of a normal black hole.

What would a white hole look like if it did exist?

The person/people who came up with the term 'white hole' was actually being quite literal. A white hole is pretty much like an 'anti-black hole'. A black hole is a place where matter can be lost from the universe. A white hole is a place where (if it could exist with any matter in it - which it can't) matter would pop out into the universe. This has many similarities to the Big Bang singularity (although it's not quite the same, since there was nothing before the Big Bang).

Try Exploring Black Holes by Taylor and Wheeler for a good undergraduate level text book on black holes and relativity. I also like Black Holes and Time Warps by Kip Thorne, which has a nice history of the subject.

Footnote: This is my second attempt at an explanation. In the interest of openness I leave the first attempt here also:

"In the full, and most simple General Relativistic solution for a space-time which has a Black Hole (in a vacuum), there are two singularities. One is in what we call the 'future-light cone' and this is the Black Hole. The other is in the 'past-light cone' and is called a White Hole. This solution is however completely unphysical in many ways and in a real Black Hole (formed from the collapse of a star for example) we cannot use the vacuum solution as there is matter present, and the White Hole singularity disappears.

"So the answer to your question is that there is only such a thing as a White Hole in the theory of Black Holes and no such thing is possible physically."

This page was last updated on June 27, 2015.

About the Author

Karen Masters

Karen was a graduate student at Cornell from 2000-2005. She went on to work as a researcher in galaxy redshift surveys at Harvard University, and is now on the Faculty at the University of Portsmouth back in her home country of the UK. Her research lately has focused on using the morphology of galaxies to give clues to their formation and evolution. She is the Project Scientist for the Galaxy Zoo project.


A star is about to plunge head first toward a monster black hole. Astronomers are ready to watch.

Sometime over the next few months, a massive star is going to take a deep plunge near a gigantic black hole in our galaxy. As it screams past the monster at terrific speed, astronomers all over the Earth will be watching it carefully to see how it behaves.

A new set of observations of the star made over the past few years as it’s been picking up speed has given astronomers the go-ahead to make precise measurements of just how the black hole's excruciatingly fierce gravity will affect the star’s orbit. This is good news, because it means there won’t be any added complications to what will already be difficult observations.

The star is called S2 (or sometimes S0-2). It's a bit of a bruiser, a B-type star about 15 times more massive than the Sun. That means it's pretty luminous, which is convenient: That makes it easier to spot from its distance of 26,000 light-years away.

There are likely upwards of a billion stars like that in our galaxy, but S2 is special: It orbits very, very close to the supermassive black hole in the center of the Milky Way.

An infrared image of the galactic center, showing the position of Sgr A* and S2. Credit: ESO/MPE/S. Gillessen et al.

All big galaxies have a huge black hole like this in their hearts. Ours, called Sagittarius A* (pronounced, literally, "Sagittarius A star," or Sgr A* for short) is roughly 4 million times the mass of the Sun, which is staggering on a human scale, but actually a bit of a lightweight when it comes to black holes like these. The exact mass of Sgr A* would be very nice to know, because we think the formation of a central supermassive black hole and the galaxy around it are tied together. Several galactic features tend to scale with the mass of the black hole, so the more we know about ours the better we'll understand our galaxy.

And that’s where S2 comes in. It orbits Sgr A* on an ellipse that takes about 15 years to complete. The diameter of its orbit is about 300 billion kilometers, which may sound like a lot, but we're talking about a supermassive black hole here! That’s close!

And it gets closer. Because the orbit is an ellipse, the star drops down to a mere 18 billion kilometers from the black hole, a positively terrifying close approach. That’s only four times farther from the black hole than Neptune is from the Sun.

When it does this, the gravity of the black hole is so fierce it'll accelerate the star to about 6,000 kilometers per second — fast enough to cross the continental U.S. in less than a second! That's an entire star ripping through space at 1/50th the speed of light, in case you were fretting over some mundane issue in your life right now. Perspective.

The radial velocity (the speed toward and away from Earth) of the star S2 as it orbits around the Milky Way’s supermassive black hole Sgr A*. In a matter of weeks it went from 4,000 km/sec away from us to 2,000 km/sec toward us, a change of 6,000 km/sec due to the black hole’s gravity. Credit: Chu et al.

By making exacting measurements of the star over time, the mass of Sgr A* can be found. Using observations from the last pass of S2 in 2002, and also using observations of the motions of other stars orbiting the black hole, astronomers have found the mass of the black hole to be 4.15 million solar masses.

That close to a black hole, relativistic effects predicted by Einstein’s equations start to become important. For example, the light from the star will have to fight the gravity of the black hole to get to us, losing energy on its way out. This is called a gravitational redshift. When an object approaches or moves away from an observer, the light shifts in wavelength a bit, which is just plain old redshift. The amount of the shift depends on the velocity.

In the case of S2, at closest approach gravitational redshift acts like another 200 kilometers per second added to the star's motion! That's enough to measure pretty easily, so astronomers should definitely see that.

Another effect is that the orbit of the star shifts, rotating a little bit. What this means is that if you draw a line through the long axis of the star's orbit, every time it whips around the black hole that line rotates a bit. This is called "relativistic precession" of the orbit. This effect is small and more difficult to detect, but it’s precisely what astronomers hope to see when S2 makes its close encounter sometime over April to June of this year.

The orbit of S2 (teal) around Sgr A*, the supermassive black hole in the Milky Way’s center, along with a handful of other stars. The dots represent actual measurements of the stars’ positions over time. Credit: S. Sakai/A.Ghez/W. M. Keck Observatory / UCLA Galactic Center Group

And that (finally) brings me back to the new observations. Stars like S2 are commonly binary, that is, have a companion star orbiting them. If S2 has one, that can mess up the observations. For example, at closest approach to the black hole, the gravity of the black hole can affect the orbit of the companion, changing its shape significantly. This in turn will affect the motion of S2, and that motion is the key factor to perform all these calculations!

So a team of astronomers used old data as well as making new observations of S2 to look for a companion. They took spectra, breaking up the light from the star into thousands of colors, to see if they could detect a redshift in the light due to the motion of a companion around the star. Long story short, they didn't see one. Also, a companion star gives off its own signature of light, different than the primary star, but they didn't see any hint of that either.

However, that doesn't mean there isn't one! Maybe it's too low mass to have much of an effect. So the scientists did the math, and determined that the star can't be any more massive than roughly 1.6 times the Sun, or else they would have seen it (also, a more massive star would have to be farther out from the primary star to hide its effects, and if it's too far out the black hole would have torn the binary apart ages ago).

Even if there is a companion, what they found is that it won’t affect the relativistic measurements very much, far less than the overall relativistic effects in the first place. However, astronomers need to be aware of any potential perturbations by a possible companion, so this study will be very helpful in the coming months as scientists pore over the data they'll be taking.

That's good to know. The only bad news here is that the astronomers who took the spectroscopic observations were hoping to get some insight on how S2 formed. Was it born from material swirling around the back hole perhaps tens of millions of years ago? Did it form in a star cluster, several of which are seen near the black hole? Another possibility is that it used to be a binary on a longer orbit around the galactic center on a close approach to the black hole the secondary companion would have been torn from the star and flung away at high speed, leaving S2 in its weird looping 15-year orbit around Sgr A*.

Unfortunately, the new observations don't help narrow down S2’s origins. Perhaps further observations may help us understand where it came from we still don't truly understand the environments around these monster black holes, and knowing how stars get there would be helpful.

But that doesn't cast any shade on the big story, which is that we're ready for S2 to take its Einsteinian plunge down the steep well of Sgr A*'s intense gravity. Will we be able to see its orbit change, and the implacable force of gravity affecting its light, and get better constraints on just how whopping a black hole Sgr A* is?


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