How black holes devour stars

The trouble is, because supermassive blacks don't emit any light at all, they spend most of their time dormant and hidden from our eyes. They only come to life when they eat, but their meals are surprisingly rare: most of the gas, dust, and stars swirling around them reside in stable orbits and will never be engulfed by them. However, they are always hungry, and whenever a piece worthy of honor does happen to fall in, you can see this gluttony from very great distances.

For most of the past fifty years, scientists have mostly observed one and only one type of black hole at dinner: Quasars. These ultra-bright celestial bodies, discovered by the astronomer Martin Schmidt In 1963, they are the centers of active galaxies. They can be seen even from the outskirts of the visible universe, each of them shining with a light brighter than a billion suns. Quasars are thought to form when massive clouds of gas and dust plunge into a black hole over hundreds and thousands of millions of years, compressing, heating and emitting light as they circle the black hole's mouth. However, quasars are not ideal objects for research. They are extreme events that usually occur at great distances from us. They are relatively rare and constitute only a small fraction of the lifespan of any supermassive black hole. Quasars thus provide a limited view, leaving astronomers blind to how supermassive black holes feed and grow during routine periods in the local universe. Scientists have also studied supermassive black holes by recording the velocities of stars revolving around them, but these measurements only work for objects that are close to us - in the Milky Way or in one of the galaxies closest to us - places where the telescopes we have are able to separate different stars.

In 1988 the astronomer suggested Martin Rees A third way to study supermassive black holes, a method that has recently begun to bear fruit. Instead of observing the steady glow of quasars or the velocities of passing stars, astronomers can look for bright, short bursts of light coming from around a black hole. These eruptions, called tidal disturbance events (TDEs), occur as a supermassive black star devours a lucky star. Since these events last a few months rather than thousands of years, they allow researchers to follow the feasting process from start to finish, and they are bright enough to be observed as they occur in both distant and nearby galaxies.

How to destroy a star

A tidal disturbance event is much more dramatic than a gentle sea tide that might sweep your beach towels onto the beach, but they are not very different in principle. Tides on Earth are mainly caused by the gravitational pull of the Moon, which pulls more strongly on the side of Earth closest to the Moon. This difference between the gravitational pull of the moon on the near side and on the far side of the earth is called tidal force. This force lifts a humpback, the machine tide, on the side facing the moon, and also, although it may seem a little strange, on the opposite side of the earth. In addition, it also creates a corresponding depression in places oriented at an angle of 90 degrees from the axis between the Earth and the Moon. When a star comes close to a supermassive black hole, perhaps due to a gravitational push from a nearby star, the massive tidal forces can tear it apart.

The details of the star's end depend on both the size of the infalling star and the size of the supermassive black hole. A compressed and small bone such as for example A white dwarf star would be much more immune to tidal forces than a large, inflated, more sun-like star. It's like it's harder to split a bowling ball than a block of cotton candy. The largest supermassive black holes, those containing billions of solar masses, are too large to easily create tidal disruption events: they swallow stars in their belly and knees before the tidal forces become large enough to tear those stars apart. The tidal forces around a black hole of millions of solar masses, however, will tear apart most stars that come within 50 million kilometers of it: roughly the distance between the planet Mercury and the sun

You might think that the complete disintegration of a star sounds like a spectacular sight, but that's just the beginning of the fireworks show. After the initial disturbance, the remnants of the star will disperse and gradually deviate from the star's original orbit. The basic mechanics of rotation dictates that about half of the remnants will be blown away in the form of long beams of material flowing from the region near the black hole, while the other half will curl back and form Adsorption disk: A structure of spiral rings that flow in a funnel motion into the black hole. As the material from the disk falls in, it accelerates almost to the speed of light and emits light as gravitational and frictional forces compress and heat it to temperatures approaching 250,000 degrees Celsius. Over a period of weeks or months, a typical tidal disruption event will cause a black hole, until then dormant and invisible, to briefly outshine all the stars in its galaxy.

The first discoveries

Although theorists predicted decades ago that tidal disruption events would occur, astronomers did not detect any such events until the 90s and early 2000s. This delay is caused in part by the rarity of tidal disruption events: they are estimated to occur once every 100,000 years in a galaxy similar to the Milky Way. Also, they may be hard to see. According to simple theoretical models, the glow of an accretion disk formed following a tidal disturbance event will peak in the areas of the electromagnetic spectrum known as soft x-rays or far ultraviolet rays: wavelengths that are difficult to observe from the ground due to interference caused by interstellar dust and by the Earth's atmosphere. The same models also raise the possibility that astronomers could use a tidal disruption event to obtain relatively accurate estimates for the mass of the black hole accompanying the event, an essential figure if we want to learn how exactly the size of a black hole changes its behavior and its effects on its galactic environment. To measure the mass of a black hole, astronomers can simply measure how long it takes for a tidal disruption event created by it to reach its peak brightness (a figure that reveals how quickly the accretion disc forms and feeds the black hole). Because tidal disruption events are so bright, researchers can accurately determine the masses of a wide range of black holes - a wider range than is possible with any other known phenomenon.

The first candidates to be tidal disruption events were found in space telescope data ROSAT, the X-ray scanner, andGALEX, the scanner in the ultraviolet range. The documented cases appeared as passionate events that lasted weeks to months, and which originated in the centers of galaxies that had been dormant until then. These discoveries, which were the first appearance of a phenomenon predicted many years before, were especially important because they helped establish an entirely new field of research. However, since such events were mostly found in old data, astronomers could not study them across several wavelengths in real time to extract their deepest secrets. To catch tidal disruption events as they happen, astronomers would have to be extremely lucky or, alternatively, be able to continuously scan vast swathes of the sky.

As fate would have it, the steady progress of the last ten years in the fields of data storage and sensors made such ambitious surveys feasible. Optical cameras of the first class today can image square height Or more of the sky in a single photograph, a jump that parallels the situation where one can suddenly see the sky with the help of a panoramic lens after years of studying it by looking through a hole in a drinking straw. By repeatedly scanning large areas of sky and digitally combining the resulting images to extract faint, transient features, astronomers can now more easily detect and study both tidal disruption events and a large group of other transient astrophysical phenomena. These new wide-field surveys, with names like the Panoramic Scanning Telescope and Rapid Response System (Pan-STARRS), the Palomar Synoptic Sky Survey (TFP), and the automatic comprehensive sky survey for the search for supernovae (HANDLES-SN), were primarily designed to detect supernovae and asteroids, but they can do much more than that. Because they can image millions of galaxies every night, they are also sensitive to more exotic transient events, such as tidal disruption events.

New questions for a new era

In 2010, shortly after Pan-STARRS began actually scanning the sky, a team led by AstronomerSubi Gazzari, a tidal disruption event called PS1-10jh, which occurred around a black hole the size of about two million solar masses in a galaxy 2.7 billion light years from Earth. Because this tidal disruption event was detected shortly after the data was collected, Gazzari and her colleagues were the first in the world to be able to watch such an event unfold before their eyes in the visible light and then in the ultraviolet. What they saw struck them with amazement.

Based on careful measurements of the spectrum of this particular tidal disruption event, it appears far too cold. Its temperature, about 30,000 degrees Celsius, was more than eight times lower than predicted by most basic theories of accretion discs. Furthermore, rather than fading within a few weeks as its adsorption disk cooled and dissipated, PS1-10jh maintained a constant temperature for many months after it was first discovered. And the strangest thing: the Pan STARRS survey detected in the glow left after the event signs of ionized helium, a phenomenon that can only occur at temperatures much higher than 100,000 degrees Celsius. And while the tidal disruption event appeared to be rich in helium, it also appeared to be devoid of hydrogen, the most common element in the universe and the main component of stars. Theorists sat down to work and test what could create such confusing results.

Investigating tidal disruption events allows astronomers to learn not only about black holes but also about stars being torn apart billions of light years away.

To explain the absence of hydrogen in PS1-10hj, the Pan-STARRS team proposed that the disrupted star had lost its thick hydrogen envelope at some point in its past, perhaps following a previous interaction with the black hole, so that whatever was left to feed the observed accretion disc was its core, rich in helium. However, this possibility alone could not explain the strange thermal contradictions of the tidal disruption event: its surprisingly low temperature on the one hand, and its abundance of helium, ionized by much higher temperatures, on the other. To solve the mystery, other theorists have hypothesized that, in fact, the accretion disk surrounding the PS1-10hj black hole has not been directly observed. Instead, they hypothesized that the astronomers must have observed a blanket of gas enveloping the region far from the black hole, absorbing the strong radiation produced by the accretion disc and re-emitting it at lower temperatures. Such a veil has another advantage: it explains why hydrogen is apparently not found there, without needing an exotic, helium-rich core to cause the tidal disruption event. Given the appropriate temperature and fairly high density, such a veil could in principle mask the presence of hydrogen, so that it would not be possible to see it directly.

The only problem was that a thick screen of gas would not be stable at the required distance from the central black hole in the galaxy. Eventually, the gas will fall over time into the black hole or dissipate until it is invisible. The obscure origins of this material are still subject to intense research and debate, but broadly, they fall into two possibilities, both of which concern the dynamics of black hole feeding. As the remnants of a disrupted star swirl around a black hole and form a growing accretion disc, shock waves may travel outward far from the disc, preventing some of the debris at the edge from immediately falling in, thus creating a temporary screen of matter. Another possibility is that an accretion disk from a newly born tidal disruption event may initially pour so much material inward that it briefly exceeds the feeding capacity of a black hole, creating temporary external winds or currents right at the edge of the black hole, and they sweep away the remnants The star out, beyond the accretion disc, to much greater distances.

As astronomers tried to sort out these confusing possibilities for PS1-10hj and for new tidal disruption events discovered soon after, one thing became abundantly clear: tidal disruption events are a far more complicated phenomenon than anyone had ever thought. But the biggest surprise was still waiting for us.

Swift strikes in amazement

The surprise came in the pre-dawn hours of March 28, 2011 with an automatic alert sent to pagers and cell phones of a dedicated team of astronomers around the world. the satellite Swift He discovered at that moment a pulse of high-energy radiation from the depths of space. Swift, built by NASA in collaboration with research institutes in Italy and England, is a fast-reacting space telescope designed to study all kinds of exploding objects across the sky. But its main target is gamma ray bursts (GRB extension), destructive interstellar explosions that are the brightest astrophysical events in the universe. Every time a burst of gamma rays seeps into Swift's sensors, the telescope quickly repositions itself to observe the source of the radiation in X-rays and visible light, calling home to trigger a complex chain of events on Earth. Minutes after Swift's warning, astronomers rush to take control of the largest and most powerful telescopes to search for the faint ember-like afterglow associated with gamma rays, before it fades and disappears from view forever. Since Swift's launch in 2004, it had detected more than 1,000 gamma-ray bursts, but as it would turn out, this particular event, later named Swift J1644+57, was unlike anything the satellite had seen before.

As evidenced by the number of gamma ray bursts, they tend to be short, usually lasting from a fraction of a second to a few minutes. When we pointed our telescopes at Swift J1644+57 that early Martian morning, we expected to see the usual, fading glow left over from a short-lived gamma-ray burst. Instead, we saw a bright, raging gamma-ray flare that lasted a day, followed by months of strong but fading X-ray emissions. We soon found that the source of the outburst was a galaxy 3.8 billion light-years away in the constellation Draco. one of our colleagues, Joshua S. Bloom from the University of California, Berkeley, raised the possibility that we were witnessing a tidal disruption event, and predicted, a prediction that turned out to be correct, that this particular gamma-ray source would be found at the center of the galaxy, home to supermassive black holes. But compared to all previous tidal disruption events detected at longer, lower-energy wavelengths, which are the wavelengths at which the thermal emission from the accretion disk of a torn apart star was observed, this event was something quite different.

How can a tidal disturbance event create gamma rays? The best answer we could give to that question was that when black people eat, they make a lot of dirt. A black hole can gobble up most of the gas of a disrupted star and trap it forever behind it the event horizon (The limit beyond which the black hole's gravitational pull is so great that even light cannot escape from there). However, apparently, all black holes are also spinning, and this spinning can repel a few percent of the total gas of the star that has undergone disturbance towards the poles of the black hole, outside the event horizon, and there the gas will be accelerated and ejected in the form of a beam of parallel particle beams moving at almost the speed of light. These fast beams emit gamma rays and X-rays as they race through the cosmos. Apparently, Swift accidentally fell into the orbit of the beam of Swift J1644+57. And he was lucky: apparently, not all tidal disturbance events generate such relative floods, and most of those that do do not cross our line of sight.

The discovery of Swift J1644+57 inspired the Swift team to join forces and start searching for more events. Already at the beginning of 2017, two more tidal disturbance events emitting jets of gamma rays were discovered. These phenomena are the fiercest and rarest death cries of stars, and are a new way to investigate one of the topics at the forefront of research in high-energy astrophysics: the creation and behavior of relativistic particle jets.

The death of worlds

Whether through the thermal emissions that crush the adsorption of stellar remnants or with the help of the gamma ray fluxes coming from the relativistic jets of a black hole devouring a star, tidal disruption events open a new window for the behavior and development of supermassive black holes and their environment. And most importantly, unlike the jets and accretion discs of quasars, which are much larger and last much longer and are created by huge clouds of gas plunging disorderly into a supermassive black hole over very long timescales, tidal disruption events are short, clean events that can be studied more easily. No human will ever live long enough to witness the full life cycle of a single quasar, but astronomers have already found and studied more than 20 tidal disruption events from start to finish. And among the details of these stellar catastrophes, they were able to discern jarring oddities that beg for further study. By precisely measuring the fluctuations of the flashes coming from tidal disruption events, astronomers are learning not only about black holes but also about the details of the components and internal structures of stars being torn apart billions of light-years away.

They may even eventually learn about stellar companions as well: planets that have also been swallowed by black holes along with their suns. Every flickering flash from a distant galactic center may be a signal that signals the death of entire worlds. Surveys of stars in our galaxy have found that almost every star has planets around it. Planets probably also accompany most, and perhaps even all the stars in other galaxies, including those that fell victim to a tidal disturbance event. Even if these planets were not directly engulfed, they could still find themselves in the orbit of transient relativistic jets created by some of the tidal disruption events, reaching light-years beyond the black hole that generated them. Life in any planetary system that is lucky enough to absorb such a beam will quickly become extinct. One day, astronomers may witness a tidal disruption event right in our backyard, when the four-million-solar-mass black hole lurking dormant in the gas-poor center of our Milky Way will flare to life as it swallows some stray star. Such an event would be very bright but would not put us in danger at all, because our distance from the galactic center is very great, so the most dangerous consequences of tidal disturbance events cannot reach us.

New and more powerful surveys expected to begin in the future herald a new era of tidal disturbance event discoveries. The Large Synoptic Survey Telescope (LST), an eight-meter diameter telescope currently under construction in Chile, whose field of view covers 10 square degrees of sky, will itself reveal thousands of such eruptions within ten years of its launch. In some ways, the most challenging aspect for LSST scientists will be to sort out and sort through the vast number of discoveries of transient phenomena. Future radio observatories such as the square kilometer array (BE) that is currently being built in Australia and South Africa will be particularly suitable for detecting relativistic jets, even if such jets are "off-axis", meaning that their beam is not directed directly along our line of sight. In the not-so-distant future, astronomers may compile a catalog of tidal disruption events that will contain thousands upon thousands of entries, an amount that no one in the world could explore in a lifetime, and it will shed new light on the masses and behaviors of those hungry and elusive ghosts, those super black holes. - massive ones that reside in the heart of galaxies all over the universe, and if it weren't for such phenomena we would have no access to them. This rich and growing pool of knowledge could be the source of more revolutionary discoveries, which we never dreamed of.