# Why isn't the star that created the black hole a black hole?

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If the mass of a black hole is creating so much gravity that light cannot escape, why isn't the mass of the star that created the black hole (before it went supernova) trapping light as well?

By all accounts, that pre-supernova star should have boatloads more mass than the black hole post supernova, right? Doesn't the star lose most of it's mass when going supernova?

You are correct in saying that a star loses a lot of its mass in a supernova. However, there is a reason why the star still becomes a black hole. Actually, I suppose the question here is "Why doesn't a star become a black hole before it even undergoes a supernova?"

There is a reason for a supernova (I'll assume you're talking about type II supernovae, which result from incredibly massive stars). Stars undergo nuclear fusion, and this leads to "thermal pressure", which counteracts the force of gravity. Without this pressure, gravity would indeed make a sufficiently large star collapse upon itself. Gravitational collapse occurs when there is not enough pressure to counteract gravity; the result is a spectacular supernova. So stars only become black holes (or other compact objects, such as neutron stars) when they cannot produce enough energy to counteract the force of gravity due to their own mass.

As for the first part of your question (sorry for answering in reverse), light in the area of a black hole cannot escape if it is inside its event horizon or on a trajectory towards it. The radius of the event horizon for a non-rotating black hole is its Schwarzschild radius, which is proportional to the mass of the black hole. The reason this is non-applicable in stars is because the Schwarzschild radius in stars is deep inside its interior, and there is not a strong enough gravitational field to produce an event horizon to trap any light near it.

Thermal pressure reference: https://en.wikipedia.org/wiki/Gravitational_collapse

I hope this helps.

Black holes are created because the core of the star becomes very dense, not just because the star is massive. Prior to the creation of the black hole, the core is able to create enough outward pressure to prevent the core from gravitationally collapsing to the density needing to create a black hole.

The star before it turns into a black hole has what is known as radiation pressure, i.e. the fusing of elements that creates nuclear explosions. This outward radiation pressure counter balances the inward force of gravity but when the star finally runs out of fuel that radiation pressure stops. Thus the only remaining force is gravity. Gravity then attracts what is left of the star after the super nova inwards into a deep dense core which then forms the black hole.

A supernova may actually be necessary in the creation of a stellar black hole.

At the ends of their lives the cores of massive stars are made mostly of iron-peak nuclei from which you cannot extract more fusion energy. To support their weight, these stars rely on electron degeneracy pressure - the pressure caused by the Pauli exclusion principle allowing no more than one electron to share the same quantum state.

In principle a star might be supported by degeneracy pressure forever as it gradually cools - this is the fate of most white dwarfs.

However, the core of a massive star is just too big for that to work. The density increases until all the electron are moving at close-to the speed of light and that's as high as the degeneracy pressure can get. If the core exceeds the Chandrasekhar mass, it will collapse and as it does so, the rest of the star collapses with it (a little more slowly).

The collapse is triggered by the removal of electrons by electron capture into nuclei to form neutrons. At some point enough neutrons are produced for neutron degeneracy pressure to halt or at least slow the collapse. This and the release of a lot of gravitational potential energy are ultimately what power a supernova explosion. But if the collapse is not halted then even neutron degeneracy pressure will not support the star and collapse to a black hole becomes inevitable. A black hole status is reached once a proportion of its mass is compressed inside its Schwarzschild radius $r_s = 2GM/c^2$. i.e. once its density achieves $$ho > frac{3M}{4pi r_s^{3}}$$ i.e. when a central mass $M$ has a density that exceeds $$ho > frac{3}{32pi} frac{c^6}{G^3 M^2} = 1.8 imes10^{19} left(frac{M}{M_{odot}} ight)^{-2} { m kg/m}^3$$ This is a ball park figure and assumes spherical symmetry and neglects any detailed GR treatment, but is more or less correct - a few times higher than typical neutron star densities.

In other words it is the density of the material that largely determines whether something becomes a black hole. The mass is only an indirect parameter.

Whether an object is a black hole is not just determined by its mass. It's determined by whether that mass is entirely within its Schwarzschild radius.

In principle, any object can be a black hole if all its mass is concentrated into a sufficiently small volume.

For example, the Sun's Schwarzschild radius is about 3.0 km -- but its actual radius is about 700,000 km. It could become a black hole only if it were compressed down to a radius of 3.0 km.

A much easier way to think about this is to consider clouds in the sky. They contain hundreds to thousands of gallons of water molecules but they are very spread out. Same as a hydrogen cloud before it creates a star. When you compact molecules in a tighter space you get a stronger magnetic field in the near vicinity of the object. The amount of molecules in a specific space dictate the "strength" of the magnetic field. When a star explodes it does lose mass but what remains is compacted into an infinitely small space to create a stronger magnetic field. A Swarzchild radius is calculated by using existing mass of a star and seeing how much space do we need to cram that amount of mass into to overcome the speed of light but it does not consider how much mass remains after a supernova explosion. And as for "forever and ever"… Our ideas are constantly changing. Remember, we used to think the planet was flat… I hope this helps, aloha.

## Milky Way’s central black hole flings star out of galaxy

An artist’s impression of a star being thrown out of the Milky Way after a close encounter with the supermassive black hole at the core of the galaxy. Image: James Josephides (Swinburne Astronomy Productions)

Five million years ago, a binary star system wandered too close to the supermassive black hole lurking at the core of the Milky Way. The hole’s ferocious gravity likely captured one of the stars, but the other was flung away with a velocity of more than 6 million kilometres per hour (3.7 million mph).

That’s fast enough to escape the Milky Way, but even so, it will still take the outward-bound star, known as S5-HVS1, some 100 million years to pass through the galaxy’s outskirts and into the great void of intergalactic space.

“We traced this star’s journey back to the centre of our galaxy, which is pretty exciting,” said Gary Da Costa, an astronomer at the Australian National University. “This star is travelling at record-breaking speed, 10 times faster than most stars in the Milky Way, including our Sun.

“In astronomical terms, the star will be leaving our galaxy fairly soon and it will likely travel through the emptiness of intergalactic space for eternity. It’s great to be able to confirm a 30-year-old prediction that stars can be flung out of a galaxy by the supermassive black hole at its centre.”

Using the 3.9-metre Anglo-Australian Telescope at Sliding Spring Observatory, an international team spotted the star by accident while searching for remnants of small galaxies orbiting the Milky Way.

“The star is only 29,000 light years away, quite close by galactic standards, which means the team could measure its trajectory very precisely,” said ANU’s Dougal Mackey.

The black hole at the heart of the Milky Way, known as Sagittarius A*, has the mass of four million Suns. ANU astronomer Thomas Nordlander said if a binary system gets too close to such a massive object, “the black hole can capture one of the stars into a close orbit and kick out the other at very high speed.”

The process is known as the Hills Mechanism, proposed three decades ago by astronomer Jack Hills.

“This is super exciting, as we have long suspected that black holes can eject stars with very high velocities,” said Sergey Koposov, an astronomer with the Carnegie Mellon University working with the Southern Stellar Stream Spectroscopic Survey. “However, we never had an unambiguous association of such a fast star with the galactic center.

“We think the black hole ejected the star with a speed of thousands of kilometres per second about five million years ago. This ejection happened at the time when humanity’s ancestors were just learning to walk on two feet.”

Ting Li from Carnegie Observatories and Princeton University, leader of the observations of S5-HVS1, said the the star’s trajectory is the first “clear demonstration’ of the Hills Mechanism in action.

“Seeing this star is really amazing as we know it must have formed in the galactic center, a place very different to our local environment,” he said. “It is a visitor from a strange land.”

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

## Dying Star Screams As It Falls Into Black Hole

As a doomed star spirals closer and closer to a black hole that's about to gobble it up, it lets out periodic bursts of light that scientists liken to dying screams, scientists say.

The star is falling into a gigantic black hole in the center of a distant galaxy that lies 3.9 billion light-years away in the direction of the constellation Draco. As the remains of the star get pulled in, it releases blips of light about every 200 seconds, with occasional lags.

"You can think of it as hearing the star scream as it gets devoured, if you like," Jon Miller, a University of Michigan astronomer, said in a statement. Miller was part of a team that detected the light blips using two orbiting X-ray telescopes: NASA and Japan's Suzaku, and Europe's XMM-Newton.

The researchers think the star has been pulled apart by the black hole's gravity into a disk of material that orbits the black hole. [The Strangest Black Holes in the Universe]

"In order for the black hole to feed from a star that its gravity has broken apart, the remains of the star must form an accretion disk surrounding the black hole," said the study's leader, University of Michigan astronomer Rubens Reis. "The disk gets heated up and we can see emissions from the disk very close to the black hole in X-rays. As this matter is falling in, it gives a quasiperiodic wobble and that's the signal we detected."

Though the dying star's signal comes to us in the form of light, the researchers liken it to sound because it comes at a characteristic frequency that, if converted to sound, would make an ultra-low D-sharp.

Similar oscillations have been observed in matter falling into smaller black holes in our own galaxy, and a huge black hole like this one in the center of a nearby active galaxy.

Never before have such screams been heard from a star falling prey to a black hole so distant, or one that had been thought to be dormant, like this one.

"This is telling us that the same physical phenomenon we observe in stellar mass black holes is also observed in black holes a million times the mass of the sun, and also for black holes that were previously asleep," Reis said. "It speaks to the invariant nature of physics, which I think is very beautiful."

The findings are described in a paper published in the Aug. 3 issue of the journal Science.

## When is a black hole not a black hole? When it’s a boson star

FASCINATING, bamboozling, vaguely terrifying: black holes are the love-to-hate monsters of the universe. These insatiable cosmic cannibals are concrete predictions of Einstein’s general theory of relativity, the best theory of gravity we have. Even so, theorists long debated whether they could exist – until astronomers saw the first signs of them. Now we see black hole paw prints all over: in huge stars collapsing in on themselves, in distant collisions of massive objects that set the universe quivering, and in the dark hearts of galaxies including our own.

#### Read more: The space-time echoes that point to a new theory of reality

This year, we should have the clincher: the first direct image of the supermassive black hole at the Milky Way’s centre. But as we gear up for that shadowy mugshot, some physicists are entertaining a maverick thought: what if it isn’t there?

The new word is that our obsession with black holes might have blinded us to the existence of something even stranger – a basic phenomenon of particle physics whose significance we have failed to grasp. After all, there’s good reason to want whatever is at our galaxy’s heart not to be a black hole. For a start, black holes make a nonsense of quantum mechanics, the best theory of everything-besides-gravity that we have.

It is a speculative idea as yet, to be sure, but there are sound reasons to contemplate it. “We scientists tend to be completely arrogant about what we think we know,” says theorist Luciano Rezzolla of the Frankfurt Institute for Advanced Studies in Germany. “I don’t want to find myself in 10 years’ time saying &hellip

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## 'Spaghettified' star wrapped around a black hole spotted for the first time

2 & 76 More

Astronomer here! I actually study this object myself and have a paper on it going through the referee process right now, and know the guys who have been working on this!

AT 2019dsg is a Tidal Disruption Event (TDE), which is when a star wanders too close to a supermassive black hole and gets ripped apart by tidal forces. It was discovered in May 2019 by an optical survey and was 230 Mpc (

750 million light years) from us, but stood out because it was really bright in optical- like, top 3% of all TDEs or some such. Obviously the question then is why, and I don't think anyone had a very good potential explanation there. Here is a really impressive animation though for how AT 2019dsg might have played out!

In particular, there are questions about the "outflow" of TDEs and how the one in AT 2019dsg worked. Specifically, when a TDE happens we think about half the mass of the star falls into the event horizon, and half gets into some sort of accretion disk and outflow (a spherical shockwave like a supernova, or in very rare cases a relativistic jet). Unfortunately this thing is so far away it's not like you can just take a picture of it, and the outflows can be "thick" at specific parts of the spectrum (aka the light is blocked) based on how the physics goes. To date, no one's really been able to get such good sampling of the disc, until this work! So this is really exciting! (So to be clear this is not a new event that the group discovered, and in fact I think this isn't the first time AT 2019dsg has reached the top of r/space, just this is a new observation of what is going on in it.)

As an aside, one random detail about AT 2019dsg is a group a few months ago that also looked into this event published claiming an association with a neutrino. That would be a really exciting thing if it were true, but assumes certain energies to be feasible. and my own work shows it doesn't appear to be there. Science! My own research is in modeling the radio outflows of AT 2019dsg with the best sampled radio observations from the VLA and ALMA, which we can use to probe the outflow at much greater distances than the innermost accretion disc. If you're interested in that, here is a link to it and a laymen's explanation of my work. But happy to answer any questions you might have about AT 2019dsg in specific or TDEs in general! They're really amazing objects and I'm rather fond of this one!

Edit: when the star is ripped apart, it is done so thoroughly that fusion ends in the star during the TDE.

Edit 2: the photo is accurate to what we simulate these events to look like up close, but is not a “real” picture. This event is far too far away to be anything but a point source (like stars are- no resolution) on Earth. That’s why things like the spectral information are so important!

## New study proves Hawking was right, black holes only get bigger

In a study published on Monday, scientists from Massachusetts Institute of Technology (MIT) and Cornell University shared their findings from a research project which analyzed ripples in spacetime created by two black holes that spiraled inward and eventually merged into one bigger black hole.

The ripples studied were the first gravitational waves ever identified, detected by the Advanced Laser Interferometer Gravitational-Wave Observatory in 2015.

Splitting the gravitational wave data into time segments before and after the black holes merged, the researchers calculated the surface areas of the black holes in both periods.

The physicists found that the surface area of the new black hole was actually greater than the two initial black holes combined. The finding confirms a prediction made by famed scientist, Stephen Hawking, in the 1970s, in which he stated that black holes cannot decrease in surface area as it mirrored a rule in physics, that entropy or disorder can&rsquot decrease over time.

Hawking&rsquos law states that the surface area of the black hole won&rsquot increase on its own, but when things enter it, it gains mass and correspondingly its surface area increases.

While incoming objects can make the black hole spin, which decreases surface area, the size increase due to the additional mass will always be greater than the size lost to spinning.

&ldquoIt&rsquos the first time that we can put a number on this,&rdquo said Maximiliano Isi of MIT.

The premise was also born out of Albert Einstein&rsquos general theory of relativity which describes the physics and relationship behind black holes and gravitational waves.

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## Black Holes Really Know How to Savor Their Meals

Astronomers have detected the glow of a star being slowly devoured millions of light-years from Earth.

In 2005, astronomers detected a burst of infrared light coming from the heart of a galaxy nearly 150 million light-years from Earth. They had been studying the night sky for supernovae, the glittering explosions that mark the deaths of stars, but this seemed different. Intrigued, they decided to keep an eye on it.

After years of observations, the astronomers have determined the source of this burst was indeed the death of a star. But the star hasn’t exploded. It’s being eaten.

The star drifted too close to a supermassive black hole, the vacuum cleaner of the universe. The black hole’s fierce gravity dragged the star toward its invisible mouth, toward a point from which nothing, not even light, can return. The tugging stretched and shredded the star, producing a bright tail of light that could be detected by powerful telescopes on Earth.

An international team of astronomers—led by Seppo Mattila of the University of Turku in Finland and Miguel Pérez-Torres of the Astrophysical Institute of Andalusia in Spain—tracked this event over the course of a decade. Six years into their monitoring, the glow of light began to change shape and lengthen—a tell-tale sign that this wasn’t a one-off star explosion.

Their observations produced a ghostly GIF of this violent cosmic meal:

Mattila, Perez-Torres, et al. / Bill Saxton / NRAO / AUI /NSF

The discovery, described in a paper published Thursday in Science, marks the first time that astronomers have directly imaged the formation and shape-shifting expansion of the jet of stellar material that forms when black holes devour stars. The stellar debris grows brightly as it swirls around the black hole, emitting light across many different wavelengths. This interaction occurred at the site of another cosmic mashup: in one of the galaxies that makes up a pair of colliding galaxies known as Arp 299, shown below:

Matilla, Pérez-Torres, and their colleagues first spotted the light from the black hole’s dinner with the William Herschel Telescope in the Canary Islands, and followed up with observations with networks of radio telescopes in Europe, Asia, and North America, as well as NASA ’s Spitzer space telescope.

Such bright streams of stellar debris are known to emit intense X-rays and visible light, but this particular case revealed itself to astronomers only through infrared and radio wavelengths. The astronomers say that could be explained by substantial amounts of interstellar gas and dust around the black hole and the crumbling star. The gas and dust absorbed the X-rays and visible light coming from the star, and then radiated it into the cosmos in these other wavelengths which, even at unfathomable distances, can still reach us here on Earth.

“This seems like a plausible explanation, as we know that this is a persistently accreting supermassive black hole, where only the high-energy X-ray emission can escape and most of the light at longer wavelengths is absorbed,” said Erin Kara, an astronomer at the University of Maryland and NASA ’s Goddard Space Flight Center who studies black holes and was not involved in the study.

For black holes, consuming stars can be a drawn-out process. As the black hole spins, it takes the shredded star along for the ride, producing a hot, glowing disk of stellar material, like a merry-go-round that won’t stop. “In some cases … it takes a lot of time—years—for all the debris to fall into the black holes,” Kara said.

Astronomers hope to detect more cosmic meals like this one, and there’s a good chance they already have. Sky surveys are like fishing nets a search for one astrophysical phenomenon often catches others. At first glance, a burst of radio emissions, or a pinprick of light, could be many things. For astronomers, few things beat the feeling of taking a closer look and finding something unexpected—and scientifically useful.

“This is a very exciting result,” Kara said. “What I love about this result is that it was serendipitous, as many of the most exciting astronomical results are.”

## Black hole devouring a neutron star caused ripples in space and time, scientists say

A black hole swallowing a neutron star has likely been detected for the first time, according to scientists.

The Australian National University (ANU), which participated in the research, explains that the “cataclysmic event” was detected on Aug. 14, 2019, by gravitational-wave discovery machines in the U.S. and Italy. The machines detected ripples in space and time from an event that happened about 8,550 million trillion kilometers away from Earth, ANU said in a statement.

“Neutron stars and black holes are the super-dense remains of dead stars,” it explained, noting that scientists are still studying the data to work out the size of the two objects.

"About 900 million years ago, this black hole ate a very dense star, known as a neutron star, like Pac-man -- possibly snuffing out the star instantly," said Professor Susan Scott, leader of the General Relativity Theory and Data Analysis Group at ANU and a chief investigator with the Australian Research Council's Centre of Excellence for Gravitational Wave Discovery (OzGrav), in the statement.

Artist's impression of a black hole about to swallow a neutron star. (Credit: Carl Knox, OzGrav ARC Centre of Excellence)

Initial findings suggest “a very strong likelihood” of a black hole devouring a neutron star, according to ANU. "Scientists have never detected a black hole smaller than five solar masses or a neutron star larger than about 2.5 times the mass of our Sun,” said Scott. "Based on this experience, we're very confident that we've just detected a black hole gobbling up a neutron star.”

However, Scott also acknowledged the slight possibility that the swallowed object was “a very light black hole.”

The Advanced Laser Interferometer Gravitational-wave Observatory (LIGO), which is operated by Caltech and MIT, detected the event, as did its Virgo sister facility near Pisa, Italy.

LIGO uses identical detector sites in Washington State and Louisiana to operate as a single "observatory.” Virgo is located at the site of the European Gravitational Observatory (EGO).

In a separate project, scientists released the first-ever image of a black hole earlier this year, revealing the distant object in stunning detail.

The groundbreaking discovery was made by the Event Horizon Telescope, an international project involving telescopes across the globe that describes itself as a “virtual Earth-sized telescope.” Telescopes in Hawaii, Arizona, Chile, Mexico, Spain and the South Pole participated in the ambitious research project.

The black hole was spotted in galaxy Messier 87 (M87), 55 million light-years away. A light-year, which measures distance in space, equals 6 trillion miles.

## 'The universe is a violent place:' Black hole collides with neutron star, scientists say

For the first time, an image of a black hole has been unveiled by the Event Horizon Telescope, but it’s not what you might think. Here’s why. USA TODAY

Forget Game of Thrones, now we may have detected a real-life "battle" going on in outer space between a black hole and a neutron star.

Scientists this week announced not only that mind-blowing news but also said they'd detected a distant collision between a pair of neutron stars, as well as the potential mergers of three black holes.

“The universe is keeping us on our toes,” said Patrick Brady, a professor of physics at the University of Wisconsin-Milwaukee, in a statement.

And, much like the too-dark battle scenes in "GOT," none of this can be "seen." Rather, these collisions can be "heard" using sensitive equipment used to detect gravitational waves, the strange ripples in space-time first foreseen by Albert Einstein a century ago,

Gravitational waves occur when neutron stars collide and are detected using both the USA's mammoth LIGO observatory and Italy's Virgo observatory. This is the third observational campaign of the two groups, which began on April 1.

Neutron stars are small yet incredibly dense stellar objects, and are the collapsed remains of imploded stars. Black holes are also collapsed stars with gravity so strong that even light cannot escape their grasp. Scientists last month captured the first-ever photo of a black hole.

An artist's conception of two merging neutron stars. (Photo: NSF/LIGO/Sonoma State University/A. Simonnet)

The neutron star-black hole collision is estimated to have taken place in a distant galaxy, roughly 1.2 billion light-years away, according to the National Science Foundation.

This is "further evidence that our universe regularly rings with the aftershocks of colossal astronomical events,” said Professor Sheila Rowan, director of the University of Glasgow’s Institute for Gravitational Research. “We’d been deaf to those sounds before the detectors equipped us with the opportunity to hear them, and each event gives invaluable new data points to expand our understanding of our cosmos.”

It's more evidence that "the universe is a violent place," according to the United Kingdom's Science and Technology Facilities Council.

But this first-ever black hole – neutron star collision isn't a slam dunk yet: "Unfortunately, the signal is rather weak," Brady said. "It's like listening to somebody whisper a word in a busy café it can be difficult to make out the word or even to be sure that the person whispered at all. It will take some time to reach a conclusion about this."

In total, since making history with the first-ever direct detection of gravitational waves in 2016, the network has spotted evidence for two neutron star mergers 13 black hole mergers and now this one possible black hole-neutron star merger.

The discoveries kicked off a new field of astronomy involving gravitational waves, CNN said.

## Star kicked out of Milky Way by giant black hole

A super-hot blue star careening at 1.6 million miles per hour was ejected from our galaxy after a brush with a monster black hole, astronomers believe.

A runaway star found racing through space at an incredible 2.5 million km an hour (1.6 million mph), suffered a brush with a monster black hole at the heart of our galaxy, astronomers believe.

Data collected using the Hubble space telescope suggests the speeding sun was part of a triple-star system that was drifting through the Milky Way a hundred million years ago.

But the threesome passed dangerously close to the centre of our galaxy where the supermassive black hole lurks.

The space scientists say it swallowed up one of the stars and booted the other two out of the Milky Way. As they flew, the two stars merged to form one super-hot blue star.

Labelled HE 0437-5439, it is now speeding about 200,000 light-years from galactic central point and high above the whirling disk of the Milky Way, according to a Hubble news release.

Around 16 hypervelocity stars are known, all of which have been discovered in the past five years. They are all suspected to have been flung from our galaxy’s centre but this is the first time the theory has been successfully tested.

As we reported two years ago, another team of astronomers previously suggested the star had been fired on its path by another yet-undiscovered black hole in a satellite galaxy of our own called the Large Magellanic Cloud.

But US Astronomer Warren Brown, of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, a member of the Hubble team that observed the star, said: “Using Hubble, we can for the first time trace back to where the star comes from. Its motion points directly from the Milky Way center,”

“The star is travelling at an absurd velocity, twice as much as the star needs to escape the galaxy’s gravitational field. There is no star that travels that quickly under normal circumstances – something exotic has to happen.”

Watching hypervelocity stars is not simply a curiosity for astronomers. They believe that studying their trajectories could help them to learn how invisible dark matter is distributed around the galaxy. Astronomers claim to have proved the supermassive black hole exists by studying the motions of stars around it at the centre of the Milky Way.

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