Does A Black Hole Die If It Has No Fuel?

If black holes are in the centers of galaxies, can the black hole die if it does not suck anything?
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). Image Credit: NASA/JPL-Caltech

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). Image Credit: NASA/JPL-Caltech

Originally posted on Forbes!

Black holes are indeed at the centers of every large galaxy like our own Milky Way, and our Milky Way's black hole is a good example of 'normal' black hole behavior, at least for galaxies living near us.

But as a first note, we have to clear up a few misconceptions. Black holes don't inhale material from their surroundings; they're extreme gravitational objects, but the force of gravity is their only mechanism for drawing material inwards towards them. A black hole won't inhale planets that orbit around it any more than a star will. The supergiant black holes at the centers of galaxies, by the same token, won't inhale stars and gas at a faster rate than any other object of the same mass.

The other facet to your question is on the 'death' of a black hole, which is an interesting concept. What would it mean for a black hole to die? Will the gravitational distortion associated with a black hole disappear if it fails to bring in new material? To that question we can give a definitive answer. Black holes are very stable objects, and will remain static and unchanging if there is no new material which interacts with the black hole. If you sit a black hole in the void of space, out on its lonesome, it will simply sit there. It won't be able to grow any more massive (and by proxy it won't grow physically larger either), but large black holes won't disappear, either. It's very difficult to destroy mass in our universe, and as one way to think about black holes is as a very dense concentration of matter, the destruction of a black hole is no simple thing.

This illustration shows a glowing stream of material from a star as it is being devoured by a supermassive black hole in a tidal disruption flare. Image credit: NASA/JPL-Caltech

This illustration shows a glowing stream of material from a star as it is being devoured by a supermassive black hole in a tidal disruption flare. Image credit: NASA/JPL-Caltech

If, on the other hand, we expand a definition of the black hole's death to include a black hole going from a luminous, bright object in the astrophysical sky to something that is invisible by almost all methods of looking at it, then yes; the lack of material around a black hole can cause a black hole to die. This is a dying down, rather than a fading from existence in the Universe, but if the black hole has been trying to grow its mass, it glows brightly, and if that material is suddenly consumed, that glow can be shut down rapidly, the death of the black hole as a light source.

Going back to our own Milky Way's black hole - currently it has nothing to feast on. Its existence is under no threat, and the black hole, as a large astrophysical object, will remain in place and unperturbed until some star or gas cloud drifts too near. At that point, the black hole will shred the star or the looser cloud of gas, heating it until it glows brightly to a number of telescopes based here on Earth. (Not to worry; anything that happens in our Galaxy's black hole is sufficiently far away that it should pose no danger to us.) Once that gas is consumed? The black hole will subside into darkness once more, perhaps a little larger after its meal; dark but not lost.

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What happens when one star in a binary turns into a red giant?

When you have a binary star system with the stars close together, what happens when one of the stars starts to turn into a red giant?


Binary stars are actually pretty common (about a third of the stars in our galaxy are in some kind of binary), although the very close pairs are a little less common.

For the majority of the lifetime of the stars in a binary, both stars just spin around each other, burning their own hydrogen and orbiting tranquilly. However, unless the two stars are exactly the same size when they’re formed (not generally the case), then one star will run out of hydrogen before the other, and will transform into a red giant star.

Red giants are pretty fuzzy stars, only loosely held together by their own gravity. For some scale, our own Sun will become a red giant in its future. It will expand from its current (quite large) radius of 695,500 kilometers to something 200 times that large. But it won’t have gained any material to grow that much bigger - it just spreads out what’s already there.

If the star is on its own, this newfound wispiness doesn’t change much. But if there’s another star nearby, the other star can begin to tug on the outer layers of the red giant, pulling some of the outer layers towards itself. (This is the same thing that the moon does to our oceans.) Because the red giant is so big, the outer layers are only weakly tied to the centre of the star. If the companion star in the binary is close enough or massive enough, it can begin to tear the outer layers of the red giant away, and pull them towards itself. This will change the shape of the red giant from a diffuse sphere into a diffuse teardrop, with the point of the teardrop facing the companion star.

What the companion star does with that surface material depends on what kind of star the companion is. By the time one star has aged its way to being a red giant, its companion is in one of two states, which will be dictated by how massive the companion was when it began its life.

If the companion started out with a lower mass than its red giant partner, it will still be burning hydrogen in its core when the red giant begins to form. As it siphons gas off of the red giant, this companion star will grow in mass. Depending on how rapidly the red giant is growing, and how quickly the companion is siphoning material, the lower mass star can actually grow so large that it becomes more massive than the red giant. After having so much of its material drawn away from itself and onto its neighbor, the red giant will slow its donation of mass. This can result in a fairly stable configuration - the red giant, having lost a good amount of its atmosphere to its neighbor, will continue to slowly bleed gas into its neighbor’s gravitational well, and the neighbor will continue burning hydrogen until it has exhausted its own resources.

The other option for these binaries is if the red giant was the smaller of the two stars when they started their lives. This means that the star now turning into a red giant took much longer to reach the end of its life than its neighbor. (The bigger your mass, the shorter your life, if you’re a star.) So the companion star has already gone through its death throes, and can be one of a number of interesting stellar remnants.

The main options for your stellar companion in this case are: a white dwarf, a neutron star, or a black hole.



If your red giant is pouring gas down onto a white dwarf, you will eventually trigger some kind of explosion: either a nova or a supernova. A nova is a thermonuclear detonation on the surface of a white dwarf, and can recur multiple times, as it’s just a surface explosion. This kind of behavior makes these binaries fairly noticeable, because the brightness of the star will flare to many times its original brightness. A supernova, on the other hand, will detonate the entire white dwarf, blasting itself apart, and leaving nothing behind (also quite noticeable). This kind of supernova occurs when the white dwarf gains too much material to be stable (these stars are balancing gravity against an electron’s unwillingness to be pushed too close to another electron), and some trigger in the core sparks a runaway burning of material.

If the other object is a black hole or a neutron star, you’ll wind up with what’s called an X-ray binary, for the somewhat boring reason that it produces a lot of X-rays. For these objects, as the gas from the red giant is pulled off of the red giant star, it gets pulled into a very thin disk, surrounding the black hole or neutron star. The disk forms because it’s very hard for gas to lose a lot of momentum all at once and plunge straight down onto the black hole, but as a result, the gas winds up heating up to an incredible temperature before it makes it all the way to the neutron star or black hole. This heat causes the X-ray glow, and keeps the disk itself almost invisible in optical light.

So there you have it! A binary system of stars with one red giant will result in the companion tearing the outer layers of the red giant star away. From there, you wind up growing the object nearby, or causing a nova, a supernova, or the creation of a lot of X-rays.

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Are black holes real?

They sure are! And a lot of the best evidence we have for them comes from the centre of our own galaxy. With the telescopes we have now, we can actually watch individual stars zip around some extremely heavy, but physically small, completely invisible object. From the orbits of the stars, we can figure out how much matter the dark object must have. We also know how big it can be, since some of these stars pass very close to the mystery object, and aren’t being torn to shreds, so the object must be smaller than that. With the mass and the size, we can have a guess at the density of the object. If the density is way too high to be anything other than a black hole, then the only reasonable conclusion is that our mystery dark object is indeed a black hole! In the case of our galaxy, the weight and size of our central black hole is about 4 million times the mass of the sun in a space that’s less than the size of our solar system. The only way you can pack things that tightly is if you crush it to the point where the object has to be a black hole.

Incidentally, we’ve recently discovered that there’s a pretty sizeable cloud of gas about to have a strong interaction with the black hole in the center of our galaxy, which might cause the black hole to light up rather dramatically. (This poses no threat to us, but it will make for some pretty cool looking images.)

We know that other galaxies have black holes too, so it’s not just that our galaxy is weird or special in some way. With really nearby galaxies, we can look at the way that the galaxy is rotating. (We can also do this with our galaxy, which gives us an extra data point.) If you’re trying to figure out how massive the galaxy is, if you don’t include a whole pile of matter right at the center (like a black hole), any model you create to try and reproduce the way the galaxy rotates just won’t work quite as well as if you included the mass of the black hole.

We also have some pretty solid evidence for star sized black holes, not just the supermassive ones in the centres of galaxies. We’ve seen lots of stars that appear to be in a pair with a mostly invisible object (the invisible object sometimes glows in X-rays), and again, you can calculate the orbits of the normal stars and find the mass of the object it was going around. Again, you’ll find that the only object that we know of that can pack that much mass into a small space is a black hole!

So yes, black holes are very real, and they’re everywhere.


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If two black holes collide, do they go crunch or bang?

Simulation of the merger of two black holes and the resulting emission of gravitational radiation. The colored fields represent a component of the curvature of space-time. The outer red sheets correspond directly to the outgoing gravitational radiation that one day may be detected by gravitational-wave observatories. The brighter yellow areas near the black holes do not correspond to physical structures but generally indicate where the strong non-linear gravitational-field interactions are in play. Image Credit: NASA/C. Henze

Simulation of the merger of two black holes and the resulting emission of gravitational radiation. The colored fields represent a component of the curvature of space-time. The outer red sheets correspond directly to the outgoing gravitational radiation that one day may be detected by gravitational-wave observatories. The brighter yellow areas near the black holes do not correspond to physical structures but generally indicate where the strong non-linear gravitational-field interactions are in play. Image Credit: NASA/C. Henze

They go chirp!

Or they would, if you could listen to gravitational waves.  Black holes go through a long spiraling wind-down before they actually collide, and the distortions they make in space as they circle each other at high speeds winds up creating waves in space-time, very like what happens if you swirl your finger in a pond.  The formation of these waves carries energy away from the two black holes, which helps the two of them merge together.  Like sound waves, gravitational waves have a frequency and amplitude.  These are at extremely long wavelengths, so it’s nothing the human ear could hear normally, but if you scale the frequencies up by a couple thousand, you can make a sound file of what it would sound like.  And it is a chirp, or, more onomatopoetically, a vwooop!

A research group at MIT has come up with a great series of audio clips of what this would sound like, based on theoretical models.  (These will come in handy, as we’d like to find direct evidence of this happening, so we need to know what to look for.)

This is what it sounds like when a black hole collides with another black hole 10000 time larger than itself, and here’s what it sounds like when it merges with one that’s only 3 times larger than itself.  That little chirp at the end is the gravity-sound of the black holes colliding, and becoming one larger object.

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