If black holes are so bad at growing, how do we get supermassive black holes?

If supermassive black holes are so bad at feeding from the stars around them, how did they get so big?
This illustration shows a glowing stream of material from a star, disrupted as it was being devoured by a supermassive black hole. The feeding black hole is surrounded by a ring of dust.   Image credit:  NASA/JPL-Caltech

This illustration shows a glowing stream of material from a star, disrupted as it was being devoured by a supermassive black hole. The feeding black hole is surrounded by a ring of dust. Image credit: NASA/JPL-Caltech

This is such a good question that the answer is not particularly settled yet - we have some ideas for how some supermassive black holes may have gotten so large, but it’s not clear that the explanations we’ve come up with so far hold true for all supermassive black holes.

So a few definitions before we launch into our ideas - by and large, all massive galaxies have a gigantic black hole at their centers. To distinguish these central black holes from the type of black hole created when a single, massive star ends its life, we named the big ones “supermassive”. The black holes created from single stars are less entertainingly named “stellar-mass black holes”.

This artist concept of the local galaxy Arp 220, captured by the Hubble Space Telescope,  shows the bright core of the galaxy, paired with an overlaid artist's impression of jets emanating from it, to indicate that the central black hole's activity is intensifying.  Image Credit:  NASA/JPL-Caltech

This artist concept of the local galaxy Arp 220, captured by the Hubble Space Telescope,  shows the bright core of the galaxy, paired with an overlaid artist's impression of jets emanating from it, to indicate that the central black hole's activity is intensifying. Image Credit: NASA/JPL-Caltech

Black holes of all sizes are extremely inefficient at gathering new material to themselves. Rather than absorbing nearby material in one fell swoop, the material is more often than not pulled into a tight disk of extremely hot material, or flung away from the black hole entirely, sometimes at relativistic speeds. For all black holes have a reputation of being cosmic garbage disposals, if you had a garbage disposal this terrible in your own kitchen, after the first time it blasted superheated onion bits on your ceiling you’d call a professional to have it removed ASAP.

So it is an eminently reasonable question - how do you make a black hole that’s thousands of times larger than the ones that are made by stars, when they’re exceptionally bad at growth? This question is made more complicated because right now, getting a star to explode in a supernova, and the remnant collapsing down into a black hole is the most robust theoretical model we have to make a black hole, so you have to somehow bridge the gap between something that contains a few times the mass of the Sun and something that contains thousands of times the mass of the Sun, while that thing steadfastly refuses to grow rapidly by accreting new material onto itself.

What can we do? Well, we can either 1) start larger, or 2) grow differently. If you start larger, then you have the benefit of not having to grow by a factor of several thousand but only by a couple. This method means you have to start with “seed black holes” very early in the universe. You can do this in one of two ways. First is the direct collapse model - the thinking goes that it might have been possible, very early in the universe for enough gas to collect together that its gravity would just collapse all the way down to a black hole, skipping the star phase entirely. The second method is effectively to go through a star first, but to go through a very, very large star - something much larger than the Universe makes nowadays, which would burn through its hydrogen much faster, and would make a larger black hole as a remnant.

Artist's illustration of supermassive black hole pair.Image credit:  NASA/CXC/A.Hobart

Artist's illustration of supermassive black hole pair.Image credit: NASA/CXC/A.Hobart

What about growing differently? It’s possible that instead of growing by slurping tiny fractions of gas and dust from its surroundings, the black holes grow by absorbing other black holes. This raises a whole string of other questions, like “are there even enough black holes around for that to work?”. We know that there should be times when galaxies gain a second supermassive black hole - during galaxy collisions. The remnant of the two galaxies should have two black holes which sink together, immediately doubling their mass. But it’s unclear if you can guarantee that enough galaxies will smash together enough times for this to work for all the galaxies we see, especially since not all galaxies are expected to merge with another galaxy the same number of times.

There’s no reason the answer will wind up being one or the other - some combination is likely to be in play. If you can start with a larger seed in the early Universe, you can grow more easily through a combination of colliding with the supermassive black holes in other galaxies, and by gathering gas inefficiently to themselves.


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How Do Black Holes Get Started?

How do black holes get started?
This artist's impression shows the orbits of three of the stars very close to the supermassive black hole at the centre of the Milky Way. The position of the supermassive black hole is marked with a white circle with a blue halo. Image credit: ESO/M. Parsa/L. Calçada

This artist's impression shows the orbits of three of the stars very close to the supermassive black hole at the centre of the Milky Way. The position of the supermassive black hole is marked with a white circle with a blue halo. Image credit: ESO/M. Parsa/L. Calçada

Originally posted on Forbes!

It depends on how big the black hole is! If you’re dealing with a small black hole, then we have a pretty good understanding of how the black hole forms. The smallest astrophysical black holes are objects that form during the catastrophic explosions of dying, large stars.

These stars contain so much mass that when they begin to explode in a supernova, the shock wave of the explosion can ricochet down into the core of the star, compressing it down to the point that the object in the center of the star becomes too dense for electrons to hold atoms apart (the end point for a white dwarf), further down until the star becomes too dense for neutrons to hold each other apart (the end point for a neutron star) and after that point, the object becomes so dense that even light can’t escape it. At that point, it seems logical to assume that the object itself will continue to press itself inwards, subject to its own ever-increasing gravity, until it takes up no more space than an infinitely tiny point - a singularity.

Left behind, far outwards, is the contour in space which marks the threshold of no return for light - if light travels closer than this horizon, it’s not coming back. This spherical contour surrounding the black hole is known as the event horizon, and often this whole region of space is called the black hole, as it’s a region where the existence of the black hole is the most important thing around.

This artist’s impression depicts the newly discovered stellar-mass black hole in the spiral galaxy NGC 300. The black hole has a mass of about twenty times the mass of the Sun and is associated with a Wolf–Rayet star : a star that will become a black hole itself.  IMage credit: ESO/L. Calçada/M.Kornmesser

This artist’s impression depicts the newly discovered stellar-mass black hole in the spiral galaxy NGC 300. The black hole has a mass of about twenty times the mass of the Sun and is associated with a Wolf–Rayet star : a star that will become a black hole itself.  IMage credit: ESO/L. Calçada/M.Kornmesser

The black holes formed this way are a few times larger than our own Sun, made out of stars at least eight times larger than our own Sun. When you want to create black holes which are larger than that, like the supermassive black holes at the centers of galaxies, the situation has to be slightly different. These black holes are many, many times larger than our own Sun - millions to billions of times more massive than the black holes which form from individual stars. How did they form? If we want black holes to grow to this size, we’re going to have to give them a lot of time.

Fundamentally, though, we still need to take a lot of mass, and compress it somehow down to a sufficiently high density that it will continue to collapse down into a black hole. This is a tricky thing to do, because matter tends to resist collapse, so you need to have quite a bit of force involved. There are a few possibilities, though. One is to scale up the mechanism that we know works for smaller black holes, and start with a larger star.

This artist’s impression depicts a Sun-like star close to a rapidly spinning supermassive black hole, with a mass of about 100 million times the mass of the Sun, in the centre of a distant galaxy. Its large mass bends the light from stars and gas behind it. Despite being way more massive than the star, the supermassive black hole has an event horizon which is only 200 times larger than the size of the star. Image credit: ESO, ESA/Hubble, M. Kornmesser

This artist’s impression depicts a Sun-like star close to a rapidly spinning supermassive black hole, with a mass of about 100 million times the mass of the Sun, in the centre of a distant galaxy. Its large mass bends the light from stars and gas behind it. Despite being way more massive than the star, the supermassive black hole has an event horizon which is only 200 times larger than the size of the star. Image credit: ESO, ESA/Hubble, M. Kornmesser

On the scale of a supermassive black hole, this pathway is still starting pretty small.  We even have the advantage of starting with the stars in the very earliest Universe, which are thought to likely be hundreds of times more massive than the stars we see near us now. When those larger stars explode, they should leave behind a black hole, which, while larger than the black holes we typically see from stellar explosions nowadays, would still need to grow considerably to reach the size of a supermassive black hole. You’d have to do some combination of feeding that black hole a lot of gas, or merging it with other black holes. But black holes are terrible at gathering gas efficiently into itself in order to grow in mass, and the mergers between black holes are also thought to take quite a long time, though they do happen if you leave them long enough.

Another option is to start large. How do you do that? Well, you could possibly build a tremendously large star, and let it collapse at the end of its life. This collapsing star would have to be tens of thousands times larger than our own Sun (and considerably larger than your standard early-universe star), but that would allow for a black hole many thousands of times more massive than our Sun to form when the star inevitably explodes at the end of its short lifetime. From that larger starting point, you would still need to grow a lot, over time, but if you start large you’d need less building. Going from 10,000 times larger than the Sun to the 1,000,000 times larger than the sun is much, much easier than going from 100 times larger than the Sun to 1,000,000 times larger than the Sun.

These large black holes are probably built through a combination of these possibilities, and potentially some other possibilities we haven’t yet constructed. These questions are part of why we built LIGO, and have plans to build an even more sensitive machine in LISA - those devices will allow us to figure out how common it is for black holes to merge together, and that can help us figure out what the population of black holes looks like in the first place. After all, LIGO has been full of surprises already!

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What's in a Jet from a Black Hole?

Originally posted at Forbes!

As far as we can tell, it’s mostly electrons, neutrons, and protons getting flung out into these jets. In the end, it turns out that the electrons are the key factor in making the jets so visible to our telescopes.

The jets themselves are interesting objects. Not every galaxy’s black hole produces a jet, even though all sizable galaxies have black holes. It seems that in order to produce this jet, the supermassive black hole in the very center of the galaxy has to be actively trying to gather new material into itself. Supermassive black holes are hilariously inefficient at growing larger, even when there’s material around for it to work with. Our Milky Way’s supermassive black hole isn’t growing at the moment because there’s no material nearby, but even if there were a lot more gas and dust very near the black hole, the black hole wouldn’t be able to grow very fast.

Part of the reason a black hole doesn’t grow very fast is that the material orbiting a black hole has to continue to lose energy to keep falling into the black hole, and that process of energy loss is driven by inefficient things like friction and heat. On top of all of this, there are probably crazy things happening with the magnetic fields within the rapidly rotating material around the black hole. Magnetic fields are a bit of a bugbear for studies of galaxies – we know that there are magnetic fields around, but we’re not quite sure how much of an effect they have on the galaxy, and they’re stupendously difficult to model correctly.

False-colour X-ray image of the giant elliptical active galaxy Centaurus A (NGC 5128) taken with the orbiting Chandra X-ray Observatory, featuring its 30,000 light-years long jet. Credit: NASA/SAO/R.Kraft et al.

False-colour X-ray image of the giant elliptical active galaxy Centaurus A (NGC 5128) taken with the orbiting Chandra X-ray Observatory, featuring its 30,000 light-years long jet. Credit: NASA/SAO/R.Kraft et al.

In the case of jets, we know that there must be strong magnetic fields, because we observe a type of glow that only happens if you have both very rapidly moving electrons and a magnetic field. It’s called synchrotron radiation, and its happens when you get a relativistic electron (which means that it’s moving at a significant fraction of the speed of light) caught in a twisting orbit around a magnetic field line. The electron moves in a helix around the magnetic field line, and emits light that we can observe with a wide range of different telescopes.

This VLA radio composite image shows the active galaxy 3C 348, also known as Hercules A. The VLA data, which record frequencies from 4-9 GHz, were taken in 2010-2011. Image Credit: R. Perley and W. Cotton (NRAO/AUI/NSF)

This VLA radio composite image shows the active galaxy 3C 348, also known as Hercules A. The VLA data, which record frequencies from 4-9 GHz, were taken in 2010-2011. Image Credit: R. Perley and W. Cotton (NRAO/AUI/NSF)

It’s usually this synchrotron radiation that we see in the images of jets coming from a supermassive black hole. If you’re looking at a radio image or an optical image, what you’re looking at is the glowing byproducts of nearly speed-of-light electrons bending under the influence of a magnetic field.

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Does a star gain mass in order to make a black hole?

If a black hole is the remnants of an extremely large collapsed star, how does gravity of the dead star increase to create the black hole? Light could obviously escape the gravity of the star and the black hole is made from the dead star but, once the black hole is created, light can no longer escape. Does the mass somehow increase at the death of the star?
In this artist’s portrayal of the IC 10 X-1 system, the black hole lies at the upper left and its companion star is on the right. The two objects orbit around a center of gravity once every 34.4 hours. The stellar companion is a type known as a Wolf-Rayet star. Such stars are highly evolved and destined to explode as supernovae. The black hole companion is shedding its outer envelope in a powerful wind, and some of this gas is captured by the black hole’s powerful gravity. redit: Aurore Simonnet/Sonoma State University/NASA.

In this artist’s portrayal of the IC 10 X-1 system, the black hole lies at the upper left and its companion star is on the right. The two objects orbit around a center of gravity once every 34.4 hours. The stellar companion is a type known as a Wolf-Rayet star. Such stars are highly evolved and destined to explode as supernovae. The black hole companion is shedding its outer envelope in a powerful wind, and some of this gas is captured by the black hole’s powerful gravity. redit: Aurore Simonnet/Sonoma State University/NASA.

Originally posted at Forbes! 

Your standard black hole is indeed the remnants of an extremely large, collapsed star, with the remnant sitting somewhere between 5-20 times more massive than our own sun. However, if we let nature produce a black hole, the black hole that is produced at the end of a supernova explosion is actually significantly less massive than the star that it once was.

The Hubble Space Telescope imaged this view in February 1995. The arcing, graceful structure is actually a bow shock about half a light-year across, created from the wind from the star L.L. Orionis colliding with the Orion Nebula flow. Image Credit: NASA

The Hubble Space Telescope imaged this view in February 1995. The arcing, graceful structure is actually a bow shock about half a light-year across, created from the wind from the star L.L. Orionis colliding with the Orion Nebula flow. Image Credit: NASA

Part of the drop in mass between star and black hole comes in the years before the supernova, when the star typically sheds a sizable fraction of its mass. Since the star has expanded so much as part of the red giant phase, it only has a loose gravitational grasp on the outer layers of its atmosphere, and they are easily pushed away from the star by the star’s stellar wind. Our sun has a stellar wind as well, and it’s one of the reasons Mars is still losing its atmosphere to space - it’s also the reason Earth’s magnetosphere is such a nice feature of our planet; we’re protected from this kind of atmospheric blasting by the sun. However, the Sun’s stellar wind is dragging many fewer particles along with it, compared to a red giant star, so the sun’s mass loss is much less than it would be if it were a red giant.

The star has pre-emptively lost some of the mass it contained before it became a red giant, but there’s also the supernova explosion itself to consider. A good chunk of the material that was left within the star goes blasting outward, fast and hot enough to barrel into any gas and dust nearby and produce X-rays. It’s really only the very core of the star that stays put, and can be compressed into the black hole.

This composite image of a supernova remnant combines infrared and X-ray observations. The explosion left a blazing hot cloud of expanding debris (green and yellow). The location of the blast's outer shock wave can be seen as a blue sphere of ultra-energetic electrons. Newly synthesized dust in the ejected material and heated pre-existing dust from the area around the supernova radiate at infrared wavelengths of 24 microns (red). Foreground and background stars in the image are white. Image Credit: MPIA/NASA/Calar Alto Observatory.

This composite image of a supernova remnant combines infrared and X-ray observations. The explosion left a blazing hot cloud of expanding debris (green and yellow). The location of the blast's outer shock wave can be seen as a blue sphere of ultra-energetic electrons. Newly synthesized dust in the ejected material and heated pre-existing dust from the area around the supernova radiate at infrared wavelengths of 24 microns (red). Foreground and background stars in the image are white. Image Credit: MPIA/NASA/Calar Alto Observatory.

If the mass of the star is actually only partially transformed into a black hole, then I’ve actually made the paradox in your question worse. How is the gravitational pull of a much bigger star (which light can escape from) so much weaker than a black hole made of only a fraction of the star (which light can’t escape from)?

The gravitational pull from a large object on a small one, at any point in space, is only determined by the mass of the heavy object, the mass of the smaller object, and the distance between the centers of the two objects. So, by this logic, if you were a cosmic wizard and replaced the sun with a black hole of equal mass, none of these parameters have changed for the planets. The planets haven’t changed their mass, or their distance from the center of the solar system where the sun used to be, and if the sun and the black hole are the same mass, then the whole system is gravitationally identical.

Obviously, there are some cosmetic differences between the black hole and the star in this scenario, but gravitationally speaking, differences only arise when you start to get very close to the objects. At the surface of our sun, which is where light escapes from the star and streams out towards the rest of the Universe, we are still 432,700 miles (696,000 kilometres) away from the center of the sun. A black hole, on the other hand, is a much denser object, so you can get far closer to its center while still having the entire mass of the black hole to contend with. It’s this density that makes the difference between light being able to escape or not.

For our magical swap scenario, you would have to get within 1.83 miles (2.95 kilometers) of the coordinates marking the very center of the Sun (or where it used to be) before you would cross the event horizon, where light would no longer be able to escape. Within that sphere, 3.66 miles from edge to edge, is the entire mass of our current sun, packed into a single pinprick’s worth of space, instead of filling 432,700 miles of space.

It’s the same scenario with the black hole produced at the end of a supernova; the black hole hasn’t grown in mass, or expanded its gravitational reach- it’s simply much more dense, so you (or light) can get much closer to the center of the black hole, while still being pulled on by the full mass of the black hole. It’s this combination of proximity and mass concentration that produces the gravitational extremes we’ve come to associate with black holes!

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Why don't black holes grow faster?

If black holes are infinitely dense and the laws of physics break down, why are they only as massive as the stars that created them? Why can’t they draw more mass from the bulk?
In this artist's illustration, turbulent winds of gas swirl around a black hole. Some of the gas is spiraling inward toward the black hole, but another part is blown away. Image credit:  NASA, and M. Weiss (Chandra X -ray Center)

In this artist's illustration, turbulent winds of gas swirl around a black hole. Some of the gas is spiraling inward toward the black hole, but another part is blown away. Image credit: NASA, and M. Weiss (Chandra X -ray Center)

Originally posted at Forbes!

A black hole is a rather complicated cosmic object, and one of the most common misconceptions about them is that they’re some kind of universal vacuum cleaner, constantly sucking material in towards their centers.

A black hole is formed when a star, which is at least 8 times more massive than our own sun, reaches the end of its lifetime and implodes on itself in a supernova. Supernovae release a huge amount of energy, and a large quantity of material is blown away from the star. The very core of the star, however, is undergoing a runaway collapse- the star is no longer producing energy in its core due to fusion (as our star currently is), so there’s no outward pressure to balance the inward crushing force of gravity. Gravity is able to overwhelm the repelling force of electrons between atoms (the same force that keeps you from falling through your chair), and also able to crush neutrons together, which is an even more difficult feat. The force of of gravity is so overwhelmingly dominant that the core of the old star is compressed to a density high enough that the escape velocity is greater than the speed of light.

At this point, the runaway collapse should continue, so that the core of the star collapses into a singularity, which is an object which takes up no space (we call it a point source) and is of infinite density. Infinities and physics currently do not play well together, so our descriptions of how space and time behave immediately surrounding the singularity begin to fail. (The singularity itself is currently most accurately described by a large number of question marks.)

Unfortunately for clarity, “black hole” has become a bit of a catch-all term that can apply to any part of the entire black hole. Astronomers tend to use it to refer to the entire area near and within the event horizon (the point of no return for light), which is fairly distant from the actual singularity.

If we are looking much further away from the singularity of the black hole (in other words, beyond the event horizon), physics is still able to describe the shape of the black hole’s distortion to space-time. At these distances, the black hole’s influence on the surrounding space is creating more of a gentle gravitational slope than a steep cliff. In fact, it has no more extreme an influence on the space surrounding it than a star of the same mass. If something happens to be in the area, it might roll towards the black hole, but if it doesn’t come close enough, it might just swing right on past, leaving the black hole untouched. If the material (gas, star, whatever it might be) wasn’t going to collide with the star, it probably won’t make it into the black hole.

Stars are usually relatively isolated – they may form within a cluster of other stars, but these clusters tend to be relatively loose, with large distances between stars. And since black holes form only at the end of an old star’s lifetime, any dust or gas that might have surrounded the star when it formed would have been blown away by a lifetime’s worth of stellar wind. By the time the black hole forms, there’s not going to be a lot of material hanging around for it to get close enough to trap. So there really isn’t an accessible “bulk” of material for the black hole to draw on!

Pictured above is an artistic impression of the Cygnus X-1 binary star system. It contains one of the best candidates for a black hole. Image credit:  ESA, Hubble

Pictured above is an artistic impression of the Cygnus X-1 binary star system. It contains one of the best candidates for a black hole. Image credit: ESA, Hubble

There is, however, one major exception to this idea; stars which are found in binary systems. These are systems where two stars are in orbit around each other, and typically pretty close in age. If one of the two stars goes supernova first, the other star (which is probably also reaching the end of its life) may become a source of fuel to the black hole as it expands. Our star’s atmosphere will reach out to the orbit of Mars by the time it reaches full red giant phase; a more massive star would extend even further. If the ballooning star reaches out far enough, its outer edges might begin to fall towards the black hole instead of staying attached to the star itself. Even then, though, black holes are very, very bad at getting material that’s handed to them this way to get all the way down past the event horizon, which would be the moment the black hole can actually grow in mass.

So, not only are the black holes formed from a single star generally lacking access to a “cosmic bulk” of material, but even their gathering of the closest material is so inefficient that it’s very hard for them to grow quickly.