“Black holes, which have no memory, are said to contain the earliest memories of the universe, and the most recent, too, while at the same time obliterating all memory by obliterating all its embodiments. Such paradoxes characterize these strange galactic monsters, for whom creation is destruction, death life, chaos order.” –Robert Coover
Our Milky Way, the swath of light and dark that dominates the darkest skies here on Earth, contains a huge variety of stars: large and small, red and blue, from young to old to ancient.
But the very center of our galaxy contains, deep within its heart, one object that’s unlike any other. Completely dark in the visible part of the light spectrum, due to the neutral gas and dust that prevent the light from getting through, we can successfully peer through to the center of the galaxy only in very specific wavelengths, and only from space.
When we do so, one region stands out from all the others, no matter how we look.
That bright region, outshining all the others in each wavelength? That’s the innermost, supremely central region of the Milky Way. If we take our highest-resolution telescope, in the wavelengths most transparent to light, what do we find in this central region?
In short: a mess. Thousands upon thousands of stars crowded into a region of space that — if it were of our local neighborhood instead — would contain only our Sun.
Yet at one very small, special location, known as Sagittarius A*, an incredibly bright radio source lives among the young, hot, but otherwise relatively normal stars.
What’s going on in that region marked by those two yellow arrows? The stars there are orbiting something with incredible speeds, something that, according to the known laws of gravity, is millions of times the mass of the Sun.
Oh, and also, it emits no light. That, my friends, is how we know we have a supermassive black hole at the center of our galaxy. It turns out that practically every galaxy has one, and the larger the galactic bulge, the larger the black hole at the center.
Most supermassive black holes are dormant — or not active — most of the time, and thus, like our own, are very difficult to detect. But looking deep into the Universe with the Chandra X-ray telescope, we were able to see a large number of active supermassive black holes.
When we extrapolate what we see here to the entire Universe, we find that about 300 million supermassive black holes are active and pointed at us at any given time.
That correlation of bulge-size to black hole mass is a good one, and there are only three known exceptions: one is when a merger kicks a black hole out of a galaxy, one is when a galaxy has some of its mass stripped away, as in the case of the galaxies below…
and perhaps most bizarrely, one is in the very early stages of the Universe, when the earliest supermassive black holes are more massive than you’d naïvely expect they would be. I was recently asked about this, and it is a very good question. So why do these supermassive black holes form, and why do they grow so massive so quickly?
After all, when we see supernovae, if they create a black hole at all, they only create a black hole that’s a few times the mass of our Sun.
Maybe the largest supernovae we’ve ever seen created a black hole 20 or 30 times as massive as our Sun, but that hardly explains how we get up to millions or (in the largest cases) billions of solar-mass black holes.
To understand how this happens, I need to you imagine all the way back to the early stages of the Universe: before our Solar System existed, before large clusters of galaxies merged and formed, before generations of stars lived and died, before even the first clouds of gas and dust collapsed to form stars. Imagine back to when the Universe was relatively uniform, dark, and only beginning to have the most overdense regions begin to contract down, where they will eventually form the very first stars in the Universe.
Eventually, the hydrogen and helium gases in the densest locations achieve high enough densities to ignite the first nuclear fusion reactions, causing stellar burning and star formation for the very first time. This is a Universe containing maybe 1023 solar masses worth of hydrogen and helium, and the very first locations that are lucky to form stars will do it in chunks that range from many millions of solar masses down to may be as small as a few hundred thousand times the mass of our Sun.
But it is these most massive stars that are most interesting: the hottest, brightest, bluest and also shortest lived, these are the stars that will form heavy elements in their core (allowing for the eventual formation of planets), go supernova (enriching the Universe), and — in certain cases — collapse to form black holes.
But it is the most extreme stars — the ones that are over 130 times the mass of our Sun — that are the most interesting as far as forming a supermassive black hole goes. Sure, less massive stars can form smaller black holes, but once you get above 130 masses, the interior of your star becomes so hot and energetic that the highest-energy radiation particles you create can form matter-antimatter pairs, which create an instability in the star that wind up blowing the entire thing into smithereens!
Of course, this doesn’t help you create a supermassive black hole at all, does it?
The thing is, this is only true for stars with masses above 130 solar masses and below 250 solar masses. If we get even more massive than that, we begin to create gamma rays that are so energetic that they cause photodisintegration, where these gamma rays cool down the interior of the star by blowing the heavy nuclei back apart into light (helium and hydrogen) elements.
In a star with more than 250 Solar Masses, it simply collapses entirely into a black hole. A 260 solar mass star would create a 260 solar mass black hole, a 1000 solar mass star would make a 1000 solar mass black hole, etc.
The question, of course, is do we actually make these, and do we make them in abundance that it’s reasonable they would grow to form early supermassive black holes? To answer this, we turn to the largest star-forming region in our wimpy local group: the Tarantula Nebula located in the Large Magellanic Cloud.
This region of space is nearly 1000 light years across, with the massive star-forming region in the center — R136 — containing about 450,000 solar masses worth of new stars. This entire complex is active, forming new, massive stars.
It should be no surprise that our most recent, close supernova — SN 1987a — originated on the outskirts of the Tarantula nebula.
But it’s not the outskirts, but the central, R136 region that is most spectacular. Let’s dive right in to the hot, bright blue stars.
You may notice, right away, that there’s one bright star that isn’t blue at all, but rather red. This is an important star that may, in fact, become our night sky’s next supernova.
But this red giant, even if it forms a black hole, won’t form a very massive one. Remember, we’re on the lookout for stars with masses over 250 times the mass of our Sun.
The amazing thing is this: there is one in here!
The most massive star known to humanity, R136a1, weighs in at 265 solar masses, and if its core gives out right now, it will collapse immediately to a black hole of 265 solar masses! This star, in particular, may not do that because of the heavy elements present, but in the early Universe, there are no heavy elements present, and so all stars above this mass threshold will simply get converted into black holes!
Because of how galaxies are thought to form in the early Universe — by the rapid merger and accretion of collapsed, star-forming regions — it’s unthinkable that these early, large black holes wouldn’t merge with one another and grow, forming increasingly larger and larger black holes at the centers of these objects: the Universe’s first large galaxies.
If we can get just one 250 solar mass black hole for every 500,000 solar masses worth of stars, that means by time we get up to a Milky Way-sized galaxy, with maybe 2-400 billion solar masses worth of stars, we’d expect a central supermassive black hole of 100-200 million solar masses! This is exactly the type of growth we need to create the largest of the supermassive black holes we observe today, and we’ve got the evidence that the right type of stars form right in our own backyard.
So that’s where the earliest supermassive black holes come from, and I hope you enjoyed your journey through time, space, stars and the elements: they brought you the Universe you get to have today!