Exploring Black Holes: Part 2
The Care and Feeding of Black Holes
How intrinsically invisible objects become the brightest things in the universe
Part 2 of a four-part series on black holes. Part 1 can be found here.
In the late 1950s, astronomers began spotting a number of bright sources of radio waves and visible light. These sources were pinpoints resembling blue stars, but further investigation showed they had to be something very different. For one thing, these quasi-stellar objects, as they were known then, were extraordinarily distant, much farther than any single star would be visible.
The spectra of these new quasi-stellar objects, or quasars, as physicist Hong-Yee Chiu abbreviated their name in 1964, showed they were emitting light through a completely different mechanism than starlight. The quantity of light quasars emitted to be visible across the universe meant they had to be driven by gravity.
Based on the data, astronomers concluded that each quasar was powered by a black hole millions or billions of times the mass of our sun. These supermassive black holes pull huge amounts of matter onto themselves, accelerating it until it glows very brightly. Additionally, the black hole jets a lot of matter away from itself rather than eating it, and those jets also glow intensely. These processes turn the ordinarily invisible black hole into something bright enough to see from billions of light-years away, outshining whole galaxies.
In fact, researchers discovered that nearly every large galaxy harbors a supermassive black hole at its heart. Quasars and other superbright black holes are known as active galactic nuclei, but not every supermassive black hole is gorging itself. Our own Milky Way, for instance, has a quiet black hole. (I’ll talk more about it in Part 3 of this series.)
But quiet today doesn’t mean it always has been. Comparing the supermassive black holes around today with those in earlier times, astronomers know they were more active in the past. Most enigmatically, supermassive black holes have been around almost as long as the universe itself, challenging researchers to figure out exactly how they formed and when.
As we learned in Part 1 of this series, black holes are defined by their event horizon, a boundary separating the inside of the black hole from the rest of the cosmos. Once anything, including light, crosses that event horizon, it can never cross back out. Outside the black hole, however, gravity is just a stronger version of its normal self. Stars and other forms of matter can orbit a black hole, just like planets and moons orbit in a solar system. (If we had a black hole for a sun with the same mass, we’d freeze to death, but Earth would keep orbiting without any noticeable changes.)
But black holes are small compared to other objects of the same mass. Four million stars the mass of our sun would occupy a very large volume, but a supermassive black hole 4 million times the mass of the sun is smaller than our solar system. That means when objects get close, they feel a stronger gravitational force than they would feel from another object. A larger force means greater acceleration and more energy.
When gas gets close to a black hole, the gravity pulls it into orbit. Fast rotation flattens the gas into pancake called an accretion disk. As gas particles bump into each other, they heat up and glow brightly. But then the physics gets really exciting: Hot gas becomes plasma as electrons get stripped off the atoms. As the free electrons accelerate, they create magnetic fields.
The sheer number of electrons all moving more or less in the same direction around the black hole collectively generates a very powerful magnetic field. That magnetic field funnels some of the hot plasma out of the accretion disk and away from the black hole. The result is a jet, which can stretch for thousands or millions of light-years, often extending far beyond the border of the galaxy hosting the black hole.
When particles in a jet hit clouds of gas inside the galaxy, the collision can create shockwaves: places where the galaxy’s gas is abruptly compressed. That compression can make new stars, so black holes are indirectly responsible for at least some growth in galaxies. However, gas falling toward the supermassive black hole can also starve a galaxy of the materials it needs to make new stars. We still don’t know exactly where that switch is between black holes making new stars and black holes throttling their creation.
Astronomers have also seen chemicals in black hole jets (through the types of light they absorb and emit) that came from the death of stars near the center of the galaxy. This means black holes are responsible for cycling atoms through the entire galaxy, including atoms like carbon, iron, and other elements that compose planets like Earth. For that reason, some researchers even think black holes in galaxies could be the primary driver for making stars like our sun, meaning the Milky Way’s monster black hole could (very indirectly) be responsible for the existence of life on Earth.
The electrons inside the jets emit light called synchrotron radiation, which was first discovered in early particle accelerators called—wait for it—synchrotrons. This light is most intense along the jet itself, so if Earth happens to be lined up with the jet, we see a very bright point of light.
In 1995, Meg Urry and Paolo Padavani created a unified model for active galactic nuclei, including quasars. They showed that these objects could be explained by the glow of the accretion disk and the angle at which we view the jet. If the black hole’s jet is aimed directly at Earth, it appears as a bright point of light: a quasar or, if the jet is particularly energetic, something called a blazar.
On the other hand, if we see the jet from another angle, we mostly will see either the accretion disk or how the jet’s particles interact with other gases in the galaxy. These observations are variously called Seyfert galaxies, radio galaxies, and other names. The key to the unified model is the way supermassive black holes manipulate matter around them. But not all galaxies are active. The black hole at the center of the Milky Way, for example, doesn’t have a huge accretion disk or jets (to the point where some astronomers once doubted the galaxy even had a supermassive black hole).
It comes down to feeding. Quasars are almost all very distant, which means their light traveled a long time to reach us. If a quasar is 5 billion light-years away, we’re seeing it as it was 5 billion years ago. As a result, we can conclude that quasars were more common in the early universe than they are today. The reason we see fewer of them now is because supermassive black holes have less raw material for their accretion disks and jets.
How did objects millions of times the mass of our sun form so early in the history of the universe? That’s a mystery we can’t yet solve. Small black holes (which I’ll talk more about in the next installments) are born when a star more than 20 times the mass of our sun dies and collapses on itself, but even the most massive stars are at most about 200 times heavier than the sun. Bright quasars may eat the equivalent of a sun or two every year, but that’s not fast enough for them to have grown out of black hole born from a star.
Maybe early in the universe, huge clouds of gas collapsed into black holes instead of forming stars. Or maybe the first stars in the cosmos were extra-massive and unstable and formed black holes that could quickly eat each other until they made supermassive monsters. Both these possibilities and other guesses have their difficulties, but researchers have ideas about how to settle the issue, including studying how often black holes collide with each other.
We’ll see how that works — and how to study black holes up close — in the next installment.