What Are Quasars? A Look at One of the Most Mysterious Objects in the Universe
Astronomers are still working behind the scenes to understand the science behind mysterious objects detected in deep space called "quasars," also known as "quasi-stellar radio sources," or "quasi-stellar objects."
They are among the brightest and most captivating objects in the known universe. So bright in fact, some are able to emit ten to one hundred times more energy than the entire Milky Way Galaxy, in an area so compact, it's equivalent in size to our Solar System. Can you even imagine how blindingly bright it would be to see one up close? Probably not.
To understand how quasars are believed to function, we must first delve into how black holes work, as the two are intrinsically linked.
What Are Black Holes?
Scientists have a lot of theories about black holes, and many of them are conflicting, but the current understanding of black holes, which is consistent with Einstein's theory of general relativity, says they are infinitely dense points in space where gravity is so strong, it warps the very fabric of spacetime and creates a so-called "escape horizon," the point where gravity is so strong that nothing — not even light — can escape.
There are micro black holes, which are so tiny that none have been detected yet (and are thus, still theoretical); intermediate black holes, which are between 100 to 1 million solar masses; stellar black holes, formed from giants stars that collapsed in on themselves at the end of their lives (these are thought to sometimes mass 20 times more than the Sun, but the black hole itself is so compact, it could fit in a ball approximately 10 miles across); and supermassive black holes. The last one is the most common type and they are believed to exist at the center of most, if not all, medium-to-large spiral and elliptical galaxies, including our own. Therefore, they are easier to observe and detect, although this may mean that they are only observationally more common.
The black hole at the center of our galaxy, known as Sagittarius A* (Sgr A*), is a very compact radio source in the constellation of Sagittarius, located approximately 25,800 light-years from Earth. That may seem pretty close (and it is in the grand scheme of space), but keep in mind, one light-year is a little over 6 trillion miles (9.5 trillion kilometers). So there's no danger of Earth becoming collateral damage in the black hole's destruction, at least not for a while.
Unfortunately, Sagittarius A* is shrouded by immense clouds of gas and dust, not to mention all of the stars and planets between us and it, or the fact that it isn't static in the sky, making it impossible to get a good direct image of the black hole within. However, we can still see its emissions by using special tools that can detect x-ray and radio wavelengths.
Sagittarius A* is believed to be fairly small as far as black holes go. It's estimated to weigh approximately 4 million solar masses (or the mass of four million suns combined), all within a radius of 120 astronomical units (AUs). One AU is the distance between the Sun and Earth. The largest black hole yet discovered, on the other hand, known as TON 618, masses approximately 66 billion times more than the Sun, and it just keeps growing. It's estimated to eat one Sun's worth of material every day — it also happens to be a very powerful quasar.
How is that possible? Well, not all black holes are quasars, but all quasars are also accompanied by black holes.
What Are Quasars Exactly?
Quasars come alive when supermassive black holes start consuming matter at an incredible rate, so fast, in fact, that the black hole within can't consume everything in its periphery. Therefore, the material loops around the black hole and creates something called an accretion disk. It begins to heat up dramatically as it spins around the black hole — releasing bright, visible light and emissions that can only be seen in gamma, radio, and x-ray wavelengths by special equipment on our telescopes. Light at visible wavelengths cannot escape from black holes themselves, at least according to the models predicted by Albert Einstein, so remember that the light isn't coming from the black hole itself, but from immense gravitational friction between the materials within the accretion disk (such as gas and dust) that surrounds the black hole are producing the light.
Quasars are among the oldest, brightest, and most distant objects in space. They can outshine the galaxies they exist in, or even burn brighter than one trillion stars combined (roughly one hundred times brighter than their home galaxies). The material in the disks spin at speeds slightly below the speed of light, often moving in jets that stream from the north and south poles of supermassive black holes in ancient galaxies. The friction produces radio waves, detectable in "radio lobes" in the jets. When they have consumed the material around them, they dim and merely become supermassive black holes.
Take TON 618 — the quasar we were talking about earlier. It's located approximately 10.5 million light-years from Earth. Since looking back into the furthest reaches of space is like looking back in time, this quasar was formed when the universe was merely 2.5 billion years old. Not only is it one of the most distant quasars, but it's also one of the most luminous objects in the sky — shining 140 trillion times brighter than the Sun.
How Are They Detected?
Quasars are difficult to detect because they're so far away; there are so many obstacles between Earth and them. They're so bright that they sometimes overwhelmingly outshine their galaxies; and sometimes, they appear to simply be really bright stars. As mentioned, material gathers around the event horizon — a one-way ticket into the belly of a black hole — and spirals inward. As it gradually moves toward the black hole, the orbital energy of gas transforms into heat, until the temperatures are so high, the gas begins to glow.
As such, once the material is sucked in, an immense amount of radiation is ejected from the once orbiting clouds of gas and dust. This material is spat out and follows along the quasar's magnetic field, throughout its poles; it can be seen at multiple wavelengths — particularly through optical and radio wavelengths.
To reliably say, "Hey, this is a quasar!" astronomers look at a potential candidate using different types of telescopes that see visible light, and satellites that can picture objects at X-ray, radio, ultraviolet, and infrared wavelengths. Since the luminosity varies based on how much material is in the accretion disk, astronomers must make precise measurements and compare them to earlier data.
Besides short-lived phenomena like gamma-ray bursts and supernovae, quasars are the most energetic and brightest objects in the universe. There's a lot still left unknown about these objects, but science is to the rescue!