Quasars are the most powerful objects in the universe, shining with the power of a trillion stars, blasting out huge jets of matter, completely reshaping the cosmos around them. Everywhere we look, we find mysterious loud radio-waves coming from spots all over the sky. These quasi-stellar radio sources or quasars are incredibly bright and violent, and are thought to be the active cores of galaxies billions of lightyears away. To appear so bright and loud, given these vast distances, they must be thousands of times brighter than the entire Milky Way. It is believed that quasars use the same fuel as stars to shine, but are the most efficient engines for converting matter into energy in the universe. This is because the energy released by matter falling into a black hole can be 60 times greater than that released by nuclear fusion in the core of a star. Matter falling into a black hole speeds up to almost the speed of light before it crosses the event horizon, buzzing with an incredible amount of kinetic energy. Of course, once inside the black hole, it takes that energy with it. You only get to witness this energy if you drop your matter in the right way. Fall straight down and the outside universe gets nothing. But when you have a lot of matter, it spirals in incredibly fast towards the event horizon forming a disk. Collisions between particles and friction heat it up to hundreds of thousands of degrees. In a space not much bigger than our solar system, the core of a galaxy can release many times more energy than all its stars combined. This is what a quasar is, a super massive black hole having a feast. And these black holes eat a lot. Typical quasars consume one to a hundred Earth masses of gas per minute!
Ten billion years ago, the universe was about a third of its current size, so the intergalactic medium was much less spread out, meaning the filaments of gas around quasars could feed them a banquet, making them vomit insane amounts of light and radiation. The brightest quasars power jets, tangling the magnetic field of the matter around them into a narrow cone. Like a particle accelerator they launch enormous beams of matter out, plowing through the circumgalactic medium, forming plumes of matter that grow to hundreds of thousands of lightyears in size. It’s almost unfathomable in scale. A tiny spot in a galaxy carving out patches of the universe 100,000s of light years long.
But quasars can’t eat for long, maybe a few million years, because their feast ultimately kills their galaxy. How Quasars Kill Galaxies Okay, maybe “killing” is a bit of an exaggeration. A galaxy is still there after its quasar turns off. But it will never be the same again. Quasars, being among the hottest and brightest things in the universe, break their galaxies by heating them up too much and stopping star formation. Hot gas cannot form stars. This sounds odd, because Stars are gas that collapsed in on itself and then got really hot. But in a cloud of gas that is already hot, atoms are moving quickly. When they collide, they hit hard, exerting pressure that resists gravity’s squeeze – so hot gas can’t form stars. Instead, the best gas for forming stars is already cold, and won’t put up a fight when it’s time to collapse into a star. On top of that, quasars push gas out of their galaxies. Not only does this starve the quasar, but its galaxy loses the raw materials for new stars.
As sad as this sounds, it might be a good thing for life. The alternative can be far more dangerous: too many stars. New stars forming is usually followed by massive stars exploding in supernovae, so planets would be burned sterile. But of course it’s more complicated. Like the intricacies of our own planet’s biosphere, every piece of the galaxy is dependent on and influencing every other part of the galactic environment. While hot things, like quasars and supernovae, tend to push gas out of the galaxy, shockwaves and quasar jets can also compress gas, making new stars at least for a short time. And gas that leaves will mix with gas coming back in and recycle it back into the galaxy. But in general we can say that without things becoming a bit more chill, we would not exist today.
Which brings us to our final question: Did the Milky Way Have a Quasar in the Past? It’s unclear if every galaxy went through a quasar phase, but understanding distant quasars may provide clues to the history of the Milky Way. Galaxies don’t do a good job of preserving their history. Like sand on a beach the endless churning mixes away the clues to their past. It’s possible the Milky Way was once a quasar, which may have allowed our supermassive black hole Sagittarius A star to have grown to 4 million times the mass of the sun. But sadly we don’t know its ancient history. Although Sagittarius A star is dormant now, it could potentially become a quasar in the future. In a few billion years, the Milky Way and Andromeda galaxies will merge, and we have already seen over a hundred ‘double quasars’ in galaxies colliding, providing fresh gas for the central black holes. However, the merging of the super massive black holes will cause them to sink into the center of the new galaxy, resulting in dust and stars being kicked up in every direction. We can’t be certain of this happening, but it would be an incredible sight to behold.
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