What is a Quasar?

August 22nd, 2008

When I travel and meet a lot of new people, they often ask what I do. When I tell them that I’m an astronomer, and they don’t immediately mumble something about liking astrology or make some other outrageous statement as sometimes happens, they’ll often follow up and ask what, specifically, is my specialty.

“Quasars,” I usually answer, and usually fail to see the light of understanding ignite in their eyes.

Sometimes I wait for them to ask, and sometimes I just launch into answering the question: What is a Quasar?

Scientists, like novelists and would-be producers, need an elevator pitch to quickly describe their thing. Sometimes you need two or three depending on who you’re talking to and how much time you have. I’ll give you my two-part answer on what’s a quasar, assuming a little more background than I would normally.

Observationally

Quasars were identified as special objects in the sky years before we had an inkling about what they actually were. The word “quasar” is short for “quasi-stellar radio source” and the first quasars were the faint blue stars — or quasi-stellar objects anyway (AKA QSOs) — associated with astronomical radio emission. They showed emission lines in their spectra, and the redshifts determined from their spectra, in conjunction with Hubble’s Law that shows the expansion of the universe, indicated that they were at great distances. They’re faint compared to visible stars in the sky, but given how far away they are, they are actually incredibly intrinsically luminous and capable of outshining entire galaxies. In fact, when we look very carefully with our sharpest telescopes like Hubble, we do in fact see that they are the active nuclei of distant galaxies, shining as brightly as a trillion suns or more.

Theoretically

This is what I usually tell people when I’m short on time and they’re unlikely to be interested in the observational side of things, and the “how we know it” sort of details can be discussed later if I’m wrong.

Quasars are supermassive blacks holes in the centers of distant galaxies tearing apart and consuming the equivalent of about a sun a year. The gas in the vicinity of the black hole is accelerated to very high speeds by the intense gravity, and this fat moving gas heats up as it spirals in. What is shining so brightly as a quasar is not the black hole itself, but the disk of hot gas around it. That disk, which is something about the size of our solar system, outshines the entire surrounding galaxy of hundreds of billions of stars.

My job is cool.

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