Q is for Quasar

In the middle of the 20th Century, astronomers were astounded to find a very bright radio source--stunningly bright. In fact, they were brighter than anything else previously known  (or since, really).  They called them Quasi-stellar Objects (QSOs).

But what were they? Astronomers captured their spectra and had a look. The spectra looked really odd, until they figured out that they were extremely redshifted. Redshift is a nifty astronomy tool. Here's how it works:

Original spectrum    vs   a redshifted spectrum

See the absorption lines in the rainbow spectrum on the left? This is a normal spectrum where some of the light has been absorbed by an element (say, hydrogen and its Balmer Lines).  When the light that makes this spectrum comes from very far away, the expansion of the universe stretches it out and makes the frequency drop. The result is the spectrum on the right, where the absorption lines move in a red-ward direction. This is redshift. Think Doppler Effect.

The greater the redshift, the farther away an object is. The spectrum sampled from quasars were so redshifted it took us a while to figure out just how redshifted they were.

In other words, these really bright radio sources were really, really far away. That was doubly-amazing because of the Inverse Square Law--intensity reduces with distance.

"Dude!" we cried.  What could possibly create so much energy to be so bright from so far away?

Eventually, we figured out quasars were really, really active distant galaxies and that the extreme energy output came from the accretion disks of the  supermassive black holes in the middle of those galaxies. Those suckers can really put on a shine when actively consuming some poor star.

(Is our own Milky Way a quasar? Currently no, as our Sagittarius A* is a rather quiet supermassive black hole at the moment.) Quasars are believed to be an early universe phenomenon because we observe them as coming from waaay back in time. But that doesn't mean that Sgr A* couldn't become a quasar in the future, given enough to eat.

So we figured out that quasars are:

• really old
• really distant
• really bright

Once we established that, we realised they could be used as a kind of anchor point in mapping of the skies. See, the farther away something is, the less likely it is to apparently move.

Want to know more about quasars?

• Easy tricycle level (with training wheels)
• Mountain bike level (for the average science-curious layman) 
• Hardcore Harley Davidson level:
• First papers on quasars
• Subsequent papers on quasars
• More modern explorations on quasars 

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Her Grace prefers to study objects that are a little bit closer than z=7.085.