Thursday, 20 April 2017

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?

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


Rebecca M. Douglass said...

I like your designation of the levels of papers on quasars :) I've been reading science stuff (for lay people) for the last 20+ years, but this stuff still bends my brain. I'll take the training wheels.

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Sandra Williamson said...

A very succinct explanation. Great info.
Dropping by from the A to Z Challenge


Sandra, Aspiring family historian, fellow participant in the #AtoZchallenge

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FinnBadger said...

I'm loving the science - I know some science, but I had no idea what Quasars were - thanks for the info.

Phillip | Q is for Quidditch

Anonymous said...

“Quasi-stellar Objects” just sounds cool, but that aside your post is really fascinating. Thanks for sharing.

Thanks for visiting my post: Q for Quiet.

Sara C. Snider said...

Fascinating. I was recently wondering about quasars, so I'm glad to have read this. :)

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sage said...

I could have written about quasars, but instead added "Queen" to Cassiopeia (there were just to many C constellations to write about, so I had to share the wealth with other letters). Besides, you understand this a lot better than me. An interesting post.