
The Wonder of Quasars - Thevet
http://www.intelligentlifemagazine.com/the-music-of-science/the-wonder-of-quasars
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antognini
An interesting historical byline was the debate over the distance of quasars
in the 1960s. What made quasars so unusual was that their spectrum resembled
absolutely nothing that had been seen before. A typical quasar spectrum has a
broad continuum with a number of emission lines on top of it. This should have
made it fairly easy to identify the composition of quasars since the emission
lines tell you what elements are nearby, but the observed emission lines
matched up with no known elements. Maarten Schmidt realized that the most
prominent emission lines could be those of hydrogen if the recessional
velocity was about 15% of the speed of light.

This sparked a big debate in astronomy because whether you believed Schmidt or
not, something crazy had to be going on. If Schmidt was right, these objects
would have to extremely far away --- much further than anything that had ever
been observed before. And if they were that distant and that bright, that
would imply that they would have to be extraordinarily luminous. The timescale
of the variability of the spectrum implied that whatever the source was, it
had to be less than the size of Solar System. Yet if they were really that far
away, something of that volume had to outshine an entire galaxy!

On the other hand, if Schmidt was wrong and the quasars were really local, you
still had to explain just what exactly they were. One plausible explanation
was that the emission lines were generated deep in a gravitational well (i.e.,
there was gravitational redshift rather than cosmological redshift). But no
known objects could stably produce emission at such specific gravitational
redshifts.

Ultimately the cosmological view of quasars prevailed because theorists could
show that accretion of gas onto a supermassive black hole could produce the
required luminosity in the required volume, whereas there was no good
explanation for how you could produce a non-cosmological quasar.

The story of non-cosmological quasars turns out to be an excellent example of
science progressing one funeral at a time. Even when most of the field had
moved on by the late 1970s and 1980s, a small cadre of vocal astronomers
continued to insist on the non-cosmological quasar model even into the 1990s
until they started to die. Halton Arp was the most well known of these (he is
better known for the Arp Atlas of Peculiar Galaxies) and even published a book
in 1987 arguing in favor of the non-cosmological model.

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sidcool
Great response. Makes the articles even more amazing.

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davesque
There's nothing that boggles my mind like astronomy. Nothing delivers the same
mix of terror, awe, and resigned bewilderment.

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ldpg
I agree, that's a great way of putting it.

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revscat
I had no idea that GPS was based upon the locations of quasars. Fascinating.

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noselasd
Here's a bit more info on
that[http://www.nasa.gov/centers/goddard/news/topstory/2009/icrf2...](http://www.nasa.gov/centers/goddard/news/topstory/2009/icrf2.html)

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chopin
I wonder why pulsars where not chosen. Are they too nearby for the purpose?
Shouldn't they be easier to distinguish than quasars?

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sidcool
A fascinating read. Such articles make me enthusiastic like Feynman. Thanks
for sharing.

