How do we know we live in a spiral galaxy?

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Charlie Ebright

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How do we know we live in a spiral galaxy?
« on: 13/02/2009 09:52:08 »
Charlie Ebright asked the Naked Scientists:
Dr. Chris:
My name is Charlie Ebright from Bandon, Oregon, USA.  I am a truck driver and listen to every one of your Naked Scientist podcasts.  They keep me very entertained on these US highways.

My question is this, "How do we know we live in a spiral galaxy and in particular one of the arms,if we can't see from the outside of this galaxy, to see our own shape?"
Thank you,
Charlie E Ebright

What do you think?



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How do we know we live in a spiral galaxy?
« Reply #1 on: 13/02/2009 11:01:43 »
Good point, this has to be worked out 'from the inside, looking out'.

The Milky Way has a thin stellar disc that resembles the spiral galaxies seen beyond our galaxy and the evidence comes from various observations (made within the Galaxy) consistent with being a spiral galaxy.
It's believed that the spiral arms stand out mainly because they contain concentrations of bright objects associated with recent star formation rather than being strong concentrations of mass.
At optical wavelengths, dust makes it difficult to see stars in the disc of the MW if they are more than a few kiloparsecs didtant. Consequently the spiral structure of the Galaxy is mapped by using a combination of optical and radio 21cm observations (since dust doesn't affect the radio stuff) and a range of objects has been observed (spiral-arm tracers) that appear to map out spiral structure.
Our Sun appears to be located in the Orion arm although it's not clear whether this is a spiral arm in its own right or a side-spur belonging to some other arm (Source: Jones & Lambourne, Galaxies and Cosmology. CUP 2004).
If there are any professional astronomers on the forum they will enlarge on this but the message is - it's probably still work in progress.
« Last Edit: 13/02/2009 11:04:45 by blakestyger »