h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

How do we know that the Milky Way is a spiral galaxy?

We can see it. It's obvious enough that it is a flat disc, because the Milky Way forms a ring around the Earth. Either the Earth is in a special position at the centre of a ring, or we're in a disc.

All other disc-shaped galaxies are either a plain spiral or a barred spiral.

More detailed examination can show the distances of the stars, which has led the scientists to tentatively conclude that our galaxy is a barred spiral.

Here's a picture to prove it
http://antwrp.gsfc.nasa.gov/apod/ap050825.html

Note the link to HGTTG

But that's only an artist's impression.

We don't have the advantage they had in Star Wars (Empire Strikes Back) of being able to fly out to outside of the galaxy and look down on it and see it turning very quickly ...

More's the pity...

Do we know how many spiral arms our galaxy has?

This page -- http://seds.lpl.arizona.edu/messier/more/mw.html -- says they can work it out from radio surveys of hydrogen clouds...

This one -- http://cassfos02.ucsd.edu/public/tutorial/MW.html -- has a map/diagram which seems to suggest that they can observe arm structures from local star positions...

I don't think they can map enough stars accurately enough to be able to extend this sort of idea out to the whole galaxy (which is presumably why they're relying on the hydrogen distributions).

Wanna go for a ride? (It took me a while to find this again)

http://www.ifa.hawaii.edu/~tully/outreach/movie.html

... the link is to W*k*...

Tut!

Thanks for locating the flight to the Virgo cluster, was looking for that some while ago...

Where is the link to w*k*? I don't promote them

It's link you mentioned earlier.

Two spiral galaxies go into a pub.

First spiral galaxy says to the landlord, "Pint of bitter for me, pint of lager for my mate."

"I'll serve you," says the landlord, "but he's got to leave."

"Why?" says the first spiral galaxy.




"He's BARRED."

















Ay theng yo.

SoRB