The Milky Way

[BertiWooster]

How do we know the following :

The shape of the MW

The size of the MW

( By the way I didn't know that there are galaxies orbiting the MW - you learn something new every day - courtesy of the Discovery Channel )

[jake-the-peg]

Those are very good questions and they're quite hard to answer partially because they are put together from a number of pieces of information and indeed the actual answers as to exact shape and size are not agreed on

Firstly what we first called the milky way are stars in our galaxy but very far away in other arms. We know how far away they are because of what are called standard candles. These are stars who's beightness can be determined from their spectra. If you know how bright a star should be from it's spectra and you know how bright it appears to be to us you can work out it's distance.

You can also use the spectra to work out how fast it's moving towards or away from us

Mapping stars by their distance and speed you can start to work out what sort of shape the Galaxy must be.

I hate to refer you to Wikipedia but the article really is quite good

http://en.wikipedia.org/wiki/Milky_Way

As for other galaxies orbiting us - not quite sure that's the best way of explaining the situation.

There are a number of Galaxies in what is called the local Group. The Magallenic clouds and the quite beautiful Andromeda Galaxy for example. All of these are moving through space influenced by each others' gravities - so they're not really orbiting us so much.

Then there are Globular clusters which are clumps of stars orbitting galaxies. These are confusingly called population II stars confusing because they are first generation stars they're normally very old - mostly made of the Hydrogen and Helium from the big bang - whereas most in the galactic disk have many metals in them which was formed in earlier stars.

Way Way to much to say for an answerbank question

Of course it's difficult because we cant see a lot of it

[R1Geezer]

further to jake's answer above, did you know that we are on a collision course with Andromeda? It's one of the few galaxies we can see that are blue shifted, ie heading towards us. In about 3 Billion years, we hit!

[starone]

So will Andromeda get bigger and bigger as it gets nearer geezer? And any idea what will happen to the sun and its planets as it comes closer? Not that I care, mind you, since I and perhaps the whole human race will be gone by then, but it is interesting.

[wildwood]

So is it worthwhile renewing my subscription to Nat.Geographic or not?

These vast distances and times never fail to amaze me. How little we are in the total Universe! Do we know that the Andromeda galaxy has not imploded in itself? Is it not so that what we see now of it is as it was 2.5 million years ago?

[jake-the-peg]

We don't know that the Andromeda Galaxy hasn't suddenly vanished any more than we don't know the Earth won't suddenly vanish tomorrow. Both would violate the known laws of physics.

That's unfair actually because studying galaxies has caused us to revise our notions of physics in the past.

Measuring the speed that stars go around galaxies showed us that at the heart of all galaxies seems to be something big and dense - *really* big - millions of solar masses dense!

These are believed to be supermassive black holes - we don't know that much about them as in how they come to be formes but they are some of the most awesome (and I don't use that word lightly) things we know of.

Also measuring Galaxies gave us the first inklings of dark matter and later dark energy.

We can see many currently colliding galaxies such as these:

http://www.theregiste...4/colliding_galaxies/

And here you can see a computer simulation of colliding galaxies (of course the exact nature depends on a lot of conditions like the angle and degree of overlap etc)

http://www.redorbit.c...laxies_collide/32809/

[R1Geezer]

yes star buck andromeda will get larger but for us imperceptably so. The actual collision is the subject of a lot of speculation, if's and but's but there is a lot of space between stars so plenty of room as it where for a gentle merge but it will probably be a series of swipes before both galaxies settle into a massive globular cluster. few actual collisions will occur but stars and planets will get rip out of their current paths by the gravity. The two centres have, we believe, super massive black holes which would eaither merge or create a binary. Google for galaxy collision simulations.