How big is our Galaxy.

Hi Jar, astronomy is all about order of magnitude so if we say 100,000, you can be fairly sure the real answer is somewhere between 10,000 and 1,000,000 At least, that is how it used to be... in my glory days we tended to think what's in a factor of 2?But to be fair, we're pretty good these days. I would suggest perhaps +/- 10,000 lyrs. As with terrestrial dating methods, we have a wide variety of distance measures which are compared, contrasted, and calibrated.[Edited once I had actually thought about this for 5 mins]This message has been edited by cavediver, 01-13-2006 04:21 PM

One of my favourite lessons I liked to teach was on how small the Universe is!For now I will just comment on the size of the Galaxy, the size of Andromeda, and their separation distance.Galaxy ~ 90,000 lyrsAndromeda ~ 110,000 lyrsSeparation = 2,200,000 lyrsThink about it... Andromeda is only 20 times further away, than it is across!!! Name some other distance structure in the physical hierarchy from planck scale to obs universe where there is such extreme proximity of objects.If you could turn down all the lights and turn up Andromeda's brightness, you would realise that it is four times wider than the moon on the night sky. How cool would that look?

Well, we have the Local Group, of which Andromeda and the Galaxy are two of the principal members, amongst a grouping of probably 30 or so galaxies. I think we're about 10,000,000 lyrs across. And we are part of a larger "cluster" and this cluster forms part of the Virgo supercluster.You are now into what we call the large-scale structure of the Universe. It is phenomenally well mapped out. I remember using a database of known galactic objects for a project back in 1991, and I look at similar databases today and they are incomparable... orders of magnitude more objects, with much greater certainty on distances.

I can get back to this in more detail tomorrow, but for now...Andromeda and the Galaxy are pretty much in the centre, so we are 1 MLyr from the that centre, so around 4 MLyr from the edge, assuming a spherical boundary to the Local Group. Distances are probably around 10-20%.Cepheid variables have been mentioned as a distance measure and these are used extensively for close by galaxies, maybe even out to the edge of the Local Group... incredible that we use individual stars in other galaxies!The other methods I will leave till tomorrow as I'm getting nagged by the wife Have a good evening!

That's larger than our universe! Did you mean that? You could shrink the universe considerably before there would be much of a problem...

Beautiful picture - thanks. Wish I'd thought of it Don't get me wrong, the night sky is one of the most beautiful aspects of creation... but sometimes I wish we could do away with the stars and just see the galactic background in all its glory. I guess a trip to the observation point that makes an equi triangle with Andromeda and the Galaxy would be truly awe-inspiring

No, pretty much spot on. Though the 2 billion lyrs figure is rather inflation dependent. If we could see beyond last scattering right back to pre-inflation, the initial distance would be zero.

Iblis is right... the cosmological horizon is precisely that which represents infinite red shift (recession at c) and the big bang. It's not a coincidence.

Well, what about photons (not that there were any photons) facing away from us That's right, though as I said, that is an inflationary based figure, and is certainly not known with any kind of certainty, especially as we now have lambda screwing up all of the old calculations. No, the cmb came much later... around 270,000 yrs later. It is the image of the surface of last scattering... the point where the universe first became transparent.This message has been edited by cavediver, 01-14-2006 10:46 AM

Hi SG. Don't confuse the obs universe with "now", the comoving hypersurface we ride upon. The 78 billion lyrs is the size of the universe on that surface, though is obviously highly lambda dependent, so we don't really have a clue. Yes, I'm sure you're right. Though of course we could be "small" and multiply connected...

Prior to 270,000 yrs, space was filld with a plasma, with free electrons and protons. This is opaque to photons, as they continually scatter off these particles. At 270,000 yrs, temperatures dropped to the point of recombination, where the electrons and protons hooked up to form neutral hydrogen. Suddenly, the universe became transparent and the photons were finally free. The CMB are the photons that have not interacted since leaving this moment of "last scattering". You are right, the homogeneity of the CMB is a result of the homogeneity of that plasma which in turn was a result of the inflationary period 270,000 yrs earlier.This message has been edited by cavediver, 01-15-2006 12:37 PM

Ahh, you see, this is what changes everything! Yes, you are quite right, Lambda screws all of this up. With Lambda=0, then it is as I have explained. When I last wrote a paper, Lambda did equal zero !!!Let me go and think of FRW with Lambda and I will get back soon...

Only in their own comoving frame, which is not the same as our comoving frame. No, the galaxies have observable time-dilation. If their light is redshifted, there is time dilation. If there is extreme red-shifting there is extreme time dilation. They are one and the same concept. This is exactly the same principle behind gravitational red-shift, in which case you need no relative motion. Go hang off a black hole for a few minutes and give everyone a shock when you get back (assuming you can find anyone alive that you know!)

Another good reason not to overly trust Wiki. I have not seen many better examples of a non-sequitur. To have an idea of what we talk about in terms of radii, you need to sit down with a good space-time diagram of the particluar model you are considering and look at all the different possible "distances" you are considering.

My basic point was that their comment about the (erroneous idea) of the universe expanding no faster than c does not logically follow from their first point.Their first point is also completely confusing. They say that the radius of the observable universe is not 26 Glyrs but go on to discuss something which is not the observable universe (the radius of the universe "now").There are some correct points, some incorrect points, and some confused points. I think the confusion is the dominating feature