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peter eldergill
2009-Aug-05, 04:20 AM
I read in my recent copy of Astronomy magazine that Hubble was the first to determine that Andromeda was not merely a Nebula, but a full galaxy on its own.

My question is, Why did it take so long? Why wasn't this realized sooner?

Now I know that technology hos obviously increased, but I figured that by the 30's (or so) the astronomers at least could have got photos to show that Andromeda was a galaxy

Note: I do know that he deduced this using Cepheid variables and I understand the basic mechanics of it, I'm just surprised that it took so long.

Note 2: Can someone explain how an astronomer can resolve a single star out of an entire galaxy?

I LOVE this stuff!

Pete

AndrewJ
2009-Aug-05, 05:16 AM
The idea that our galaxy was one of a myriad and not the whole of the Universe was not established until surprisingly recently. Shapley and Curtis were debating this in the early twentieth-century. One might argue that acceptance of General Relativity heralded a fresh look at the cosmos.

WayneFrancis
2009-Aug-05, 05:25 AM
Visit Berkeley's web site. It has been a while since I watched Alex's course but I'm sure he goes into how we do the measurements.
http://webcast.berkeley.edu/course_details.php?seriesid=1906978237

But to your point, nebulas do exist and there was no big reason to think that other galaxies where not just interesting shaped nebulas. He measured the Cepheid Variable in the early 20s. The problem about photographing individual stars isn't what most people think. Just like all of the stars in our night sky are points of light the same goes for stars in other galaxies. IE a star in Andromeda is a point of light much like a star in our own galaxy is. The big difference is how bright they are. They still resolve to a point like object.

Since a cephied's luminosity can be calculated based on their period you can then work out how far away they must be based on the inverse square law based on its observed brightness.

As for your 2nd question. It is all dependent on the angular resolution of your telescope. For a telescope like Hubble that is outside our atmosphere there is no problem with photons altering its path based on atmospheric effects. The rest of it is just optics of getting the photons to land on the receiver. CCDs where invented for astronomy. With telescopes they resolve individual stars by collecting lots of photons over long periods of time. For near by galaxies the longer exposures allow you to start seeing individual stars because the contrast of the image increases the longer you capture the photons.

Tim Thompson
2009-Aug-05, 06:53 AM
My question is, Why did it take so long? Why wasn't this realized sooner?
Plenty of people thought that the spiral nebulae were actually "island universes" made of stars, starting with Immanuel Kant (http://en.wikipedia.org/wiki/Immanuel_Kant) in the 1700's. The "great debate (http://en.wikipedia.org/wiki/The_great_debate)" between Harlow Shapley (http://en.wikipedia.org/wiki/Harlow_Shapley) & Heber Curtis (http://www.astro.virginia.edu/research/observatories/26inch/history/curtis.php) in 1919 & 1920, which we think of as famous in hindsight, but was somewhat boring at the time, is an example of competing theories of the time (Shapley, 1919 (http://adsabs.harvard.edu/abs/1919JRASC..13..438S), Curtis, 1920 (http://adsabs.harvard.edu/abs/1920JRASC..14..317C), The Shapley - Curtis debate, 1920 (http://antwrp.gsfc.nasa.gov/diamond_jubilee/debate20.html); Shapley thought that spiral nebulae were not extragalactic stellar systems, Curtis thought they were).

But until the 100-inch Hooker Telescope (http://www.mtwilson.edu/vir/100in.php) came to life in 1917, there was no telescope in the world with high enough resolution to resolve individual stars as far away as M31. Hubble spotted his first extragalactic cepheid in M31 in 1923, but published NGC6822 (Hubble, 1925 (http://adsabs.harvard.edu/abs/1925ApJ....62..409H)) & M33 (Hubble, 1926 (http://adsabs.harvard.edu/abs/1926ApJ....63..236H)) before publishing M31 (Hubble, 1929 (http://adsabs.harvard.edu/abs/1929ApJ....69..103H)).

That's why it took so long. Until the 100-inch was available it was not possible to see individual stars in the spiral nebulae and so not possible to confidently determine whether or not they were extragalactic stellar systems. This is just one of the many firsts which make Mt. Wilson Observatory (http://www.mtwilson.edu/) the single most significant observatory in the history of astronomy, save perhaps Galileo (http://en.wikipedia.org/wiki/Galileo_Galilei)'s (although he was not the first astronomer with a telescope; Thomas Harriot (http://en.wikipedia.org/wiki/Thomas_Harriot)'s observations pre-date Galileo's), in my opinion.

astromark
2009-Aug-05, 09:45 AM
For the understanding you expect you would need more resolution than was available. We have at a local observatory a beautiful old refractor type Cook telescope of just 9.5 inches.. Its iron and brass and still works very well indeed. It was first built and installed in 1860. As its motor driven and is polar axis mounted.. its a fine old machine. I use this old tool regularly for its fine movement is excellent for searching for those hard to find objects. My point is.. When we look at a galaxy we see a faint smudge not your stacked long exposure image. I have never seen Andromeda. It never gets high enough for us to see. Those galactic smudges do not look any different than a much nearer to us Nebular. It was not until the finer resolution of the big reflectors did come the understanding of the sea of Galaxies we are now aware of.

trinitree88
2009-Aug-05, 01:27 PM
For the understanding you expect you would need more resolution than was available. We have at a local observatory a beautiful old refractor type Cook telescope of just 9.5 inches.. Its iron and brass and still works very well indeed. It was first built and installed in 1860. As its motor driven and is polar axis mounted.. its a fine old machine. I use this old tool regularly for its fine movement is excellent for searching for those hard to find objects. My point is.. When we look at a galaxy we see a faint smudge not your stacked long exposure image. I have never seen Andromeda. It never gets high enough for us to see. Those galactic smudges do not look any different than a much nearer to us Nebular. It was not until the finer resolution of the big reflectors did come the understanding of the sea of Galaxies we are now aware of.

astromark. Nice post Mark, must transport you back 150 years when you do your observing. pete

alexwagner
2009-Sep-02, 04:39 PM
I think the big problem in realizing that Andromeda is a separate galaxy starts with the difficulty recognizing the nature of the galaxy we're in. It's a problem of, as the saying goes, not being able to see the forest for the trees. Beyond artists' impressions, we don't even know what our own galaxy looks like. If we couldn't see other galaxies, we'd still be fumbling around trying to figure out what ours might look like from the outside.