# So, just how big is the galaxy, anyway? **URL:** \ **Category:** Factual Questions\ **Created:** [September 11, 2001, 8:17am UTC](https://boards.straightdope.com/t/so-just-how-big-is-the-galaxy-anyway/80844 "2001-09-11T08:17:40Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![SPOOFE](https://avatars.discourse-cdn.com/v4/letter/s/c5a1d2/32.png) [@SPOOFE](https://boards.straightdope.com/u/SPOOFE)\ **Post date:** [September 11, 2001, 8:17am UTC](https://boards.straightdope.com/t/so-just-how-big-is-the-galaxy-anyway/80844/1 "2001-09-11T08:17:40Z")
Since childhood, I’ve always heard that the Milky Way is 100,000 lightyears across. That’s an impressive number, of course (and I’m not easy to impress). However, during a Google image search for depictions of the Milky Way (which I intend to use to map out a galactic civilization for a space opera story I am writing, if you must know), I came across a few sites: [This one](http://www.uwinnipeg.ca/~vincent/Cosmology/long_address.htm) which indicates that the Milky Way is 150,000 lightyears in diameter (scroll about 2/5ths of the way down, to the large box marked “The Milky Way Galaxy”). Then I also came across a [SETI site](http://www.seti.org/game/c20.html) which lists the diameter as 120,000 lightyears. Now, I understand that the galaxy isn’t perfectly round… but a discrepancy of 20,000 (and 50,000!) lightyears is nothing to sneeze at. Can someone enlighten this poor, dumbfounded SPOOFE? Are they just upper and lower estimates? Or are the larger sizes due to globular clusters and the like which extend beyond the diameter of the galaxy proper? And I have another, only-slightly-related question which I’ve been meaning to ask for a while… aside from time and speed constraints, is there any reason why a starship wouldn’t be able to leave the galaxy? Any theories about radiation or strong gravity pulls that would make it extremely difficult to leave the galactic disk? ---
**Author:** ![Ashtar](https://avatars.discourse-cdn.com/v4/letter/a/9fc29f/32.png) [@Ashtar](https://boards.straightdope.com/u/Ashtar)\ **Post date:** [September 11, 2001, 8:24am UTC](https://boards.straightdope.com/t/so-just-how-big-is-the-galaxy-anyway/80844/2 "2001-09-11T08:24:56Z")
How big is the galaxy? Big. Reaaaaally big. To borrow an analogy from the late Mr. Adams, you might think it’s a long way down the street to the 7-11, but that’s just -peanuts- compared to the galaxy. It’s enormously–phenomenally—gigantically—unbelievably big. Bigger than a breadbox. Bigger than Michael Jackson. Bigger than you. It’s a great big helping of ‘Big’, with a little more ‘Big’ as an encore. It is the “McDonalds” of Bigness. I imagine it might even be almost as big as The Master’s gargantuan cranium. [Come on, Unca Cece knows everything. He must have a really big head.] It’s really really really big. Ya know? ---
**Author:** ![Fern\_Forest](https://avatars.discourse-cdn.com/v4/letter/f/71c47a/32.png) [@Fern\_Forest](https://boards.straightdope.com/u/Fern_Forest)\ **Post date:** [September 11, 2001, 10:03am UTC](https://boards.straightdope.com/t/so-just-how-big-is-the-galaxy-anyway/80844/3 "2001-09-11T10:03:58Z")
Well there’s no real distinct border to the Milky Way, much as there’s no real distinct border to our solar system. So different sources may include different fringe objects. Here’s a good point from the site you gave. > [@](#): > > The halo is a component of the Milky Way that predates its spiral structure. It consists of the oldest stars in the galaxy. Most of these stars are in groups containing on the order of 100,000 stars. These groups are called Globular Clusters. The diameter of the halo is about 100,000 to 150,000 L.Y. The halo is probably more ovalish then round since it is tugged by certain extra galactic structures like our smaller orbiting galaxies like the Large and Small Magallenic Clouds. Probably it would be useful to redefine the diameter of spiral galaxies a little better. Perhaps as the point at which the circle described would include 99% of all matter bound by gravity to said galaxy? ---
**Author:** ![bryanmaguire](https://avatars.discourse-cdn.com/v4/letter/b/d07c76/32.png) [@bryanmaguire](https://boards.straightdope.com/u/bryanmaguire)\ **Post date:** [September 11, 2001, 10:05am UTC](https://boards.straightdope.com/t/so-just-how-big-is-the-galaxy-anyway/80844/4 "2001-09-11T10:05:37Z")
How come their estimate is so vague? 100,000 to 150,000 LY is a big margin of error. ---
**Author:** ![Fern\_Forest](https://avatars.discourse-cdn.com/v4/letter/f/71c47a/32.png) [@Fern\_Forest](https://boards.straightdope.com/u/Fern_Forest)\ **Post date:** [September 11, 2001, 10:15am UTC](https://boards.straightdope.com/t/so-just-how-big-is-the-galaxy-anyway/80844/5 "2001-09-11T10:15:32Z")
> [@](#): > > \*Originally posted by bryanmaguire \* > \*\* > > How come their estimate is so vague? 100,000 to 150,000 LY is a big margin of error. \*\* Because it depends on the direction they use. It’s not an estimate but rather a complete answer. For instance if you look up into the southern skies you can see two large splotches of stars which are small galaxies called the Large and Small Magellenic Clouds (LMC & SMC). They orbit the Milky Way. Eventually they’ll be absorbed but for the moment they still orbit. Anyway the parts of our galaxy closest to these clouds get pulled toward them stretching out our galaxy. Something most pronounced in the halo. This effect doesn’t not happen in other directions. So the distance from the galactic center to the edge of the halo is further in the direction for the LMC and SMC then they are in the opposite direction. ---
**Author:** ![Popup](https://avatars.discourse-cdn.com/v4/letter/p/94ad74/32.png) [@Popup](https://boards.straightdope.com/u/Popup)\ **Post date:** [September 11, 2001, 11:15am UTC](https://boards.straightdope.com/t/so-just-how-big-is-the-galaxy-anyway/80844/6 "2001-09-11T11:15:59Z")
according to **Monty Python** > [@](#): > > (_found at [http://perso.infonie.fr/edwood/monty\_python\_songs.htm](http://perso.infonie.fr/edwood/monty_python_songs.htm) )_ > THE GALAXY SONG > > \ > Whenever life gets you down, Mrs. Brown, > And things seem hard or tough, > And people are stupid, obnoxious or daft, > \ > And you feel that you’ve had quite eno-o-o-o-o-ough, > > \ > > \ > > Our universe itself keeps on expanding and expanding, > In all of the directions it can whiz; > As fast as it can go, that’s the speed of light, you know, > Twelve million miles a minute and that’s the fastest speed there is. > So remember, when you’re feeling very small and insecure, > How amazingly unlikely is your birth; > And pray that there’s intelligent life somewhere out in space, > 'Cause there’s bugger all down here on Earth! And now, let me explain how sheep bladders can be used to prevent earthquakes [Edited by bibliophage on 09-11-2001 at 07:00 PM] ---
**Author:** ![Phobos](https://avatars.discourse-cdn.com/v4/letter/p/3d9bf3/32.png) [@Phobos](https://boards.straightdope.com/u/Phobos)\ **Post date:** [September 11, 2001, 1:46pm UTC](https://boards.straightdope.com/t/so-just-how-big-is-the-galaxy-anyway/80844/7 "2001-09-11T13:46:34Z")
Although I love Monty Python, let me nitpick that song… [http://itss.raytheon.com/cafe/qadir/q1402.html](http://itss.raytheon.com/cafe/qadir/q1402.html) (this is the SDMB after all). To the OP…good question. I agree with the others here that are saying that part of the problem is that the boundary is non-distinct. I’d also add that it is difficult to take such measurements from the inside of the galaxy. Our view is necessarily obscured, especially in the direction toward the center (since there is more stuff that way). ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [September 11, 2001, 2:08pm UTC](https://boards.straightdope.com/t/so-just-how-big-is-the-galaxy-anyway/80844/8 "2001-09-11T14:08:28Z")
> [@](#): > > but a discrepancy of 20,000 (and 50,000!) lightyears is nothing to sneeze at. As others said, the galaxy doesn’t have a distinct boundary. It’s like trying to define the diameter of a metropolitan area. > [@](#): > > …aside from time and speed constraints, is there any reason why a starship wouldn’t be able to leave the galaxy? None at all. Intergalactic space shouldn’t be any more or less friendly to us than interstellar space. ---
**Author:** ![muttrox](https://avatars.discourse-cdn.com/v4/letter/m/a8b319/32.png) [@muttrox](https://boards.straightdope.com/u/muttrox)\ **Post date:** [September 11, 2001, 7:21pm UTC](https://boards.straightdope.com/t/so-just-how-big-is-the-galaxy-anyway/80844/9 "2001-09-11T19:21:20Z")
I believe intergalactic space has even less matter per cubic whatever than interstellar. If you use a ramjet system, this would be a minus… that much harder to accelerate and decelerate. On the other hand, it’s that much less stuff to have to shield against in one way or another. But generally, I agree with scr4. Space is space.