# Modelling the Milky Way **URL:** \ **Category:** Factual Questions\ **Created:** [April 13, 2015, 2:30am UTC](https://boards.straightdope.com/t/modelling-the-milky-way/717541 "2015-04-13T02:30:27Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![blood63](https://avatars.discourse-cdn.com/v4/letter/b/97f17d/32.png) [@blood63](https://boards.straightdope.com/u/blood63)\ **Post date:** [April 13, 2015, 2:30am UTC](https://boards.straightdope.com/t/modelling-the-milky-way/717541/1 "2015-04-13T02:30:27Z")
I did a cool modelling activity with my students. I took them to the gym and told them to imagine that the gym was the Milky Way galaxy. A beach ball would then the area of a our local stars, the stars we can see in the night sky. I was wondering how to represent the amount of matter in the Milky Way. Perhaps collecting all the stars in one place would be the size of a marble or a BB at that scale. I would hate for anyone to do any fancy math but how close would I be? ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [April 13, 2015, 3:54am UTC](https://boards.straightdope.com/t/modelling-the-milky-way/717541/2 "2015-04-13T03:54:22Z")
> [@blood63](#): > > I did a cool modelling activity with my students. I took them to the gym and told them to imagine that the gym was the Milky Way galaxy. A beach ball would then the area of a our local stars, the stars we can see in the night sky. > I was wondering how to represent the amount of matter in the Milky Way. Perhaps collecting all the stars in one place would be the size of a marble or a BB at that scale. I would hate for anyone to do any fancy math but how close would I be? If we say the length of the gym corresponds to the diameter of the Milky Way, then on that scale the mass of the Milky Way is about the mass as a single cell of _[Prochlorococcus](http://en.wikipedia.org/wiki/Prochlorococcus)_ cyanobacteria. If you include the mass of just the stars, then it is the mass of a single virus. ---
**Author:** ![septimus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/septimus/32/410_2.png) [@septimus](https://boards.straightdope.com/u/septimus)\ **Post date:** [April 13, 2015, 4:05am UTC](https://boards.straightdope.com/t/modelling-the-milky-way/717541/3 "2015-04-13T04:05:13Z")
I get the same answer as **Asympotically fat** but will show my work. I’ll guesstimate the Milky Way --\> gymnasium equivalence as 100,000 light-year --\> 50 meters, or 1 light-year --\> half a millimeter. The Milky Way has about 200 billion ( 6000^3) stars, some much bigger than the sun, some much smaller, but let’s assume they all have the sun’s radius, or 2.3 light-seconds. So all the stars clumped together have radius 6000\*2.3 light-seconds, or about .0004 light-years, or 0.2 microns scaled. That’s about the radius of a largish virus. ---
**Author:** ![Quartz](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/quartz/32/267_2.png) [@Quartz](https://boards.straightdope.com/u/Quartz)\ **Post date:** [April 13, 2015, 9:06am UTC](https://boards.straightdope.com/t/modelling-the-milky-way/717541/4 "2015-04-13T09:06:28Z")
According to Google the mass of the Milky Way is between 0.8 and 1.5 trillion solar masses. Call it 1T for simplicity - 10,000^3 - so that’s 23000 light seconds or 0.0007 light-years. ---
**Author:** ![blood63](https://avatars.discourse-cdn.com/v4/letter/b/97f17d/32.png) [@blood63](https://boards.straightdope.com/u/blood63)\ **Post date:** [April 13, 2015, 5:33pm UTC](https://boards.straightdope.com/t/modelling-the-milky-way/717541/5 "2015-04-13T17:33:40Z")
Wow. That is truly amazing. Thanks for your work!! ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [April 13, 2015, 6:10pm UTC](https://boards.straightdope.com/t/modelling-the-milky-way/717541/6 "2015-04-13T18:10:48Z")
I did this for the visible Universe in a thread a yew years ago, and got very similar results - if the universe was the size of a large room, the individual galaxies would be bacteria-sized. ---
**Author:** ![leahcim](https://avatars.discourse-cdn.com/v4/letter/l/b4bc9f/32.png) [@leahcim](https://boards.straightdope.com/u/leahcim)\ **Post date:** [April 13, 2015, 6:45pm UTC](https://boards.straightdope.com/t/modelling-the-milky-way/717541/7 "2015-04-13T18:45:05Z")
> [@septimus](#): > > …but let’s assume they all have the sun’s radius, or 2.3 light-seconds… It is not quite that simple to estimate. XKCD what-if did an [exposé](https://what-if.xkcd.com/83/) on the distribution of star sizes. Summary, while most stars are around sun-sized or smaller, stars many orders of magnitude larger exist and while they are relatively uncommon, they are common enough that they make up the bulk of the stellar volume of the galaxy. Not that they push it up above “pathetically small compared to the volume of the galaxy itself”, but the viruses might be surprised about how big it is.