# Galaxies closer together than stars

I heard yesterday that relative to their size, galaxies are much closer together than stars. I’d never heard that, so I looked into it. Just using orders of magnitude, the sun is 109 meters wide and the nearest star is 1016 meters away. The Milky Way is 1021 meters wide, and the Andromeda galaxy is 1022 meters away. So stars are millions of diameters apart, but galaxies are tens of diameters apart.

## 14 thoughts on “Galaxies closer together than stars”

1. S. Walker

Is this just something you heard, or do you have a source? (Either Google or ask.com is usually my friend, but not this time.)

I wonder if this poses any problem for the BBT. One would think galaxies would be radiating out from one another at a rapid pace, if the BBT were correct.

2. I heard the comparison of the relative distances between stars and galaxies in The Canon by Natalie Angier. I tested out the comparison by getting the raw numbers in my post from Wikipedia and doing a little arithmetic. As far as the big bang theory, it’s very well established empirically. Indeed the galaxies are rushing away from each other, on average. But some galaxies are getting closer, even passing through each other. Since galaxies are closer packed than stars, relative to their size, it’s more common for galaxies to collide than stars. And because stars are so loosely packed, galaxies can pass through each other without stars colliding.

3. Bill the Lizard

The sizes of and distances between celestial objects always fascinates me (to say nothing of the time scales of astronomy). To try and make those numbers more fathomable I did a quick calculation of my own. If the sun were the size of a tennis ball, the nearest star would be 635 meters (about 0.4 miles) away. If the Milky Way were a tennis ball, the nearest galaxy would be about two feet away.

4. Ian Mulvany

Stars are really small. Essentially euclidean points in the grand scheme of things.

5. Scott

No one seems to talk about the relative nearness of galaxies much. I only became aware of it a couple of years ago. But it makes sense. Galaxies collide all the time, but stars never do. The Andromeda Galaxy is predicted to be on a collision course with the Milky Way, so the distance will only get closer over the next few billions years.

6. That’s an interesting discrepancy, but how about this twist: Assuming that there is indeed a massive black hole at the center of each galaxy, would the average distance between these black holes relative to their event horizons match up with the average distance between stars?

7. EastwoodDC

This is sort of relevant, at least it describes why galaxies cluster:
http://www.nature.com/news/dark-matter-s-tendrils-revealed-1.10951

@Chris Nahr: A black hole will be much denser than a star. A little Googling around gives 30 million kilometers as the radius of a galaxy-sized black hole, while the sun has a radius of 0.7 million kilometers. If I have done my BOTE calculation correctly, then the relative distance between the largest black holes is 1 or 2 orders of magnitude less than the relative distance between stars.

It might be interesting to consider relative mass and distance. Maybe after I have more coffee.

8. @EastwoodDC: Thanks for looking up the numbers! However, in this sentence:

“If I have done my BOTE calculation correctly, then the relative distance between the largest black holes is 1 or 2 orders of magnitude less than the relative distance between stars.”

Surely you mean greater since there are far fewer black holes than stars, and they are not that much bigger? Or am I confused here?

9. EastwoodDC

I can see how that might be confusing.
Galaxies are relatively less far apart than stars (relative to their diameters). For galactic-center black holes compared to stars, that relative difference is less (less than galaxies –> stars).

Fewer should not enter into it; my understanding is that every galaxy is believed to have a super-massive black hole at the center. Only the really big black holes would be larger than our sun.

10. I agree with that, but my point was that black holes are much rarer than stars, not galaxies; and since you say they are rarely bigger than stars, the relative distance between black holes should be even greater than that between stars.

11. There’s a striking bit of evidence for this that hit me one dark night in Wales (where there is little light pollution). The Andromeda Galaxy subtends a really large angle, comparable to the Moon! On a really dark, clear night you can see it as a smudge – whereas all stars are pinpricks.

12. Tab Atkins

Score another one for the Copernican principle – though these numbers were based on the nearest stars/galaxies to our own, it looks like they’re roughly equal to the true average.

Unrelated: I think this is apples to oranges. You shouldn’t compare the size of the sun to the size of a galaxy; you should either compare the sun to the galactic core, or the solar system to the galaxy.

The size of the solar system is, of course, a little hard to determine; it depends on your definition. If we include the Oort Cloud, the size is about a lightyear, putting it on the same size/distance realm as galaxies. Alternatively we can draw the boundary somewhere around Pluto, which puts the size at about 1e13m, making the ratio in the thousands range; higher than galaxies, but by only two orders of magnitude.

13. Isn’t it that bigger masses tend to be more closer together than smaller masses ? If yes, than the closness of galaxies makes perfect sense.