Notes from Under Sky

Stars That Are Out of this Galaxy

Are all of the stars we see part of the Milky Way?

Esmail Bonakdarian asked:

When I look at the sky at night and see stars, am I right in assuming that all of the stars I see are those in our Milky Way Galaxy? By stars I mean points of light, not collection of stars such as in other galaxies that are visible from earth.
I can also see for example, the Andromeda Galaxy, another huge conglomeration of stars far far away.
Are there any stars that are not part of a galaxy? I.e., stars that are just floating in intergalactic (?) space? What are they doing there?

If by "see stars," you mean see them with the unaided eye, then yes, all of those stars are in fact in the Milky Way Galaxy.

Well, usually. In 1885, there was a supernova in the Andromeda Galaxy, which now goes by the name of S Andromedae. That peaked at magnitude 6.5 or so, if I recall correctly, which means that it could have been seen by the unaided eye. The Andromeda Galaxy (M31) is bigger in apparent size than the supernova, of course, but its light is spread out over a larger area, and the supernova would not have blended in. (Just the same, I'm not aware of any ironclad reports of seeing it with the unaided eye.)

The distance to M31 is still in dispute, but it's approximately 800 kiloparsecs. Its distance modulus is therefore about 24.5, meaning that objects at that distance have an apparent magnitude 24.5 higher than their absolute magnitude. To put it another way, the supernova of 1885 would, if placed at a distance of 10 parsecs (32.6 light-years), shine at a brilliant magnitude –18.

However, ordinary non-detonating stars have no chance of glowing that brightly. The best such a star can do is probably in the neighborhood of absolute magnitude –10, and there's no way the unaided eye can make out a star of magnitude –10+24.5 or 14.5.

On the other hand, there are closer stars than that, that are not in the Milky Way galaxy. In 1987, a supernova blew up in the Large Magellanic Cloud (LMC), and that star got as bright as apparent magnitude 2.8. The LMC has a distance modulus of about 18.5, so that means the supernova of 1987 had an absolute magnitude of –15.7—not as bright as the Andromeda supernova of 1885, but still respectable.

Again, though, the brightest an ordinary star can get at that distance is about magnitude –10+18.5 or 8.5, and if by any chance you could see a star that dim, it would surely be lost in the glow of the LMC.

How does one compute a distance modulus? If you know the distance d in parsecs, it's easy. The expression is simply

DM = –5 + 5 log d

For example, the Andromeda Galaxy's distance modulus is computed as

DM = –5 + 5 log 800000 = –5 + 5 (5.9) = 24.5

If we again say that the brightest an ordinary (and by that, I mean not currently exploding as a supernova) star can get is absolute magnitude –10, then in order to see such a star by the unaided eye (which we'll say can go down to about magnitude 7.5), the highest the distance modulus can get is 17.5. From the above formula for DM, that corresponds to a distance of 104.5 or around 30,000 parsecs. That's equivalent to about 100,000 light-years.

Are there any other galaxies within 100,000 light-years?

Sure there are! The problem is, they are mostly sparse, and if they ever had supermassive stars that could glow at absolute magnitude –10, those stars probably blew themselves up billions of years ago. Moreover, many of the galaxies can be seen only through some of the gas and dust of the Milky Way itself, and that causes stars in them to appear even dimmer than they already are. The distance modulus only accounts for the dimming due to distance—it does not consider dimming (sometimes called "extinction") due to intervening gas and dust.

There is another possibility, though. All of these nearby galaxies, which are small in comparison with our own Milky Way and are therefore sometimes called "dwarf galaxies," are gravitationally bound to the Milky Way. When they get close enough, tidal forces exerted by the massive gravity of the Milky Way tear them apart. It might not happen all in one pass, but eventually, stars get strewn out and separated from their original galactic host. This is expected to happen to the Magellanic Clouds sometime in the next few billion years, and it is already happening, reportedly, to the Sagittarius Dwarf.

Even after the stars are separated from their home galaxy, they are almost always still gravitationally bound to the Milky Way. All the same, its nearest neighbor might not be for hundreds or even thousands of light-years around, and for many purposes we could consider it free-floating, even though it could easily be within 100,000 light-years. And being separate from any galactic glow, it would be easier to see with the unaided eye.

Unfortunately, stars of magnitude –10 are few and far between, even in a relatively dense and raucous place as the Milky Way Galaxy. They must be fewer and further between in sparser galaxies like the dwarfs, and fewer and further between still in intergalactic space. In fact, over the period of human history, we don't know of one. (To be sure, it would be difficult to tell. It could make its enormous distance apparent only by being a very bright and therefore very long-period Cepheid, and it might be far enough not to be conspicuous. We wouldn't notice it until it blew up as a supernova, and then photographs might reveal what kind of star was there beforehand.)

In short, it seems as though the unaided eye is incapable of seeing stars in galaxies other than the Milky Way, except for the occasional supernova. (Very occasional indeed. Between 1885 and 1987, I don't think there were any unaided eye supernova.)

Incidentally, speaking of intergalactic stars (weren't we?), there is a globular cluster in Lynx, NGC 2419, which is sometimes called the Intergalactic Wanderer, because it used to be thought to be aimlessly wandering amongst the galaxies of the Local Group. I believe Burnham's Celestial Guide describes it thus. However, more recent measurements seem to indicate that it, too, is gravitationally bound to the Milky Way and therefore not a true wanderer as its name suggests.

Copyright (c) 2001 Brian Tung