Can we see individual stars that are outside our galaxy?

Yes and No. The faintest individual stars we can see with our naked eyes are brighter than an apparent magnitude of about +6.5. The bright star Betelgeuse in Orion is at a distance of 1500 light years and is 10,000 times as luminous as the Sun. To be outside our galaxy, the star has to be at least 3000 light years above the plane of the Milky Way in the so-called Halo region, which is still technically a part of our Milky Way, but at least not a part of the spiral disk. There are no supergiants like Betelgeuse in the halo of the Milky Way. The most common stars are only a little more brighter than the Sun, which means they would be much fainter than the faintest star you could see with your eye. The famous Supernova of 1987 is, of course, the only recent exception. It was a single star located 160,000 light years outside the Milky Way which we could see easily with the naked eye. That's what made it doubly spectacular.

The most distant, individual star known was discovered by astronomer Bruce Margon while searching for the optical candidate for an X-ray source. He found a very red, carbon star about 400,000 light years from the Milky Way. With a velocity of 40 kilometers per second, at its distance it is only weakly bound to the Milky Way. It lies not far from the orbital track of the Magellanic Clouds and may be an escaped star from one of them. It is an 18th magnitude star, even though it is nearly 1000 times more luminous than the Sun. See Sky and Telescope magazine, April 1984, p. 316 for more details.

Astronomers such as Saul Perlmutter have also seen supernova in galaxies that are billions of light years distant, so these are the truly most distant individual stars we know about at least at the time that they detonate! The above figure is a supernova seen by Perlmutter's Supernova Cosmology Project team. In their caption they note:

The third image shows the same supernova as observed with the Hubble Space Telescope. This much sharper picture allows a much better measurement of the apparent brightness and hence the distance of this supernova. Because their intrinsic brightness is predictable, such supernovae help to determine the deceleration, and so the eventual fate, of the universe.

Lets take a walk through the universe to see what individual stars look like in various Hubble Space Telescope and ground-based telescope images!

The stellar stream in the halo of the nearby dwarf starburst galaxy NGC 4449 is resolved into its individual starry constituents in this exquisite image taken with the 8.2-meter Subaru Telescope and Suprime-Cam. The distance to this galaxy is about 12 million light years away.

At a distance of 56 million light years you can still resolve some of the brightest stars in the barred spiral NGC 1365 shown above.

NGC 1309 is located 130 million light years from the sun and as this Hubble Heritage image shows, you can still see individual bright stars in its spiral arms at this distance. These stars are thousands of times brighter than our own sun and are giant or supergiant stars.

Return to Dr. Odenwald's FAQ page at the Astronomy Cafe Blog.