Expert: Courtney Seligman - 3/5/2010

Question
When you look at a picture of Hoag's Object you can clearly see the blue stars in the ring, but in the center you just see a glob of bright yellow.  Do we know what the center is like, I mean I assume is isn't just one great big yellow star because it is light years in diameter, isn't it? And yet I thought huge blue stars are brighter (LBVs) than Yellow and yet you can't see individual stars in the center. So I don't understand. What do you suppose the center of Hoag's Object might be like?

Answer
The yellow object in the middle is the nucleus of the galaxy, and consists not of a single star, but billions of middle and lower Main Sequence stars like the Sun and other yellow dwarfs, and red dwarfs. Such stars are too faint to see as individual objects at the Hoag's Object's half a billion light year distance, but their combined light can be seen.

If the nucleus were all that we could see, we would assume Hoag's Object is an elliptical galaxy, or an S0 spiral galaxy (one in which the disk is too faint, in comparison with the nucleus, to notice). It is the ring of hot, young, bright blue stars surrounding the nucleus that is the puzzle, not the nucleus. Obviously, the ring is there, but why it is there -- how what should presumably be an undetectable disk has become so full of bright stars -- is unknown. That is partly because given the distance, it was hard (until recently) to even be sure if the ring and nucleus were part of the same structure, so it was easy to ignore the problem. But the more difficult part of the problem is that the dynamics of spiral galaxies, though well understood in general terms, are very complicated, and are not well understood as far as complex or unusual details are concerned. All we know is that such galaxies are probably rare -- which makes it ironic that another such galaxy, much more distant (if the same actual size, at least ten times farther away), is visible between the ring and nucleus (!)