Expert: Courtney Seligman - 12/8/2008

Question
I am trying to rank the relative sizes of the following:
Jupiter, Sun, Spiral Galaxy, Neutron Star, White Dwarf, Black hole, Giant Star, Nebula, Galaxy Cluster and Globular Cluster.  
I know some of the objects are larger than others and vise versa.  However, to order them together is giving me a hard time.  Can you help?  Thanks!

Answer
In order of increasing size:

Black holes have very little size (in fact none, if you mean the singularity at their center), but the size of their event horizons depends upon their masses. Low-mass black hole event horizons are only a few miles across, which would make them smaller than neutron stars; but black holes with a few tens of solar masses would have event horizons a few tens of miles across, making them larger than neutron stars. So you'd need to know the mass, to know the ranking. However, I'd assume that in most circumstances, you'd be thinking of the actual object inside the black hole, in which case, as stated above, it would have no size, at all.

Neutron stars have diameters of about ten miles.

White dwarfs are typically about five to ten thousand miles in diameter.

Jupiter is just under a hundred thousand miles in diameter.

The Sun is a little less than a million miles in diameter.

Giant stars can be as much as several hundred million miles diameter.

Nebulae range from a fraction of a light-year, up to several tens of light-years. Galactic clusters are in this same size range, so it would depend upon the specific clusters and nebulae, as to which is larger.

Globular clusters are usually 50 to 150 light-years in diameter.

Galaxies such as our own are around a hundred thousand light-years across, but dwarfs can be less than a tenth that size, all the way down to the size of very large globular clusters. Giant galaxies, on the other hand, range up to a million light-years or more in diameter.

Clusters of galaxies are a few millions of light-years in diameter for small ones, such as our Local Group, to several tens of millions of light-years across, for "rich" clusters with thousands of members.

Courtney Seligman