Links | Offers an interactive lesson on the Hubble System of classifying galaxies. |
 www.smv.org/hastings/galaxy.htm| Essay on these large systems of stars and interstellar matter, typically containing several million to some trillion stars, of masses between several million and several trillion times that of our Sun, of an extension of a few thousands to several 100,00 |
www.seds.org/messier/galaxy.html | |
