Sagittarius Dwarf Irregular Galaxy
This image from the Hubble Space Telescope shows a small galaxy called the Sagittarius dwarf irregular galaxy, or "SagDIG" for short. SagDIG is relatively nearby, and Hubble's sharp vision is able to reveal many thousands of individual stars within the galaxy.
The brightest stars in the picture (easily distinguished by the spikes radiating from their images, produced by optical effects within the telescope), are foreground stars lying within our own Milky Way galaxy. Their distances from Earth are typically a few thousand light-years. By contrast, the numerous faint, bluish stars belong to SagDIG, which lies some 3.5 million light-years (1.1 Megaparsecs) from us. Lastly, background galaxies (reddish/brown extended objects with spiral arms and halos) are located even further beyond SagDIG at several tens of millions parsecs away.
As their name implies, dwarf irregular galaxies are unlike their spiral and elliptical cousins, because of their much smaller physical size and lack of definite structure. Using Hubble, astronomers are able to resolve dwarf irregular galaxies that are at very large distances from Earth, into individual stars. By examining properties of the galaxy, such as distance, age and chemical composition, the star formation history of the whole galaxy is better understood, and reveals how, where, and when active star formation took place.
The main body of SagDIG shows a number of star-forming complexes that cover an appreciable fraction of the galaxy surface area. The presence of on-going star formation in a gas-rich galaxy such as this makes SagDIG an excellent laboratory where scientists can test present-day theories of what triggers star-formation in galaxies (without companions) and how this propagates throughout the galaxy.
Image Credit: NASA, ESA, and The Hubble Heritage Team STScI/AURA)
Explanation from: http://hubblesite.org/image/1603/news_release/2004-31
