Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2004 November 16
See Explanation.  Clicking on the picture will download
 the highest resolution version available.

Sagittarius Dwarf Irregular Galaxy
Credit: Hubble Heritage Team (AURA / STScI), Y. Momany (U. Padua) et al., ESA, NASA

Explanation: How old is this galaxy? The nearby Local Group galaxy dubbed the Sagittarius Dwarf Irregular Galaxy (SagDIG) is not only very small but also has relatively few elements more massive than helium. Now the lack of heavy elements might mean that SagDIG is very young, so that component stars had little time to create and disperse massive elements. Conversely, SagDIG's diminutive size could indicate that it formed in the early universe, being a surviving building block of modern large galaxies. The above detailed image from the Hubble Space Telescope has now resolved enough stars to solve this mystery: SagDIG is ancient. Although SagDIG does have some groups of young stars, many stars are very old, and the galaxy as a whole helps astronomers to understand how the universe evolved, and show that at least one metal-poor galaxy is almost as old as the universe. Pictured above, SagDIG spans about 1,500 light years and lies about 3.5 million light years away toward the constellation of Sagittarius.

Tomorrow's picture: colorful memory

< | Archive | Index | Search | Calendar | Glossary | Education | About APOD | >

Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA)
NASA Web Site Statements, Warnings, and Disclaimers
NASA Official: Jay Norris. Specific rights apply.
A service of: LHEA at NASA / GSFC
& Michigan Tech. U.