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Donald Savage
Headquarters, Washington          May 7, 2003
(Phone: 202/358-1547)

Ray Villard
Space Telescope Science Institute, Baltimore
(Phone: 410/338-4514)

RELEASE: 03-160

DEEPEST VIEW OF SPACE YIELDS YOUNG STARS IN ANDROMEDA HALO

     Relying on the deepest visible-light images ever taken 
in space, astronomers using NASA's Hubble Space Telescope 
(HST) have reliably measured the age of the spherical halo of 
stars surrounding the neighboring Andromeda galaxy (M31). 

To their surprise, they have discovered that approximately 
one-third of the stars in Andromeda's halo formed only 6 to 8 
billion years ago. That's a far cry from the 11-to-13 
billion-year age of the stars in the Milky Way's halo.

Why the difference in halo ages? You might call it a tale of 
rich galaxy/poor galaxy. Apparently, M31 must have gone 
through a major "corporate merger" with another large galaxy, 
or a series of mergers with smaller galaxies, billions of 
years ago. Astronomers cannot yet tell whether this was one 
tumultuous event or a more continual acquisition of smaller 
galaxies. 

The newly discovered younger stars in Andromeda's halo are 
richer in heavier elements than the stars in our Milky Way's 
halo, or in most of the small dwarf galaxies that surround 
the Milky Way. Indeed the level of chemical enrichment seen 
in these younger stars is characteristic of relatively 
massive galaxies, containing at least a billion stars. 

This suggests three possibilities: (1) collisions destroyed 
the young disk of M31 and dispersed many of its stars into 
the halo; (2) a single collision destroyed a relatively 
massive invading galaxy and dispersed its stars and some of 
Andromeda's disk stars into the halo; and/or (3) many stars 
formed during the collision itself. 

Astronomers say it will take more detailed observations to 
unravel the "acquisition history" of these early cataclysmic 
events. Located only 2.5 million light-years away, the 
magnificent Andromeda galaxy, visible to the naked eye as a 
spindle of light in the autumn sky, has long been considered 
a near twin to our Milky Way in terms of size, shape and age. 
This new finding promises to offer new clues about how giant 
galaxies, like M31 and our Milky Way, formed by 
gravitationally shredding galaxies, like a cosmic Cuisinart, 
and then devouring them.

Dr. Tom Brown of the Space Telescope Science Institute 
(STScI) is reporting the findings today in Baltimore at the 
STScI May Symposium, "The Local Group as an Astrophysical 
Laboratory." His team used Hubble's Advanced Camera for 
Surveys (ACS) to peer into a small sample of the Andromeda 
halo for 120 Hubble orbits. This allowed for a study of the 
entire demographics of the halo population, down to its 
extremely faint stars. 

Previously, telescopes could only see the bright giant stars 
in the halo population, but the population of "normal" stars 
like our own sun was beyond our grasp, because such stars in 
M31 are so faint. The ACS is the first astronomical camera to 
combine ultra-sharp vision and sensitivity to ferret out 
M31's faint halo population.

An estimated 300,000 of these never-before-seen halo stars 
can be resolved, peppering Hubble's narrow sample of the halo 
population. Looking far beyond the halo stars, Hubble reveals 
thousands of background galaxies (down to 31st magnitude) 
billions of light-years away.

A large fraction of the background galaxies in the image also 
have peculiar shapes due to collisions. This reinforces the 
fact: we live in a vibrant and dynamic universe undergoing 
constant change.

Electronic image files and additional information are 
available on the Internet at:

http://hubblesite.org/news/2003/15

The Association of Universities for Research in Astronomy, 
Inc. operates the STScI for NASA, under contract with the 
Goddard Space Flight Center, Greenbelt, Md. The HST is a 
project of international cooperation between NASA and the 
European Space Agency.

For information about NASA and space science on the Internet, 
visit:

http://www.nasa.gov

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