Donald Savage
Headquarters, Washington, DC January 9, 1998
(Phone: 202/358-1547) EMBARGOED UNTIL 8:30 AM EST

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

RELEASE: 98-3

ASTRONOMERS DISCOVER AN INFRARED BACKGROUND GLOW IN THE UNIVERSE

Astronomers have assembled the first definitive detection
of a background infrared glow across the sky produced by dust
warmed by all the stars that have existed since the beginning of time.

For scientists, the discovery of this "fossil radiation" is
akin to turning out all the lights in a bedroom only to find the
walls, floor and ceiling aglow with an eerie luminescence.

The telltale infrared radiation puts a limit on the total
amount of energy released by all the stars in the universe.
Astronomers say this will greatly improve development of models
explaining how stars and galaxies were born and evolved after the
Big Bang.

The discovery reveals a surprisingly large amount of
starlight in the universe cannot be seen directly by today's
optical telescopes, perhaps due to stars being hidden in dust, or
being too faint or far away to be seen.

The discovery culminates several years of meticulous data
analysis from the Diffuse Infrared Background Experiment aboard
NASA's Cosmic Background Explorer (COBE), which was launched in
1989. The difficulty in making the discovery is analogous to
listening for a faint background hum in a shopping mall full of
people talking, music playing, and other noises.

"This is another big step in bringing cosmology to a
science based on observation as well as theory," said Michael
Hauser of the Space Telescope Science Institute, Baltimore, MD,
principal investigator on the Diffuse Infrared Background Experiment.

Reporting the results at the meeting of the American
Astronomical Society in Washington, DC, Hauser added: "We set out
to do this 23 years ago, and these results show it was worth it.
Our discovery fulfills the third and final cosmology objective of
the Cosmic Background Explorer mission."

LOST STARLIGHT

The unexpected preponderance of far infrared light implies
that many stars have "fallen between the cracks" in ultra-
sensitive visible light probes of the distant corners of the
universe, such as the Hubble Deep Field.

One possible explanation is that the universe is very
dusty, with many stars hidden in blankets of dust. Alternatively,
many stars may have been born in a flurry of activity in the very
early universe but faded away at earlier times than yet reached
with large telescopes.

In either scenario, the existence of hidden stars is
revealed by telltale dust which absorbs and re-radiates their
light at infrared wavelengths, and so a permanent record of their
existence is encoded in the infrared background.

UNCOVERING THE INFRARED BACKGROUND

Data from two other instruments on the Cosmic Background
Explorer have already yielded the precise spectrum and a detailed
map of another cosmological fossil (first discovered in the
1960s), the microwave background radiation from the Big Bang.

But finding the infrared background was not easy. Unlike
the cosmic microwave background, which at millimeter wavelengths
outshines everything else in the universe, the infrared background
is masked by infrared light from nearby dust in our solar system,
stars and interstellar dust in the Milky Way Galaxy, and, for
ground-based instruments, emission from the Earth's atmosphere and
from the instrument itself. The COBE mission overcame the last
two problems by observing from space using a small telescope and
instrument cooled to within a few degrees of absolute zero.

The COBE science team began by using the Diffuse Infrared
Background Experiment to scan half the entire sky once a week,
over a 10-month period from Dec. 1989 to Sept. 1990. Astronomers
then modeled and subtracted the infrared glow from foreground
objects in our solar system, our galaxy's stars, and vast clouds
of cold dust between the stars of our Milky Way.

Solar system dust was fairly easy to identify in the data
because its brightness changes from week to week as Earth orbits
the Sun. The interstellar dust of our Galaxy was identified in
the data because it has structure, and so looks different across
the sky. Light from stars was removed using a detailed model
based on counts of the many types of stars in the various parts of
the Galaxy.

When infrared light from these sources was subtracted from
the all-sky maps, the astronomers found a smooth background of
residual infrared light in the 240 and 140 micrometer wavelength
bands in "windows" near the north and south poles of the Milky
Way, which provided a relatively clear view across billions of
light years.

Astronomers next plan to probe the early formation of stars
and galaxies using infrared telescopes on new space missions such
as the Space Telescope Infrared Facility, Wide Field Infrared
Explorer, Next Generation Space Telescope and the Far Infrared
Space Telescope, and hope some day to make more infrared
background measurements using instruments launched deep into the
solar system to escape the interplanetary dust.

- end -

EDITOR'S NOTE: Images accompanying this release can be obtained
from the internet at the following URLs:

http://oposite.stsci.edu/pubinfo/pr/1998/01.html
http://oposite.stsci.edu/pubinfo/gif/dirbe.gif
http://oposite.stsci.edu/pubinfo/jpg/dirbe.jpg