SWEEPING AWAY DUST TO UNCOVER ANCIENT STARS

For decades astronomers had a much simpler picture of galactic
bulges. They thought bulges were amorphous, old stellar populations
similar in content to large elliptical galaxies. This picture grew
more complicated when astronomers probed the center of our Milky Way
and found bright young clusters intermixed with older stars.

Hubble's sharp resolution and infrared visibility allowed astronomers
to look far beyond the Milky Way and probe the hubs of over 200 galaxies
out to over 100 million light-years from Earth.  Exact ages were
determined by precisely measuring star color. The older the star, the
redder it is.

However, starlight can also be reddened by interstellar dust, just as
the Sun looks red at sunsets because of blue light being scattered by
atmospheric dust.

But Hubble's Near Infrared Camera and Multi-Object Spectrometer (NICMOS)
was able to peer past the dust to directly observe starlight. When these
results were combined with complementary observations taken in visible
light by the Wide Field Planetary Camera 2 (WFPC2), the stars' true
colors could at last be measured.

The Hubble observations allowed astronomers to study the true
variability of galactic bulges without the interfering effects of dust.
When astronomers examined the real colors of the bulges, they found that
a class of old bulges are very similar to each other. These ancient
bulges are also similar in color to elliptical galaxies found in galaxy
clusters.  This suggests that the bulges and the ellipticals probably
were all formed 10 billion years ago, within 2 billion years of each
other.

"Hubble has cleared away the dust from the history of the bulges of
spiral galaxies and shown that, at least in the case of spirals with
substantial bulges, these formed nearly contemporaneously with the
ellipticals in clusters," says Andrew Fruchter of the Space Telescope
Science Institute, in Baltimore, Md.

Hubble has also uncovered spectacular complexity in the smaller hubs of 
another class of spiral galaxies. Hubble reveals a bewildering richness
of structure: bright star clusters, star-forming rings, and other
complicated structures. Some of these bulges were not seen as anything
but featureless elliptical blobs from ground-based telescopes.

The environment around a galaxy can trigger star formation in a bulge.
If a smaller galaxy collides with a larger galaxy, it will provide fresh
"fuel" in the form of dust and gas to trigger new star birth in the hub.

Instabilities within the broad, flattened disk of an isolated spiral
galaxy can also fuel star birth inside the hub by "shoveling" gas into
the hub along a bar-like structure.  This makes the bulge grow like an
inflating balloon.

As young star clusters grow at the center, they disrupt the bar, thus
stopping the infall of gas and any further bulge growth. This process
can take a few hundred million years.