Amid a backdrop of far-off galaxies, the majestic dusty spiral,
NGC 3370, looms in the foreground in this NASA Hubble Space
Telescope image. Recent observations taken with the Advanced
Camera for Surveys show intricate spiral arm structure spotted
with hot areas of new star formation. But this galaxy is more
than just a pretty face. Nearly 10 years earlier NGC 3370, in the
constellation Leo, hosted a bright exploding star.

In November 1994, the light of a supernova in nearby NGC 3370
reached Earth. This stellar outburst briefly outshone all
of the tens of billions of other stars in its galaxy. Although
supernovae are common, with one exploding every few seconds
somewhere in the universe, this one was special. Designated
SN 1994ae, this supernova was one of the nearest and best
observed supernovae since the advent of modern, digital
detectors. It resides 98 million light-years (30 megaparsecs)
from Earth. The supernova was also a member of a special
subclass of supernovae, the type Ia, the best tool astronomers
have to chart the growth rate of the expanding universe.

Recently, astronomers have compared nearby type Ia supernovae
to more distant ones, determining that the universe is now
accelerating in its expansion and is filled with mysterious
“dark energy.” Such measurements are akin to measuring the
size of your room by stepping it off with your feet. However,
a careful measurement of the length of your foot (to convert
your measurements into inches or centimeters) is still needed
to know the true size of your room. Similarly, astronomers
must calibrate the true brightness of type Ia supernovae to
measure the true size and expansion rate of the universe.

The very nearest type Ia supernovae, such as SN 1994ae, can
be used to calibrate distance measurements in the universe,
because other, fainter stars of known brightness can be
observed in the same galaxy. These stellar “standard candles”
are the Cepheid variable stars, which vary regularly in
brightness with periods that are directly related to their
intrinsic brightness, and thus allow the distance to the
galaxy–and the supernova–to be determined directly. However,
only the Hubble Space Telescope, equipped with its new Advanced
Camera for Surveys, has the capability to resolve these
individual Cepheids.

Adam Riess, an astronomer at Space Telescope Science Institute
in Baltimore, Md., observed NGC 3370 a dozen times over the
course of a month and has seen many Cepheid variables. Already
he and his colleagues can see that these Cepheids are the most
distant yet observed with Hubble. Because of their need to observe
this galaxy with great frequency to record the variation of the
Cepheids, the total exposure time for this galaxy is extremely
long (about one full day), and the combined image provides one
of the deepest views taken by Hubble. As a result, thousands of
distant galaxies in the background are easily discernable.

Dr. Riess imaged NGC 3370 with Hubble in early 2003. His science
only required looking at NGC 3370 in two filters that covered
the visual and infrared portions of the spectrum. By teaming up
with the Hubble Heritage Project, a third blue filter was added
to the data to produce the composite three-color image that is shown.

Image Credit: NASA, The Hubble Heritage Team and A. Riess (STScI)

NOTE TO EDITORS: For additional information, please contact
Adam Riess, Hubble Heritage Team, Space Telescope Science
Institute, 3700 San Martin Drive, Baltimore, MD 21218, (phone)
410-338-4509, (fax) 410-338-4579, (e-mail) or

Keith Noll, Hubble Heritage Team, Space Telescope Science
Institute, 3700 San Martin Drive, Baltimore, MD 21218, (phone)
410-338-1828, (fax) 410-338-4579, (e-mail)

Electronic images and additional information are available at

The Space Telescope Science Institute (STScI) is operated by the
Association of Universities for Research in Astronomy, Inc. (AURA),
for NASA, under contract with the Goddard Space Flight Center,
Greenbelt, MD. The Hubble Space Telescope is a project of
international cooperation between NASA and the European Space
Agency (ESA).