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