The explosion of a massive star blazes with the light of 200 million
Suns in this NASA Hubble Space Telescope image. The arrow at top right
points to the stellar blast, called a supernova. The supernova is so
bright in this image that it easily could be mistaken for a foreground
star in our Milky Way Galaxy. And yet, this supernova, called SN 2004dj,
resides far beyond our galaxy. Its home is in the outskirts of NGC 2403,
a galaxy located 11 million light-years from Earth. Although the
supernova is far from Earth, it is the closest stellar explosion
discovered in more than a decade.

The star that became SN 2004dj may have been about 15 times as massive
as the Sun, and only about 14 million years old. (Massive stars live
much shorter lives than the Sun; they have more fuel to “burn” through
nuclear fusion, but they use it up at a disproportionately faster rate.)
A team of astronomers led by Jesus Maiz of the Space Telescope Science
Institute discovered that the supernova was part of a compact cluster of
stars known as Sandage 96, whose total mass is about 24,000 times the
mass of the Sun. Many such clusters — the blue regions — as well as
looser associations of massive stars, can be seen in this image. The
large number of massive stars in NGC 2403 leads to a high supernova
rate. Two other supernovae have been seen in this galaxy during the past
half-century.

The heart of NGC 2403 is the glowing region at lower left. Sprinkled
across the region are pink areas of star birth. The myriad of faint
stars visible in the Hubble image belong to NGC 2403, but the handful of
very bright stars in the image belong to our own Milky Way Galaxy and
are only a few hundred to a few thousand light-years away. This image
was taken on Aug. 17, two weeks after an amateur astronomer discovered
the supernova.

Japanese amateur astronomer Koichi Itagaki discovered the supernova on
July 31, 2004, with a small telescope. Additional observations soon
showed that it is a “Type II supernova,” resulting from the explosion of
a massive, hydrogen-rich star at the end of its life. The cataclysm
probably occurred when the evolved star’s central core, consisting of
iron, suddenly collapsed to form an extremely dense object called a
neutron star. The surrounding layers of gas bounced off the neutron star
and also gained energy from the flood of ghostly “neutrinos” (tiny,
almost non-interacting particles) that may have been released, thereby
violently expelling these layers.

This explosion is ejecting heavy chemical elements, generated by nuclear
reactions inside the star, into the cosmos. Like other Type II
supernovae, this exploding star is providing the raw material for future
generations of stars and planets. Elements on Earth such as oxygen,
calcium, iron, and gold came long ago from exploding stars such as this
one.

Astronomers will continue to study SN 2004dj over the next few years, as
it slowly fades from view, in order to gain a better understanding of
how certain types of stars explode and what kinds of chemical elements
they eject into space.

This color-composite photograph was obtained by combining images through
several filters taken with the Wide Field Camera of the Advanced Camera
for Surveys. The colors in the image highlight important features in the
galaxy. Hot, young stars are blue. Older stars and dense dust lanes near
the heart of the galaxy are red. The hydrogen-rich, star-forming regions
are pink. The dense concentration of older stars in the galaxy’s central
bulge is yellow.

In addition to the visible-light image shown here, ultraviolet images
and spectra are being obtained with Hubble’s Advanced Camera for
Surveys. Astronomers are also using ground-based telescopes to study the
supernova.

Credit: NASA, ESA, A.V. Filippenko (University of California, Berkeley),
P. Challis (Harvard-Smithsonian Center for Astrophysics), et al.

Electronic images and additional information are available at:
http://hubblesite.org/news/2004/23

For additional information, please contact:
Dr. Alex Filippenko, Department of Astronomy, 601 Campbell Hall,
University of California, Berkeley, CA 94720-3411, (phone)
510-642-1813, (e-mail) alex@astron.berkeley.edu

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).