Paula Cleggett-Haleim
Headquarters, Washington, D.C.                             April 6, 1993
(Phone:  202/358-0883)

Michael Finneran
Goddard Space Flight Center, Greenbelt, Md.
(Phone:  301/286-5565)

RELEASE:  93-63

SCIENTISTS REVEAL NATURE OF EXPLODING, RED SUPERGIANT STAR

        NASA scientists have direct evidence that red supergiants --  the
largest stars known -- end their existence in massive explosions known as
supernovae.  Until this week, astronomers could only speculate that these
explosions represented the death of such stars.

        "It's a very exciting result and a tremendous advance for stellar
astronomy," said Dr. George Sonneborn, of NASA's Goddard Space Flight Center,
Greenbelt, Md.

        "This substantiates decades of work in stellar structure theory," said
Sonneborn, a research scientist for NASA's International Ultraviolet Explorer
(IUE) satellite, which obtained the new evidence through observations of a new
supernova on March 30.  "It has long been suspected that red supergiants
explode to become supernovae.  Now we have first-hand evidence of that."

        "This clearly is the second most important supernova of the century,"
said Sonneborn.  "It's of major significance."

        The Type II supernova took place about 12 million light-years from
Earth in a galaxy known as M81 in the Ursa Major or Big Bear constellation.
It has been designated SN 1993J because it was the tenth supernova discovered
this year.

IUE's Fast Response Was Critical

        The supernova's nearness and the quickness with which IUE was able to
observe it were critical factors that enabled scientists to verify an aspect
of stellar evolution theory.

        A supergiant is massive -- about the diameter of the solar system out
to the planet Jupiter.  Stellar evolution theory long has taught that red
supergiants can explode to become supernovae.  But in the only previous case
in which astronomers definitively determined the type of star that produced a
supernova explosion, it turned out to be a smaller and hotter blue supergiant.
That supernova occurred in 1987, 160,000 light-years away and also was observed
by IUE.

        The difference between red and blue supergiants is that the blue
variety are believed to have evolved from red supergiants after shedding much
of their extended atmosphere.  Thus, blue supergiants are smaller than red
supergiants.

        The satellite's observations of the supernova revealed that the
exploding star is surrounded by a thick shell of slowly expanding gas.  Heated
to very high temperatures by the enormous energy released in the stellar
explosion, the ultraviolet emissions from this glowing gas were detected by
IUE.

        A red supergiant loses large amounts of material through a slowly
moving wind flowing outward from the star.  The presence of this glowing gas
in the first observations of the supernova means that it must be close to the
explosion and that the star must have been in a red supergiant phase shortly
before its demise.

        In a Type II supernova explosion, the central core of the supergiant
star collapses after the star uses up its nuclear fuel.  This central implosion
sets off an explosion of the outer layers of the star, leaving behind a small,
incredibly dense remnant body called a neutron star or possibly a black hole.
A black hole derives its name from the theory that its gravity is so powerful
not even light can escape it.

        "A supernova is the most cataclysmic event in the universe," said
Goddard's Dr. Yoji Kondo, IUE Project Scientist.  "The light it produces for a
few weeks is roughly equivalent to the brightness of the whole Milky Way
galaxy, which contains a few hundred billion stars."

        Explosions of these huge stars are not uncommon, but rarely are they
observed so close by, Kondo added.  "On a cosmic scale, it's practically a
next-door neighbor," he said, of SN 1993J which IUE observed after it was
first spotted by amateur astronomers in Spain.

        Prior to the 1987 blue supergiant explosion, the most recent nearby
supernova that could be seen without a telescope took place in 1604 and was
observed by Johannes Kepler, one of the great German astronomers.

        Unlike the 1604 and 1987 supernovae, however, the one viewed March
30 was not close enough to be  visible to the naked eye.  Its brightness was
of 9.2 magnitude.  A  brightness of at least the sixth magnitude would have
been necessary for the March 30 supernova to be seen without using a telescope.

Learning From SN 1993J

        Kondo and Sonneborn said much stands to be learned from the March 30
supernova.  Because it is so close and because IUE observed it so quickly,
scientists will obtain data about the explosion that they otherwise would
not have gotten.

        "We'll learn things about supernovae that we never could have until
now," said Sonneborn.

        For instance, astronomers for the first time anticipate studying the
stellar wind -- the outflowing of energy -- from this type of star in a
fashion not possible before.  That's because the light from the supernova
illuminated the red supergiant's stellar wind in a way that scientists can see
it, Sonneborn said.

        "The stellar wind will tell us about the late stages of the star's
life prior to the explosion," Sonneborn said.  "This also is a rare opportunity
to study the tenuous gases in the far reaches of the Milky Way galaxy and in
the M81 galaxy by observing the absorption caused by such gases upon the
spectrum of this bright supernova."

        NASA's International Ultraviolet Explorer was launched into modified
synchronous Earth orbit in January 1978 by a Delta rocket from Cape Canaveral,
Fla., in cooperation with the European Space Agency (ESA) and the British
Science and Engineering Research Council.  It is managed by Goddard Space
Flight Center for the Office of Space Science at NASA Headquarters in
Washington, D.C.  Goddard shares operational control of the satellite with the
ESA in Villafranca, Spain.