KEPLER'S SUPERNOVA REMNANT
TURNS 400
On the night of October 9, 1604, sky watchers looking
at a rare clustering of Mars, Jupiter and Saturn,
were amazed by the appearance of a "new star"
as bright as the planets. The famous astronomer,
Johannes Kepler, heard about it, but had to wait
a week for the skies to clear over Prague before
he had a chance to see the phenomenon. From then
on he observed the new star regularly for a year
until, in October 1605, it became too faint to see
with the naked eye. Telescopes were first used in
astronomy only four years after that. It was not
until the mid-20th century that astronomers, using
large telescopes, searched for and found a cloud
of glowing gas around the location of the new star
of 1604. Today this nebula is known as Kepler's
supernova remnant: the remnants of a stellar explosion
that was the last supernova seen in our Milky Way
galaxy.
Four hundred years after its discovery, astronomers
are using the combined power of NASA's three Great
Observatories (Hubble, Spitzer and Chandra), the
Very Large Array radio telescope, and ground-based
telescopes with modern spectrographs to unravel
intricate features of Kepler's supernova remnant.
Multiwavelength images show a bubble-shaped shroud
of gas and dust, 14 light-years wide. There is a
fast-moving shell of iron-rich material surrounded
by the primary shock wave from the supernova, expanding
at 4 million miles per hour (2000 kilometers per
second) that is sweeping up gas and dust from the
surrounding medium.
The Hubble Telescope image, obtained with the Advanced
Camera for Surveys in August 2003, shows regions
that have been lighted up by the passage of the
supernova shock wave. Knots and filamentary sheets
of emission viewed edge-on are revealed in glorious
detail. The bright knots are dense clumps that form
behind the outward moving shock wave. The shock
plows into material lost from the progenitor in
a stellar wind prior to the supernova explosion,
and instabilities cause the swept up gas to fragment
into clumps. The thin filaments trace regions where
the shock front is encountering more uniform, lower
density material.
Filters onboard Hubble have been used to isolate
light emitted by hydrogen atoms, and nitrogen and
oxygen ions present in the gas. These filters also
transmit starlight from foreground and background
stars. Regions in which there is only glowing hydrogen
are red, the yellow regions are strong in nitrogen,
and regions where oxygen emission is present are
pink or white.
The multi-wavelength study, for which the Hubble
observations provide a crucial component, will help
astronomers identify the type of star that produced
the explosion. There are two different types of
supernovas: one formed by the thermonuclear explosion
of an accreting white dwarf star, and the other
formed by the rebound explosion following the collapse
of the core of a massive star. Of the six known
supernovas in our Milky Way in the last 1000 years,
SN 1006, and SN 1572 (Tycho's supernova) are of
the former type, while SN 1054 (Crab Nebula), SN
1181 and SN 1680? (Cassiopeia A) are of the latter
type, and SN 1604 (Kepler's supernova) is the only
one for which the type is as yet unknown.
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