Four hundred years ago, sky watchers,
including the famous astronomer Johannes Kepler, were startled by the
sudden appearance of a "new star" in the western sky, rivaling the
brilliance of the nearby planets. Now, astronomers using NASA's three
Great Observatories are unraveling the mysteries of the expanding
remains of Kepler's supernova, the last such object seen to explode in
our Milky Way galaxy.
When a new star appeared alongside Jupiter,
Mars, and Saturn on Oct. 9, 1604, observers could use only their eyes to
study it. The telescope would not be invented for another four years.
Modern-day astronomers, on the other hand, have the combined abilities
of the Spitzer Space Telescope, the Hubble Space Telescope, and the
Chandra X-ray Observatory at their disposal. A team of astronomers, led
by Ravi Sankrit and William Blair of Johns Hopkins University in
Baltimore, Md., are using the Great Observatories to analyze the
remains, called Kepler's supernova remnant, in infrared radiation,
visible light, and X-rays.
The combined image unveils a bubble-shaped
shroud of gas and dust that is 14 light-years wide and is expanding at 4
million miles per hour (2,000 kilometers per second). Observations from
each telescope highlight distinct features of the supernova remnant, a
fast-moving shell of iron-rich material from the exploded star,
surrounded by an expanding shock wave that is sweeping up interstellar
gas and dust.
"Multiwavelength studies are absolutely
essential for putting together a complete picture of how supernova
remnants evolve," said Sankrit, an associate research scientist in the
Center for Astrophysical Sciences at Johns Hopkins and the lead
astronomer on the Hubble observations. "The glow from young remnants,
such as Kepler's supernova remnant, comes from several components. Each
component shows up best at different wavelengths."
"For instance, the infrared data are
dominated by heated interstellar dust, while optical and X-ray
observations sample different temperatures of gas," added Blair, a
research professor in the Physics and Astronomy Department at Johns
Hopkins, and the lead astronomer on the Spitzer observations. "A range
of observations is needed to help us understand the complex relationship
that exists among the various components."
The explosion of a star is a catastrophic
event. The blast rips the star apart and unleashes a roughly spherical
shock wave that expands outward at more than 22 million miles per hour
(10,000 kilometers per second), like an interstellar tsunami. This wave
spreads out into surrounding space, sweeping up any tenuous interstellar
gas and dust into an expanding shell. In certain cases, the surrounding
regions include material shed by the progenitor star in a stellar wind
before the explosion, in earlier phases of its evolution. The stellar
ejecta from the explosion initially trails behind the shock wave but
eventually catches up with the inner edge of the shell and is heated to
X-ray temperatures.
Visible-light images from the Hubble
telescope's Advanced Camera for Surveys reveal where the supernova shock
wave is slamming into the densest regions of surrounding gas. The bright
glowing knots are dense clumps that form behind the shock wave. As the
shock plows into material lost from the progenitor star, instabilities
left in its wake cause the swept-up gas to fragment into clumps. This
clumping process is similar to the patterns made by oil and vinegar (a
mix of two liquids of different densities) in a shaken bottle of salad
dressing. The Hubble data also show thin filaments of gas that look like
rippled sheets seen edge-on. These filaments reveal where the shock wave
is encountering lower-density, more uniform interstellar material.
Sankrit and Blair also compared their Hubble observations with those
taken with ground-based telescopes to obtain a more accurate distance to
the supernova remnant of about 13,000 light-years.
The astronomers used the Spitzer telescope
to probe the material that radiates in infrared light. These
observations show heated microscopic dust particles that have been swept
up by the supernova shock wave. The Spitzer data are brightest in the
densest regions seen by the Hubble telescope. Whereas Hubble sees only
the brightest, densest regions, the Spitzer telescope is sensitive
enough to detect the entire expanding shock wave, a spherical cloud of
material. Recent spectroscopic observations from Spitzer also reveal
information about the chemical composition and physical environment of
the expanding clouds of gas and dust that were ejected into space. This
dust is similar to dust that was part of the cloud of dust and gas that
condensed to form the Sun and planets in our solar system.
The Chandra X-ray data show regions of very
hot gas. The hottest gas (higher-energy X-rays) is located primarily in
the regions directly behind the shock front. These regions also show up
in the Hubble observations, and also align with the faint rim of glowing
material seen in the Spitzer data. Cooler X-ray gas (lower-energy
X-rays) resides in a thick interior shell and marks the location of
heated material expelled from the exploded star. In some other supernova
remnants, the ejecta also can be seen in visible light, but in Kepler it
is seen only in X-rays.
This broad study of the supernova remnant
also may help astronomers identify the type of star that produced the
explosion. Supernovas arise from two very different types of stars:
low-mass, white-dwarf stars and massive stars. Of the six known
supernovas in our Milky Way over the last 1,000 years, Kepler's
supernova is the only one for which astronomers are uncertain of the
type of star that exploded.
By combining information from all three
Great Observatories, astronomers are obtaining a much clearer picture of
Kepler's supernova remnant. "It's really a situation where the total is
greater than the sum of the parts," Blair said. "When the analysis is
complete, we will be able to answer several important questions about
this enigmatic object."
Electronic images and additional
information are available at:
http://hubblesite.org/news/2004/29
http://chandra.harvard.edu
http://spitzer.caltech.edu
http://www.jhu.edu/news_info/news/
http://heritage.stsci.edu/2004/29
http://www.nasa.gov/vision/universe/starsgalaxies/kepler.html
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).
NASA's Marshall Space Flight Center, Huntsville, Ala., manages the
Chandra program for NASA's Office of Space Science, Washington. Northrop
Grumman of Redondo Beach, Calif., formerly TRW, Inc., was the prime
development contractor for the observatory. The Smithsonian
Astrophysical Observatory controls science and flight operations from
the Chandra X-ray Center in Cambridge, Mass. JPL manages the Spitzer
Space Telescope mission for NASA's Science Mission Directorate,
Washington, D.C. Science operations are conducted at the Spitzer Science
Center at the California Institute of Technology in Pasadena. JPL is a
division of Caltech. Spitzer's Infrared Array Camera was built by NASA
Goddard Space Flight Center, Greenbelt, Md.
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