On the night of Oct. 9, 1604, sky watchers —
including Johannes Kepler, an astronomer best known for
discovering the laws of planetary motion — were
startled by the sudden appearance in the western sky of a
"new star" that rivaled the brilliance of the nearby
planets. Now, exactly 400 years later, a pair of
astronomers at Johns Hopkins is using NASA's three Great
Observatories to unravel still-mysterious aspects of the
remains of this supernova, the last such object seen to
explode in our Milky Way galaxy.
When this bright object — now called Kepler's
supernova remnant — appeared alongside Jupiter, Mars
and Saturn on that long-ago October evening, observers had
only their naked eyes with which to study it because the
telescope would not be invented for another four years.
Johns Hopkins astronomers Ravi Sankrit and William P.
Blair, however, have the combined abilities of the Spitzer
Space Telescope, the
Hubble Space Telescope and the Chandra X-ray
Observatory at their disposal and are using them to analyze
the continuously expanding supernova remnant's appearance
three ways: in infrared radiation, visible light and
X-rays.
"Multi wavelength studies are absolutely essential for
putting together a complete picture of how supernova
remnants evolve," said Sankrit, an associate research
scientist in the Krieger School's
Center for
Astrophysical Sciences and the lead astronomer on the
Hubble observations.
"The glow from young remnants, such as Kepler's
supernova remnant, comes from several components," he said.
"Each component shows up best at different wavelengths."
The resulting combined image unveils a bubble-shaped
shroud of gas and dust that is 14 light-years wide and is
expanding at four million miles per hour (2,000 kilometers
per second). Observations from each telescope highlight
distinct features of the supernova, 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.
"A range of observations is needed to help us
understand the complex relationship that exists among the
various components," said Blair, a research professor in
Physics and Astronomy and the lead astronomer on the
Spitzer observations. "For instance, the infrared data is
dominated by heated interstellar dust, while optical and
X-ray observations sample different temperatures of
gas."
The explosion of a star is a catastrophic event. The
blast rips it 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 any tenuous interstellar gas and dust up 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 material from the explosion, called ejecta,
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
in a pattern similar to that made by oil and vinegar
— two liquids of different densities — in a
shaken bottle of salad dressing.
The Hubble data also show thin filaments of gas that
resemble rippled sheets viewed edge-on. These filaments
reveal where the shock wave is encountering lower density,
more uniform interstellar material. In an effort to obtain
a more accurate estimate of the distance to the supernova
remnant, Sankrit and Blair also compared their Hubble
observations with those taken with ground-based telescopes.
They concluded that Kepler is about 13,000 light-years
away.
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 (emitting 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 (emitting lower-energy
X-rays) resides in a thick interior shell and marks the
location of the 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.
Sankrit and Blair said they hope that this broad study
of the supernova remnant also may help astronomers identify
the variety of star that produced Kepler's original
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 past
1,000 years, Kepler's remains the only one whose type still
puzzles astronomers. By combining information from all
three Great Observatories, Blair and Sankrit believe they
are close to being able to identify Kepler's supernova's
progenitor star. But not quite yet.
"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."