Sept. 9, 1998: Most objects
in the sky can be pigeonholed into a few of the hundreds of categories
that classify stars, galaxies, and other bodies. Every now and
then, you get one that changes its colors - literally - and seems
to beg for closer examination.
That's the case with a five-year-old supernova known as SN
1993J.
"It started out as a classic Type II supernova,"
said Dr. Doug Swartz of NASA's Marshall Space Flight Center,
"with hydrogen lines in its spectrum. These weakened in
a few weeks and helium lines appeared, more like a Type Ib supernova."
In effect, SN 1993J changed
its appearance from a supernova caused by an ordinary massive
star to a supernova caused by a dense helium stellar core.
To help resolve the apparent conflict, Swartz was recently
selected as a guest observer who will use the Advanced X-ray
Astrophysics Facility (AXAF), NASA's next Great Observatory for
space astrophysics. The first two have been the Hubble Space
Telescope and the Compton Gamma Ray Observatory. Both have revolutionized
astronomy with their observations in the visible and highest-energy
portions of the spectrum.
AXAF, managed by NASA's Marshall Space Flight Center, is designed
to do the same in the X-ray portion of the spectrum. AXAF is
scheduled for launch on the Space Shuttle in January 1999. After
a checkout phase and initial observations by the principal investigators,
NASA will allocate observing time to guest investigators like
Swartz.
Swartz worked on the AXAF calibration,
with AXAF project scientist Dr. Martin Weisskopf of NASA/Marshall,
when the telescope and instruments were at Marshall's X-ray Calibration
Facility. Those tests measured the instrument's sensitivities
under a range of conditions so scientists will be able to adjust
the data to produce a truer understanding of objects under study. |
NASA's
NEXT
Great Observatory
The world's largest and finest X-ray
telescope - the Advanced X-ray Astrophysics Facility (AXAF) -
is scheduled for launch aboard Space Shuttle Columbia in January
1999. With AXAF, astrophysicists at NASA's Marshall Space Flight
Center and around the world will observe energetic bodies ranging
from quasars down to dust clouds in a quest to understand more
of how and why the universe operates.
To help the public understand the purpose and value of AXAF,
we are running a series of stories that describe the science
that AXAF will support, and the investigations that will be carried
out by scientists at NASA/Marshall.
Other stories in the
series:
- How
hot is the Crab?: NASA's
next Great Observatory takes aim at the Crab Nebula pulsar
- Why
did the supernova change colors?
SN 1993J was seen to be one kind of massive explosion, but then
seemed to morph into a distinctly different kind. Scientists
using NASA's Advanced X-ray Astrophysics Facility, launching
in January, 1999, think they can discover why.
- Looking
for Pulsars in the Fast Lane
Scientists are looking for bizarre, short-lived, powerhouse stars
that burst with some of the brightest energy in the universe.
Using AXAF, they hope to find some of the few that may exist.
(this
story)
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![](https://webarchive.library.unt.edu/eot2008/20080921105548im_/http://science.nasa.gov/newhome/headlines/axaf_pix/m81wwo.tnl.jpg)
Left: Before-and-after
visible light images of galaxy M81, taken by David McDavid of
the D. Nelson Limber Memorial Observatory, show the appearance
of SNR 1993J |
Swartz also has been studying carbon monoxide emissions from
SN 1987A, the supernova that made headlines in early 1987. Carbon
monoxide is the deadly gas produced from inefficient combustion
in furnaces and car engines. It is also produced by stars as
carbon and oxygen atoms - the "ash" from helium fusion
in aging stars - which are blown into space, cool, and then collide
with each other. |
With the rest of the astrophysics community, Swartz's interest
has been drawn to SN 1993J, the 10th supernova to be discovered
in 1993. It was found on March 28, 1993 by Francisco Garcia Diaz,
an amateur astronomer in Lugo, Spain, in an arm of a beautiful
spiral galaxy, M81, about 11 million light-years from Earth.
SN 1993J is the second brightest
supernova discovered since telescopes were invented; No. 1 is
SN 1987A (shown at right by the Hubble Space Telescope). |
|
Its change from Type II to Type
Ib emissions is strange because the two supernova types are distinctly
different. A Type II supernova happens when a lone, massive star
has burned everything in its core. The furnace inside turns off
and the star collapses to become a massive piston that blows
most of its mass into space and compresses the core into a neutron
star. A Type Ib is believed to result from a star that has somehow
lost its entire hydrogen envelope, probably as a result of mass
transfer in a binary system, before collapse.
Because the supernovae have different origins, they emit light
differently as they explode. But SN 1993J is a transition object
which had lost most, but not all, of its hydrogen envelope.
"The idea is that supernovae, when they go off, are surrounded
by whatever materials the star emitted in the last 10,000 to
100,000 years of its life," Swartz explained. All stars
have stellar winds - just as our sun has a solar wind - that
carry away parts of the star's mass. Stars in binary systems
can lose material to a companion. A star destined to become a
Type IIb supernova will start off with perhaps 15 times as much
mass as our Sun, and lose about 9 solar masses over the course
of its life. When it finally explodes, the outrushing blast wave
- about 2.5 solar masses' worth in the case of SN 1993J - will
run into and energize the hydrogen cloak, or circumstellar medium,
around the star and emit x-rays.
Left: Visible
and ultraviolet views of galaxy M81 (several years before
the supernova) help illustrate how objects present different
faces in different parts of the spectrum. This image was taken
with the Ultraviolet Imaging Telescope during the Astro mission
on the Space Shuttle.
"Supernova 1993J is one of the few
that has made the transition from one type to another,"
Swartz explained. Only one other supernova, SN 1987K, has been
seen making such a change.
To help see this "missing" hydrogen envelope, and
to study its composition and shape, Swartz has been allocated
50,000 seconds (13.9 hours) of observing time with the AXAF CCD
Imaging Spectrometer (ACIS), one of two principal instruments
aboard AXAF (the other is a high-resolution camera). AXAF also
carries two spectral gratings that will spread incoming X-rays
in much the same way that a prism breaks white light into colors.
Supernova 1993J is one of the few that has made the transition
from one type to another," Swartz explained. Only one other
supernova, SN 1987K, has been seen making such a change. Obviously,
the star did not suddenly develop a white dwarf at its former
core. But some other mechanism is at work.
Right:
The National Radio Astronomy Observatory produced this series
of images showing SN1993J as it expands to a diameter of
1/10th of a light year in 18 months.
ACIS actually is a two-in-one camera designed to make high-resolution
images and moderate-resolution spectra of interesting X-ray sources
like galaxies, pulsars, and supernovae. One CCD - a charge-coupled
device, similar to those in TV camcorders - will produce images
while the other measures energy levels within the objects being
studied. The camera will observe an area of sky just 16.9 arc-minutes
across (that' a little more than half the apparent diameter of
the Moon). The images will be 2,048 by 2,048 pixels in size,
thus making highly detailed images. The spectrometer will divide
the X-ray spectrum into 8,192 slices - in effect, 8,192 X-ray
"colors" - for precise measurements of a source's energy
and chemical makeup.
With ACIS, Swartz hopes to see more signatures, in X-rays,
of the chemical makeup of the outrushing material, including
the hydrogen that disappeared a few weeks after the blast.
"It's still there," he said. "You just don't
see it. Eventually it will run into the circumstellar medium
and become visible again. Then the whole thing becomes transparent
and just fades away."
Already, SN 1993J has stirred interest among astronomers using
radio and optical telescopes, and the few X-ray telescopes now
in orbit have found a number of intriguing details. With AXAF,
Swartz expects to take the closest look yet, in x-rays, at this
oddity. |