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The Question
(Submitted November 12, 1998)
I'm a 14 year old student and I recently read that there was some evidence
that a supernova should have occurred in the year of 1250, but that there was
no records showing a supernova in that year. The supernova should have lasted
for a year. Do you have any information about this topic; why it is believed
to have occurred, possibilities about why there are no records, how vivid it
was, etc...?
The Answer
You are right, this has generated some news recently. I can only relay
what was in the news stories: there may be historical reasons
why there is no record that has survived (wrong part of sky, records
destroyed,
gap in record keeping ...), or that the light was absorbed
by intervening gas, or that the estimates for the age are wrong.
I append the articles from AP and the press release from
the Max Planck Institute.
I hope this helps.
Allie Cliffe, Koji Mukai, David Palmer, Mike Arida and Tim Kallman
for Ask a High-Energy Astronomer
How come no one saw big supernova explosion?
1.39 p.m. ET (1840 GMT) November 11, 1998
By Jeff Donn, Associated Press
(AP) - Astronomers have discovered evidence that hundreds of years ago a
star exploded closer to Earth than any other known supernova. They just
can't figure out why no one back in the 13th century seems to have recorded
the blast, which should have been a spectacular sight.
The mystery suggests, among other things, that the astronomic records are
incomplete or that scientists have come across a new celestial phenomenon:
invisible supernovas.
The supernova should have appeared for a year or more as the brightest
object in the night sky, except for the moon, around the year 1250.
Presumably, the great astronomers of the Orient should have been able to
see it just above the horizon, sometimes even during the day, according to
Bernd Aschenbach, one of the chief researchers.
But they made no known observations of it.
The explanation may be that the explosion of superheated gas and radiation
may not have given off visible light. The new work "offers the possibility
that we're looking at a new and different phenomenon," said astrophysicist
Robert Petre at NASA's Goddard Space Flight Center in Greenbelt, MD.
Or the explanation might be much more mundane, like bad record-keeping, the
researchers said.
The discovery was made by scientists at the Max Planck Institute in Germany
and was reported Thursday in the journal Nature.
Only a few exploded stars within our Milky Way galaxy have been at least
potentially visible to the naked eye on Earth.
The supernova revealed itself when researchers scanned the constellation
Vela for invisible X-rays and gamma rays, which can be byproducts of a
supernova explosion.
In analyzing the radiation, the astronomers found that the explosion's gas
cloud is still out there and is twice as hot as the sun's core and
stretches up to 25 light-years across.
Though it is the nearest known supernova, it is still 650 light-years away,
or about 41 million times farther from Earth than the sun. (That means that
the explosion actually occurred around the year 600, but its light took
about 650 years to reach Earth.)
Aschenbach said the 13th century supernova should have been spectacular. He
said the lack of any historical record has him "very puzzled."
However, one other known supernova, known as Cas-A from the year 1680,
appears strangely dark. So maybe it and the newfound supernova suggest a
previously unknown group of invisible, "subluminous" exploding stars,
researchers said.
Another one of the researchers, A.F. Iyudin, suggested that maybe the
explosion was simply hidden behind some expanse of absorbing cosmic material.
Aschenbach also said that some Chinese astronomical records are missing
from the time, so it's possible that Mongol rule disrupted the work of
astronomers.
Or maybe astronomers did record the supernova, but their records have been
lost, said Edwin Krupp, director at the Griffith Observatory in Los Angeles.
The last plainly observable supernova in this galaxy was recorded by
Johannes Kepler and other astronomers in 1604. It was so dramatic to the
naked eye that Kepler predicted it would be hailed symbolically "as a
spectacle of public triumph or the entry of a mighty potentate."
Max Planck Institute for Extraterrestrial Physics,
Garching/Germany
Contact: Bernd Aschenbach
Phone: +49-89-3299-3561 Fax: +49-89-3299-3569
13-11-98
Discovery of a Young Near-By Supernova Remnant
Young supernova remnant close to Earth discovered in hard X-rays towards the
edge of the "Vela" supernova remnant confirmed by detection of gamma-ray lines
from titanium-44
X-ray astronomers and gamma-ray astronomers of the Max Planck Institute for
Extraterrestrial Physics in Garching/Germany, have discovered a young
supernova remnant which is exceptionally close to Earth (Nature, Vol. 396,
12 November 1998). The remnant is just 700 light years away and it was
created about 700 years ago when a star exploded in the southern sky in
the constellation Vela ("sail").
"Our analysis shows that this is the nearest supernova remnant to have
occurred during recent human history; other similarly close remnants in the
Milky Way are of age of at least 10.000 years and more", Dr. Bernd Aschenbach
from the Garching Max Planck Institute explains. "Now, it is up to optical
astronomers and radio astronomers to confirm and extend our results."
The discoveries are being reported in two separate contributions in
"Nature's" issue of November 12, 1998, volume 396. The data on which these
discoveries rest have been taken with the German X-ray astronomy satellite
ROSAT, which has been developed and built under the direction of the
Garching Max Planck Institute, and by the COMPTEL instrument, built by an
international collaboration under the leadership of the Max Planck Institute
as well, on board of the U.S. American gamma-ray astronomy Observatory "COMPTON".
During the first all-sky survey with imaging X-ray telescopes also the Vela
constellation has been mapped by ROSAT. This is a region in the sky well
known to astronomers. In soft X-rays the Vela region is dominated by a huge
and bright supernova remnant, the Vela supernova remnant, with a diameter of
almost 200 light years, which still continues to expand at supersonic speed.
More than 10.000 years ago a star exploded as a gigantic supernova and it
gave rise to the clouds of hot gas which we see today. (Outside of the
boundary of the explosion cloud Dr. Bernd Aschenbach has discovered numerous
fragments of the progenitor star, a result which has been published in
"Nature" in March 1995.)
When Dr. Aschenbach was analyzing the Vela supernova remnant in a way
differing from standard software routines developed for ROSAT, in particular
by extracting only the highest energy photons accessible with ROSAT, the
image of the Vela supernova remnant changed drastically. For photon energies
greater than 1300 electron volts the soft X-rays of the Vela supernova
remnant had disappeared almost completely and a previously unknown, fairly
circular emission region of about 2 degrees diameter, which is about four
times the size of the full moon, emerged at the south-east corner of the
Vela remnant (c.f. the pictures attached).
"We were stunned; there is no way around, this is a new supernova remnant",
Dr. Aschenbach says enthusiastically. "There are no other X-ray sources in
the sky we know of which show this sort of shape and brightness distribution,
except supernova remnants". The previously unknown object was named
"RX J0852.0-4622" according to the position in the sky.
Further analysis showed: "RX J0852.0-4622" is extremely hot at a temperature
of about 30,000,000 Kelvin. This means: "RX J0852.0-4622" is a very young
object, otherwise it would have cooled down to much lower temperatures
already. But because "RX J0852.0-4622" is young it could have reached the
angular extent of 2 degrees only if it is relatively close to Earth,
otherwise just a small patch of X-ray emission would have been visible.
"Detailed analysis and comparison with the well-studied remnant of the
supernova which occurred in the year 1006 demonstrate that the new
supernova remnant can not be significantly older than 1500 years and it
can not be located at distances greater than 1000 parsec or 3300 light
years", Dr. Aschenbach explains. "And the low X-ray surface brightness of
"RX J0852.0-4622" can be attributed to a low matter density of just 0.04
gas and dust particles per cubic-centimeter, surrounding the star before it
exploded. This is indeed low compared with standard values being about 20
times higher."
The case that "RX J0852.0-4622" is a supernova remnant was finally settled
by the gamma-ray astronomers. They specialize in studies of the gamma-ray emission
from radioactive decay of atomic nuclei. During the sudden death of a star
in a supernova, which takes a fraction of a second, matter density and
temperature in the star reach levels at which atomic nuclei change and
reformat. Most of the chemical elements including their isotopes are being
released from the star to the world at supernova explosions, without these
stellar ashes no life would have been possible. The matter expelled in the
supernova process is further being used, it is the "raw material" for
formation of the next generation of stars and planets.
Many isotopes are not stable; they decay at a variety of times scales,
which can be measured as "life-time" and which is a characteristic of each
individual isotope. Eventually only the known "natural" chemical elements
remain. Often the decay of an isotope is accompanied by the emission of
gamma-rays of very specific energies. These gamma-ray lines are as unique as a
finger-print for each radioactive isotope. Among other elements titanium-44
forms in a supernova explosion. It is produced exclusively during "silicon
burning" and it decays over scandium to calcium by emitting a gamma-ray line
of 1.156 million electron volt. The same group of gamma-ray astronomers had
discovered this line for the first time from the well known young galactic
supernova remnant "Cassiopeia A" already back some years ago.With the
discovery of the titanium-44 gamma-ray line now from "RX J0852.0-4622" it was
clear: "RX J0852.0-4622" is a young closeby supernova remnant.
Despite the fact that the production yield of titanium-44 in
"RX J0852.0-4622" is not known -- it is being produced in every type of
supernova but at different rates -- the gamma-ray measurements could be used to
further constrain the age and distance of "RX J0852.0-4622" making use of
the "life-time" of titanium-44. It is concluded that the supernova occurred
in the 13th century at a distance of about 700 light years from Earth. "This
is the first time that a previously unknown supernova remnant has been found
by means of the titanium-44 gamma-ray line", Dr. Anatoli Iyudin from the Max
Planck Institute for Extraterrestrial Physics comments about this success.
Coming years are very likely to show further discoveries of supernova
remnants. In our Milky Way two to three stars in every 100 years are
expected to explode; this follows from a comparison with external galaxies.
But for the last 1000 years only seven remnants have been found so far. The
remaining supernovae and their remnants might have escaped detection because
the optical light might have gone lost in intervening interstellar gas and
dust clouds. Hard X-rays and gamma-rays are not blocked by these clouds, so that
the missing supernova remnants might be found soon by the instruments on
board of the next X-ray astronomy and gamma-ray astronomy satellites, which are
already being built and which are close to launch.
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