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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 an Astrophysicist

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


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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