The Question

I was in South Carolina last night Oct 15th on a lake looking at the night sky. I was looking straight up and slightly to the north and I saw a medium sized star get really bright and then it disappeared as if it exploded. This event occurred around 7:00pm. Was this a supernova???? If not what was it????? Please let me know.

The Answer

What you saw was probably an "Iridium Flare." This is caused by the satellites of the Iridium mobile phone system, when they happen to line up just right. The Iridium consortium is now bankrupt (apparently nobody wanted to carry around a 10 kg mobile phone when 90% of where most business travelers go is served by cell phones), and the satellites will be deorbited unless a buyer is found. In the meantime, there are web-based programs to predict when you might see such a flare in the future (unfortunately, we don't know of one to tell you if what you saw in the past was a flare).

Anything lasting just a few seconds is much too short a time to be a supernova, or any other such astronomical event. It could have been a meteor that happened to be heading directly toward you so that you didn't see any streak, but your description sounds like an Iridium flare.

One prediction service is at

http://www.heavens-above.com/

You can read about the Iridium satellites and why they flare, as well as look at some photos at

http://www.satobs.org/iridium.html
http://antwrp.gsfc.nasa.gov/apod/ap980402.html

Hope that helps.

-Kevin Boyce and Martin Still,
for "Ask an Astrophysicist"

The Question

I'm in 8th grade in a Science class and my question is what is a supernova and how does ita ffect us?

The Answer

A supernova is an explosion at the very end of the life of certain types of stars. You can find more information about supernovae at

http://starchild.gsfc.nasa.gov/docs/StarChild/universe_level2/stars.html

in the section about massive stars.

There is much more information also included in our Imagine the Universe! web site you can find by going to:

http://imagine.gsfc.nasa.gov/

and click on the search button and asking it to search for "supernova".

These pages explain what a supernova is, how they happen, and the different kinds of supernovae. These will answer your question about what a supernova is.

Your other question was about how supernovae affect us. They can affect us in some important ways. First and foremost, we and much of the Earth are made of the material supernovae created. According to current theories about the formation of the universe, all of the original material in the Universe was hydrogen and helium, with very slight traces of some other materials. All the stuff we, and the Earth around us, are made of, like iron and oxygen and carbon, has come from that initial material being fused to form heavier elements in the cores of stars. But the heaviest elements, like iron, are only formed in the massive stars which end their lives in supernovae. Our blood has iron in the hemoglobin which is vital to our ability to breath. So without supernovae, most forms of life on Earth, including us, would not be possible. And much of the material the Earth is made of would not exist.

Supernovae also create shock waves through the interstellar medium (the stuff between stars), compressing material there. Astronomers believe that these shock waves are vital to the process of star formation, causing large clouds of gas to collapse and form new stars. No supernovae, no new stars.

Supernovae throw much of the material from their parent star back out into the interstellar medium, changing its chemical composition. This adds many elements to the interstellar medium which were not present before, or were only present in trace amounts. Other less massive stars also enrich the interstellar medium, but lack many of the heavier elements. The gradual enrichment of the interstellar medium with heavier elements has made subtle changes to how stars burn: the fusion process in our own Sun is moderated by the presence of carbon. The first stars in the Universe had much less carbon and their lives were somewhat different from modern stars. Stars which will be formed in the future will have even more of these heavier elements and will have somewhat different life cycles. So supernovae play a very important part in this chemical evolution of the Universe.

Jesse Allen
for Ask an Astrophysicist

The Question

What is a nova? Can you please simplify this for me since I'm new to astronomy? What is the relationship between a Nova and a supernova?

The Answer

A nova is a sudden brightening of a star. Novae are thought to occur on the surface of a white dwarf star in a binary system with another star. If these two stars are close enough to each other, material from one star can be pulled off its surface and onto the white dwarf. Occasionally, the temperature of this new material on the surface of the white dwarf may become hot enough to start nuclear fusion and suddenly the surface of the white dwarf will start to fuse the hydrogen into helium over its surface. This causes the white dwarf to suddenly become very bright. Ancient astronomers, who did not have telescopes and other instruments modern astronomers now have, did not realize that there was a star already there, and so they would just see a new star where they had not seen one before. "Stella Nova" means "new star" in Latin and this is where novae got their name. Supernovae were once thought to just be really bright novae (hence the addition of "super" to their name). If you look at

http://imagine.gsfc.nasa.gov/science/objects/supernovae2.html

you can see there are two types of supernova, one of which occurs in the same binary systems as nova. If a nova fails to clear enough material off the surface of the white dwarf, enough may collect for the entire star to be destroyed by a very large (a "super") nova explosion. The other type of supernova is from the end of a massive isolated star and is not related to nova at all.

Jesse Allen
for Ask an Astrophysicist

The Question

Who discovered what a supernova is? How did they discover it?

The Answer

Fritz Zwicky, Walter Baade, and Rudolph Minkowski developed the theories about what supernovae are in the 1930s.

http://lycos.infoplease.com/ce5/CE050177.html

It took a while for these to be confirmed by various techniques. (e.g. the discovery of pulsars inside supernova remnants.)

David Palmer and Samar Safi-Harb
for Ask an Astrophysicist

The Question

I know that there are different types of supernova explosions:

• Ia
• Ib
• Ic
• II

What do these mean? Do they account for how big the explosion is?

The Answer

Thank you for your question. As is the case with most astronomical objects and phenomena, supernovae are classified according to their basic observable characteristics -- in this case particular features of their optical spectra and behavior of their "light curves" (how their brightness changes with time).

For example, the Type I (Ia, Ib and Ic) are distinguished from Type II by the absence of hydrogen spectral emission lines in the former (and their presence in the latter). In addition, type Ia are always the same mass and thus the same luminosity, which is why they are considered to be standard candles.

For more details, please see one or more of the following sites:

http://imagine.gsfc.nasa.gov/science/objects/supernovae2.html,
http://en.wikipedia.org/wiki/Supernova,
http://rsd-www.nrl.navy.mil/7212/montes/snetax.html,
and
http://www.jca.umbc.edu/~george/html/courses/2002_phys316/lect12/lect12_sn_basics.html,

where you can also find information on the different physical characteristics of these different types.

Most supernovae of all classification are believed to have roughly the same amount of energy released.

-- Michael Loewenstein and Amy Fredericks for "Ask an Astrophysicist"

The Question

How long does a supernova last?

The Answer

There is no single answer to your question.

In a sense, the explosion itself is over within a matter of seconds.

But the envelope of the dying star is expelled with such speed that, when it ploughs into the interstellar gas, it is heated to millions of degrees and remain bright in X-rays for tens of thousands of years.

In the visual light, how long you can track supernovae depends on their distance. Supernova 1987A in the Large Magellanic Cloud (which is relatively nearby as these things go) is still being followed from the ground and from the Hubble Space Telescope:

http://oposite.stsci.edu/pubinfo/pr/1998/08/

Best wishes,

Koji Mukai
for Ask an Astrophysicist

The Question

When a star of adequate mass ceases to have enough fusion to support its self, collapses and goes supernovae just exactly WHERE does the energy come from that causes the explosion? The answer provided on your site simply stated that the star collapsed and energy was produced. So where does it come from?

The Answer

The short answer is that the energy released in a supernova explosion comes from the gravitational energy released as the star collapses. Think about an object falling from a very high building. You may know that this object as it is falling is transforming its potential energy into kinetic energy --- The same happens for a star iron core which collapses: it transforms its gravitational energy to heat and motion --- When the collapse is stopped by the formation of a neutron star able to sustain its own gravitational pressure because of the fermion pressure, there is a "bounce" and the star explodes.

Now the detailed answer is much, much more complicated than that and in fact, is a field of research by itself.

The large amount of gravitational energy available from the collapse of the core of the star is indeed transformed to heat and kinetic energy but the problem was that this is not the part which explode. The part which goes flying off the surface is a relatively loosely bound envelope of matter (hydrogen and heavier elements up to silicon)--- How the heat from the gravitational collapse of the core is transfered to the rest of the star was a heated discussion (no pun intended :-)) --- The solution of the "transfer" of energy may lie in neutrinos, these elusive particles which are supposedly massless, neutral and react very weakly with matter. These neutrinos are ejected carrying most of the energy of the gravitational collapse and in the process they give a little (around 1%) of their energy to the envelope which then "explodes."

The detection of a burst of neutrinos (7 total compared to the billions ejected in the explosion) right before the detection of SN1987A is considered to be the most impressive confirmation of the theory.

For more information you may want to check scientific articles dated from around the discovery of the neutrino burst (check "Scientific American" and "Sky and Telescope" index for neutrinos and SN 1987A).

We hope this answers your question.

Koji and Ilana for
the "Ask an Astrophysicist" team.

The Question

How are supernovae detected and how can I find out when the last one occurred?

The Answer

Most bright supernovae (SNe) are discovered by amateur astronomers. People who know the sky really well go out and look at many galaxies with their small telescopes to see if any look unusual. Then they contact an observatory, which then notifies the astronomical community via e-mail.

In addition to the amateur astronomers there are a number of new automatic searches which find many more, fainter, supernovae. We saw an announcement in February for supernova 1997fg. This implies that at least 163 SNe were discovered last year. Most of them were very faint, and reported a dozen or so at a time.

In 1987 there was a supernova in a companion galaxy to ours, that is still a very interesting object (SN1987A). This was the last "nearby" (i.e. less than a million light years away) supernova.

The last known supernova in our galaxy was in 1680. However it was so dim that it was thought to be a star, and historically shows up only as an anomaly in the sky charts of John Flamsteed. It is now extremely bright in radio waves and X-rays and is known as "Cassiopeia A."

Most recently, 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 is claimed to have been created about 700 years ago when a star exploded in the southern sky in the constellation Vela ("sail")

Good luck,
Jonathan Keohane, David Palmer & Karen Smale
for Ask a NASA scientist

The Question

Can a supernova be seen with the naked eye? What are the odds of seeing a supernova with the naked eye?

The Answer

Several supernovae throughout history have been seen with the naked eye. However no naked eye supernovae have been seen since 1987 (and then only in the southern hemisphere, and the last one before that was about 400 years ago). You can see the most recent listing at:

http://cfa-www.harvard.edu/iau/lists/RecentSupernovae.html

Under the darkest conditions we can't really see anything dimmer than about magnitude 6.5 with the naked eye, but here are details of some of the brighter (< Mag 17) ones that you may be able to see with a telescope:

http://www.rochesterastronomy.org/supernova.html

Hope this helps,
Mike and Georgia
For "Ask an Astrophysicist"

The Question

You mentioned in the introduction to supernovae that they are "relatively rare in our own galaxy". I am taking an introductory astronomy course and learned that a galaxy the size of our Milky Way should have about five supernovae per century. so why haven't more supernovae been seen in our galaxy in the past century and even in the last 1,000 years?!

The Answer

This question has perplexed astronomers studying our Galaxy for years. We think, based on theory, that a certain amount of stars would explode in our galaxy during a century. This number then seems to match other galaxies reasonably well. However, for our own Galaxy there hasn't been much evidence for a supernova in hundred of years.

One answer is just that the number 5 is an average. Meaning that in some centuries you'll have a few more, and in others a few less. We may just be at one of the low points, and then in a century or two there will be more that we can see. Another explanation is that there may have been several supernovas, but they were blocked out by huge clouds of gas and dust which absorb visible light.

There could be a "deeper" answer....i.e. the formation of massive stars in our neighborhood or whole Galaxy may have been "shut off" at some point, only to have turned back on more recently. But since we see very many massive stars (the kind that become supernovae), it seems unlikely that their production would have been shut off at any time in the past or that it would ever bee noticeable to us if it had.

Steve Bloom
for Ask an Astrophysicist

The Question

I am an educated layman with a question. I have often read that supernovae are the prime source of elements heavier than helium in the ism, but I wonder about formation of planetary nebulae and novae. They may be less efficient at adding to the metal inventory of the ISM, but they are more numerous. How then do astrophysicists identify supernovae as the better source overall? Are statements like "Most of the atoms of carbon, oxygen and nitrogen in our bodies came from supernovae." justified?

The Answer

What a good question! It is absolutely true that virtually all of the elements heavier than iron are from supernovae, because stars do not make these, since the element with the highest binding energy per nucleon is iron.

As far as other elements (Fe and lighter), they are mostly made in stars. The heavier the star, the more different elements it will make, and the shorter its life. Our Sun, on the other hand, will not make much past helium. In addition, it is only the more massive stars that go supernova. So, if a star makes iron, it will likely go supernova.

In short, elements below Fe are mostly made in stars, while heavier elements are made predominately in supernovae. Now, your question is about the distribution mechanism. Supernovae are by far the best mechanism for ejecting these elements very far away from the star.

In addition, most all of the other mechanisms like stellar winds involve the outer layers of a star. Because they are made toward the center of the star, one would expect heavy elements to stay toward the middle of stars where they are produced, while the outer layers blow off lots of hydrogen.

That said, it is also thought that much of the CNO elements come from asymptotic giant stars burning helium, convectively dredging it to the surface, and then blowing it off in a wind.

So, all in all, it is safe to say that supernovae are the primary distribution mechanism for heavy elements in our galaxy, but they are not the only one.

Thanks for the good question.

Jonathan Keohane, Mark Kowitt and David Palmer
for Ask a NASA scientist

The Question

How could an element heavier than iron be created? I ask this question because I read somewhere that when a star could not burn iron it exploded into a supernovae.

The Answer

When a supernova explodes, it gets hot enough to drive nuclei and particles together. Nuclei absorb neutrons and protons and can grow to be heavier than iron. This absorbs rather than releases energy, but there is enough energy available in a supernova that this happens anyway.

David Palmer
for "Ask an Astrophysicist"

The Question

Do any of your programs and/or archives address the X-ray/gamma-ray spectra of supernovae? Specifically, I am interested in determining the isotopic abundances that might be apparent in the effluvia surrounding supernovae and, especially, the one that occurred just a few years ago and was visible from, say, the observatory in Chile. Do you have any information and/or references that might be of value to me? Please note, again, that I am interested in isotopes, not elements. The latter would be apparent in visible spectra. Rather, I am interested in the relative abundances of the various isotopes.

The Answer

X-rays and gamma-rays can help us understand the isotopic abundances from supernovae. However, only for particular radioactive isotopes. Let me explain:

In a supernova explosion many elements and isotopes are created, mostly through the rapid process (r process). However, most of these are unstable so they decay down to a stable or more stable isotope. It is during this decay process that they give off gamma-rays. The energy of the gamma-ray will depend on the particular isotope and the process involved. These gamma-rays are observed as spectral lines within the overall spectrum.

Now a word about 1/2 life. The 1/2 life of a particular isotope is the time it takes for half of the initial quantity of the isotope to decay. So, observing a supernovae early on one can observe a number of these spectral lines, however over time they fade as there become fewer radioactive isotopes and more stable isotopes.

For example, the element Titanium 44 decays with a 1/2 life of about 70 years. The decay process gives off 1 MeV, 60 and 70 keV photons. By counting these photons, the mass of Ti 44 produced in the supernova explosion can be estimated.

Thank you for your question. As far as particular details of the supernova 1987A, I suggest that you do a literature search on this topic in the leading astronomical journals. This can be easily done on-line from the URL http://adsabs.harvard.edu/

Jonathan Keohane
(for Imagine the Universe!)

The Question

From what I've read, the elements heavier than Iron in atomic weight can only be created by supernovae. Similarly, elements beyond Hydrogen, helium and Lithium can only be created in the core of stars during the process of their lives.

The constituent elements of stars including our Sun can be determined by spectrographic analysis. I believe that our Sun, and many other stars already have heavy elements within them which indicates that they were, at least in part, formed by material from ancient Supernovae. I believe spectrographic analysis can also be used to determine the overall distribution of matter in whole galaxies just like it is used on stars (I'm speculating this, since I've never heard it before, but it seems logical).

Since Supernovae, and stellar evolution occur over time, there should be more heavy elements now (at our present point in cosmic history), than there were near the beginning. So what do we see when we look back in time at distant galaxies.

My questions are:

If we do spectrographic analysis on galaxies from oldest to youngest, does a change in the distribution of heavy elements show up?

If so, what does it tell us about the rate at which Supernovae occur on average?

How many Supernovae per galaxy per century are required to account for the current distribution of heavy elements found in stars?

The Answer

First, it does not require a supernova to create elements heavier than iron. Heavy elements can also form in the cores of massive stars before they go supernova (s-process isotopes). Secondly, some elements beyond helium are formed in planetary nebulae. Some can also be formed through cosmic ray collisions. So the picture is a bit more complicated.

Now for your questions. Galaxy metallicity (the fraction of heavy elements) can be derived from emission line spectroscopy of planetary nebulae and H-II regions in nearby spirals and irregulars, and from absorption line spectroscopy of large ensembles of stars in elliptical galaxies. The metallicities of galaxies depends on the star formation history (how many generations of supernova producing stars) and whether the newly synthesized metals can be retained. The latter depends mostly on mass (i.e., gravitational binding energy) --- low mass galaxies lose a lot of metals in galactic winds.

On getting metallicity of distant galaxies from optical spectroscopy: it's a much tougher thing than getting the metallicity of a star. For a star, you solve for temperature, gravity, and chemical abundances. But a galaxy has a population of stars of different temperatures, gravities, and chemical abundances (from old M dwarfs that have been around for billions of years to O stars that were borne yesterday). You need a pretty good idea of the stellar populations before you can make any inferences on the metallicity.

Another problem: you need high resolution, high signal-to-noise spectra. cosmologically distant galaxies (the ones which may show a significant difference) are all too faint (at least they were, before the days of 10m class telescopes).

For a typical stellar population one supernova is produced for every 100 solar masses of stars formed --- this can give a rough idea of the rate in a galaxy of a given age and mass (in a galaxy like the Milky Way, 1-10 per century or so). The rate of Supernovae is probably not constant over the age of the galaxy, and also is not uniform across the width of the galaxy. Metallicity increases the closer you get to galactic center, due to the higher density of stars.

Cheers,

Hans Krimm, Bram Boroson, Eric Christian, Kazunori Ishibash, Mike Loewenstein, and Koji Mukai for "Ask an Astrophysicist"

The Question

Approximately how much do supernovae expand when they blow up?

The Answer

A supernova remnant can become very large, but depends on its age. There are large bubbles of hot gas within our galaxy that extend for hundreds of light years from older supernovae. The Crab nebula, which is believed to have been formed from a supernova in 1054, is currently expanding at 1450 km/s which, after a little hand waving math, makes it roughly five light years in radius (assuming a constant expansion velocity, which is not correct in the early phase of the supernova, but is roughly correct over most of the 1,000 year age of this remnant). So had the Crab supernova been at Alpha Centauri distance when it went off, the blast wave would be coming at us or recently past us.

Jesse Allen
for "Ask an Astrophysicist"

The Question

I just watched a movie on your site showing a Hubble view of a1987 supernova. My question is why the energy expanding around the supernova is in the form of a ring, and not a sphere? The only possible answer I can think of is that it has something to do with the rotation of the star.

The Answer

This is a very good question. This ring around SN1987A already existed before the supernova explosion; the light from the supernova event just lit it up. The actual supernova is the point-like source in the center of the picture. It has only recently begun to be resolved with the highest resolution telescopes.

The explanation of one article I read, and the one that makes the most sense to me, is more-or-less as follows. The very massive star that existed before the supernova explosion, blew off very strong stellar winds. Probably because this star was spinning a denser region formed in the equatorial plane around the star -- kind-of like a planetary disk. Then as the star changed from a red supergiant to a blue supergiant, the winds changed. The winds then became focused to the poles of the star, making an hour-glass shaped region of material. This material was not observed until the light from the supernova hit it.

What we see is like an echo. The initial blast of radiation from the supernova ionizes the material, so it emits light.

Something you should know, however, is that spherical shells usually look like rings on the sky. In the case of 1987A it is probably really a ring (at least the inner ring is), but often something is not. This is because on the edge of a shell lots of material is along our line on sight, while in the middle has less. So, we see a ring not a shell even when something really is a shell. This is often observed in stars and older supernova remnants.

Good luck,

Jonathan Keohane
for Ask an Astrophysicist

The Question

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

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.

The Question

I am writing an article that will attempt to refute a creationist claim that all supernova remnants in our galaxy appear to be less than 10,000 years old, based on the "well-established" decay pattern of a supernova's light intensity in the radio-wave frequency range. I would appreciate any information that you could give me, even just a reference or two, or a referral.

The Answer

Thanks again for asking such a thoughtful question. I have broken my answer into two parts: I. My Personal Opinions and II. A Scientific Answer. The first part is based on my experience in my discussions with creationists, while the second part is the real scientific answer to your question.

I. My Personal Caveat

Before I give you your scientific answer about supernova remnants, I want to say from my personal experience you are probably bound to fail in your attempt to change the mind of any true creationist. The reason is simply that the evidence against the age of the universe being only 10,000 years old is so overwhelming, that yet another piece of evidence will not sway any "dyed in the wool" creationist.

In addition, no scientific argument can counter the non-scientific "theory" that God made the world 10,000 years ago (or any other length of time) exactly as it was then. That is to say, with the particular abundance ratios of radioactive elements, stars that appear to be billions of years old, and an expanding universe. By definition this "theory" is untestable, therefore it is unscientific in the true sense of not being testable.

In my experience, the only arguments that have worked with my creationist friends are ones that are completely non-scientific, but theological, in nature. Arguments that imply that the creation stories in the Bible were not written to actually explain how, or when, the Earth was created (why would there be 2 of them if that were the case?), but were written to say something to us about our own human condition, our faith, or even why the Earth was made. Science explicitly makes no effort to answer these "why" questions, so whatever answer a person comes up with is compatible with the scientific data.

II. The Scientific Answer to your question

The gist of the creationist's argument is right, observations of ongoing radioactive decay in supernova remnants can only date the very young ones. The expansion method of dating a supernova remnant (as describe in the www page from my last message) similarly only works for young supernova remnants (up to about 10,000 years old).

There are ways to date older supernova remnants (ages > 10,000 years), however they are not very accurate. These methods involve X-ray observations which measure the temperature of these supernova remnants. From the temperature, one can estimate the speed of the shock wave, from the speed of the shock wave one can estimate the age. Using these methods, we observe supernova remnants up to abound 100,000 years old, when they fade into the interstellar medium.

Now if the goal of this is to find the age of the universe, supernova remnants are not the objects to look at. This is simply because they become mixed up with the interstellar medium after only about 100,000 years. The universe is much older than that, which we know from the oldest stars (on the order of 10,000,000,000 years old). In addition, we observe distant objects that are billions of light years away.

The most solid evidence for the Earth being old are the products from long half-life radioactive decay found in meteorites and rocks on the Earth. For example: Potassium 40 (40K) decays into the gas Argon 40 (40Ar) with a 1/2 life of 1.3 billion years. As long as a rock remains a rock, this 40 Ar remains trapped. If the rock melts, the Argon escapes. So, by measuring the amount of 40K and 40Ar in a rock, geologists can measure its age.

Common elements used for this are Potassium 40 (1/2 life = 1,300,000,000 years), Uranium 238 (1/2 life = 4,500,000,000 years), Rubidium 87 (1/2 life = 47,000,000,000 years).

These studies clearly show that the Earth is at least 3.9 Billion years old, because that is the age of the oldest rocks. The oldest meteorites are about 5 Billion years old.

Good luck,

Jonathan Keohane
-- for "Ask an Astrophysicist"

The Question

There are some scientists that proposed that a supernova occurred 65 million years ago at a distance of 130 light years from Earth that could be the engine of dinosaur extinction.

Is it possible to determine if an event of this nature occurred during this time frame? Or at another earlier time?

The Answer

Supernova have been suggested as possible culprits in mass extinctions many times. For example, there's a paper about how supernovae could cause mass extinctions on the Los Alamos National Labs e-print server by Juan Collar at

http://xxx.lanl.gov/abs/astro-ph/9505028
(be sure to follow the 'cited-by' links to get differing viewpoints).

However, in the case of the Cretaceous-Tertiary (KT) extinction that killed the dinosaurs 65 million years ago, there is ample evidence of both an asteroid strike and catastrophic volcanic upheaval (the Deccan Traps). Adding a supernova to these events is probably unnecessary. This is not to say that a supernova could not cause a mass extinction, just that it probably didn't cause that one.

As for evidence, supernova remnants only remain detectable for a few tens of thousands of years. When supernovae form pulsars, their typical velocities are a few thousandths of lightspeed, so in 65 million years, a pulsar could have traveled from near Earth to any point in the galaxy and had numerous encounters with other stars, randomizing its velocity, so we could not find a particular pulsar and discover that it was in the right place at the right time.

Cosmic rays from a nearby supernova could cause a detectable change in isotopes on Earth. That was one of the first explanations Luis Alvarez thought of when he found the iridium at the KT boundary. However, other elements and isotopes which would be expected from such an event were not found, leading to the meteor theory.

David Palmer
for Ask an Astrophysicist

The Question

Is there a possibility that a nearby star could go supernova and destroy the earth? Or have other bad effects on us?

The Answer

To destroy the Earth itself, the Sun will have to go supernova (which it never will).

If you are talking about the life on Earth, then there is a detailed calculation of the risks due to a nearby supernova on the web:

http://stupendous.rit.edu/richmond/answers/snrisks.txt

The author concludes that a supernova has to be within 10 parsecs (30 light years) or so to be dangerous to life on Earth. This is because the atmosphere shields us from most dangerous radiations. Astronauts in orbit may be in danger if a supernova is within 1000 parsecs or so.

No stars currently within 20 parsecs will go supernova within the next few million years.

There are some indirect effects, though, which are harder to evaluate: the possible effects on the Earth ozone layer is listed in the article above. Additionally, according to one calculation, the neutrino flux from a nearby supernova might heat up the Sun.

Best wishes,

Koji Mukai & Eric Christian
for Ask an Astrophysicist

The Question

I have just read that our solar system was likely formed from the effects of an earlier supernova. Does this mean that there should be some residual object like a neutron star (or even a black hole) somewhere in the neighborhood?

The Answer

Thank you for your question. There indeed is some speculation that the compression of Giant Molecular clouds by supernova explosions is instrumental in causing these clouds to collapse and form stars and solar systems (such as our own). This is called stochastic star formation, and is thought to cause the "starburst" regions seen in other galaxies.

Any ancient remnant from such an explosion would be quite inconspicuous. Solitary black holes are practically unobservable, as are isolated old neutron stars (young neutron stars may shine as pulsars). They could continue to shine by proxy if they were encircled by hot disks of gas continually fed by a binary companion, but it may be difficult for such systems to remain intact after the explosion.

Any such object is unlikely to still be in our neighborhood anyway. The Sun has orbited the center of our galaxy a couple of dozen times or so since it was formed and any object nearby at that time that is not bound to the Sun (as the Earth is) is extremely unlikely to be nearby now. Moreover, the explosion might have imparted a high speed to the collapsed remnant so that it is not even in its original galactic orbit.

We're afraid the chances of identifying the culprit are rather remote.

-- Michael Loewenstein and David Marsden
for "Ask an Astrophysicist"