* Astronomy

If you've ever wondered where the iron in your blood comes from, it was forged in the heart of a massive star. But the gold and silver in your jewellery, plus mercury, lead and other heavy metals we find useful, were created when that star exploded in a supernova. When a supernova explodes, it ejects huge amounts of gas and dust into interstellar space, where they become the building blocks of stellar systems like our solar system. Remnants of recent supernova explosions therefore have much to tell about the origins of our world.


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Position(2000): RA 05 : 34.5 Dec +22 : 01

A University of Minnesota team of astronomers has studied the Crab Nebula, a filamentous remnant of a star that exploded in A.D. 1054 in the constellation Taurus.
Using the new Spitzer Space Telescope, which operates at infrared wavelengths, the team found that a crucial type of dust has gone missing. The astronomers will present their findings at 10 a.m. Wednesday, June 1, at the American Astronomical Society meeting in the Minneapolis Convention Centre.

The team leader was graduate student Tea Temim. Her advisers, astronomy professors Robert Gehrz and Charles Woodward, are among the co-authors of the paper. They are all infrared astronomers interested in how cosmic dust forms and recondenses into a new generation of stars with planets.
Our sun is at least a second-generation star, built from the dusty, gaseous wreckage of previous stars.
Cosmic dust is composed of any fine solid particles in space except ices. It is much finer than beach sand; it's more like smoke particles. It forms from the aggregation of gas molecules and very fine particles into even bigger particles--like the nucleation of water to make raindrops. Since very fine dust has been observed to form in the ejected material of another supernova only a few years after it exploded (Supernova 1987A, detected in 1987), the scientists expected to find plenty of it in a relatively young supernova remnant like the Crab Nebula.
Instead, the Spitzer data showed only much coarser dust--particles that, although only a few millionths of a meter in size, were still 10 to 100 times larger than the fine stuff.
The big question is, what could have destroyed or spirited away the smallest dust particles?
One possible culprit is the rapidly spinning neutron star at the core of the Crab Nebula. It is pumping out intense ultraviolet radiation, which might be evaporating small particles. The core is also throwing out protons and electrons at speeds approaching the speed of light, and these could also be destroying the fine dust. One reason this is an attractive explanation is that fine dust forms within a year of an explosion, but the intense radiation from the core doesn't develop until long after coarse dust has coalesced.

Temim has mapped the energy distribution from these ultra fast particles (called synchrotron radiation) coming from the neutron star. This will show how these particles spread out and mix with the rest of the ejected material.
The Crab Nebula( also know as M1) is 6,500 light-years away--that's 30,000 times the distance of Earth from the sun, in the constellation Taurus. The nebula is five light-years across, which is bigger than the distance between the sun and its nearest star.
The Crab Nebula has been photographed in the X-ray, radio and visible parts of the spectrum.

"The images are exciting because we're filling in the puzzle with infrared. Infrared is where we can find out information about the formation of dust" - Tea Temim.
These images could not have been made from a ground-based telescope because Earth's atmosphere filters out much infrared light. Spitzer can pick up faint infrared signals only because it is cooled to minus 450 degrees F; if it were much warmer, its own heat would drown out the signals from the Crab.

A note on how dust forms in a supernova. A massive star, during the course of its life, derives its energy from synthesizing elements as heavy as iron (of the 92 naturally occurring elements, iron is the 26th heaviest). When the star uses up its fuel, its core collapses, forcing protons and electrons together, forming neutrons. But, as Isaac Newton said, every action has an equal and opposite reaction.
After collapsing, the star explodes, shooting neutrons and small atomic nuclei into each other to form new nuclei. Some of these nuclei are bigger than iron. Ordinarily, they would undergo radioactive decay, but as they are shot away from their parent star by the explosion they cool before decay can occur.
All these elements coalesce to form dust. The iron in our blood comes from the iron forged in these big stars, and the gold, silver, mercury, lead and other heavy elements we find so useful are forged in the supernova explosion.


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This spectacular supernova explosion was recorded by Chinese and (quite probably) Anasazi Indian astronomers. According to the records, it was visible in daylight for 23 days, and was about four times brighter than Venus( about mag -6) , and 653 days to the naked eye in the night sky.
The filaments are mysterious because they appear to have less mass than expelled in the original supernova and higher speed than expected from a free explosion. In the above picture taken recently from a Very Large Telescope, the colour indicates what is happening to the electrons in different parts of the Crab Nebula.
Red indicates the electrons are recombining with protons to form neutral hydrogen, while blue indicates the electrons are whirling around the magnetic field of the inner nebula. In the nebula's very centre lies a pulsar: a neutron star rotating, in this case, 30 times a second.
In the visible light, the pulsar is of 16th apparent magnitude. This means that this very small star is roughly of absolute magnitude +4.5, or about the same luminosity as our sun in the visible part of the spectrum !