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The Beginner's Guide to Making a Star

Astronomers using the Chandra X-ray Observatory are seeing how supernovae spray the essential elements of life into interstellar space.

by Dr. Tony Philips

In order to make an apple pie from scratch, you must first create the universe. Carl Sagan

  Medical students toiling over their homework in organic chemistry might be excused for day dreaming about a universe without carbon or oxygen. University essentials like spaghetti and baked beans would be unheard of in a carbon-deprived cosmos, but even the toughest chemistry course would be a snap!

Such a universe existed about 10 billion years ago. According to the Big Bang theory, just before the first stars formed the cosmos was made entirely of the three simplest atoms: hydrogen (H), helium (He) and small amounts of lithium (Li). It was every beleaguered chemistry student's dream come true. There were no oxygen or carbon atoms or any of the elements that make up the bulk of the periodic table today.

A quick glance around the room is proof that times have changed. Light elements that filled the early universe are rare on Earth, while our planet and we ourselves consist substantially of the heavier atoms, like oxygen and nitrogen, which were missing at the dawn of cosmic history.

What happened between then and now? Star formation.

Periodic Table

The first stars that collapsed from primordial gas clouds 10 billion years ago were made of hydrogen and helium. As gravity caused these newborn stars to contract, temperatures near their cores reached levels that triggered nuclear fusion, the same process that powers our Sun today. In a stellar fusion reactor, lightweight atoms like H and He are smashed together to form heavier species. Hydrogen and helium is transmuted to elements like carbon, nitrogen, and oxygen in the cores of mid-sized stars. Stars that are ten times or so more massive than the Sun can assemble atoms as heavy as iron (Fe) before they explode as cataclysmic supernovae. Atoms more massive than Fe form in supernova blast waves (accounting for useful substances like plutonium and uranium). The debris from supernovae seed interstellar clouds with the raw materials for rocky worlds and carbon-based life.

In the immortal words of Carl Sagan, "we are star stuff."

Scientists using NASA's Chandra X-ray Observatory are enjoying a close up look at star stuff emerging from one of the youngest supernova remnants in our Galaxy. Cassiopeia A - Cas A for short - is the remnant of a star that blew itself apart about 9,400 years ago. It is relatively close - only about 9,100 light years away - so it should have been bright enough to cause a stir when it appeared in the night sky in the mid-1600s, but most astronomers missed the explosion. Sir John Flamsteed, Britain's Astronomer Royal, may have seen Cas A in 1670. If so, he misidentified it as a star and made no follow-up observations. No other sightings are known.

Cas A is very faint when viewed in visible light but its huge shell (13 light years across) is filled with 30 million °K gas that glows brilliantly at x-ray energies. The remnant is big, about one-sixth the width of the full moon as seen from Earth, so it makes an attractive target for the high-powered Chandra X-ray telescope.

Moon v Cas A

The Cas A Supernova inset against a picture of the moon to show their comparative sizes.

Chandra carries an instrument called the Advanced CCD Imaging Spectrometer (ACIS) that can measure the energy of incoming x-ray photons and associate them with specific chemical elements. Using the ACIS, astronomers are taking pictures of Cas A that reveal the distribution of heavy atoms like oxygen, silicon and iron in the supernova's rapidly expanding shell that show how those elements are mixing into the ambient interstellar medium of gas and dust.

Recently, NASA released images of Cas A at x-ray wavelengths emitted by ions of silicon (Si), calcium (Ca), and iron (Fe). On the eastern side of the supernova's shell, Ca and Si images reveal a high speed jet erupting into a relatively low-density region of the interstellar medium. Scientists speculate that the jet might signify an asymmetry in the original supernova explosion. On the opposite side, observations at radio and other wavelengths indicate that Cas A is plowing into an interstellar molecular gas cloud that confines the shell's outward flow.

X-ray Images NASA/GSFC/U. Hwang et al

Chandra X-ray Images of the Cas A supernova remnant

There are intriguing differences between the maps of Ca and Si and the map of Fe, which is clumpier and does not show the jet so clearly. Material rich in iron comes from the inner core of the star where fusion temperatures were highest. Scientists have examined these maps carefully and note that iron-containing knots from deepest in the star seem to be nearest the outer edge of the remnant. This means they were flung the furthest by the explosion that created Cas A.

Cas A's outer envelope is expanding at 800 km/s (about 1.73 million mph). That's rapid enough that images taken by Chandra over the years will show how knots in the shell change and cool. By monitoring these changes, Chandra scientists hope to learn more about how quickly and in what form different elements are deposited into the interstellar medium.

Star Map

Cassiopeia A is too faint to view with the unaided eye, but it's easy to see where the remnant is in the northern summer sky. Simply go outside just after sunset and look 45 degrees above the north-north east horizon between the constellations Cepheus and Cassiopeia.

Even after more than 10 billion years of star formation, hydrogen and helium still are overwhelmingly the dominant atoms in the cosmos. Heavier atoms like the ones we see in the shell of Cas A are over represented on Earth because H and He are volatile gases that solar heating drives from the low-gravity terrestrial planets. Massive Jupiter, on the other hand, is made up almost entirely of hydrogen, as is the Sun.

Heavy elements may be no more than rare cosmic pollutants, but they are exceedingly important to us. Without them, solid, rocky planets would be impossible, and the prospects for Earth-like life would be correspondingly dim. As it is, the iron we see now in Cas A might one day flow as hemoglobin in the blood of some future alien species. Fast moving knots of silicon from the supernova could provide the raw material for sand on otherworldly shores, where crashing waves of H2O send thunderous sound waves through a nitrogen-rich atmosphere. And just perhaps, on that fanciful alien world, hardworking science students distracted by the beckoning sounds of distant waves might wish for less organic chemistry and more time on the beach.

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First Science 2014