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.

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.

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.