What did the first stars look like?

You get no credit if you only said big and round! That much is a given.

The first stars were indeed very big, perhaps over 100 times more massive than our sun. They were also composed solely of the hydrogen and helium created in the big bang. These two features---very massive and pure---make them different from all the stars in existence today. The first stars were so big and hot that they burned blue, not yellow like our sun.

Let's start from the beginning---I mean, really the beginning. Scientists say that space, time and matter as we know it were created 13.7 billion years ago in a big bang. Another 200 million years would pass before stars formed. During much of this time, between the fireball of the big bang and the era of first starlight, the universe was steeped in darkness.

Star formation occurred when hydrogen and helium created in the big bang slowly condensed into gas clouds under the force of gravity. There were no other elements in the universe at this time aside from a tiny amount of lithium. That is, no oxygen, carbon or the other elements that make life possible.

When the cloud grew dense enough, high pressure and temperature initiated nuclear fusion in the cloud core, the signature of star birth.

Stars glow by first burning hydrogen. The "ash" of all that hydrogen burning produces helium. Once the hydrogen is gone, the star starts burning the helium. The "ash" of helium burning is carbon and oxygen. You might see where this is going. As stars burn, they create new (heavier) elements from lighter ones.

When a star runs out of fuel, it explodes. The explosion sends all the newly minted elements into space. This means that the stars that came after the first generation of stars were "dirty," containing the dozens of chemical elements that were floating in space instead of just hydrogen and helium.

Scientists have not yet seen these first stars. Being so hot, the stars burned quickly. They were gone in just a few millions years, as opposed to our sun, which is about five billion years old.

There is a debate about how big a star can get. Some scientists say that when stars have only hydrogen and helium, then can get very big. Most scientists say the first stars were 50 to 200 times as massive as the sun, yet some speculate that 1,000-solar-mass stars existed. The presence of heavier elements forces modern stars to be smaller, no larger than 50 solar masses, according to theory.

Clearly there are many unanswered questions!

In October, scientists at NASA Goddard using the Spitzer telescope said they might have detected a diffuse glow of the very first stars. They described the observation as seeing the glow of a distant city at night from an airplane. The light is too distant and feeble to resolve individual objects. To actually see the first stars, they said, they will need to wait for the successor to the Hubble Space Telescope, a mission called the James Webb Space Telescope.

In September scientists using the Swift telescope detected the most distant explosion ever seen, a gamma-ray burst from a very early star that exploded nearly 13 billion years ago. While this wasn't part of the first generation of stars, it was sure close. The observation implies that Swift could detect the first stars when they explode.

These telescopes are like time machines. The firsts stars exploded 13.5 billion years ago, but because their light takes so long to reach us, we can still see what they looked like and even watch them explode.

This week's question comes from Christopher Wanjek. Mr. Wanjek is a science writer supporting the Beyond Einstein initiative, a roadmap to understand the forces of nature beyond General Relativity and Quantum Mechanics through the study of the Universe from the Big Bang to black holes.