Looking up in the sky, the stars seem unchanging and timeless.
But that's an illusion. Like humans, stars are born, live their lives, and die. That simple truth holds now and will hold for the future of the Universe itself.
But what about the past? We know the Universe started in the Big Bang 13.7 billion years ago. When it did, the expanding chaos was almost perfectly random and uniform, and stars came later. So when did the first star turn on?
For all of history, this question had no satisfactory answer. It was more of a fairy tale question than a scientific one; all we could say in response was "a long time ago." But that's all changed now! The Wilkinson Microwave Anisotropy Probe (for obvious reasons, usually just called WMAP), is a NASA mission that peered deep into space to observe the fires left over from the Big Bang. This light, which has traveled billions of years to reach us here at Earth, has embedded in it the fingerprints of the early Universe. By studying that light carefully, astronomers can tease out details of what the conditions were like all those eons ago.
When the first stars were born, things were different. The Universe was simpler in many ways, making it easier to scientifically and mathematically model what those stars were like. We are pretty sure they were monsters, these early stars: 100 times the mass of the Sun or more, and blazing hundreds of thousands of times brighter, they profoundly affected the environment around them. When light passed through that environment, it was altered very slightly, changed by the presence of those stars.
It is this change that has now been seen by WMAP. That imprint on the light is faint, ten million times fainter than the light itself, but it can be detected. After the fierce cosmological math is applied to the data, scientists can finally answer the age old question: the first stars were born about 13.3 billion years ago, 400 million years after the Big Bang.
Our own Sun is a Johnny-come-lately, born from gas and dust just 4.5 billion years ago, 9 billion years or so (give or take a few million) after those first stellar pioneers. We've benefited from them as well: those stars created the first heavy elements like iron and calcium, the elements in our blood and bones we need to survive. We owe our existence to those long-dead monsters. It's possible that in the near future we'll get a direct observation of the light from one of those early stars, enfeebled by the billions of light years of travel, but it's also possible they are forever beyond our grasp. But their effect, their fingerprints on the cosmic backdrop still reveal their story.
And there's more of this story to tell as well. WMAP measured far more than just the ages of those stars... and you can read more about it on my Bad Astronomy Blog, or at the WMAP website.