And God said, Let there be light: and there was light.
And God saw the light, that it was good: and God divided the light from the darkness
(Genesis 1:3-4, King James Version)

It's a beautiful phrase, isn't it? First light. It makes me think of sunrise, my favorite part of the day. For some, "first light" has religious connotations. A Google search reveals several Christian congregations and churches named "First Light", and the phrase is also another name for the holiday of Imbolc, a pagan sabbat marking the early stirrings of Spring. For those of a scientific inclination, first light is also a very literal description of the beginning of all things -- in the beginning, there was matter and energy. The big bang. Light and heat. The universe was born in fire.

We talk about first light in astronomy often. This shouldn't be too surprising -- everything we do involves light in one way or another. In the cosmological sense, we wonder where the first light in the universe came from. It's not an easy question to answer -- what kind of light do you mean? Probably the earliest light was the relic radiation left over from the big bang, which we now see as the cosmic microwave background. Once matter and energy decoupled, all of the heat left over from the big bang began freely propagating through the universe. As the universe expanded, this radiation cooled, and we see it today not as visible light, but as microwave photons. The spectrum of this light has that of a black body, feebly radiating with a temperature of a frosty 2.73 kelvins.

Then we ask when were the first stars born? The first galaxies? Today, we "see" the universe in the form of light being emitted by luminous objects -- stars, and galaxies made up of stars mostly. When did these objects first come into being? We don't really know. As of the writing of this work, the world record for the most distant quasar known has a redshift of about 6.7, placing it about 13 billion light years away from us. It took 13 billion years for the light of that quasar to speed through space to get to us. But a quasar isn't really a star. They're believed to be enormous black holes at the centers of galaxies, accreting huge amounts of matter into themselves, while blasting some of it away into space at nearly the speed of light. The light from the stuff that gets blasted away is what appears to us as a quasar. While they're not stars, quasars do give some indication of when organized galaxies had begun to form.

The very first stars probably formed a few hundred million years after the big bang, after gravity was able to gather together enough matter to form galaxies, giant matter clouds, and eventually protostellar nebulae. The first stars, sometimes called Population III stars, were not much like the ones we see today. In the early universe, there wasn't much other than hydrogen and helium -- big bang nucleosynthesis simply didn't result in much else. Most of these very early stars were probably huge objects, having dozens of times the mass of our Sun. Becuase of their giant sizes, they raced through their supply of nuclear fuel within a few million years, and then exploded as supernovae. In so doing, they scattered the ashes of themselves throughout the early universe, seeding the galaxies with carbon, nitrogen, iron, silicon, and nearly everything else. Eventually, this stuff collected into new generations of stars, into planets and solar systems, and eventually, into you and me.

We could talk about the first light of our own solar system -- when did the Sun first start to shine? Both astronomers and geologists seem to have reached a happy agreement on this question. Some time between 4.5 and 5 billion years ago, the Sun and solar system formed from a protostellar nebulae, with the Sun growing at the center of the gravitationally contracting cloud, and the seeds of the planets growing within the pancake of material that rotated around it. Eventually, the nine planets (that we know of) grew into what we see today. When was the first "sunrise"? The Sun probably "turned on" within a few million years of the gravitational collapse of the protosolar nebula. At first, it would've shined from the transformation of potential energy into thermal radiation. Eventually, the center of the Sun commenced burning its own store of nuclear fuel, and has been ever since. It might continue to do so for another five billion years or so.

We see the result of that event every day, as our world spins once every 24 hours, rotating the Sun into and out of view. First light in a more practical context is also known as twilight, or sunrise. First comes astronomical twilight; once the Sun reaches a point about 18 degrees below the horizon, we can start to see a faint trickle of photons being scattered from points east of us. Once the Sun reaches 12 degrees below the horizon, we call it nautical twilight, as we can now see the horizon, and sailors are then able to take their bearings properly. Once the Sun is six degrees below the horizon -- civil twilight -- you should really consider getting out of bed. Otherwise, you'll miss our daily first light.

First light also has a technical meaning, too. It's used to describe the very first attempt at acquiring an image through a new telescope.

A lot of design work has to go into building a telescope: How big will the mirror be? What focal length? What should the mirror figure be? What sort of mount should the telescope have? How will we point, control, and guide it? and so on. Most of the time, first light is a way of confirming our careful calculations as to the mirror's figure and optical system design, or at most adjusting a few screws here and there. Usually, it's a happy time -- lots of hard work and careful calculations paying off in that first wash of photons through the system and into your detector. Rarely, it's not so happy. When the Hubble Space Telescope was launched, a severe abberation was discovered in the primary mirror, and the first light images were quite poor. It was later determined that though HST's primary mirror was one of the most precisely figured optical instruments in history, it was ground to the wrong figure. It wasn't discovered until after the telescope was launched because there wasn't enough money in the budget to properly test the mirror prior to assembly. The first Hubble Servicing Mission installed some eyeglasses for Hubble, known as COSTAR, and it has worked fine ever since.

The concept of first light isn't limited to optical telescopes though. Other kinds of astronomical facilities have first light, too. Both the Chandra X-ray Observatory and the Spitzer Space Telescope had their own "first light" events, only in X-rays and infrared light. Radio telescopes and interferometers have first light of a sort. Instead of taking true images, radio interferometers measure objects with two or more widely-spaced telescopes, and then combine and interfere the received radio waves to create interference fringes. You can then convert those interference patterns into images. When such a telescope is brought on line, the event is usually called first fringe.

Even space missons to other planets have a sort of "first light" too. Venera, Viking, Sojourner, and now Spirit and Opportunity had first lights of their own, opening their (and our) eyes to strange new worlds. How did you feel when you saw the first pictures from the surface of Mars recently? Thrilled? Awestruck? Disappointed there were no Martians walking by? Chilly? Imagine how they felt at the Jet Propulsion Laboratory as that first light was sent home bit by bit, photon by photon.

The discussion about the first stars was partly inspired by a talk given by Volker Bromm of Harvard University. He spoke at the American Astronomical Society meeting in January 2004, giving a talk entitled "The First Stars and Quasars and the End of the Cosmic Dark Ages".