The Orion Nebula: A Star Nursery by Andrew Fraknoi
Stars don't last forever. Old stars eventually die and new ones are born from great clouds of gas and dust in space. (See our page on the Lives of Stars.) The best known and closest site where astronomers can regularly observe the birth of stars is 1500 light years away from us and is called the Orion Nebula.
On winter evenings, Orion is one of the easiest star patterns to find in the sky. Its identifying feature is a row of three stars which make up the belt of the legendary hunter, Orion. Above the belt is the bright reddish star Betelgeuse (pronounced "Beetle-juice"). Below the belt is the bluish star Rigel ("Rye-gil"), and hanging from the belt is a straight "sword" of dimmer stars. On this sword of stars, ancient observers were very impressed to find what looked like a splotch of blood, as if the hunter were returning from fresh victory. That fuzzy splotch, visible to the naked eye when it is really dark (and easily seen through binoculars) is the Orion Nebula.
Through larger telescopes and with carefully processed images, the Nebula reveals itself to be a magnificent web of glowing gas and dark dust, illuminated by the energy of the adolescent stars that recently formed inside it. The group of four bright stars noticeable in the center is called the Trapezium; telescopes have revealed that there are actually 11 stars in the tight central group and over 2,000 new stars in the immediate neighborhood. (These stars are all less than a million years old, mere babies by astronomical standards.) Just one of the stars is the dominant source of energy for the glowing nebula. Called by the technical name Theta-1C Orionis, this star has enough material to make 40 stars like the Sun and shines as brightly as 210,000 Suns. Like all "superstars," it will go through its life very quickly and is likely to explode in a few million years. Such explosions compress nearby regions of gas and allow them to start forming the next generation of stars. Chemical analysis of comets and asteroids in the solar system indicate that our sun and its planets formed in one such second-generation event, 4.5 billion years ago.
Throughout the nebula (which is about 30 light years wide), we see a significant number of infant stars that are surrounded by disks of darker material, many of which are likely to make a system of planets like the one that surrounds our Sun. Observations with the Hubble Space Telescope have revealed over 150 such "getting-ready-to-make-planets" disks, an indication that the birth of stars is frequently accompanied by the birth of planets.
Imposing as it is, the Orion Nebula is merely a visible "blister" on a much larger structure of cosmic raw material that lies behind it from our perspective on Earth. Part of this cloud is visible, and can be seen in wide-angle shots of the Orion Nebula's surroundings in the film. But most of this Orion Molecular Cloud (as it is called, because the gas and dust has been found to contain an assortment of primitive molecules) is invisible to the human eye. It stretches over 100 light years and contains enough gas to make 200,000 Suns. We know the cloud is there because the materials reveal themselves with infra-red light and radio waves. Less than 1% of the gas has been turned into stars, both because star formation is still continuing and because it is a messy, inefficient process—leaving lots of unused gas and dust behind.
Indeed, astronomers have recorded a wave of star formation moving from northwest to southeast through the region of the sky where we see the ancient hunter figure. Stars near the shoulder of the hunter formed about 12 million years ago, the stars that make up the belt are about 8 million years old, and there are stars in the Orion Nebula that are still forming today. Millions of years from now, future astronomers will record the wave of new stars passing deeper into the great molecular cloud, its creative and destructive energies far from spent.