October 21, 1997
Space between stars in a galaxy is nearly empty, except for a scattering of hydrogen atoms. The atoms are so far apart that, if an atom were an average- size person, each person would be separated by about 465 million miles, which is the distance between our Sun and Jupiter. These atoms are moving very fast because they are extremely hot, baked by ultraviolet radiation from stars. This makes it difficult for atoms to bond to form molecules. Those that do form don't last for long. If radiation doesn't break these molecules apart, a chance encounter with another atom will.
Some parts of space, however, are not wide open frontiers containing a few atoms. These cosmic spaces comprise dense clouds of dust and gas left over from galaxy formation. Since these clouds are cooler than most places, they are perfect breeding grounds for star birth. When the density is 1,000 times greater than what is found in normal interstellar space, many atoms combine into molecules, and the gas cloud becomes a molecular cloud. Like clouds in our sky, these molecular clouds are puffy and lumpy. Molecular clouds in our Milky Way Galaxy have diameters ranging from less than 1 light-year to about 300 light-years and contain enough gas to form from about 10 to 10 million stars like our Sun. Molecular clouds that exceed the mass of 100,000 suns are called Giant Molecular Clouds.
A typical full-grown spiral galaxy contains about 1,000 to 2,000 Giant Molecular Clouds and many more smaller ones. Such clouds were first discovered in our Milky Way Galaxy with radio telescopes about 25 years ago. Since the molecules in these clouds do not emit optical light, but do release light at radio wavelengths, radio telescopes are necessary to trace the molecular gas and study its physical properties. Most of this gas is very cold (about -440 degrees Fahrenheit) because it's shielded from ultraviolet light. Since gas is more compact in a colder climate, it is easier for gravity to collapse it to form new stars.
Ironically, the same climate that is conducive to star formation also may shut off the star birth process. The problem is heat. Young stars are very hot and can heat the molecular gas to more than 1,000 degrees Fahrenheit, which is an unfavorable climate for star birth. When the temperature exceeds about 3,000 degrees Fahrenheit, the gas molecules break down into atoms.
The density of the gas can increase considerably near the centers of some Giant Molecular Clouds: Gas as dense as 1 billion molecules per cubic inch has been observed. (Though dense by astronomical standards, such gas is still 100 billion times thinner than the air we breathe here on Earth at sea level!) In such dense regions, still denser blobs of gas can condense and create new stars. Although the star formation process is not fully understood, there is observational evidence that most stars are born in the densest parts of molecular clouds.
What happens when stars begin forming in Giant Molecular Clouds depends on the environment. Under normal conditions in the Milky Way and in most other present-day spiral galaxies, star birth will stop after a relatively small number of stars have been born. That's because the stellar nursery is blown away by some of the newly formed stars. The hottest of these heat the surrounding molecular gas, break up its molecules, and drive the gas away. As the celestial smog of gas and dust clears, the previously hidden young stars become visible, and the molecular cloud and its star-birthing capability cease to exist. Two years ago the Hubble Space Telescope revealed such an emerging stellar nursery in the three gaseous pillars of the Eagle Nebula.
Giant Molecular Clouds in colliding galaxies may experience a different fate. As the collision crunches the interstellar gas and stars form at an accelerating rate, the gas pressure around the surviving Giant Molecular Clouds increases one-hundred- to one-thousand-fold. Calculations suggest that the hot surrounding gas can trigger rapid star birth throughout the clouds by driving shock waves into them. The several hundred thousand stars that form from the cold molecular gas in such clouds use up most of the gas before it has time to be heated and dispersed. The result of such violent events is the nearly complete conversion of Giant Molecular Clouds into rich star clusters, each containing up to 1 million stars. Observations by the Hubble telescope suggest that many of these newly born star clusters remain bound by their own gravity and evolve into globular clusters, like those observed in the halo of our Milky Way.