Giant Molecular Clouds and Protostars:
Animation: Giant Molecular Cloud in Orion
Credit: CfA
Huge complexes of interstellar gas and dust left over from the formation of the galaxy, called molecular clouds, are composed mostly of molecular hydrogen. These clouds are the coolest (10 to 20 K) and densest (106 to 1010 particles/cm3) portions of the interstellar medium. Since these clouds are cooler than most places, they are perfect locations for star formation. The molecular clouds are puffy and lumpy, with 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 (GMC's). A typical full-grown spiral galaxy contains about 1,000 to 2,000 Giant Molecular Clouds and many 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 gases and study their physical properties. The image above shows the distribution of GMC's within the Orion and neighboring constellations; produced by radio mapping of carbon monoxide (CO) gas. A map of stars, bright nebulae, and cold clouds within 2000 LY of the Orion spiral arm of the Milky Way Galaxy can be seen at http://www.anzwers.org/free/universe/2000lys.html
Animation: Star Formation in Spiral Galaxy
Credit: NASA/SAO/CXC/D.Berry
Milky Way Galaxy Molecular Map
Credit: T. Dame (CfA) et al., Columbia 1.2-m Radio Telescopes
Star-forming molecular clouds are mostly found along spiral arms, as seen in the CO molecular map showing the distribution of these clouds in the Milky Way Galaxy. Individual giant molecular clouds are internally violent and turbulent. The self-gravitational energy of the clumps is counter-balanced by pressure from both the supersonic velocity of the gases and magnetic field lines. Pertubations from the spiral density wave within the spiral arm structure, collisions between clouds, supernovae shockwaves, and nearby massive star formation are some of the possible triggers that eventually cause an imbalance within the GMC's and the clumps begin to collapse. Individual stars within clumps form within their own smaller gaseous structures, called cores.
Proplyds in Orion
Credit: NASA, HST, WFPC 2, C.R. O'Dell (Rice U.)
Protoplanetary Disks
Credit: J. Bally (U. Colorado), H. Throop (SwRI), C.R. O'Dell (Vanderbilt U.), NASA
As a gas clump collapses it heats up due to friction as the gas particles bump into each other. The energy the gas particles had from falling under the force of gravity (gravitational potential) gets converted to heat (thermal) energy. The gas clump becomes warm enough to produce infrared and microwave radiation. During the initial collapse, the clump is transparent to radiation and the collapse proceeds fairly quickly. As the clump becomes more dense, it becomes opaque. Infrared radiation is trapped, and the temperature and pressure in the center begin to increase. As the clump starts evolving into a protostar, at first it only has about 1% of its final mass; however the envelope of the star continues to grow as infalling material is accreted. After a few million years, thermonuclear fusion begins in its core, and a strong stellar wind is produced which stops the infall of new mass. Other material in the disk may coalesce to form other stars and/or planets. Protostars reach temperatures of 2000K to 3000K - hot enough to glow red - but the cocoon of gas and dust surrounding them blocks visible light from escaping. The proplyds in Orion are protostars embedded within protoplanetary disks. The close-up of two of these young disks in Orion reveals the torturous conditions they must face while trying to grow into full-fledged planetary systems. Ultraviolet radiation from one of Orion's nearby hot stars is rapidly destroying the disks surrounding the protostars. Only ~10% if all protostars survive the harsh conditions within stellar nurseries to become stars.
Revised: March 19, 2008