Molecular Clouds

Large, dense molecular clouds are very special environments in space. Composed mainly of molecular hydrogen and helium, with small amounts of heavier gases, they are the birth place of new stars and planets. Molecular clouds that exceed the mass of 100,000 suns are called giant molecular clouds. Giant molecular clouds are the largest inhabitants of galaxies, reaching up to 300 light years in diameter. They contain enough dense gas and dust to form hundreds of thousands of Sun-like stars. These stars are formed in the densest parts of the clouds. Molecular clouds are very cold, having temperatures ranging from about -440 to -370 degrees Fahrenheit (-263 to -223 degrees Celcius or 10 to 50 degrees Kelvin). They usually do not radiate their own visible light and appear dark when viewed with an optical telescope. In these cold, dense environments, many atoms can combine into molecules. Giant molecular clouds can last for 10 to 100 million years before they dissipate, due to the heat and stellar winds from newly formed stars within them. An average spiral galaxy, like our own Milky Way, contains about 1,000 to 2,000 Giant Molecular Clouds in addition to numerous smaller clouds.

Visible light images of the dark molecular cloud Barnard 68 (FORS Team, 8.2-meter VLT Antu, ESO),
and the well known Horsehead Nebula (Nigel Sharp (NOAO), NSF; Copyright: AURA).

Although visible light is usually blocked by molecular clouds, infrared light can pentrate the thick molecular clouds where new stars are forming. Infrared telescopes can be used to peer directly into the star forming regions of these clouds, giving us valuable, new information about these environments. In the infrared can learn about the regions which are collapsing to form new stars. Through infrared spectroscopy we can gain knowledge about the temperature and density of a collapsing region as well as it's rate of collapse and velocity structure. We can also find out which molecules are present in star forming regions and determine their abundance and distribution. In the image to the left, you can see an infrared view (right side) of a portion of the Orion Nebula (shown in visible light on the left side of the image). The infrared image from the NICMOS camera onboard the Hubble Space Telescope clearly shows how we can use infrared to peer deeply into a molecular cloud.

The Spitzer Space Telescope will study the temperature and density of molecular clouds and gather data about the physical conditions and chemical compositions from which new stars are formed.