 | Interstellar Medium |
| Proto-stars |
Composition:
| 74% H |
| 25% He |
| dust (silicates,
carbon and iron compounds). |
The atoms are usually bound together in molecules and concentrated in giant molecular clouds, like the Orion molecular cloud complex, of which the famous Orion Nebula is a part. Sometimes, like the Orion Nebula and the gas that envelopes the Pleiades, these clouds reflect the light of newly created stars.
These clouds are extremely tenuous, having a density of about 200 hydrogen molecules per cubic centimeter. But the clouds are immense: they have masses between about 100,000 to 2 million solar masses, and are between 50 to 300 light-years across. The Horsehead Nebula is an example of a region that appears to comprise mainly dust. Proto-stars
Density fluctuations in the gas clouds can lead to gravitational compression and star formation. Density fluctuations is just a fancy way of saying that the density varies from place to place.Gravitational Compression
The basic idea of gravitational compression is this: if a gas cloud contains density fluctuations, that is, some regions are denser than others, the denser regions will tend to draw more gas towards them, because their gravitational pull is stronger than that of the less dense regions. Over hundreds of thousands to millions of years, this causes the gas clouds to fragment into smaller and smaller regions. And the denser regions get progressively denser because of gravitational compression.
But there is a snag: the denser regions can become denser only if they are dense enough to overcome the opposing tendency of the gas cloud to disperse. (Even at a temperature of 10K the molecules are moving at 500 m/s, about 1100 mph.)There is a contest between the pressure of the gas and the pressure due to the gravitational force. If the gravitational pressure exceeds the gas pressure then the gas will gradually collapse.
Shock Waves
One effective way to create sufficiently large density fluctuations is through shock waves. A shock wave is any rapidly moving, thin shell, of dense fluid. A familiar example (at least for those unfortunate enough to live near a military airbase) is the highly compressed (cone-shaped) shell of air created by supersonic jets. As the shell passes your ears the rapid change in density, and therefore pressure, is sensed as a loud bang: a sonic boom!Dense Cores
In interstellar space, shock waves can be caused by the rapidly expanding gas shells from the explosion of a nearby massive star. When the shock wave hits the molecular cloud it compresses the cloud in the region of contact thereby making that region denser. These denser regions of gas in the molecular clouds are called dense cores. It is from these dense cores that stars form. The remnants of the dead stars are called supernova remnants.Stars tend to form in groups because of the tendency of gravity to fragment a larger dense region into smaller denser regions that remain gravitational bound to each other.
Accretion
The center of the dense cores get more and more dense by drawing material into the core. This is called accretion. This process forms accretion disks of which many have been identified with the Hubble Telescope. The core heats up (to about 1500 K) because of the compression, caused by gravity, and begins to shine in the infrared and at radio wavelengths.Pre-Main Sequence Star
After about 100,000 years of accretion the proto-star has as much mass as the Sun. Eventually, the wind of radiation and particles flowing from the proto-star prevents further material from accreting. The proto-star becomes a pre-main sequence star.Coulomb Repulsion
Gravitational compression continues until the core is hot enough (about 7 million K) to fuse hydrogen into helium. At that temperature the ionized atoms of hydrogen (that is, protons) are traveling so fast that they have enough energy to just overcome the Coulomb repulsion (that tends to keep the protons as far apart as possible) and thus get close enough to fuse. The repulsion is due to the electric force between the positively charged protons. Recall that like charges repel, while unlike charges attract.Brown Dwarfs
If, however, the temperature at the core is too low then the protons (hydrogen nuclei) will be moving too slowly and will have insufficient energy to overcome the Coulomb repulsion. The electric force wins out. Calculations suggest that stars that are less than 0.08 solar masses cannot trigger fusion reactions in their cores because their cores are too cool. Such stars contract to planet-like objects called brown dwarfs. Brown dwarfs are basically giant Jupiters.Some young stars have very powerful stellar winds and erratic light outputs (T Tauri Stars). These appear to be proto-stars that are clearing away their surrounding gas and dust.
