Protostar

Star Formation

Interstellar medium Molecular cloud Bok globule | Dark nebula Young stellar object Protostar T Tauri star Herbig Ae/Be stars Nebular hypothesis

Classes of Object

Herbig-Haro object

Theoretical Concepts

Initial mass function Jeans Instability Kelvin-Helmholtz mechanism

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A protostar into a Bok globule (Artist's image).

A protostar is a large object that forms by contraction out of the gas of a giant molecular cloud in the interstellar medium. The protostellar phase is an early stage in the process of star formation. For a one solar-mass star it lasts about 100,000 years. It starts with a core of increased density in a molecular cloud and ends with the formation of a T Tauri star, which then develops into a main sequence starbubbles. This is heralded by the T Tauri wind, a type of super solar wind that marks the change from the star accreting mass into radiating energy.

Observations have revealed that giant molecular clouds are approximately in a state of virial equilibrium—on the whole, the gravitational binding energy of the cloud is balanced by the thermal pressure of the cloud's constituent molecules and dust particles. Although thermal pressure is likely the dominant effect in counteracting gravitational collapse of protostellar cores, magnetic pressure, turbulence and rotation can also play a role (Larson, 2003). Any disturbance to the cloud may upset its state of equilibrium. Examples of disturbances are shock waves from supernovae; spiral density waves within galaxies and the close approach or collision of another cloud. If the disturbance is sufficiently large, it may lead to gravitational instability and subsequent collapse of a particular region of the cloud.

The British physicist Sir James Jeans considered the above phenomenon in detail. He was able to show that, under appropriate conditions, a cloud, or part of one, would start to contract as described above. He derived a formula for calculating the mass and size that a cloud would have to reach as a function of its density and temperature before gravitational contraction would begin. This critical mass is known as the Jeans mass. It is given by the following formula:

where n is the particle number density, m is the mass of the 'average' gas particle in the cloud and T is the gas temperature.

Fragmentation

Stars are often found in groups known as clusters which appear to have formed at around the same time. This can be explained if it is assumed that as a cloud contracts it does not do so uniformly. In fact, as first pointed out by Richard Larson, the giant molecular clouds in which stars are formed are universally observed to have turbulent velocities imposed on all scales within the cloud. These turbulent velocities compress the gas in shocks, which generate filaments and clumpy structures within the giant molecular cloud over a wide range of sizes and densities. This process is referred to as turbulent fragmentation. Some clumpy structures will exceed their Jeans mass and become gravitationally unstable, and may again fragment to form a single or multiple star system.

Whatever the reason, the cloud breaks up into smaller, denser areas which may again break into still smaller areas - the outcome being a cluster of protostars. This certainly agrees with the observation that star clusters are common, but not always huge.

Heating due to gravitational energy

As the cloud continues to contract it begins to increase in temperature. This is not caused by nuclear reactions but by the conversion of gravitational energy to thermal kinetic energy. As a particle (atom or molecule) decreases its distance from the centre of the contracting fragment this will result in a decrease in its gravitational energy. The total energy of the particle must remain constant so the reduction in gravitational energy must be accompanied by an increase in the particle's kinetic energy. This can be expressed as an increase in the thermal kinetic energy, or temperature, of the cloud. The more the cloud contracts the more the temperature increases.

Collisions between molecules often leave them in excited states which can emit radiation as those states decay. The radiation is often of a characteristic frequency. At these temperatures (10 to 20 kelvins) the radiation is in the microwave or infrared range of the spectrum. Most of this radiation will escape, preventing the rapid rise in temperature of the cloud.

As the cloud contracts the number density of the molecules increases. This will eventually make it more difficult for the emitted radiation to escape. In effect, the gas becomes opaque to the radiation and the temperature within the cloud will begin to rise more rapidly.

The fact that the cloud becomes opaque to radiation in the infrared makes it difficult for us to observe directly what is happening. We must look to longer wavelength radio radiation which does escape even the densest clouds. In addition, theory and computer modelling are necessary to understand this phase.

As long as the surrounding matter is falling onto the central condensation, it is considered to be in protostar stage. When the surrounding gas/dust envelope disperses and accretion process stops, the star is considered as pre-main sequence star. In HR diagram then it appears to be on the stellar birthline.

Notes

See also

Space portal

• Herbig-Haro object
• Pre-main sequence star
• Protoplanetary disk
• NGC 7538, home of the largest discovered protostar which is about 300 times the size of our Solar System.

References

• Larson, R.B. (2003), The physics of star formation, Reports on Progress in Physics, vol. 66, issue 10, pp. 1651-1697

External links

• Planet-Forming Disks Might Put Brakes On Stars (SpaceDaily) Jul 25, 2006
• Planets could put the brakes on young stars Lucy Sherriff (The Register) Thursday 27 July 2006 13:02 GMT
• Why Fast-Spinning Young Stars Don't Fly Apart (SPACE.com) 24 July 2006 03:10 pm ET