The Formation of Protostars

The mass of the Milky Way is about 1011 solar masses. Since its age is about 1010 years, stars have been forming at the average rate of 10 Mq per year. This estimate is only an upper limit for the present rate, because earlier the rate of star formation must have been much higher. Since the lifetime of O stars is only about a million years, a better estimate of the star formation rate can be made, based on the observed number of O stars. Accordingly, it has been concluded that at present, new stars are forming in the Milky Way at a rate of about three solar masses per year.

Stars are now believed to form inside large dense interstellar clouds mostly located in the spiral arms of the Galaxy. Under its own gravity, a cloud begins to contract and fragment into parts that will become protostars. The observations seem to indicate that stars are not formed individually, but in larger groups. Young stars are found in open clusters and in loose associations, typically containing a few hundred stars which must have formed simultaneously.

Theoretical calculations confirm that the formation of single stars is almost impossible. An interstellar cloud can contract only if its mass is large enough for gravity to overwhelm the pressure. As early as in the 1920's, James Jeans calculated that a cloud with a certain temperature and density can condense only if its mass is high enough. If the mass is too small the pressure of the gas is sufficient to prevent the gravitational contraction. The limiting mass is the Jeans mass (Sect. 6.11), given by

Mj « 3 x 10%/ — Mq , n where n is the density in atoms/m3 and T the temperature.

In a typical interstellar neutral hydrogen cloud n = 106 and T = 100 K, giving the Jeans mass, 30,000 Me. In the densest dark clouds n = 1012 and T = 10 K and hence, MJ = 1 Mq.

It is thought that star formation begins in clouds of a few thousand solar masses and diameters of about 10 pc. The cloud begins to contract, but does not heat

up because the liberated energy is carried away by radiation. As the density increases, the Jeans mass thus decreases. Because of this, separate condensation nuclei are formed in the cloud, which go on contracting independently: the cloud fragments. Fragmentation is further advanced by the increasing rotation velocity. The original cloud has a certain angular momentum which is conserved during the contraction; thus the angular velocity must increase.

This contraction and fragmentation continues until the density becomes so high that the individual fragments become optically thick. The energy liberated by the contraction can then no longer escape, and the tem perature will begin to rise. In consequence the Jeans mass begins to increase, further fragmentation ceases and the rising pressure in existing fragments stops their contraction. Some of the protostars formed in this way may still be rotating too rapidly. These may split into two, thus forming double systems. The further evolution of protostars has been described in Sect. 11.2.

Although the view that stars are formed by the collapse of interstellar clouds is generally accepted, many details of the fragmentation process are still highly conjectural. Thus the effects of rotation, magnetic fields and energy input are very imperfectly known. Why a cloud begins to contract is also not certain; one theory

Fig. 15.22. The Helix nebula (NGC7293). The planetary layers into space. (National Optical Astronomy Observatories, nebulae are formed during the final stages of evolution of Kitt Peak National Observatory) solar-type stars. The centrally visible star has ejected its outer is that passage through a spiral arm compresses clouds and triggers contraction (see Sect. 17.4). This would explain why young stars are predominantly found in the spiral arms of the Milky Way and other galaxies. The contraction of an interstellar cloud might also be initiated by a nearby expanding H II region or supernova explosion.

Star formation can be observed particularly well in the infrared, since the temperatures of of the condensing clouds and protostars are of the order 100-1000 K and the infrared radiation can escape even the densest dust clouds. For example, in connection with the Orion nebula there is a large cloud of hydrogen, found

in radio observations, containing small infrared sources. E.g. the Becklin-Neugebauer object has a temperature of a couple of hundred kelvins but a luminosity that is thousandfold compared with the Sun. It is a strong H2O maser source, located next to a large HII region.

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