Australia: The Land Where Time Began

A biography of the Australian continent 

Protostars

Stars form in regions of clouds in which the gas and dust are particularly dense where it is easier for gravity to attract the particles together. The very low temperature of the clouds is an additional factor that will assist in the formation. In these cold clouds the thermal pressure of the interstellar medium is low. A cold temperature of a cloud is a prerequisite for star formation because if the thermal pressure is high enough it will tend overcome any gravitational collapse. There is a delicate balance between gravity and pressure with stars forming only where gravity is the dominant force. It is also believed that an outside agency is necessary to trigger the initiation of star formation.

The dark nebulae that are located within molecular clouds are places where the conditions are conducive for the formation of a star. If the balance between pressure and gravity permit, the dust and gas cloud becomes very opaque and is the precursor to star formation. Barnard objects is the name that is often given to these regions, in honour of the astronomer who first catalogued them. Located within a Barnard object there are sometimes objects that are even smaller. These objects resemble small dark blobs, Bok globules. A Bok globule may be thought of as a Barnard object that has its outer layers dispersed as less dense regions.

It is indicated by radio measurements that their internal temperature is as low as 10 K, and their density is much greater than that of the interstellar medium, though there are only about 100-20,000 particles of dust grains, gas atoms and molecules per cm³. A Bok globule is on average about 1 parsec in diameter, with anywhere from 1-1,000 solar masses. On the other hand, the larger Barnard objects can have masses of about 10,000 solar masses and a diameter of about 10 parsecs. The sizes of these objects vary greatly, being determined by the local conditions in the interstellar medium.

The densest areas within these objects and globules will, if conditions permit, contract further under gravitational attraction. The material of the blob heats up as a consequence of the contraction; however, this thermal energy can be radiated away by the cloud which prevents the building up of pressure to the point where it is high enough to resist contraction. The temperature remains below 100 K during the early phase of collapse, and the thermal energy is transported by convection from the warmer interior to the exterior of the cloud which causes the cloud to glow in the infrared band of radiation. The density of the cloud increases as a result of this ongoing collapse, though this makes it difficult for the radiation to escape from the object. As a consequence, the central regions of the cloud become opaque, and this traps nearly all of the thermal energy that is produced by the gravitational collapse.

A dramatic increase in temperature and pressure results from this trapping of the energy. Eventually a point is reached where the ever-increasing pressure counteracts the overpowering crush of gravity, and the fragment of the cloud that is now denser becomes a protostar – the seed from which the star forms. At this point the protostar may resemble a star, though there are no nuclear reactions occurring at its core.

The time taken for this scenario to occur can be extremely short, in astronomical terms, possibly of the order of a few thousand years. The protostar is still quite large. After about 1,000 years, a protostar of 1 solar mass can be 20 times larger than the radius of the Sun and be about 100 times more luminous, 100 L_☉.

Sources & Further reading

1. Inglis, Michael, 2015, Astrophysics is Easy, An Introduction for the Amateur Astronomer, 2^nd Edition, Springer International Publishing.