galaxies & stars form

Star formation followed several stages. Firstly, clouds in the interstellar medium, with radius of roughly a Jeans Length, became unstable and started contracting under the gravitational field. Following this, as the density increased and gravitational attraction became stronger, the clouds commenced collapsing. The clouds remained at cool temperatures, however, as radiation escaped. As the relationship between Jeans length and cloud radius changed, they broke into collapsing fragments, resulting in radiation being trapped and thus an increase in temperature. As fragmentation concluded, the clouds became clusters of protostars. Such collapse ceased and slow contraction commenced after the pressure inside the protostars increased, the temperature reaching as high as three thousand degrees Kelvin. Hydrogen was then dissociated and ionised, as the protostars collapsed rapidly, before recommencing a slow contraction. Finally, nuclear engergy became available, when the temperature had risen to several million degrees Kelvin.

After this, gravity amplified slight irregularities in the density of the primordial gas, pockets of gas become more and more dense and these stars ignite within these pockets, as stars begin formation in regions where the interstellar medium is rich in gas and dust, whereby groups of stars become the earliest galaxies. A galaxy is a system of many thousands of millions of stars, together with interstellar gas and dust. Galaxies may be spiral, spherical, elliptical or irregular in shape.

Jeans Length: The critical radius of a spherical region of gas and dust, such that regions greater than this are unstable and those smaller are stable and don't collapse.

- Tt, ASh (ed)