Protostars, nebulas and Brown dwarfs

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Stars form inside relatively dense concentrations of interstellar gas and dust known as molecular clouds. These regions are extremely cold (temperature about 10 to 20K, just above absolute zero). At these temperatures, gases become molecular meaning that atoms bind together. CO and H2 are the most common molecules in interstellar gas clouds. The deep cold also causes the gas to clump to high densities. When the density reaches a certain point, stars form. Since the regions are dense, they are opaque to visible light and are known as dark nebula. Since they don't shine by optical light, we must use IR and radio telescopes to investigate them.

Emission nebulae are heated by the formation of stars nearby. In these images, we see the parent cloud in stage 1, contracting fragments between stages 1 and 2, and a new star in stage 6 or 7. The new star is the one heating the nebula.

19.4 Observations of Cloud Fragments and Protostars

The evolution of young stars is from a cluster of protostars deep in a molecular clouds core, to a cluster of T-Tauri stars whose hot surface and stellar winds heat the surrounding gas to form an HII region (HII, pronounced H-two, means ionized hydrogen). Later the cluster breaks out, the gas is blown away, and the stars evolve as shown below.

The Orion Nebula has many contracting cloud fragments, protostars, and newborn stars

19.4 Observations of Cloud Fragments and Protostars

Protostars: Once a clump has broken free from the other parts of the cloud core, it has its own unique gravity and identity and we call it a protostar. As the protostar forms, loose gas falls into its center. The infalling gas releases kinetic energy in the form of heat and the temperature and pressure in the center of the protostar goes up. Several candidate protostars have been found by the Hubble Space Telescope in the Orion Nebula.

These are two protostars in the Orion Nebula, at around stage 5 in their development

19.4 Observations of Cloud Fragments and Protostars

Protostars are believed to have very strong winds, which clear out an area around the star roughly the size of the solar system

19.4 Observations of Cloud Fragments and Protostars

The T-Tauri phase is when a star has:1. vigorous surface activity

(flares, eruptions) 2. strong stellar winds 3. variable and irregular light

curves

Once a protostar has become a hydrogen-burning star, a strong stellar wind forms, usually along the axis of rotation with a flow of gas out the poles of the star.. This early phase in the life of a star is called the T-Tauri phase.

These two jets are matter being expelled from around an unseen protostar, still obscured by dust on Orion’s molecular cloud.

Protostar outflow

Discovery 19-1: Observations of Brown Dwarfs

Brown dwarfs are difficult to observe directly, as they are very dim. These images are of two binary-star systems, each believed to contain a brown dwarf. The difference in luminosity between the star and the brown dwarf is apparent.

Brown star formation

If a protostar forms with a mass less than 0.08 solar masses, its internal temperature never reaches a value high enough for thermonuclear fusion to begin. This failed star is called a brown dwarf, halfway between a planet (like Jupiter) and a star. The core becomes degenerate before the start of fusion. There is enough energy from the collapse to cause the brown dwarf to shine for over 15 million years. Brown dwarfs fade and cool to become black dwarfs.

Brown dwarf

In galaxies we find clusters of young stars near other young stars, called supernova induced star formation. The very massive stars form first and explode into supernova. This makes shock waves into the molecular cloud, causing nearby gas to compress and form more stars, young stars are found near other young stars and causes the pinwheel patterns we see in galaxies.