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.

19.4 Observations of Cloud Fragments and Protostars 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. Figure 19-8. Star Formation Phases (a) The M20 region shows observational evidence for three broad phases in the birth of a star. The parent cloud is stage 1 of Table 19.1. The region labeled “contracting fragment” likely lies between stages 1 and 2. Finally, the emission nebula (M20 itself) results from the formation of one or more massive stars (stages 6 and 7). (b) A closeup (including Hubble inlays) of the area near region B outlines (in drawn ovals) especially dense knots of dusty matter. (c) A Spitzer Telescope infrared image of the same scene reveals those cores thought to be stellar embryos (arrows). (AURA; NASA)

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.

19.4 Observations of Cloud Fragments and Protostars The Orion Nebula has many contracting cloud fragments, protostars, and newborn stars Figure 19-9. Orion Nebula, Up Close (a) The constellation Orion, with the region around its famous emission nebula marked by a rectangle. The Orion Nebula is the middle “star” of Orion’s sword (see Figure 1.8). (b) Enlargement of the framed region in part (a), suggesting how the nebula is partly surrounded by a vast molecular cloud. Various parts of this cloud are probably fragmenting and contracting, with even smaller sites forming protostars. The three frames at the right show some of the evidence for those protostars: (c) false-color radio image of some intensely emitting molecular sites, (d) nearly real-color visible image of embedded nebular “knots” thought to harbor protostars, and (e) high-resolution image of one of many young stars surrounded by disks of gas and dust where planets might ultimately form. (Astrostock-Sanford; SST; CfA; NASA)

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.

19.4 Observations of Cloud Fragments and Protostars These are two protostars in the Orion Nebula, at around stage 5 in their development Figure 19-10. Protostars (a) An edge-on infrared image of a planetary system-sized dusty disk in the Orion region, showing heat and light emerging from its center. On the basis of its temperature and luminosity, this unnamed source appears to be a low-mass protostar on the Hayashi track (around stage 5) in the H–R diagram. (b) An optical, face-on image of a slightly more advanced circumstellar disk surrounding an embedded protostar in Orion. (NASA)

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 Figure 19-11. Protostellar Wind (a) The nebular disk around a protostar can be the site of intense heating and strong outflows, forming a bipolar jet perpendicular to the disk. (b) As the disk is blown away by the wind, the jets fan out, eventually (c) merging into a spherical wind. In contrast to this art, part (d) is an actual infrared image of a hot young star (at right) whose powerful winds are ripping away the disk (at left) surrounding a Sun-like star (at center). This system is located about 750 pc away in the star-forming cloud IC 1396. (SST)

The T-Tauri phase is when a star has: vigorous surface activity (flares, eruptions) strong stellar winds 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.

Protostar outflow These two jets are matter being expelled from around an unseen protostar, still obscured by dust on Orion’s molecular cloud. Figure 19-13. Protostellar Outflow This view of the Orion molecular cloud shows the outflow from a newborn star, still surrounded by nebular gas. The inset shows a pair of jets called HH1 and HH2, formed when matter falling onto another protostar (still obscured by the dusty cloud fragment from which it formed) creates a pair of high-speed gas jets perpendicular to the flattened protostellar disk. The jets are nearly 1 light-year across. Several more Herbig–Haro objects can be seen at the top right. One of them, the oddly shaped “waterfall” at top right, may be due to an earlier outflow from the same protostar responsible for HH1 and HH2. (AURA; NASA)

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.