Star Formation

Interstellar medium Molecular cloud Bok globule | Dark nebula Young stellar object Protostar T Tauri star Herbig Ae/Be stars Nebular hypothesis

Classes of Object

Herbig-Haro object

Theoretical Concepts

Initial mass function Jeans Instability Kelvin-Helmholtz mechanism

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Star Formation is the process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. As a branch of astronomy star formation includes the study of the interstellar medium and giant molecular clouds as precursors to the star formation process and the study of young stellar objects and planet formation as its immediate products. Star formation theory, as well as accounting for the formation of a single star, must also account for the statistics of binary stars and the initial mass function.

Collapse of molecular clouds

Star formation begins in the interstellar medium of a galaxy. In addition to the stars, which make up 85% of the mass of the Milky Way, diffuse gas and dust containing around 0.1 to 1 particle per cm³ is spread thought the disk of spiral galaxies. The interstellar medium is typically composed of roughly 70% hydrogen (by mass), with most of the remaining gas consisting of helium; trace amounts of heavier elements, called metals, are present. Some of the interstellar medium consists of far denser clouds, or nebulae. Stars form in these nebulae.^[1]

In the dense nebulae where stars form, much of the hydrogen is in molecular (H₂) form, so the clouds are called molecular clouds.^[1] The largest molecular clouds, called giant molecular clouds, have typical densities of 100 particles per cm³, diameters of 100 light-years (9.5×10¹⁴ km), and masses of up to 6 million solar masses.^[2] The nearest star-forming nebula to the Sun is the Orion nebula, 1,300 ly (1.2×10¹⁶ km) away.^[3]

If an interstellar cloud is massive enough that the gas pressure is insufficient to support it, the cloud will undergo gravitational collapse. The mass above which a cloud will undergo such collapse is called the Jeans mass. The Jeans mass depends on the temperature and density of the cloud, but is typically thousands to tens of thousands of solar masses.^[1] In so-called triggered star formation, one of several events might occur to compress a molecular cloud and initiate its gravitational collapse. Molecular clouds may collide with each other, or a nearby supernova explosion can be a trigger, sending shocked matter into the cloud at very high speeds.^[1] Finally, galactic collisions can trigger massive starbursts of star formation as the gas clouds in each galaxy are compressed and agitated by tidal forces.

As it collapses, a molecular cloud breaks into smaller and smaller pieces. In each of these fragments, the collapsing gas releases gravitational potential energy as heat. As its temperature and pressure increase, the fragments condense into rotating spheres of gas. Once the gas is hot enough for the internal pressure to support the fragment against further gravitational collapse (hydrostatic equilibrium), the object is known as a protostar.^[1]

Accretion of material onto the protostar continues partially through a circumstellar disc. When the density and temperature are high enough, deuterium fusion begins, and the outward pressure of the resultant radiation slows (but does not stop) the collapse. Material comprising the cloud continues to "rain" onto the protostar. In this stage bipolar flows are produced, probably an effect of the angular momentum of the infalling material.

The protostar follows a Hayashi track on the Hertzsprung-Russell (H-R) diagram.^[4] The contraction will proceed until the Hayashi limit is reached, and thereafter contraction will continue on a Kelvin-Helmholtz timescale with the temperature remaining stable. Stars with less than 0.5 solar masses thereafter join the main sequence. For more massive protostars, at the end of the Hayashi track they will slowly collapse in near hydrostatic equilibrium, following the Henyey track.^[5]

Finally, hydrogen begins to fuse in the core of the star, and the rest of the enveloping material is cleared away. This ends the protostellar phase and begins the star's main sequence phase on the H-R diagram.

The stages of the process are well defined in stars with masses around one solar mass or less. In high mass stars, the length of the star formation process is comparable to the other timescales of their evolution, much shorter, and the process is not so well defined. The later evolution of stars are studied in stellar evolution.

Observations

The Orion Nebula is a picture book of star formation, from the massive, young stars that are shaping the nebula to the pillars of dense gas that may be the homes of budding stars. This turbulent star formation region is one of astronomy's most dramatic and photogenic celestial objects.

Key elements of star formation are only available by observing in wavelengths other than the optical. The protostellar stage of stellar existence is almost invariably hidden away deep inside dense clouds of gas and dust left over from the GMC. Often, these star-forming cocoons can be seen in silhouette against bright emission from surrounding gas; they are then known as Bok globules.^[6] Early stages of a stars life can be seen in infrared light, which penetrates the dust more easily than visible light.^[7]

The structure of the molecular cloud and the effects of the protostar can be observed in near-IR extinction maps (where the number of stars are counted per unit area and compared to a nearby zero extinction area of sky), continuum dust emission and rotational transitions of CO and other molecules; these last two are observed in the millimeter and submillimeter range. The radiation from the protostar and early star has to be observed in infrared astronomy wavelengths, as the extinction caused by the rest of the cloud in which the star is forming is usually too big to allow us to observe it in the visual part of the spectrum. This presents considerable difficulties as the atmosphere is almost entirely opaque from 20μm to 850μm, with narrow windows at 200μm and 450μm. Even outside this range atmospheric subtraction techniques must be used.

The formation of individual stars can only be directly observed in our Galaxy, but in distant galaxies star formation has been detected through its unique spectral signature.

Notable Pathfinder Objects

• MWC 349 was first discovered in 1978, and is estimated to be only 1,000 years old.
• VLA 1623 -- The first exemplar Class 0 protostar, a type of embedded protostar that has yet to accrete the majority of its mass. Found in 1993, is possibly younger than 10,000 years [1].
• L1014 -- An incredibly faint embedded object representative of a new class of sources that are only now being detected with the newest telescopes. Their status is still undetermined, they could be the youngest low-mass Class 0 protostars yet seen or even very low-mass evolved objects (like a brown dwarf or even an interstellar planet). [2].
• IRS 8* -- The youngest known main sequence star, discovered in August 2006. It is estimated to be 3.5 million years old [3].

Low Mass and High Mass Star Formation

Stars of different masses are thought to form by slightly different mechanisms. The theory of low-mass star formation, which is well-supported by a plethora of observations, suggests that low-mass stars form by the gravitational collapse of rotating density enhancements within molecular clouds. As described above, the collapse of a rotating cloud of gas and dust leads to the formation of an accretion disk through which matter is channeled onto a central protostar. For stars with masses higher than about 8 solar masses, however, the mechanism of star formation is not well understood.

Massive stars emit copious quantities of radiation which pushes against infalling material. In the past, it was thought that this radiation pressure might be substantial enough to halt accretion onto the massive protostar and prevent the formation of stars with masses more than a few tens of solar masses.^[8] Recent theoretical work has shown that the production of a jet and outflow clears a cavity through which much of the radiation from a massive protostar can escape without hindering accretion through the disk and onto the protostar. ^[9][10] Present thinking is that massive stars may therefore be able to form by a mechanism similar to that by which low mass stars form.

There is mounting evidence that at least some massive protostars are indeed surrounded by accretion disks. Several other theories of massive star formation remain to be tested observationally. Of these, perhaps the most prominent is the theory of competitive accretion, which suggests that massive protostars are "seeded" by low-mass protostars which compete with other protostars to draw in matter from the entire parent molecular cloud, instead of simply from a small local region.^[11][12]

Another theory of massive star formation suggests that massive stars may form by the coalescence of two or more stars of lower mass.^[13]

References