Formation of low and intermediate mass stars
The formation paradigm for low and intermediate mass (less than roughly 8 times the mass of our own sun) has been assembled over many years through the combination of observational and theoretical work. In isolated star formation, which takes place in the nearest molecular clouds such as Taurus and Ophiuchus, a cold clump of gas and dust collapses under self-gravity and forms a central star, still surrounded by its birth cloud. Due to conservation of angular momentum, some of the material collapsing onto the star forms a flattened disk, while other material falls directly onto the star from a surrounding envelope. At this stage, it is commonly accepted that a high velocity jet forms, which ejects only a small mass of material, but serves to rid of the star of much of its initial angular momentum and in the case of energetic outflows, injects energy back into the cloud. After roughly 1 million years (for the case of a 1 solar mass star; evolution proceeds more quickly for higher masses) the combination of outflow and infall disperses the majority of the envelope and the star is optically revealed. For solar-mass stars, this is the T Tauri phase, while for intermediate masses, these stars are referred to as Herbig Ae/Be stars. A substantial circumstellar disk is still present and many objects at this stage continue to power jets and winds. After several million years (again for a 1 solar mass star) the primordial disk is mostly depleted.
Effect of mulitiplicity and clusters
This sequence applies primarily to relatively isolated single stars. However, the majority of stars are known to be in binary or higher multiple systems and the details of binary formation are not as well understood. Very close (less than a few astronomical units (AU), where 1 astronomical unit is the distance between the earth and the sun) binaries generally follow the sequence above, and in particular, these stars are observed to have disks and jets with the same frequency as single stars. Very wide (more than a few hundred AU) binaries also have disks and jets. At intermediate separations, disks are observed less frequently, probably because the companion star disrupts the disk which are generally 100 AU in radius.
In a dense cluster environment, such as the Orion molecular cloud, the formation of low mass stars is also influenced by the presence of nearby massive stars. The radiation field and shocks can both trigger new star formation and influence the formation process.
Formation of high mass stars
The formation of high mass stars is considerably less well understood. A simple scaling of the low and intermediate mass scenario runs into the problem of radiation pressure from the massive protostar halting the accretion before enough material has accumulated to match the most massive stars in the galaxy.
There are two primary approaches to overcoming this difficulty. The first is for the accretion to continue through modifications of the collapse scenario, including accretion through a disk, higher accretion rates and radiatively driven instabilities. The second scenario involves a completely different paradigm in which the most massive stars are formed through mergers of lower mass stars or formation in clusters so dense that the accretion rate depends on the total cluster mass.
High mass stars can still be embedded and accreting after joining the main sequence. Hypercompact and ultracompact ionized hydrogen (HII) region stages, where the massive star is surrounded by a shell of ionized gas, follow the protostellar phase and massive young stellar objects drive jets and outflows as well.
Observations of young stars
Observations of young stellar objects are done with a variety of techniques. In the youngest stages, the optical and near-infrared light is absorbed by the surrounding cloud and envelope, and these objects are observed primarily at wavelengths longer than 100 microns. Millimeter and radio interferometry are often used to provide the resolution necessary to measure the disk and envelope sizes and to determine how many young stars are forming. After some of the primordial material has been accreted or dispersed, the young star becomes optically visible. Images in the optical and near-infrared can reveal the distribution of dust and gas around the star, as well as the presence of a jet or outflow. X-rays and ultraviolet observations are used to probe the most energetic aspect, the accretion of material directly onto the central star. Near-infrared interferometry can trace the inner disk and spectroscopy at all wavelengths is used to measure the gas and dust composition. The use of all these methods gives researchers a comprehensive view of young star, which is used to test theories of how they form and evolve.
Preview Image
NGC 2080, nicknamed "The Ghost Head Nebula," is one of a chain of star-forming regions lying south of the 30 Doradus nebula in the Large Magellanic Cloud. 30 Doradus is the largest star-forming complex in the local group of galaxies. This "enhanced color" picture is composed of three narrow-band-filter images obtained by Hubble on March 28, 2000. View full-size image. (Source: NASA/JPL - Photo Journal.)
Citation
Akeson, Rachel, Ph.D. (Contributing Author); Bernard Haisch (Topic Editor). 2009. "Star Formation." In: Encyclopedia of the Cosmos. Eds. Bernard Haisch and Joakim F. Lindblom (Redwood City, CA: Digital Universe Foundation). [First published December 13, 2007].
<http://www.cosmosportal.org/articles/view/138333/>
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Formation of low and intermediate mass stars
The formation paradigm for low and intermediate mass (less than roughly 8 times the mass of our own sun) has been assembled over many years through the combination of observational and theoretical work. In isolated star formation, which takes place in the nearest molecular clouds such as Taurus and Ophiuchus, a cold clump of gas and dust collapses under self-gravity and forms a central star, still surrounded by its birth cloud. Due to conservation of angular momentum, some of the material collapsing onto the star forms a flattened disk, while other material falls directly onto the star from a surrounding envelope. At this stage, it is commonly accepted that a high velocity jet forms, which ejects only a small mass of material, but serves to rid of the star of much of its initial angular momentum and in the case of energetic outflows, injects energy back into the cloud. After roughly 1 million years (for the case of a 1 solar mass star; evolution proceeds more quickly for higher masses) the combination of outflow and infall disperses the majority of the envelope and the star is optically revealed. For solar-mass stars, this is the T Tauri phase, while for intermediate masses, these stars are referred to as Herbig Ae/Be stars. A substantial circumstellar disk is still present and many objects at this stage continue to power jets and winds. After several million years (again for a 1 solar mass star) the primordial disk is mostly depleted.
Effect of mulitiplicity and clusters
This sequence applies primarily to relatively isolated single stars. However, the majority of stars are known to be in binary or higher multiple systems and the details of binary formation are not as well understood. Very close (less than a few astronomical units (AU), where 1 astronomical unit is the distance between the earth and the sun) binaries generally follow the sequence above, and in particular, these stars are observed to have disks and jets with the same frequency as single stars. Very wide (more than a few hundred AU) binaries also have disks and jets. At intermediate separations, disks are observed less frequently, probably because the companion star disrupts the disk which are generally 100 AU in radius.
In a dense cluster environment, such as the Orion molecular cloud, the formation of low mass stars is also influenced by the presence of nearby massive stars. The radiation field and shocks can both trigger new star formation and influence the formation process.
Formation of high mass stars
The formation of high mass stars is considerably less well understood. A simple scaling of the low and intermediate mass scenario runs into the problem of radiation pressure from the massive protostar halting the accretion before enough material has accumulated to match the most massive stars in the galaxy.
There are two primary approaches to overcoming this difficulty. The first is for the accretion to continue through modifications of the collapse scenario, including accretion through a disk, higher accretion rates and radiatively driven instabilities. The second scenario involves a completely different paradigm in which the most massive stars are formed through mergers of lower mass stars or formation in clusters so dense that the accretion rate depends on the total cluster mass.
High mass stars can still be embedded and accreting after joining the main sequence. Hypercompact and ultracompact ionized hydrogen (HII) region stages, where the massive star is surrounded by a shell of ionized gas, follow the protostellar phase and massive young stellar objects drive jets and outflows as well.
Observations of young stars
Observations of young stellar objects are done with a variety of techniques. In the youngest stages, the optical and near-infrared light is absorbed by the surrounding cloud and envelope, and these objects are observed primarily at wavelengths longer than 100 microns. Millimeter and radio interferometry are often used to provide the resolution necessary to measure the disk and envelope sizes and to determine how many young stars are forming. After some of the primordial material has been accreted or dispersed, the young star becomes optically visible. Images in the optical and near-infrared can reveal the distribution of dust and gas around the star, as well as the presence of a jet or outflow. X-rays and ultraviolet observations are used to probe the most energetic aspect, the accretion of material directly onto the central star. Near-infrared interferometry can trace the inner disk and spectroscopy at all wavelengths is used to measure the gas and dust composition. The use of all these methods gives researchers a comprehensive view of young star, which is used to test theories of how they form and evolve.
Preview Image
NGC 2080, nicknamed "The Ghost Head Nebula," is one of a chain of star-forming regions lying south of the 30 Doradus nebula in the Large Magellanic Cloud. 30 Doradus is the largest star-forming complex in the local group of galaxies. This "enhanced color" picture is composed of three narrow-band-filter images obtained by Hubble on March 28, 2000. View full-size image. (Source: NASA/JPL - Photo Journal.)
Citation
Akeson, Rachel, Ph.D. (Contributing Author); Bernard Haisch (Topic Editor). 2009. "Star Formation." In: Encyclopedia of the Cosmos. Eds. Bernard Haisch and Joakim F. Lindblom (Redwood City, CA: Digital Universe Foundation). [First published December 13, 2007].
<http://www.cosmosportal.org/articles/view/138333/>
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