Star Formation
Stars are born when the densest, coldest parts of interstellar molecular clouds become unable to support themselves against gravity and collapse, building up protostars that accrete from disks and eventually ignite nuclear fusion.
Definition
Star formation is the process by which dense, cold concentrations of interstellar gas collapse under gravity and accrete mass to form new stars, together with their attendant disks and outflows.
Scope
The area covers the conditions and physics of stellar birth: the structure and gravitational stability of molecular clouds, the collapse of dense cores, the formation and growth of protostars by accretion, the circumstellar disks and bipolar outflows that accompany them, and the distribution of stellar masses captured by the initial mass function.
Sub-topics
Core questions
- What conditions allow interstellar gas to collapse into stars?
- How does a protostar grow and reach the main sequence?
- What role do disks and outflows play in stellar birth?
- What sets the distribution of masses of newborn stars?
Key concepts
- molecular cloud
- Jeans mass
- dense core
- protostar
- accretion disk
- bipolar outflow
- initial mass function
Key theories
- Gravitational collapse and the Jeans criterion
- A cloud collapses when its self-gravity overcomes the support from thermal pressure, turbulence, and magnetic fields; the Jeans criterion sets the mass and size above which collapse runs away, and dense cores within molecular clouds are the immediate sites of star birth.
- Inside-out collapse and disk accretion
- A marginally stable core collapses from the inside out, depositing matter onto a central protostar through a rotationally supported disk while bipolar outflows remove angular momentum, a picture developed in the standard model of low-mass star formation.
Mechanisms
Within cold molecular clouds, regions where gravity exceeds the combined support of pressure, turbulence, and magnetic fields begin to collapse; conservation of angular momentum flattens the infalling gas into a disk that feeds a growing protostar, while magnetized outflows shed angular momentum and disperse surrounding material until the new star is revealed.
Clinical relevance
Star formation governs the evolution of galaxies, the recycling of interstellar gas, and the production of new stars and planetary systems; the initial mass function it sets is a key input to models of galactic chemical and luminosity evolution and to interpreting the integrated light of distant galaxies.
History
Jeans formulated the gravitational-instability criterion in the early twentieth century, the standard inside-out collapse model of low-mass star formation was developed by Shu and collaborators in the 1970s and 1980s, and modern theory increasingly emphasizes turbulence and magnetic fields as reviewed by McKee and Ostriker.
Debates
- The role of turbulence versus magnetic support
- Whether molecular cloud support and the regulation of star formation are governed primarily by supersonic turbulence or by magnetic fields, and how the two interact to set the low efficiency of star formation, remains an active question in the field.
Key figures
- Frank Shu
- Christopher McKee
- Eve Ostriker
- Edwin Salpeter
Related topics
Seminal works
- shu1987
- mckee2007
Frequently asked questions
- Why don't all molecular clouds quickly turn into stars?
- Clouds are supported against collapse by turbulence and magnetic fields as well as pressure, and feedback from young stars disperses gas, so star formation is inefficient and only a small fraction of a cloud's mass becomes stars over its lifetime.
- Why do young stars have disks and jets?
- Collapsing gas carries angular momentum that prevents it from falling straight onto the star, so it settles into a rotating disk; accretion through the disk and associated magnetic fields launch bipolar jets that carry away angular momentum and allow the star to grow.