Pre-main sequence star
Encyclopedia
A pre-main-sequence star (also known as a PMS star and PMS object) is a star
in the stage when it has not yet reached the main sequence
. It can be a T Tauri star
or FU Orionis star (<2 solar
mass) or an Herbig Ae/Be stars (2-8 solar
mass).
The energy source of these objects is gravitational contraction (as opposed to hydrogen burning
in main-sequence stars). On the Hertzsprung–Russell diagram
, the pre-main-sequence stage of stars with masses>0.5 Solar mass
translates into a move along Hayashi track
s (almost vertically down) and later along Henyey tracks (almost horizontally to the left, towards the main sequence).
PMS stars can be differentiated from main-sequence dwarf stars by using stellar spectra to measure the correlation between gravity and temperature. A PMS star will have a larger radius than a main-sequence star, and thus be less dense and have lower surface gravity.
While the surrounding matter is falling onto the central condensation, it is considered to be a protostar
. When the surrounding gas/dust envelope disperses and accretion process stops, the star is considered as pre-main-sequence star.
Pre-main-sequence stars become optically visible after stellar birthline
. Pre-main-sequence stage will last less than 1% of a star's life (in contrast, the star will spend about 80% of its life on the main sequence).
It is believed that during this stage all stars have dense circumstellar disk
s, probable sites of planet formation.
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
in the stage when it has not yet reached the main sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...
. It can be a T Tauri star
T Tauri star
T Tauri stars are a class of variable stars named after their prototype – T Tauri. They are found near molecular clouds and identified by their optical variability and strong chromospheric lines.-Characteristics:...
or FU Orionis star (<2 solar
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
mass) or an Herbig Ae/Be stars (2-8 solar
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
mass).
The energy source of these objects is gravitational contraction (as opposed to hydrogen burning
Proton-proton chain reaction
The proton–proton chain reaction is one of several fusion reactions by which stars convert hydrogen to helium, the primary alternative being the CNO cycle. The proton–proton chain dominates in stars the size of the Sun or smaller....
in main-sequence stars). On the Hertzsprung–Russell diagram
Hertzsprung–Russell diagram
The Hertzsprung–Russell diagram is a scatter graph of stars showing the relationship between the stars' absolute magnitudes or luminosities versus their spectral types or classifications and effective temperatures. Hertzsprung–Russell diagrams are not pictures or maps of the locations of the stars...
, the pre-main-sequence stage of stars with masses>0.5 Solar mass
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
translates into a move along Hayashi track
Hayashi track
The Hayashi track is a path taken by protostars in the Hertzsprung–Russell diagram after the protostellar cloud has reached approximate hydrostatic equilibrium...
s (almost vertically down) and later along Henyey tracks (almost horizontally to the left, towards the main sequence).
PMS stars can be differentiated from main-sequence dwarf stars by using stellar spectra to measure the correlation between gravity and temperature. A PMS star will have a larger radius than a main-sequence star, and thus be less dense and have lower surface gravity.
While the surrounding matter is falling onto the central condensation, it is considered to be a protostar
Protostar
A protostar is a large mass that forms by contraction out of the gas of a giant molecular cloud in the interstellar medium. The protostellar phase is an early stage in the process of star formation. For a one solar-mass star it lasts about 100,000 years...
. When the surrounding gas/dust envelope disperses and accretion process stops, the star is considered as pre-main-sequence star.
Pre-main-sequence stars become optically visible after stellar birthline
Stellar birthline
The stellar birthline is a predicted line on the Hertzsprung–Russell diagram that relates the colour and luminosity of protostars at the end of the initial phase of accretion onto them. Prior to this point, the protostar is deeply embedded in the cloud of dust and gas from which it formed, and so...
. Pre-main-sequence stage will last less than 1% of a star's life (in contrast, the star will spend about 80% of its life on the main sequence).
It is believed that during this stage all stars have dense circumstellar disk
Circumstellar disk
A circumstellar disk is a torus, pancake or ring-shaped accumulation of matter composed of gas, dust, planetesimals, asteroids or collision fragments in orbit around a star. Around the youngest stars, they are the reservoirs of material out of which planets may form...
s, probable sites of planet formation.
See also
- Protoplanetary diskProtoplanetary diskA protoplanetary disk is a rotating circumstellar disk of dense gas surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star...
- ProtostarProtostarA protostar is a large mass that forms by contraction out of the gas of a giant molecular cloud in the interstellar medium. The protostellar phase is an early stage in the process of star formation. For a one solar-mass star it lasts about 100,000 years...
- Young stellar objectYoung stellar objectYoung stellar object denotes a star in its early stage of evolution.This class consists of two groups of objects: protostars and pre–main sequence stars. Sometimes they are divided by mass - massive YSO , intermediate mass YSO and brown dwarfs....