Onset of Collapse:
- The interstellar cloud: 10's
of pc across, usually a molecular cloud in which H2
and other molecules form [CO, H2O,
...] as well as dust, with T = 10-100 K, about 1000 atoms/cc.
- Helping factors: The main one
is gravity; Collapse can possibly be triggered by shock waves
from events like supernova explosions (or galaxy collisions,
and possibly large "objects" such as globular clusters
passing by), which can start a chain reaction.
- Opposing factors: The main ones
are heat and rotation [and magnetism]; Nearby massive stars can
also prevent star formation by heating and stirring the interstellar
matter; Turbulence can compress insterstellar matter in some
locations, but its overall effect is to oppose collapse.
- Rates: A few
stars are probably born each year on average in our galaxy; The number
is much higher in active galaxies.
Early Stages and Newborn
- Initial collapse: The cloud
gets warmer but energy escapes as (infrared) radiation.
- Fragmentation: The cloud splits
into fragments, 0.01 pc wide or so, each of which will produce
a star, or a multiple system, depending on its rotation.
- Protostar: Dense enough not
to be transparent; Becomes hotter, and is initially very bright;
Has a surface, emits IR and protostellar wind.
- Newborn star: T above
10 MK in the core ignites fusion of H into He and halts collapse.
- Arrival on the Main Sequence: After
a total of 40-50 Myr for the Sun; (more than 100 Myr for small stars,
less than 1 Myr for massive ones).
- Range of sizes: Masses range
from 0.08 solar masses (below that value objects do not have
enough mass for H fusion to start and are not considered stars)
to about 100 solar masses (more massive objects would be unstable,
the heat and pressure of the forming stars ejecting any extra
- Large stars: Some very large
stars exist; the Pistol Star has 100 solar masses and LBV 1806-20,
150 (it is 40 million times brighter than the Sun); Even more
massive stars may have formed in the early universe, when the
abundance of heavy elements was very low.
Brown dwarfs: Objects below 0.08 solar
masses (between 13 and 74 Jupiter masses), in which there can be some
D burning (collapse halted not by temperature); They are faint and hard
to see, but may be very common (for example in stellar nurseries like r Oph).
- Features of young stars: Most
are surrounded by a protostellar disk of leftover matter, flattened
by rotation and swept by a strong stellar wind; Sometimes we
see jets and blobs of matter.
- Herbig-Haro objects: Young stars
emitting high-velocity gas [near 300 km/s], which collides with
a surrounding nebula of interstellar material, heating it to
sufficiently high temperatures to make it glow and produce X-rays.
- [Other special type: T Tauri
stars, erratically variable pre-main sequence stars.]
- Fate of disk: Forms planets
(evidence accumulates!), or is blown away by stellar wind and
Result of Star Formation
- Size distribution: The star
formation process leads to star associations and clusters, with
a few bright stars and many small ones; A good example is the
Orion Nebula nursery. Stars above 100 solar masses don't
form (they would be unstable) and Objects below 0.08
solar masses (about 80 Jupiters) are not stars; they are hard
to see but may be common.
- Speed of evolution: The same difference
in the rate of evolution before the main
sequence applies to later stages – the more
massive the star, the faster it evolves.
- Fate of clusters: The more massive
stars explode before clusters can change significantly; Smaller
stars go their own way later or are kicked out by a gravitational encounter
(possibly what happened to the Sun), and open clusters dissolve over
billions of years.
page by luca bombelli <bombelli at olemiss.edu>,
modified 29 sep 2012