Protostars, nebulas and Brown
dwarfs
• http://www.space.com/18698-brown-dwarfsmake-rocky-planets-too-video.html
Stars form inside relatively dense concentrations of interstellar gas and dust
known as molecular clouds. These regions are extremely cold (temperature about
10 to 20K, just above absolute zero). At these temperatures, gases become
molecular meaning that atoms bind together. CO and H2 are the most common
molecules in interstellar gas clouds. The deep cold also causes the gas to clump to
high densities. When the density reaches a certain point, stars form.
Since the regions are dense, they are opaque to visible light and are known as
dark nebula. Since they don't shine by optical light, we must use IR and radio
telescopes to investigate them.
19.4 Observations of Cloud Fragments
and Protostars
Emission nebulae are heated
by the formation of stars
nearby. In these images, we
see the parent cloud in stage
1, contracting fragments
between stages 1 and 2, and a
new star in stage 6 or 7. The
new star is the one heating the
nebula.
The evolution of young stars is from a cluster of protostars deep in a molecular
clouds core, to a cluster of T-Tauri stars whose hot surface and stellar winds heat
the surrounding gas to form an HII region (HII, pronounced H-two, means ionized
hydrogen). Later the cluster breaks out, the gas is blown away, and the stars evolve
as shown below.
19.4 Observations of Cloud Fragments
and Protostars
The Orion Nebula has many contracting cloud fragments,
protostars, and newborn stars
Protostars:
Once a clump has broken free
from the other parts of the cloud
core, it has its own unique
gravity and identity and we call
it a protostar. As the protostar
forms, loose gas falls into its
center. The infalling gas
releases kinetic energy in the
form of heat and the
temperature and pressure in the
center of the protostar goes up.
Several candidate protostars
have been found by the Hubble
Space Telescope in the Orion
Nebula.
19.4 Observations of Cloud Fragments
and Protostars
These are two protostars in the Orion Nebula, at around
stage 5 in their development
19.4 Observations of Cloud Fragments
and Protostars
Protostars are believed
to have very strong
winds, which clear out
an area around the star
roughly the size of the
solar system
The T-Tauri phase is when a
star has:
1. vigorous surface activity
(flares, eruptions)
2. strong stellar winds
3. variable and irregular light
curves
Once a protostar has
become a hydrogenburning star, a strong
stellar wind forms, usually
along the axis of rotation
with a flow of gas out the
poles of the star.. This
early phase in the life of a
star is called the T-Tauri
phase.
Protostar outflow
These two jets are matter being expelled from around
an unseen protostar, still obscured by dust on Orion’s
molecular cloud.
Discovery 19-1:
Observations of Brown Dwarfs
Brown dwarfs are difficult to observe directly, as they are very
dim. These images are of two binary-star systems, each
believed to contain a brown dwarf. The difference in luminosity
between the star and the brown dwarf is apparent.
Brown star formation
If a protostar forms with a mass less than 0.08 solar
masses, its internal temperature never reaches a value
high enough for thermonuclear fusion to begin. This
failed star is called a brown dwarf, halfway between a
planet (like Jupiter) and a star. The core becomes
degenerate before the start of fusion. There is enough
energy from the collapse to cause the brown dwarf to
shine for over 15 million years. Brown dwarfs fade and
cool to become black dwarfs.
Brown dwarf
In galaxies we find clusters
of young stars near other
young stars, called
supernova induced star
formation. The very
massive stars form first
and explode into
supernova. This makes
shock waves into the
molecular cloud, causing
nearby gas to compress
and form more stars,
young stars are found
near other young stars
and causes the pinwheel
patterns we see in
galaxies.