Transcript Lecture Notes-PPT

Star Formation
•Processes in Stellar Formation
•Sequence of Events
•Role of Mass in Stellar Formation
•Observational Evidence
•New Theories
Stellar Formation
Stages of Stellar Evolution
There are 7 distinct stages of
stellar development from
interstellar cloud to main
sequence star
These stages are characterized
by differing core and surface
temperatures and radii of the
prestellar object
Gravitational attraction drives
the evolutionary tract, leading
ultimately to nuclear fusion,
signaling the birth of a star
Stage 1—Interstellar Cloud
Dense, dark, and cold
interstellar cloud
Large—10-100 parsecs
across (1014 – 1015 km)
1000X mass of our Sun
Mainly atomic and
molecular gas
Gravitational instability in
cloud-caused by some
external event-triggers cloud
collapse
Interstellar Cloud Collapse-Stage 2
Stars form inside relatively dense
concentrations of interstellar gas known as
molecular clouds.
These regions are extremely cold, causing
the gas to clump to high densities.
Star formation begins when the denser parts
of the cloud core collapse under gravity.
These cores typically have masses around
104 solar masses.
As the cores collapse they fragment into
clumps around 0.1 parsecs in size and 10 to
50 solar masses in mass.
These clumps then form into protostars and
the whole process takes about 10 million
years.
Stage 3 to 5
Protostar H-R Diagram-Stage 4
Evolutionary tract followed by
contracting interstellar cloud
fragment
High luminosity results from
large size of gas cloud
Evolutionary track known as the
Kelvin-Helmholtz contraction
phase
Internal heat gradually diffuses
out and is radiated away
Evolutionary Time Scale
Route to Main Sequence
The track from stage 4 to stage 6 is
known as the Hayashi track
Stars on this track are called T
Tauri stars
Luminosity drops dramatically as
contraction occurs;core
temperature rises to 5 million K
Heat and gravity compete between
stages 6 and 7 until core reaches
about 10 million K; nuclear fusion
begins.
Stars of Different Masses
Features of the Hayashi Track
similar for each mass star
However, the time required to
arrive on the main sequence
differs considerably, decreasing
rapidly as the mass increases
Stars do not “evolve” along the
main sequence; they arrive at
some point on it depending on
their mass and composition
Relative Sizes of Different Mass Stars
Conditions for Stellar Stability
Conditions for Stellar Stability
Star Cluster Formation
When stars are born they develop from large clouds of molecular
gas. After the remnant gas is heated and blow away, the stars
collect together by gravity. During the exchange of energy
between the stars, some stars reach escape velocity from the
protocluster and become runaway stars. The rest become
gravitationally bound, meaning they will exist as collection
orbiting each other forever.
Star Clusters
Jewel Box-Young Cluster
M80-Old Cluster
Brown Dwarfs—Failed Stars
If a protostar forms with less
than 0.08 solar masses, nuclear
fusion never begins
This failed star is called a brown
dwarf, a planet sized object
Brown dwarfs still emit energy,
due to gravitational collapse
Brown dwarfs are important to
astronomy since they may be the
most common type of star out
there and solve the missing mass
problem
Brown dwarfs eventual fade and
cool to become black dwarfs.
Evidence of Stellar Formation
The region surrounding the
nebula M20 shows evidence of
contraction
A huge, dark molecular cloud
surrounds the visible nebula
Density and temperature are low
The glowing region of ionized
gas results directly from a
massive O-type star at stage 6 or
7 on its evolutionary track.
Evidence of Protostars
Star forming regions known
as "EGGs" are uncovered at
the end of this giant pillar of
gas and dust in the Eagle
Nebula (M16)
EGGs, short for evaporating
gaseous globules, are dense
regions of mostly molecular
hydrogen gas that fragment
and gravitationally collapse to
form stars.
Shock Waves and Star Formation
Carbon Star