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Unformatted text preview: Proto-Stars Star or Gas? Gas Cloud
Contraction Explosion Star How Are Stars Born? Stellar Evolution
The energy radiated by stars comes from thermonuclear reactions, which
consume hydrogen each second and convert it into helium.
The amount of hydrogen in a star is vast, but it is not infinite.
Therefore, stars must not have always been shining, nor can they continue
to shine forever.
Thus, stars must have had a beginning as well as (someday) have an end. Variety of Stellar Ages Components of the ISM
H II Regions
H I Regions
Cold Clouds Interstellar Dust
Reddening Giant Molecular Clouds H II Regions
The process for producing clouds
of glowing gas near hot stars is
called fluorescence. The light
emitted from regions of ionized gas
consists largely of emission lines,
so they are also called emission
nebulae. Dark Nebulae
While dust clouds are invisible in the
optical region of the spectrum, they
glow brightly in the infrared. Small
dust grains absorb optical and UV
radiation very efficiently. The
grains are heated by the absorbed
radiation, typically to between 20
and 500 K, and reradiate this heat at
IR wavelengths. Giant Molecular Clouds
In many cases, individual clouds
have gathered into large complexes
containing a dozen or more discrete
clumps. Since the large molecular
clouds and complexes are the sites
where star formation occurs, most
young stars are also to be found in
spiral arms. Hydrostatic Equilibrium
What conditions must exist for star formation to begin? Sir James Jeans
(1877 - 1946) first investigated this problem in 1902 by considering the
effects of small deviations from hydrostatic equilibrium. Hydrostatic Equilibrium – At each
layer, the downward force of gravity is
just balanced by the upward pressure of
the material. Virial Theorem
The Virial Theorem describes the condition of equilibrium for a stable,
2K + U = 0
2 (Kinetic Energy) + (Potential Energy) = 0 Jean’s Instability Conditions
If 2 x Pressure > Gravity Then Gas Pressure dominates and no collapse If 2 x Pressure < Gravity Then Self gravity dominates and collapse occurs Example 1
T = 50 K n = 500 /cm3 All Hydrogen o = mH nH = 8.4 x 10-22 g/cm3
MJ = 1500 M
which is much greater than the 1 - 100 M in an HI cloud Example 2
Giant Molecular Cloud
T = 150 K n = 108 /cm3 All Hydrogen o = mH nH = 2 x 10-16 g/cm3
MJ ~ 17 M
which is much less than the 100 - 1000 M in a GMC
Tff = 4700 year Proto-Star Stages
1. The initial gravitational collapse from interstellar matter is relatively quick.
Once the condensation is about 1000 AU in diameter, the time for it to reach
hydrostatic equilibrium is measured in thousands of years.
2. Pre-main-sequence gravitational contraction is much more gradual. From
the onset of hydrostatic equilibrium to the main sequence requires millions of
years. For stars with masses just barely high enough to ignite hydrogen
burning, this phase of evolution can take as long as 100 million years. Hayashi Limit Proto-Star Stages
3. Subsequent evolution on the main sequence is very slow, for a star changes
only as thermonuclear reactions alter its chemical composition. For a star of
1 solar mass, this gradual process requires billions of years. All evolutionary
stages are relatively faster in stars of high mass and slower in those of low
mass. Evolutionary Tracks Function of Mass
In general, the pre-main-sequence evolution of a star slows down as the
star moves along its evolutionary track toward the main sequence.
The time for the whole evolutionary process is highly mass-dependent.
Stars of mass much higher than the Sun’s reach the main sequence in a
few thousand to a million years.
The Sun required millions of years; tens of millions of years are required
for stars to evolve to the lower main sequence. PRS Question
1. The source of a protostar’s heat is
a. Nuclear reactions converting H into He in its core
b. Gravitational energy released as the protostar expands
c. Gravitational energy released as the protostar contracts
d. Nuclear reactions converting He to C and O in its core
e. Radioactive decay of particular isotopes Not Understood
1. Lower mass limit Objects under 0.08 M are not able to generate
thermonuclear reactions. These “brown dwarfs” are technically not
stars nor are they planets, for Jupiter’s mass is only 0.001 M. Many
brown dwarfs should exist, but only a few candidates have been
2. Upper mass limit The upper mass limit is harder to calculate, but it is
somewhere from 50 to 100 M. (Stars above 30 M are extremely
rare.) The internal pressures are so much greater than the self-gravities
that these stars are blown apart from within. Not Understood
3. Mass distribution The cluster of stars that is formed contains many
more low mass stars than intermediate mass ones. Likewise, there are
more intermediate mass objects than high mass stars. As stellar mass
increases, the number of stars per that mass decreases.
4. Multiple stars About half of the protostars form gravitationallybound binary star systems. It is believed that single stars are the only
ones that can be encircled with planets. ...
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This note was uploaded on 03/04/2012 for the course PHYS 2022 taught by Professor Jarrio during the Spring '12 term at Central GA Tech.
- Spring '12