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Star Processes and Formation
The Interstellar Medium (ISM)
Composition of the ISM
A. Composition by mass
- 75% H, 23% He, and
2% heavier elements such as C, N, O, etc.
- mostly gas plus a little dust
Space is not Empty!
Examples
Atmosphere
Vacuum cleaner
Incandescent light bulb
Ultra high vacuum chamber
Interplanetary space
Interstellar space
Intergalactic space
atoms / cm3
1019
8 x 1018
1015
106
10
0.1 to 1
10-6
Molecular Clouds
Molecular Clouds
Higher density regions of the ISM, often dark
A giant molecular cloud:
- 50 ly to 300 ly diameter
- 100,000 to 10,000,000 solar masses
- Higher Density than typical ISM
- Temperature = 10 K to 30 K (cool enough
for molecules to form)
Star Formation
Supernova explosions (exploding stars) produce shock
waves that compress the interstellar medium.
Star Formation
Another source of shock waves may be the birth of very
hot stars. When such a star is born, the sudden blast of
light, especially ultraviolet radiation, can ionize and drive
away nearby gas—forming a shock wave that could
compress nearby clouds and trigger further star
formation.
Star Formation
Even the collision of two interstellar clouds can produce a
shock wave and trigger star formation.
Star Formation
The dominant trigger of star formation in
our galaxy may be the spiral pattern itself.
As interstellar clouds encounter these spiral
arms, the clouds are
compressed, and star
formation can be
triggered.
Star Formation
Once begun, star formation can spread like
a grass fire.
Formation of Stars – Star Clusters
• A collapsing cloud of gas does not form a single object.
Due to instabilities, the cloud breaks into fragments—
producing hundreds to thousands of stars.
• http://www.astro.ex.ac.uk/people/mbate/Animations/
Formation of Stars – Emission Nebulae
• Star forming regions heat the gas to the point
where hydrogen is ionized, producing a typically red
color. These regions are known as H II regions or
emission nebulae.
Star Formation and MCC’s
• Observations of small very dense zones of
gas and dust called Bok globules within
larger nebulae probably represent the very
first stages of star formation. These are
also the same as an
MCC (molecular cloud
complex).
Formation of Stars- Protostars
• The initial collapse of the gas in an MCC forms a
dense core.
• As more gas falls in, a warm protostar develops—
buried deep in the dusty gas cloud that continues to
contract.
• Throughout its contraction, the protostar converts
its gravitational energy into thermal energy.
• As the internal temperature climbs, the center gets
hot enough to begin nuclear reactions, and a star is
born!
The Formation of Protostars
• The time a protostar
takes to contract from
a cool interstellar gas
cloud to a mainsequence star depends
on its mass.
• The more massive the
star, the stronger its
gravity and the faster
it contracts.
The Formation of Protostars
– The sun took about 30
million years to reach
the main sequence.
– In contrast, a 15-solarmass star can contract in
only 160,000 years.
– Conversely, a star of 0.2
solar mass takes 1 billion
years to reach the main
sequence.
Protostars
• Only when the protostar is hot enough to
drive away its enveloping cloud of gas
and dust does it become easy to observe
at wavelengths your
eye can see. But
before that it may be
observed in infrared
wavelengths, which
penetrate dust.
Young Stellar Objects (YSO’s)
• Observations of jets coming from hidden
protostars show that protostars are often
surrounded by disks of gas and dust.
• These jets appear to produce small flickering
nebulae called Herbig–Haro objects.
Young Stellar Objects (YSO’s)
• T Tauri stars: protostars surrounded by thick
disks of gas and dust that are very bright at
infrared wavelengths.
Star Formation in the Orion Nebula
Left: The Trapezium (hot young stars). Top Right: Proplyds (dusty sacks).
Bottom Right: Protoplanetary disk with protostar.
Evaporating Gaseous Globules:
EGGs
Baby stars are embedded in
thick clouds of gas and dust.
At their birth, they begin to
emit high amounts of UV
radiation and strong
superwinds, which help to
evaporate the cloud and
drive the dust away.
Young Stars, Young Planets
• Some young star systems are surrounded by
dusty rings of proto-solar systems.
Disk around Beta Pictoris. Star is occulted (blocked) for a better view.
Star Birth
• Stars are “born” when the core gets hot enough
to begin nuclear fusion.
• When fusion begins, its outward push generates
enough pressure to stop gravitational
contraction of the forming star and its size
stabilizes. This is called hydrostatic equilibrium.
• This balance of fusion vs. gravitational forces
keeps the star a stable size until late in its life.
During this time, the star is on the Main
Sequence of the H-R Diagram.
Hydrostatic Equilibrium
Brown Dwarfs
• Some protostars are not massive enough to ever
begin nuclear fusion, since they will never
achieve high enough temperatures and
pressures in their cores.
• These “wanna-be” stars still glow red from light
generated due to gravitational contraction. They
are known as brown dwarfs, but aren’t really
brown!
• How does gravity create heat? When a gas is
compressed, it converts some of its kinetic
energy to heat.
Brown Dwarfs
Basic Fusion Reactions
• The sun fuses four hydrogen nuclei to
make one helium nucleus.
– As one helium nucleus contains 0.7 percent less mass than
four hydrogen nuclei, some mass vanishes in the process.
– That mass is converted to energy.
– You can figure out how much by using Einstein’s famous
equation, E = mc2.
Basic Fusion Reactions
• The Sun has a voracious appetite and needs
1038 reactions per second, transforming 5
million tons of mass into energy every
second, just to replace the energy pouring
into space from its surface.
• It might sound as if the Sun is losing mass at a
furious rate. However, during its entire 10-billionyear lifetime, the Sun will convert less than 0.07
percent of its mass into energy.
Basic Fusion Reactions
Fusion happens only in the core. The simplest
and first reaction is:
4 1H → 4He + energy
– 1H represents a proton, the nucleus of a hydrogen atom.
– 4He represents the nucleus of a helium atom.
– The superscripts indicate the total number of protons and
neutrons in each nucleus.
Basic Fusion Reactions
Four H atoms becomes one He atom through
the proton-proton chain.
Advanced Fusion: The C-N-O Cycle
A second set of
fusion reactions
begin later at
higher
temperatures
and pressures.
They are known
as the C-N-O
Cycle.
The Life of a Main Sequence Star
• Stars begin their stable
lives fusing hydrogen
on the lower edge of
the main sequence,
known as the Zero-Age
Main Sequence (ZAMS)
The Life of a Main Sequence Star
• As stars age, gradual
changes in luminosity
and surface
temperature move
them upward and
slightly to the right.
• By the time they
reach the upper edge
of the main
sequence, they have
exhausted nearly all
the hydrogen in their
centers.
The Life of a Main Sequence Star
• A star’s main sequence
position indicates how
much hydrogen has
been converted to
helium in their cores.
• You can use the
position of a star in the
band, combined with
stellar evolution
models, as one way to
estimate the star’s age.
Life Spans of Main Sequence Stars
The H-R Diagram by Population