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Star Formation
Life begins in a dense region of
gas and dust
Molecular Clouds
In these nebulae much of the
hydrogen is in the molecular (H2)
form, so these nebulae are called
molecular clouds.
The largest such formations are
called giant molecular clouds
(GMC).
GMCs typically have diameters of
100 light-years, masses of up to 6
million solar masses, and an average
interior temperature of 10 K.
The nearest nebula to the Sun where
massive stars are being formed is the
Orion nebula, 1,300 ly away.
Orion
Nebula
Eagle Nebula
GMC Collapse
GMCs can begin to collapse, where
denser areas begin to gravitationally
attract more gas and dust.
The collapse can also be triggered:
- Supernova shock wave
- Collision of galaxies
The collapse continues until the
temperature and pressure climb high
enough to stop the collapse.
The central core of this collapse
become a “protostar”.
Stars form in clusters of 100 to 1000.
The Interstellar Medium
• Today stars form in a complex environment
• Hydrogen and helium are the predominant
components of the ISM, but it is enriched with
heavier elements from earlier stars (created in
stellar fusion and supernova explosions).
• Stellar formation also must deal with the
presence of energetic stars, magnetic fields
and supernovae explosions.
Protostar Becomes A Star
• Once sufficient temperature (>= 10 million
degrees K) and pressure is reached in the core of
the protostar, nuclear fusion begins and the
protostar has now officially become a star
• 2/3 of the stars form in multiple star systems
• Stars range from 0.08 to ~100 solar masses
• <0.08 solar mass = brown dwarf
Radiation from hot, young stars has blown gas
away from the center of the Rosetta Nebula
Hertzsprung-Russell Diagrams
• H-R diagrams are
graphs that plot the
luminosity (i.e. the
total energy output of
a star in watts) versus
surface temperature
•When looking at a
group of stars, the data
points do not fall
randomly.
Main-Sequence Lifetimes
Future
of Our
Sun
Lifetime of a Star Depends on its Mass
• Mass is essentially the sole determining
factor in how long a star lives
•The more massive a star, the greater the temp
and press in its core and the greater the rate of
hydrogen fusion
•25 solar mass star: 3 million year lifetime
•1 solar mass star: 10 billion year lifetime
•0.5 solar mass star: 200 billion year lifetime
Post MainSequence
Evolution
The
Instability
Strip
Period-Luminosity Relation of Cepheid Variable Stars
Cepheid Variables Help Measure
Large Distances
• Cepheid variable stars have a well defined
relationship between their energy output (i.e.
luminosity) and their period of getting
brighter, then dimmer, then brighter again.
•Cepheid variable are very bright (102 – 104
solar luminosity), so can be seen from a long
distance
Thank you! You’ve been
a stellar audience!
The End