Download April 1st

Survey of the Universe
Tom Burbine
[email protected]
Quiz #4
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This Wednesday
Covers up to last Wednesday
Cumulative
You can bring in one 8 ½ by 11 inch piece of
paper with anything written on it
Life of a Star
• A star-forming interstellar cloud is called a
molecular cloud because low temperatures allow
Hydrogen to form Hydrogen molecules (H2)
• Temperatures like 10-50 K
• Contain Hydrogen and Helium with particles of
silicates, carbon, and iron coated with irces
Region is approximately 50 light years across
Condensing
• Molecular clouds tends to be lumpy
• These lumps tend to condense into stars
• That is why stars tend to be found in clusters
Protostar
• The dense cloud fragment gets hotter as it
contracts
• The cloud becomes denser and radiation cannot
escape
• The thermal pressure and gas temperature start to
rise and rise
• The dense cloud fragment becomes a protostar
When does a protostar become a star
• When the core temperatures reaches 10 million K,
hydrogen fusion can start occurring
• http://www.youtube.com/watch?v=7YhRW9kLghY
T-Tauri Stars
• Pre-main-sequence stars that vary erratically in
brightness
• Because nuclear reactions have not yet begun in the
protostar’s core, this luminosity is due entirely to the
release of gravitational energy as the protostar continues
to shrink and material from the surrounding fragment
• http://www.youtube.com/watch?v=W13ZYepDB
vo
• http://www.youtube.com/watch?v=QFklLMB_Z
OI&feature=related
Brown Dwarfs
• Failed stars
• Not enough mass for fusion
• Minimum mass of gas need for fusion is 0.08
solar masses (80 times the mass of Jupiter)
Main Sequence
• Is not an evolutionary track
– Stars do not evolve on it
• Stars stop on the main sequence and spend most
of their lives on it
Hydrostatic Equilbrium
Fusion reaction in high mass -stars
Main Sequence Lifetimes
• The more massive a star on the main sequence,
the shorter its lifetime
• More massive stars do contain more hydrogen
than smaller stars
• However, the more massive stars have higher
luminosities so they are using up their fuel at a
much quicker rate than smaller stars
Main Sequence Lifetime (t)
• t = M/L x 1010 years
• M is in Solar masses and L is in Solar Luminosities
• For example, Sirius has a mass of 2 solar masses
and a luminosity of 20 solar luminosities
• t = (2/20) x 1010 years = 0.1 x 1010 years
Ages
• Universe is thought to be about 14 billion years
old
• So less massive stars have lifetimes longer than
the age of the universe
• More massive stars have ages much younger
• So stars must be continually forming
• Blue stars have formed recently since they have
young main sequence lifetimes
Core Uses up hydrogen
• When the core uses up its hydrogen
• Gravity causes the core to shrink
• The star leaves the main sequence
Any Questions?