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Stellar Evolution
(for low mass stars
like the Sun)
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Stellar Evolution... Refers to the life cycle of stars
is based on the H-R diagram. Theoretical plots of the H-R diagram are compared
to real star data and observations.
(Computer simulations of the nuclear reactions
inside a star match up with what we observe. )
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I. First stage of a stars life:
Protostar A. protostar formed by
gravitational contraction(accretion) of dust/gas
B. contraction causes
1. Pressure to rise
2. Temp. to rise
C. Infrared and
microwave radiationgiven off; radiationsweeps away gases andmay cause interstellargases to glow
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http://oposite.stsci.edu/pubinfo/pr/1997/orion/orion.html8/2/2019 Stellar Evolution of Low Mass Stars
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Star-disk systems in Orion's Trapezium
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II. Protostar to Main Sequence Star
D. At temps. around10 million K, nuclearfusion (hydrogen to
helium) takes place. E. diagram
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F. hydrostaticequilibrium--a balancebetween pressure of
gases and gravity
Hydrostatic
equilibrium: the star isnow stable and remains
main sequence for themajority of its lifeour
Sun is a class G2V star
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G. How does massaffect the time of starformation?
--larger massescontract faster and
burn brighter
H. How does themass of mainsequence star affect
its lifetime? --larger masses diefaster (run out ofhydrogen faster)
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The Main Sequence Lifetime for
Stars of Different Masses
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Red Giants
C. energy released causes 1. Star to be brighter
2. Size to increase
D. Expansion causes the
overall surface temperatureto decrease
E. Further contraction at thecore causes Heavier
elements to form (carbon &oxygen)
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Red Giants
F. Examples are
1. Betelgeuse(actually a red
supergiant)
2. Arcturus
(appear red/orange
in the sky)
IV Pl t b l t
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IV. Planetary nebula stage
Red Giant star loses mass, forming
a planetary nebula
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V. White dwarf starsthese are Ancient, White Dwarf
Stars in the Milky Way Galaxy
A. Whitedwarfsradiate theirenergy andwill fadeout
over time(over fewmillionyears).
Their
collapse isprevented bydegenerateelectrons.
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For our sunall this takes ~10 billion years from beginning to end.
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Whats a brown dwarf?A failed star too small to start H fusion to He
Hard to find because they are very faint
and cool; emit mostly in the infrared.Many have been detected in star forming
regionslike the Orion Nebula.