stellar evolution of low mass stars

8/2/2019 Stellar Evolution of Low Mass Stars

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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.html


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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.

stellar evolution of low mass stars

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