gmu college of science - astro 113 week6physics.gmu.edu/~pesce/astro113/astro_113_week6.pdf ·...
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© Dr. Joseph E. Pesce, Ph.D.
Astronomy 113Dr. Joseph E. Pesce, Ph.D.
Travels on the H-R Diagram
© Dr. Joseph E. Pesce, Ph.D.
9-4
The Hertzsprung-Russell Diagram
© Dr. Joseph E. Pesce, Ph.D.
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Main Sequence
³ Most of a star’s life is spent on M.S.² More massive, the faster it �lives�
q High mass, means high gravity leading to higher T in core, so faster fusion rates• O & B stars, 106 yrs on M.S.
q Low mass, just the opposite• M stars, trillions of yrs on M.S.
L, M
T
Main Sequence, Stars fusing H in cores
H He
H
© Dr. Joseph E. Pesce, Ph.D.
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Main-Sequence Lifetimes
© Dr. Joseph E. Pesce, Ph.D.
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Hydrogen Conversion
³ When Hydrogen is completely converted to Helium -fusion stops
³ Why? ³ No radiation pressure to support gravitational collapse
H
H He
He
Shell H fusionMore efficient radiative
transfer
Envelope expands (cools) to 3500 K - red, but L is high (surface area bigger) = Red Giant
© Dr. Joseph E. Pesce, Ph.D.
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Red Giant
³ Envelope becomes so extended, it �leaks� away Mass Loss
L, M
T
Red Giants
³ Helium �ash� dumps onto core, increasing mass, causing it to shrink and heat up
The core is about the size of the
Earth© Dr. Joseph E. Pesce, Ph.D.
12-6
Helium Core Flash³ Core heats and when T ~ 108 K, Core Helium Fusion begins
3 He 1 C + g1 C + 1 He 1 O + g
³ Helium fusion only lasts ~20% the Main Sequence lifetime² Why?
³ In ordinary gas, contraction leads to heating leading to expansion leading to cooling, a safety valve
³ Core contracts so much to heat up it squeezes electrons together and core becomes degenerate
³ Degenerate gas does not act like an ordinary gas = no safety valve. So core FLASHES in seconds, explosively, and degeneracy is lifted
After core helium ignition, star is smaller, dimmer, & hotter
T
L, M
* He Flash
H
He
C,OHe
© Dr. Joseph E. Pesce, Ph.D.
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Globular Clusters³ Old star clusters outside of, but orbiting galaxies³ 100,000s of stars all born simultaneously³ By plotting their HR diagrams, we can determine ages by
their turn-off points³ Turn off point = age of stars here (where H fusion is just
finishing in core), and thus the age of the cluster
Horizontal Branch = post-helium flash are of H-R diagram (He fusion in core)
Open Clusters = Typically younger and smaller star clusters within a galaxy
© Dr. Joseph E. Pesce, Ph.D.
© Dr. Joseph E. Pesce, Ph.D.
© 2007-2014 Joseph E. Pesce, Ph.D.© Dr. Joseph E. Pesce, Ph.D.
© 2007-2014 Joseph E. Pesce, Ph.D.© Dr. Joseph E. Pesce, Ph.D.
© 2007-2014 Joseph E. Pesce, Ph.D.© Dr. Joseph E. Pesce, Ph.D.
© 2007-2014 Joseph E. Pesce, Ph.D.© Dr. Joseph E. Pesce, Ph.D.
© 2007-2014 Joseph E. Pesce, Ph.D.© Dr. Joseph E. Pesce, Ph.D.
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Dating clusters
© Dr. Joseph E. Pesce, Ph.D.
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Main Sequence Turnoff
© Dr. Joseph E. Pesce, Ph.D.
12-15
Variable Stars
³ Core fusion in red giant branch changes lead to varying energy output
² Stars pulsate in brightness
³ Cepheid variables have a relationship between period of pulsation and luminosity
³ Why important?
² Bright and can see them at large distances
² Measure L for objects with known distance
q Then measure period for more distant objects and get M
q Measure m and get D!!
² Hubble Key Project
© Dr. Joseph E. Pesce, Ph.D.
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Variable Stars
© Dr. Joseph E. Pesce, Ph.D.
Thank You!
© Dr. Joseph E. Pesce, Ph.D.