GMU College of Science - astro 113 week6physics.gmu.edu/~pesce/astro113/astro_113_week6.pdf ·...

© Dr. Joseph E. Pesce, Ph.D.

Astronomy 113Dr. Joseph E. Pesce, Ph.D.

Travels on the H-R Diagram


9-4

The Hertzsprung-Russell Diagram


12-2

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


12-3

Main-Sequence Lifetimes


12-4

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


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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


12-7

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


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Dating clusters


12-14

Main Sequence Turnoff


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


12-16

Variable Stars


Thank You!


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