Download ASTR 200 : Lecture 15 Ensemble Properties of Stars

ASTR 200 : Lecture 15
Ensemble Properties of Stars
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Midterm exam
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In lecture, October 28
Will contain short answer questions and problems
Formula sheet will be provided
No calculators
Will cover lectures and associated textbook readings
listed on the course web site up to and including
Friday Oct 21
• A previous year's midterm will be posted, but it is a
different format and course topic order
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Absence
• I will be away Monday-Thursday inclusive at the
Division of Planetary Sciences meeting
• Monday and Wednesday's lecture will be given by
Professor Hickson (last year's ASTR 200
instructor)
• HW4 will not be due until FRIDAY Oct 21, 4 PM
• TA office hours Wed+Thurs as normal; Prof
Gladman's Thursday office hour will be moved to
Friday Oct 21, 1:30-2:30
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We can measure stellar T and L
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In 1910, Ejnar Hertzsrung and Henry Norris Russell both independently decided to look at the dependence of luminosity on stellar surface temperature.
● Note, in practice that getting luminosity is tricky, and they used 'spectral class' as a proxy for temperature (see ASTR 205)
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The discovered that this was NOT a uniform diagram.
● Stars of a given temperature are not at all luminosities
● Given L = 4 R2T4 , one would think a star of fixed T could have any R, and thus any L, but no....
The HR diagram
100x Sun
Luminosity
Sun
0.01 Sun
~20,000 K
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~3,000 K
Temperature
The HR diagram
100x Sun
Luminosity
Sun
M
ain
Se
qu
en
ce
0.01 Sun
~20,000 K
6
~3,000 K
Temperature
Hipparcos
stars
Relatively close stars, so rare very luminous stars are sparse
● Temperature (top scale) increases to the left!
● The main sequence is the most populated because stars spend most of their life there
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The most luminous main sequence stars have ~3000 solar luminosities
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Mass-Luminosity relation
• Text gives:
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L~M3.9 for stars >0.7 solar masses
L~M2.6 for stars <0.7 solar masses
Mass-Luminosity relation
• Text gives:
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L~M3.9 for stars >0.7 solar masses
L~M2.6 for stars <0.7 solar masses
So 10x
So larger stars burn much brighter
• We haven't discussed the energy source yet
• But, since the energy source is inside the star, it is
reasonable to assume that the amount of energy E for
the star's life is proportional to the mass
• For large stars, L∝ M 4 is the rate energy is used
• So the total lifetime t must go as
M
−3
t∝ 4∝M
M
• The Main Sequence lifetime of the Sun is ~10 Gyr
so a 10 solar mass star will only live ~10 Myr (!)
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Along the main sequence:
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Masses:
● 60 to 0.05
solar masses ●Lifetimes:
● ~Myr to a
>trillion yr
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Off the main sequence:
Radii vary a lot
Super giants
~1000 times Sun
White Dwarfs
~0.01 time Sun Post main-sequence evolution
Most stars will leave the main sequence and evolve to become giants for a while
● Driven by changes in energy production in core
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The previous HR diagrams were for 'all stars
nearby', which have a variety of ages
• But star clusters exist, where all the stars are
thought to have been formed in a short interval of
time
M11 open cluster
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HR diagram for M11 star cluster
Luminosity
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Temperature
Luminosity
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This is the positions of a set of stars of a large mass range, all 200 Myr after start of collapse of their interstellar clouds, based on equations of stellar structure
(next week)
Temperature
Cluster+stars
Was formed ~200 Myr ago
Luminosity
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Temperature
?
Cluster+stars
Was formed ~200 Myr ago
Luminosity
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Temperature
Gas Cloud → Main Sequence → ???
• So, a large cloud (1000s to ~million solar masses) gets cold
enough that many cores collapse into stars, giving a cluster
• Each star clears gas disk away, but the cluster as a whole
also blows out all the remaining interstellar gas, shutting
down star formation
• The stars settle onto the main sequence for their `hydrogen
burning phase'
• When they finish, they leave and become giants, and then
– Explode in supernovae (outshine a galaxy!)
• leaving neutron stars or black holes behind
– Turn into planetary nebulae, ejecting their outer layers
• leaving a white dwarf
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Post main-sequence evolution
This is, thus, actually the `evolutionary track' on the HR diagram of a solar­mass star
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