Title: The Family of Stars
1Midterm Review
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Please press 1 to test your transmitter.
2Sirius, the brightest star in the sky, has a
trigonometric parallax of p 0.385 arc seconds.
What is its distance from Earth?
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- 0.385 pc
- 0.80 light years
- 1.255 pc
- 2.60 light years
- 8.47 light years
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3Distances of Stars
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d in parsec (pc) p in arc seconds
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1
d
p
Trigonometric Parallax
Star appears slightly shifted from different
positions of the Earth on its orbit
1 pc 3.26 LY
The further away the star is (larger d), the
smaller the parallax angle p.
4Star A has an apparent magnitude of mA 5.6 and
an absolute magnitude of MA 2.3. Star B has an
apparent magnitude of mB 0.6 and an absolute
magnitude of MB 2.3.Which of the following
statements is true?
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- The flux received from both stars is the same,
but star B is 5 times more luminous than star A,
so star B must be further away. - The flux received from both stars is the same,
but star B is 100 times more luminous than star
A, so star B must be further away. - Both stars are equally luminous, but the flux
received from star A is 5 times less than from
star B, so star A must be further away. - Both stars are equally luminous, but the flux
received from star A is 100 times less than from
star B, so star A must be further away. - Both stars are equally luminous, but the flux
received from star A is 5 times more than from
star B, so star B must be further away.
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5Absolute Magnitude
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Absolute Magnitude Magnitude that a star would
have if it were at a distance of 10 pc.
- The absolute magnitude measures a stars
intrinsic brightness ( luminosity).
If we know a stars absolute magnitude, we can
infer its distance by comparing absolute and
apparent magnitudes.
6Which of these spectral types describes a Red
Giant?
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- O3V
- F9V
- B2Ia
- K5III
- G2V
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7Spectral Classification of Stars
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Temperature
8Spectral Classification of Stars
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Mnemonics to remember the spectral sequence
Oh Oh Only
Be Boy, Bad
A An Astronomers
Fine F Forget
Girl/Guy Grade Generally
Kiss Kills Known
Me Me Mnemonics
9Luminosity Classes
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Ia Bright Supergiants
Ia
Ib
Ib Supergiants
II
II Bright Giants
III
III Giants
IV Subgiants
IV
V
V Main-Sequence Stars
10Masses of Stars in the Hertzsprung-Russell Diagram
lt 100 solar masses
Masses in units of solar masses
40
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High masses
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The higher a stars mass, the more luminous it is.
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gt 0.08 solar masses
1.7
1.0
High-mass stars have much shorter lives than
low-mass stars
Mass
0.8
0.5
Sun 10 billion yr.
Low masses
10 Msun 30 million yr.
0.1 Msun 3 trillion yr.
11In a binary star system
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- The less massive stars orbits around the more
massive one. - The more massive star orbits around the less
massive one. - Both stars orbit on identical orbits around the
mid-point between them. - Both stars orbit around their center of mass,
which is closer to the less massive star. - Both stars orbit around their center of mass,
which is closer to the more massive star.
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12The Center of Mass
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center of mass balance point of the system.
Both masses equal gt center of mass is in the
middle, rA rB.
The more unequal the masses are, the more it
shifts toward the more massive star.
13Which law allows astronomers to calculate the
masses of stars in binary systems?
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- Newtons first law
- Keplers third law
- Einsteins theory of general relativity
- Newtons third law
- Keplers second law
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14Estimating Stellar Masses
Rewrite Keplers 3. Law as 1 aAU3 / Py2
Valid for the Solar system star with 1 solar
mass in the center.
We find almost the same law for binary stars with
masses MA and MB different from 1 solar mass
aAU3
____
MA MB
Py2
(MA and MB in units of solar masses)
15Which is the most common type of binary star
systems?
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- Spectroscopic binaries
- Eclipsing binaries
- X-ray binaries
- Visual binaries (where both stars and their
motion can be resolved) - Binary neutron stars
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16Spectroscopic Binaries
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The approaching star produces blue shifted lines
the receding star produces red shifted lines in
the spectrum.
Doppler shift ? Measurement of radial velocities
? Estimate of separation a
? Estimate of masses
17Which of these fusion mechanisms does NOT fuse
Hydrogen to Helium?
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- Proton-proton chain
- CNO Cycle
- Triple-Alpha Process
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18The CNO Cycle
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In the sun, energy production is dominated by
direct fusion of H into He (PP chain).
In stars slightly more massive than the sun, a
more powerful energy generation mechanism than
the PP chain takes over The CNO Cycle.
19Energy Transport Structure
Inner convective, outer radiative zone
Inner radiative, outer convective zone
CNO cycle dominant
PP chain dominant
20Summary Stellar Structure
Convective Core, radiative envelope Energy
generation through CNO Cycle
Sun
Mass
Radiative Core, convective envelope Energy
generation through PP Cycle
21What are globules?
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- Small planetary bodies, still in the process of
growing into planets (globes) - Large, cold, uncompressed molecular clouds that
may eventually form thousands of stars. - Small, compressed pockets of dense gas that may
form stars. - The remnants of the explosions of sun-like stars.
- The remnants of the explosions of high-mass stars.
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22(Bok) Globules
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Compact, dense pockets of gas which may contract
to form stars.
10 1000 solar masses
Contracting to form protostars
23Jets of gas ejected from protostellar disks are
called
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- Globules
- Planetary Nebulae
- Novae
- Herbig-Haro Objects
- Pulsars
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24Herbig-Haro Objects
25What happens in the Triple-Alpha Process?
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- Fusion of Hydrogen to Helium
- Fusion of Helium to Carbon
- Fusion of Carbon to Neon
- Fusion of Silicon to Iron
- Nuclear fission of Uranium
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26Red Giant Evolution
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He-core gets denser and hotter until the next
stage of nuclear burning can begin in the core
4 H ? He
He
He fusion 3 4He ? 12C Triple-Alpha
Process Fusion of Helium into Carbon
27What is a white dwarf?
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- A failed star that does not become hot enough to
ignite nuclear fusion. - The burned-out remnant of a very low-mass star
that never ignites Helium fusion. - The collapsed Carbon/Oxygen core of a sun-like
star. - The collapsed iron core of a high-mass star.
- The collapsed iron core of a sun-like star.
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28White Dwarfs
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Degenerate stellar remnant (C,O core)
Extremely dense 1 teaspoon of WD material mass
16 tons!!!
Chunk of WD material the size of a beach ball
would outweigh an ocean liner!
White Dwarfs Mass Msun Temp. 25,000
K Luminosity 0.01 Lsun
29Summary of Post-Main-Sequence Evolution of Stars
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Fusion proceeds to formation of Fe core.
Evolution of 4 - 8 Msun stars is still uncertain.
Fusion stops at formation of C,O core.
M gt 8 Msun
Red dwarfs He burning never ignites
M lt 4 Msun
M lt 0.4 Msun
30Which was the first method that allowed
astronomers to measure the distances to other
galaxies?
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- Light-travel time measurements
- Gravitational-lensing measurements
- Trigonometric parallax
- Using Cepheid Variables
- Warp-Drive travel
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31Cepheid VariablesThe Period-Luminosity Relation
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The variability period of a Cepheid variable is
correlated with its luminosity.
The more luminous it is, the more slowly it
pulsates.
gt Measuring a Cepheids period, we can determine
its absolute magnitude!
32If you plot all stars of a star cluster on a
Hertzsprung-Russell diagram Which feature will
allow you to determine the clusters age?
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- The brightness of red giants.
- The number of white dwarfs.
- The average surface temperature of neutron stars.
- The turn-off point from the Main Sequence.
- The minimum mass of stars at the lower end of the
main sequence.
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33ExampleHR diagram of the star cluster M 55
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High-mass stars evolved onto the giant branch
Turn-off point
Low-mass stars still on the main sequence
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The lower on the MS the turn-off point, the older
the cluster.