Download - Midterm Review
Midterm Review
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Sirius, 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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1. 0.385 pc2. 0.80 light years3. 1.255 pc4. 2.60 light years5. 8.47 light years
Distances of Stars
Trigonometric Parallax:Star appears slightly shifted from different
positions of the Earth on its orbit
The further away the star is (larger d), the smaller the parallax angle p.
d = __ p 1
d in parsec (pc) p in arc seconds
1 pc = 3.26 LY
Star 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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1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
Absolute Magnitude
The absolute magnitude measures a star’s intrinsic brightness (= luminosity).
Absolute Magnitude = Magnitude that a star would have if it were at
a distance of 10 pc.
If we know a star’s absolute magnitude, we can infer its distance by comparing
absolute and apparent magnitudes.
Which of these spectral types describes a Red Giant?
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1. O3V2. F9V3. B2Ia4. K5III5. G2V
Spectral Classification of Stars
Tem
pera
ture
Spectral Classification of Stars
Mnemonics to remember the spectral sequence:
Oh Oh OnlyBe Boy, BadA An AstronomersFine F ForgetGirl/Guy Grade GenerallyKiss Kills KnownMe Me Mnemonics
Luminosity ClassesIa Bright Supergiants
Ib Supergiants
II Bright Giants III Giants
IV Subgiants
V Main-Sequence Stars
IaIb
IIIII
IVV
Masses of Stars in the
Hertzsprung-Russell Diagram
0.5
18
6
31.7
1.00.8
40
Masses in units of solar masses
Low m
asses
High masses
Mass
The higher a star’s mass, the more luminous it is.
High-mass stars have much shorter lives
than low-mass stars
Sun: ~ 10 billion yr.10 Msun: ~ 30 million yr.0.1 Msun: ~ 3 trillion yr.
< 100 solar masses
> 0.08 solar masses
In a binary star system …
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1. The less massive stars orbits around the more massive one.
2. The more massive star orbits around the less massive one.
3. Both stars orbit on identical orbits around the mid-point between them.
4. Both stars orbit around their center of mass, which is closer to the less massive star.
5. Both stars orbit around their center of mass, which is closer to the more massive star.
The Center of Masscenter of mass =
balance point of the system.
Both masses equal => center of mass is in the middle, rA = rB.
The more unequal the masses are, the more
it shifts toward the more massive star.
Which law allows astronomers to calculate the masses of stars in binary systems?
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1. Newton’s first law2. Kepler’s third law3. Einsteins theory of general relativity4. Newton’s third law5. Kepler’s second law
Estimating Stellar MassesRewrite Kepler’s 3. Law as
1 = aAU3 / Py
2
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:
MA + MB = aAU
3 ____ Py
2
(MA and MB in units of solar masses)
Which is the most common type of binary star systems?
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1. Spectroscopic binaries2. Eclipsing binaries3. X-ray binaries4. Visual binaries (where both stars and
their motion can be resolved)5. Binary neutron stars
Spectroscopic Binaries
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
Which of these fusion mechanisms does NOT fuse
Hydrogen to Helium?
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1. Proton-proton chain2. CNO Cycle3. Triple-Alpha Process
The CNO Cycle
In stars slightly more massive than the
sun, a more powerful energy generation
mechanism than the PP chain takes over:
The CNO Cycle.
In the sun, energy production is
dominated by direct fusion of H into He
(PP chain).
Energy Transport Structure
Inner radiative, outer convective
zone
Inner convective, outer radiative
zone
CNO cycle dominant PP chain dominant
MassSun
Radiative Core, convective envelope;
Energy generation through PP Cycle
Convective Core, radiative envelope;
Energy generation through CNO Cycle
Summary:
Stellar Structure
What are “globules”?
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1. Small planetary bodies, still in the process of growing into planets (“globes”)
2. Large, cold, uncompressed molecular clouds that may eventually form thousands of stars.
3. Small, compressed pockets of dense gas that may form stars.
4. The remnants of the explosions of sun-like stars.5. The remnants of the explosions of high-mass stars.
(Bok) Globules
~ 10 – 1000 solar masses;
Contracting to form protostars
Compact, dense pockets of gas which
may contract to form stars.
Jets of gas ejected from protostellar disks are called …
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1. Globules2. Planetary Nebulae3. Novae4. Herbig-Haro Objects5. Pulsars
Herbig-Haro Objects
What happens in the Triple-Alpha Process?
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1. Fusion of Hydrogen to Helium2. Fusion of Helium to Carbon3. Fusion of Carbon to Neon4. Fusion of Silicon to Iron5. Nuclear fission of Uranium
Red Giant Evolution
4 H → He
He
He-core gets denser and hotter
until the next stage of nuclear burning can begin in the
core:
He fusion:
3 4He → 12C
“Triple-Alpha Process”
Fusion of Helium into Carbon
What is a “white dwarf”?
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1. A failed star that does not become hot enough to ignite nuclear fusion.
2. The burned-out remnant of a very low-mass star that never ignites Helium fusion.
3. The collapsed Carbon/Oxygen core of a sun-like star.
4. The collapsed iron core of a high-mass star.5. The collapsed iron core of a sun-like star.
White DwarfsDegenerate stellar remnant (C,O core)
Extremely dense:
1 teaspoon of WD material: mass ≈ 16 tons!!!
White Dwarfs:
Mass ~ Msun
Temp. ~ 25,000 K
Luminosity ~ 0.01 Lsun
Chunk of WD material the size of a beach ball would outweigh an ocean liner!
Summary of Post-Main-Sequence Evolution of Stars
M > 8 Msun
M < 4 Msun
Evolution of 4 - 8 Msun
stars is still uncertain.Fusion
stops at formation
of C,O core.
Fusion proceeds to formation of Fe core.
Red dwarfs: He burning
never ignitesM < 0.4 Msun
Which was the first method that allowed astronomers to measure the distances to other galaxies?
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1. Light-travel time measurements2. Gravitational-lensing measurements3. Trigonometric parallax4. Using Cepheid Variables5. Warp-Drive travel
Cepheid Variables:The Period-Luminosity Relation
The variability period of a Cepheid variable is correlated
with its luminosity.
=> Measuring a Cepheid’s period, we
can determine its absolute magnitude!
The more luminous it is, the more slowly it pulsates.
If you plot all stars of a star cluster on a Hertzsprung-Russell diagram:
Which feature will allow you to determine the cluster’s age?
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1. The brightness of red giants.2. The number of white dwarfs.3. The average surface temperature of neutron
stars.4. The turn-off point from the Main Sequence.5. The minimum mass of stars at the lower end
of the main sequence.
Example:HR diagram of the star cluster M 55
High-mass stars evolved onto the
giant branch
Low-mass stars still on the main
sequence
Turn-off point
The lower on the MS the turn-off
point, the older the cluster.