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Imagine three identical stars (meaning that they have the same intrinsic luminosity). The apparent magnitudes of the stars are given below; which star is the most distant?
A)m=8B)m=10C)m=12
Quiz Question: Magnitudes
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Imagine three identical stars (meaning that they have the same intrinsic luminosity). The apparent magnitudes of the stars are given below; which star is the most distant?
A)m=8B)m=10C)m=12
Star C has the largest apparent magnitude, hence it is the most distant.
Quiz Question: Magnitudes
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The absolute magnitude of a star is its luminosity.
The apparent magnitude of a star is its perceived brightness.
Brightnesses, Luminosities, & Magnitudes
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•How do astronomers measure the distances to nearby stars?•How do we get stellar masses from binary stars?•How do we classify stars?
Questions
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Fig. 16.3
Using the motion of the Earth to measure distances to stars
Recall: 1" = (1/60) of 1'1' = (1/60) of 1°
Distance (parallax seconds) = 1/parallax(")
1 parallax second = 1 parsec = 3.26 light years = 3.1x1016 m
d(pc) = 1/p(")
Stellar Parallax: Measuring the Distances to Nearby Stars
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The closest star to the Sun
Sun Proxima CentauriEarth
GolfBall
Grain of
Sand
GolfBall
1m 270km
Proxima Centauri is a little closer to the Sun than the average separation between stars in the Milky Way.
Case Study: Proxima Centauri
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d(pc) = 1/p(“)
What is the distance, in parsecs, to Proxima Centauri, given that it’s parallax angle is 0.76“?
A)0.76 pcB)1.3 pc
Concept Question: ProximaCentauri Parallax
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d(pc) = 1/p(“)
What is the distance, in parsecs, to Proxima Centauri, given that it’s parallax angle is 0.76“?
A)0.76 pcB)1.3 pc
d(pc) = 1/0.76“ = 1.3 pc
Concept Question: ProximaCentauri Parallax
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Although you cannot see it, stars are moving in all 3 dimensions.
α Cen22 km/s
-20 km/s(blueshift)
30 km/s net motion
Sun
Stellar Motions: α Centauri
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Astronomers use the relative motion of stars in binary systems, along with Kepler’s laws, to measure the masses of stars.
(Approximately half of all stars are in binary systems.)
Fig.
16.
09 &
Mat
h. In
sigh
t 16.
4
Restpositions
Determining Stellar Masses: Binary Stars
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Restpositions
Which star is more massive?A)AB)B
Concept Question: Binary Stars Masses
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Restpositions
Which star is more massive?A)A—Star A has a slower orbital velocityB)B
Concept Question: Binary Stars Masses
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Stars are classified based on their spectra, which are determined by their temperatures.
Fig. 16.04
1. Stars are like blackbodies: color ⇒temperature
2. Spectra absorption features allow a finer classification
Stellar Classification
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Study this tableStellar Classification: Spectral Sequence
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Study this table
OhBeAFineGirlKissMe
Stellar Classification: Spectral Sequence
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Fig. 16.10
The Hertzprung-Russell Diagram