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
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
The absolute magnitude of a star is its luminosity.
The apparent magnitude of a star is its perceived brightness.
Brightnesses, Luminosities, & Magnitudes
•How do astronomers measure the distances to nearby stars?•How do we get stellar masses from binary stars?•How do we classify stars?
Questions
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
The closest star to the Sun
Sun Proxima CentauriEarth
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Proxima Centauri is a little closer to the Sun than the average separation between stars in the Milky Way.
Case Study: Proxima Centauri
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
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
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
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.)
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Restpositions
Determining Stellar Masses: Binary Stars
Restpositions
Which star is more massive?A)AB)B
Concept Question: Binary Stars Masses
Restpositions
Which star is more massive?A)A—Star A has a slower orbital velocityB)B
Concept Question: Binary Stars Masses
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
Study this tableStellar Classification: Spectral Sequence
Study this table
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Stellar Classification: Spectral Sequence
Fig. 16.10
The Hertzprung-Russell Diagram
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