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=8 B)m=10 C)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=8 B)m=10 C)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 pc B)1.3 pc

Concept Question: Proxima Centauri 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 pc B)1.3 pc

d(pc) = 1/0.76“ = 1.3 pc

Concept Question: Proxima Centauri 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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Rest positions

Determining Stellar Masses: Binary Stars

Rest positions

Which star is more massive? A)A B)B

Concept Question: Binary Stars Masses

Rest positions

Which star is more massive? A)A—Star A has a slower orbital velocity B)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 table Stellar Classification: Spectral Sequence

Study this table

Oh Be A Fine Girl Kiss Me

Stellar Classification: Spectral Sequence

Fig. 16.10

The Hertzprung-Russell Diagram