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### Transcript of Quiz Question: jglenn/courses/astr_1120/ppt_lectures/classification.pdf same intrinsic luminosity)....

• 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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Grain of

Sand

Golf Ball

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

• 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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1 6.

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4

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