# Chapter 15 Surveying the Stars Pearson Education, Inc.

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1 Chapter 15 Surveying the Stars

2 15.1 Properties of Stars Our goals for learning: How do we measure stellar luminosities? How do we measure stellar temperatures? How do we measure stellar masses?

3 1. How do we measure stellar luminosities?

4 The brightness of a star depends on both distance and luminosity.

5 Luminosity: Amount of power a star radiates (energy per second = watts) Apparent brightness: Amount of starlight that reaches Earth (energy per second per square meter)

6 brightness ~ luminosity (radius) 2 For Sphere 1: brightness ~ luminosity (1) 2 For Sphere 2: brightness ~ luminosity (2) 2 = luminosity 4 For Sphere 3: brightness ~ luminosity (3) 2 = luminosity 9

7 The relationship between apparent brightness and luminosity depends on distance: Brightness = Luminosity 4π (distance) 2 We can determine a star s luminosity if we can measure its distance and apparent brightness: Luminosity = 4π (distance) 2 (brightness)

8 Thought Question How would the apparent brightness of Alpha Centauri change if it were three times farther away? A. It would be only 1/3 as bright. B. It would be only 1/6 as bright. C. It would be only 1/9 as bright. D. It would be three times brighter.

9 2. How do we measure the distance to a star? So how far away are these stars?

10 Parallax is the apparent shift in position of a nearby object against a background of more distant objects.

11 Apparent positions of nearest stars shift by about an arcsecond as Earth orbits Sun. Earth in Jan.

12 Parallax angle depends on distance. Earth in July

13 Picture 1 (Dic.) Picture 2 (May) Parallax is measured by comparing snapshots taken at different times and measuring the shift in angle to star.

14

15 Parallax and Distance p = parallax angle d (in parsecs) = 1 p (in arcseconds) d (in light-years) = p (in arcseconds)

16 3. How do we measure stellar temperatures?

17 How do we measure stellar temperatures? Way 1: The color High energy (hot) Blue Low energy (cold) Red Every object emits thermal radiation with a spectrum that depends on its temperature.

18 How do we measure stellar temperatures? Way 2: Level of ionization Absorption lines in star s spectrum tell us its ionization level.

19 Lines in a star s spectrum correspond to a spectral type that reveals its temperature. (Hottest) O B A F G K M (Coolest) Oh, Be A Fine Girl, Kiss Me

20 Thought Question Which kind of star is hottest? A. M star B. F star C. A star D. K star

21 The orbit of a binary star system depends on strength of gravity (masses).

22 Types of Binary Star Systems Visual binary Eclipsing binary Spectroscopic binary About half of all stars are in binary systems.

23 Visual Binary We can directly observe the orbital motions of these stars.

24 Eclipsing Binary We can measure periodic eclipses.

25 Spectroscopic Binary We determine the orbit by measuring Doppler shifts.

26 We measure mass using gravity. Direct mass measurements are possible only for stars in binary star systems. 4π 2 p 2 = a 3 G (M 1 + M 2 ) p = period a = average separation

27 What is a Hertzsprung-Russell diagram?

28 The H-R (Hertzsprung-Russell) diagram - An H-R diagram plots the luminosity and temperature of stars. Luminosity - Most stars fall somewhere on the main sequence of the H-R diagram. - Stars with lower T and higher L than mainsequence stars must have larger radii. These stars are called giants and supergiants. Temperature - Stars with higher T and lower L than mainsequence stars must have smaller radii. These stars are called white dwarfs.

29 What is the significance of the main sequence?

30 Main-sequence stars are fusing hydrogen into helium in their cores like the Sun. - Luminous main-sequence stars are hot (blue). Less luminous ones are cooler (yellow or red). Mass measurements of main-sequence stars show that the hot, blue stars are much more massive than the cool, red ones. - Massive blue stars die first, followed by white, yellow, orange, and red stars.

31 Main-Sequence Star Summary High-Mass Star: High luminosity Short-lived Larger radius Blue Low-Mass Star: Low luminosity Long-lived Small radius Red

32 What are giants, supergiants, and white dwarfs? Off the Main Sequence Stellar properties depend on both mass and age: Those that have finished fusing H to He in their cores are no longer on the main sequence. All stars become larger and redder after exhausting their core hydrogen: giants and supergiants. Most stars end up small and white after fusion has ceased: white dwarfs.

33 Variable Stars Any star that varies significantly in brightness with time is called a variable star. Most pulsating variable stars inhabit an instability strip on the H-R diagram. The most luminous ones are known as Cepheid variables.

34 There are the two types of star clusters Insert TCP 6e Figure Open cluster: A few thousand loosely packed stars Globular cluster: Up to a million or more stars in a dense ball bound together by gravity

35 What have we learned? How do we measure stellar luminosities? If we measure a star s apparent brightness and distance, we can compute its luminosity with the inverse square law for light. Parallax tells us distances to the nearest stars. How do we measure stellar temperatures? A star s color and spectral type both reflect its temperature.

36 What have we learned? How do we measure stellar masses? Newton s version of Kepler s third law tells us the total mass of a binary system, if we can measure the orbital period (p) and average orbital separation of the system (a).

37 What have we learned? What is a Hertzsprung-Russell diagram? An H-R diagram plots stellar luminosity of stars versus surface temperature (or color or spectral type). What is the significance of the main sequence? Normal stars that fuse H to He in their cores fall on the main sequence of an H-R diagram. A star s mass determines its position along the main sequence (high-mass: luminous and blue; low-mass: faint and red).

38 What have we learned? What are giants, supergiants, and white dwarfs? All stars become larger and redder after core hydrogen burning is exhausted: giants and supergiants. Most stars end up as tiny white dwarfs after fusion has ceased. Why do the properties of some stars vary? Some stars fail to achieve balance between power generated in the core and power radiated from the surface.

39 What have we learned? What are the two types of star clusters? Open clusters are loosely packed and contain up to a few thousand stars. Globular clusters are densely packed and contain hundreds of thousands of stars.

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