# Chapter 11 Surveying the Stars

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1 Chapter 11 Surveying the Stars

2 Luminosity Luminosity: Rate of energy emitted by star every second. Apparent brightness (flux): Amount of energy passing through every second per unit area. Luninosity = (Brightness) 4π (distance) 2 F = L 4 d 2

3 Clicker 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 as bright.

4 Which star is most luminous? Need to know distances!

5 Measuring Distances using Parallax Parallax is the apparent shift in position of a nearby object (against a background of more distant objects) due to Earth s orbit. sin(p) = 1AU d d 1AU p (p expressed in radians) Parallax of the nearest stars is about an arcsecond (1/3600 th of 1 degree).

6 Clicker Question As the distance between us and a star increases, the parallax angle a) increases b) decreases c) Heck if I know; this is another math question!

7 If a star has a parallax angle of 1 arc-second (4.8 x 10-6 Rad), then d = 1AU = m=3.26 ly We define 1 parsec (pc) to be 3.26 ly, so 1 d(pc) = p(arcsec) Nearest star is roughly 1 pc away; Hipparcos satellite: can detect down to arcsec stars over thousands pc away are too far to resolve parallax. GAIA: Launched in 2013; 20 micro-arcsec ( ) resolution!

8 Clicker Question Star A has a parallax angle of 2 and star B has a parallax 4. The apparent brightness of star A is twice that of star B. Which star is more luminous? a) Star A b) Star B c) They have the same luminosity d) Heck if I know; this is another math question!

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10 Result: Star luminosities range from 10 6 L sun L Sun

11 Clicker Question Which of the following stars is the hottest? a) bright red star b) dim blue star c) bright white star d) impossible to determine based on information given

12 Temperature of Stars Result: Temperature ranges from 50,000 K to 3000 K (Sun s photosphere is 5800 K)

13 Mass Astronomers measure mass using Kepler s 3 rd Law (as described by Newton). Direct mass measurements are possible only for stars in binary star systems. Visual Binary: Both orbiting stars are visible we can observe both semi-major axis and orbital period.

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

15 Stellar Masses Result: The range of masses of stars is roughly 100 M Sun to 0.1 M Sun

16 Stellar Properties Review Luminosity: from brightness and distance 10 4 L Sun 10 6 L Sun Temperature: from color and spectral type 3,000 K 50,000 K Mass: from period and semi-major axis of binary-star orbit 0.08M Sun 100M Sun

17 Hertzsprung Russell diagram Plot showing luminosity vs. spectral type (temperature) for a collection of stars Most stars lie in the main sequence strip.

18 Which star has the largest radius? a) Rigel b) Antares c) Proxima Centauri d) Sirius B e) Vega

19 Stellar Radii Radius can be determined from luminosity and temperature. Flux proportional to temperature to the fourth power: F = σt 4 (Watts per meter squared) Total luminosity L = F 4π R 2 = σt 4 4π R 2 Thus, Luminous, cold stars are big R = r L 4 T 4

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21 Giants and supergiants are far larger than main-sequence stars and white dwarfs.

22 Main Sequence Stars Mass determines spectral type and luminosity (higher mass means hotter/more luminous)

23 Mass of Main-Sequence Stars L / M 4

24 The core temperature and density of a higher-mass star needs to be higher due to stronger force of gravity A higher core temperature boosts the fusion rate, leading to a greater luminosity.

25 Clicker Question Sun s life expectancy: 10 billion years What is the life expectancy of a 10M Sun star (with a luminosity 10 4 L Sun? a) 100 billion years b) 10 billion years c) 1 billion years d) 100 million years e) 10 million years

26 massive stars are like Hummers: Big gas tanks but still run out of gas quickly

27 If two stars are on the main sequence, and one is more luminous than the other, we can be sure that the a) more luminous star will have the longer lifespan. b) fainter star is the more massive. c) more luminous star is the more massive. d) more luminous star will have the redder color. e) more luminous star is younger

28 Clicker Question As a main sequence star ages, it a) progresses up the main sequence strip in the HR diagram (becoming more luminous and hotter) b) progresses down the main sequence strip in the HR diagram (becoming less luminous and colder) c) doesn t move in the HR diagram (until it begins to die)

29 Relative abundance of stars

30 Open cluster: A few thousand loosely packed stars

31 Globular cluster: Up to a million or more stars in a dense ball bound together by gravity

32 Which cluster is older? Clicker Question (a) (b)

33 Visual Representation of a Star Cluster Evolving

34 How are cluster ages determined? Make an HR diagram for stars in a globular cluster. Determine the mainsequence turnoff point. Determine mass of a MS star at turnoff point. The life expectancy of a star with this mass is the age of the cluster.

35 M13 - Hercules Result: The oldest globular clusters in our galaxy are about billion years old.

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