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The Family of Stars Chapter 9

Science is based on measurement, but measurement in astronomy is very difficult. Even with the powerful modern telescopes described in Chapter 6, it is impossible to measure directly simple parameters such as the diameter of a star. This chapter shows how we can use the simple observations that are possible, combined with the basic laws of physics, to discover the properties of stars. With this chapter, we leave our sun behind and begin our study of the billions of stars that dot the sky. In a sense, the star is the basic building block of the universe. If we hope to understand what the universe is, what our sun is, what our Earth is, and what we are, we must understand the stars. In this chapter we will find out what stars are like. In the chapters that follow, we will trace the life stories of the stars from their births to their deaths. Guidepost

I. Measuring the Distances to Stars A. The Surveyor's Method B. The Astronomer's Method C. Proper Motion II. Intrinsic Brightness A. Brightness and Distance B. Absolute Visual Magnitude C. Calculating Absolute Visual Magnitude D. Luminosity III. The Diameters of Stars A. Luminosity, Radius, and Temperature B. The H-R Diagram C. Giants, Supergiants, and Dwarfs Outline

D. Luminosity Classification E. Spectroscopic Parallax IV. The Masses of Stars A. Binary Stars in General B. Calculating the Masses of Binary Stars C. Visual Binary Systems D. Spectroscopic Binary Systems E. Eclipsing Binary Systems V. A Survey of the Stars A. Mass, Luminosity, and Density B. Surveying the Stars Outline

Light as a Wave (1) We already know how to determine a star’s surface temperature chemical composition surface density In this chapter, we will learn how we can determine its distance luminosity radius mass and how all the different types of stars make up the big family of stars.

Distances to Stars Trigonometric Parallax: Star appears slightly shifted from different positions of the Earth on its orbit The farther away the star is (larger d), the smaller the parallax angle p. d = __ p 1 d in parsec (pc) p in arc seconds 1 pc = 3.26 LY

The Trigonometric Parallax Example: Nearest star,  Centauri, has a parallax of p = 0.76 arc seconds d = 1/p = 1.3 pc = 4.3 LY With ground-based telescopes, we can measure parallaxes p ≥ 0.02 arc sec => d ≤ 50 pc This method does not work for stars farther away than 50 pc.

Proper Motion In addition to the periodic back-and- forth motion related to the trigonometric parallax, nearby stars also show continuous motions across the sky. These are related to the actual motion of the stars throughout the Milky Way, and are called proper motion.

Intrinsic Brightness/ Absolute Magnitude The more distant a light source is, the fainter it appears.

Brightness and Distance (SLIDESHOW MODE ONLY)

Intrinsic Brightness / Absolute Magnitude (2) More quantitatively: The flux received from the light is proportional to its intrinsic brightness or luminosity (L) and inversely proportional to the square of the distance (d): F ~ L __ d2d2 Star A Star B Earth Both stars may appear equally bright, although star A is intrinsically much brighter than star B.

Distance and Intrinsic Brightness Betelgeuse Rigel Example: App. Magn. m V = 0.41 Recall that: Magn. Diff. Intensity Ratio *2.512 = (2.512) 2 = 6.31 …… 5(2.512) 5 = 100 App. Magn. m V = 0.14 For a magnitude difference of 0.41 – 0.14 = 0.27, we find an intensity ratio of (2.512) 0.27 = 1.28

Distance and Intrinsic Brightness (2) Betelgeuse Rigel Rigel is appears 1.28 times brighter than Betelgeuse, Thus, Rigel is actually (intrinsically) 1.28*(1.6) 2 = 3.3 times brighter than Betelgeuse. But Rigel is 1.6 times further away than Betelgeuse

Absolute Magnitude To characterize a star’s intrinsic brightness, define Absolute Magnitude (M V ): Absolute Magnitude = Magnitude that a star would have if it were at a distance of 10 pc.

Absolute Magnitude (2) Betelgeuse Rigel BetelgeuseRigel mVmV MVMV d152 pc244 pc Back to our example of Betelgeuse and Rigel: Difference in absolute magnitudes: 6.8 – 5.5 = 1.3 => Luminosity ratio = (2.512) 1.3 = 3.3

The Distance Modulus If we know a star’s absolute magnitude, we can infer its distance by comparing absolute and apparent magnitudes: Distance Modulus = m V – M V = log 10 (d [pc]) Distance in units of parsec Equivalent: d = 10 (m V – M V + 5)/5 pc

The Size (Radius) of a Star We already know: flux increases with surface temperature (~ T 4 ); hotter stars are brighter. But brightness also increases with size: A B Star B will be brighter than star A. Absolute brightness is proportional to radius squared, L ~ R 2. Quantitatively: L = 4  R 2  T 4 Surface area of the star Surface flux due to a blackbody spectrum

Example: Star Radii Polaris has just about the same spectral type (and thus surface temperature) as our sun, but it is 10,000 times brighter than our sun. Thus, Polaris is 100 times larger than the sun. This causes its luminosity to be = 10,000 times more than our sun’s.

Organizing the Family of Stars: The Hertzsprung-Russell Diagram We know: Stars have different temperatures, different luminosities, and different sizes. To bring some order into that zoo of different types of stars: organize them in a diagram of Luminosity versus Temperature (or spectral type) Luminosity Temperature Spectral type: O B A F G K M Hertzsprung-Russell Diagram or Absolute mag.

The Hertzsprung-Russell Diagram Most stars are found along the Main Sequence

The Hertzsprung-Russell Diagram (2) Stars spend most of their active life time on the Main Sequence (MS). Same temperature, but much brighter than MS stars  Must be much larger  Giant Stars Same temp., but fainter → Dwarfs

The Radii of Stars in the Hertzsprung-Russell Diagram 10,000 times the sun’s radius 100 times the sun’s radius As large as the sun 100 times smaller than the sun RigelBetelgeuse Sun Polaris

Luminosity Classes Ia Bright Supergiants Ib Supergiants II Bright Giants III Giants IV Subgiants V Main-Sequence Stars Ia Ib II III IV V

Example Luminosity Classes Our Sun: G2 star on the Main Sequence: G2V Polaris: G2 star with Supergiant luminosity: G2Ib

Spectral Lines of Giants => Absorption lines in spectra of giants and supergiants are narrower than in main sequence stars Pressure and density in the atmospheres of giants are lower than in main sequence stars. => From the line widths, we can estimate the size and luminosity of a star.  Distance estimate (spectroscopic parallax)

Binary Stars More than 50 % of all stars in our Milky Way are not single stars, but belong to binaries: Pairs or multiple systems of stars which orbit their common center of mass. If we can measure and understand their orbital motion, we can estimate the stellar masses.

The Center of Mass center of mass = balance point of the system. Both masses equal => center of mass is in the middle, r A = r B. The more unequal the masses are, the more it shifts toward the more massive star.

Center of Mass (SLIDESHOW MODE ONLY)

Estimating Stellar Masses Recall Kepler’s 3rd Law: P y 2 = a AU 3 Valid for the Solar system: star with 1 solar mass in the center. We find almost the same law for binary stars with masses M A and M B different from 1 solar mass: M A + M B = a AU 3 ____ Py2Py2 (M A and M B in units of solar masses)

Examples: Estimating Mass a) Binary system with period of P = 32 years and separation of a = 16 AU: M A + M B = = 4 solar masses ____ 32 2 b) Any binary system with a combination of period P and separation a that obeys Kepler’s 3. Law must have a total mass of 1 solar mass.

Visual Binaries The ideal case: Both stars can be seen directly, and their separation and relative motion can be followed directly.

Spectroscopic Binaries Usually, binary separation a can not be measured directly because the stars are too close to each other. A limit on the separation and thus the masses can be inferred in the most common case: Spectroscopic Binaries

Spectroscopic Binaries (2) The approaching star produces blue shifted lines; the receding star produces red shifted lines in the spectrum. Doppler shift  Measurement of radial velocities  Estimate of separation a  Estimate of masses

Spectroscopic Binaries (3) Time Typical sequence of spectra from a spectroscopic binary system

Eclipsing Binaries Usually, inclination angle of binary systems is unknown  uncertainty in mass estimates. Special case: Eclipsing Binaries Here, we know that we are looking at the system edge-on!

Eclipsing Binaries (2) Peculiar “double-dip” light curve Example: VW Cephei

Eclipsing Binaries (3) From the light curve of Algol, we can infer that the system contains two stars of very different surface temperature, orbiting in a slightly inclined plane. Example: Algol in the constellation of Perseus

The Light Curve of Algol

Masses of Stars in the Hertzsprung- Russell Diagram The higher a star’s mass, the more luminous (brighter) it is: High-mass stars have much shorter lives than low-mass stars: Sun: ~ 10 billion yr. 10 M sun : ~ 30 million yr. 0.1 M sun : ~ 3 trillion yr Masses in units of solar masses Low masses High masses Mass L ~ M 3.5 t life ~ M -2.5

Maximum Masses of Main-Sequence Stars  Carinae M max ~ solar masses a) More massive clouds fragment into smaller pieces during star formation. b) Very massive stars lose mass in strong stellar winds Example:  Carinae: Binary system of a 60 M sun and 70 M sun star. Dramatic mass loss; major eruption in 1843 created double lobes.

Minimum Mass of Main-Sequence Stars M min = 0.08 M sun At masses below 0.08 M sun, stellar progenitors do not get hot enough to ignite thermonuclear fusion.  Brown Dwarfs Gliese 229B

Surveys of Stars Ideal situation: Determine properties of all stars within a certain volume. Problem: Fainter stars are hard to observe; we might be biased towards the more luminous stars.

A Census of the Stars Faint, red dwarfs (low mass) are the most common stars. Giants and supergiants are extremely rare. Bright, hot, blue main-sequence stars (high- mass) are very rare

stellar parallax (p) parsec (pc) proper motion flux absolute visual magnitude (M v ) magnitude–distance formula distance modulus (m v – M v ) luminosity (L) absolute bolometric magnitude H–R (Hertzsprung– Russell) diagram main sequence giants supergiants red dwarf white dwarf luminosity class spectroscopic parallax binary stars visual binary system spectroscopic binary system eclipsing binary system light curve mass–luminosity relation New Terms

1. If someone asked you to compile a list of the nearest stars to the sun based on your own observations, what measurements would you make, and how would you analyze them to detect nearby stars? 2. The sun is sometimes described as an average star. What is the average star really like? Discussion Questions

Quiz Questions 1. The parallax angle of a star and the two lines of sight to the star from Earth form a long skinny triangle with a short side of a km. b. 1 Earth diameter. c. 1 AU. d. 2 AU. e. 40 AU.

Quiz Questions 2. What is the distance to a star that has a parallax angle of 0.5 arc seconds? a. Half a parsec. b. One parsec. c. Two parsecs. d. Five parsecs. e. Ten parsecs.

Quiz Questions 3. Why can smaller parallax angles be measured by telescopes in Earth orbit? a. Telescopes orbiting Earth are closer to the stars. b. Earth's atmosphere does not limit a telescope's resolving power. c. Earth's atmosphere does not limit a telescope's light gathering power. d. Earth's atmosphere does not limit a telescope's magnifying power. e. They can be connected to Earth-based telescopes to do interferometry.

Quiz Questions 4. At what distance must a star be to have its apparent magnitude equal to its absolute magnitude? a. One AU. b. Ten AU. c. One parsec. d. Ten parsecs. e. One Megaparsec.

Quiz Questions 5. Which magnitude gives the most information about the physical nature of a star? a. The apparent visual magnitude. b. The apparent bolometric magnitude. c. The absolute visual magnitude. d. The absolute bolometric magnitude. e. None of the above tells us anything about the physical nature of a star.

Quiz Questions 6. For which stars does absolute visual magnitude differ least from absolute bolometric magnitude? a. Low surface temperature stars. b. Medium surface temperature stars. c. High surface temperature stars. d. Stars closer than 10 parsecs. e. Stars farther away than 10 parsecs.

Quiz Questions 7. The absolute magnitude of any star is equal to its apparent magnitude at a distance of 10 parsecs. Use this definition, how light intensity changes with distance, and how the stellar magnitude system is set up to determine the following. If a star's apparent visual magnitude is less than its absolute visual magnitude, which of the following is correct? a. The distance to the star is less than 10 parsecs. b. The distance to the star is 10 parsecs. c. The distance to the star is greater than 10 parsecs. d. Its bolometric magnitude is greater than its visual magnitude. e. Its bolometric magnitude is less than its visual magnitude.

Quiz Questions 8. What is the distance to a star that has an apparent visual magnitude of 3.5 and an absolute visual magnitude of -1.5? a. 100 parsecs. b. 50 parsecs. c. 25 parsecs. d. 10 parsecs. e. 5 parsecs.

Quiz Questions 9. What is the luminosity of a star that has an absolute bolometric magnitude that is 10 magnitudes brighter than the Sun (-5.3 for the star and +4.7 for the Sun)? a. 1 solar luminosity. b. 10 solar luminosities. c. 100 solar luminosities d solar luminosities. e solar luminosities.

Quiz Questions 10. How can a cool star be more luminous than a hot star? a. It can be more luminous if it is larger. b. It can be more luminous if it is more dense. c. It can be more luminous if it is closer to Earth. d. It can be more luminous if it is farther from Earth. e. A cool star cannot be more luminous than a hot star.

Quiz Questions 11. A star has one-half the surface temperature of the Sun, and is four times larger than the Sun in radius. What is the star's luminosity? a. 64 solar luminosities. b. 16 solar luminosities. c. 4 solar luminosities. d. 2 solar luminosities. e. 1 solar luminosity.

Quiz Questions 12. The Sun's spectral type is G2. What is the Sun's luminosity class? a. Bright Supergiant (Ia) b. Supergiant (Ib) c. Bright Giant (II) d. Giant (III) e. Main Sequence (V)

Quiz Questions 13. A particular star with the same spectral type as the Sun (G2) has a luminosity of 50 solar luminosities. What does this tell you about the star? a. It must be larger than the Sun. b. It must be smaller than the Sun. c. It must be within 1000 parsecs of the Sun. d. It must be farther away than 1000 parsecs. e. Both a and b above.

Quiz Questions 14. In addition to the H-R diagram, what other information is needed to find the distance to a star whose parallax angle is not measurable? a. The star's spectral type. b. The star's luminosity class. c. The star's surface activity. d. Both a and b above. e. All of the above.

Quiz Questions 15. What is the radius and luminosity of a star that is classified as G2 III? a. About 0.1 solar radii and solar luminosities. b. About 1 solar radii and 1 solar luminosity. c. About 10 solar radii and 100 solar luminosity. d. About 100 solar radii and 10,000 solar luminosities. e. About 1000 solar radii and 1,000,000 solar luminosities.

Quiz Questions 16. For a particular binary star system star B is observed to always be four times as far away from the center of mass as star A. What does this tell you about the masses of these two stars? a. The total mass of these two stars is four solar masses. b. The total mass of these two stars is five solar masses. c. The ratio of star A's mass to star B's mass is four to one. d. The ratio of star B's mass to star A's mass is four to one. e. Both b and c above.

Quiz Questions 17. For a particular binary star system the ratio of the mass of star A to star B is 4 to 1. The semimajor axis of the system is 10 AU and the period of the orbits is 10 years. What are the individual masses of star A and star B? a. Star A is 1 solar mass and star B is 4 solar masses. b. Star A is 4 solar masses and star B is 1 solar mass. c. Star A is 2 solar masses and star B is 8 solar masses. d. Star A is 8 solar masses and star B is 2 solar masses. e. None of the above.

Quiz Questions 18. To which luminosity class does the mass-luminosity relationship apply? a. The Supergiants. b. The Giants. c. The Subgiants. d. The Main Sequence. e. The mass-luminosity relationship applies to all luminosity classes.

Quiz Questions 19. Which luminosity class has stars of the lowest density, some even less dense than air at sea level? a. The Supergiant. b. The Bright Giant. c. The Giant. d. The Subgiant. e. The Main Sequence.

Quiz Questions 20. In a given volume of space the Red Dwarf (or lower main sequence) stars are the most abundant, however, on many H-R diagrams very few of these stars are plotted. Why? a. Photographic film and CCDs both have low sensitivity to low- energy red photons. b. They are so very distant that parallax angles cannot be measured, thus distances and absolute magnitudes are difficult to determine precisely. c. They have so many molecular bands in their spectra that they are often mistaken for higher temperature spectral types. d. They have very low luminosity and are difficult to detect, even when nearby. e. Most of them have merged to form upper main sequence stars.

Answers 1.c 2.c 3.b 4.d 5.d 6.b 7.a 8.a 9.e 10.a 11.e 12.e 13.a 14.d 15.c 16.c 17.d 18.d 19.a 20.d