Midterm Review

1. Midterm Review 0 Please press “1” to test your transmitter.

2. 0 Sirius, the brightest star in the sky, has a trigonometric parallax of p = 0.385 arc seconds. What is its distance from Earth? • 0.385 pc • 0.80 light years • 1.255 pc • 2.60 light years • 8.47 light years

3. 0 Distances of Stars d in parsec (pc) p in arc seconds __ 1 d = p Trigonometric Parallax: Star appears slightly shifted from different positions of the Earth on its orbit 1 pc = 3.26 LY The further away the star is (larger d), the smaller the parallax angle p.

4. 0 Star A has an apparent magnitude of mA = 5.6 and an absolute magnitude of MA = 2.3. Star B has an apparent magnitude of mB = 0.6 and an absolute magnitude of MB = 2.3.Which of the following statements is true? • The flux received from both stars is the same, but star B is 5 times more luminous than star A, so star B must be further away. • The flux received from both stars is the same, but star B is 100 times more luminous than star A, so star B must be further away. • Both stars are equally luminous, but the flux received from star A is 5 times less than from star B, so star A must be further away. • Both stars are equally luminous, but the flux received from star A is 100 times less than from star B, so star A must be further away. • Both stars are equally luminous, but the flux received from star A is 5 times more than from star B, so star B must be further away.

5. 0 Absolute Magnitude Absolute Magnitude = Magnitude that a star would have if it were at a distance of 10 pc. The absolute magnitude measures a star’s intrinsic brightness (= luminosity). If we know a star’s absolute magnitude, we can infer its distance by comparing absolute and apparent magnitudes.

6. 0 Which of these spectral types describes a Red Giant? • O3V • F9V • B2Ia • K5III • G2V

7. 0 Spectral Classification of Stars Temperature

8. 0 Spectral Classification of Stars Mnemonics to remember the spectral sequence:

9. 0 Luminosity Classes Ia Bright Supergiants Ia Ib Ib Supergiants II II Bright Giants III III Giants IV Subgiants IV V V Main-Sequence Stars

10. < 100 solar masses Masses of Stars in the Hertzsprung-Russell Diagram Masses in units of solar masses 40 18 High masses 6 The higher a star’s mass, the more luminous it is. 3 > 0.08 solar masses 1.7 1.0 High-mass stars have much shorter lives than low-mass stars Mass 0.8 0.5 Sun: ~ 10 billion yr. Low masses 10 Msun: ~ 30 million yr. 0.1 Msun: ~ 3 trillion yr.

11. 0 In a binary star system … • The less massive stars orbits around the more massive one. • The more massive star orbits around the less massive one. • Both stars orbit on identical orbits around the mid-point between them. • Both stars orbit around their center of mass, which is closer to the less massive star. • Both stars orbit around their center of mass, which is closer to the more massive star.

12. 0 The Center of Mass center of mass = balance point of the system. Both masses equal => center of mass is in the middle, rA = rB. The more unequal the masses are, the more it shifts toward the more massive star.

13. 0 Which law allows astronomers to calculate the masses of stars in binary systems? • Newton’s first law • Kepler’s third law • Einsteins theory of general relativity • Newton’s third law • Kepler’s second law

14. Estimating Stellar Masses RewriteKepler’s 3. Law as 1 = aAU3 / Py2 Valid for theSolar system: star with1 solar massin the center. We findalmost the same lawfor binary starswith massesMA and MBdifferent from 1 solar mass: aAU3 ____ MA + MB = Py2 (MA and MB in units of solar masses)

15. 0 Which is the most common type of binary star systems? • Spectroscopic binaries • Eclipsing binaries • X-ray binaries • Visual binaries (where both stars and their motion can be resolved) • Binary neutron stars

16. 0 Spectroscopic Binaries 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

17. 0 Which of these fusion mechanisms does NOT fuse Hydrogen to Helium? • Proton-proton chain • CNO Cycle • Triple-Alpha Process

18. 0 The CNO Cycle In the sun, energy production is dominated by direct fusion of H into He (PP chain). In stars slightly more massive than the sun, a more powerful energy generation mechanism than the PP chain takes over: The CNO Cycle.

19. Energy Transport Structure Inner convective, outer radiative zone Inner radiative, outer convective zone CNO cycle dominant PP chain dominant

20. Summary: Stellar Structure Convective Core, radiative envelope; Energy generation through CNO Cycle Sun Mass Radiative Core, convective envelope; Energy generation through PP Cycle

21. 0 What are “globules”? • Small planetary bodies, still in the process of growing into planets (“globes”) • Large, cold, uncompressed molecular clouds that may eventually form thousands of stars. • Small, compressed pockets of dense gas that may form stars. • The remnants of the explosions of sun-like stars. • The remnants of the explosions of high-mass stars.

22. 0 (Bok) Globules Compact, dense pockets of gas which may contract to form stars. ~ 10 – 1000 solar masses; Contracting to form protostars

23. 0 Jets of gas ejected from protostellar disks are called … • Globules • Planetary Nebulae • Novae • Herbig-Haro Objects • Pulsars

24. Herbig-Haro Objects

25. 0 What happens in the Triple-Alpha Process? • Fusion of Hydrogen to Helium • Fusion of Helium to Carbon • Fusion of Carbon to Neon • Fusion of Silicon to Iron • Nuclear fission of Uranium

26. 0 Red Giant Evolution He-core gets denser and hotter until the next stage of nuclear burning can begin in the core: 4 H → He He He fusion: 3 4He → 12C “Triple-Alpha Process” Fusion of Helium into Carbon

27. 0 What is a “white dwarf”? • A failed star that does not become hot enough to ignite nuclear fusion. • The burned-out remnant of a very low-mass star that never ignites Helium fusion. • The collapsed Carbon/Oxygen core of a sun-like star. • The collapsed iron core of a high-mass star. • The collapsed iron core of a sun-like star.

28. 0 White Dwarfs Degenerate stellar remnant (C,O core) Extremely dense: 1 teaspoon of WD material: mass ≈ 16 tons!!! Chunk of WD material the size of a beach ball would outweigh an ocean liner! White Dwarfs: Mass ~ Msun Temp. ~ 25,000 K Luminosity ~ 0.01 Lsun

29. 0 Summary of Post-Main-Sequence Evolution of Stars Fusion proceeds to formation of Fe core. Evolution of 4 - 8 Msun stars is still uncertain. Fusion stops at formation of C,O core. M > 8 Msun Red dwarfs: He burning never ignites M < 4 Msun M < 0.4 Msun

30. 0 Which was the first method that allowed astronomers to measure the distances to other galaxies? • Light-travel time measurements • Gravitational-lensing measurements • Trigonometric parallax • Using Cepheid Variables • Warp-Drive travel

31. 0 Cepheid Variables:The Period-Luminosity Relation The variability period of a Cepheid variable is correlated with its luminosity. The more luminous it is, the more slowly it pulsates. => Measuring a Cepheid’s period, we can determine its absolute magnitude!

32. 0 If you plot all stars of a star cluster on a Hertzsprung-Russell diagram: Which feature will allow you to determine the cluster’s age? • The brightness of red giants. • The number of white dwarfs. • The average surface temperature of neutron stars. • The turn-off point from the Main Sequence. • The minimum mass of stars at the lower end of the main sequence.

33. 0 Example:HR diagram of the star cluster M 55 High-mass stars evolved onto the giant branch Turn-off point Low-mass stars still on the main sequence

34. 0 The lower on the MS the turn-off point, the older the cluster.