1. The Hertzsprung-Russell Diagram The HR diagram separates The effects of temperature And surface area on stars’ Luminosity and sorts the Stars according to their size

2. The Hertzsprung-Russell Diagram The HR diagram separates The effects of temperature And surface area on stars’ Luminosity and sorts the Stars according to their size

3. The Hertzsprung-Russell Diagram The Main Sequence - all main sequence stars fuse H into He in their cores - this is the defining characteristic of a main sequence star.

4. The Hertzsprung-Russell Diagram Red Giants - Red Giant stars are very large, cool and quite bright. Ex. Betelgeuse is 100,000 times more luminous than the Sun but is only 3,500K on the surface. It’s radius is 1,000 times that of the Sun.

5. The Hertzsprung-Russell Diagram

6. White Dwarfs - White Dwarfs are hot but since they are so small, they are not very luminous.

7. The Hertzsprung-Russell Diagram Size of Star Mass of Star The HR diagram separates The effects of temperature And surface area on stars’ Luminosity and sorts the Stars according to their size

8. Mass-Luminosity relation Most stars appear on the Main Sequence, where stars appear to obey a Mass-Luminosity relation: L  M 3.5 For example, if the mass of a star is doubled, its luminosity increases by a factor 2 3.5 ~ 11. Thus, stars like Sirius that are about twice as massive as the Sun are about 11 times as luminous. The more massive a Main Sequence star is, the hotter (bluer), and more luminous. The Main Sequence is a mass sequence!

9. Review Questions 1. What is the Hertzsprung-Russell Diagram? 2. Why are most stars seen along the so-called main sequence? 3. What makes more massive stars hotter and brighter?

10. To calculate a star's radius, you must know its 1) temperature and luminosity. 2) chemical composition and temperature. 3) color and chemical composition. 4) luminosity and surface gravity. L=4 π R 2 σ T 4

11. 3) 4 times larger 4) the same 1) ½ times as large 2) ¼ times as large If a star is half as hot as our Sun, but has the same luminosity, how large is its radius compared to the Sun? L=4 π R 2 σ T 4

12. What is burning in stars? Gasoline Gasoline Nuclear fission Nuclear fission Nuclear fusion Nuclear fusion Natural gas Natural gas

13. Stars More in Depth: To fully characterize stars we need to know their Four Basic Parameters Luminosity Size Mass Surface Temperature

14. Measurements of Star Properties Apparent brightness Distance Luminosity Temperature Radius Direct measurent Parallax Distance + apparent brightness ( L=4  D 2 f) Spectral type (or color) Luminosity + temperature (L=4  R 2  T 4 ) Luminosity and temperature are the two independent intrinsic parameters of stars.

15. Mass: how do you weigh a star? Mass is the single most important property in how a star’s life and death will proceed. Mass is the single most important property in how a star’s life and death will proceed. We can “weigh” stars that are in binary systems (two stars orbiting each other). Fortunately, most stars fall into this category. We can “weigh” stars that are in binary systems (two stars orbiting each other). Fortunately, most stars fall into this category. Most stars in binary systems have a mass that is very similar to its companion … we’ll see why this is soon! Most stars in binary systems have a mass that is very similar to its companion … we’ll see why this is soon!

16. Big p = Small Masses Small p = Big Masses rara Star A Star B Binary Stars -Each star in a binary system moves in its own orbit around the system's center of mass. -Kepler’s Third Law: the orbital period depends on the relative separation and the masses of the two stars: p 2 = 4π24π2 G(M 1 +M 2 ) a3a3 rbrb M a /M b = r b /r a Center of mass (or baricenter)

17. I. Visual Binaries

18. Spectroscopic binaries: Doppler Shift tells if it is moving toward or away 1.The total spread (size) of the Doppler shift gives velocities about center of mass (gives orbit sizes, r A +r B ) 2.The time to complete one repeating pattern gives period, P 1 2 3 4 5

19. Eclipsing Binaries: Best binaries to measure mass

20. Classification of Stars: the H-R diagram 1)Collect information on a large sample of stars. 2)Measure their luminosities (need the distance!) 3)Measure their surface temperatures (need their spectra)

21. The Interstellar Medium

22. Assigned Reading Chapter 10 Chapter 10

23. A World of Dust and Gas We are interested in the interstellar medium because a) dense interstellar clouds are the birth place of stars b) Dark clouds alter and absorb the light from stars behind them The space between the stars is not completely empty, but filled with very dilute gas and dust, producing some of the most beautiful objects in the sky.

24. Bare-Eye Nebula: Orion One example of an interstellar gas cloud (nebula) is visible to the bare eye: the Orion nebula

25. The ISM Space between stars not empty Space between stars not empty Gas, dust; Gas is mostly Hydrogen (80%) and Helium (20%) Gas, dust; Gas is mostly Hydrogen (80%) and Helium (20%) Physical status of the gas characterized by: Physical status of the gas characterized by: Temperature Temperature Density Density Chemical composition Chemical composition ISM and stars are the components of the “ machine ” that makes the universe evolve: the cycle of star formation and death, and the chemical enrichment of the cosmos. ISM and stars are the components of the “ machine ” that makes the universe evolve: the cycle of star formation and death, and the chemical enrichment of the cosmos. ISM also “ disturbs ” observations, since it absorbs light and modifies (reddens) colors ISM also “ disturbs ” observations, since it absorbs light and modifies (reddens) colors

26. Red light can more easily penetrate the cloud, but is still absorbed to some extent. Interstellar Reddening Blue light is strongly scattered and absorbed by interstellar clouds. Infrared radiation is hardly absorbed at all. Interstellar clouds make background stars appear redder. Visible Barnard 68 Infrared

27. Interstellar Reddening (2) The Interstellar Medium absorbs light more strongly at shorter wavelengths.

28. Interstellar Reddening (3) Nebulae that appear as dark nebulae in the optical, can shine brightly in the infrared due to blackbody radiation from the warm dust.

29. The ISM Main Components (Phases) Phase Phase Dust Dust Present in all phases Present in all phases “ Metals ” “ Metals ” Everything that is not hydrogen or Helium is a metal Everything that is not hydrogen or Helium is a metal HI Clouds HI Clouds Inter-cloud Medium Inter-cloud Medium Coronal Gas Coronal Gas Molecular clouds Molecular clouds This what forms stars This what forms stars T (K) Density a/cm 3 20-100 size: a few  m 50-500 1-1000 10 3 -10 4 0.01 10 5 -10 6 10 -4 -10 -3 20-50 10 3 -10 5

30. How did a star form? A cloud of hydrogen gas began to gravitationally collapse. A cloud of hydrogen gas began to gravitationally collapse. To start the collapse, the gas needs to loose pressure, i.e. needs to become cold To start the collapse, the gas needs to loose pressure, i.e. needs to become cold If the gas does not cool, it cannot collapse If the gas does not cool, it cannot collapse It also needs to become dense, i.e. to have more gravity It also needs to become dense, i.e. to have more gravity As more gas fell in, it ’ s potential energy was converted into thermal energy. As more gas fell in, it ’ s potential energy was converted into thermal energy. As it collapses, the gas gets hotter and hotter As it collapses, the gas gets hotter and hotter Eventually the in-falling gas was hot enough to ignite nuclear fusion in the core. Eventually the in-falling gas was hot enough to ignite nuclear fusion in the core. Gas that continued to fall in helped to establish gravitational equilibrium with the pressure generated in the core. Gas that continued to fall in helped to establish gravitational equilibrium with the pressure generated in the core.

33. Conservation of Angular Momentum (L) L is conserved during the collapse: if R decreases, T has to decrease, too, to keep L constant Only, the rate of change of T is faster to keep pace with the rate of change of R 2 (because of the second power) The collapsing object spins up rapidly during the collapse

34. O

35. Molecular cloud Cool molecular clouds gravitationally collapse to form clusters of stars Stars generate helium, carbon and iron through stellar nucleosynthesis The hottest, most massive stars in the cluster supernova – heavier elements are formed in the explosion. New (dirty) molecular clouds are left behind by the supernova debris. The Stellar Cycle

37. The Four Components of the Interstellar Medium ComponentTemperature [K] Density [atoms/cm 3 ] Main Constituents HI Clouds50 – 1501 – 1000Neutral hydrogen; other atoms ionized Intercloud Medium (HII) 10 3 - 10 4 0.01Partially ionized H; other atoms fully ionized Coronal Gas10 5 - 10 6 10 -4 – 10 -3 All atoms highly ionized H Molecular Clouds20 - 5010 3 - 10 5 Neutral gas; dust and molecules

40. Interstellar Absorption Lines The interstellar medium produces absorption lines in the spectra of stars. These can be distinguished from stellar absorption lines through: a) Absorption from wrong ionization states Narrow absorption lines from Ca II: Too low ionization state and too narrow for the O star in the background; multiple components b) Small line width (too low temperature; too low density) c) Multiple components (several clouds of ISM with different radial velocities)

41. Observing Neutral Hydrogen: The 21-cm (radio) line Electrons in the ground state of neutral hydrogen have slightly different energies, depending on their spin orientation. Magnetic field due to proton spin Magnetic field due to electron spin Opposite magnetic fields attract => Lower energy Equal magnetic fields repel => Higher energy 21 cm line

42. The 21-cm Line of Neutral Hydrogen Transitions from the higher-energy to the lower- energy spin state produce a characteristic 21-cm radio emission line. => Neutral hydrogen (HI) can be traced by observing this radio emission.

43. Observations of the 21-cm Line All-sky map of emission in the 21-cm line G a l a c t i c p l a n e

44. Observations of the 21-cm Line HI clouds moving towards Earth (from redshift/blueshift of line) HI clouds moving away from Earth Individual HI clouds with different radial velocities resolved

45. Interstellar Dust Formed in the atmospheres of cool stars Mostly observable through infrared emission Spitzer Space Telescope (infrared) image of interstellar dust near the center of our Milky Way (Right:) Infrared Emission from interstellar dust and gas molecules in the “Whirlpool Galaxy” M51.

46. Molecules in Space In addition to atoms and ions, the interstellar medium also contains molecules. Molecules also store specific energies in their a) rotation b) vibration Transitions between different rotational / vibrational energy levels lead to emission – typically at radio wavelengths.

47. The Most Easily Observed Molecules in Space CO = Carbon Monoxide  Radio emission OH = Hydroxyl  Radio emission The Most Common Molecule in Space: Difficult to observe! Use CO as a tracer for H 2 in the ISM! H 2 = Molecular Hydrogen  Ultraviolet absorption and emission: But: Where there’s H 2, there’s also CO.

48. Molecular Clouds Molecules are easily destroyed (“dissociated”) by ultraviolet photons from hot stars.  They can only survive within dense, dusty clouds, where UV radiation is completely absorbed.  “Molecular Clouds”: Largest molecular clouds are called “Giant Molecular Clouds”: Diameter ≈ 15 – 60 pc Temperature ≈ 10 K Total mass ≈ 100 – 1 million solar masses Cold, dense molecular cloud core HI Cloud UV emission from nearby stars destroys molecules in the outer parts of the cloud; is absorbed there. Molecules survive

49. Molecular Clouds (2) The dense cores of Giant Molecular Clouds are the birth places of stars.

50. The Coronal Gas Additional component of very hot, low-density gas in the ISM: n ~ 0.001 particles/cm 3 Observable in X-rays Called “Coronal gas” because of its properties similar to the solar corona (but completely different origin!) Probably originates in supernova explosions and winds from hot stars Our sun is located within (near the edge of) a coronal gas bubble. T ~ 1 million K

51. The Gas-Star-Gas Cycle All stars are constantly blowing gas out into space (recall: Solar wind!) The more luminous the star is, the stronger is its stellar wind. These winds are particularly strong in aging red giant stars.

52. The Gas-Star-Gas Cycle Stars, gas, and dust are in constant interaction with each other.