Starry Monday at Otterbein

1. Welcome to Starry Monday at Otterbein Astronomy Lecture Series -every first Monday of the month- May 6, 2013 Dr. Uwe Trittmann

2. Today’s Topics • Understanding the Stars II • The Night Sky in May

3. Reminder: Last Starry Monday • Limitations: Astronomy is not an experimental Science – It is an observational science • We observe (largely) the electromagnetic radiation we receive from an object

4. Two Interwoven Strands • If we can measure distances to stars we might be able to understand how they work • If we understand how stars work, we might be able to use this knowledge to measure (larger) distances

5. Appearance vs. “the real thing” Angular size of an object cannot tell us its actual size – depends on how far away it is Sun and Moon have very nearly the same angular size (30' = ½) when viewed from Earth They APPEAR to have the same size, but ARE of different size

6. Understanding the Stars? • What does understanding mean? • We can predict their brightness from other properties? • We can calculate their masses from other properties? • We understand how energy is being produced? • We understand how stars from, live and die? • Something else/ All of the above?

7. Brightness is not Brightness?! • Must not confuse: • apparent brightness B • intrinsic brightness L (luminosity) • Only the latter is a property of the star • Cf: the sun and the moon have the same size?!

8. Luminosity and Brightness • Luminosity L is the total power (energy per unit time) radiated by the star, actual brightness of star, cf. 100 W lightbulb • Apparent brightness B is how bright it appears from Earth • Determined by the amount of light per unit area reaching Earth • B L / d2 • Just by looking, we cannot tell if a star is close and dim or far away and bright

9. Distance Determination Method • Understand how bright an object is (L) • Observe how bright an object appears (B) • Calculate how far the object is away: B L / d2 So L/B d2 or d  √L/B

10. Distances to the Stars • Parallax can be used out to about 100 light years • The parsec: • Distance in parsecs = 1/parallax (in arc seconds) • Thus a star with a measured parallax of 1” is 1 parsec away • 1 pc is about 3.3 light years • The nearest star (Proxima Centauri) is about 1.3 pc or 4.3 lyr away • Solar system is less than 1/1000 lyr

11. How are temperature and color related? • Thermodynamics has a clear answer from the lab • Physics is an experimental science! • Do an experiment: heat up an iron rod and see how its color and luminosity changes!

12. Color of a radiating blackbody as a function of temperature • Think of heating an iron bar in the fire: red glowing to white to bluish glowing • Every THING radiates roughly like a BB

13. Understand Star Brightness: Classify Stars by their Temperature (Color) Class Temperature Color Examples O 30,000 K blue B 20,000 K bluish Rigel A 10,000 K white Vega, Sirius F 8,000 K white Canopus G 6,000 K yellowSun,  Centauri K 4,000 K orange Arcturus M 3,000 K red Betelgeuse The hotter  the bluer!

14. Color “=“ Temperature! • What about the other properties? • Temperature “=“ Luminosity? • Mass “=“ Luminosity? • Size “=“ mass?  Observe! • Measure properties of many stars, plot them against each other

15. Color-Luminosity Correlation • Hertzprung-Russell Diagram is a plot of absolute luminosity (vertical scale) against spectral type or temperature (horizontal scale) • Most stars (90%) lie in a band known as the Main Sequence

16. Indirect Measurement of Sizes • Distance and brightness can be used to find the luminosity: L  d2 B(1) • The laws of black body radiation also tell us that amount of energy given off depends on star size and temperature: L R2  T4 (2) • We can compare two values of absolute luminosity L to get the size

17. Sizes of Stars • Dwarfs • Comparable in size, or smaller than, the Sun • Giants • Up to 100 times the size of the Sun • Supergiants • Up to 1000 times the size of the Sun • Note: Temperature changes – no clear correlation between size and temperature

18. Main Sequence Sizes

19. Two Ways to Continue • Take this “understanding” of stars’ properties as new baseline to develop distance measurement methods that work farther out  Cosmology • Try to “explain” these empirical findings by uncovering the physical mechanism generating all this energy  Astrophysics

20. First Path: Energy Generation in Stars • Use nuclear physics and thermodynamics to understand energy production • Then go on to uncover the lifecycle of stars

21. How do we know how much energy the Sun produces each second? • The Sun’s energy spreads out in all directions • We can measure how much energy we receive on Earth • At a distance of 1 A.U., each square meter receives 1400 Watts of power (the solar constant) • Multiply by surface of sphere of radius 149.6 bill. meter (=1 A.U.) to obtain total power output of the Sun: 4  1026 Watts

22. The Sun • Diameter: 100  that of Earth • Mass: 300,000  that of Earth • Density: 0.3  that of Earth • Temperature of visible surface = 5800 K (about 10,000º F) • Conclusion: • The sun is big, hot, massive • But not crazy big, or insanely hot, or grotesquely massive

23. Where does the Energy come from? • Anaxagoras (500-428 BC): Sun a large hot rock – No, it would cool down too fast • Combustion? • No, it could last a few thousand years • 19th Century – gravitational contraction? • No! Even though the lifetime of sun would be about 100 million years, geological evidence showed that Earth was much older than this

24. What process can produce so much power? • For the longest time we did not know • Only in the 1930’s had science advanced to the point where we could answer this question • Needed to develop very advanced physics: quantum mechanics and nuclear physics • There is virtually only one type of process that can do the job

25. Atom:Nucleus and Electrons The Structure of Matter Nucleus: Protons and Neutrons (Nucleons) Nucleon: 3 Quarks | 10-10m | Atomic Energy: 1 eV, Visible light | 10-14m | Nuclear energy: 10000 x atomic energy! |10-15m|

26. Nuclear fusion reaction • In essence, 4 hydrogen nuclei combine (fuse) to form a helium nucleus, plus some byproducts (actually, a total of 6 nuclei are involved) • Mass of products is less than the original mass • The missing mass is emitted in the form of energy, according to Einstein’s famous formulas: E = mc2 (the speed of light is very large, so there is a lot of energy in even a tiny mass)

27. Hydrogen fuses to Helium Start: 4 + 2 protons End: Helium nucleus + 2 protons Hydrogen fuses to Helium

28. Hydrostatic Equilibrium • Two forces compete: gravity (inward) and energy pressure due to heat generated (outward) • Stars neither shrink nor expand, they are in hydrostatic equilibrium, i.e. the forces are equally strong Heat Gravity Gravity

29. More Mass means more Energy • More mass means more gravitational pressure • More pressure means higher density, temperature • Higher density, temp. means faster reactions & more reactions per time • This means more energy is produced

30. Understanding the Stars • Stars are “suns”: hot glowing gas balls made up of hydrogen and helium initially • Some stars have more mass than others, hence: • Some are hotter, some are cooler • Some look blue, some red • Some live shorter, others longer • Some end up as black holes, some as neutron stars, some as white dwarfs

31. Lifecycle • Lifecycle of a main sequence G star • Most time is spent on the main-sequence (normal star) 250 million years

32. Baby Stars Gas cloud becomes smaller, flatter, denser, hotter  Star

33. A Newborn Star • Main-sequence star; pressure from nuclear fusion and gravity are in balance • Duration ~ 10 billion years (much longer than all other stages combined) • Temperature ~ 15 million K at core, 6000 K at surface • Size ~ Sun

34. Mass Matters • Larger masses • higher surface temperatures • higher luminosities • take less time to form • have shorter main sequence lifetimes • Smaller masses • lower surface temperatures • lower luminosities • take longer to form • have longer main sequence lifetimes

35. Mass and the Main Sequence • The position of a star in the main sequence is determined by its mass All we need to know to predict luminosity and temperature! • Both radius and luminosity increase with mass

36. Stellar Lifetimes • From the luminosity, we can determine the rate of energy release, and thus rate of fuel consumption • Given the mass (amount of fuel to burn) we can obtain the lifetime • Large hot blue stars: ~ 20 million years • The Sun: 10 billion years • Small cool red dwarfs: trillions of years The hotter, the shorter the life!

37. Main Sequence Lifetimes Mass(in solar masses)LuminosityLifetime 10 Suns 10,000 Suns 10 Million yrs 4 Suns 100 Suns 2 Billion yrs 1 Sun 1 Sun 10 Billion yrs ½ Sun 0.01 Sun 500 Billion yrs

38. Old Stars • Leave the main sequence when they run out of hydrogen fuel • For sun-like stars (0.08-8 Msun): puff up into Red Giant, etc. • Explode into white dwarf (ex core) and planetary nebula

39. The life of Stars – pretty well understood

40. Second Path: Distance Measurements lead to Cosmology • Apparent brightness B is obvious – it’s what we “see” • We use some additional insight (“This is a blue MS star”, “This is a Cepheid variable star”) to deduce the absolute brightness or luminosity • Then, from the apparent brightness compared to absolute luminosity, we can determine the distance (d2  L/B)

41. Use Insight to come up with another Method: Spectroscopic Parallax • From the color of a main sequence star we can determine its absolute luminosity • Then, from the apparent brightness compared to absolute luminosity, we can determine the distance (B  L / d2 ) • Good out to 3000 ly or so; accuracy of 25%

42. More Insight: Understanding Variable Stars yields another Method • Two useful types: • Cepheids • RR Lyrae • Again, method uses insight to get absolute brightness, then concludes distance from apparent brightness

43. Cepheids • Henrietta Leavitt (1908) discovers the period-luminosity relationship for Cepheid variables • Period thus tells us luminosity, which then tells us the distance • Since Cepheids are brighter than RR Lyrae, they can be used to measure out to further distances

44. Properties of Cepheids • Period of pulsation: a few days • Luminosity: 200-20000 suns • Radius: 10-100 solar radii

45. Cepheids and RR Lyrae: Yard-Sticks • Normal stars undergoing a phase of instability • Cepheids are more massive and brighter than RR Lyrae • Note: all RR Lyrae have the same luminosity • Apparent brightness thus tells us the distance to them! • Recall: B  L/d2

46. Distance Measurements with variable stars • Extends the cosmic distance ladder out as far as we can see Cepheids – about 50 million ly • In 1920 Hubble used this technique to measure the distance to Andromeda (about 2 million ly) • Works best for periodic variables

47. Last Rung of Distance Ladder • This one works differently! • Due to the expansion of the universe, we expect distances between objects to increase with time • Also, the “speed” of objects should be proportional to the distance of the object

48. Aside: The Expanding Universe • Except for a few nearby galaxies (like Andromeda), all the galaxies are seen to be moving away from us • Generally, the recession speed of a galaxy is proportional to its distance from us; that is, a galaxy that’s twice as far away is moving twice as fast (aside from local motions within galaxy clusters)

49. The Expanding Universe This expansion pattern (speed proportional to distance) actually implies that galaxies are all moving away from each other Milky Way Expansion

50. The Expanding Universe This expansion pattern (speed proportional to distance) actually implies that galaxies are all moving away from each other Milky Way Expansion Twice as far away, so moves twice as fast