1. Stars, our Friends in the Universe

2. The Nearest Stars Distance to Alpha or Proxima Centauri is ~4 x 10 13 km or ~4.2 light years Distance between Alpha and Proxima Centauri is ~23 AU

3. The Solar Neighborhood Some stars within about 2 x 10 14 km (~ 20 light years)

4. What are Stars? What are they made of? What are their life cycles? How do we know what we know about them?

5. What is a Star? Stars are huge balls of hot gas, heated from inside by nuclear energy. Many are similar to our Sun, but there are giants as big as our solar system and dwarfs the size of Earth.

6. Life Cycles of Massive (> 8 Suns) Stars

7. Life Cycles of Stars

8. Classifying Stars Hertzsprung-Russell diagram

9. Classes of Stars Bigger stars are brighter than smaller stars because they have more surface area Hotter stars make more light per square meter. So, for a given size, hotter stars are brighter than cooler stars. White dwarfs - small and can be very hot Main sequence stars - range from hotter and larger to smaller and cooler Giants - rather large and cool Supergiants - cool and very large

10. How Stars are Born Pillars of dense gas Newly born stars seen to emerge at the ends of the pillars About 7000 light years away HST/Eagle Nebula in M16

11. How Stars are Born Orion nebula/Trapezium stars (in the sword) About 1500 light years away HST / 2.5 light years Chandra/10 light years

12. Main Sequence Stars Stars spend most of their lives on the “main sequence” where they burn hydrogen in nuclear reactions in their cores Burning rate is higher for more massive stars - hence their lives are much shorter and they are rather rare Red dwarf stars are the most common as they burn hydrogen slowly and live the longest Often called dwarfs (but not the same as White Dwarfs) because they are smaller than giants or supergiants

13. Properties of Stars Temperature (degrees K) - color of star light. All stars with the same blackbody temperature are the same color. Specific spectral lines appear for each temperature range classification. Astronomers name temperature ranges in decreasing order as: Surface gravity - measured from the shapes of the stellar absorption lines. Distinguishes classes of stars: supergiants, giants, main sequence stars and white dwarfs. O B A F G K M

14. Properties of Stars Luminosity (Watts) - absolute brightness; independent of distance. Derived from spectral type and surface gravity classification. Mass (kg or Solar mass units) - can be derived from spectrum or measured directly in binaries Radius (m) - usually derived, but can be measured directly for close, very large stars.

15. Brightness Luminosity = absolute brightness (How much energy does the star emit each second?) Flux - How much energy from the star hits a square meter located at a distance d? Apparent brightness (or magnitude) - How bright does the star appear (from the Earth)? Absolute magnitude - the apparent magnitude of a star if it were located at 10 pc. A logarithmic measure of its absolute brightness.

16. Sun Facts Mass of Sun 1.989 x 10 30 kg Diameter of Sun 1,390,000 km Distance to Sun 1 A. U. or 93 x 10 6 miles or ~1.5 x 10 11 m Rotation Rate of Sun 25.4 d (equator) 36 d (poles) Surface Temperature of Sun 5800 K (yellow visible light)

17. Star Power A star is powered by nuclear fusion reactions in its core The gravity from the star’s mass squeezes the nuclei together so that they can overcome electrostatic repulsion and fuse But high pressure and temperature encourage impact Electrostatic repulsion stops impact

18. Star Power Hydrogen nuclei fuse to Deuterium and then Helium, releasing about 7 MeV each The released radiation keeps the star from collapsing due to its own gravity Start with 4 protons under enormous pressure and temperature End up with a ìnormalî helium nucleus, two gamma rays, two positrons and two neutrinos Several Reactions

19. Features of a Main Sequence Star

20. Regions of a Main Sequence Star Core - dense region consisting of plasma of electrons and protons which undergo nuclear fusion reactions to power the star. Temperature is greater than 15,000,000 K. Radiation zone - region containing both plasma and atoms. The atoms slowly (170,000 y) absorb and reradiate the energy created in the core, transporting it to the outer layers. Temperature is around 5,000,000 K. Convection zone - turbulent region where the solar material “boils” to quickly (1 week) move heat to the outer layers. T ~ 2,000,000 K

21. Regions of a Main Sequence Star Photosphere - “surface” of the star that radiates visible light. Convection cells can be seen as granules - T ~ 5800 K Sunspots - highly variable, dark, cool regions in the photosphere. T ~ 3500 K Chromosphere - thin (2000 km) layer outside photosphere in which Hydrogen absorbs radiation and reemits it as red light (H-alpha). Jagged outer edge has dancing “flames” or spicules.

22. Regions of a Main Sequence Star Transition region - very thin (100 km) layer in which temperature rises from 20,000 to 10 6 K Corona - very sparse outer ionized gas region with loops and streamers of magnetic field. Temperature ~ 10 6 K Solar Movie shows: 1) Photosphere 2) Chromosphere 3) Corona

23. Sunspot and Convection Cells Optical sunspot image from the Vacuum Tower telescope at the Sacramento Peak National Solar Observatory with100 km resolution Shows granules from convection - each is about 1000 km across and lasts for about 10 minutes

24. Solar Chromosphere Maps of the solar chromosphere are made by observing light in the H-alpha line Light is emitted in the H-alpha line when an electron jumps down from the n=3 shell to the n=2 shell in Hydrogen

25. Solar Corona Only easily visible during solar eclipse Eclipses can be created artificially in coronographs SOHO/LASCO movie

26. Eruptions on the Sun Sunspots - concentrations of magnetic flux on the solar disk, which appear dark because they are cooler Prominences - loops and streamers of magnetic field which channel electrons in the corona Coronal Mass Ejections - (CMEs) violent flares which eject particles from the sun at millions of miles per hour Solar magnetic field loops

27. Solar Flares Solar prominence seen by Skylab in 1973 SOHO/MDI 11th magnitude earthquake on Sun following solar flare

28. Coronal Mass Ejections CMEs are the cause of major geomagnetic storms on Earth CMEs are NOT caused by solar flares, although they may both be signatures of rapid changes in the magnetic field 10 15 - 10 16 g of material is ejected into space at speeds from 50 to >1200 km/s Can only be observed with coronagraphs

29. Coronal Mass Ejections Coronal mass ejection in UV from SOHO Solar Maximum Mission CME in 1989

30. Let’s Take a Break

31. How Stars Die Stars that are below about 8 M o form red giants at the end of their lives on the main sequence Red giants evolve into white dwarfs, often accompanied by planetary nebulae More massive stars form red supergiants Red supergiants undergo supernova explosions, often leaving behind a stellar core which is a neutron star, or perhaps a black hole

32. Red Giants and Supergiants Hydrogen burns in outer shell around the core Heavier elements burn in inner shells

33. Planetary Nebulae Planetary nebulae are not the origin of planets Outer ejected shells of red giant illuminated by a white dwarf formed from the giant’s burnt-out core Not always formed HST/WFPC2 Eskimo nebula 5000 light years

34. White Dwarf Stars Red giants (but not supergiants) turn into white dwarf stars as they run out of fuel White dwarf mass must be less than 1.4 M o White dwarfs do not collapse because of quantum mechanical pressure from degenerate electrons White dwarf radius is about the same as the Earth A teaspoon of a white dwarf would weigh 10 tons Some white dwarfs have magnetic fields as high as 10 9 Gauss White dwarfs eventually radiate away all their heat and end up as black dwarfs in billions of years

35. Supernovae Supergiant stars become (Type II) supernovae at the end of nuclear shell burning Iron core often remains as outer layers are expelled Neutrinos and heavy elements released Core continues to collapse Chandra X-ray image of Eta Carinae, a potential supernova

36. Three Views of a Supernova

37. Neutron Stars Neutron stars are formed from collapsed iron cores All neutron stars that have been measured have around 1.4 M o (Chandrasekhar mass) Neutron stars are supported by pressure from degenerate neutrons, formed from collapsed electrons and protons A teaspoonful of neutron star would weigh 1 billion tons Neutron stars with very strong magnetic fields - around 10 12-13 Gauss - are usually pulsars due to offset magnetic poles

38. Cas A ~320 years old 10 light years across 50 million degree shell Radio/VLA X-ray/Chandra neutron star

39. Binary Star Systems Often stars are formed in binary systems Since they have unequal masses, the more massive star will evolve faster - and reach the end of its main sequence lifetime In some cases, the supernova of the primary star will not disrupt the binary system and a COMPACT BINARY is formed Mass transfer can then occur from the main sequence star onto the collapsed, compact companion star - which can be a WHITE DWARF, NEUTRON STAR or BLACK HOLE

40. X-ray Binary

41. Pulsars Radio pulses are powered by the energy released as the pulsar’s spin slows We see the brightness change in a periodic way….we see this in their light curves!

42. Crab Nebula Observed by Chinese astronomers in 1054 AD Age determined by tracing back exploding filaments Crab pulsar emits 30 pulses per second at all wavelengths from radio to TeV

43. Crab Nebula Radio/VLA Infrared/Keck

44. Crab Nebula Optical/HST WFPC2 Optical/Palomar

45. Crab Nebula and Pulsar X-ray/Chandra

46. Black Holes Final state of stellar collapse After supernova, if cores are larger than 3 M o, a black hole will be formed Escape velocity from a black hole is greater than the speed of light, once inside the event horizon

47. Some Other Stellar Types and Groups …and a little reminder

48. Variable Stars Most stars vary in brightness Periodic variability can be due to: Eclipses by the companion star Repeated flaring Pulsations as the star changes size or temperature Novae are stars which repeatedly blow off their outer layers in huge flares – they are NOT supernovae! Flare stars have regions which explode Pulsating stars have an unstable equilibrium between the competing forces of gas pressure and gravity

49. Cepheid Variables Henrietta Leavitt studied variable stars that were all at the same distance (in the LMC or SMC) and found that their pulsation periods were related to their brightnesses L =K P 1.3 Polaris (the North Star) is not constant, it is a Cepheid variable!

50. Open Star Clusters Open Cluster NGC 3293 d = 8000 c-yr 20 -1000 stars young stars mostly located in spiral arms of our Galaxy and other galaxies solar metal abundance

51. Globular Star Clusters Globular Cluster 47 Tuc d=20,000 c-yr 10 4 - 10 6 stars centrally condensed old stars galaxy halo low in metals

52. Pleiades Star Cluster A star cluster has a group of stars which are all located at approximately the same distance The stars in the Pleiades were all formed at about the same time, from a single cloud of dust and gas

53. Light-years 1 light-year is the distance light will travel in one year 1 light-year = (2.998 x 10 8 m/s)(86400 s/d)(365 d/y) = 9.46 x 10 12 km A LIGHTYEAR IS A DISTANCE, NOT A TIME!