Chapter 15: Surveying the Stars

  1. Variable Stars
    • any star that varies significantly in brightness with time
    • some stars vary in brightness b/c they cannot achieve proper balance b/w power welling up from the core and power radiated from the surface
    • such a star alternately expands and contracts, varying in brightness as it tries to find a balance
  2. Pulsating Variable Stars
    the light curve of this pulsating variable star shows that its brightness alternately rises and falls over a 50-day period
  3. Cepheid Variable Stars
    • most pulsating variable stars inhabit an instability strip on the H-R diagram
    • most luminous ones are known as Cepheid variables
  4. Open Cluster
    a few thousand loosely packed stars
  5. Globular Cluster
    • up to a million or more stars in a dense ball bound together by gravity
    • detailed modelling of the oldest globular clusters reveals that they are about 13 billion years old
  6. Measuring the age of a star cluster...
    • massive blue stars die first, followed by white, yellow, orange, and red stars
    • Pleiades doesn't have a star with a life expectancy less than ~ 100 million years
    • main-sequence turnoff point of a cluster tells us its age

    to determine accurate ages, we compare models of stellar evolution to cluster data.
  7. Measuring Luminosity and Brightness and their Relationship
    • luminosity = 4pi(distance)² x (brightness)
    • brightness = luminosity ÷ 4pi(distance)²
    • relationship between apparent brightness and luminosity depends on distance
    • brightness of a star depends on both distance and luminosity
    • the further the distance, the less bright it is
  8. Luminosity
    • amount of power a star radiates (energy er second = Watts)
    • an object of fixed size grows more luminous as its temperature rises
    • Most luminous: 10Lsun
    • Least luminous: 10-4 Lsun
  9. Apparent Brightness
    amount of starlight that reaches Earth (energy per second per square meter)
  10. Parallax
    • the apparent shift in position of a nearby object against a background of more distance objects
    • apparent positions of nearest stars shift by about an arc-second as earth orbits the sun
    • parallax angle depends on distance
    • parallax is measured by comparing snapshots taken at different times and measuring the shift in angle to star
  11. Parallax and Distance
    • = parallax angle
    • d (in parsecs)= 1 ÷ in arc-seconds)
    • (in light years) = 3.26 x 1/p (in arc-seconds)
  12. How do we measure stellar temperatures?
    every object emits thermal radiation with a spectrum that depends on its temperature
  13. Properties of Thermal Radiation
    • hotter objects emit...
    • more light per unit area at all frequencies
    • photons with a higher average energy
  14. Temperatures of Stars
    • hottest stars: 50 000 K
    • coolest stars: 3 000 K
    • sun's surface is about 5 800 K

    (Hottest) O B A F G K M (Coolest)
  15. Ionization
    • reveals a star's temperature
    • 10 K: solid
    • 102 K: molecules
    • 103 K: Neutral gas
    • 105 K: ionized gas (plasma)

    absorption lines in a star's spectrum tell us ionization level
  16. Spectral Type
    • lines in a star's spectrum
    • reveals the star's temperature
  17. Pioneers of Stellar Classification
    Annie Jump Cannon and the "calculators" at Harvard laid the foundation of modern stellar classification
  18. Binary Star Systems
    • visual
    • eclipsing
    • spectroscopic

    • the orbit of a binary star system depends on strength of gravity
    • about half of all stars are in binary systems
  19. Visual Binary
    • large orbital periods
    • we can directly observe the orbital motions of these stars
  20. Eclipsing Binary
    • don't see 2 stars, we see a light curve
    • know 2 stars are present and closer together
    • plane of orbit is close
    • spectroscopic binary as well
    • we can measure periodic eclipses
    • get doppler shift, size of planet from width of eclipse
  21. Spectroscopic Binary
    • red shift/blue shift, depending on the moving distance
    • gives us spectral information
    • we determine the orbit by measuring Doppler shifts
  22. Spectrum Binary
    • non eclipsing
    • 1 set of lines moving back and forth
    • orbits a black hole
    • through spectrum binary we are able to tell the mass of a black hole
  23. Light Curve
    • goes up before going down because the light from the back of the blue star is bouncing off of the ionized red star
    • ∴ we see more light
  24. Measuring Mass
    • direct mass measurements are possible only for stars in binary star systems
    • you need 2 out of 3 observables to measure mass: 
    • 1) orbital period (p)
    • 2) orbital separation (a or r)
    • 3) orbital velocity (v) 

    for circular orbits, v= 2pi⋅r/p
  25. Hertzsprung-Russell diagram
    • plots the luminosity and temperature of stars
    • most stars fall somewhere on the main sequence
    • depicts: temperature, colour, spectral type, luminosity, radius 

  26. Giants/Supergiants
    • stars with lower temperature and higher luminosity than main-sequence stars
    • have a larger radius
  27. White Dwarfs
    • stars with higher temperature and lower luminosity than main-sequence stars
    • have a smaller radius
  28. Different kinds of stars
    • a star's full classification includes spectral type (line identities) and luminosity class (line shapes related to the size of the star): 
    • I - super-giant
    • II - bright giant
    • III - giant
    • IV - sub-giant
    • V - main sequence

    • Examples... 
    • Sun - G2 V
    • Sirius - A1 V
    • Proxima Centauri - M5.5 V
    • Betelgeuse - M2 I
  29. Significance of the Main Sequence
    • Main-sequence stars are fusing hydrogen into helium in their cores like the sun
    • luminous main-sequence stars are hot (blue)
    • less luminous stars are cooler (yellow or red)
    • mass measurements of main-sequence stars show that the hot stars are much more massive than the cool ones
    • the mass of a normal hydrogen-burning star determines its luminosity and spectral type
    • core pressure and temperature of a higher-mass star need to be larger in order to balance gravity
    • higher core temperature boosts fusion rate, leading to a larger luminosity
  30. Mass and Lifetime
    • Sun's life expectancy: 10 billion years (until core hydrogen (10% of total) is used up)
    • Life expectancy of 10Msun star: 10 times as much fuel, uses it 104 times as fast 
    • 10 million years ~ 10 billion years x 10/10
    • Life expectancy of a 0.1Msun star: 0.1 times as much fuel, uses it 0.01 times as fast
    • 100 billion years ~ 10 billion years x 0.1/0.01
  31. Main Sequence Star Summary
    • high mass: high luminosity, short-lived, large radius, blue in colour
    • low mass: low luminosity, long-lived, small radius, red in colour
  32. Stars Off the Main Sequence
    • stellar properties depend on both mass and age: those that have finished fusing H to He in their cores are no longer on the main
    • all stars become larger and redder after exhausting their core hydrogen: giants and supergiants
    • most stars end up small and white after fusion has ceased: white dwarfs
Card Set
Chapter 15: Surveying the Stars
Lecture 8