Jessie Horn: Chapter 11 Notes

Chapter 11: Surveying the Stars
    Properties of Stars

• The brightness of stars depend on the distance and luminosity (the amount of power a star radiates)
• Apparent brightness: amount of starlight (energy per second = watts) that reaches Earth (energy per second per square meter)
• Luminosity passing through each sphere is the same
• Parallax: apparent shift in position of nearby object against background of more distant objects
• Apparent positions of the nearest stars shift by about an arcsecond as Earth orbits the Sun.
• the parallax angle depends on distance.
• Parallax is measured by comparing snapshots taken at different times and measuring shifts in angles compared to the stars.

   The Magnitude Scale

• m = apparent magnitude
• M = absolute magnitude
• Every object emits thermal radiation with a spectrum that depend on its temperature
• An object of fixed size grows more luminous as its temperature rises

   Properties of Thermal Radiation

1. Hotter objects emit more light per unit area at all frequencies
2. Hotter objects emit photons with a higher average energy

• Hottest stars: 50,000 k
• Coolest stars: 3,000 k
• Sun's surface is 5,800 k
• Level of ionization also reveals a star's temperature.
• Absorption lines in a star's spectrum tell us its ionization level

(Hottest) O B A F G K M (Coolest)

• Line in a star's spectrum corresponds to a spectral type that reveals its temperature

   Binary Star Orbits

• Orbit of a binary star system depends on gravity
• Types: visual binary, eclipsing binary, spectroscopic binary
• *About half of all stars are in binary systems
• We measure mass using gravity
• 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 =radius0
3. Orbital velocity (v)

11.2 Patterns Among Stars

• An H-R diagram plots the luminosities and temps of stars
• Most stars fall somewhere on the main sequence of the H-r diagram
• Stars with lower Ts and higher Ls than main-sequence stars must have larger radii: giants and supergiants
• Stars with higher T and lower L than main-sequence stars must have smaller radii: white dwarfs
• A star's full classification includes spectral type (line identities) and luminosity class (line shapes, related to size of star)

l- supergiant
ll- bright giant
lll- giant
lV- subgiant
V- main sequence

• H-R diagram depicts: temp, color, spectral type, luminosity, radius
• Main-sequence stars are fusing hydrogen into helium in their cores, like the Sun
• Mass measurements of main-sequence stars show that the hot, blue stars are much more massive than the cool, red ones
• The mass of a normal, hydrogen-fusing star determines its luminosity and spectral type
• 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
• Giants and s. giants are far larger than main-sequence stars and white dwarfs

   11.3 Star Clusters

• Open cluster: a few thousand loosely packed stars
• Globular cluster: up to a million or more stars in a dense ball bound together by gravity
• The dying process (from left to right, sequentially) looks like...massive blue stars, white, yellow, orange, and lastly red stars
• The main-sequence turnoff point of a cluster tells us its age
• To determine accurate ages, we compare models of stellar evolution to the cluster data
• The oldest globular clusters reveal that they are approximately 13 billion years old