Amber Reed's Notes

Chapter 11: Surveying the Stars

Properties of Stars
How do we measure stellar luminosities ?

-The brightness of stars depend on the distance and luminosity (the amount of power a star radiates)
-Apparent brightness is the amount of starlight (energy per second = watts) that reaches Earth (energy/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
- positions of the nearest stars shift by about an arcsecond as Earth orbits the Sun.
-Parallax is measured by comparing snapshots taken at different times and measuring shifts in angles compared to the stars.

How do we measure stellar temperatures?

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

-Hotter objects emit more light per unit area at all frequencies
-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

How do we measure stellar masses?

-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

-Orbital period (p)
-Orbital separation (a or r =radius)
-Orbital velocity (v)

11.2 Patterns Among Stars

What is Hertzsptung- Russell diagram?

-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
-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 these are ;giants and supergiants
-Most stars end up small and white after fusion is done: white dwarfs
Giants and s. giants are far larger than main-sequence stars and white dwarfs
  
11.3 Star Clusters

What are the two types of 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
-dying process (from left to right, sequentially) looks like ;massive blue stars, white, yellow, orange, and lastly red stars

How do we measure the age of a star cluster?
- main-sequence turnoff point of a cluster tells us its age
-determining accurate ages, we compare models of stellar evolution to the cluster data
- oldest globular clusters reveal that they are approximately 13 billion years old


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