ASTR 3 Lecture Notes - Lecture 12: Hertzsprung (Crater), Main Sequence, Star Cluster
Luminosity: amount of power a star radiates (energy per second)
Apparent Brightness: amount of starlight that reaches Earth 0 energy per second per square meter
• Depends on luminosity and distance
• Divide luminosity by area to get brightness
o Area falls by d2
o Area of sphere = 4pi(r2)
We can determine luminosity if we can measure its distance and apparent brightness:
Luminosity = 4pi (distance)2 (brightness)
*brightness is easily measured with a photometer, an instrument that detects and counts photons
The parsec distance gives a parallax angle of 1 arcsec (= 206265 AU = 3.26 LY)
d(parsec) = 1/p(arcsec)
Ex: if P = 0.01 = 0.01 1/100
d = 100/1 = 100 parsecs or d = 326 LY
HOW HOT ARE THE STARS?
Every object emits thermal radiation with a spectrum that depends on its temperature
• At low temps, the emitted radiation is infrared, which we can't see --> hottest objects we
can see are bluish white
• Hotter objects emit more light per unit area at all wavelengths
o Luminosity per square meter = constant x T4
• Hotter objects emit photons with higher average energy
o Weins: T (K) = 1,000,000/wavelength(nm)
Hottest stars: 40,000K - blueish white
Coolest stars: 3,000K - red
Sun's surface: 5,800K
Lines in a star's spectrum correspond to a spectral type that reveals its temperature
Spectral type (letter/number) is shorthand for temperature:
Hottest: O B A F G K M :Coolest
Coolest stars you see molecules --> middle stars you see metals --> in the hottest, you see
hydrogen
Sun is a G star
________________________________________________________________________________
FIGURING OUT MASS
Two stars that orbit each other are called binary stars
Types of binary star systems:
• Visual Binary --> seen
• Eclipsing Binary --> think planetary transit
• Spectroscopic Binary --> detect only because of doppler shift
Visual Binary we can directly observe the orbital motions
• We get the orbital period and projected size of the orbit directly
Document Summary
Luminosity: amount of power a star radiates (energy per second) Apparent brightness: amount of starlight that reaches earth 0 energy per second per square meter: depends on luminosity and distance, divide luminosity by area to get brightness, area falls by d2, area of sphere = 4pi(r2) We can determine luminosity if we can measure its distance and apparent brightness: *brightness is easily measured with a photometer, an instrument that detects and counts photons. The parsec distance gives a parallax angle of 1 arcsec (= 206265 au = 3. 26 ly) d(parsec) = 1/p(arcsec) Ex: if p = 0. 01 = 0. 01 1/100 d = 100/1 = 100 parsecs or d = 326 ly. Lines in a star"s spectrum correspond to a spectral type that reveals its temperature. Hottest: o b a f g k m :coolest. Coolest stars you see molecules --> middle stars you see metals --> in the hottest, you see hydrogen.
