what is a star? wednesday, october 8 next planetarium shows: tonight 7 pm, thurs 7 pm
TRANSCRIPT
What is a Star?What is a Star?
Wednesday, October 8Wednesday, October 8Next Planetarium Shows: Tonight 7 pm, Thurs 7 pmNext Planetarium Shows: Tonight 7 pm, Thurs 7 pm
Light source moving towardtoward you: wavelength is shorter (blueshift).
Light source moving away away from you: wavelength is longer (redshift).
Doppler shift review:
Size Size of Doppler shift is proportional to radial velocity:
c
V
0
Δλ = observed wavelength shift = λ-λ0
λ0 = wavelength if source isn’t moving
V = radial velocity of moving source
c = speed of light = 300,000 km/sec
Example:
Hydrogen absorbs light with λ0 = 656.3 nm.
You observe a star with a hydrogen absorption line at λ = 656.2 nm.
What is a star?Examples of stars:
Sun Betelgeuse
What is a star?
A large, hot, luminous ball of gas.
“Why do stars shine?”
Stars are dense (Sun is 40% denser than liquid water).
Stars are opaque (you can’t see to the Sun’s center).
Stars are hot.
What happens when a dense, opaque
object becomes hot?
It emits light.
What do I mean by ““HOT”HOT”?
90°F 9980°F212°F
At the submicroscopic level: atoms in a gas
Object is hothot when the atoms of which it’s made are in rapid random motion.
TemperatureTemperature: measure of typical
speed of the atoms.
Random motions stop at absolute zeroabsolute zero temperature.
Kelvin = Celsius + 273Water boils: 373 Kelvin (K)
Water freezes: 273 K
Absolute zero: 0 K
Room temperature: ~300 K
Surface of Sun: ~5800 K
Different elements respond in different ways to changes in temperature.
At high density & low temperature, hydrogen is a gas of molecules.
Molecular hydrogen = H2 = two H atoms bonded together
(This assumes there’s no oxygen for the hydrogen
to bond with.)
At low density & low temperature, hydrogen is a gas of atoms.
Much of the interstellar gas in our Galaxy is
atomic hydrogen.
density ≈ 10 atoms/cm3 T ≈ 100 K
At high density & high temperature, hydrogen is an ionizedionized gas.
Much of the Sun’s interior is ionized
hydrogen.
Sun’s center: density ≈ 150 tons/m3
T ≈ 15 million K
A blackbody is an object that absorbs allall the light that hits it.
Heat a blackbody: it emits light of all wavelengths (a continuous continuous spectrum).
Wavelength at which spectrum peaks depends only on temperatureonly on temperature.
A star is an approximate blackbodyblackbody..
Blackbody spectra
21,100°F
10,300°F
4900°F
Wavelength of peak emission for a blackbody is inverselyinversely related to
temperature.
T
nm 000,900,2peak
emission maximum ofh wavelengtpeak
(Kelvin) re temperatuT
Examples:You:
Sun’s surface:
310
nm 000,900,2peak
5800
nm 000,900,2peak
Kelvin 310 C 37 F 6.98 T
Kelvin 5800T
9400 nm (Infrared)
500 nm (Visible)
2 am tomorrow, looking east
Another example: taking the temperature of a star!
Betelgeuse is redred.
Rigel is blueblue.
(Hard to see colors with the naked eye – binoculars help!)
Betelgeuse:
Rigel:
1000
K 000,900,2T
200
K 000,900,2T
nm 1000peak
nm 200peak
2900 K (4800°F)
14,500 K (26,000°F)
The Sun’s actual spectrum:
Close to a blackbody, but not perfect.
Perfect blackbody
Inte
nsity
Friday’s Lecture:
Reading:
Chapter 3
What is a Galaxy?