What is a Star?

1
What is a Star?
Wednesday, October 8Next Planetarium Shows
Tonight 7 pm, Thurs 7 pm
2
Doppler shift review
Light source moving toward you wavelength is
shorter (blueshift).
Light source moving away from you wavelength is
longer (redshift).
3
Size of Doppler shift is proportional to radial
velocity
?? observed wavelength shift ?-?0 ?0
wavelength if source isnt moving V radial
velocity of moving source c speed of light
300,000 km/sec
4
Example
Hydrogen absorbs light with ?0 656.3 nm.
You observe a star with a hydrogen absorption
line at ? 656.2 nm.
5
What is a star?
Examples of stars
Sun
Betelgeuse
6
What is a star?
A large, hot, luminous ball of gas.
7
Why do stars shine?
Stars are dense (Sun is 40 denser than liquid
water).
Stars are opaque (you cant see to the Suns
center).
Stars are hot.
8
What happens when a dense, opaque object becomes
hot?
It emits light.
9
What do I mean by HOT?
90F
9980F
212F
10
At the submicroscopic level atoms in a gas
11
Object is hot when the atoms of which its made
are in rapid random motion.
Temperature measure of typical speed of the
atoms.
Random motions stop at absolute zero temperature.
12
(No Transcript)
13
Kelvin Celsius 273
Water boils 373 Kelvin (K) Water freezes 273
K Absolute zero 0 K
Room temperature 300 K Surface of Sun 5800 K
14
Different elements respond in different ways to
changes in temperature.
15
At high density low temperature, hydrogen is a
gas of molecules.
Molecular hydrogen H2 two H atoms bonded
together
16
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
17
At high density high temperature, hydrogen is
an ionized gas.
Much of the Suns interior is ionized hydrogen.
Suns center density 150 tons/m3 T 15
million K
18
A star is an approximate blackbody.
A blackbody is an object that absorbs all the
light that hits it.
Heat a blackbody it emits light of all
wavelengths (a continuous spectrum).
Wavelength at which spectrum peaks depends only
on temperature.
19
Blackbody spectra
20
Wavelength of peak emission for a blackbody is
inversely related to temperature.
21
Examples
You
9400 nm (Infrared)
Suns surface
500 nm (Visible)
22
(No Transcript)
23
2 am tomorrow, looking east
24
Another example taking the temperature of a star!
Betelgeuse is red.
(Hard to see colors with the naked eye
binoculars help!)
Rigel is blue.
25
Betelgeuse
2900 K (4800F)
Rigel
14,500 K (26,000F)
26
The Suns actual spectrum
Close to a blackbody, but not perfect.
27
Fridays Lecture
What is a Galaxy?
Reading
Chapter 3