1. ASTR 1020 – February 16 { } O B A F G K M .
First Exam: Tuesday, February 21
Material through Feb 9’s lecture
O B A F G K M
Oh Be A Fine
Gal
Guy
Kiss Me
Coolest
Hottest { }
Website

2. Stellar Classification
Full range of surface temperatures from 2000 to 40,000K
Spectral Classification is Based on Surface Temperature
O B A F G K M
Oh Be A Fine
Gal
Guy
Kiss Me
Coolest
Hottest { }
Each Letter has ten subdivisions from 0 to 9
0 is hottest, 9 is coolest

3. The Spectral Types O Stars of Orion's Belt >30,000 K
Lines of ionized helium, weak hydrogen lines
<97 nm (ultraviolet)* B
Rigel
30,000 K-10,000 K
Lines of neutral helium, moderate hydrogen lines
nm (ultraviolet)* A
Sirius
10,000 K-7,500 K
Very strong hydrogen lines
nm (violet)* F
Polaris
7,500 K-6,000 K
Moderate hydrogen lines, moderate lines of ionized calcium
nm (blue)* G
Sun, Alpha Centauri A
6,000 K-5,000 K
Weak hydrogen lines, strong lines of ionized calcium
nm (yellow) K
Arcturus
5,000 K-3,500 K
Lines of neutral and singly ionized metals, some molecules
nm (red) M
Betelgeuse, Proxima Centauri
<3,500 K
Molecular lines strong
>830 nm (infrared)
*All stars above 6,000 K look more or less white to the human eye because they emit plenty of radiation at all visible wavelengths.

4. Stellar Classification (2)
Sun G2
a Cen G2 + K5
Sirius A1
Antares M1
Rigel B8
O5 40,000K
B5 15,500
A5 8500
F5 6580
G5 5520
K5 4130
M5 2800
Letters are odd due to confusion in sorting out temperature scale
between 1900 and 1920

5. Question
Bellatrix (g Orionis) has a "surface" temperature of about 22,000K. What color do we expect it to be?
A: Yellow
B: White
C: Blue
D: Green

6. The Doppler Shift Another Powerful Tool
Frequency of light changes depending on velocity of source.
Similar to sound wave effect
Higher pitch when vehicle approaches
Lower when it recedes.

7. Spectral Shifts Spectrum is identifiable as
known element, but lines appear
shifted.
Measure the shift, and we get
velocity information!
Shift to blueward implies approach
Shift to redward implies departure

8. The Doppler Shift vt ct Observer D
During t seconds, source emits n waves of wavelength l.
They move ct during that time.
But source also moves vt during that time.
So the n waves are scrunched into ct-vt instead of the usual ct
Thus the wavelength is reduced from l to

9. The Doppler Formula v is positive if coming toward us
Wavelength l decreases from lab value
Frequency shifts up as source approaches

10. Doppler Examples I run toward you with laser at 3m/s
c = 3x108m/s, l = 6328Å
v/c = 10-8
So dl = l x v/c = 6328 x 10-8 = 6.3x10-5
l = Å That’s why we can’t sense a change
Shuttle orbits at 6km/s
v/c = 6/300,000 = 2x10-5
100MHz becomes 100MHz x 2x10-5 = 100,002,000Hz if coming
at you.

11. Another Doppler Example
Star has known hydrogen line at 6563Å
Detect line at 6963Å
dl = 400Å
Star is receding at 18,000km/s !!
In some cases astronomers can detect shifts as small as one part in a million.
That implies detection of motion as small as 300m/s.

12. What about that #@&! radar gun?
Cop uses radar which typically operates near l = 1cm
If you are going 65mph = 65 mi/hr x 1600m/mi / (3600 s/hr) = 30m/s
This creates a shift of dl = 30/3x108 = 10-7 in the wavelength
1cm shifts to cm. Not much.
To say you were 5mph over the limit needs to measure one part in 100million!

13. Example of How Its Used in Astronomy
Stellar lines are broadened by star’s rotation.

14. Stellar Luminosity The H-R Diagram
By 1915 had lots of spectra and classifications
Had a few distances from parallax
Once distance was available, luminosity and Absolute Magnitude
could be calculated.
Herzsprung and Russel, working independently both plotted
absolute magnitude (luminosity) vs classification (temperature)
The H-R Diagram

15. The H-R Diagram Plot of Brightness vs Temperature-5
Rigel
Giants
Capella
Sirius
Brightness
Procyon
Sun
Main Sequence +5
a Cen B
White Dwarfs
+10
Sirius B
Prox Cen
+15 O B A F G K M
Spectral Type

16. The H-R Diagram

17. The Main Sequence Stars Differ By: Mass Age Composition Nothing else!
And composition doesn’t vary
Age and Mass only.
Those on main sequence are all
burning H so age drops out.
MS is function of MASS only!!!

18. Full, Artistic H-R As mass of MS star increases, both R and T increase
increasing
size
sAT4
T constant
on any vertical
line

19. Newly Formed Star M Spectral Type O +10 +5 -5 +15 B A F G K Protostar
+15 B A F G K
Sun
Sirius
a Cen B
Prox Cen
Procyon
Rigel
Capella
Sirius B
Main Sequence
Giants
White Dwarfs
Protostar
Large,
Low T.
Settles down
to MS
Then sits while
burning H

20. MS Lifetime
What determines amount of time a star stays on Main Sequence?
Just like a kerosene heater: Amount of fuel and rate of burn.
More Mass = More Fuel
More Luminosity = Greater Burn Rate
We can scale from the Sun: M = 1M L = 1L
Sun lasts 1010 years
M in solar masses
L in solar luminosities

21. Some Lifetimes Mass Luminosity Lifetime in Billion Years Sun 1 1 10
Sirius
Prox Cen
Rigel 8 10,
Dinky little stars like Prox Cen will last trillions of years
Huge stars like Rigel are gone in a few million
There aren’t many large stars out there, because they don’t last.
10,000 O stars of the 100,000,000,000 Milky Way stars