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 http://casa.colorado.edu/~wcash/APS1020/APS1020.html

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

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 97-290 nm (ultraviolet)* A Sirius 10,000 K-7,500 K Very strong hydrogen lines 290-390 nm (violet)* F Polaris 7,500 K-6,000 K Moderate hydrogen lines, moderate lines of ionized calcium 390-480 nm (blue)* G Sun, Alpha Centauri A 6,000 K-5,000 K Weak hydrogen lines, strong lines of ionized calcium 480-580 nm (yellow) K Arcturus 5,000 K-3,500 K Lines of neutral and singly ionized metals, some molecules 580-830 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.

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

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

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.

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

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

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

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 = 6328.000063Å ---- That’s why we can’t sense a change Shuttle orbits at 6km/s v/c = 6/300,000 = 2x10-5 100MHz becomes 100MHz + 108 x 2x10-5 = 100,002,000Hz if coming at you.

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.

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 .9999999 cm. Not much. To say you were 5mph over the limit needs to measure one part in 100million!

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

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

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

The H-R Diagram

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!!!

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

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

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

Some Lifetimes Mass Luminosity Lifetime in Billion Years Sun 1 1 10 Sirius 2 10 2 Prox Cen .4 .001 4000 Rigel 8 10,000 .008 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