The H-R Diagram Physical Astronomy Professor Lee Carkner Lecture 8

Questions 1)Temperature by spectrum shape  2, 5, 3, 1, 4  Where spectrum peaks, from short to long wavelength 2)Rank the stars in order of the strength of hydrogen lines (strongest to weakest)  5, 3, 1, 4, 2  5 and 3 are strong, 1,4,2 hard to see 3)Rank the stars in order of temperature (hottest to coolest)using the color of the images  2 and 5, 3, 1 and 4  2 and 5 are blue, 3 is white,1 and 4 are reddish

Questions 4)Rank the stars in order of temperature (hottest to coolest) by combining all the information together  2, 5, 3, 1, 4  2 and 5 are hottest, but 2 has no H lines 5)Estimate a spectral type for each star  1 – G  2 – O  3 – F  4 – K  5 – A

 Star 2  Shape: 1  peak < 4000  H lines: 5  Color: 1  blue  Overall temp: 1  Spectral type: O  Hot, weak H

 Star 5  Shape: 2  peak ~4000  H lines: 1  Color: 1  blue  Overall temp: 2  Spectral type: A  Hot, strong H

 Star 3  Shape: 3  peak ~5000  H lines: 2  Color: 3  white  Overall temp: 3  Spectral type: F  Medium hot, medium H

 Star 1  Shape: 4  peak ~6500  H lines: 3  Color: 5  reddish?  Overall temp: 4  Spectral type: G  cool, strong Ca?

 Star 4  Shape: 5  peak ~8000  H lines: 4  Color: 5  red-orange  Overall temp: 5  Spectral type: K  coolest, strong Ca

Spectral Type   Start by ordering by strength of hydrogen lines (A, B, C …)   From high temperature (early-type) to low temperature (late-type): O B A F G K M L T  e.g., B9, A0, A1, A2 … A9, F0, F1

The H-R Diagram  We can measure m and d to get M   We can find spectral type   How are they related?   Luminosity increases for increasing temperature  Main sequence

Luminosity Class   These stars are more or less luminous than main sequence stars of the same temperature   Main sequence stars are dwarfs   Giant stars also tend to have narrower lines   Represent stars in different stages of their lives

Luminosity Classes 0 – hypergiant I – II – bright giant III – IV – subgiant V – Red dwarf Brown dwarf IV – D (VII) – white dwarf Red dwarf Brown dwarf

Mass/Luminosity  We can use binary stars to find mass for each spectral type   There is a mass-luminosity relationship for main sequence stars: L = M  (in solar units)   = 1.8 for   = 4.0 for 0.3   = 2.8 for 3  Note the different exponents  Different mass stars must have different interiors

Density   Main sequence stars have similar density  Giants have low density   White dwarfs have high density  Similar masses to MS stars, but much smaller radius

Spectroscopic Parallax  If we take a star’s spectrum we can find temperature and luminosity class   d = 10 (m-M+5)/5  Not very accurate, but easy to do 

Next Time  Read:  Homework: 10.3, 10.4, 10.22