The H-R Diagram

Physical Astronomy

Professor Lee Carkner

Lecture 8

Questions1) 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

Questions4) 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 – hypergiantI – II – bright giantIII – IV – subgiantV –

Red dwarfBrown 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: 10.1-10.2 Homework: 10.3, 10.4, 10.22