Let’s review some important things we want to know about stars…

Given enough time and information, we can figure out their…

• Brightness - easily observed• Parallax to measure distance• Spectral type - can get from the spectrum• Brightness + Distance = Luminosity• Temperature - can get from spectrum• Temperature + distance = Size• Mass - hard to figure out, but there are binary stars• Age - exact age is hard, but can estimate

What do you do when you have data and you don’t know what to do with it and you don’t understand it?

CLASSIFY!HOPE:

We just might get to know THE universe better?

Stars can be arranged into categories based on the features in their spectra…

This is called “Spectral Classification”

1. by the “strength” (depth) of the absorption lines in their spectra 2. by their color as determined by their blackbody curve 3. by their temperature and luminosity

How do we categorize stars?A few options:

• Much of the work in classifying and explaining stellar spectra and brightness was done by women at Harvard around the turn of the century.

Harvard Computers (1890)

Annie Jump Cannon (1863-1941)

• Single-handedly classified more than 250,000 stellar spectra.

Henrietta Leavitt (1868-1921)

Stars are classified by their spectra as

O, B, A, F, G, K, and M spectral types

• O B A F G K M• hottest to coolest• bluish to reddish• An important sequence to remember:

–Oh Boy, An F Grade Kills Me

The Spectral SequenceClass Spectrum Color Temperature

O ionized and neutral helium, weakened hydrogen bluish 31,000-49,000 K

B neutral helium, stronger hydrogen blue-white 10,000-31,000 K

A strong hydrogen, ionized metals white 7400-10,000 K

F weaker hydrogen, ionized metals yellowish white 6000-7400 K

G still weaker hydrogen, ionized and neutral metals yellowish 5300-6000 K

K weak hydrogen, neutral metals orange 3900-5300 K

M little or no hydrogen, neutral metals, molecules reddish 2200-3900 K

L no hydrogen, metallic hydrides, alkalai metals red-infrared 1200-2200 K

T methane bands infrared under 1200 K

Eventually, the connection was made

between the observables and the theory.

Observable:• Strength of Hydrogen Absorption Lines• Blackbody Curve (Color)

Theoretical:• Using observables to determine things

we can’t measure: Temperature and Luminosity

Cecilia Payne

Hertzsprung-Russell (H-R ) Diagram

• done independently by Enjar Hertzsprung and Henry Norris Russell

• graph of luminosity versus temperature (or spectral class)

Categorizing the stars…

Henry Norris Russell dissuaded Cecilia Payne-Gaposchkin from concluding that the composition of the Sun is different from that of the Earth in her papers, as it contradicted the accepted wisdom at the time. However, he changed his mind four years later after deriving the same result by different means. After Payne was proven correct, Russell briefly credited Payne for discovering that the Sun had a different chemical composition from Earth in his paper. However the credit was still generally given to him instead.[11]

Temperature

10-4

10-2

102

104

106

L/L Θ

40,000 20,000 10,000 5,000 2,500

WHITE DWARFS

MAIN SEQUENCE

SUPER GIANTS

GIANTS

O B A F G K M

Shematic H-R Diagram

BRIGHT

FAINT

HOT COOL

15

10

5

– 5

– 10

0

None of these “extra”

stars are Hydrogen burning!

Most stars are found along the

Main Sequence

Stars spend most of their

active life time on the Main

Sequence (MS).

Same temperature,

but much brighter than

MS stars→ Must be much larger► Giant Stars

“Red Giants” “Supergiants”

Same temp., but fainter → Dwarfs

the stars aren’t randomly scattered on this graph-- they form a line!

WHAT IS AMAZING: Stars of different masses fall along a narrow path in L/T diagram

If you measure the luminosity and the color of a star, you know its mass!!!

If a random star falls on

the Main Sequence, you also

know that it’s Hydrogen

burning!

The more massive a star is, the more luminous it is…

• Hotter• Brighter• Bigger• Shorter-lived

More massive stars are…

But a higher rate of fusion means it’s burning its fuel faster!

Low mass stars have lifetimes comparable to the Age of the Universe

High mass stars have very short lifetimes, and disappear quickly!

The Mystery of Red Giants and White Dwarfs…Many of these stars have the same temperature as normal Main

Sequence stars, but they’re much brighter or fainter!How is this possible???

Hot. Cool.Cool, but bigLuminous!Same Luminosity

Same Temperature & Surface Brightness

Hot, but tinyFaint.

If the size of the star changes, its luminosity changes

L = b x Area

L T4 4R2

After time passes…

Only long-lived low mass stars are left on the main sequence!

The high mass stars are gone!

• Cool, but bright.• Same temp as some

main sequence stars same surface brightness!

Must be bigger AREA BIGGER star!(and thus the name, red giant)

Red Giants:

L T4 4R2

as gravity caused the collapseStars are formed by a cloud of gas and dust that collapsed inward and began to spin. These clouds are called nebula. About 30 million years after the cloud collapsed, its center has reached 15 million kelvin and has become a protostar. As stars continue to go through nuclear fusion from hydrogen gas combining to make deuterons and then two deuterons making helium, the star will eventually run out of hydrogen.

First, there was a nebula. Or: last, there was a nebula.

The birth of stars in the M16 Eagle Nebula

and the cycle starts again

RED GIANT PHASE of star’s existanceA star experiences an energy crisis and its core collapses when the star's basic, non-renewable energy source - hydrogen - is used up. A shell of hydrogen on the edge of the collapsed core will be compressed and heated. The nuclear fusion of the hydrogen in the shell will produce a new surge of power that will cause the outer layers of the star to expand until it has a diameter a hundred times its present value.