Astronomy 141
Life in the Universe
Prof. Scott Gaudi

Lecture 1: Stars: Masses, Luminosities, Tempertatures

Key Ideas

Color of a star depends on its Temperature

-Red Stars are Cooler, Blue Stars are Hotter

Spectral Classification

--Classify stars by their spectral lines

--Spectral differences are due to Temperature

Spectral Sequence (Temperature Sequence)

O B A F G K M L T

Stars shine because they are hot

Stars stay hot via nuclear fusion energy

--Energy from fusion of 4 Hydrogen into 1 Helium

Main Sequence stars "burn" Hydrogen into Helium in their cores.

--Get slowly brighter with age

The Main Sequence is a Mass Sequence

Observable Properties of Stars

Luminosity

Rate of Energy Output

Brightness and distance

Stars span an enormous range of luminosity

-- From 1/10,000 to one million solar luminosities

Colors of Stars

Stars are hot, dense balls of gas

--Continuous spectrum from the lowest visible layers ("photosphere").

--Approximates a blackbody spectrum with a single temperature.

Color of a star is related to its temperature:

--hotter stars appear BLUE (T=10,000-50,000K)

--middle stars appear YELLOW (T~6000K)

--cool stars appear RED (T~3000K)

Spectra of Stars

Hot, dense lower photosphere of a star is surrounded by thinner (but hot) atmosphere

--Produces an Absorption-Line Spectrum superimposed on a Continous Spectrum

--Lines come from the elements in the stellar atmosphere

Can we use stellar spectra to distinguish among different types of stars?

Spectra of Stars

Henry Draper Memorial Survey

Led by Edward Pickering

Objective prism photography of the sky from Harvard and Arequipa, Peru

Spectra of 220,000 stars:

Hired women as "computers" to analyze the stellar spectra

Harvard Classification

Edward Pickering & Williamina Fleming made a first attempt to classify ~10,000 stars by their spectra:

--Sorted by Hydrogen absorption-line strength

--Spectral Type "A" = strongest Hydrogen lines

--followed by types B, C, D, etc. (weaker)

Problem:

--The other lines didn't fit into this sequence.

Annie Jump Cannon

In 1901, Annie Jump Cannon noticed that stellar temperature was the principal distinguishing feature:

--Re-ordered the ABC types by temperature.

--Many classes thrown out as redundant.

Left with 7 primary classes:

O B A F G K M

Later work added the classes R, N, and S.

Henry Draper Catalog of Stars

Cannon further refined the spectral classification system by dividing the classes into numbered subclasses:

For example, A was divided into

-- A0 A1 A2 A3 ... A9

Between 1911 and 1924, she classified about 220,000 stars, published as the Henry Draper Catalog.

New Spectral Types: L & T

Coolest stars (<2500K) discovered by recent digital all-sky surveys.

L stars:

--Temperatures ~1300-2500K

--Strong lines of metal hydrides & neutral metals.

T dwarfs:

--Strong Methane (CH4) bands, like Jupiter.

--Most likely failed stars ("Brown Dwarfs")

Cecilia Payne Gaposhkin

Harvard graduate student in 1920s

1925 Ph.D. dissertation was a classic:

--First comprehensive theoretical interpretation of spectral spectra.

--Based on the then new atomic physics.

Showed that stars are mostly Hydrogen and Helium with traces of all of the other metals.

The Spectral Sequence

O B A F G K M L

Hottest <----------> Coolest

Bluest <----------> Reddest

Spectral Sequence is a Temperature Sequence

Hertzsprung-Russell Diagram

Hertzsprung-Russell Diagram

Plot of Luminosity versus Temperature:

--Temperture (T) from Spectral Type

--Luminosity (L) from apparent brightness & distance

Most stars fall on the Main Sequence

Other stars:

Giants

Supergiants

White dwarfs

Energy Generation in Stars

Why do stars shine?

Stars shine because they are hot.

--Emit light with a roughly thermal (blackbody) spectrum

--Internal heat "leaks" out of their surfaces.

Luminosity = rate of energy loss

To stay hot, stars must make up for the lost energy, otherwise they would cool and eventually fade out.

Fusion Energy

Fuse 1 gram of Hydrogen into 0.993 grams of Helium.

Leftover 0.007 grams is converted into energy

Enormous amount of energy!

Hydrogen Fusion

Question:

How do you fuse 4 1H (p) into 4He (2p+2n)?

Issues:

--Four protons colliding at once is unlikely.

--Must turn 2 of the protons into neutrons.

--Must be hot: >10 Million K to get protons close enough to fuse together.

Proton-Proton Chain

3-step Fusion Chain

Bottom Line Fuse 4 protons (1H) into one 4He nucleus plus the following reaction by-products: --2 photons = Energy --2 positrons (positive electrons) --2 neutrinos that leave the Sun carrying energy

Main Sequence

Definition: Stars which generate energy by fusion of H into He

The Main Sequence is a Mass Sequence

The location of a star along the M-S is determined by its Mass.

--Low-Mass Stars: Cool & Low Luminosity

--High-Mass Stars: Hot & High Luminosity

Result of the Mass-Luminosity Relation:

Which are the best stars to look for life?

Factors we want to consider:

--Habitable Zones

--Lifetime

--Frequency

--Tidal Locking

--Other peculiarities

See A Note about Graphics to learn why the graphics shown in the lectures are generally not reproduced with these notes.

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