The Nature of Stars

MRS 2

• Test 3 Thursday April 24• Part 1 Due 1 week from Today (4/17)• Read http://www.planetary.org/blogs/emily-lakdawalla/2013/12091832-curiosity-results-at-agu-age-

dating.html

• Read http://www.sciencemag.org/content/343/6169/1247166.abstract• Answer questions on website• If you do not understand/have questions

contact us!!!

Pollux has an apparent magnitude of 1.1 and an absolute magnitude of 1.1. Epsilon Eridani has an apparent

magnitude of 3.72 and an absolute magnitude of 6.1. From which of these stars do we receive more light?

a) Pollux

b) Epsilon Eridani

Imagine that you are viewing a star that has an apparent magnitude of 0.2 and is located about 100 parsecs away from us. Which of the following is most likely the star’s

absolute magnitude?

a) -4.8

b) 0.1

c) 0.2

d) 0.3

e) 5.2

A star’s color reveals its surface temperature

What color is this star?

Diversity Leads to Revolution

• Annie Jump Cannon• Meghnad Saha• Cecilia Payne-Gaposchkin

Women Computers (1890)

Annie Jump Cannon (1863-1941)

OBAFGKM

A Revolution

• Most astronomers believed that the differences in spectral classes (O-M) were due to differences in chemical abundance.

• Indian physicist Meghnad Saha offered another explanation, which was confirmed at Harvard by Cannon

Meghnad Saha (1893-1956)Theory of thermal ionization of atoms

Cecelia Payne-Gaposchkin (1900-1979)First PhD in Astronomy from Harvard/Radcliffe

Together Saha and Payne-Gaposchkin

• Gave theoretical explanation for Cannon’s classification scheme.

• Showed that the differences in spectra (absorption lines) are due to temperature and thermal ionization of atoms not abundance of elements

• Provided a convincing argument that stars are mostly made of hydrogen.

Stars are classified by their spectra as O, B, A, F, G, K, and M spectral types

What does this give us?• a new way to classify stars• color, peak wavelength of the black

body curve, and spectral class all of which are indicators of a star’s temperature

Summary of Spectral Classes

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 Be a Fine Guy (or Girl), Kiss Me

For thousands of nearby stars we can find:

• the total luminosity• the temperature (color or spectral

type) • the size (radius)• the distance

CAN WE FIND ANY RHYME, REASON, OR RELATIONSHIPS?

Looking for correlations:Height vs. IQ ?

Height vs. Weight ?

QUESTIONS:• Are more luminous stars always

larger?• What combinations of temperature

and luminosity are possible?

THE H-R DIAGRAM• Done independently by Enjar

Hertzsprung and Henry Norris Russell• Graph of luminosity (or absolute

magnitude) versus temperature (or spectral class)

The Hertzsprung-Russell (H-R)

diagram identifies a definite relationship between temperature

and absolute magnitude

HR DIAGRAM

absolute magnitude vs temperature

or

luminosity vs spectral type

MAIN SEQUENCE• Goes from top left (hot and

bright) to bottom right (cool and dim).

• 90% of the stars are in the Main Sequence stage of their lives

• Burning Hydrogen to Helium in core

• Includes our Sun.

• Main Sequence stars are found in a band from the upper left to the lower right

RED GIANTS• Really Big, Not Very Hot but

VERY BRIGHT!• Betelgeuse: 3500 K , 100,000

times more luminous than the sun• Radius must be 1000x that of Sun!

• Red Giant and Supergiant stars are found above and to the right of the Main Sequence stars

WHITE DWARFS• Very Small, Very Hot but

Not Very Bright• Sirius B: 27,000 K, but gives

off 1000 times less light than the Sun

• 100 times smaller than the Sun

• Tiny White Dwarf stars are found in the lower left corner of the HR diagram

Determining the Sizes of Stars from an HR Diagram

• The Smallest stars are the tiny White Dwarf stars and are found in the lower left corner of the HR diagram

• Main sequence stars span a range of sizes from the small found in the lower right to the large found in the upper left

• The largest stars are the Giant and Supergiant stars which are found in the upper right corner

Tutorial: H-R Diagram (p.117)

• Work with a partner!• Read the instructions and questions carefully.• Discuss the concepts and your answers with one

another. Take time to understand it now!!!!• Come to a consensus answer you both agree on.• If you get stuck or are not sure of your answer, ask

another group.

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How does the size of a star near the top left of the H-R diagram compare with a star of the same

luminosity near the top right of the H-R diagram?

1. They are the same size.2. The star near the top left is

larger.3. The star near the top right

is larger.4. There is insufficient

information to determine this.

0/0

The star Rigel is about 100,000 times brighter than the Sun and belongs to spectral type B8. The star Sirius B is about 3000 times dimmer than the Sun

and also belongs to spectral type B8. Which star has the greatest surface temperature?

1. Rigel2. Sirius B3. They have the same

temperature.4. There is insufficient

information to determine this.

What about the Masses of Stars on the H-R Diagram?

• Main Sequence stars range from 0.1M to ~100M

• The masses of Main Sequence stars increase with increasing luminosity, size and temperature

• Main Sequence stars increase in mass from the lower right to the upper left of the H-R Diagram

There is a relationship

between mass and luminosity

for Main Sequence stars

Bigger (more massive) is brighter and hotter!

There is a relationship

between mass and luminosity

for Main Sequence starsthe numbers shown are masses in terms of the Sun’s mass

Bigger (more massive) is brighter and hotter!

There is not simple relationship for the Mass of Non-Main Sequence stars:

• Giants and Supergiants: range from M to about 20M

• White Dwarfs: approximately M or less

Average Densities:• SUN: about density of water• GIANTS: One thousand times less

dense than AIR!• DWARFS: about 1 million times

the Sun’s density–one teaspoon: 5 tons!!!

The evolution of stars is determined by a constant battle between gravity and pressure

gravity pulls things togetherpressure pushes things apart

40

Interstellar mediumGas = HydrogenDust = Carbon and

Silicon

Stars condense from clouds of gas and dust (the interstellar medium) that exist throughout the disk of the galaxy

Pillars of CreationEagle Nebulae

Becoming a Star Step 1 – Cloud collapses• Why do these clouds of gas and dust collapse?

– One idea is that a shockwave from the explosion at the death of a star known as a supernova cause the gas and dust cloud to become unstable and start to collapse

Becoming a Star Step 1 – Cloud collapses• As the cloud collapses, the

center becomes very very hot and very very dense -

• As the gas cloud collapses due to gravitational forces, the core becomes hotter and the density inside the core increases

• Eventually, the temperature and density reach a point where nuclear fusion can occur

Becoming a Star Step 2 – Fusion

Fusion is the combining together of light atoms, into heavier atoms

For all Main Sequence stars, the temperature and density in their cores are so great that Hydrogen atoms combine to make Helium atoms and release

energy – a process known as thermonuclear fusion

4H He + energy

The Main Sequence is defined by stars converting hydrogen to helium in their core

Becoming a Star Step 3 – BalanceAll Main Sequence stars are in

hydrostatic equilibrium

• Fusion produces radiation (light) that creates an outward pressure

• During hydrostatic equilibrium there is a balance between the gravitational collapse of the star pushing inward and the outward pressure produced by photons from nuclear fusion in the core.

It’s a matter of balance.• This balance is called hydrostatic equilibrium• gravity ( ) wants to collapse the star, but pressure

( ) pushes outward against the collapsing material

Fusion: 4H He + energy(light)

• All Main Sequence stars are in hydrostatic equilibrium because nuclear fusion of hydrogen is producing enough outward pressure to balance gravitational collapse.

It takes a few million years to get there but - stars spend most of their life time as a Main Sequence star

STELLAR LIFETIMES• Which will have a greater core temperature

and density – a high mass star or a low mass star?

• Which will then have a greater fusion rate?• Which will use up its fuel more quickly?• What is the fuel?

STELLAR LIFETIMES• Consider a main sequence star with 10 times the

mass of the Sun• It will

– have higher temps and pressures at the core – have greater fusion rates - consumes fuel at 1000

times the rate of the sun– be 1000 times as bright and last 1/100 as long

• “Burn bright, die young.”

LIFETIMES• Bright O-type stars live very short

lives (about 10 million years)• Very small stars live a long time (100

billions of years)• Our SUN: will live a total of about

10 billion years (half used up)

The more massive a star, the faster it goes through its main sequence

phase

Tutorial: Star Formation and Lifetimes (p.119)

• Work with a partner!• Read the instructions and questions carefully.• Discuss the concepts and your answers with one

another. Take time to understand it now!!!!• Come to a consensus answer you both agree on.• If you get stuck or are not sure of your answer, ask

another group.

Stars spend most of their life cycles on the Main Sequence

• Main Sequence stars are in hydrostatic equilibrium because nuclear fusion is turning hydrogen into helium and producing enough outward pressure to balance gravitational collapse.

• 90% of all stars are found on the Main Sequence• 90% of the whole life of all stars is spent on the Main

Sequence • BUT – What happens when the hydrogen runs out?