The Nature of Stars

EXAM #3 – WILL NOT BE APRIL 5th it has been re-scheduled for THURSDAY APRIL 12th 4/12 (Wednesday)

Tailgate Party!! For Exam #3 Wed April 11th

Observing Logs Due IN CLASS May 1st

red

A star’s color reveals its surface temperature

What color is this star?

bluegreen

Spectra of Stars

Diversity Leads to Revolution

• Annie Jump Cannon

• Antonia Maury

• Meghnad Saha

• Cecilia Payne-Gaposchkin

Women Computers (1890)

Annie Jump Cannon (1863-1941)

Classification SchemeA

B

C

D

E

.

.

.

S

Antonia Maury (1866-1952)

Spectral classes might make more sense if arranged by temperature

O

B

A

F

G

K

M

A Revolution

• Most astronomers believed that the differences in spectral lines were due to subtle differences in chemical abundance.

• Indian physicist Meghnad Saha offered another explanation, which was confirmed at Harvard by Cannon and Maury’s work.

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

O hotter than 25,000 KB 11,000 - 25,000 KA 7500 - 11,000 KF 6000 - 7500 KG 5000 - 6000 KK 3500 - 5000 KM cooler than 3500 K

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:

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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

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

• Includes our Sun.

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

RED GIANTSRED GIANTS• Really Big, Not Very Hot but Really Big, Not Very Hot but

VERY BRIGHT!VERY BRIGHT!

• Betelgeuse: 3500 K , 100,000 Betelgeuse: 3500 K , 100,000 times more luminous than the suntimes more luminous than the sun

• radius must be 1000x that of 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 Very Small, Very Hot but

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

off 1000 times less light than off 1000 times less light than the Sunthe Sun

• 100 times smaller than the Sun100 times smaller than the Sun

• Tiny White DwarfWhite 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

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.73)

• 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.

c

e

Temperature (K)

Lu

min

osit

y (s

olar

un

its)

Spectral Type

Ab

solute m

agnitu

de

20,000 10,000 5,000

O5 B5 A5 F5 G5 K5 M5

10,000

1,000

100

10

1

.1

.01

.001

.0001

a

b

d

f

-5

0

5

10

15

Main Sequence

Red Giants

White Dwarfs

0/0

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.

Which is hotter a B3 or an A7? Which is hotter a B0 or a B9?

c

e

Temperature (K)

Lu

min

osit

y (s

olar

un

its)

Spectral Type

Ab

solute m

agnitu

de

20,000 10,000 5,000

O5 B5 A5 F5 G5 K5 M5

10,000

1,000

100

10

1

.1

.01

.001

.0001

a

b

d

f

-5

0

5

10

15

Main Sequence

Red Giants

White Dwarfs

Which is hotter a B3 or an A7? Which is hotter a B0 or a B9?

c

e

Temperature (K)

Lu

min

osit

y (s

olar

un

its)

Spectral Type

Ab

solute m

agnitu

de

20,000 10,000 5,000

O5 B5 A5 F5 G5 K5 M5

10,000

1,000

100

10

1

.1

.01

.001

.0001

a

b

d

f

-5

0

5

10

15

Main Sequence

Red Giants

White Dwarfs

c

e

Temperature (K)

Lu

min

osit

y (s

olar

un

its)

Spectral Type

Ab

solute m

agnitu

de

20,000 10,000 5,000

O5 B5 A5 F5 G5 K5 M5

10,000

1,000

100

10

1

.1

.01

.001

.0001

a

b

d

f

-5

0

5

10

15

Main Sequence

Red Giants

White Dwarfs

B9B0

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!!!