The Nature of Stars

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The Nature of Stars
Courtesy of Ed Prather and Center for Astron.
Educ.
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A stars color reveals its surface temperature
What color is this star?
red
blue
Green or White
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Crystal Star has an apparent magnitude of 1.1
and an absolute magnitude of 1.1. Sydney Star
has an apparent magnitude of 3.72 and an
absolute magnitude of 6.1.Crystal use ASydney
use B
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Spectra of Stars
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Diversity Leads to Revolution

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

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Women Computers (1890)
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Annie Jump Cannon (1863-1941)
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Classification Scheme

• A
• B
• C
• D
• E
• .
• .
• .
• S

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Antonia Maury (1866-1952)

• Spectral classes might make more sense if
  arranged by temperature

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O
B
A
F
G
K
M
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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 Maurys work.

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Meghnad Saha (1893-1956)

• Theory of thermal ionization of atoms

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

• Gave theoretical explanation for Cannons
  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.

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

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Summary of Spectral Classes
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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
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• One Boring Afternoon, Frank Grew Killer Marijuana

19
J. Davy Kirkpatrick -- Annual Reviews of
Astronomy and Astrophysics
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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?
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Looking for correlations
Height vs. IQ ?
Height vs. Weight ?
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QUESTIONS

• Are more luminous stars always larger?
• What combinations of temperature and luminosity
  are possible?

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THE H-R DIAGRAM

• done independently by Enjar Hertzsprung and Henry
  Norris Russell
• graph of luminosity (or absolute magnitude)
  versus temperature (or spectral class)

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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
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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
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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.

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

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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!

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

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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

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The brightest star in the sky with its white
dwarf companion Sirius B
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• Tiny White Dwarf stars are found in the lower
  left corner of the HR diagram

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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

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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

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Tutorial H-R Diagram (p.109)

• 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?

• They are the same size.
• The star near the top left is larger.
• The star near the top right is larger.
• There is insufficient information to determine
  this.

0/0
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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?

• Rigel
• Sirius B
• They have the same temperature.
• There is insufficient information to determine
  this.

0/0
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Which is hotter a B3 or an A7? Which is hotter a
B0 or a B9?
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Which is hotter a B3 or an A7? Which is hotter a
B0 or a B9?
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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

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There is a relationship between mass and
luminosityfor Main Sequence stars
Bigger (more massive) is brighter and hotter!
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There is a relationship between mass and
luminosityfor Main Sequence starsthe numbers
shown are masses in terms of the Suns mass
Bigger (more massive) is brighter and hotter!
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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

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Average Densities

• SUN about density of water
• GIANTS One thousand times less dense than AIR!
• DWARFS about 1 million times the Suns density
• one teaspoon 5 tons!!!