11.2 Classifying Stars

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11.2 Classifying Stars

• Our Goals for Learning
• How do we classify stars?
• Why is a stars mass its most important property?
• What is a HertzsprungRussell diagram?

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What is the stellar Main Sequence?
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Most of the brightest stars are reddish in
color Color and luminosity are closely related
among the remaining normal stars
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Main-sequence stars (like the Sun) are fusing
hydrogen into helium in their cores Luminous
main-sequence stars are hot (blue) Less luminous
ones are cooler (yellow or red)
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Why are some red stars so much more
luminous? Theyre bigger! L Luminosity T
Temperature LArea T4 L4pr2 T4 (page
312) Biggest red stars 1000 Rsun Smallest
red stars 0.1 RSun (Ranking Task)

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Which star is the least luminous?

1. A
2. B
3. C
4. D
5. E
6. F

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Which star is the most luminous?

1. A
2. B
3. C
4. D
5. E
6. F

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Temperatures and sizes of stars
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1) Hotter objects emit more light per area at all
wavelengths 2) Hotter objects tend to emit light
at shorter wavelengths and higher frequencies
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Why are some red stars so much more
luminous? Theyre bigger! L Luminosity T
Temperature LArea T4 L4pr2 T4 (page
312) Biggest red stars 1000 Rsun Smallest
red stars 0.1 RSun (Ranking Task)

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Activity 10, Part II (pages 32-34, questions 6-15)
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6) Stars E and F in Figure 2a are the same size.
Which gives off more red light?

1. E
2. F
3. Both give off the same amount of red light

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7) Stars E and F in Figure 2a are the same size.
Which gives off more blue light?

1. E
2. F
3. Both give off the same amount of blue light

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8) Which star from Figure 2a looks red, and which
looks blue?

1. E looks blue and F looks red
2. E looks red and F looks blue
3. Both look red
4. Both look blue

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11) In Figure 2b, how must star C be different
from star E to account for their difference in
energy output?

1. Star C must be cooler than Star E
2. Star C must be smaller than Star E
3. Both 1 and 2
4. Either 1 or 2

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1) Hotter objects emit more light per area at all
wavelengths 2) Hotter objects tend to emit light
at shorter wavelengths and higher frequencies
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14) In Figure 2c, which star has the higher
surface temperature?

1. Star D
2. Star E
3. Both have the same temperature

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1) Hotter objects emit more light per area at all
wavelengths 2) Hotter objects tend to emit light
at shorter wavelengths and higher frequencies
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15) In Figure 2c, which star is larger?

1. Star D
2. Star E
3. Both are the same size

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How do we classify stars?
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Most of the brightest stars are reddish in
color Color and luminosity are closely related
among the remaining normal stars
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Main-sequence stars (like the Sun) are fusing
hydrogen into helium in their cores Luminous
main-sequence stars are hot (blue) Less luminous
ones are cooler (yellow or red)
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Properties of Main Sequence Stars - Review
Luminosity from brightness and
distance 10-4 LSun - 106 LSun Temperature
from color and spectral type 2,000 K -
50,000 K Mass from period (p) and average
separation (a) of binary-star orbit 0.08
MSun - 150 MSun
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Main Sequence Stellar Properties Review
Luminosity from brightness and
distance 10-4 LSun - 106 LSun Temperature
from color and spectral type 2,000 K -
50,000 K Mass from period (p) and average
separation (a) of binary-star orbit 0.08
MSun - 150 MSun
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Why is a stars mass its most important property?
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Core pressure and temperature of a higher-mass
star need to be larger in order to balance
gravity Higher core temperature boosts fusion
rate, leading to larger luminosity
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A stars mass largely determines how much light
it gives off (its luminosity) and for how long it
will shine (its lifetime).
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What can we learn about stars from a
Hertzsprung-Russell Diagram?
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An H-R diagram plots the luminosity versus
temperature of stars
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Normal hydrogen- fusing stars reside on the main
sequence of the H-R diagram
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Stars with low temperature and high luminosity
must have large radius
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H-R diagram depicts Temperature, Colour, Luminos
ity, Radius, Spectral Type Mass Lifespan
Age
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Which star is the hottest?
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Which star is the hottest?

1. A
2. B
3. C
4. D

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Which star is the hottest?
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Which star is the most luminous?
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Which star is the most luminous?

1. A
2. B
3. C
4. D

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Which star is the most luminous?
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Which star is a main-sequence star?
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Which star is a main sequence star?

1. A
2. B
3. C
4. D

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Which star is a main-sequence star?
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Which star has the largest radius?
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Which star has the largest radius?

1. A
2. B
3. C
4. D

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Which star has the largest radius?
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Which star is most like our Sun?
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Which star is most like our Sun?

1. A
2. B
3. C
4. D

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Which star is most like our Sun?
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What have we learned?

• How do we classify stars?
• We classify stars according to their spectral
  type and luminosity class.
• The spectral type tells us the stars surface
  temperature
• The luminosity class how much light it puts out.

• Why is a stars mass its most important
  property?
• A stars mass at birth determines virtually
  everything that happens to it throughout its
  life.

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What have we learned?

• What is a Hertzsprung-Russell diagram?
• An HR diagram plots stars according to their
  surface temperatures and luminosities.

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Activity 9, questions 10-15,pages 29-30
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10) Which star is larger, star S or star T?

1. Star S
2. Star T
3. They are the same size

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11) Which star is larger, star S or star X?

1. Star S
2. Star X
3. They are the same size
4. Cannot answer with information given

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12) Which star is larger, star X or star Y?

1. Star X
2. Star Y
3. They are the same size
4. Cannot answer with information given

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14) Star Z has the same luminosity as star W but
is smaller than star W. It could be located

1. Between S and X
2. Between T and Y
3. Between S and T
4. Between X and Y
5. 1 or 2 above
6. 3 or 4 above

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15) Star S has a temperature twice as high as
star W does. Using L4pR2T4, which star is
larger star S or star W?

1. Star S
2. Star W
3. They are the same size