11.2 Classifying Stars

Our Goals for Learning

• How do we classify stars?

• Why is a star’s mass its most important property?

• What is a Hertzsprung–Russell diagram?

What is the stellar Main Sequence?

Most of the brightest stars are reddish in color

Color and luminosity are closely related among the remaining “normal” stars

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)

Why are some red stars so much more luminous?They’re bigger!

L = LuminosityT = Temperature

L=Area × T4

L=4πr2 × T4

(page 312)

Biggest red stars: 1000 Rsun

Smallest red stars: 0.1 RSun

(Ranking Task)

Which star is the least luminous?

A B C D E F

94%

3%0%0%2%0%

1. A

2. B

3. C

4. D

5. E

6. F

Which star is the most luminous?

A B C D E F

0% 0%

31%

1%

67%

0%

1. A

2. B

3. C

4. D

5. E

6. F

Temperatures and sizes of stars

1) Hotter objects emit more light per area at all wavelengths

2) Hotter objects tend to emit light at shorter wavelengths and higher frequencies

Laws of Thermal Radiation

Why are some red stars so much more luminous?They’re bigger!

L = LuminosityT = Temperature

L=Area × T4

L=4πr2 × T4

(page 312)

Biggest red stars: 1000 Rsun

Smallest red stars: 0.1 RSun

(Ranking Task)

Activity 10, Part II (pages 32-34, questions 6-15)

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

E F

Both

giv

e of

f the

s...

63%

2%

35%

1. E

2. F

3. Both give off the same amount of red light

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

E F

Both

giv

e of

f the

sa..

89%

0%10%

1. E

2. F

3. Both give off the same amount of blue light

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

E lo

oks b

lue

and

F loo..

E lo

oks re

d and

F loo..

Both

look

red

Both

look

blue

86%

1%2%11%

1. E looks blue and F looks red

2. E looks red and F looks blue

3. Both look red

4. Both look blue

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

Sta

r C m

ust b

e co

oler..

.

Sta

r C m

ust b

e sm

alle

...

Both

1 a

nd 2

Eith

er 1

or 2

35%

26%

18%21%

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

1) Hotter objects emit more light per area at all wavelengths

2) Hotter objects tend to emit light at shorter wavelengths and higher frequencies

Laws of Thermal Radiation

14) In Figure 2c, which star has the higher surface temperature?

Sta

r D

Sta

r E

Both

hav

e th

e sa

me.

..

17%7%

75%1. Star D

2. Star E

3. Both have the same temperature

1) Hotter objects emit more light per area at all wavelengths

2) Hotter objects tend to emit light at shorter wavelengths and higher frequencies

Laws of Thermal Radiation

15) In Figure 2c, which star is larger?

Sta

r D

Sta

r E

Both

are

the

sam

e si

ze

28%

69%

3%

1. Star D

2. Star E

3. Both are the same size

How do we classify stars?

Most of the brightest stars are reddish in color

Color and luminosity are closely related among the remaining “normal” stars

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)

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

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

(0.08 MSun) (150 MSun)

(150 MSun)(0.08 MSun)

Each star’s properties depend mostly on mass and age

Why is a star’s mass its most important property?

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

A star’s mass largely determines how much light it gives off (its luminosity) and for how long it

will shine (its lifetime).

What can we learn about stars from a Hertzsprung-Russell Diagram?

Temperature

Lu

min

osi

tyAn H-R diagram plots the luminosity versus temperature of stars

Normal hydrogen-fusing stars reside on the main sequence of the H-R diagram

Stars with low temperature and high luminosity must have large radius

Large radius

Small radius

Temperature

Lu

min

osi

tyH-R diagram depicts: Temperature,

Colour,

Luminosit

y,

Radius,

Spectral Type

*Mass

*Lifespan

*Age

Temperature

Lu

min

osi

ty

Which star is the hottest?

A

BC

D

Which star is the hottest?

0%

0%

0%

0%

:10

1. A

2. B

3. C

4. D

Temperature

Lu

min

osi

tyWhich star is the hottest?

A

BC

A

D

A

BC

D

Temperature

Lu

min

osi

ty

Which star is the most luminous?

A

BC

D

Which star is the most luminous?

0%

0%

0%

0%

:10

1. A

2. B

3. C

4. D

Temperature

Lu

min

osi

tyWhich star is the most luminous?

C

A

BC

D

Temperature

Lu

min

osi

tyWhich star is a main-sequence star?

A

BC

D

Which star is a main sequence star?

0%

0%

0%

0%

:10

1. A

2. B

3. C

4. D

Temperature

Lu

min

osi

tyWhich star is a main-sequence star?

D

A

BC

D

Temperature

Lu

min

osi

tyWhich star has the largest radius?

A

BC

D

Which star has the largest radius?

0%

0%

0%

0%

:10

1. A

2. B

3. C

4. D

Temperature

Lu

min

osi

tyWhich star has the largest radius?

C

A

BC

D

Temperature

Lu

min

osi

ty

Which star is most like our Sun?

A

B

C

D

Which star is most like our Sun?

0%

0%

0%

0%

:10

1. A

2. B

3. C

4. D

Temperature

Lu

min

osi

tyWhich star is most like our Sun?

B

A

B

C

D

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 star’s surface temperature

• The luminosity class how much light it puts out.

• Why is a star’s mass its most important property?

• A star’s mass at birth determines virtually everything that happens to it throughout its life.

What have we learned?• What is a

Hertzsprung-Russell diagram?

• An H–R diagram plots stars according to their surface temperatures and luminosities.

Activity #9, questions 10-15,pages 29-30

10) Which star is larger, star S or star T?

Sta

r S

Sta

r T

They

are

the

sam

e si

ze

0% 0%0%

:10

1. Star S

2. Star T

3. They are the same size

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

Sta

r S

Sta

r X

They

are

the

sam

e si

ze

Can

not a

nswer

with

...

0% 0%0%0%

:10

1. Star S

2. Star X

3. They are the same size

4. Cannot answer with information given

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

Sta

r X

Sta

r Y

They

are

the

sam

e si

ze

Can

not a

nswer

with

...

0% 0%0%0%

:10

1. Star X

2. Star Y

3. They are the same size

4. Cannot answer with information given

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

It could be located…

Bet

wee

n S a

nd X

Bet

wee

n T a

nd Y

Bet

wee

n S a

nd T

Bet

wee

n X a

nd Y

1 o

r 2 a

bove

3 o

r 4 a

bove

0% 0% 0%0%0%0%

:10

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

15) Star S has a temperature twice as high as star W does. Using L=4πR2T4, which star is larger: star S or star W?

Sta

r S

Sta

r W

They

are

the

sam

e si

ze

0% 0%0%

:10

1. Star S

2. Star W

3. They are the same size