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?
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