8. Classification of stars
•Stars also have each properties
•How do we classify stars?
•What are important properties of stars?
9. Classification of stars
•Stars also have each properties
•How do we classify stars?
•What are important properties of stars?
10. Classification of stars
•Stars also have each properties
•How do we classify stars?
•What are important properties of stars?
We usually focus on temperature and luminosity（光度）
of stars
16. HR diagram
HR diagram of well-known stars
•Instead of temperature, we can also use
spectrum classification.
Or
T = 4600 K
(
1
0.92(B − V) + 1.7
+
1
0.92(B − V) + 0.62 )
.
•We can also use color index instead of
temperature
17. HR diagram
•If we plot more and more stars, we can find a relationship between luminosity and
temperature.
Higher temperature
Larger luminosity
18. HR diagram
•If we plot more and more stars, we can find a relationship between luminosity and
temperature.
Higher temperature
Larger luminosity
Red Giant
（红巨星）
Main sequence
（主序）
White dwarf
（⽩矮星）
19. HR diagram
•If we plot more and more stars, we can find a relationship between luminosity and
temperature.
Higher temperature
Larger luminosity
20. HR diagram
•If we plot more and more stars, we can find a relationship between luminosity and
temperature.
Higher temperature
Larger luminosity
21. HR diagram
•If we plot more and more stars, we can find a relationship between luminosity and
temperature.
Higher temperature
Larger luminosity
22. HR diagram
•If we plot more and more stars, we can find a relationship between luminosity and
temperature.
Higher temperature
Larger luminosity
24. HR diagram
•What can we know from HR diagram??
•The temperature of main sequence star is
ranged from 3’000K to 30’000K.
L = 4πσR2
T4
25. HR diagram
•What can we know from HR diagram??
•The temperature of main sequence star is
ranged from 3’000K to 30’000K.
•The main sequence stars with higher
temperature(Bluer stars) tend to be larger
luminosity.
Luminosity ∝ (Radius)2
× (Temperature)4
(Remember )
F ∝ σT4
L = 4πσR2
T4
26. HR diagram
•What can we know from HR diagram??
•The temperature of main sequence star is
ranged from 3’000K to 30’000K.
•The main sequence stars with higher
temperature(Bluer stars) tend to be larger
luminosity.
Luminosity ∝ (Radius)2
× (Temperature)4
(Remember )
F ∝ σT4
L = 4πσR2
T4
27. HR diagram
•What can we know from HR diagram??
•The temperature of main sequence star is
ranged from 3’000K to 30’000K.
•The main sequence stars with higher
temperature(Bluer stars) tend to be larger
luminosity.
Luminosity ∝ (Radius)2
× (Temperature)4
(Remember )
F ∝ σT4
•Bluer luminous stars are called “Blue giant”
L = 4πσR2
T4
28. HR diagram
•What can we know from HR diagram??
•The temperature of main sequence star is
ranged from 3’000K to 30’000K.
•The main sequence stars with higher
temperature(Bluer stars) tend to be larger
luminosity.
Luminosity ∝ (Radius)2
× (Temperature)4
(Remember )
F ∝ σT4
•Bluer luminous stars are called “Blue giant”
•Redder faint stars are called “Red dwarf”
L = 4πσR2
T4
30. HR diagram
•What can we know from HR diagram??
•The temperature of red giant stars are low
compared with main sequence stars which have
same luminosity.
31. HR diagram
•What can we know from HR diagram??
•The temperature of red giant stars are low
compared with main sequence stars which have
same luminosity.
32. HR diagram
•What can we know from HR diagram??
•The temperature of red giant stars are low
compared with main sequence stars which have
same luminosity.
•The luminosity of white dwarf stars is
smaller than main sequence stars, but
they have high temperature.
33. HR diagram
•What can we know from HR diagram??
•The temperature of red giant stars are low
compared with main sequence stars which have
same luminosity.
•The luminosity of white dwarf stars is
smaller than main sequence stars, but
they have high temperature.
34. HR diagram
•What can we know from HR diagram??
•The temperature of red giant stars are low
compared with main sequence stars which have
same luminosity.
•The luminosity of white dwarf stars is
smaller than main sequence stars, but
they have high temperature.
•As you can see, HR diagram tells us the
properties of stars. We can also learn
stellar interior and evolution from HR
diagram.
35. Extending the cosmic distance scale
m − M = 5 log10 (
d[pc]
10pc )
•You remember how to measure distance to
stars by using following relation
•We can also evaluate the distance to stars if we
know the luminosity of stars ( ) and apparent
brightness .
L
b
b =
L
4πR2
•If we use the spectral type (or temperature) of
the star, we can estimate the luminosity from
HR diagram..
L
36. Luminosity class
•Astronomers developed a system for
classifying stars called “Luminosity class”.
Class Description
Ia Bright supergiants
Ib Supergiants
II Bright giants
III Giants
IV Subgiants
V
Main sequence stars
and dwarfs
Luminous
37. Stellar masses
•HR diagram apparently provides relation between
temperature and luminosity of stars.
•But, remember that the mass is also important
property of stars. Where does HR diagram
include the information of mass?
•Astronomers checked the mass of stars on the main
sequence and found that lower temperature stars
have smaller mass.
Low mass stars are cool and faint.
Massive stars are hot and bright.
But, why?? > it is related how stars form
38. Mass determination
How can we determine the mass of stars?
•We know roughly 50% of stars are binary（双星）
•Two stars are in orbital motion around the center of
mass
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T2
/ R3
Do you remember Kepler’s third law?
•I skip the detail, but we can determine the mass of stars when we apply Kepler’s third law
to binary.
39. Initial mass function
•Initial mass function(IMF) describes initial distribution of masses for stars during star formation.
•The number of massive stars is small and the number of less massive stars is large.
多
少
41. Mass and other stellar properties
•The mass-radius relation on main sequence stars.
Massive stars are larger
•The mass-luminosity relation on main sequence stars.
Massive stars are brighter
42. Mass and other stellar properties
•We would like to evaluate lifetime of stars. How long can stars burn??
•Because stars burn by nuclear fusion process, we can estimate a main
sequence star’s lifetime by dividing the amount of fuel available (the
mass of star) by the rate at which the fuel is being consumed (the star’s
luminosity)
stellar lifetime ∝
stellar mass
stellar luminosity
.
∝
1
(stellar mass) 3
.
stellar luminosity
∝ (stellar mass)4
Larger mass or brighter stars die earlier!
43. Mass and other stellar properties
Let’s calculate the lifetime of Proxima Centauri!
•The lifetime of the sun is roughly year (100亿年)
1010
•The mass of Proxima Centauri is roughly 0.12 (solar mass)
M⊙
stellar lifetime ∝
stellar mass
stellar luminosity
.
∝
1
(stellar mass) 3
.
What is the lifetime of Proxima Centauri?
44. Mass and other stellar properties
Let’s calculate the lifetime of Proxima Centauri!
•The lifetime of the sun is roughly year (100亿年)
1010
•The mass of Proxima Centauri is roughly 0.12 (solar mass)
M⊙
stellar lifetime ∝
stellar mass
stellar luminosity
.
∝
1
(stellar mass) 3
.
What is the lifetime of Proxima Centauri?
(Answer)
45. Summary
• HR diagram provides us the relation between luminosity
and temperature of stars.
• HR diagram tells us properties of stars.
• Let’s understand how to read HR diagram.
![Classification（分类）
Weight[kg]
Height[m]
•Weight: 80kg
•Height: 1.6m
•Fat…](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)