Properties of stars

1. The Properties of Stars
Stellar Spectra
& The H. R. Diagram

2. Stellar Spectra
There is a lot of information hiding in
individual stellar spectra besides color! We
can also learn much about the chemical
composition of stars by investigating the
spectral lines that show up in the spectra.

3. “spectral types”
Annie Jump Cannon
Born 1863, Dover Delaware
She developed the widely
adopted classification scheme in
1910.
Strength of Hydrogen Balmer
(to n=2) Lines along with the
appearance of other chemical line
features
This is a surface temperature
scale based on spectral lines
features.

4. The Sun’s Complete Spectra!

5. O B A F G K M
Early Types Late Types
Hot Cool
Bluish Reddish

6. Hertzsprung-Russell Diagram:
Ejmar Hertzsprung (1873-1967) – Copenhagen –
Began his career as a Chemical Engineer. While
working and independently at the same time…
Henry Norris Russell (1877-1957) – Princeton –
Student then professor.
A graph that separates the effects of temperature
and surface area on stellar luminosities.
The HR Diagram is much like the same thing as
producing a graph of people’s height vs. weight.

7. What does the luminosity of a star
depend on?
Temperature (proportional to T
4
)
Size (proportional to R
2
)
Full blown formula? L=4pR
2
sT
4

9. Spectral Types

10. White Dwarfs
Supergiants
Red Giants
Main
Sequence
A
b
s
o
l
u
t
e

M
a
g
n
i
t
u
d
e
Bright
Dim
L
u
m
i
n
o
s
i
t
y
O B A F G K
M
Early Types
Hot
Late Types
Cool

11. Main Sequence
Extends from the hot, bright, bluish stars in
the upper left to cool, dim, reddish stars in the
lower right.
Size (Radius) of the stars: R~R

(or slightly
bigger/smaller) ~90% of the stars in space.

13. Red Giants
Cool, luminous stars. They are very luminous
because of their large size.
Size R~100R
  but they are only about
0.9% of the stars by number.

14. Supergiants
Exceptionally luminous extra large sized
stars! (humongous!)
Size R~1000R
  but they are only about
0.1% of the stars by number.

16. Where is the missing 9%?
White Dwarfs:
HOT but faint stars.
The surface temperatures of these stars are
very hot, but since they are so small they are
not very luminous.
Size R~(1/100)R

~R
Å
 but they are only
about 0.9% of the stars by number.

18. Spica B1 ~20,000 K 13 M

M.S.
Bluegiant
Sun G2 ~5,800 K 1 M

Main
Sequence
Betelguese M2 ~ 3,000 K 14 M

Red
Supergiant
Sirius B A0 ~18,500 K 1 M

White
Dwarf
Stellar Information: Some Example Stars
Name
Spectral
Type
Temp (K) Mass Kind?

19. Caution!
Do not confuse the size of an object with the
mass of an object. Just because an object is
large in dimension does not necessarily mean
it is also large in mass. For example, you can
have a forty foot tall by three foot across
marshmallow that looks “large,” but that
does not mass as much as that of a “small”
football sized hunk of lead.

20. Mass Ranges of Main Sequence Stars
Most Massive Stars ~ 55-100(?) times more
massive than the Sun (very rare)
Least Massive Stars ~ 0.1-0.08 times the
mass of the Sun. (very common)
This pattern stretches from the higher mass
O,Bs to lower mass K,Ms.

21. Mass-Luminosity Relationship
The Main Sequence also exhibits a “Mass-
Luminosity Relationship.” Simply, a star’s
location on the main sequence depends on its
mass. The more luminosity a star has the
larger its mass. The lower the star’s mass the
smaller its luminosity. But this is true only on
the main sequence.

22. A
b
s
o
l
u
t
e

M
a
g
n
i
t
u
d
e
Bright
Dim
L
u
m
i
n
o
s
i
t
y
O B A F G K M

23. Mass-Luminosity Relationship
Main Sequence Stars ONLY!!
L = M
3.5

24. Examples
Suppose the mass of a star is 4 M

then
L = (4)
3.5
= (4) × (4) × (4) × (4)
0.5
= 128 times
more luminous than Sun.
L = 128 L

Suppose the luminosity of a star is 5 L

then 5
= (M)
3.5
 M=(5)
-3.5
= 1.6 times more mass
than Sun.
M = 1.6 M


25. Density ranges of Stars
Sun ~ 1.0 grams/cm
3
(The same density of
water!)
Giants ~ 0.01 grams/cm
3
 0.1 grams/cm
3

Supergiants ~ 0.000001 grams/cm
3
(That is
less than the density of air!)
White Dwarfs ~ 10,000,000 grams/cm
3
(1
sugar-cubed size of white dwarf material
would weigh nearly 20 tons!)