Which kind of star is hottest?
A.
B.
C.
D.
M star
F star
A star
K star
Which kind of star is hottest?
A.
B.
C.
D.
M star
F star
A star
K star
The luminosity of a star:
The luminosity of a star:
A. Is another name for its apparent brightness.
B. Can be calculated from the apparent
brightness and distance to the star.
C. Depends on star’s surface temperature only.
D. Depends on star’s size only.
A. Is another name for its apparent brightness.
B. Can be calculated from the apparent
brightness and distance to the star.
C. Depends on star’s surface temperature only.
D. Depends on star’s size only.
If the Sun were twice as far away
from us (at 2AU), then how would
its luminosity change?
If the Sun were twice as far away
from us (at 2AU), then how would
its luminosity change?
A.
B.
C.
D.
E.
It would decrease 4 times.
It would decrease 2 times
It would increase 4 times.
It would increase 2 times.
It would not change.
A.
B.
C.
D.
E.
It would decrease 4 times.
It would decrease 2 times
It would increase 4 times.
It would increase 2 times.
It would not change.
1
If the Sun were twice larger in
diameter, then how would its
luminosity change?
A.
B.
C.
D.
E.
It would decrease 4 times.
It would decrease 2 times
It would increase 4 times.
It would increase 2 times.
It would not change.
If the Sun were twice larger in
diameter, then how would its
luminosity change?
A.
B.
C.
D.
E.
It would decrease 4 times.
It would decrease 2 times
It would increase 4 times.
It would increase 2 times.
It would not change.
Basic Properties of Stars
How do we measure star’s
mass?
We measure star’s mass using the
Law of Gravity.
Direct mass measurements are
possible only for stars in binary star
systems.
p2 =
Isaac Newton
4π2
G (M1 + M2)
Types of Binary Star Systems
• Visual Binary
• Eclipsing Binary
• Spectroscopic Binary
a3
About half of all stars are in binary systems!
p = period
a = average distance
2
Eclipsing Binary
Visual Binary
Observations of Sirius A & B
We can directly observe the orbital motions of these stars.
We can measure periodic eclipses.
Spectroscopic Binary
We need 2 out of 3 observables
to measure mass:
1) Orbital Period (p)
2) Orbital Separation (a or r=radius)
3) Orbital Velocity (v)
For circular orbits, v = 2πr / p
Therefore: r = v p / (2π)
v
r
M
We determine the orbit by measuring Doppler shifts.
Most massive stars:
100 MSun
Least massive stars:
0.08 MSun
(MSun is the mass
of the Sun)
Classifying Stars
• How do we classify stars?
• Why is a star’s mass its most important
property?
• What is a Hertzsprung–Russell diagram?
3
Most of the brightest
stars are reddish in
color.
How do we classify stars?
Color and luminosity
are closely related
among the remaining
“normal” stars.
Main-sequence stars
(or “normal” stars)
are fusing
hydrogen into
helium in their
cores like the Sun.
Why are some red stars so
much more luminous?
They’re bigger!
Biggest red stars:
Luminous mainsequence stars are
hot (blue).
1,000 Rsun
Smallest red stars:
Less luminous
ones are cooler
(yellow or red).
A star’s full classification includes spectral type (line
identities) and luminosity class (related to the size
of the star):
I
II
III
IV
V
Examples:
- supergiant
- bright giant
- giant
- subgiant
- main sequence
Sun - G2 V
Sirius - A1 V
Proxima Centauri - M5.5 V
Betelgeuse - M2 I
0.1 RSun
Stellar Properties Review
Luminosity: from brightness and distance
10-4 LSun - 106 LSun
Temperature: from color and spectral type
3,000 K - 50,000 K
Mass: from period (p) and average separation (a)
of binary-star orbit
0.08 MSun - 100 MSun
4
Stellar Properties Review
Luminosity: from brightness and distance
(0.08 MSun) 10-4 LSun - 106 LSun (100 MSun)
Temperature: from color and spectral type
(0.08 MSun) 3,000 K - 50,000 K (100 MSun)
Why is star’s mass its most
important property?
Mass: from period (p) and average separation (a)
of binary-star orbit
0.08 MSun - 100 MSun
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.
Each star’s properties depend mostly on its mass and age.
Mass & Lifetime
Sun’s life expectancy: 10 billion years
Mass & Lifetime
Sun’s life expectancy: 10 billion years
Until core hydrogen
(10% of total) is
used up
5
Mass & Lifetime
Sun’s life expectancy: 10 billion years
Until core hydrogen
(10% of total) is
used up
Life expectancy of 10 MSun star:
Mass & Lifetime
Sun’s life expectancy: 10 billion years
Until core hydrogen
(10% of total) is
used up
Life expectancy of 10 MSun star:
10 times as much fuel, uses it 104 times as fast
10 times as much fuel, uses it 104 times as fast
10 million years ~ 10 billion years x 10 / 104
10 million years ~ 10 billion years x 10 / 104
Life expectancy of 0.1 MSun star:
0.1 times as much fuel, uses it 0.01 times as fast
100 billion years ~ 10 billion years x 0.1 / 0.01
Main-Sequence Star Summary
High Mass:
High Luminosity
Short-Lived
Large Radius
Blue
What is a Hertzsprung-Russell
Diagram?
Low Mass:
Luminosity
Low Luminosity
Long-Lived
Small Radius
Red
An H-R diagram
plots the luminosity
and temperature of
stars.
Normal hydrogenburning stars
reside on the
main sequence of
the H-R diagram.
Temperature
6
Large radius
An H-R diagram
depicts:
Temperature
Luminosity
Stars with low
temperature and
high luminosity
must have large
radius.
Color
Spectral Type
Luminosity
Radius
*Mass
Small radius
*Lifespan
*Age
Temperature
C
D
Which star is
the hottest?
A
C
B
Luminosity
Luminosity
B
D
Which star is
the hottest?
A
A
Temperature
C
D
Which star is
the most
luminous?
A
C
B
Luminosity
B
Luminosity
Temperature
D
Which star is
the most
luminous?
A
C
Temperature
Temperature
7
C
C
B
D
Which star is a
main-sequence
star?
A
Luminosity
Luminosity
B
D
Which star is a
main-sequence
star?
A
D
Temperature
Temperature
C
C
B
D
Which star has
the largest
radius?
A
Luminosity
Luminosity
B
D
Which star has
the largest
radius?
A
C
Temperature
Temperature
A
A
D
B
C
Which star is
most like our
Sun?
Luminosity
Luminosity
D
B
C
Which star is
most like our
Sun?
B
Temperature
Temperature
8
A
A
B
C
D
Which of these
stars will have
changed the
least 10 billion
years from now?
Luminosity
Luminosity
D
B
C
Which of these
stars will have
changed the
least 10 billion
years from now?
C
Temperature
Temperature
A
A
B
C
D
Which of these
stars can be no
more than 10
million years
old?
Luminosity
Luminosity
D
B
C
Which of these
stars can be no
more than 10
million years
old?
A
Temperature
Temperature
9