ASTR100 (Spring 2008)
Introduction to Astronomy
Classifying Stars
Prof. D.C. Richardson
Sections 0101-0106
What is a Hertzsprung-Russell
Diagram?
Luminosity
An H-R
diagram plots
the
luminosities
and
temperatures
of stars.
Temperature
Most stars fall
somewhere on
the main
sequence of
the H-R
diagram.
Large Radius
Stars with
lower T and
higher L must
have larger
radius R:
giants and
supergiants.
L = 4R2  T4
Stars with
higher T and
lower L must
have smaller
radius R:
white
dwarfs.
L = 4R2  T4
Small Radius
Giants and
Supergiants
White Dwarfs
Add luminosity class to spectral class:
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
H-R diagram
depicts:
Temperature
Color
Spectral Type
Luminosity
Luminosity
Radius
Temperature
C
Luminosity
B
D
A
Temperature
Which star
is the
hottest?
C
Luminosity
B
Which star
is the
hottest?
A
D
A
Temperature
C
Luminosity
B
D
A
Temperature
Which star
is the
most
luminous?
C
Luminosity
B
D
A
Temperature
Which star
is the
most
luminous?
C
C
Luminosity
B
D
A
Temperature
Which star
is a mainsequence
star?
C
Luminosity
B
D
A
Temperature
Which star
is a mainsequence
star?
D
C
Luminosity
B
D
A
Temperature
Which star
has the
largest
radius?
C
Luminosity
B
D
A
Temperature
Which star
has the
largest
radius?
C
What is the significance of the
main sequence?
Main-sequence
stars are fusing
hydrogen into
helium in their
cores, like the
Sun.
Luminous mainsequence stars
are hot (blue).
Less luminous
ones are cooler
(yellow or red).
High Mass
Low Mass
Mass
measurements of
main-sequence
stars show that
the hot, blue
stars are much
more massive
than the cool, red
ones.
High Mass
Low Mass
The mass of a
normal,
hydrogen-burning
star determines
its luminosity and
spectral type!
The core pressure
and temperature
of a higher-mass
star need to be
higher in order to
balance gravity.
A higher core
temperature
boosts the fusion
rate, leading to
higher luminosity.
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 million years.
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 million years.
Life expectancy of 0.1 MSun star:
0.1 times as much fuel, uses it 0.01 times as fast.
 100 billion years.
Main-sequence Star Summary
 High mass:




High luminosity
Short-lived
Large radius
Blue
 Low mass:




Low luminosity
Long-lived
Small radius
Red
Concept Check
 Two stars have the same surface
temperature but different luminosities.
How can that be?
 Answer: one is bigger than the other!
 Why?
 Thermal radiation law: objects at a given
temperature emit a certain luminosity per
unit surface area.
 Hence the more luminous star has a larger
surface area, and so a larger radius.
What are giants, supergiants, and
white dwarfs?
Off the Main Sequence
 Stellar properties depend on both mass
and age: stars that have finished fusing
H to He in their cores are no longer on
the main sequence.
 All stars become larger and redder after
using up their core hydrogen: giants
and supergiants.
 Most stars end up small and white after
fusion has ceased: white dwarfs.
Main-sequence stars (to scale)
Giants, supergiants, white dwarfs
A
Which star
is most like
our Sun?
Luminosity
D
B
C
Temperature
A
Which star
is most like
our Sun?
Luminosity
D
B
C
Temperature
B
A
Luminosity
D
B
C
Temperature
Which star
will have
changed
the least
10 billion
years from
now?
A
Luminosity
D
B
C
Temperature
Which star
will have
changed
the least
10 billion
years from
now?
C
A
Luminosity
D
B
C
Temperature
Which star
can be no
more than
10 million
years old?
A
Luminosity
D
A
B
C
Temperature
Which star
can be no
more than
10 million
years old?
What are the two types of star
clusters?
Open cluster: A few thousand loosely packed stars.
Globular cluster: Up to a million or more stars in a
dense ball bound together by gravity.
How do we measure the age of a
star cluster?
Massive
blue stars
die first,
followed
by white,
yellow,
orange,
and
red stars.
The Pleiades
cluster now
has no stars
with life
expectancy
less than
around 100
million years.
Main-sequence
turnoff
The mainsequence
turnoff
point of a
cluster tells
us its age.
To determine
accurate ages,
we compare
models of
stellar evolution
to the cluster
data.
Detailed
modeling of
the oldest
globular
clusters
reveals that
they are
about 13
billion years
old…
Surprise Quiz!! (10 points)
 Take out a piece of paper, print your
name and section number on it.
 Sketch an H-R diagram…
1.
2.
3.
4.
5.
6.
Label the temperature & luminosity axes.
Sketch the main sequence.
Plot a point representing the Sun.
Plot a main-sequence B star.
Plot a main-sequence M star.
Indicate where giants & supergiants are
found.
7. Indicate where white dwarfs are found.
B Star
Giant and
Supergiants
Sun
(G2 V)
White Dwarfs
M Star
Here’s
what your
sketch
should
look like!