Lecture 10: The
Hertzsprung-Russell Diagram
Reading: Sections 19.7-19.8
Key Ideas
The Hertzsprung-Russell (H-R) Diagram
Plot
of Luminosity vs. Temperature for stars
Features:
Main
Sequence
Giant
& Supergiant Branches
White
Dwarfs
Luminosity classes
Review of what we know about Stellar Properties
Large range of Stellar
Luminosities
10-4 to
106 Lsun
Large range of Stellar Radii
10-2 to
103 Rsun
Modest range of Stellar
Temperature
3000 to > 50,000
K
Moderate range of Stellar
Masses
0.1 to 50 Msun
Reminder: Luminosity-Radius-Temperature Relation
Box 19-4 in the book.
In words: if two stars have
the same temperature, the larger one will be more luminous.
Hertzsprung-Russell Diagram
Plot of Luminosity versus Temperature
Temperature (T) from
spectral type
Luminosity (L) from
apparent brightness and distance
Diagram was drawn independently in 1912 by:
Eijnar Hertzsprung
for star clusters (all stars at same distance)
Henry
Norris Russell for nearby stars (stars close enough to have good parallax
measurements)
It could have turned out that
stars could have had any combination of luminosity and temperature. Then if we
plotted them up, theyÕd look something like this.
However, when we actually make the plot, we find that
stars fall only in
certain areas of the temperature-luminosity plot. See
Figure 19-14 as well.
The Major Regions of the
Hertzsprung-Russell Diagram
Main Sequence
Most nearby stars (85%) lie along a diagonal band
called the Main Sequence.
Range of properties
L=10-2 to 106 Lsun
T=3000 to > 50,000 K
R=0.1
to 10 Rsun
The Sun is a Main Sequence Star.
Giants & Supergiants
Stars brighter than Main Sequence stars of the same
Temperature.
Means they must be
larger in radius
Giants:
R=10-100 Rsun
L=103-105
Lsun
Supergiants
R>103 Rsun
L=105-106
Lsun
White Dwarfs
Stars on the lower left of
the H-R Diagram fainter than Main Sequence stars of the same Temperature
Means they must be
smaller in radius
L-R-T relation
predicts R~0.01 Rsun = about the size of Earth
These are very
unusual objects
Luminosity Classes
Absorption lines are
Pressure-sensitive
Lines broader as the
pressure increases
Larger stars
puffier, hence lower pressure
Implications
Larger Stars have
Narrower Lines
Larger Stars are
brighter for the same Temperature
Way to assign a luminosity to
stars based on their spectra.
Consistency Checkˆ both the L-T-R relation and the spectral classes say
that giant and supergiant stars are big. Whew! See Figure 19-16.
Spectral Classes from some
well-known stars
Sun G2V
Betelgeuse M2Ib
Rigel B8Ia
Sirius A1V
Aldebaran K5III
H-R Diagram for the Brightest
Stars
Shows that these
stars tend to be intrinsically luminous
H-R Diagram for the Nearest
Stars
Shows
that these stars tend to be intrinsically un-luminous. Lots of G, K and M
dwarfs.
Note: if we relied on the
brightest stars to tell us about the Universe, we would underestimate the
number of low-luminosity dwarfs (both white and red) by a lot!
Mass and the Main Sequence
We know the masses for a few
of the stars on the H-R diagram. When we plot the masses of the stars, we see
that the main sequence is actually a mass sequence.
More massive stars on the
main sequence are hotter, low mass stars are cooler.
Why? This is one of the
things our model of how stars work need to explain.
See also Figure 19-21 in your book
There is a mass-luminosity
relation on the main sequence. We can use that + a sample of stars where we get
all the stars within a certain distance of the Sun to figure out how many stars
of what masses are out there.
Answer: Lots of low-mass
stars! Very few high-mass stars. This is something our theory of star formation
will need to explain.
Questions:
1. Why donÕt stars have just any luminosity and
temperature?
2. Why is there a distinct Main Sequence?
3. What makes on main-sequence star different from
another?
4. Are giants, supergiants,
and white dwarfs born that way, or is something else going on?
Patterns on the H-R Diagram
are telling us about the internal physics of stars.