The Four Basic Parameters of Stars
Luminosity
Size
Mass
Surface
Temperature
Measurements of Star Properties
Apparent brightness Direct measurent
Parallax
Distance
Distance + apparent brightness
Luminosity
( L=4D2 f)
Spectral type (or color)
Temperature
Luminosity + temperature
Radius
(L=4R2 T4)
Luminosity and temperature are the two
independent intrinsic parameters of stars.
Mass: how do you weigh a star?

Mass is the single most important property in how a
star’s life and death will proceed.

We can “weigh” stars that are in binary systems (two
stars orbiting each other). Fortunately, most stars
fall into this category.

Most stars in binary systems have a mass that is very
similar to its companion … we’ll see why this is
soon!
Binary Stars
Center of mass
(or baricenter)
Star A
ra
rb
Star B
Ma/Mb = rb/ra
-Each star in a binary system
moves in its own orbit around
the system's center of mass.
-Their orbital period depends on
their separation and their masses.
2
4π
3
2
a
p = G(M +M )
1
2
Big p = Small Masses
Small p = Big Masses
I. Visual Binaries
1
4
2
5
1.
3
2.
The total spread (size) of the
Doppler shift gives velocities
about center of mass (gives orbit
sizes, rA+rB )
The time to complete one repeating
pattern gives period, P
Spectroscopic binaries: Doppler Shift tells if it is moving toward or away
Eclipsing Binaries:
Best binaries to measure mass
In Review

There are four principal characteristics of a
star:
Luminosity
 Surface Temperature
 Size
 Mass

How may we classify stars?
We can take a census of stars and see what’s
out there.
But first, let’s do some sociology in the classroom.
Discussion Question
Make a plot that shows the general
relationship between height and weight for
humans.
- now add to your plot the population of
basketball players who are very tall and
very thin.
- now add the population of obese
children
Classification of Stars
1) Collect information on
a large sample of stars.
2) Measure their
luminosities
(need the distance!)
3) Measure their surface
temperatures
(need their spectra)
The Hertzsprung-Russell Diagram
The HR diagram separates
The effects of temperature
And surface area on stars’
Luminosity and sorts the
Stars according to their size
The Hertzsprung-Russell Diagram
The HR diagram separates
The effects of temperature
And surface area on stars’
Luminosity and sorts the
Stars according to their size
The Hertzsprung-Russell Diagram
The Main Sequence
- all main sequence
stars fuse H into He
in their cores
- this is the defining
characteristic of a
main sequence star.
The Hertzsprung-Russell Diagram
Red Giants
- Red Giant stars
are very large, cool
and quite bright.
Ex. Betelgeuse is
100,000 times more
luminous than the Sun
but is only 3,500K on
the surface. It’s radius
is 1,000 times that of the
Sun.
The Hertzsprung-Russell Diagram
The Hertzsprung-Russell Diagram
White Dwarfs
- White Dwarfs
are hot but since
they are so small,
they are not very
luminous.
The Hertzsprung-Russell Diagram
The HR diagram separates
The effects of temperature
Mass of
Star
And surface area on stars’
Luminosity and sorts the
Stars according to their
size
Size of Star
Mass-Luminosity relation





Most stars appear on the Main Sequence, where stars
appear to obey a Mass-Luminosity relation:
L  M3.5
For example, if the mass of a star is doubled, its
luminosity increases by a factor 23.5 ~ 11.
Thus, stars like Sirius that are about twice as massive as
the Sun are about 11 times as luminous.
The more massive a Main Sequence star is, the hotter
(bluer), and more luminous.
The Main Sequence is a mass sequence!
Review Questions
1.
2.
3.
What is the Hertzsprung-Russell Diagram?
Why are most stars seen along the so-called
main sequence?
What makes more massive stars hotter and
brighter?