The Life Cycles of Stars
and our Sun
Gravity
a= GM/R2
a= GM’/(1/2 R)2 ~ R
1/2 R
R
a=0
Astronomers (and what it takes…)
1.
2.
3.
4.
Passion!
Determination
Do only what you like to do
Flexibility
The Many Opportunities for
Astronomers
1. Academic life
2. National or International Research
Centers/Observatories
3. Industry
4. Education (e.g. Planetariums, Museums, etc.)
Space Telescope Science Institute
NICMOS (Near Infrared Camera and
Multi-Object Spectrometer)
One of the Instruments
on the Hubble Space
Telescope
The many facets of Astronomers
1. Observational (telescopes, ground and space;
data)
2. Theoretical (numerical, analytical)
3. Instrumentalist
4. Laboratory Scientist
The Typical Day of an Astronomer
…but there are also special days
Meeting of the American
Astronomical Society
June 2002
Some of the important ingredients
1. Collaborations (with peers)
2. Interactions (with younger/future peers)
3. Living above your Universe Island
Astronomers and Family
1. Perfectly compatible carriers
2. Most of the astronomers I know have families to
which they contribute their half share
3. Many places are supportive of `dual career’ needs
Stars
“The stars are distant and
unobtrusive, but bright and
enduring as our fairest and most
memorable experiences.”
Henry David Thoreau (1849)
Are Stars similar to our Sun?
How far away are they?
Where did they come from?
What do they do?
Do they live forever?
Panorama view of the sky
The Four Basic Parameters of Stars
»Luminosity
»Size
»Mass
»Surface Temperature
Brightness, Distance, and
Luminosity
L=4D2 l
luminosity
distance
l =L/(4 D2 )
apparent brightness
or flux
There is a Big Range of
Stellar Luminosities Out there!
Star
Sun
Proxima Centauri
Rigel (Orion)
Deneb (Cygnus)
Luminosity (in
units of solar)
1
0.0006
70,000
170,000
How to measure the surface
temperature of a star?
1. Overall spectral shape (the peak of the
blackbody continuous spectrum)
2. More accurately, spectroscopically
Spectral Types
For historical reasons,
astronomers classify the
temperatures of stars on a
scale defined by spectral
types, called O B A F G
K M, ranging from the
hottest (type O) to the
coolest (type M) stars.
The sun has a spectral type: G2
Stellar Size
• Stars are very spherical so we
characterize a star’s size by its radius.
R
Stellar Radii vary in size
from ~1500xRSun for a
large Red Giant to
0.008xRSun for a White
Dwarf.
Temperature, Luminosity, and
Size – pulling them all together
A star’s luminosity, surface temperature, and size
are all related by the Stefan-Boltzmann Law:
Stefan-Boltzmann Law
L=4πR2 σT4
Luminosity
Stellar
radius
Surface
temperature
Measurements of Star Properties
Apparent brightness Direct measurent
Parallax
Distance
Distance + apparent brightness
Luminosity
( L=4D2 l)
Spectral type (or color)
Temperature
Luminosity + temperature
Radius
(L=4R2 T4)
Luminosity and temperature are the two
independent intrinsic parameters of stars.
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 …
I. Visual Binaries
Eclipsing Binaries
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.
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 Hertzsprung-Russell Diagram
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
Mass of
Star
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