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READING:
Units: 59, 60
The Family of Stars
Stars come in all sizes…
The Mass-Luminosity Relation
• If we look for trends in
stellar masses, we notice
something interesting
– Low mass main
sequence stars tend to be
cooler and dimmer
– High mass main
sequence stars tend to be
hotter and brighter
• The Mass-Luminosity
Relation:
L  M 3.5
Massive stars burn brighter!
Massive stars burn brighter
L~M3.5
Luminosity Classes
Stellar Evolution –
Models and Observation
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Stars change very little over a human lifespan, so it is impossible to
follow a single star from birth to death.
We observe stars at various stages of evolution, and can piece together
a description of the evolution of stars in general
Computer models provide a “fast-forward” look at the evolution of
stars.
Stars begin as clouds of gas and dust, which collapse to form a stellar
disk. This disk eventually becomes a star.
The star eventually runs out of nuclear fuel and dies. The manner of
its death depends on its mass.
Evolution of low-mass stars
Evolution of high-mass stars
Tracking changes with the
HR Diagram
• As a star evolves, its
temperature and luminosity
change.
• We can follow a stars
evolution on the HR
diagram.
• Lower mass stars move on
to the main sequence, stay
for a while, and eventually
move through giant stages
before becoming white
dwarfs
• Higher mass stars move
rapidly off the main
sequence and into the giant
stages, eventually exploding
in a supernova
Our Sun will eventually
A. Become white dwarf
B. Explode as a supernova
C. Become a protostar
D. Become a black hole
The spectral type of a star is most directly related to
its
a. Absolute magnitude
b. Surface temperature
c. Size or radius
d. Luminocity
Which two vital parameters are used to describe the
systematics of a group of stars in the HR diagram?
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a. Mass and weight
b. Luminocity and radius
c. Surface temperature and mass
d. Luminocity and surface temperature
Which is one of Kepler’s laws:
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a. For every action has an equal and opposite reaction
b. Planets move in elliptical orbits
c. F=ma
d. Planets move in perfect circles around the sun
A solar exlipse can occur ONLY when
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a. the Moon comes between the Earth and the Sun
b. the Sun comes between the Moon and the Earth
c. the Earth comes between the Moon and the Sun
d. the Sun, Moon and Earth form a precise rightangled triangle
When dropped by an astronaut on the Moon, two
objects of different mass will
• a. Have different accelerations proportional to their
masses
• b. Have different accelerations, the more massive
object having the smaller acceleration
• c. Have the same acceleration
• d. Have no acceleration at all in the airless space
According to Newton's laws, a force must be acting
whenever
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a. an object's position changes
b. the direction of an object's motion changes
c. time passes
d. an object moves with non-zero speed
Kepler's first law states that a planet moves around
the Sun
• a. in a circle with the Sun at the center
• b. in an elliptical orbit, with the Sun at the center of
the ellipse
• c. in an elliptical orbit, with the Earth at the center of
the ellipse
• d. in an elliptical orbit, with the Sun at one focus
If an object has an orbit around the Sun that has an
essentricity of 0.1, then the orbit is
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a. a straight line
b. exactly circular
c. almost circular, but not quite
d. a long, thin ellipse
What causes sunspots?
• a. differential rotation on the Sun creates vortices, or
eddies, which are cooler and darker than the rest of
the solar surface
• b. solar flares cause the photoshere to expand and cool
in the vicinity of the flare
• c. magnetic fields breaking through the photosphere
inhibit heat conduction where the field is strong
• d. masses of heavy elements occlude solar light
Spectral types (e.g. O, B, A, F, G, K, M) define
uniquely their
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a. surface temperatures
b. luminosities
c. sizes of radii
d. brightnesses
Which of the following astronomical systems are held
together by gravity
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a. The Sun
b. The Solar System
c. The Milky Way
d. All of them
If a new planet were found with a period of revolution
of 6 years, what would be its average distance from
the Sun?
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a. About 1AU
b. About 3.3 AU
c. About 6 AU
d. About 36 AU
In order of increasing wavelength the electromagnetic spectrum is
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a. gamma rays, blue light, red light, radio waves;
b. ultraviolet, gamma rays, blue light, radio waves;
c. red light, radio waves, X rays, blue light;
d. visible, ultraviolet, X-rays, radio
Light has properties
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a. of waves;
b. of particles;
c. none of the above;
d. both a. and b.
What is the Law of Inertia?
• A body at rest stays at rest unless acted on by an
outside force
• b. F=ma
• c. P^2=A^3
• d. Fg=mMG/R^2
What is retrograde motion?
• a. “backward moving”/ or interrupted movement of a
planet on the sky
• b. Clockwise rotation of the moon around the earth
• c. Rotation of planets around the sun
• d. Large elliptical movements of comets