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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:

Massive stars burn brighter!

5.3ML ≈

Massive stars burn brighter

L~M3.5

Luminosity Classes

Stellar Evolution – Models and Observation

• 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?

• 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:

• 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

• 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 right-angled 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

• 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

• 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

• a. surface temperatures

• b. luminosities

• c. sizes of radii

• d. brightnesses

Which of the following astronomical systems are held together by gravity

• 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?

• a. About 1AU

• b. About 3.3 AU

• c. About 6 AU

• d. About 36 AU

In order of increasing wavelength the electro-magnetic spectrum is

• 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

• 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