Galaxies are classified according to —

A. size
Incorrect. The three types of galaxies are spiral, elliptical, and irregular.

B. shape
Correct! The three types of galaxies are spiral, elliptical, and irregular.

C. number of stars
Incorrect. The three types of galaxies are spiral, elliptical, and irregular.

D. type of gas
Incorrect. The three types of galaxies are spiral, elliptical, and irregular.

The H-R diagram shows the relationship between what two criteria?

A. Brightness and surface temperature
Correct!

B. Brightness and distance from the earth
Incorrect. Brightness is one of the variables on the H-R Diagram but distance from the earth is not.

C. Surface temperature and distance from the earth
Incorrect. Surface temperature is one of the variables on the H-R Diagram but distance from the earth is not.

D. Surface temperature and distance from the sun
Incorrect. Surface temperature is one of the variables on the H-R Diagram but distance from the sun is not.

Which of the following statements about the Sun are correct?

1. The Sun is a medium sized-star.
2. The Sun is the hottest star in the universe.
3. The Sun is the largest star in our galaxy.
4. The Sun is the closest star in our galaxy.

A. I and III are correct
Incorrect. The Sun is a medium sized star but it is not the largest in our galaxy.

B. II and III are correct
Incorrect. The Sun is not the hottest in the universe and it is not the largest in our galaxy either.

C. I and IV are correct
Correct! Our Sun is considered a medium sized star and is many thousand times closer to the Earth than any other star.

D. All are correct
Incorrect. The Sun is not the hottest in the universe and it is not the largest in our galaxy either.

All stars start out as a ball of gas and dust in nebulae. Gravity pulls the gas and dust together into a sphere. At this stage it is called a —

A. red giant
Incorrect. A red giant is a very large star of high luminosity and low surface temperature.

B. protostar
Correct! A protostar is a ball of gas and dust and is the first stage in the formation of a star.

C. white dwarf
Incorrect. A white dwarf is a small and very dense star.

D. neutron star
Incorrect. Neutron stars are formed when large stars run out of fuel and collapse.