Question

1. White dwarfs are not included in the luminosity classification because they are:
a) no longer producing energy by nuclear reactions
b) too small
c) not yet active stars; they have not yet begun nuclear reactions
d) no longer radiating energy away

2. Absolute magnitude is defined as the apparent magnitude that a star would have if
a) it were located at exactly 10 pc from Earth
b) it were located at exactly 10 AU from Earth
c) all the energy from the star was concentrated in the visual region
d) it were located at exactly 10 ly from Earth

3. What is a dwarf star?
a) main-sequence star
b) a star that is significantly smaller than a giant or supergiant star
c) large, planetary object, such as Jupiter
d) a star of about the same size (diameter) as Earth

Answer 1

1. White dwarfs are not included in the luminosity classification because they are:
Ans. (a) no longer producing energy by nuclear reactions.

The material in a white dwarf no longer undergoes fusion reactions, so the star has no source of energy. As a result, it cannot support itself by the heat generated by fusion against gravitational collapse, but is supported only by electron degeneracy pressure, causing it to be extremely dense.

A white dwarf's faint luminosity comes from the emission of stored thermal energy; no fusion takes place in a white dwarf wherein mass is converted to energy

2. Absolute magnitude is defined as the apparent magnitude that a star would have if
Ans. (a) it were located at exactly 10 pc from Earth

Absolute magnitude is defined to be the apparent magnitude an object would have if it were located at a distance of 10 parsecs. So for example, the apparent magnitude of the Sun is -26.7 and is the brightest celestial object we can see from Earth.

3. What is a dwarf star?
Ans. (d) a star of about the same size (diameter) as Earth

A white dwarf, also called a degenerate dwarf, is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to that of the Sun, while its volume is comparable to that of Earth.