How is it possible for white dwarfs to have a lower absolute magnitude than the sun?

Answer 1

A white dwarf star is hotter than the Sun, so you would think that it ought to give off more light. And it does, on a size basis. But a white dwarf is much SMALLER that the Sun, so even though each square mile of surface is brighter, a dwarf has many fewer square miles of surface to radiate.

A small bright light may not give off as much total light as a very large dimmer light.

Answer 2

White Dwarfs are degenerate stars. They are created when the host star has exhausted it's supply of hydrogen. Its luminosity comes from the emission of stored thermal energy, it has no other means of energy as all fusion has ceased nor can it generate any energy from gravitational collapse.

A white dwarf is very hot when it is formed, but as it has no source of energy, it will gradually cool down. This means that in time it will lessen and redden with time and eventually cool to become a black dwarf.

Answer 3

Because they are small.

As stars (or solid things) get hotter it gives off more light and at overall shorter

wavelengths (blue is higher energy than red.) At the temperature of white

dwarfs they give off a lot of green light but because green is in the middle of

what we can see they give off so much of the other colors that they look white.

Without fusion though they can only cool down as they send away so much

heat in the form of light.

They are also much dimmer if they are further away, just like how a lightbulb

might hurt your eyes right in front of your face but is a mere speck several

blocks away.

Answer 4

A dwarf star will have an "average" absolute magnitude, as they have an average temperature.

You have to understand what absolute magnitude is [See related question] and what a dwarf star is [See related question].

I have a feeling that the question should be "Why do reddwarfs have a low absolute magnitude".

Answer 5

Temperature is only part of it. Another part is surface area. White dwarfs are a lot hotter, and they're emitting more light per square meter, but there are a lot less square meters of surface (by a factor of around 10,000) on a white dwarf, because they're so much smaller than the sun.

(By the way, I assume you mean "white dwarf" as in "Earth-sized, no fusion, slowly cooling" as opposed to "Slightly more than Sun-sized, main sequence, happens to be white." The second type have a higher luminosity than the Sun.

Answer 6

They aren't generating all that much energy from nuclear fusion, because much of the fuel has already been fused. If a star is radiating a lot of energy, it will expand from its own internal pressure, but as the radiation falls off, the internal pressure isn't enough to support the star's mass against its own gravity. So as the energy production falls, the star shrinks, which also reduces the surface area which is the part of the star that actually radiates.

Answer 7

Because they are very small - about the size of the Earth.

Answer 8

White dwarfs are less bright than other stars because they have a relatively small surface ares from which to emit light.

Answer 9

White dwarfs are much, much smaller than the Sun (by about a factor of a million in surface area).

Answer 10

White dwarves have a very small surface, in the order of around 1/10,000 of the Sun's surface.