Why a white dwarf has a low luminosity even though it is very hot?

White dwarfs have an extremely small surface area to radiate this heat from, so they cool gradually, remaining hot for a long time. As a white dwarf cools, its surface temperature decreases, the radiation which it emits reddens, and its luminosity decreases.

Why is a white dwarf so hot?

Such stars eventually blow off the material of their outer layers, which creates an expanding shell of gas called a planetary nebula. Within this nebula, the hot core of the star remains—crushed to high density by gravity—as a white dwarf with temperatures over 180,000 degrees Fahrenheit (100,000 degrees Celsius).

Why are white dwarf stars hotter than the sun?

A typical white dwarf has a carbon and oxygen mass similar to the Sun, but is much smaller in size (similar to the Earth). It is much hotter (25,000 K), but because of its small size its luminosity is low.

Why are white dwarfs hotter than main sequence stars?

Stars smaller than a quarter the mass of the sun collapse directly into white dwarfs. White dwarfs no longer burn fusion at their center, but they still radiate heat. The new star is far dimmer than it was as a main sequence star.

Are white dwarfs hotter than red giants?

A white dwarf has a higher surface temperature than a red giant star.

What is the luminosity of a white dwarf?

After a billion years the typical white dwarf is down to 0.001 the luminosity of the Sun.

What is the luminosity of white dwarf?

What is the luminosity class of most white dwarfs?

D
The temperature defines the star’s “color” and surface brightness….A note on the spectral atlas and spectral classification.

Luminosity Class	Description	Comments
IV	Sub-Giants
V	Dwarfs	Main Sequence
sd	Sub-Dwarfs
D	White Dwarfs

How does the luminosity of white dwarfs compare to main sequence stars?

Although they are cool [red], they are very luminous, and therefore bright. In the Main Sequence, stars that are cool are not as luminous. White dwarf stars are very hot [blue], but dim because they are so small.

What is the luminosity of the Sun?

1 L☉Sun / Luminosity

How do white dwarfs compare with super giants in terms of temperature and luminosity?

How do white dwarfs compare with supergiants, in terms of temperature and luminosity? White dwarfs are cooler and have higher luminosity than supergiants. White dwarfs are cooler and have lower luminosity than supergiants.

What is the average temperature of a white dwarf star?

Most observed white dwarfs have relatively high surface temperatures, between 8,000 K and 40,000 K. A white dwarf, though, spends more of its lifetime at cooler temperatures than at hotter temperatures, so we should expect that there are more cool white dwarfs than hot white dwarfs.

Why do white dwarfs take so long to cool down?

Because of its small surface area, and large thermal mass (typically about a half the mass of the sun) the cooling time of white dwarfs is billions of years. As he says they do cool, however the universe isn’t old enough to have created condensed red dwarfs.

What is the relationship between the mass and radius of white dwarfs?

The relationship between the mass and radius of white dwarfs can be derived using an energy minimization argument. The energy of the white dwarf can be approximated by taking it to be the sum of its gravitational potential energy and kinetic energy.

Can a white dwarf become a neutron star as it cools?

The answer is no. The white dwarf’s pressure is maintained by electron degeneracy pressure, they do not contract appreciably as they cool down, and dense as they are they are orders of magnitude less dense than nuclear matter. A white dwarf has to exceed the Chandrasekhar mass for core collapse in order to become a neutron star.