Good question. Three part answer.
First, all stars convert gravitational potential energy into radiative energy. White dwarfs have a mass that's in the same order of magnitude as the stars that became them, but with a much higher density and a much smaller surface area. So the amount of energy radiated per inch would be higher, resulting in higher surface temperatures.
Second, the degenerate matter that makes up the bulk of a white dwarf has a very low opacity, because any absorption of a photon requires an electron transition to a higher empty state, which may not be available given the energy of the photon.
Third, since the heat-generating capacity of the white dwarf is not replenished by nuclear fusion - and IF there is no companion star present from which the dwarf gains new mass - the star will slowly cool; the high surface temperatures do not last.
white dwarfs
White dwarfs have a broad spectrum, ranging from ultraviolet to near-infrared. However, they are most prominent in the blue and ultraviolet part of the spectrum, due to their high surface temperatures.
Dwarfs, such as white dwarfs or brown dwarfs, are small in size but can still be very bright because they have high temperatures that produce intense luminosity. The brightness of a dwarf is determined by its surface temperature, not its physical size. Therefore, even though dwarfs are smaller than the sun, their high temperatures allow them to emit significant amounts of light.
The average temperature for a white dwarf star is around 10,000 to 100,000 Kelvin, which is significantly cooler than other types of stars. Despite their high temperatures, white dwarfs are considered "dead" stars as they no longer undergo nuclear fusion reactions in their cores.
White dwarfs are found in the bottom left portion of the H-R diagram, characterized by high surface temperatures and low luminosities. They are the end stage of evolution for low to medium mass stars like the Sun.
Small and hot stars are typically classified as white dwarfs. These stars are the end stage of evolution for stars with low to medium mass, such as the Sun, and are characterized by their high temperature and small size. White dwarfs are very dense and can appear white in color due to their high surface temperature.
White dwarfs remain hot after exhausting their nuclear fuel because of residual heat from their previous nuclear reactions. The heat produced during their formation and contraction also contributes to their high temperatures. Additionally, white dwarfs have no way to generate new energy, so they gradually cool over billions of years.
White dwarfs.
White dwarfs.
Small hot stars are classified as type O, B, or A stars based on their spectral characteristics. These stars are typically blue-white in color and have high surface temperatures and luminosities. They are also referred to as main sequence stars because they are actively fusing hydrogen into helium in their cores.
White dwarfs are not very luminous compared to other stars. While they can be thousands of times more luminous than the Sun due to their high surface temperatures, their small size limits their overall brightness. They are often dimmer than main sequence stars of similar mass.
The stars in the Pegasus constellation vary in color from white to blue, indicating high surface temperatures. The surface temperatures of these stars can range from around 6,000 to 25,000 degrees Celsius.