White dwarf


The white dwarfs are those that represent the final stage with the evolution of a star . They are small, compact stars that have the ability to spin quickly. Their radius can easily be compared to that of the earth and they are stars that eventually collapse.


What is a white dwarf?

white dwarf is a stellar remnant that forms when a star that has a lower mass has exhausted all its nuclear fuel , they are hot and small with a very low luminosity and are considered low planetary mass stars.



A white dwarf is basically what stars like the Sun become after they have used up their nuclear fuel . Near the end of its nuclear combustion stage , this type of star expels most of its material to the outside, giving rise to a nebula age plan, leaving only the hot core of the star. This core then becomes a white dwarf which is very hot, with a temperature even exceeding 100,000 degrees Kelvin. Unless it is accumulating matter from a nearby star, the white dwarf cools for the next billion years.


Characteristics of the white dwarf

The main characteristics of the white dwarf are as follows:

  • A typical white dwarf is half as massive as the Sun , but is only slightly larger than Earth .
  • They are stars small and hot , whose masses are comparable with the sun.
  • Their temperature is around 000 Kelvin and that is why they appear white.
  • They represent the last phase of the life of stars that are similar to the sun.
  • They are formed when nuclear fuel runs out.
  • They are considered one of the densest bodies of matter in space, second only to neutron stars .
  • Because a white dwarf can not create internal pressure the gravity compacts the material inwardly to crush even electrons that form.
  • As it does not have thermonuclear reactions in its nucleus , this type of star does not have energy sources so it compresses on its own weight. 


The first white dwarf was the 40 Eridani triple star system orbiting the binary system formed by the 40 Eridani B , and 40 Eridani white dwarf William Herschel on January 31, 1783 was the one who discovered the system that was later observed by Friedrich Georg Wilhelm von Struve and Otto Wilhelm . In the 19th century , the techniques of measuring the position of the stars were able to detect small changes in the position of the stars and in 1844, Friedrich Bessel , perceived that the stars Sirius and Procyontheir positions varied, and he thought that these position changes were due to an invisible star. CHF Peters calculated an orbit for this star in 1851. This star that was talked about was Sirius B or Cachorro , the second discovered white dwarf. On January 31, 1862, Alvan Graham Clark observed a kind of dark star near Sirius which he identified as the star predicted by Bessel.

The first white dwarfs to be discovered after them were discovered in the 1930s . In 1939, 18 white dwarfs were discovered. Several scientists continued to search for white dwarfs in the 1940s. In 1950 there were almost 100 known white dwarfs, and in 1999, 2,000. Since then, the Sloan Digital Sky Survey has found 9,000 new white dwarfs .


White dwarfs are composed of atoms in a plasma state and emit only stored thermal energy and for this reason they have a rather weak luminosity . When the intermediate white dwarfs finish with the fusion of hydrogen , they manage to expand like red giants and go on to fuse helium into carbon and oxygen for their nucleus. White dwarfs are basically composed of carbon and oxygen, which then result from the melting of ice.On them, we can also find a layer of hydrogen and helium. degenerates that shape the atmosphere of the white dwarf.

Formation of the white dwarf

Once the smallest stars reach the end of their evolution, they transform into white dwarfs . They are stars that have used up all the amount of hydrogen they have and that they used as a type of nuclear fuel . The fusion that happens in the core of the star produces heat and pressure towards its exterior, which is balanced by the force of gravity that is generated thanks to the mass that the star has. Once the hydrogen has been used as fuel it runs out and fusion slows down, causing gravity to of the star collapsing.

As the star condenses and compacts , it gains more heat , burning the hydrogen and causing the outer layers of the star to expand outward, transforming it into a red giant star first . Due to its large size, the heat expands and the temperature of its surface is very cold, but its core is hot. These stars are so hot that they convert the helium in the nucleus into different heavy elements like carbon. Then they expel material from its outer layers creating a sheath of gaswhere the nucleus will continue to heat up and contract to form the white dwarf .


The types of white dwarf that can be found are the following:

  • dA : they are white dwarfs that only have Balmer lines and have no metals present.
  • dB : in this type there are no metals present .
  • AD : They have a continuous spectrum , and few or none of them have a visible line.
  • dO : possess helium or hydrogen
  • dZ: they have only some metal lines .
  • dQ : they have characteristics of carbon either atomic or molecular in any part of the spectrum.


The habitable zone that a white dwarf has is close to it, so if a planet revolves around it it could be swallowed by the star during the red giant phase  To solve this problem, one could think that the planet has migrated from a more distant orbit or that it is in that place due to several planetary carambola . In any case, such a planet would have to endure high doses of ultraviolet radiation and X-rays from the white dwarf, in addition to the increase in temperature that can heat the internal part. of the planet in an extreme way.

How is it different from a red giant?

The red giant stars have extremely large sizes and during the life of the star, H2 is the element that is responsible for giving them their brightness . When fuses the hydrogen occurs Helio and when reserves hydrogens end, the star used helium to continue the fusion process , increasing its volume and descending its temperature , making the star has a tone reddish . The white dwarf stars are the phase after the red giant that after exhausting itsnuclear fuel explodes producing a supernova , leaving behind a tiny star and dense , made mostly of O2 and C in plasma state. They are smaller in size and have a white or bluish color . They do not produce energy , nor do they compensate for its gravity , so they are in a constant process of ” compression ” and shutdown.

Featured white dwarfs

Among the main white dwarfs that we can find we mention the following:

  • 40 Eridani B
  • Sirius B
  • Van Maanen’s star


Some of the curiosities of the white dwarf are:

  • The small stars that are less than 8 solar masses end up becoming dwarf white what is considered a natural process that happens almost all the stars we know, including the sun .
  • They are excessively dense . Although their radius is smaller than that of the Sun , these bodies have the same solar density.
  • The white dwarfs have burned all their nuclear fuel and for that reason gravity causes the star to collapse in on itself.
  • During the cooling process of the stars , a gaseous material known as a planetary nebula is released in which the stellar core of a high density persists due to gravity , which would then be what we know as a white dwarf.

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