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White dwarfs are stellar remnants produced when a star with around 8 solar masses or less sheds its outer layers into a planetary nebula. The leftover core becomes the white dwarf. It is thought that white dwarfs cool down over quadrillions of years to produce a black dwarf. [15] Neutron star: RX J0720.4−3125: 0.0000064683 – 0.0000077332
1930 – Subrahmanyan Chandrasekhar discovers the white dwarf maximum mass limit. 1933 – Fritz Zwicky and Walter Baade propose the neutron star idea and suggest that supernovae might be created by the collapse of normal stars to neutron stars—they also point out that such events can explain the cosmic ray background.
The highly magnetized white dwarf in the binary system AR Scorpii was identified in 2016 as the first pulsar in which the compact object is a white dwarf instead of a neutron star. [120] A second white dwarf pulsar was discovered in 2023.
The Crab Nebula is a supernova remnant containing the Crab Pulsar, a neutron star. In certain binary stars containing a white dwarf, mass is transferred from the companion star onto the white dwarf, eventually pushing it over the Chandrasekhar limit. Electrons react with protons to form neutrons and thus no longer supply the necessary pressure ...
An intermediate-mass binary pulsar (IMBP) is a pulsar-white dwarf binary system with a relatively long spin period of around 10–200 ms consisting of a white dwarf with a relatively high mass of approximately . [7] The spin periods, magnetic field strengths, and orbital eccentricities of IMBPs are significantly larger than those of low mass binary pulsars (LMBPs). [7]
A neutron star is the collapsed core of a massive supergiant star. It results from the supernova explosion of a massive star—combined with gravitational collapse—that compresses the core past white dwarf star density to that of atomic nuclei.
The result is a star with a diameter on the order of a thousandth that of a white dwarf. The properties of neutron matter set an upper limit to the mass of a neutron star, the Tolman–Oppenheimer–Volkoff limit, which is analogous to the Chandrasekhar limit for white dwarf stars.
Sub-dwarf classes have also been used: VI for sub-dwarfs (stars slightly less luminous than the main sequence). Nominal luminosity class VII (and sometimes higher numerals) is now rarely used for white dwarf or "hot sub-dwarf" classes, since the temperature-letters of the main sequence and giant stars no longer apply to white dwarfs.