SDSS J1240+6710 or SDSS J124043.01+671034.68, nicknamed Dox by its discoverers, is a

white dwarf A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes fro ...

in the constellation

Draco Draco is the Latin word for serpent or dragon. Draco or Drako may also refer to: People * Draco (lawgiver) (from Greek: Δράκων; 7th century BC), the first lawgiver of ancient Athens, Greece, from whom the term ''draconian'' is derived * D ...

. It was discovered by Kepler de Souza Oliveira, Detlev Koester and Gustavo Ourique. It is unusual for having an atmosphere of almost pure

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...

. The atmosphere also has a detectable amount of

magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ta ...

neon Neon is a chemical element with the symbol Ne and atomic number 10. It is a noble gas. Neon is a colorless, odorless, inert monatomic gas under standard conditions, with about two-thirds the density of air. It was discovered (along with krypton ...

(under 4%) and

silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic tab ...

, but no hydrogen, helium or carbon. A possible explanation for the unusual composition would be if its mass were close to the

limit Limit or Limits may refer to: Arts and media * ''Limit'' (manga), a manga by Keiko Suenobu * ''Limit'' (film), a South Korean film * Limit (music), a way to characterize harmony * "Limit" (song), a 2016 single by Luna Sea * "Limits", a 2019 ...

for collapsing to a

neutron star A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Except for black holes and some hypothetical objects (e.g. white ...

. However, its mass is only 0.56 solar masses, below the mass expected for a star that could convert carbon to oxygen, neon and magnesium. The star was originally catalogued in the catalogue of new white dwarf stars from the Data Release 12 of the

Sloan Digital Sky Survey The Sloan Digital Sky Survey or SDSS is a major multi-spectral imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. The project began in 2000 a ...

. Gustavo Ourique, an undergraduate in Physics at Universidade Federal do Rio Grande do Sul, identified the star's unique spectrum, working under the advice of Dr. Kepler Oliveira (S.O. Kepler). The star's spectrum was modeled by Dr. Detlev Koester, characterizing its composition, temperature and mass. In 2020, the star was discovered to be moving at an unusually high velocity. There is no fully-confirmed explanation for this. There are various theories put forth as to possible reasons for this. Some researchers indicate this may be due to a supernova that has not been observed yet, and which would cause this star to move at high velocities. However, there is no definitive data that can absolutely prove that this is what happened. Astronomers note that some supernovas are difficult to detect, especially if they do not contain the element

nickel Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow to ...

, which is a key indicator for such events. Nuclear blast sends star hurtling across galaxy
By Paul Rincon, Science editor, BBC News website, 15 July 2020.

References

{{Reflist White dwarfs Draco (constellation) 2020 in science Unexplained phenomena