LP 768-500 - Smallest Known Star?
RA = 1h 48m 12.35 sec m = +18.3 m = 1.18"/yr. in P.A. 188°
DEC = -17° 11' 5.0" Distance = 18 pc (59 light years)
LP 768-500 is the designation for a star discovered by Wilhelm J. Luyten in his search for high proper motion stars in the 1950’s and 60’s. It is a white dwarf star in the constellation Cetus and was discovered in November 1963. It is one of the smallest and densest stars known with a computed diameter of about 1,400 km. (See diagram below). Another one of the smallest stars (excluding neutron stars) is LP 327-16, also discovered by Luyten in May 1962 in Minneapolis with an estimated diameter of 1,700 km (half the size of the Moon).
Comparative size of white dwarf star LP 768-500 with Texas. LP 768-500's estimated size is just 900 miles !!
This faint star (LP 768-500) has an annual proper motion of 1.18"/yr. in P.A. 188° (position angle). Its high proper motion resulted in a parallax determination placing it at a distance of about 18 pc (59 light years). Luyten introduced the letter “P” in his labeling of small stars to indicate their “pigmy” status. Due to its incredibly small size and mass, LP 768-500 has a density of 18,000 tons/cubic inch. Such extreme densities place these stars nearly in a state of degeneracy.
Degeneracy is a state of matter in which the only forces acting are those from quantum effects. Degeneracy occurs following gravitational collapse in which the electrons are stripped from their host atoms, thus only electrons and atomic nuclei can only exist in a closely packed highly dense state. As the density increases, the number of electrons per unit volume increases to a point where they can exert a considerable pressure, called degenerate pressure. This pressure is the result of laws of quantum mechanics. Unlike normal pressure, quantum pressure is independent of temperature, depending entirely on density. With the intense densities typical of white dwarfs (107 kg/cubic meter, or more) it becomes sufficiently large to counter the gravitational force and thus prevents the star from collapsing any further.
Above a certain stellar mass limit (1.44 M¤*, the Chandrasekhar Limit), equilibrium cannot be maintained between gravity pulling the star inward and the electron degeneracy pressure outward. The star must collapse further to become a neutron star. It is then the degeneracy pressure exerted by the tightly packed neutrons that balances the gravitational force. An example of a neutron star is the remaining collapsed core of the massive star that resulted in the Supernova explosion of July 4, 1054, resulting in the Crab Nebula remnant in Taurus.
*M¤= Mass of Sun
Burnham, R, Jr., 1978, Burnham's Celestial Handbook, Dover Publications, New York., p. 1462.