GJ 742
Encyclopedia
GJ 742 is a white dwarf
star
located about 43 light years from Earth
in the constellation
Draco
. With a magnitude
of about 13 it is visible only through a large telescope
.
project, it was not determined to be a white dwarf until observed in 1934 by G. P. Kuiper. This makes it the fifth or sixth white dwarf discovered. At first, its spectrum
was thought to be almost featureless, but later observation showed it to have unusual broad, shallow absorption bands
., p. 28; In 1970, when light it emitted was observed to be circularly polarized, it became the first white dwarf known to have a magnetic field
. In the 1980s it was realized that the unusual absorption bands could be explained as hydrogen
absorption lines shifted by the Zeeman effect
.
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
located about 43 light years from Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
in the constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....
Draco
Draco (constellation)
Draco is a constellation in the far northern sky. Its name is Latin for dragon. Draco is circumpolar for many observers in the northern hemisphere...
. With a magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
of about 13 it is visible only through a large telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...
.
Properties
Although photographed in the 19th century as part of the Carte du CielCarte du Ciel
The Carte du Ciel and the Astrographic Catalogue were two distinct but connected components of a massive international astronomical project, initiated in the late 19th century, to catalogue and map the positions of millions of stars as faint as 11th or 12th magnitude...
project, it was not determined to be a white dwarf until observed in 1934 by G. P. Kuiper. This makes it the fifth or sixth white dwarf discovered. At first, its spectrum
Spectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
was thought to be almost featureless, but later observation showed it to have unusual broad, shallow absorption bands
Spectral line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from a deficiency or excess of photons in a narrow frequency range, compared with the nearby frequencies.- Types of line spectra :...
., p. 28; In 1970, when light it emitted was observed to be circularly polarized, it became the first white dwarf known to have a magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
. In the 1980s it was realized that the unusual absorption bands could be explained as hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
absorption lines shifted by the Zeeman effect
Zeeman effect
The Zeeman effect is the splitting of a spectral line into several components in the presence of a static magnetic field. It is analogous to the Stark effect, the splitting of a spectral line into several components in the presence of an electric field...
.