Substellar object
Encyclopedia
A substellar object, sometimes called a lump, is an astronomical object
whose mass
is smaller than the smallest mass, approximately 0.08 solar mass
es, at which a star
can sustain hydrogen fusion. This definition includes brown dwarf
s, former stars similar to EF Eridani B
, and can also include objects of planetary mass, regardless of their formation mechanism and whether or not they are associated with a primary star
.
Assuming that a substellar object has composition similar to the Sun
's and at least the mass of Jupiter
(approximately 10−3 solar masses), its radius will be comparable to that of Jupiter (approximately 0.1 solar radii). This is because the center of such a substellar object just below the hydrogen-burning limit is quite degenerate, with a density
of ≈103 g/cm3, but this degeneracy lessens with decreasing mass until, at the mass of Jupiter, a substellar object has central density less than 10 g/cm3. The density decrease balances the mass decrease, keeping the radius approximately constant.
A substellar object with mass just below the hydrogen-fusing limit may ignite hydrogen fusion temporarily at its center, but although this will provide some energy, it will not be enough to overcome the object's ongoing gravitational contraction; likewise, although an object with mass above approximately 0.013 solar masses will be able to fuse deuterium
for a time, this source of energy will be exhausted in approximately 106 to 108 years. Apart from these sources, the radiation of an isolated substellar object comes only from the release of its gravitational potential energy, which causes it to gradually cool and shrink. A substellar object in orbit
about a star will shrink more slowly as it is kept warm by the star, evolving towards an equilibrium
state where it emits as much energy as it receives from the star.
Astronomical object
Astronomical objects or celestial objects are naturally occurring physical entities, associations or structures that current science has demonstrated to exist in the observable universe. The term astronomical object is sometimes used interchangeably with astronomical body...
whose mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
is smaller than the smallest mass, approximately 0.08 solar mass
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
es, at which a 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...
can sustain hydrogen fusion. This definition includes brown dwarf
Brown dwarf
Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...
s, former stars similar to EF Eridani B
EF Eridani
EF Eridani is a variable star of the type known as polars, AM Herculis stars, or magnetic cataclysmic variable stars. Historically it has varied between apparent magnitudes 14.5 and 17.3, although since 1995 it has generally remained at the lower limit...
, and can also include objects of planetary mass, regardless of their formation mechanism and whether or not they are associated with a primary 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...
.
Assuming that a substellar object has composition similar to the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
's and at least the mass of Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
(approximately 10−3 solar masses), its radius will be comparable to that of Jupiter (approximately 0.1 solar radii). This is because the center of such a substellar object just below the hydrogen-burning limit is quite degenerate, with a density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
of ≈103 g/cm3, but this degeneracy lessens with decreasing mass until, at the mass of Jupiter, a substellar object has central density less than 10 g/cm3. The density decrease balances the mass decrease, keeping the radius approximately constant.
A substellar object with mass just below the hydrogen-fusing limit may ignite hydrogen fusion temporarily at its center, but although this will provide some energy, it will not be enough to overcome the object's ongoing gravitational contraction; likewise, although an object with mass above approximately 0.013 solar masses will be able to fuse deuterium
Deuterium burning
Deuterium burning is a nuclear fusion reaction that occurs in stars and some substellar objects, in which a deuterium nucleus and a proton combine to form a helium-3 nucleus...
for a time, this source of energy will be exhausted in approximately 106 to 108 years. Apart from these sources, the radiation of an isolated substellar object comes only from the release of its gravitational potential energy, which causes it to gradually cool and shrink. A substellar object in orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
about a star will shrink more slowly as it is kept warm by the star, evolving towards an equilibrium
Radiative equilibrium
Radiative equilibrium is one of the several requirements for thermodynamic equilibrium, but it can occur in the absence of thermodynamic equilibrium. There are various types of radiative equilibrium, which is itself a kind of dynamic equilibrium....
state where it emits as much energy as it receives from the star.
See also
- Brown dwarfBrown dwarfBrown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...
- Planemo
- Sub-brown dwarfSub-brown dwarfA sub-brown dwarf is an astronomical object of planetary mass that is not orbiting a star and is not considered to be a brown dwarf because its mass is below the limiting mass for thermonuclear fusion of deuterium ....
- PlanetarPlanetar (astronomy)Planetar is a term used in astronomy that refers to one of two things:* Brown dwarfs — objects intermediate in size between planets and stars — but having formed similarly to planets....