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physical object > natural object > particle > hypothetical particle > dark matter > massive compact halo object

massive compact halo object (MACHO)
subject			fact

massive compact halo object	is a part of halo
	has acronym MACHO
	has definition These are black holes, neutron stars and brown dwarfs, none of which are luminous and all of which are postulated to exist in the halos of galaxies. They are a form of dark matter.
	is a kind of dark matter
dark matter	is a part of dark halo
	has synonym hidden mass
hypothetical particle	has acceptance status hypothetical
particle	obeys uncertainty principle
	has frequency inversely proportional to the wavelength
	has wavelength inversely proportional to its momentum
	has charge
	has mass
physical object	has location or center of gravity
	has angular momentum
	has velocity
	has momentum
	has temperature
	has volume
	has extent
	has material

Kinds of massive compact halo object :

• black dwarf (3 facts) - The final stage in the evolution of a star of roughly 1 M_sun. It is a mass of cold, electron-degenerate gas, and can no longer radiate energy, because the whole star is in its lowest energy state. No black dwarfs have ever been observed. Also, an object (M < 0.085 M_sun) that is not massive enough to achieve nuclear chain reactions.
• black hole (7 kinds, 22 facts) - An object that is maximally gravitationally collapsed, and from which not even light can escape., An object with such a strong gravitational field that even light cannot escape. Matter can fall into a black hole, but according to classical physics no matter or energy can leave it. (Hawking has used quantum theory to show that black holes emit blackbody radiation, but the effect is significant only for black holes much smaller than those that are expected to form by the collapse of stars, which have masses of several solar masses or more.), A mass that is sufficiently compact that not even light can escape its intense gravity. Thus it appears black from the outside. If the sun were compressed to a sphere about four miles in diameter, it would become a black hole. It is believed that some massive stars, after exhausting their nuclear fuel, collapse under their own weight to form black holes., A singularity in space, surrounded by an event horizon, caused by the collapse of a small but massively dense star through the effects of its own increasing gravity. By the time the state of singularity is reached, the remnants of the star may be minimal, but the gravitational force is so strong it prevents even light from escaping. Black holes may form the "power centers" of galaxies, thus explaining infrared radiation detected in several galactic centres. The properties of matter entering a black hole are the theme of John Wheeler's no hair theorem., A gravitationally collapsed mass inside the Schwarzschild radius (q.v.), from which no light, matter, or signal of any kind can escape. A black hole occurs when the escape velocity of a body becomes the velocity of light (2GM / R = c²). If an object with the mass of the Sun had a radius of 2.5 km, it would be a black hole. Black holes represent one of the possible endpoints of stellar evolution for stars very much more massive than the Chandrasekhar limit.
• brown dwarf (6 facts) - Star with too little mass to ignite its hydrogen 1 fuel. If brown dwarfs exist, they shine faint red for a time, as they convert gravitational energy into heat, and then fade and cool., A substellar object that is below the minimum mass required for nuclear fusion reactions to occur in its core., A self-gravitating, self-luminous gaseous object which is not sufficiently massive to result in thermonuclear hydrogen fusion reactions in its core and cannot therefore be considered a star. Such objects are expected to have a mass less than 7% of the Sun's mass and represent a "missing link" between low-mass stars and gas giant planets like Jupiter (at 0.1% of the Sun's mass).
• neutron star (10 kinds, 60 facts) - A dead, collapsed star that consists mostly of neutrons and is only about 20 kilometers across. Neutron stars are much denser than white dwarfs., A star whose core is composed primarily of neutrons, as is expected to occur when the mean density is in the range 10¹³-10¹⁵ g cm^-3. Under current theories pulsars are assumed to be rotating magnetic neutron stars. A neutron star would probably be only 10-15 km in diameter with a magnetic field of about 10¹² gauss, a density of 10¹³-10¹⁵ g cm^-3 (compared with a white dwarf's maximum density of about 10⁸ g cm^-3) and a central temperature of about 10⁹ K and thus would be both bluer and dimmer than a white dwarf., Remnant of a star after it has exploded as a supernova. Usually optically dim, a neutron star sends out regular or irregular radio emissions and is therefore also called a pulsar. The density of such a star may be unimaginably great although the diameter is generally around only 10 km; the gravitational and magnetic forces are correspondingly vast. It is called a neutron star because in such density, protons fuse with electrons to form neutrons, of which the star is almost entirely composed.

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