Black holes are heaps of concentrated matter whose gravity is so strong
that even light cannot escape. Supermassive black holes are believed to
reside at the cores of every galaxy, though some are thought to be more
active than others. Active black holes drag surrounding material into
them, heating it up and causing it to glow. Dormant black holes, like
the one in our Milky Way galaxy, hardly make a peep, so they are
difficult to study.
- See more at: file:///C:/Users/hp/Desktop/Black%20Hole.htm#sthash.QknTFDBn.dpuf
Black holes are heaps of concentrated matter whose gravity is so strong
that even light cannot escape. Supermassive black holes are believed to
reside at the cores of every galaxy, though some are thought to be more
active than others. Active black holes drag surrounding material into
them, heating it up and causing it to glow. Dormant black holes, like
the one in our Milky Way galaxy, hardly make a peep, so they are
difficult to study.
- See more at: file:///C:/Users/hp/Desktop/Black%20Hole.htm#sthash.QknTFDBn.dpuf
Black
holes are heaps of concentrated matter whose gravity is so strong that even
light cannot escape. Supermassive black holes are believed to reside at the
cores of every galaxy, though some are thought to be more active than others.
Active black holes drag surrounding material into them, heating it up and
causing it to glow. Dormant black holes, like the one in our Milky Way galaxy,
hardly make a peep, so they are difficult to study.
History of discovery of Black hole
Black holes are heaps of concentrated matter whose gravity is so strong
that even light cannot escape. Supermassive black holes are believed to
reside at the cores of every galaxy, though some are thought to be more
active than others. Active black holes drag surrounding material into
them, heating it up and causing it to glow. Dormant black holes, like
the one in our Milky Way galaxy, hardly make a peep, so they are
difficult to study.
- See more at: file:///C:/Users/hp/Desktop/Black%20Hole.htm#sthash.QknTFDBn.dpuf
Objects whose gravity fields are too strong for light to escape were first considered in the 18th century by John Michell and Pierre-Simon Laplace. The first modern solution of general relativity that would characterize a black hole was found by Karl Schwarzschild in 1916, although its interpretation as a region of space from which nothing can escape was first published by David Finkelstein
in 1958. Long considered a mathematical curiosity, it was during the
1960s that theoretical work showed black holes were a generic prediction
of general relativity. The discovery of neutron stars sparked interest in gravitationally collapsed compact objects as a possible astrophysical reality.
How do Black holes form and grows?
Black holes of stellar mass
are expected to form when very massive stars collapse at the end of
their life cycle. After a black hole has formed it can continue to grow
by absorbing mass from itssurroundings. By absorbing other stars and
merging with other black holes, supermassive black holes of millions of solar masses may form. There is general consensus that supermassive black holes exist in the centers of most galaxies.
How are Black holes spotted by scientist?
Despite its invisible interior, the presence of a black hole can be inferred through its interaction with other matter and with electromagnetic radiation such as light. Matter falling onto a black hole can form an accretion disk heated by friction, forming some of the brightest objects in the universe.
If there are other stars orbiting a black hole, their orbit can be used
to determine its mass and location. Such observations can be used to
exclude possible alternatives (such as neutron stars). In this way,
astronomers have identified numerous stellar black hole candidates in binary systems, and established that the core of the Milky Way contains a supermassive black hole of about 4.3 million solar masses.
How can the properties of a black hole be ascertained?
The no-hair theorem states that, once a black hole achieves a
stable condition after formation, it has only three independent
physical properties
- mass,
Any two black holes that share the
same values for these properties, or parameters, are indistinguishable
according to classical (i.e.
non-quantum)
mechanics.These properties are special because
they are visible from outside a black hole. For example, a charged black hole
repels other like charges just like any other charged object. Similarly, the
total mass inside a sphere containing a black hole can be found by using the
gravitational analog of Gauss's law, the ADM mass, far away from the black hole. Likewise, the angular momentum can be
measured from far away using frame dragging by the gravitomagnetic field.
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| NASA's telescope sees a black hole munch on a star |
When an object falls into a black
hole, any information
about the shape of the object or distribution of charge on it is evenly
distributed along the horizon of the black hole, and is lost to outside
observers. The behavior of the horizon in this situation is a dissipative system
that is closely analogous to that of a conductive stretchy membrane with
friction and electrical resistance—the
membrane paradigm.This is different from other field theories
like electromagnetism, which do not have any friction or resistivity at the
microscopic level, because they are time-reversible. Because a black hole eventually
achieves a stable state with only three parameters, there is no way to avoid
losing information about the initial conditions: the gravitational and electric
fields of a black hole give very little information about what went in. The
information that is lost includes every quantity that cannot be measured far
away from the black hole horizon, including approximately conserved quantum numbers such as the total baryon number and lepton number. This behavior is so puzzling that
it has been called the black hole
information loss paradox.
Physical properties
The simplest static black holes have
mass but neither electric charge nor angular momentum. These black holes are
often referred to as Schwarzschild black holes
after Karl Schwarzschild who discovered this solution
in 1916. According to Birkhoff's
theorem, it is the only vacuum
solution that is spherically
symmetric.This means that there is no observable
difference between the gravitational field of such a black hole and that of any
other spherical object of the same mass. The popular notion of a black hole
"sucking in everything" in its surroundings is therefore only correct
near a black hole's horizon; far away, the external gravitational field is
identical to that of any other body of the same mass.
Solutions describing more general
black holes also exist. Charged black holes
are described by the Reissner–Nordström
metric, while the Kerr metric describes a
rotating black hole.
The most general stationary
black hole solution known is the Kerr–Newman metric,
which describes a black hole with both charge and angular momentum.
Due to the relatively large strength of the electromagnetic force,
black holes forming from the collapse of stars are expected to retain
the nearly neutral charge of the star. Rotation, however, is expected to
be a common feature of compact objects.
NASA
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