 A nebula is a cloud of hydrogen gas and dust in space.
 Nebulae are the birthplaces of stars.
 There are different types of nebula: An Emission Nebula: such as Orion nebula,
glows brightly because the gas is energized by the stars that have formed in it.
 In a Reflection Nebula, light reflects on the grains of dust in a nebula.
 A protostar is a large object that forms by
contraction of the gas of a giant molecular
cloud in the interstellar medium.
 The protostellar phase is one of the early
stages in the process of forming a star.
 It starts with a core of increased density in
a molecular cloud and ends with the
formation of a T Tauri star, which then
becomes a main sequence star. This is held
by the T Tauri wind, a type of super solar
wind that shows the change from the star
accreting mass into radiating energy.
 A black dwarf is a hypothetical stellar
remnant, it is created when a white dwarf
becomes cool to no longer give out significant
heat or light.
 Since the required time for a white dwarf to
reach the given state is calculated to be
longer than the age of the universe of 13.7
billion years.
 No black dwarfs are expected to exist in the
universe yet, but the temperature of the
coolest white dwarfs is one observational
limit on the age of the universe.
 After a star has formed, it creates energy at the core region of the star by fusing
hydrogen atoms into helium. During this stage of the star's lifetime, it is determined
by its mass, but also based upon its chemical composition and other factors.
 All main sequence stars are in hydrostatic equilibrium, where outward thermal
pressure from the core is balanced by the inward gravitational pressure from the
overlying layers.
 In the basic Hydrogen fusion cycle, four Hydrogen nuclei come together to
make a Helium nucleus.
 There are actually electrons, neutrinos and photons involved that make
up the fusion of Hydrogen into Helium.
 This is a large bright star with a cool surface.
 It is formed during the later stages of the evolution of a star like the Sun, as it runs out of
hydrogen fuel at its centre.
 Red giants are 10 and 100 times larger than our Sun.
 They are very bright because they are so large, although their surface temperature is lower
than that of the Sun, about 2000-3000�C.
 Very large stars (red giants) are often called Super Giants.
 The fusion of helium-4 nuclei (alpha particles) is known as the triple-alpha
process, because fusion of just two helium nuclei only produces beryllium-8. If
the core temperature of a star exceeds 100 million Kelvin's (100 megakelvins.
 Depending upon the temperature and density, an additional helium nucleus may
fuse with carbon-12 to form oxygen-16, and at very high temperatures, additional
fusions of helium to oxygen and heavier nuclei may occur.
 A planetary nebula is an emission nebula consisting of a glowing shell of gas and
plasma formed by certain types of stars when they die.
 The name originated in the 18th century because of their similarity in
appearance to giant planets when viewed through small optical telescopes, and
is unrelated to the planets of the solar system.
 They are a relatively short-lived phenomenon, lasting a few tens of thousands of
years, compared to a typical stellar lifetime of several billion years.
 A white dwarf, also called a degenerate dwarf, is a
small star composed mostly of electron-degenerate
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 thermal energy.
White dwarfs comprise roughly 6% of all known stars
in the solar neighborhood.
The unusual faintness of white dwarfs was first
recognized in 1910 by Henry Norris Russell, Edward
Charles Pickering, and Williamina Fleming. the name
white dwarf was coined by Willem Luyten in 1922.
 A black dwarf is a hypothetical stellar
remnant, created when a white dwarf
becomes sufficiently cool to no
longer emit significant heat or light.
 Since the time required for a white
dwarf to reach this state is
calculated to be longer than the
current age of the universe of 13.7
billion years, no black dwarfs are
expected to exist in the universe yet,
and the temperature of the coolest
white dwarfs is one observational
limit on the age of the universe.
 A massive main sequence star is just a bigger main
sequence star that dies faster because it is bigger
therefore it uses hydrogen and helium faster.
 Red supergiant (RSGs) are supergiant stars (luminosity class I) of
spectral type K or M. They are the largest stars in the universe in terms of
volume, although they are not the most massive. Betelgeuse and Antares
are the best known examples of a red supergiant.
 A supernova is a stellar explosion. Supernova are extremely luminous and
cause a burst of radiation that often briefly outshines an entire galaxy,
before fading from view over several weeks or months. During this short
interval, a supernova can radiate as much energy as the Sun could emit
over its life span.
 A neutron star is a type of remnant that can result from the gravitational collapse of a massive
star during a Type II, Type Ib or Type Ic supernova event.
 Such stars are composed almost entirely of neutrons, which are subatomic particles without
electrical charge and roughly the same mass as protons.
 Neutron stars are very hot and are supported against further collapse because of the Pauli
exclusion principle.
 This principle states that no two neutrons can occupy the same place and quantum state
 In general relativity, a black hole is a region of space in which the gravitational
field is so powerful that nothing, not even light, can escape. The black hole has a
one-way surface, called an event horizon, into which objects can fall, but out of
which nothing can come.
 It is called "black" because it absorbs all the light that hits it, reflecting nothing,
just like a perfect black-body in thermodynamics.
 Quantum analysis of black holes shows them to possess a temperature and
Hawking radiation.
 These are very cool, faint and small stars,
approximately one tenth the mass and diameter of
the Sun. They burn very slowly and have estimated
lifetimes of 100 billion years. Proxima Centauri and
Barnard's Star are red dwarfs.
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