FREE ESSAY ON STARS

STARS

STARS
A star is a large celestial body composed of gravitationally contained hot gases giving
off electromagnetic radiation, especially light. The sun is actually a star. Unlike the
sun the stars seem to be fixed, but in fact stars are in rapid motion, but their
distances are so great no relative changes can be seen with the naked eye.
The sun is a typically star, with a visible surface called a photosphere, which an
atmosphere of hot gases, and above them an outflowing stream of particles called the
solar (stellar) winds. Cooler spots called sunspots are likely present on other stars. We
come to know this by a technique called speckle interferometry. Internal structures of
the sun and other stars cannot be directly observed but some studies indicate convection
currents and layers of increasing density and temperature. Stars consist mainly of
hydrogen and helium, with varying amounts of other elements. Star brightness is described
in terms of magnitude. 
The brightest star may be as much as 1,000,000 times brighter than the sun. The number of
stars visible to the naked eye from earth has been estimated to a total of 8000. 4000 are
visible from the Northern Hemisphere and 4000 from the Southern Hemisphere. At any time
in either hemisphere only 2000 stars are visible the other 2000 are located in the
daytime sky and are obscured by much brighter light. 
A star begins its life as a large and comparatively cool mass of gas. The contraction of
gases and rise of temperature continue until the interior temperature of the star reaches
about 1,000,000c (about 1,800,000f). At this point a nuclear reaction takes place in
which the nuclei of hydrogen atoms combined with heavy hydrogen deuterons form the
nucleus of the inert gas called helium. The later reaction liberates large amounts of
nuclear energy and further contractions of the stars are halted. When the release of
energy from the deuteron-hydrogen nucleus reaction ends it begins a new until it reaches
a point at which a nuclear reaction occurs between hydrogen and lithium and other light
materials. Again energy is released and contraction stops. When the lithium and other
light materials are consumed contraction resumes and stars enter the final stage of
development in which hydrogen is transformed into helium at extremely high temperatures
through the action of carbon and nitrogen. This thermonuclear reaction is characteristic
of the main sequence of stars. The star gradually swells and becomes a red giant. It
becomes its greatest size when all its hydrogen has been converted into helium. If it
wants to continue shining like a star its temperature must rise high enough to cause
fusion of the helium nuclei. During this process the star probably becomes much smaller
and dense.
When it has exhausted all possible sources of nuclear energy it may contract further and
become a white dwarf. This final stage may be called a stellar explosion. When a star
sheds its outer shell it returns to the interstellar medium elements which it started off
as. Stars that shed their outer shell in a nonexplosive reaction become planetary nebulas
or old stars surround by spheres and radiate gases. Massive stars run through their cycle
of evolution rapidly. This may cause them to become a neutron star. Such stars are
gravitationally bound to keep contracting until they become black holes, from which light
can not escape. The final fate of low-mass dwarfs is unknown, except they cease to
radiate.
The birth of stars is connected with the presence of dust grains and molecules, as in the
Orion nebula region of earth's galaxy. Here, molecular hydrogen is compressed to high
densities and temperatures at which the molecules spread apart. The atomic hydrogen then
recollapes and forms a dense stellar core that attracts materials. The hot core dispels
the overlying molecules and a new star emerges. Further gravitational heating raises the
temperature until nuclear processes can occur. Stars are generally born in to small
groups at one end of a large molecular cloud. Successive generations of stars eat into
the edge of the cloud more and more, leaving a trail of stars of increasing ages. 
Some different classes of stars are as followed:
Class O-which is primarily characterized by the lines of helium, oxygen, and nitrogen,
besides the hydrogen lines. The O group is composed of extremely hot stars, this
including the bright -line spectra of hydrogen and helium, as well as dark lines of the
same elements.
Class B-in this group the helium lines attain maximum intensity at the subdivision B2 and
fade progressively in to higher subdivisions. The intensity of the hydrogen lines
steadily increases throughout the subdivisions. The group is typified by the star Epsilon
Orionis.
Class A-this group comprises the so -called hydrogen stars with spectra dominated by the
absorption lines of hydrogen. A typical star of the group is Sirius, the Dog Star.
Class F-this group comprises stars in which the so-called H and K lines of calcium and
characteristic lines of hydrogen are strong. A notable star for this is Aquilae.
Class G-This group of stars is prominent with H and K calcium lines and less prominent
hydrogen lines. The spectra of many metals, notably iron, are also present. The sun
belongs to this group, and the G stars are therefore frequently called solar stars.
Class K-this group comprises stars having strong calcium lines and line indicating the
presence of other materials and metals. The violet light of the spectrum is less intense,
compared with the red light, than in the classes previously mentioned. The group is
typified by Arcturus.
Class M-This group comprises stars with spectra dominated by bands resulting from the
presence of metallic-oxide molecules, notably those of titanium oxide. The violet end of
the spectra is less intense than that in the K stars. The star Orionis is very typical of
this group.
All these characteristics are compatible with the conclusion that stars of these classes
are all similar chemicals composed and arranged in temperature order from hottest to
coolest. The absolute surface temperature of the various star groups are as followed:
O- 22,200c
B-13900c
A-10000c
F-6650c
G-5540c
K-3870c
M-1760c
All go from hottest too coolest. The interior temperature of the average star is about
20,000,000c. 
Scientists have found a lot out about stars and we even have more to figure out.