Download Basics of Astronomy - Surrey Sports Park

Asteroids
What we refer to as an ‘Asteroid’ is actually one of the small or minor planets that make
up the solar system and move in elliptical orbits primarily between the orbits of Mars and
Jupiter.
Most of the meteorites recovered on earth are thought to be fragments of asteroids. They
reveal that asteroids, like meteorites, can be classified into a few distinct types.
C Type
About 75 % of the asteroids visible from earth, belong to the ‘C type’, which appear to
be related to a class of stony meteorites known as carbonaceous chondrites.
S Type
Asteroids of the ‘S Type’, related to the stony iron meteorites, make up about 15% of the
total population.
M Type
Much rarer are the ‘M Type’ objects, corresponding in composition to the meteorites
known as ‘irons’ - which consist of an iron-nickel alloy.
Achondrites
The ‘Achondrites’ are the rarest class of meteorite - very few asteroids are related to this
type.
Comets
A 'Comet' (Latin name 'stella cometa', meaning hairy star) is a nebulous celestial body
revolving around the Sun. A comet is characterised by a long, luminous tail, but only in
the segment of the comet's orbit when it passes closest to the Sun.
Appearances of large comets were regarded as atmospheric phenomena until 1577, when
the Danish astronomer Tycho Brahe proved that they were, in fact, celestial bodies. In the
17th century the British scientist Sir Isaac Newton demonstrated that the movements of
comets are subject to the same laws that control the planets in their orbits.
By comparing the orbital elements of a number of earlier comets, British astronomer
Edmund Halley showed the comet of 1682 to be identical with the two that had appeared
in 1607 and 1531, and he successfully predicted the return of the comet in 1759. This
comet is now known as Halley's comet and has been identified from records dating from
as early as 240 BC.
A comet is generally considered to consist of a small, sharp nucleus embedded in a
nebulous disk called the coma. American astronomer Fred Whipple proposed in 1949 that
the nucleus, containing practically all the mass of the comet, is a "dirty snowball"
conglomerate of ices and dust.
Major proofs of the snowball theory rest on various data. For example, of the observed
gases and meteoric particles that are ejected to provide the coma and tails of comets, most
of the gases are fragmentary molecules, or radicals, of the most common elements in
space: hydrogen, carbon, nitrogen, and oxygen. The radicals, for example, of CH, NH,
and OH may be broken away from the stable molecules CH4 (methane), NH3 (ammonia),
and H2O (water), which may exist as ices or more complex, very cold compounds in the
nucleus. Another fact in support of the snowball theory is that the best-observed comets
move in orbits that deviate significantly from Newtonian gravitational motion. This
provides clear evidence that the escaping gases produce a jet action, propelling the
nucleus of a comet slightly away from its otherwise predictable path. In addition, shortperiod comets, observed over many revolutions, tend to fade very slowly with time, as
would be expected of the kind of structure proposed by Whipple. Finally, the existence of
comet groups shows that cometary nuclei are fairly solid units.
The head of a comet, including the hazy coma, may exceed the planet Jupiter in size. The
solid portion of most comets, however, is equivalent to only a few cubic kilometers. The
dust-blackened nucleus of Halley's comet, for example, is about 15 by 4 km (about 9 by
2.5 mi) in size.
Solar Effects
As a comet approaches the Sun, the solar heat evaporates, or sublimates, the ices so that
the comet brightens enormously. It may develop a brilliant tail, sometimes extending
many millions of kilometers into space. The tail is generally directed away from the Sun,
even as the comet recedes again. The great tails of comets are composed of simple
ionized molecules, including carbon monoxide and dioxide. The molecules are blown
away from the comet by the action of the solar wind, a thin stream of hot gases
continuously ejected from the solar corona, the outermost atmosphere of the Sun, at a
speed of 400 km (250 mi) per sec. Comets frequently also display smaller, curved tails
composed of fine dust blown from the coma by the pressure of solar radiation.
Black Holes
The ‘Black Hole’ is a theoretical concept in the study of gravitation. A Black Hole is an
extremely dense body and its gravitational field is so strong that, if the body is large
enough, nothing, including electromagnetic radiation, can escape from its vicinity. The
Black Hole derives its name from the spherical boundary (called a ‘horizon’) which
surrounds the body. This ‘horizon’ appears totally black as it allows light to enter but not
escape.
Properties
The concept of a black-hole was developed by the German astronomer Karl
Schwarzschild in 1916, based on Albert Einstein's theory of general relativity. According
to general relativity, gravitation severely modifies space and time near a black hole. As
the horizon is approached from outside, time slows down relative to that of distant
observers, stopping completely on the horizon. Once a body has contracted within its
Schwarzschild radius, it would theoretically collapse to a singularity, that is, a
dimensionless object of infinite density.
Formation
Black holes may form during the course of stellar evolution. As nuclear fuels are
exhausted in the core of a star, the pressure associated with their heat is no longer
available to resist contraction of the core to ever higher densities. Two new types of
pressure arise at densities a million and a million billion times that of water, respectively,
and a compact white dwarf or a neutron star may form. If the core mass exceeds about 1.7
solar masses, however, neither electron nor neutron pressure is sufficient to prevent
collapse to a black hole.
Physicists such as Stephen Hawking have suggested that many black holes may have
formed in the early universe and they could even compose a significant fraction of the
total mass of the universe. Hawking has also proposed that black holes do not collapse
but instead form “worm holes” to other universes besides our own.
Stars
A ‘Star’ is a large celestial body composed of gravitationally contained hot gases emitting
electromagnetic radiation, especially light, as a result of nuclear reactions inside the star.
The sun is a star. With the exception of the sun, stars appear to be fixed, maintaining the
same pattern in the skies year after year. However, stars are actually in rapid motion, but
their distances are so great that their relative changes in position become apparent only
over the centuries.
The number of stars visible to the naked eye from earth has been estimated to total 8000,
of which 4000 are visible from the northern hemisphere and 4000 from the southern
hemisphere. At any one time in either hemisphere, only about 2000 stars are visible. The
other 2000 are located in the daytime sky and are obscured by the much brighter light of
the sun.
Astronomers have calculated that the stars in the Milky Way, number in the hundreds of
billions. The Milky Way, in turn, is only one of several hundred million such galaxies
within the viewing range of the larger modern telescopes. The individual stars visible in
the sky are simply those that lie closest to the solar system in the Milky Way.
The star nearest to our solar system is the triple star ‘Proxima Centauri’, which is about
40 trillion km (about 25 trillion mi) from earth.