Geography is the study of the Earth as a home for mankind. It deals with the material and human phenomena in the space accessible to human beings and their instruments especially the patterns of and variation in their distribution in that space on all scales in the past or present.

THEORIES OF SPACE

Modern theories of the Universe are based on this flight of galaxies, that is, on the assumption that matter is in a state of rapid expansion.

The Expanding Universe

It is a general law that all material bodies are heated when compressed and cooled when expanded. The primordial Universe, being highly compressed, must have experienced high temperatures.  Heat, as we know, tends to expand matter.  High temperatures, therefore, must have, at some point, started an expansion of the Universe.  It is this expansion which is continuing even now. All theories of space (Universe) seek to explain the nature and consequences of this expansion.

Big-Bang Theory

In evolving cosmological theories, the first credit goes to a Belgian astronomer-priest, Abbe Georges Lemaitre.  He explained this process of expansion, in what is known as ‘the evolutionary theory’ or ‘the big-bang theory’. He argued that billions of years ago, cosmic matter (Universe) was in an extremely compressed state, from which expansion started by a primordial explosion.  This explosion broke up the super dense and super hot ball and cast its fragments far out into space, where they are still travelling at thousands of miles per second.  It is from these speeding fragments of matter that our galaxies have been formed.  The formation of galaxies and stars has not halted the speed of expansion.  And, as it happens in all explosions, the farthest pieces are flying the fastest.

The primordial explosion is the hallmark of the big-bang theory.  It also differs from other theories in two important respects: (i) it disagrees with the Steady State claim, that new matter is being continuously created in the Universe; (ii) it differs from the Pulsating theory, in that it does not admit, that matter will revert to the original congestion point, from which the primordial explosion started.

Steady State Theory

This theory originally advanced by two astronomers, Hermann Bondi and Thomas Gold, has since received support from the British astronomer of Cambridge University.  According to this theory, which is also known as the Continuous Creation Theory, galaxies recede from one another but their spatial density remains constant.  The Universe everywhere and everytime remained relatively uniform, unchanged, without beginning or end.  That is to say, as old galaxies move apart new galaxies are being formed in the vacancies.  These new galaxies are formed from new matter which is being continuously created to replace old matter that is being dispersed.  This concept, dispersed to get around the philosophic hurdle of a Universe with finite beginning and end, is known as the ‘Steady State Theory’.

Pulsating (Oscillating) Universe Theory

According to this theory, advocated among others by Dr. Alan Sandage, the Universe expands and contracts alternately between periods running into tens of billons of years.  Dr. Sandage thinks that some 12 billion years ago a great explosion occurred in the Universe and that the Universe has been expanding ever since.  It is likely to go on expanding for 29 billion years more, when gravitation will halt further expansion.  From then on, all matter will begin to contract or collapse upon itself in a process known as ‘implosion’.  This will go on for 41 billion years compressing matter into an extremely super dense and super heated state and then it will explode once again.  This is the latest theory of the evolution of the Universe.

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SOLAR SYSTEM

The Solar System consists of the Sun, the eight planets, their satellites and other bodies[1] revolving around it. However, the size, orbit, the period of revolution and rotation, the number of sub satellites of these planets are different. The sun due to its gravitation attracts these planets etc. towards its centre. But due to the revolution of these bodies around the Sun these bodies develop a force opposite to the attracting force of the sun. The attracting force is called centripetal force and the force acting in opposite direction is called centrifugal force. As a result of balance between them, the planets etc. keep on revolving around in different orbits.

Order of the Planets

The planet closest to the sun is Mercury and that farthest away is Neptune. The order of the planets, according to their distances from the Sun is as follows.

 (1) Mercury, (2) Venus, (3) Earth, (4) Mars, (5) Jupiter, (6) Saturn, (7) Uranus,  and (8) Neptune.

The planet largest in size is Jupiter and the one with the smallest size is Mercury. The order of the planets, according to their sizes is given below:

(1) Jupiter (2) Saturn, (3) Neptune, (4) Uranus, (5) Earth (6) Venus (7) Mars, (8) Mercury.

The solar system has a diameter of about 11,790 million km. Its planets can precisely be grouped into two divisions:

(i)      Inner Planets: These planets (Mercury, Venus, Earth and Mars) are smaller in size but have higher densities. They are also known as Terrestrial Planets.

(ii) Outer Planets-These planets (Jupiter, Saturn, Uranus, and Neptune) are bigger in size but have lower densities. They are also known as Jovian Planets.

Presently there are 8 planets in our solar system^[2]

Sub Planets

There are sub planets (satellites) revolving round the planets of the Sun. Moon is such a satellite, which revolves round the Earth. Jupiter has as many as 12 sub-planets. Only mercury and Venus have no sub-planets.

Some facts about planets

• All the planets revolve around the sun from west to east except for Uranus which revolves east to west.
• All the planets rotate from west to east on their axis. The exceptions to this rule are Venus and Uranus. Venus rotates from east to west and Uranus due to its extreme axial tilt of 98 degrees appears to rotate from north to south.
• Venus takes 224 days to revolve around the sun and around 243 days to rotate on its own axis thereby making its day longer than its year.
•  Venus is sometimes called Earth's twin because Venus and Earth are almost the same size, have about the same mass (they weigh about the same), and have a very similar composition (are made of the same material). They are also neighboring planets.
• However, Venus and Earth are also very different. Venus has an atmosphere that is about 100 times thicker than Earth's and has surface temperatures that are extremely hot. Venus does not have life or water oceans like Earth do.
• Despite being the closest planet to sun, Mercury is not the hottest. This is so because it has no atmosphere to trap heat. Venus due to presence of an atmosphere and proximity to sun is the hottest planet of solar system.
• Density wise, Saturn is the lightest planet. It also has 7 rings made of primordial dust, gases and ice particles.
• Mars, very much like earth is tilted at an angle of 24 degrees on its axis and as a result has seasons.

Milky Way

It is a larger system of stars of which our solar system is a part. This system of stars appears as an arch in a clear night. It is known as Milky Way in Europe and Galaxy in Greek. The Milky Way has been compared to a flat disc whose diameter is 100,000 light years and its thickness is 5,000 light years. It has a nucleus at its centre. A large number of stars exist in the spiral arms, which emerge from its nucleus and revolve around it.  Our Sun is an ordinary star situated on a spiral arm called Orion arm of the Milky Way. It is situated at a distance of 56,000 light-years from its nucleus. The sun completes one revolution around the centre of the Milky Way in 250 million years.

THE SUN

The Sun is the most important member of the Solar system. It comprises 99.86% of the total matter of the Solar system. Its surface temperature is 6000⁰ Centigrade and the temperature in its interior is likely to be 15 million degrees centigrade. It is the largest member of the Solar system. From the point of   view of mass, it is 330,000 times that of the Earth. Its diameter is 109 times that of the Earth. It is 149 million km away from the Earth and light takes about 8.3 minutes to reach the Earth from the Sun. That is why the Sun looks so small. The weight of the Sun is 2x10²⁷ tones.

The Sun is one of more than 100 billion stars in the Milky Way.  The Sun is the centre of the Solar System.  Its mass is about 740 times as much as that of all the planets combined.  The huge mass of the Sun creates the gravitation that keeps the other objects traveling around it in an orderly manner.  Modern estimates place the Sun at a distance of about 56,000 light years from the centre of the galaxy.

The Sun continuously gives off energy in several forms – “visible light”, “invisible infra-red”, “ultraviolet”, “X-rays” and “gamma rays”, “cosmic rays”, “radio waves” and “plasma”.

The Sun and the neighbouring stars generally move in almost circular orbits around the galactic centre at an average speed of about 250 km per second.

The Sun at this rate takes 250 million years to complete on revolution round the centre.  The period is now called a COSMIC YEAR.

Like all other stars, the Sun is composed mainly of hydrogen. Sun uses Nuclear Fusion process to create energy. In Nuclear Fusion, Hydrogen nucleus combines to form helium nucleus. It is calculated that the Sun consumes about a trillion pounds of hydrogen every second.  At this rate, it is expected to burn out its stock of hydrogen in about 5 billion years and turn into a RED GIANT.  

When the Sun turns into a Red Giant, it would have swelled a hundred times in diameter and increased a thousand times in brightness – “bright red”. It will then occupy about 25 per cent of the horizon.  The nearest planets, Mercury and Venus, would melt.  The oceans of the earth would evaporate and disappear.  The earth would remain a barren rock, heated to melting point of lead.  All life on earth would cease.  The Sun will survive as a ‘red-giant’, for about a hundred million years more, slowly dissipating its enlarged outer shell leaving a tiny core. This core will be a faint, white dwarf-sun no larger than the present planet Mars.  Around this tiny star, the burnt-out earth will continue to revolve.

SOLAR CHEMISTRY
Elements	Per cent	Per cent
Hydrogen	81.760	70.7
Helium	18.170	9.1
Oxygen	0.03	0.09
Magnesium	0.02	-
Nitrogen	0.01	0.01
Silicon	0.006	-
Carbon	0.003	0.05
Iron	0.0008	0.007
Calcium	0.0003	less than 0.01
Neon	-	0.01

 

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Structure of the Sun

Structure of Sun

The Sun has a core at its center; a radiative zone surrounding the core; a convective zone surrounding the radiative zone; a thin photosphere at its surface; and a chromosphere and corona that extends beyond the photospheric surface.

Each of these zones are briefly discussed here:

Core

Solar energy is produced at the core of the sun where temperatures reach 15 million °C by nuclear fusion. This enormous energy makes the sun shine.

Radiative Zone

Energy produced in core slowly rises in the radiative zone outside the core. It takes around one million years for energy to travel out of the radiative zone.

Convection Zone

Convection zone is just beneath the Sun’s surface.

Photosphere

Photosphere is the visible surface of Sun where temperature is around 5500°C. This part gives us light, which takes around 8 minutes to reach from sun to earth.

Chromosphere

Chromosphere is a thin layer of gas above the photosphere. Along with Corona, it makes the atmosphere of Sun.

Corona

Corona is a thick layer of gas above chromosphere. It extends millions of kilometers around the sun. Corona and Chromosphere are visible during a total solar eclipse when the sun’s surface is completely hidden behind moon. We note here that Corona is much dimmer than the rest of the Sun, and can only be seen when the Sun is blocked from view—either by a scientific instrument called a coronagraph, or naturally during a solar eclipse. Even though it is thinner than the best laboratory vacuums on Earth and so far away from the Sun’s core, the corona is very energetic and very hot, with its plasma reaching temperature of millions of degrees. The scientists still have not been able to figure out how the corona gets so hot. Current research suggests that the strong electrical currents and magnetic fields in and around the Sun transfer tremendous amounts of energy to the corona, either generally or by special “hotspots” that form for short periods of time and then disappear again.

Composition of Sun

The Sun’s mass is composed of 71 percent hydrogen, 27 percent helium, and 2 percent other elements.

In terms of the number of atoms in the Sun, 91 percent are hydrogen atoms, 9 percent are helium atoms, and less than 0.1 percent are atoms of other elements. Most of the stars in the universe have similar chemical composition.

Mass of Sun

The Sun has a mass of 1.99 million trillion trillion kilograms. The most massive supergiant stars have about one hundred times more mass than the Sun. The least massive dwarf stars and brown dwarfs contain about one-hundredth the mass of the Sun.

Rotation of Sun

Sun rotates about its axis from west to east. Since the Sun is not a solid object but rather a big ball of electrically charged gas, it spins at different speeds depending on the latitude.

The Sun spins once around its axis near its equator in about 25 days, and in about 35 days near its north and south poles. This kind of spinning, in which different parts move at different speeds, is called differential rotation.

Implications of Sun’s Spin

Magnetic fields in the Sun, created by strong electric currents, are produced because of the Sun’s spin. Since Sun has differential rotation, and its interior roils with tremendous heat and energy, the magnetic field lines in the Sun get bent, twisted, knotted, and even broken; sunspots, prominences, solar flares, and coronal mass ejections are the result.

Beyond this layer (chromosphere) is the magnificent CORONA of the Sun which is visible during eclipses only, as a remarkable silver pearly radiant glow around the Sun.  The inner part of the Corona which is the brightest, gives a continuous spectrum on which there are superimposed bright lines.

Between the chromosphere and the Corona, spectroscopic investigations have identified a distinct, very narrow boundary zone known as the transition region. The temperature of the photosphere is about 6000° Celsius, that of the chromosphere about 32,400° Celsius, that of the transition region about 324,000° Celsius, and that of the Corona, which extends far into space, about 2,700,000° Celsius hot enough to emit x-rays. (The density of the gas in each layer decreases with increasing altitude, just as the earth’s atmosphere thins with height.  The corona, accordingly, is the least dense of the Sun’s layer). It is sometime said for short that 6,000° Celsius is the temperature of the Sun, although the temperature and density of the gases of the Sun vary with depth.

At the core of the Sun where thermonuclear reactions take place the temperature level is around 15 million degrees centigrade.  The density of the core is estimated at about 150 gm/cm3. Outside the core is the convection zone.  Here, like the boiling water in a kettle, turbulent motions of gases transport the energy that is generated in the core towards the photosphere.

The visible while light of the corona is made up of a continuum of colours, such as violet, indigo, blue, green, yellow, orange and red.  Superimposed on this continuum are hundreds of dark lines called the FRAUNHOFER LINES.  Each line indicates some element present in the solar atmosphere.  The intensity and width of the lines reveal the temperature and density of the element.

Moon

The Earth’s Moon appears a beautiful celestial body.  It is the only satellite or sub-planet of the Earth.  But it is a satellite

Some Important facts about the Moon
1.	Distance from the Earth	3,82,200 Km
2.	Diameter	3,475 Km
3.	Ration of Gravitational Pull of Moon & Earth	1 : 6
4.	Atmosphere	Absent
5.	Highest Mountain	Leibnitz Mts.( 35,000 fts)
6.	Time taken by moonlight to reach the Earth	1.3 seconds

 of distinction.  For, when compared to the size of earth, it is far too big to be its satellite.  All other satellites have sizes ranging below 1/8 of the sizes of mother planets.  But the moon is about ¼ of the size of the mother planet, the Earth, and one-eightieth of its mass. Its gravity being one-sixth of the Earth’s that is its gravitational pull is six times weaker.

Origin of the Moon

The incompatibility of the relative sizes of the Earth and Moon, and their separate existence at such close quarters led to the conjecture that the Moon is not a true satellite but was captured by the Earth during a close approach to the Earth.

(i)      First Theory:  The theory of the origin of the Moon is known as the SPOUSE THEORY.  It states that the Moon came from elsewhere in the solar system and sweeping too near, it was snared by the Earth’s gravity and “MARRIED” – that is locked into orbit. But some scientists claim that the moon is far too big to be captured by Earth’s gravity.

(ii)     Second Theory:  Second theory known as the DAUGHTER THEORY” says that the Earth once rotated so rapidly that it became blimp shaped and was torn into two, the smaller blob, entering into orbit as the Moon.  But some scientists contest this theory by highlighting the geological dissimilarity between the two.

(iii)    Third Theory:   The third theory – the “SISTER THEORY” – suggests that the Earth and the Moon were formed more or less at the same time from the original wheeling cloud of cosmic gas that ultimately condensed into the planets and the satellites.

But today most of the scientists believe in the Big Splat or Giant Impact theory for moon’s formation. As per this theory, around 4.4 billion years ago a body of size one to three times that of Mars collided with the earth. This collision blasted a large part of earth into the space which continued revolving around the earth and eventually took the form of moon.

Dimensions

The Moon has a diameter of 3475 kilometers as against the Earth’s 12700 kilometres.  But it has a surface less than half that of the Atlantic Ocean.  Therefore, its gravitational pull is about one-sixth of the Earth’s.  Because the orbit of the Moon round the Earth is not circular but elliptical, the maximum distance (APOGEE) which the moon may keep from the Earth is 406000 km and the minimum distance (PERIGEE) 364000 Kilometre.  The Moon revolves round the Earth in 27  1/3 days (27 day 7 hours 43 minutes and 11.47 seconds) and rotates on its own axis in exactly the same time.  This is why we see only one side of the Moon.

Topography

The near side (front side) of the Moon seems to be made up of bright and dark patches.  The bright parts are the mountains and highlands that receive the Sun’s rays, while the darker patches are low-lying plains.  These were once thought to be seas (marias) and named accordingly, though the Moon is devoid of water.  The craters are depression caused by the onslaught of meteors. They vary in size.  The moon has raised high sharp-peaked mountains, many of them rising to 6000 metres.  The highest of these are “Liebnitz  Mountains”, near the Moon’s south pole which rise to 10,660 metres higher than Mount Everest.

Weather and Climate

The Moon has no atmosphere, as its gravitational power is too weak to hold down gases.  It causes many strange phenomena.  There is no twilight, the day dawning suddenly, as there is no atmosphere to be lit up before the Sun comes over the horizon. There is no sound either as sound is a vibration transmitted through air.

The lunar day is two Earth-weeks long and during this time its surface at the equator heats up to 125 degree Celsius.  But during the two-week night the temperature drops down to minus 150 degree Celsius.  The excessively small thermal conductivity of moon is because the surface layer of the Moon is porous, pumice like matter.

 

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Eclipse

In astronomy, an eclipse is the obscuring of one celestial body by another, particularly that of the sun or a planetary satellite. Two kinds of eclipses involve the earth: those of the moon, or lunar eclipses; and those of the sun, or solar eclipses. A lunar eclipse occurs when the earth is between the sun and the moon and its shadow darkens the moon. A solar eclipse occurs when the moon is between the sun and the earth and its shadow moves across the face of the earth. Transits and occultations are similar astronomical phenomena but are not as spectacular as eclipses because of the small size of these bodies as seen from earth.

Lunar Eclipses

The earth, lit by the sun, casts a long, conical shadow in space. At any point within that cone the light of the sun is wholly obscured. Surrounding the shadow cone, also called the umbra, is an area of partial shadow called the penumbra. The approximate mean length of the umbra is 1,379,200 km (857,000 mi); at a distance of 384,600 km (239,000 mi), the mean distance of the moon from the earth, it has a diameter of about 9170 km (about 5700 mi).

A total lunar eclipse occurs when the moon passes completely into the umbra. If it moves directly through the center, it is obscured for about 2 hours. If it does not pass through the center, the period of totality is less and may last for only an instant, if the moon travels through the very edge of the umbra.

A partial lunar eclipse occurs when only a part of the moon enters the umbra and is obscured. The extent of a partial eclipse can range from near totality, when most of the moon is obscured, to a slight or minor eclipse, when only a small portion of the earth’s shadow is seen on the passing moon. Historically, the view of the earth’s circular shadow advancing across the face of the moon was the first indication of the shape of the earth.

Before the moon enters the umbra in either total or partial eclipse, it is within the penumbra and the surface becomes visibly darker. The portion that enters the umbra seems almost black, but during a total eclipse, the lunar disk is not completely dark; it is faintly illuminated with a red light refracted by the earth’s atmosphere, which filters out the blue rays. Occasionally a lunar eclipse occurs when the earth is covered with a heavy layer of clouds that prevent light refraction; the surface of the moon is invisible during totality.

Solar Eclipses

The length of the moon’s umbra varies from 367,000 to 379,800 km (228,000 to 236,000 mi), and the distance between the earth and the moon varies from 357,300 to 407,100 km (222,000 to 253,000 mi). Total solar eclipses occur when the moon’s umbra reaches the earth. The diameter of the umbra is never greater than 268.7 km (167 mi) where it touches the surface of the earth, so that the area in which a total solar eclipse is visible is never wider than that and is usually considerably narrower. The width of the penumbra shadow, or the area of partial eclipse on the surface of the earth, is about 4828 km (about 3000 mi). At certain times when the moon passes between the earth and the sun, its shadow does not reach the earth. At such times an annular eclipse occurs in which an annulus or bright ring of the solar disk appears around the black disk of the moon.

The shadow of the moon moves across the surface of the earth in an easterly direction. Because the earth is also rotating eastward, the speed of the moon shadow across the earth is equal to the speed of the moon traveling along its orbit, minus the speed of the earth’s rotation. The speed of the shadow at the equator is about 1706 km/hr (about 1060 mph); near the poles, where the speed of rotation is virtually zero, it is about 3380 km/hr (about 2100 mph). The path of a total solar eclipse and the time of totality can be calculated from the size of the moon’s shadow and from its speed. The maximum duration of a total solar eclipse is about 7.5 minutes, but these are rare, occurring only once in several thousand years. A total eclipse is usually visible for about 3 minutes from a point in the center of the path of totality.

In areas outside the band swept by the moon’s umbra but within the penumbra, the sun is only partly obscured, and a partial eclipse occurs.

At the beginning of a total eclipse, the moon begins to move across the solar disk about 1 hour before totality. The illumination from the sun gradually decreases and during totality (and near totality) declines to the intensity of bright moonlight. This residual light is caused largely by the sun’s corona, the outermost part of the sun’s atmosphere. As the surface of the sun narrows to a thin crescent, the corona becomes visible. At the moment before the eclipse becomes total, brilliant points of light, called Bailey’s beads, flash out in a crescent shape. These points are caused by the sun shining through valleys and irregularities on the lunar surface. Bailey’s beads are also visible at the instant when totality is ending, called emersion. Just before, just after, or sometimes during totality, narrow bands of moving shadows can be seen. These shadow bands are not fully understood but are thought to be caused by irregular refraction of light in the atmosphere of the earth. Before and after totality, an observer on a hill or in an airplane can see the moon’s shadow traveling eastward across the earth’s surface like a swiftly moving cloud shadow.

Frequency of Eclipses

If the earth’s orbit, or the ecliptic, were in the same plane as the moon’s orbit, two total eclipses would occur during each lunar month, a lunar eclipse at the time of each full moon, and a solar eclipse at the time of each new moon. The two orbits, however, are inclined, and, as a result, eclipses occur only when the moon or the sun is within a few degrees of the two points, called the nodes, where the orbits intersect.

Periodically both the sun and the moon return to the same position relative to one of the nodes, with the result that eclipses recur at regular intervals. The time of the interval, called the saros, is a little more than 6585.3 days or about 18 years, 9 to 11 days (depending on the number of intervening leap years) and 8 hours.

During one saros about 70 eclipses take place, usually 29 lunar and 41 solar; of the latter, usually 10 are total and 31 partial. The minimum number of eclipses that can occur in a given saros year is 2, the maximum 7, and the average is 4.

Observation of Eclipses

Many problems of astronomy can be studied only during a total eclipse of the sun. Among these problems are the size and composition of the solar corona and the bending of light rays passing close to the sun because of the sun’s gravitational field (see Relativity). The great brilliance of the solar disk and the sun-induced brightening of the earth’s atmosphere make observations of the corona and nearby stars impossible except during a solar eclipse. The coronagraph, a photographic telescope, permits direct observation of the edge of the solar disk at all times. Today, scientific solar eclipse observations are extremely valuable, particularly when the path of the eclipse traverses large land areas. An elaborate network of special observatories may provide enough data for months of analysis by scientists. Such data may provide information on how minute variations in the sun affect weather on earth, and how scientists can improve predictions of solar flares[3].

 

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Space related terms

Comet

It is a nebulous celestial body revolving around the sun. A comet is characterized by a long, luminous tail, but only in the segment of the comet’s orbit when it passes closest to the sun. A comet is generally considered to consist of a small, sharp nucleus embedded in a nebulous disk called the coma. 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. 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.

As a comet recedes from the sun, the loss of gas and accompanying dust decreases in quantity, and the tails disappear. Some of the comets with small orbits have tails so short that they are practically invisible. On the other hand, the tail of at least one comet has exceeded 320 million km (200 million mi) in length. The variation in length of the tail, together with the closeness of approach to the sun and the earth, accounts for the variation in the visibility of comets. Of some 1400 comets on record, fewer than half the tails were visible to the naked eye, and fewer than 10 percent were conspicuous.

In 1992 Comet Shoemaker-Levy 9 broke apart into 21 large fragments as it ventured into the strong gravitational field of the planet Jupiter. During a week-long bombardment in July 1994, the fragments crashed into Jupiter’s dense atmosphere at speeds of about 210,000 km/hr (130,000 mph). Upon impact, the tremendous kinetic energy of the comets was converted into heat through massive explosions, some resulting in fireballs larger than the earth.

SOME FAMOUS COMETS
Year	No.	Name of Comet	Period, Year
1744		De Cheseaux’s Comet	-
1808		Biela’s coment	6.7
1811	I	Great Comet of 1811	3000
1812		Di Vico’s Comet	70.7
1815		Olber’s Comet	74.0
1819	I	Encke’s Comet	3.3
1819		Pons-Winnexke Comet	6.0
1835	III	Halley’s Comet	76.3
1843	I	Great Comet of 1843	512.4
1844	II	Great Comet 1844	102.050
1858	VI	Donati Comet	2040 (?)
1864	II	Great Comet of 1864	2800000
1871	III	Tuttle’s Comet	13.8
1874	III	Coggia’s Comet	6000 (?)
1879		Brorson’s Comet	5.6
1881	III	Tebbutt’s Comet	-
1889	VI	Swift’s 2nd Comet	7.0
1892	III	Holme’s Comet	6.9
1911	IV	Halley’s Comet	76.3
1925	II	Comet Schwassmann- Wachmann	16.2
1975		Comet West [4]	100000

 

Black Holes

A black hole is a fascinating and mysterious astronomical body. It is so called because it gives off no light and sucks in whatever matter and energy that comes near it. It is the end product left behind after the death of a very massive star.

All stars have a life cycle—they are born, grow old and finally die. The way they end up depends on the mass they start with. If the star is very massive—more than 30 times as massive as the sun, the end comes in a blinding explosion known as supernova. After the explosion what is left behind is a tiny object called black hole. The gravitational field on a black hole is so strong that it does not allow even light to escape. Thus, a black hole cannot be seen. However, astronomers locate black holes by the gravitational and other effects they have on nearby starts.

Nova and Supernova

In astronomy, these are the names of two kinds of explosive events that take place in some stars. A nova is a star that suddenly increases greatly in brightness and then slowly fades, but may continue to exist for some time. A supernova exhibits the same pattern of behavior, but the causative explosion destroys or profoundly alters the star. Supernovas are much rarer than novas, which are observed fairly frequently in photographs of the sky.

Novas

Before the era of modern astronomy, a star that appeared suddenly where none had been seen before was called a nova, or “new star.” This is a misnomer, as the stars involved had existed long before they became visible to the naked eye. Novas may be considered variable stars in a late stage of evolution. They apparently behave as they do because their outer layers have built up an excess of helium through nuclear reactions and expand too rapidly to be contained. The star explosively emits a small fraction of its mass as a shell of gas—the cause of the increase in brightness—and then settles down. Such a star is typically a white dwarf and is commonly thought to be the smaller member of a binary (two-star) system, subject to a continuous infall of matter from the larger star. This is perhaps always the case with dwarf novas, which erupt repeatedly at regular intervals of a few to hundreds of days.

Supernovas

A supernova explosion is far more spectacular and destructive than a nova and much rarer. Such events may occur no more than once every few years in the Galaxy; and despite their increase in brilliance by a factor of billions, only a few are ever observable to the naked eye. Until 1987, only three had been positively identified in recorded history, the best known of which is the one that occurred in AD 1054 and is now known as the Crab nebula. Supernovas, like novas, are more often seen in other galaxies. Thus, the most recent supernova, which appeared in the southern hemisphere on February 24, 1987, was found located in a companion galaxy, the Large Magellanic Cloud. This supernova, which exhibits some unusual traits, is now the object of intense astronomical scrutiny.

The mechanisms that produce supernovas are less certain than those of novas, particularly in the case of stars approximately as massive as the earth’s sun, an average star. Stars that are much more massive, however, sometimes explode in the late stages of their rapid evolution as a result of gravitational collapse, when the pressure created by nuclear processes within the star is no longer able to withstand the weight of the star’s outlying layers. Little may remain after the explosion except the expanding shell of gases. The Crab nebula has left behind a pulsar, or rapidly rotating neutron star. Supernovas are significant contributors to the interstellar material that forms new stars.

Aurora (Phenomenon)

luminous atmospheric phenomenon occurring most frequently above 60° North or South latitude, but also in other parts of the world. It is named specifically, according to its location, aurora borealis (northern lights) or aurora australis (southern lights). The term aurora Polaris, polar lights, is a general name for both.

The aurora consists of rapidly shifting patches and dancing columns of light of various hues. Extensive auroral displays are accompanied by disturbances in terrestrial magnetism and interference with radio, telephone, and telegraph transmission. The period of maximum and minimum intensity of the aurora follows almost exactly that of the sunspot cycle, which is an 11-year cycle.

Studies made during and after the 1957-58 International Geophysical Year indicate that the auroral glow is triggered when the solar wind (Solar System) is enhanced by an influx of high-energy atomic particles emanating from sunspots. The electrons and protons penetrate the magnetosphere of the earth and enter the lower Van Allen radiation belt (Radiation Belts[5]), overloading it.

Connecting Concept:  Van Allen radiation belt Van Allen radiation belt, doughnut-shaped zones of highly energetic charged particles trapped at high altitudes in the magnetic field of Earth. The zones were named for James A. Van Allen, the American physicist who discovered them in 1958, using data transmitted by the U.S. Explorer satellite. The Van Allen belts are most intense over the Equator and are effectively absent above the poles. No real gap exists between the two zones; they actually merge gradually, with the flux of charged particles showing two regions of maximum density. The inner region is centred approximately 3,000 km (1,860 miles) above the terrestrial surface. The outer region of maximum density is centred at an altitude of about 15,000 to 20,000 km (9,300 to 12,400 miles), though some estimates place it as far above the surface as six Earth radii (about 38,000 km [23,700 miles]). The inner Van Allen belt consists largely of highly energetic protons, with energy exceeding 30,000,000 electron volts. The peak intensity of these protons is approximately 20,000 particles per second crossing a spherical area of one square cm in all directions. The outer Van Allen belt contains charged particles of both atmospheric and solar origin, the latter consisting largely of helium ions from the solar wind (steady stream of particles emanating from the Sun). The protons of the outer belt have much lower energies than those of the inner belt, and their fluxes are much higher.

The excess electrons and protons are discharged into the atmosphere over an area centering on the north and south magnetic poles and extending about 20° away from them. These particles then collide with gas molecules in the atmosphere, thereby exciting the molecules and causing them to emit electromagnetic radiation in the visible portion of the spectrum.

The aurora assumes an endless variety of forms, including the auroral arch, a luminous arc lying across the magnetic meridian; the auroral band, generally broader and much more irregular than the arch; filaments and streamers at right angles to the arch or band; the corona, a luminous circle near the zenith; auroral clouds, indistinct nebulous masses, which may occur in any part of the sky; the auroral glow, a luminous appearance high in the sky, the filaments diverging toward the zenith; and curtains, fans, flames, or streamers of various shapes.

Asteroid

One of the many small or minor celestial bodies/debris that are members of the solar system and that move in elliptical orbits primarily between the orbits of Mars and Jupiter. The largest representatives are Ceres, with a diameter of about 1030 km (about 640 mi), and Pallas and Vesta, with diameters of about 550 km (about 340 mi). About 200 asteroids have diameters of more than 97 km (more than 60 mi), and thousands of smaller ones exist.

The total mass of all asteroids in the solar system is much less than the mass of the Moon. The larger bodies are roughly spherical, but elongated and irregular shapes are common for those with diameters of less than 160 km (less than 100 mi). Most asteroids, regardless of size, rotate on their axes every 5 to 20 hours. Certain asteroids may be binary, or have satellites of their own. With the exception of a few that have been traced to the moon and Mars, most of the meteorites recovered on earth are thought to be asteroid fragments. Remote observations of asteroids by telescopic spectroscopy and radar support this hypothesis. They reveal that asteroids, like meteorites, can be classified into a few distinct types.

Meteor and Meteorites

Meteors

Shooting stars, or meteors, are bits of interplanetary material falling through Earth's atmosphere and heated to incandescence by friction. These objects are called meteoroids as they are hurtling through space, becoming meteors for the few seconds they streak across the sky and create glowing trails.

Scientists estimate that about 48.5 tons (44 tonnes or 44,000 kilograms) of meteoritic material falls on the Earth each day. Several meteors per hour can usually be seen on any given night. Sometimes the number increases dramatically—these events are termed meteor showers.

Some occur annually or at regular intervals as the Earth passes through the trail of dusty debris left by a comet. Meteor showers are usually named after a star or constellation that is close to where the meteors appear in the sky. Perhaps the most famous are the Perseids, which peak around 12 August every year. Every Perseid meteor is a tiny piece of the comet Swift-Tuttle, which swings by the Sun every 135 years.

Meteorites

Chunks of rock and metal from asteroids and other planetary bodies that survive their journey through the atmosphere and fall to the ground are called meteorites. Most meteorites found on Earth are pebble to fist size, but some are larger than a building. Early Earth experienced many large meteorite impacts that caused extensive destruction.

Meteorites may resemble Earth rocks, but they usually have a burned exterior. This fusion crust is formed as the meteorite is melted by friction as it passes through the atmosphere. There are three major types of meteorites: the "irons," the "stones," and the stony-irons. Although the majority of meteorites that fall to Earth are stony, more of the meteorites that are discovered long after they fall are irons—these heavy objects are easier to distinguish from Earth rocks than stony meteorites.

Meteorites also fall on other solar system bodies. Mars Exploration Rover Opportunity found the first meteorite of any type on another planet when it discovered an iron-nickel meteorite about the size of a basketball on Mars in 2005, and then found a much larger and heavier iron-nickel meteorite in 2009 in the same region. In all, Opportunity has discovered six meteorites during its travels on Mars.

Hubble Space Telescope (HST)

The first general-purpose orbiting observatory. Named after American astronomer Edwin P. Hubble, the Hubble Space Telescope was launched on April 24, 1990. The HST makes observations in the visible and ultraviolet regions of the electromagnetic spectrum. The primary mirror of the HST has a diameter of 94.5 in (240 cm), and the optics of the telescope are designed so that, theoretically, when making a visible-light observation, the telescope can resolve astronomical objects that are an angular distance of 0.05 arc second apart. (Traditional large ground-based telescopes under very good sky conditions have an image resolution of about 0.5 arcs second.) Originally, the HST was equipped with five detectors: the Wide-Field Planetary Camera, the Faint Object Camera, the Faint Object Spectrograph, the High-Resolution Spectrograph, and the High Speed Photometer. It also has three fine guidance sensors that can be used for precision astronomy measurements such as determining the distances of stars from the earth.

After the HST was launched, scientists discovered that its primary mirror had a systematic aberration, the result of a manufacturing error. A service mission was carried out in December 1993 using the space shuttle Endeavor. A corrective optical device, called the Corrective Optics Space Telescope Axial Replacement (COSTAR), was inserted in the slot for the High Speed Photometer, which had to be removed to make room for COSTAR. The Wide-Field Planetary Camera, which had a different optical path from the other four instruments, was replaced with a second camera, which has a built-in correction for the aberration in the primary mirror. The service mission, which involved numerous intricate procedures, was successful.

Even before the aberration was corrected, the HST produced many valuable images, such as images showing mysterious dark structures in the spiral galaxy M51. Now that the HST has the resolving power it was designed to have, it is capable of performing such research as significantly improving the calculation of the rate at which galaxies are receding from the Milky Way as a function of their distances. This data could then be used to calculate the age of the universe. In June 1994 a team of American scientists announced that the HST had provided the first convincing evidence of the existence of a black hole: The acceleration of gases around the center of the galaxy M87 indicates the presence of an object with a mass 2.5 billion to 3.5 billion times greater than that of the sun. In addition, the HST provided one of the best available views of the planet Jupiter when fragments of Comet Shoemaker-Levy 9 bombarded the planet in July 1994. The HST’s detailed images of the collisions provided scientists with data for a spectral analysis of the chemical makeup of Jupiter’s atmosphere.

Pulsars and Neutron Stars

A number of distinct sources of radio pulses, referred to as pulsars, have been discovered with radio telescopes. Typical pulsation periods of the pulsars are near 1 sec. The periods range from several seconds to a tiny fraction of a second, as confirmed by optical and X-ray observations. The pulsation periods are so constant that only the most precise clocks can detect a slight increase in the average pulse interval for several pulsars; this increase indicates that it would take approximately 1 million years for typical periods to double.

The evidence strongly suggests that pulsars are rotating neutron stars with diameters of perhaps only about 16 km (about 10 mi). Probably they rotate once per pulsation period. Their density is so enormous that if the volume of the ball on a ballpoint pen were packed with neutrons, as in a pulsar, it would contain more than 91,000 metric tons of mass.

Quasar

Acronym for quasi-stellar radio source, any of the blue, star like objects that are strong radio emitters and the spectra of which exhibit a strong red shift. Quasars were identified as sources of intense radio emission in the late 1950s. In 1960, using the 200-in. (508-cm) telescope on Mount Palomar in California to observe the positions of these radio sources, astronomers discovered objects the spectra of which showed emission lines that could not be identified. In 1963 the Dutch-American astronomer Maarten Schmidt discovered that these unidentified emission lines in the spectrum of quasar 3C 273 were known lines that exhibited a far stronger red shift than in any other known object.

LIFE OF STARS

Stars have its own light. Stars pass through a definite evolutionary sequence in the following manner, depending primarily on their mass and internal structure:

1.       Proto Star:  it begins to form by the compression of gases and dust particles. Compression generates heats which in turn causes hydrogen to be converted into helium in a nuclear fusion, thereby emitted large amount of heat and light. Thus, star is formed.

2.       Red Giant: Continued nuclear fusion becomes increasingly heavy, resulting into swelling and reddening of outer regions. Such stars of gigantic dimensions are rightly termed as Red Giant. This stage gives the first indication of aging.

3.       Novae and Supernovae: A giant star phase may end in a novae or supernovae stage. When brightness increases to 20 magnitudes or more it is called Supernovae.

4.       White Dwarfs: A novae or supernovae explosion in a small star like our Sun may leave behind a very dense core of that star. A star of this size cools and contracts to become white dwarf. Which is no bigger than the Earth.

5.       Neutron Star: When a novae or supernovae explosion in a bigger star than the Sun but not more than twice as big is known as Neutron Star.

6.       Black Hole: Stars having mass greater than three times that of Sun because of their great gravitational power, have contracted so much that they have developed super density. It is so dense that nothing, not even light can escape from its gravity and hence called black hole.

Unit of Measurement

Light Year: A light year is a unit of distance. It is the distance that light can travel in one year and it is 9.46 X 10¹². Light moves at a velocity of about 3, 00, 000 km per second. Solar system is less than one light day across.

Astronomical Unit (AU):  the Astronomical Unit is the average distance between the Sun and Earth. It is used to measure distances within the solar system. 1 light year = 63.24 X 10³  AU.

Parsec: it is a unit of length used in astronomy. The length of the parsec is based on the method of trigonometric parallax, one of the oldest methods for measuring the distance to stars. The actual length of the parsec is approximately 3.262 light years.

Space activity in India

Dr. Vikram Sarabhai was the leader of Indian space activities.

• Indian Space Research Organization (ISRO, 1969 Bangalore) has two major satellite systems
• Indian National Satellites (INSAT) for communication services and
• Indian Remote Sensing (IRS) satellites for management of natural resources;
• After successfully launching the Mars Orbiter Mission, India is now gearing up for its next expedition, to the Sun.
• Mars Space Laboratory (MSL) is nuclear powered American mission sent to Mars in 2011. This lab contains a rover called Curiosity Rover.
• (PSLV) for launching IRS type of satellites and (GSLV) for launching INSAT type of satellites
• In 1975, India launched its first satellite, Aryabhata
• ISRO’s Bhuvan is a satellite mapping tool similar to Google Earth and Wikimapia
• Chandrayaan-1 (2008) was India's first unmanned lunar probe.
• India’s Mars mission “Mangalyaan”took fifteen months to complete at a cost of $74 million, the cheapest Mars mission yet attempted.
• The Vainu Bappu Observatory at Kavalur has been the main optical observatory of the Institute for nighttime astronomy since the late 1960s.
• ISRO plans to carry out a mission to the Sun by the year 2019–20.The probe is named as Aditya-1 and will weigh about 400 kg. It is the First Indian-based Solar Coronagraph to study solar Corona in visible and near IR bands. The launch of the Aditya mission was planned during the high solar activity period in 2012 but was postponed to 2015–2016 due to the extensive work involved in the fabrication and other technical aspects. The main objective is to study the Coronal Mass Ejection (CME) and consequently the crucial physical parameters for space weather such as the coronal magnetic field structures, the evolution of the coronal magnetic field, etc. This will provide completely new information on the velocity fields and their variability in the inner corona having an important bearing on the unsolved problem of heating of the corona would be obtained.
• Chandrayaan-2 will be India's second mission to the Moon which will include an orbiter and Lander-rover module. Chandrayaan-2 will be launched on India's Geosynchronous Satellite Launch Vehicle (GSLV-MkIII) in the first quarter of 2019. The science goals of the mission are to further improve the understanding of the origin and evolution of the Moon.
• In its thirty ninth flight (PSLV-C37), ISRO's Polar Satellite Launch Vehicle successfully launched the 714 kg Cartosat-2 Series Satellite along with 103 co-passenger satellites today morning (February 15, 2017) from Satish Dhawan Space Centre SHAR, Sriharikota. This is the thirty eighth consecutively successful mission of PSLV.  The total weight of all the 104 satellites carried on-board PSLV-C37 was 1378 kg.

The high altitude Indian Astronomical Observatory (IAO) at Hanle in south-eastern Ladakh has High Altitude Gamma Ray (HAGAR). Hanle's high altitude, dry environment and sparse population make it an ideal place for locating a telescope.

Recently, with the launch of IRNSS-1G the seventh and final satellite of IRNSS series, India has its own Satellite based navigation system on lines of GPS (USA), GLONASS Russia), Beidou (China), GALILEO (Europe).