The minimum interval needed for two sounds to be heard distinctly is 0.1 sec. This means for the echo to be heard as a distinct sould from the original within this interval, the sound waves have to travel to the wall and back. So for these waves is reach the wall only 1/2 or 0.5 sec. is required. Within this time the sound can travel 300*0.05 m or 16.5 m where 330 m/s is the velocity of sound.
In a majority of cases a musical note consists of several different frequencies blended together. The strongest audible frequency present is called the fundamental frequency which gives the note its characteristic pitch. It is the least of the frequencies. The other frequencies are called overtones which determine the quality of the sound.
Tachometer is a device for measuring the speed of various motors i.e., the number of revolutions per minute the motor is making. The iron box acts as a magnetic shield. The magnetic flux (bundle of lines of force) passes through it and no flux crosses into the space on the other side.
In rifles spiral grooves are cut inside its barrel. When the bullet is forced along the barrel by the explosive power of the powder it is given a spin owing to the spiral groove which guides the bullet along the barrel. Spinning gives a directivity along the axis of spin, to any object in motion, and thereby the moving body does not deflect while on its course. This is known as the gyroscopic property. So a rifle is more dependable for accuracy than a smooth-bore gun.
The body as well as the liquid and beaker are falling down with the same acceleration. The liquid is not supported by the beaker and the body does not experience any upward thrust. The water drop spreads because its power of adhesion to glass is great. Int he case of the mercury drop, the cohesion of its molecules is greater than their adhesion to glass.
Due to surface tension, the surface area is reduced to the minimum and a spherical form has the least surface area for a given volume. Dialysis is an apparatus used to separate a liquid and a solid or two liquids of different densities by high speed rotation. The frictional force is greater between surfaces of the same metal than between two different metals.
Radio-telescope is an instrument used to pick up and analyse radio-frequency radiation of extra terrestrial sources.
Astronomy and Space Science
In the 2nd century A.D., Ptolmey, the great mathmatician, declared that the earth was a fixed body in the centre of the universe and that all other bodies revolve round it.
Modern astronomy was founded by Nicholas Copernicus, a polish astronomer, who postulated that the sun was the centre of the entire universe and that planets were of same size and they moved in perfect circles.
Edwin Powell Hubble (1889-1963), an American astronomer, proved the existence of galaxies. A light year is the distance covered by light in one year in vacuum.
Astronomic Unit (AU) is the mean distance between the sun and the earth. 1 AU is approximately 14,96,00,000 km (or 93 million miles). 1 light year is about 60,000 AUs.
The Big Bang theory states that some 2 × 1010 years ago, all the galaxies were close together and as a result of big explosion, fragments of this big mass thrown off in the form of galaxies. Since then galaxies are continuously moving apart from each other.
|The first persons in the world to land on the Moon|
The first man to enter space or the world's first cosmonaut The first returnable space shuttle The first disabled satellite repaired in space
The first mission of a line-up in space
by manned spaceships of the uSa
and the former Soviet Union
The first manned space
vehicle to land on moon
The first country to launch earth
satellite or “artificial baby moon”
The first woman cosmonaut of the world
The first person to float in space The first American astronaut (and second person in the world) to float in space
The first American astronaut to make two space flights The first country to launch a cosmic space rocket towards moon The first crew transfer between orbiting space ship The first manned space ship to perform the longest stay in space (11 days)
The first spacecraft to leave solar system
|Neil A. Armstrong and Edwin E. Aldrin|
Jr. of U.S.A. (Armstrong was the first to set foot on
the moon followed by Aldrin) July 21, 1969.
Yuri Gagarin (Russian)
Columbia Solar Max
Apollo-Soyuz Test Project Mission (ASTP)- launched on July 15 and linked up on July 17, 1975 Lunar Exploration Module (LEM) nick named “Eagle”
Valentina Tereshkova (Russian)
Alexie Leonov (Russian)
Gordon Cooper (USA)
Soyuz-IV and Soyuz-V (former USSR)
Apollo 7 (USA)
Galaxies are giant clusters of stars, gases and various substances.
They have three categories—
(i) Eliptical galaxies: They have little or not interstellar matter. Stars within them are predominantly old.
(ii) Spiral galaxies: Interstellar matter in the spiral arm and stars, which are bright and young, in the disc but around the central nucleus.
(iii) So galaxis: Have no definite shape. Stars within them are predominantly old.
Of the 1000 brightest galaxies, about 75% and spiral, 20% elliptical, and 5% irregular. The nearest outside galaxies to our own (the Milky Way) are the Large and Small Clouds of Magellan (about 100,000 light years distant from us). Another well-known galaxy is the Andromeda, the largest of the nearby galaxies.
A large assembly of millions of stars, dust and gas held together by gravitational forces and rotating about a common axis is called the galaxy, Milky Way or Akashganga in which we live.Milky Way is a very large band of stars, having a misty cloud like appearance, extending itself across the sky like part of a great circle. With telescope the hazy hard band gets resolved into a large number of faint stars.
It is thicker in the middle and thins out at the edges. The diameter of the Milky Way is about 100,000 light years. The sun which belongs to this galaxy is situated in north of central plane at a distance of about 27,000 light years from the centr.e The central thickness of the disc is about 5,000 light years and about 1000 light year near the sun and diminishes as we reach the edges. Its nucleus is a huge globular star cluster which ejects out one stellar mass per year.
Two spiral arms stretch out from this nucleus. These are about 1200 light years in width having gas of almost 600 light years. The sun is one of about 150 billion stars in the Milky Way. All the stars revolve about the centre. The sun moving at a speed of 250 km-1 revolves about the centre of the galaxy is about 220 million years. The Milky Way is not uniformly bright everywhere.
Clouds of gases and dusts begins to contractsunder its own gravitational force. The gradual shrinking process of the huge ball (called protostar) goes on until its core so hot (10 million degree celsius) to begin the fusion reaction.
The dwarf stars, which form more than 90% of the steller population, are found in this stage of evolution. When fuels (hydrogen) exhausted, the core begins to contract while the outer region expand. This stage of the star is called giant or supergiant.
For the sun, the giant stage will be after about 5 billion years from now. The collapsing core imparts so much energy to the exterior of the star that it explodes in the form of nova or supernova (having luminosity 10,000 times or more). Further course depends in the following way:
(i) Dwarf: If the original mass of the star was less than about 2 solar masses (mass less than 1.4 solar mass is known as the chandrasekhar Limit), a white dwarf of less than 1.2 solar mass results. As there remains no fuel, the white dwarf cools off slowly changing its colour from white to yellow, to red and finally a black body.
(ii) Neutron Stars: have very large magnetic fields. If the magnetic axis is inclined to the axis of rotation, the star emits pulses at regular period (from 30 milli seconds to 30 seconds). These are Pulsars.
(iii) Black Holes: Black Hole is the destiny of all stars whose original mass is greater than 5 solar masses.
Black hole is not a hole, but is a kind of star whose density is 1016 gms per cubic cm. The boundary of the black hole is thought to be a sphere with a radius (called the Schwartzchild radius) 2 GM/c2, where M is the mass of the region, G is the gravitational constant, and c is the velocity of light. The problem of detecting black holes is that, being unable to emit or reflect radiation, they are invisible. However, it is thought that some X-ray binary stars exist in which one member of the pair is a black hole.
The gravitational pull of the black hole is so strong, that even light or radiations cannot escape from them. So they cannot be seen by optical telescope.
Kinds of Stars
Fixed Stars: Do not appear to alter their relative position in the sky.
Binary Stars: A group of two stars revolving round a common centre under mutual gravitational attraction.
Temporary Stars: Suddenly flare up to greatly increased brightness and fade away after short time.
Variable Stars: Its brightness varies.
Red Giants: The stars, which have consumed at least 10% of its fuel, appear reddish.
Nebulae: Nebulae, which appear in the sky as bright spots, are actually clusters of stars and gaseous clouds. There are many nebulae such as the Orion Nebula within the Milky Way. Stars are formed out of condensing gases within these nebulae. Some nebulae, located far outside the limits of the Milky Way, are called extragalactic nebulae or simply galaxies.
Among numerous dim stars in the sky, there are some groups of bright stars. These groups of stars form certain shapes or patterns. These groups of stars were called constellations and were given the names of the figures they resembled. Some such groups of stars are: Ursa Major (great bear), Orion (giant hunter), Cygnus (swan), Hydra (sea serpent), Hercules, and so on. The modern definition of constellations is different. The term constellation now refers to definite regions of the sky set off by arbitrary boundary lines to include prominent groups of stars. The names of the regions or constellations have been derived from the groups of bright stars contained in them. For example, the region which encloses the group Ursa Major, along with other dim stars, is now called the constellation Ursa major. There are in all 89 constellations. The largest of these is Hydra, which contains at least 68 stars visible to the naked eye. The constellation Centaurus has 94 stars.
Quasars (Quasi-stellar Radio Sources)
In the universe, certain objects appear smallar than any galaxy, yet they emit more energy than all the stars of the Milky Way. The existence of such immensely luminous objects was first established in 1962 through their strong radio emissions. Since they resembled stars, they were called ‘quasars’. Later on, similar radioquiet objects were discovered. However, the name, quasar, has been retained. In 1983, a quasar was announced with a visual luminosity, 1.1 x 1015 times greater than the sun.
The outermost part of the sun’s atmosphere is called corona. Photosphere is the visible surface of the sun and the source of absorption spectrum. Immediately above the photosphere and below the corona is chromosphere. Hydrogen gas accounts for 70 per cent of the sun’s mass. The remainder is made up of 28 per cent helium and 2 per cent allother heavier elements from lithium to urnaium.
Controlled thermonuclear reactions are responsible for the generation of solar energy. Photographs of the sun taken in white light show many dark spots called sun spots. Sunspots appear dark because they have a lower temperature of about 4500 K. These spots are characterised by intense magnetic fields. The number of sunspots are found to vary from year to year with a period of about 11 years. This periodicity is known as sunspot cycle.
The sunspots are found to move across the disc from day to day; from this, it is inferred that the sun rotates with a period of about 25 days at its equator. Solar flare is the sudden increase of the intensity in the H * light in some places on the solar disc. During a flare the sun emits streams of protons, electrons and alpha particles which reach the earth about a day later causing global magnetic storms and radio disturbances.
All this solar activity also varies with the period of sunspot cycle. The growth of trees is also affected by the sunspot cycle. Sun is a typical star of a average brightness. It is of moderate mass and size. Distance of sun from earth is 1.49 x 1611 m. This distance is called Astronomical unit. Light takes 8 minutes to reach from the sun to earth.
The radius of sun is 6.91 x 105 km and its volume is 1.3 x 104 times that of earth. Mass of the sun is 2 x 1030 kg which is 740 times the combined mass of all the planets. Pressure at the centre of sun is 200 billion atmosphere.Average density of sun is 1.4 x 103 km-3.
Luminosity and Temperature of Sun: Luminosity is defined as the amount of energy radiated per second by the sun in all directions. It is denoted by L0. Surface temperature. The amount of heat energy received per minute by 1 square centimetre of a perfectly black body placed at a mean distance of the earth from the sun, the surface being held perpendicular to the sun’s rays is called solar constant. Its value is about 2 cal. cm-2 per minute or (2 x 104) x 4.18/16 JM-2s-1 which amounts to 1.39 x 103 Wm-2.
The sun’s path in the sky during one full year is called the ecliptic. The 12 constellations through which the sun moves define the zodiac. The 12 zodiacal constellations are: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorns, Aquarius and Pisces.
Structurally, a comet consists of three parts-a nucleus, a head and a tail. A comet may have three kinds of orbits. If the comet approaching the sun does not have enough speed to over-come the sun’s gravity, it will settle down in an elliptical orbit like our earth. A comet, which has just enough speed to counterbalance the sun’s gravity, will take on a parabolic orbit. If a comet is fast enough to overcome the sun’s attraction, it will describe a hyperbolic orbit and escape into interstellar space.
Halley’s Comet. The successive appearances of Halley’s comet have been traced back to 467 B.C. The first prediction of its return by Edmund Halley (1656-1742) proved true on the Christmas night of the year 1758, and since then it has been known by his name. Its last perihelion (closest approach to the sun) occurred on February 9, 1986, 75.81 years after the previous one which was on April 19, 1910. It was the 33rd appearance of the comet.
Meteors or Shooting Stars
Small chunks and tiny sand-like particles (remnants of comets) keep orbiting the sun. When such a particle enters the earth’s atmosphere, it evaporates almost immediately and produces a trail of hot gas often visible at night. These are meteors or shooting stars.
Russia launched the first artificial satellite, Sputnik I on October 4, 1957. A month later it was followed by Sputnik II with a dog Laika. The animal’s behaviour in space proved possibilities of survival in space. USA launched their first satellite on Jan 31, 1958, Explorer I which led to the discovery of Van Allen radiation belts around the earth.
In Oct 1959 the Russian satellite Luna III, sent pictures of the other face of the moon.
The American satellite Mariner II flew past Venus in 1962 establishing that Venus had a high temperature. Mariner IV sent back pictures of the Mars in 1965 showing that there were huge craters. USA launched in 1977 Voyager I & II which were designed essentially to study Jupiter and Saturn at close range. The American Voyager II space craft capped a historical 12 year tour to the four outer planets on Aug 25, 1989. It’s close up photographs of Neptune showed active ice volcanoes that springs nitrogen ice particles and gas.
Of two probe missions to Mars that Soviets launched in July 1988, only one, Phobos II, succeeded in approaching the red planet in six months. The Americans in 1976 had succeeded in soft landing the Viking space craft on the Mars’ surface. In October 1989, USA launched satellite space probe mission through spacecraft Atlantis. It will arrive at Jupiter in December 1995 for its two year orbit.
|Facts to be Remembered|
India’s Space Programme
The Indian space programme was launched in 1962, ISRO in 1969 and the Space Commission and Department of Space in 1972 for national socio-economic objectives.
ISRO. Indian Space Research Organisation, is responsible for planning, execution and management of space research and space applications. Head Quarters at Bangalore provides rockets, laboratory facilities scientists.
Space Missions. India has progressed steadily in the space programme, notwithstanding some failures. Aryabhatta: First Indian satellite by ISRO at Bangalore weighing 360 kg. Launched on 19-4-1975 in a near circular orbit of 600 km at an inclination of 51° to the equator from fromer Soviet cosmodrome using former Soviet rocket. Life beyond six months in orbit.
Bhaskara I—June 7, 1979 from former USSR cosmodrome for earth observation; 444 experimental satellite designed by ISRO.
Bhaskara II—Improved version. Now 20, 1981 from former USSR.
Apple: (Ariane Passenger Payload Experiment) Launched on June 19, 1981 from French Guiana. SLV Mission and Rohini Satellites— First Satellite Launch Vehicle SLV 3 which was successfully launched on July 18, 1980 from ISRO’s SHAR centre developed at VSSC. Four stage propellent rocket placed 35 kg indigenous Rohini Satellite RS D1 into a near earth orbit. RS D2 was launched successfully on April 17, 1983. ASLV mission (Augmented Satellite Launch Vehicle) mission failed in its attempt to launch 150 kg satellite on March 24, 1987 from SHAR. The second attempt on July 13, 1988 also failed.
INSAT-1-A built for the Indian Space Research Organisation by the Ford Aerospace Corporation of USA and launched by US National Aeronautics and Space Administration from Cape Canveral on April 10, 1982; combined the services of Telecommunication and meteorology, TV relay and radio broad-casting; parked 37000 km out in space over Indonesia; ended on Sept 6, 1982 prematurely.
INSAT 1 B—Multipurpose satellite—launched from cape Canaveral, Florida on Aug 30, 1983 on board the US Space Shuttle Challenger became operative on Oct 15, 1983 for mass communication, meteorology etc. India became the third Asian nation—(two others Japan/Indonesia) with a satellite in space. It enabled massive television expansion scheme, forecasting disaster systems, telecommunication link between distant cities to far flung areas of the country. Its life was extended and completed 7 years on Aug. 30, 1990.
INSAT 1-C. This could not be put into orbit because on Jan 28, 1986, Challenger space shuttle got burnt. This was launched by European Space Agency, Ariane Development agency from Kourou in French Guiana on July 22, 1988. However, it was partially crippled due to some snags.
INSAT 1-D: The last in the INSAT 1 series went up on June 12, 1990 from Cape Canaveral, became operative on July 17 at its space home, 63° of East longitude.
IRS/1A. Indian Remote Sensing Satellite was launched on March 17, 1988 from the fromer Soviet cosmodrome at Baikanour for monitoring and management of natural resources in areas of agriculture, for testing soils, water sources, etc.
First Indian in Space. On April 3, 1984 former USSR sent on Indian, Sq. Leader Rakesh Sharma into space— the space ship SOYUZ-11 was launched from Balkanour cosmodrome in former USSR, returned on April, 11, 1984.India was the 14th nation to send a man into space.
|Indian Space Programme|
|Dr Vikram Sarabhai, the founder Chief of ISRO, started the Thumba Equatorial Rocket Launching Station at Thiruvananthpuram in 1962.|
India entered “space club”, when the first indigenously-built satellite, Aryabhatta, was put in on April, 19, 1975 by a Soviet rocket.
India’s INSAT series satellites with capability to provide communications, meteorology and disaster warning systems is unique as other countries have separate satellites for each of these functions.
India pioneered the use of space technologies for large-scale societal use with the Satellite Instructional Television Experiment (SITE) in 1975 using an American Satellite, ATS-6.
GSLV’s predecessor PSLV, which can put a 1000 kg Satellite in orbit, has been successfully launched on October 15, 1994.
PSLV is the first Indian launch vehicle to use liquid propulsion (2nd and 4th stage). Its first and third stages were made of solid propellants.
India’s liquid propellent vikas, used in PSLV, is based on French liquid engine project called viking.
Liquid engines are more efficient than solid rocket.
India’s two previous launch vehicles, the SLV-3 and the ASLV, relied entirely on solid motors.
The IRS-P2, an experimental satellite which PSLV put into orbit, weighed 804 ig.
PSLV’s prime role is to place operational IRS satellites into a 900 km polar orbit.
A cryogenic engin, which burns liquid oxygen and liquid hydrogen, is essentially for the GSLV. Only the giant first stage in the GSLV will use solid propulsion, for even the boosters will have liquid engines.
PSLV was 44 m tall and 275-tonne four stage vehicle.
GSLV is a three-stage vehicle, the core being a 125-tonne solid booster as in the PSLV with four liquid strap-ons of Vikas engine, each with a propellant loading of 40 tonnes.
The most interesting aspect of GSLV is its modularity, which without strap-ons has the same capability as PSLV, with two strap-ons can launch over 1600 kg into a polar orbit and with all the four strap-ons can launch 2.5 tonnes into GTO.
|Facts to be Remembered|
|1. What is astronomy?|
|2. How are galaxies formed?|
|3. What is a comet made of?|
|4. How do stars produce light and heat?|
|5. How do astronomers determine the distance to galaxies?|