Physics Notes for NEET Chapter 31 - Universe PDF

60 1830 Universe Chapter E3 31 Universe (i) The gravitational pull of the Sun on the planets control their motion. Solar System (ii) There are other heavenly bodies (about 32) which revolve around the planets called satellites (or moons) of the planets. (iii) A planet does not emit light of its own....

60 1830 Universe Chapter E3 31 Universe (i) The gravitational pull of the Sun on the planets control their motion. Solar System (ii) There are other heavenly bodies (about 32) which revolve around the planets called satellites (or moons) of the planets. (iii) A planet does not emit light of its own. ID Universe is the limitless expanse of space around us consisting of solar system. star, galaxies etc. U (iv) A planet do not twinkle at night. (v) The planets are very small in size as compared to stars or Sun. D YG (vi) The relative positions of planets keep on changing day by day. (vii) Most of the planets move around the sun from west to east. (viii) The planets are made of rocks and metals. (ix) The temperature of planet depends upon its distance from sun. Solar system is a family of nine planets, satellites, asteroids, comets, meteors, meteroites and dust particles orbiting around the Sun. (2) Asteroids : The small pieces of planet revolving around the sun between orbits of Mars and Jupiter are called Asteroids. (1) Planets : Nine planets revolving around the sun in elliptical orbits. In order of increasing distance from Sun, these are Mercury, Venus, Earth, Mars, (i) Astronomers have identified about 2000 asteroids ranging from the Jupiter, Saturn, Uranus, Neptune and Pluto. largest 770 km diameter to bodies 1.5 km in diameter. Table 31.1 : Some information about planets Planet Radius Mercury Venus Mass as compared to earth Time of revolution around the sun Time taken to complete one rotation around its own axis Number satellites 2.4 57.9 0.055 88 days 59 days — 6.1 108.2 0.815 225 days 243 days — 6.3 149.6 1 1 Year 23 hrs. 56 min. 1 Mars 3.4 227.9 0.108 1.9 Year 24 hrs. 27 min 2 Jupiter 71.4 778.3 317.9 11.8 Year 9 hrs. 50 min 14 Saturn 60.0 1427 95.2 29.5 Year 10 hrs. 14 min 10 + Ring Uranus 23.4 2870 14.6 85 Year 10 hrs. 49 min 5 + Ring Neptune 22.3 4594 17.2 165 Year 15 hrs. 2 Pluto 3.2 5900 0.002 248 Year 6.39 days — ST Earth Mean distance from sun  106 km U R  103 km (ii) The largest asteroids are called Ceres. (iii) The largest asteroid complete one revolution around the sun in 4.6 years. of (3) Comets : These are composed of rock like materials surrounded by large masses of easily vaporisable substances like, ice, water, ammonia and methane. (i) They revolve around the Sun in highly elliptical orbits. Universe (4) Meteors and meteorites : Meteors are the smaller pieces of stones and metals which may be produced due to the breaking up of comets while approaching the Sun. When they reach earth's atmosphere due to friction they start buring. They are also called shooting stars. Sometimes, the large pieces of stones (acting as meteors) do not burn completely and reach the surface of the earth as stony, iron balls resulting in crators on the earth surface. These are called meteorites. Measurement of Size of Planet D or where 1 AU = 1.496  10 m 11 Thus sin  rps , similarly cos   (3) Kepler's law : According to Kepler's law the square of time period of planet around sun is proportional to cube of semi-major axis of the orbit a3  constant , if a1 and a2 are semi-major T2 axes of planets 1 and 2 and T and T their respective periods of revolution, then of planet around sun i.e. 1 a13 a3  22 2 T1 T2  d ~– D T  a2   2   T1  or 2/3 a1 1 2 (4) Spectroscopic method : In this method, photograph of two different planets P and P are taken on similar photographic plates from one place of the earth. Let I and I be the intensities of the images of these two planets. If R and R be the distances of these planets from the earth then 1 2 1 2 2 I1 R 22  ( intensity at a point is inversely proportional to the square of I2 R12 Fig. 31.1 Earth Measurement of Distance of Planet From the Earth 2 For circular orbits a and a represent the radii of orbits. 1 d D r PE  pe  rpe  cos  ES 1 AU the distance) U  ~– r PS  ps ES 1 AU ID We can measure the size of a planet by measuring the angle subtended by its diameter AB at a d point on the earth. This angle is A B called angular diameter of planet. If d denotes diameter of planet and D its distance from the earth sin  60 (v) Hally comet was seen in early 1986 and is expected to be seen again in 2062. maximum when earth and sun subtend an angle 90° at the planet. From figure, E3 (ii) Their time period of revolution around the Sun is very large. (iii) Comets appear to be having a bright head and a long tail while passing close to the Sun and when away from sun generally they show no tail. (iv) The tail of comet is formed when the comet is passing close to the Sun and the heat of Sun exerts a pressure on the material which gets evaporated due to heat of Sun. 1833 Stars D YG Fig. 31.1 (1) Parallax method : The planet O is observed from two points P and P on the surface of the earth. The distance between these two points, PP = b, is called basis. The angle 1 2 1 O subtended by planet at these two points is called parallax angle or parallactic angle From figure  ~– b  b D D D P1 P2 U or D ~– 2  Earth ST (2) Copernicus method : The b inferior planets (Mercury and Fig. 31.2 Venus) have nearly circular orbits. Angle between directions of observation from earth to sun and earth to planet is called planet's elongation. res  The distance of earth from Sun, rps  The distance of planet from Sun and rpe  The distance of planet from Earth A firey massive luminous heavenly body is called a star. (1) Some features (i) Stars twinkle at night. (ii) Stars are countless in number ; about 10 in a universe. 22 (iii) Stars are very big in size but they appear small because they are very far off. (iv) The relative positions of stars do not change day by day. (v) Stars appear to be moving from west to east. (vi) The temperature of stars is very high. (vii) The Sun is the nearest star to the earth. Its light reaches the earth in 8.3 minutes. (viii) After Sun the next nearest star to earth is Alpha centuri. Its distance is 4.3 light year from earth. The rps and res are fixed distances as (ix) Other bright stars in the sky are known as Spica (Chitra), Arcturus (Swati), Polaris (Dhruva), Sirius (Vydha), Canopus (Agasti) etc. orbits have been assumed to be circular. During orbital motion of the planet the distance rpe changes. Planet's elongation is S rps P res = 1AU 90° rpe  E Fig. 31.3 (x) The temperature of a star is estimated from the colour of its light received on earth. The blue coloured star is at higher temperature than red coloured star. 1834 Universe (ii) According to modern astronomy there are 88 constellations in the sky. (3) Brightness of star : Brightness of stars is represented through system of magnitudes. Magnitude of star is the measure of its brightness when observed from earth. (i) Hipporacus, a Greek astronomer divided the stars (visible with naked eye) into six magnitude classes. Brightness goes on decreasing as the magnitude increases. A first magnitude star is about 100 times as bright as a sixth magnitude star. Decrease in magnitude number by one increases brightness by ratio 1001 / 5  2.5119. In general The brightness of star in n th magnitude class  (2.512)m The brightness of star in (n  m )th magnitude class (ii) If two stars have magnitudes m and m (m > m ) and brightness l l and l (l < l ), then 1  100(m 2 m1 ) / 5 l2 1 2 2 1 1 1 Taking logarithm to base 10 of both sides, we get (m 2  m1 )   2.5 log l2 l1 s' s (7) Black hole : When the original mass of the star is more than 5 M , then on supernova explosion, the core continues suffering compression indefinitely due to recoil. This gives rise to a black hole. The mass of the black hole is greater than the mass of the Sun but its size is very small. Due to this fact, the gravitational pull of black hole is very strong. This is the reason that the photon of radiation emitted by it cannot escape from its surface. On the other hand, a photon approaching a black hole is swallowed by it. Hence it is called a black hole. s The black hole is said to have been formed if the star of mass M has 2GM contracted within a radius r which is given by r  c2 Sun (iii) For a star of zero magnitude m = 0, l = l , m = m and l = l. 1 0 2 2 The Sun called the centre of the solar system, is a star nearest to the earth. U 1  m  2.5 log (c) Neutron star : When the original mass of the star is lies between 2M and 5 M the core of the star tends to finish up as neutron star. In such a case, when super nova explosion occurs, the core of the star is compressed and electrons and protons combine to form neutrons. Due to this reason, this is called as neutron star. It is found to have a radius of about 10 km. ID 2 2 s s 60 (i) The Great-bear (Saptarishi), Taurus (Vrishabha) Aries (Mesha) etc. are the other constellations near the north and south celestial poles. (a) White dwarf : When the original mass of the star is less than 2 M (M being solar mass), the core of the star tends to die as White dwarf. It was theoretically discovered by S. Chandrasekhar in1930 and is known as Chandrasekhar limit. As the core keeps on emitting heat and light for millions of year, it colour changes from white to yellow, then to red finally it becomes black. Now this becomes invisible for ever. E3 (2) Constellation : Many of the stars appear to be bunched together in groups. These groups are called constellations. l0 l (1) Properties of the Sun (i) It's average distance from earth is 1.49  10 km = 1 AU D YG (iv) The star vega is of zero magnitude and of brightness (ii) It's mass is 1.99  10 kg l0  2.52  10 8 W / m 2. (v) A star having negative magnitude is brighter; e.g., a star having magnitude – 5 will be 100 times more bright than a star of zero magnitude. (4) Absolute luminosity : The total energy radiated into space per second from the surface of a star is called absolute luminosity of the star. 8 30 (iii) It's mean diameter is 1.392  10 km 6 (iv) The density of the Sun varies from 10 kg/m at the surface to 10 kg/m at the centre. It's mean density is 1410 kg m. –4 3 3 (v) The pressure at the centre of the Sun is about 2  10 N/m. 6 2 The absolute luminosity of the Sun is  3.9  10 J / sec. (vi) Light takes 8 minute to reach earth from the Sun. (5) Birth of a star : Star dust and gases present in interstellar space come closer together with a gravitational force in the form of a cloud. (vii) 70% of Sun's mass is H , 28% He and 2% Lithium or Uranium. 26 U (i) When the cloud is quite big, due to compression cloud heats up and starts radiating ST (ii) At this temperature, fusion of hydrogen atom into helium atom takes place and a star is said to have come into existence. 4 –3 2 (viii) The Sun is also called Yellow Dwarf. (2) Structure of Sun : The Sun structure consist of four parts : Photosphere (P), Reversing layer (R), Chromosphere (CH) and Corona (C). (iii) This process result in the release of energy, which keeps the star shining for millions of years. (6) Death of a star : When large number of hydrogen atoms of a star are converted into He, the core of star begins to contract and other layers begin to expand. At this stage star appears red, the stage is called Red Giant. P R CH C (i) Now a violet explosion occurs in star. This is called nova or super nova explosion. (3) Solar activity : The surface feature of the Sun are called Solar Fig. 31.4 activity. This can be classified as follows (ii) Due to explosion, the outer layers are thrown back into interstellar leaving behind the core of the star. This is known as death of the star. (i) Sun spots : These are dark spots on the surface of sun associated with strong magnetic fields. The sun spots move across Sun slowly, so there numbers vary over a cycle of 11 year called Sun spot cycle. After every eleven year activity of sun spots tends to be maximum. Movements of sun spots The core of the star may further end up into one of the following three dead bodies (stellar dead materials) : (a) White dwarf (b) Neutron star (c) Black hole Universe have revealed the time period of rotation of sun on its own axis as about 25 days. (ii) Faculae : These are bright patches near Sun spots. (iii) Granules : Small granules form a covering over photosphere. (iv) Flares : Sudden increase in magnetic activity is called flare. During these flares Sun emits streams of protons, -particles and electrons. Usually, the radius of star is expressed in terms of solar radius 1/2 T2  E  solar (Rs  6.95  10 8 m). Thus star radius    6.95  10 8  4  radius.  The radii of most of the stars lie in the range 0.02 to 220 solar radii. (2) Stellar masses : Let M and M be the masses of two stars revolving about their common centre of mass in circular orbits of radii r and r respectively such that r1  r2  r. Now 1 1 (vi) Prominences : Surface of photosphere is covered by rising clouds called prominences. M1  M 2  4 2 r 3  2 G T (4) Solar constant (S) : Energy falling in one second on the unit area of the earth's surface held normal to Sun's rays is called solar constant. It is 1 E3 MS  where  = Stefan's constant = 5.68  10 S.I. unit –8 Earth S Fig. 31.5 Hence equation (i) gives G  4 2 s D YG Ls  (4r 2 )S  3.9  10 26 W U (5) Solar Luminosity (L ) : It is defined as the amount of energy emitted by the Sun per second in all directions. (6) Temperature of Sun (T) : The surface temperature of the Sun is 1/4 S      (7) Mass of the Sun (M) : Let M be the mass of sun and m be the mass of a planet moving around it, then as gravitational force of attraction between them supplies the necessary centripetal force U GMm mv 2 v 2r F  M   Mass of Sun r G r2  2  r3   r r r T      G G G 2 ST 3 r3 = constant T2 In binary system, r = 1 AU, T = 1 year and M1  M 2  1 solar mass. r 2 As M is constant, it implies that which is Kepler's third law. r = Radius of Earth's orbit 2.....(ii) S R = Radius of Sun 1/2 4 2 r 3  2 G T ID T = Surface temperature of Sun r given by T    R S S equation (i) can be written as  1.388  10 3 W / m 2 r2......(i) If a planet of mass M moves round the Sun of mass M , then the mass M can be neglected in comparison with M because M S  M1. Then 1 T 4 R 2 2 where T is common period of revolution. (vii) Filaments : These are thin markings on the photosphere. given by S  2 60 (v) Spicules : Bright spikes emerging from chromosphere are termed spicules. Spicules are source of large number of charged particles into the corona. 1835 2 M1  M 2   r3 T2..... (iii) Equation (iii) can be used to find the masses of two stars in binary system. (3) Spectra of stars : The different stars are of different colours and the spectrum of a star is related to its colour, There are seven classes of stellar spectra denoted by letters O, B, A, F, G, K and M. Our sun belongs to G class star. Table 31.2 : Spectrum of stars Spectra type O Colour Dark blue 2 Surface temp (K) 3  104 Description of absorption spectra Ionized helium lines to 4  104 4 r GT 2 2 3 B where G = 6.67  10 Nm kg and r is distance between the sun and planet. T period of revolution of planet around the sun. If we consider the planet and its satellite, mass of the planet can similarly be found –11  Blue 1.s5  104 Lines of neutral helium to 2.3  104 –2 A White 9.5  103 Lines of H2 to 1.1  104 F Green Stellar Radii, Mass and Spectra 6.5  103 Lines of H2 and ionised metals to 7.5  103 (1) Stellar radii : The total energy radiated by the star per second is given by E  T 4  Surface area of the star 1/2  E   E  T 4  4R 2  Radius of star (R)     4  T2 G Yellow 5800 Lines of ionised Ca, Fe , C K Orange 4500 Bands due to hydrocarbons M Red 3500 Bands of Titanium oxide These relationship between the colour of a star and its temperature is expressed by Wien's displacement law. According to this law 1836 Universe m  Akash Ganga because the light from the various stars together gives the impression of a stream of milk flowing across the sky. b 1 or m T  b or T  ; where b = 2.89  10 mK. m T –3 Milky way is a spiral galaxy. Its mass is 150 solar masses So those stars which appear blue (minimum wavelength) such as class O and B, are very hot and which appear red (maximum wavelength) such as class M are less hot. (i.e. 3  10 kg). 41 Diameter of galactic disc (5000 LY) Galaxies C S Sun Galactice centre 2.7  10 LY 60 4 105 LY Fig. 31.6 A large group of stars is called Galaxy. Millions of galaxies are therein the sky. Each galaxy contains about 10 stars. E3 Milky way contains 150 billions sun like stars. Milky way contains clouds of dust and gases. Pulsars 11 The Sun and the planets of the solar system belong to the galaxy, called Milky way (Akash Ganga). (1) Types of galaxies : There are two types of galaxies ID (i) Normal galaxies, and (ii) Radio-galaxies. As the age of a star increases, its hydrogen content goes on decreasing. Ultimately, the star explodes as a supernova, in the universe. After explosion of a supernova, a variable star is born. It is not an ordinary star. It is the remaining part of a supernova. The variable star is called a pulsar. A pulsar emits electromagnetic waves in pulses and not continuously. The pulses are of very short duration (0.033 s to 0.088 s). The pulses may lie either in visible region or in radio region. About 50 pulsars have been detected, two in visible region and others in radio region. It is expected that there are about 100 pulsars in the universe. (i) Normal galaxies : Besides milky way, there are billions of other galaxies in the universe. All these galaxies are called normal galaxies. There are three types of normal galaxies. (a) Elliptical galaxies (18%), (ii) Spiral galaxies (80%), and (iii) Irregular galaxies (2%). Evolution of the Universe U (a) Elliptical galaxies : The galaxies which look like the flat elliptical discs are called elliptical galaxies. These generally consist of red giants, white dwarfs etc. i.e., those stars which are nearing their ends. D YG (b) Spiral galaxies : The galaxies have lens-shaped central portion surrounded by a flat disc. It has two spiral arms which spiral around the central portion. Example : Milky way and Andromeda. (c) Irregular galaxies : These have no specific form of their own. Irregular galaxies are youngest normal galaxies and are middle aged and elliptical galaxies are quite old galaxies. (2) Radio galaxies : The galaxies which emit electromagnetic radiations in the radio frequency are called radio galaxies. These have been classified as (i) Ordinary radio galaxies (ii) Quasars. U (i) Ordinary radio galaxies : A normal optical galaxy (O) which has two strong radio sources (R and R ) occurring symmetrically on either side of it, is called an ordinary radio galaxy. It appears like two ears on the two sides of the face of a person. The radio power output lies in the range 10 to 10 watt. 2 30 38 ST 1 (ii) Quasars : Quasars are quasi-stellar radio sources. They are star like in structure and they emit powerful radio waves. They have a radio output of 10 to 10 watt. Quasars are farthest objects known. They are millions of light years away from Earth. These seem to be lying at the limit of the universe. They are moving away from Earth with a velocity of about 0.9 times the velocity of light. Their size is much smaller. It is of the order of light days. They form very dense galaxies. The density is also very large and their gravitational field is also very high. The cause of tremendous energy of the quasars is unknown. About 150 quasars have been detected so far. 37 38 (3) Milky way (Akash Ganga) : It is the name of the galaxy to which our earth belongs. The milky way is the glowing belt of the sky formed by the combined light of a very large number of stars. It is called milky way or Important theories about the origin and evolution of universe are as follows. (1) Big Bang theory : The whole of the matter of the universe was concentrated in a very dense and hot fire ball about 20 billion years ago. An explosion occurred. The matter was broken into pieces in the form of stars and galaxies. The faster moving galaxies have gone farther than the slower ones. A galaxy situated at 20 billion light years is the boundary of the universe. (2) Expanding universe theory : All the galaxies would continue to move away from the Earth and we will have an empty universe because on account of continuous expansion of the universe, more and more galaxies will go beyond the boundary of the universe and will be lost. The motions of galaxies relative to the earth can be measured by observing the shifts in the wavelengths of their spectra. For distant galaxies these shifts are always toward longer wavelength, so they appear to be receding from us and from each other. Astronomers first assumed that these were Doppler shifts and used a relation between the wavelength  of light measured now from a source receding at speed v and the wavelength  measured in the rest frame of the source when it was emitted. 0 s 0  S c v c v For v