The Solar System & Universe

The Solar System & Universe

Universe

• The universe is commonly defined as the totality of everything that exists, including all physical matter and energy, the planets, stars, galaxies and the contents of intergalactic space.
• The study of universe is known as Cosmology.

Cosmology = cosmos (universe) + logos (science)

• The diameter of the universe is 10⁸ light years.
• After the birth of universe atoms came into existence. With these atoms dust and clouds of gases formed.

Galaxy

• A galaxy is a vast system of billions of stars, which also contains a large number of gas clouds (mainly of hydrogen gas) and dust, isolated in space from similar systems.
• There are about 100 billion galaxies (10¹¹ galaxies) in the universe, and each galaxy has, on an average, 100 billion stars (10¹¹ stars). So, the total number of stars in the universe is 10²² stars.
• The Milky Way Galaxy is the home of the Earth and our Solar System. It is spiral in shape.
• Milky Way Galaxy was formed 5 billion years after the Big Bang. The first person to see this galaxy was Galileo Galilei.
• Latest known galaxy is the Dwarf Galaxy.
• According to the modem thought, universe can be classified into two parts namely—(a) Atmosphere and (b) Space.
• Origin of the universe is explained by the Big Bang Theory, formulated and proposed by the Belgium Roman catholic priest, physicist, astronomer and cosmologist Georges Lemaitre.
• Andromeda is our nearest galaxy.

The Big Bang Theory

• All the matter in the universe was originally a concentrated lump called primeval atom.
• Big Bang was an explosion that occurred 13.798 billion years ago, leading to the formation of galaxies of stars and other cosmic bodies.
• Since then, all the galaxies have been flying away from one another causing expansion of the universe.

Higgs Boson

• A historical mega experiment was successfully conducted in 100 feet deep and 27 km long tunnel in Large Hadron Collider (LHC) in Geneva by European Centre for Nuclear Research (CERN) on 30th March, 2010 to unravel the mysteries of the Universe, in which more than 1000 scientists participated in the experiment. In this experiment Proton beams having the same velocity of light were collided to produce Higgs Boson. This mega experiment will unravel the unsolved mysteries related to the origin of the universe, which is so far known as Dark matter, Dark energy, Extra Dimension, Higgs Boson and God Particle. Basically through this mega experiment the Scientists tried to repeat the phenomena which occurred prior to 15 billion years ago known as Big Bang in the field of science.
• It is now considered as the Higgs Boson which is known as God Particle in which mystery of the Universe exists, because it is also considered as the most basic unit. As a result of continued effort of 50 years, CERN on 4th July, 2012 identified the particle which is almost same to Higgs Boson. It will help to unravel several aspects of the Universe. On 14th February, 2013 the large hadron collider was closed but it has been restarted in June 2015.

Star

• Clumps of dust and gas in a nebula come together due to gravity and form stars.
• Stars are made of hot burning gases. They emit light of their own and are very large and very hot.
• According to NASA, Light takes about 4.35 years to reach us from Alpha centauri and 4.25 years from the nearest star Proxima Centauri.

The Solar System

• The solar system consists of the Sun, the eight planets and their satellites (or moons), and thousands of other smaller heavenly bodies such as asteroids, comets and meteors.
• The Sun is at the centre of the solar system and all these bodies are revolving around it.
• The gravitational pull of the Sun keeps all the planets and other objects revolving around it. Thus, the motion of all the members of the solar system is governed mainly by the gravitational force of the Sun.
• Planets revolve around the Sun in elliptical orbit.

• In the solar system the planet nearest to the Sun is Mercury and the planet farthest from the Sun is Neptune (not Pluto).
• The size of solar system has been estimated to at about 10⁵ A.U.
• The solar system is dominated by the Sun which accounts for almost 99.9% of the matter in the whole solar system.
• The Sun is also the source of all the energy in the solar system.
• Pluto is a dwarf planet.
• Mercury, Venus, Earth, Mars are called terrestrial planets and Jupiter, Saturn, Uranus and Neptune are called gaseous planets.

Heliocentrism

• Heliocentrism, a cosmological model in which the Sun is assumed to lie at or near a central point (e.g, of the solar system or of the universe) while the Earth and other bodies revolve around it.
• In the 5th century BC the Greek philosophers Philolaus and Hicetas speculated separately that the Earth was a sphere revolving daily around some mystical "central fire" that regulated the universe. Two centuries later Aristarchus of Samos extended this idea by proposing that the Earth and other planets moved around a definite central object, which he believed to be the Sun.
• In the 2nd century AD, Claudius Ptolemy of Alexandria suggested that this descrepancy could be resolved if it were assumed that the Earth was fixed in position with the Sun and other bodies revolving around it. As a result, Ptolemy's geocentric (Earth-centred) system dominated scientific thought for some 1,400 years.
• In 1444 Nicholas of Cusa again argued for the rotation of the Earth and of other heavenly bodies, but it was not until the publication of Nicolaus Copernicus's De revolutionibus orbium coelestium libri VI ("Six Books Concerning the Revolutions of the Heavenly Orbs") in 1543 that heliocentrism began to be reestablished Galileo Galilei's support of this model resulted in his famous trial before the Inquisition in 1633.

Geocentric Model

• Geocentric Model, any theory of the structure of the solar system (or the universe) in which Earth is assumed to be at the centre of it all. The most highly developed geocentric model was that of Ptolemy of Alexandria (2nd century CE). It was generally accepted until the 16th century, after which it was superseded by heliocentric models such as that of Nicolaus Copernicus.

Members of the Solar System

The Sun

• The Sun is at the centre of the Solar System.
• Its size is thirteen lakh times as that of the Earth.
• It is the nearest star to the Earth.
• It is an ultimate source of energy for life on Earth.
• Its diameter is 14 lakh kms. (approx).
• It is composed of 71% Hydrogen, 26.5% Helium and 2.5% other elements.
• Hydrogen and Helium are the main gases present in the Sun.
• Within the Sun, hydrogen is converted to Helium due to nuclear fusion releasing a tremendous amount of heat and light.
• It has a surface temperature of 5778 K or 5504.85°C or 9940.73°F.
• A storm of hot atoms which dissipates from the photosphere of the Sun overcomes its gravity and goes into the outer space, is known as Solar Flares.
• When Solar Flares reach the earth's atmosphere, after colliding with the air and dust particles, it produces a spectacular colourful effect.
• In the North-Pole region, this effect can be seen as Aurora Borealis and in the South Pole region as the Aurora Australis.
• The Regions from where the solar flares originate some dark spots are seen and these are called Sun Spots. The Spots are relatively colder regions of the Sun, having temperature of about 1500°C. Sun Spots produce strong Magnetic Radiations which hamper the Wireless Communication Systems on earth. Sun Spots appear and disappear after every 11 years which is called Sun Spot Cycle.
• The temperature at the centre is around 1.571 x 10⁷ K or 15,000,000°C.
• Shining surface of the Sim is called photosphere, it appears like a disc, radiates energy and acts as a source of energy.
• The outer layer of the Sim's atmosphere made up of thin hot gases, is called Corona. Corona is visible only during a total eclipse of the Sun (or with a special solar telescope called Coronograph).
• The planet travels with the Sun through millions of stars in our galaxy at a speed of about 70,000 km per hour.
• The Sun is about 150 million kms away from the Earth.
• Light (at the speed of 3,00,000 km per second) takes about 8.5 minutes to reach the Earth from the Sun.

The Planets

• These are opaque bodies which continuously revolve around and are lighted by the Sun.
• The IAU General Assembly (2006) agreed that a 'planet' is defined as a celestial body that-
  (a) is in orbit around the Sun,
  (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
  (c) has cleared the neighbourhood around its orbit.
• This means that the Solar System consists of eight 'planets' Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. A new distinct class of objects "dwarf planets" was also decided. It was agreed that "planets" and "dwarf planets" are two distinct classes of objects. The first members of the"dwarf planet" category are Ceres, Pluto and 2003 UB (temporary name).
• A ninth planet has been recently discovered by NASA named as Carla.
• The sequence of planets according to their distance from the Sun is Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
• The sequence of planets according to their size (in descending order i.e. from big to small) is Jupiter, Saturn, Uranus, Neptune, Earth, Venus, Mars, and Mercury.
• Jupiter is the biggest and Mercury is the smallest planet of our solar system.

Classification of Planets

• The eight planets have been divided into two groups. All the planets of a particular group have some common features. 'Terrestrial planets' or 'Rocky planets' and 'Jovian planets' or 'Gaseous planets' (Gas giants) are the two groups of planets.
• The four planets nearest to the Sun-Mercury, Venus, Earth and Mars are called terrestrial planets, because their structure is similar to the earth.
• Other four planets-Jupiter, Saturn, Uranus and Neptune are called Jovian planets.
• Planets are classified into the following two groups inner and outer planets. These are separated by asteroid belt.

Some Notable Facts about Various Planets and Satellites

Mercury

• Mercury is the closest planet to the Sun.
• It is extremely hot planet. It is visible two hours before sun rise.
• Its special feature is the presence of magnetic field in it.
• It is the smallest and lightest of all the planets. It has no moon.
• The planet has no water on it.
• Mercury planet has no gases like CO₂, N₂, H₂ and O₂ which can act as building blocks of life.
• Mercury planet has no protective blanket like Ozone around it to prevent us from harmful radiations.

Venus

• Venus is the second planet in distance from the Sun. This planet is nearest to the Earth and is also the brightest planet.
• Venus is known as the 'Evening Star' as well as 'Morning Star'.
• Venus is surrounded by a thick cloud cover, hence known as the 'Veiled Planet' ('veil'means undear/cover).
• Venus is like the Earth in size and mass, and hence also known as the 'Earth's twin'. It also rotates clockwise like Uranus.
• Venus is the hottest planet (even hotter than Mercury) of our Solar System, due to its veil of cloud.
• Venus has no water on it. There is no sufficient oxygen on the Venus.

The Earth

• Earth is the largest of the inner planets and the fifth largest planet of our solar system.
• The Earth is 23° tilted on its axis and thus makes 66 ° angle. The earth rotates by 1° in 4 minutes.
• It takes 23 hours 56 minutes and 4.091 seconds to rotate on its axis with a speed 1,610 km/h.
• It takes 365 days, 5 hours and 48 minutes to revolve around the Sun. Mean velocity = 29.8 km/sec
• The Earth is known as the 'watery planet' or the 'blue planet' due to the presence of huge amount of water on it.
• The Earth is the only known planet which provides sustenance or life on it. It has a large quantity of oxygen which supports life.

The Moon

• The Moon is the only satellite of the Earth.
• It has a diameter of 3,475 km and its circumference is 10,864 km while its orbit is elliptical.
• The maximum distance (apogee) of the Moon from the Earth is 4,06,000 km, the minimum distance (perigee) is 3,64,000 km and approximate average distance is 3,84,400 km.
• It takes 27 days, 7 hours and 43 minutes to rotate on its axis (this period of about 27% days is called the sideral month) and approximately the same period of time it takes to revolve around the Earth. The Moon's period of revolution with reference to the Sim is about 29.53 days (29 days, 12 hours, 44 minutes and 2.8 seconds). This period is called a synodic month.
• Only 59% of the total surface of the Moon is visible from the Earth.
• Mass (compared to that of the Earth) - 1:81.30

Density (relative to that of the water) - 3.34

Density (relative to that of the Earth) - 0.6058

Hidden part of Moon's surface - 0.41 (41%)

Highest point on the Moon - Mt. Leibnitz (35,000 ft.) situated on the South Pole of the Moon.

• The bright part of the Moon is full of mountains whereas the dark patches are low lying plains.
• 'Sea of tranquility' made of the plain of dust particles, is on the rear side of the Moon, which always remains dark.
• The Moon has no atmosphere, no twilight and no sound.
• The temperature during daytime is about 100°C and during night it drops down to about -180°C.
• The light from the Moon takes 1.3 seconds to reach the Earth.
• The size of the Moon is one-fourth (1 /4th) the size of the Earth.
• Gravitational pull of Moon is one-sixth (l/6th) that of the Earth.
• Mainly silicon, iron, magnesium etc elements are found on the Moon's surface.
• The study of the Moon is called 'Selenology'.
• Moon is also known as the fossil planet.

Super Moon

• Super Moons happen when a full Moon is closest to the Earth, appearing bigger and brighter than normal. They are called 'super' because they are 15% brighter and 30% bigger than regular full moons.

Blue Moon

• If in a calendar month there are two full moons then the second full moon is referred as Blue Moon. It is because of the gap between the two moons which is less than 31 days. If in any specific year Blue Moon appears in two or more than two months then it is referred as Blue Moon Year.

Blood Moon

• A'Blood Moon' is the name given for a view of the Moon during a total lunar eclipse. Because of the way light passes through the Earth's atmosphere during an eclipse, red light from the Sun is reflected on to it. The Moon's reddish colour earns it the nickname—"Blood Moon".

Lunar Tetrad

• A sequence of four total lunar eclipses including no partial eclipses is called a tetrad.

Chandrayaan-1

• On 22nd October, 2008 Indian Space Research Organization (ISRO) has successfully established CHANDRAYAAN-1 in the moon's orbit from Satish Dhawan Space Centre in Sri Harikota of Andhra Pradesh by polar satellite launch vehicle PSLV-C-1.
• In this project India manufactured unmanned space vehicle moon impact probe which has been landed on moon's surface. Chandrayaan-1, is first satellite with its own type with high resolution remote sensing facility with which images of moon could be seen, having 11 extra modem apparatus including X-ray, spectrometer miniature synthetic aperture radar, moon mineralogy mapper (M-3) etc. This takes pictures of moon's surface even of the whole of its chemical environment and provides information about its soil, mineral, ice, heat and weather etc.

Chandrayaan-2 Mission

• India's Geosynchronous Satellite Launch Vehicle, GSLV MKIII-M1, successfully launched Chandrayaan-2 spacecraft on July 22, 2019 into its planned orbit with a perigee (nearest point to Earth) of 169.7 km and an apogee (farthest point to Earth) of 45,475 km. The launch took place from the Second Launch Pad at Satish Dhawan Space Centre, SHAR Sriharikota.
• Chandrayaan-2 mission comprised an Orbiter, Lander and Rover to explore the unexplored South Pole of the Moon.
• After the injection of Chandrayaan-2, a series a maneuvers were carried out to raise its orbit and oa August 14,2019, following Trans Lunar Insertion (TLI maneuver, the spacecraft escaped from orbiting the earth and followed a path that took it to the vicinity of the Moon. On August 20, 2019, Chandrayaan-2 wai successfully inserted into lunar orbit. While orbiting the moon in a 100 km lunar polar orbit, on September 0? 2019, Vikram Lander was separated from the Orbita in preparation for landing. Subsequently, two de-orbi maneuvers were performed on Vikram Lander so as to change its orbit and begin circling the moon in a 100 kn x 35 km orbit. Vikram Lander descent was as planne< and normal performance was observed up to an altitude of 2.1 km. Subsequently communication from lander to the ground stations was lost.
• But the Orbiter camera is the highest resolution camera (0.3 m) in any lunar mission so far and will provide high resolution images which will be immensely useful to the global scientific community.

Chandrayaan-3 Mission

• Chandrayaan-3 is the third lunar exploration mission developed by the Indian Space Research Organization (ISRO) after Chandrayaan-1 in 2008 and Chandrayaan-2 in 2019.
• It consists of a lander named 'Vikram' and a rover named 'Pragyan', focused on demonstrating India's capacity to make soft landings on the lunar surface - something only three nations have achieved so far: US, Russia and China.
• Set to launch in June 2023, it will be launched onboard India's powerful Geosynchronous Satellite Launch Vehicle MkIII rocket from Sriharikota spaceport.
• Chandrayaan-3 attempted to land near the same site where Vikram lander crash-landed during Chandrayaan-2 mission's final moments in September 2019.
• Costing Rs 250 crore, Chandrayaan-3 is largely similar to Chandrayaan-2 in design with some improvements based on lessons from the previous mission.
• Chandrayaan-3 mission successfully achieved its motive on 23rd August 2023 by landing successfully on moon surface.

Mars

• Iron-rich red soil and pink sky of Mars give it the name, 'Red Planet'.
• Mars has a cold desert world. It is half the size of Earth. Like Earth, Mars has seasons, polar ice caps, volcanoes, canyons and weather. It has an atmosphere made of carbon dioxide, nitrogen and argon.
• Mars is a terrestrial planet. It is small and rocky. Mars has a thin atmosphere. Mars has an active atmosphere, but the surface of the planes is not active. Its volcanoes are dead.
• One day on Mars lasts 24.6 hrs. It is just a little longer than a day on the Earth. One year on Mars's is 687 Earth days. It is almost twice as long as one year on the Earth. Mars has two moons. Their names are Phobos and Deimos. Mars is the fourth planet from the Sun. That means Earth and Jupiter are Mars's neighboring planets.
• Mars Orbiter Mission (MOM), the maiden interplanetary mission of ISRO, launched on November 5, 2013 by PSLV-C25 got inserted into Martian orbit on September 24, 2014 in its first attempt.MOM completes 1000 Earth days corresponds to 973.24 Mars Sols (Martian Solar Day) and MOM completes 388 orbits.

Jupiter

• Jupiter is the largest planet of the Solar System.
• Jupiter is also known as winter planet as its average temperature is very low (-148° C).
• Ganymede, (yellow in colour), largest satellite of Jupiter is the also largest satellite in the Solar System.

Saturn

• Saturn is the second largest planet in the Solar System.
• Saturn has bright concentric rings which are made up of ice and ice-covered dust particles which revolve around it.
• Titan is the largest satellite of Saturn,

Uranus

• Uranus is about four times the size of the Earth. This planet appears greenish in colour because of methane gas present in its atmosphere.
• Uranus was discovered in 1781 by Sir William Herschel. Uranus is the 7th planet from the Sun.
• Uranus is the first planet to have been discovered by the use of a telescope.
• Uranus is the third largest planet of the Solar System. Uranus is extremely cold, having surface temperature (-) 190°C and is surrounded by 13 rings namely zeta (G)/R1986U2,6,5,4, alpha (α), beta (β), eta (∈), gamma (Y), delta (δ), lambda (γ), epsilon (∈), nu (v) and mu (μ).
• Uranus rotates from east to west on its axis, which is opposite to other planets except Venus.
• The axis of Uranus has large inclination so that it appears to be lying down, hence it bears the name 'A Planet on its Side'.

Neptune

• Neptune is the 8th (farthest) planet of the Solar System.
• The temperature on the surface of Neptune remains low.
• Neptune is very similar to Uranus and can be considered as its twin.
• Neptune is surrounded by methane rings of sub zero temperature.

Pluto is not a Planet now

• On the basis of the new definition of planet given by the IAU (International Astronomical Union), the world's top institution on space science research, leading astronomers participating in IAU's meet at Prague (Czech Republic) on August 24, 2006, declared that Pluto would no longer remain a planet.
• Under the IAU's new guidelines, the number of planets in the Solar System has thus been reduced from nine to eight. Its merits mentioning here that, prior to this decision, Pluto had been holding the planetary status since its discovery in 1930 by Clyde Tombaugh.
• Now, with the omission of Pluto from the Solar System, its membership has been restricted to the eight 'classical' planets, namely Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

Pluto Gets a Numerical Denomination

Weeks after it was demoted to a sub-planetary status, Pluto was given a new name to reflect its new status as a dwarf planet in September, 2006. The former 9th planet was assigned the asteroid number 134340 by the Minor Planet Centre (MPC), the official organization responsible for collecting data about asteroids and comets in our Solar System.

Pluto's companion satellites, Charon (Pluto's largest moon), Nix and Hydra are considered part of the same system and will not be assigned separate asteroid numbers. Instead, they will now be called 134340 I, II and III respectively.

• Before loosing its planetary status on 24th August, 2006 Pluto was the outermost planet of the Solar System.

Some Facts and Figures about the Planets (As on August 17, 2022)

Some more elements in the Solar System

Asteroids (or Planetoids)

• Asteroids are also known as minor planets. They are objects that revolve around the Sun.
• They are mostly found between the orbits of Mars and Jupiter. They are a belt of debris which failed to assemble into planets and keeps on revolving around the Sun. This has come to be called as 'asteroid belt'.
• More than 5000 asteroids have been identified. Asteroids may be spherical, elongated or irregular in shape.
• All asteroids rotate on their axis, every 5 to 20 hours. Certain asteroids may have satellites.
• Trojan asteroids are found in two clouds moving in the orbit of Jupiter, one moving ahead of it and the other moving behind it.
• Scientists believe that these asteroids occupy a place where a planet could have existed but was prevented from its formation by the disruptive gravitational force of the nearby giant planet, Jupiter.

Meteors and Meteorites

• Meteors and Meteorites are also called shooting stars.
• Meteors are fragments of rocks coming towards the earth, formed due to the collision of asteroids with one another.
• Meteors are usually small, and due to the heat produced by air resistance, bum up before they reach the Earth's surface.
• When meteors are large and do not bum up completely, they land on the Earth's surface and are known as Meteorites.
• All meteorites are believed to originate in the asteroid belt, where a sudden collision may send them towards the Earth and the Earth's gravity attracts them towards its surface.

Comets

• Visitors of the Solar System, Comets (the name derived from the Latin words Stella cometa meaning 'hairy star') are among the most spectacular and unpredictable bodies in the Solar System.
• Comets move around the Sim in regular orbits, but their orbits are elongated ellipses that it takes them hundreds and, sometimes even thousands of years to complete one revolution around the Sun.
• Comets are made up of frozen gases which hold together rocky and metallic materials.
• A comet becomes visible only when it travels close to the Sun. Its ice melts and the gas and dust is swept back into a tail.
• The tail always points away from the Sun. So when it is travelling away from the Sun it is led by its tail.

Features of a Comet

• A comet is characterized by a long luminous tail, which emits light. But, this is visible only when the comet's orbit passes close to the Sun.
• When the comet travels close to the Sun, the ice melts to a head of gas called a Coma. The Sun's radiation sweeps this into a gas tail. Dust particles are also swept back to form a dust tail.

Stars

• Stars are heavenly bodies made up of hot burning gasea thus shining by their own light.
• Stars seem to be fixed with respect to each other, in fact they are in rapid motion but they are at such great distance that relative changes in position become noticeable only over the centuries.
• According to NASA Proxima Centauri is the closest star to the Earth after the Sun. It is about 4.24 light years away.
• Pole star (or Polaris), Sirius, Vega, Capella, Alpha centauri, Beta centauri, Proxima centauri, Spice Regulus, Pleiades, Aldebaran, Arcturus, Betelgeuse, and of course the Sun are some of the important examples of the stars.

Facts about Stars

• There are billions and billions of stars in the sky but only about 2000 stars can be seen with the naked eye on a clear moonless night.
• There are 10²² stars in the Universe.
• About 8000 stars are visible from the Earth with naked eye. Out of this, 4000 stars are visible in the Northern Hemisphere and 4000 in the Southern Hemisphere.
• In either hemisphere, only 2000 stars are visible at any given time. The other 2000 are located in the day-time sky and the brightness of the Sun renders them invisible.

Constellations

• To enable astronomers to identify roughly the position of the stars, the sky has been divided into units. These units are known as Constellations.
• These constellations were named in the honour of mythological characters.
• At present 88 constellations are recognized.

Some well known Constellations

• Some well known constellations, with their Indian names are given below :

Earth's Galaxy: The Milky Way

• We live on the outer edge of a spiral type of galaxy called the Milky Way which is about 1,00,000 light years in diameter and is rotating slowly.
• The Milky Way is a large spiral-shaped galaxy.
• It spans about 1,00,000 light-years across and is about 10,000 light-years thick at the centre.
• It is called the Milky Way because it appears as a soft flowing light of billions of stars. These stars are so far that they can be seen only in constellation, not separately.
• Galileo discovered that this band of light was produced by countless individual stars which a naked eye can not see. It takes about 250 million years to complete one revolution.

Andromeda: Earth's closest Galactic neighbour

• Andromeda is a spiral galaxy and also our closest neighbour. It appears as a fuzzy patch of light and contains millions of stars.
• It is the farthest object that can be seen with the naked eye. Along with the Milky Way, it belongs to a group of galaxies known as the Local Group, which in turn is a part of Virgo Cluster of groups.
• Like stars, galaxies are grouped into clusters. Some dusters contain thousands of galaxies. About 30 galaxies, along with the Milky Way and the Andromeda are grouped together in one cluster called the LocalGroup.
• Clusters may group together into upper clusters. Super dusters are also spread randomly throughout the universe.

Nebulae

• Nebulae are huge interstellar clouds of gas and dust that appear as faint, misty patches of light scattered all over the sky.
• They appear either as bright luminous clouds or as dark patches against a brighter background.
• A nebula depends for its luminosity upon the presence of stars that have either arisen from it or are contained in it.
• If the stars are extremely hot, the hydrogen in the nebula is ionized and emits a certain amount of light of its own.
• If a star is less hot, the nebula shines only by reflection.
• If there are no suitable stars, the nebula does not shine and remains dark and can be detected only because it blots out the light of the stars beyond.

There are five types of nebulous objects in the sky –

1. Planetary nebulae,

2. Emission nebulae,

3. Reflection nebulae,

4. Dark nebulae and

5. Supernova remnants.

The Earth: Shape Size & Statistical Data

Shape of the Earth:

Pythagoras (572-500 B.C.), a Greek philosopher and mathematician, was among the first to suggest that the Earth was shaped like a globe.

The Earth is not flat: If the Earth were a flat disc, then the rising Sun would have been seen at all places at the same time. But this does not happen. Places in the east see the rising Sun earlier.

• When a ship approaches land, its funnel or mast is seen first and then the hull. If the Earth had been flat, the whole ship would have been seen at the same time.

The Earth is a sphere: The Earth is rarely oriented in the same position during successive eclipses but it always casts a circular shadow, thus proving that the Earth is a sphere. A sphere is the only solid body that will always cast a circular shadow.

• At the North Pole, the Pole Star can always be observed at 90 degrees in the sky, since the star lies in the line with the axis of the Earth.
• As one travels southwards, the angle of Pole Star decreases. At the Equator the angle becomes zero degree. This observation proves that the path of travel is an arc of a circle.
• The Sun, Moon and all the heavenly bodies appear to be spherical when viewed from different positions. It seems logical to conclude that the Earth is no exception.
• The photographs of the Earth taken from the space prove beyond any doubt that the Earth is a sphere.

The Earth as an Oblate Spheroid: Refined measurements of the Earth have proved that the true form of the Earth resembles a sphere that has been compressed at the poles and made to bulge at the Equator. This form is known as an oblate spheroid.

The various factors which make the earth suitable for life to evolve and survive are:

• The earth has all the essential elements like carbon, hydrogen, nitrogen and oxygen, which act as building block for the origin of life.
• The earth is neither too hot nor too cold. It has the right temperature range for carrying out the life-sustaining chemical reactions.
• The earth has a lot of water in the form of lakes, rivers and oceans for the growth and survival of life.
• The earth has enough oxygen gas in its atmosphere for the survival of living beings through breathing.
• The earth has a protective blanket of ozone layer high up in its atmosphere to save life from harmful ultraviolet radiations coming from the sun.
• Biodiversity changes increase towards equator. Biodiversity is the degree of variation of life. It is a measure of variety of organisms present in different ecosystems. It is richest in the tropics.

Statistical Data of the Earth

The Earth, third planet from the Sun, is the fifth largest planet in the Solar System in terms of size and mass.

• Average diameter of Earth is 12,742 km, which was first calculated by Eratosthenes, the Greek mathematician.
• 29.1% of the total surface area of Earth is covered by continents (land), while 70.9% is covered by oceans.
• The total water area of the earth including the oceans, lakes, rivers, ice sheets and the water in the atmosphere is called hydrosphere and it covers about 71% of the earth's surface.