My Science School

What is meant by “Apogee” and “Pergiee”?

Apogee (Aphelion with reference to solar orbits) is the point in the elliptical orbit at which the orbiting body is at its farthest distance from the body around which it orbits.

Perigee (Perhelion with reference to solar orbits) refers to that point where the orbiting body is closest to the body around which it orbits. For example, in the case of an artificial satellite moving in an elliptical orbit around the Earth, its distance from the Earth would fluctuate during each orbit from a maximum at apogee to a minimum at perigee.

 

 

What is a nebula?

A nebula (Plural: nebulae or nebulas) is a vast cloud of gas and dust floating in space. Nebulae are usually detected in the night sky as bright or dark patches against the background of stars.

What are galaxies?

Galaxies are clusters of stars. The Sun and the Solar System are part of a star cluster called the Milky Way galaxy visible as a cloudy white area in the night sky and commonly called “Akash Ganga” in India.

How many galaxies are there in the Universe?

It has been estimated that there are about a hundred billion galaxies in the Universe.

What are spiral, twin spiral, and multiple spiral patterns of galactic discs?

The spiral, twin and multiple spiral patterns refer to the number of ‘arms’ of the spiral.

How are galaxies classified?

Galaxies are classified as elliptical, spiral and irregular depending on the nature of distribution of stars on the galactic disc.

The more common types have a regular structure. Elliptical galaxies arre denoted by the letter E followed by a numeral having values from 0 to 7 depending on the degree of flattening of the ellipse. Spiral galaxies arre denoted by Sa, Sb, Sc, Sba, SBb or SBc according to the pattern of the spiral arms. Barred spiral galaxies are denoted by Sba, SBb and SBc. SO denotes a class intermediate between elliptical and spiral. Irregular galaxies arre denoted by Irr.

 

 

 

 

What is Gravity?

“Gravity” or “Gravitational Force” or “Gravitation” refers to the natural force of attraction between objects in space. For example, we fall down when we jump up, because our bodies are attracted by the Earth’s gravity. The Moon orbits the Earth because the Earth’s gravity holds it in place in its orbit. The Earth, the planets and the other objects in the Solar System orbit the Sun due to the gravitational force exerted on them by the Sun. It plays a vital role in the behavior of the Universe.

 

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What are asteroids?

Asteroids are tiny heavenly bodies that move around the Sun. Most of them occur in the region between Mars and Jupiter. They are thought to be fragments left over from the formation of the solar system.

 

What is trajectory?

Trajectory is a general term for the path of any object moving in a force field such as a gravitational field that exerts a force on the object. Hence, the term trajectory can be applied to the path of space probe moving in a gravitational field.

 

 

 

 

What is an orbit?

When a small heavenly body moves around a larger one, the smaller body is said to orbit the larger one and the path followed by the smaller body is called its orbit around body. For example, the Earth orbits the Sun.

 

 

 

What are comets?

Comets are small heavenly bodies which orbit the Sun but regularly come close to the Sun. They are partially vaporized by the Sun’s heat and glow brightly for a short time when near the Sun. A comet consists of a central nucleus a few kilometers across and can be mixed with dust.

 

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What is a natural satellite?

A natural satellite is a smaller natural heavenly body that moves around a planet. For example, the Earth has one natural satellite (the Moon), Mars has two and Venus has none. Natural satellites that orbit planets are also called the moons of their planets.

What is the difference between natural and artificial satellites?

Nowadays man-made objects can be launched into space from the Earth. Today, the satellites circling the Earth in space include the Moon as well as such man-made objects. Satellites such as the Moon are called natural satellites and man-made ones are called artificial satellites.

 

What do we mean by the Solar System?

The solar system refers to the Sun and the various heavenly objects that move around the Sun.

Name the heavenly bodies that move around the Sun?

Planets, satellites, asteroids, comets etc.

What is a planet?

A planet is a major heavenly body. For example, the Earth is a planet and so are Venus and Mars.

What is a star?

A star is a massive heavenly body which emits enormous quantities of energy and appears to be glowing.

How many stars are there in a galaxy?

On an average, there are about a hundred billion stars in each galaxy.

How many stars are there in our galaxy, the Milky Way?

About four hundred billion stars.

Which star is nearest to the Earth?

The Sun.

How many stars are bigger in magnitude than the Sun?

In the Milky Way galaxy alone, there are five billion stars larger in magnitude than the Sun.

which heavenly body is nearest to the Earth?

The Moon.

 

What is the universe?

The Universe consists of everything one can cognize through one’s senses and through observations using instruments.

The Sun and the Moon are components of the Universe one can easily observe but some objects too faint or too far away can easily only be observed through instruments like telescopes.

What are the main components of the Universe is general terms?

 The Universe consists of a vast emptiness called space with unknown boundaries, containing matter and energy in various forms.

In what forms does matter exist in the Universe?

Matter exists in the form of stars, plants, comets, satellites, meteors, asteroids, galaxies etc.

In what forms does energy exist in the Universe?

Energy exists in the form of heat, light, X-rays, microwaves, gamma rays, radio waves etc. What the eyes detect is only a minute range of wavelengths and frequencies (visible light) from a vastly broader spectrum of electromagnetic radiation. The heat and light from the Sun is a part of the energy can be detected only through instruments.

Does the Universe remain steady and unchanging?

No. The Universe is constantly changing with the passage of time.

 

          The cosmic year is related to the motion of a galaxy. A galaxy is a large system of stars held together by gravitational force. There are millions of galaxies in the universe. The size of a galaxy is measured in Light Years. The diameters of different galaxies range from a few thousand Light Years to 500,000 Light Years.

          There are three main types of galaxies depending on their shapes: spiral galaxies, elliptical galaxies and irregular galaxies. The spiral galaxies are generally disc-shaped with curving arms extending from a centre bulge. The elliptical galaxies are shaped like spheres or flattened spheres. The irregular galaxies have no definite shape. 

 

 

          The Milky Way or Akash Ganga as described by ancient Indian astronomers is a spiral galaxy. The Milky Way belongs to a cluster of some 24 galaxies called ‘The local group’. Our solar system belongs to this galaxy. Sun is one of about 100,000 million stars located in the Milky Way. The modern estimates place the Sun at a distance of about 32,000 Light Years from the centre of the galaxy.

          The Milky Way is not stationary. It is rotating round an axis passing through its centre. This is the real reason for its flat disc-like shape. The whole disc of the galaxy rotates about its galactic centre —but not at a uniform speed. The rate of rotation decreases with increasing distance from the centre. The galaxy does not rotate like a solid wheel.

          Each star revolves around the central nucleus in an elliptical or circular orbit, just like the planets revolve round the sun. The stars near the centre move faster than the stars farther away. Sun moves round the centre of galaxy at a speed of 250 km per sec. It takes about 225 million years to complete one revolution around the galactic nucleus. This period is known as the cosmic year or the galactic year.

          There is evidence of galactic rotation in observations that in general stellar motions are not random, but in preferential directions. The Dutch astronomer J.C. Kapteyn first observed this effect in 1904.

 

           We can measure our body temperature with a thermometer. Thermocouples and other devices are used to measure the temperature of furnaces. But how can we measure the temperature of stars?

          The surface temperature of stars is determined by various techniques. The most conventional and fairly accurate estimate can be made by colour alone. Red-coloured stars are cool while blue ones are extremely hot. On the basis of colour, stars have been classified in the table given below.

          A more accurate determination of the temperature is made by the comparison of spectra of stars. Light, which comes from the sun and other stars, is made up of many different wavelengths. It can be separated into different wavelengths by a spectrograph (an instrument used to record spectrum). From the spectroscopic studies, it has been observed that stars are largely composed of hydrogen (about 75% on the average). Next in abundance is helium followed by various other metals. In the cooler stars, some compounds are present but at high temperatures, they disintegrate into atoms. In order to know the temperature, the spectra of stars are recorded. It will be different for different stars, depending upon their temperature.

          Moreover, the intensity of spectral lines, bright or dark, varies with the temperature. It has been found that blue stars have O-type spectra; our sun has G-type spectra and so on. Blue stars emit 20 or more times the radiation per unit area than that of our sun does, whereas a red type may emit as little as 1/20 as much per unit area.

          From these spectra, by measuring and comparing the intensity of different lines and using Wien’s Displacement Law, the temperature may be determined. Intensity of emitted light is plotted against wavelength and the curve is drawn. The temperature of the star is directly proportional to the frequency at which most of its radiation is given off, i.e. to the highest point of the curve. 

                                                                                                                                                                                            

          Earth is perhaps the only planet in the solar system where life exists. This is because the conditions favourable for the existence of life are available only on the Earth. Conditions present in other planets make life almost impossible.

          Mercury is the planet closest to the Sun. It is difficult to see it even with a powerful telescope. It does not have any atmosphere. The temperature during the daytime may even go beyond 400°C. The lack of oxygen and the extreme temperature make life on Mercury impossible.

          Venus, which comes next in terms of closeness to the Sun, is often described as the sister planet of the Earth. Venus and the Earth are almost identical in size, mass and density. Its diameter is 0.95 times and mass is 0.815 times as compared to the mass and diameter of the Earth. It is surrounded by thick clouds of carbon dioxide (95%). The temperature on its surface is about 95 times more (480°C) than that on the Earth. This makes Venus the hottest planet in the solar system. Under these conditions life is not possible here.

           Mars comes after the Earth and is much colder than the Earth. Its average temperature is about – 62°C. At night it may drop to – 101°C. It has an atmosphere much thinner than that of the Earth. It has been found that it contains 1 to 2% argon, 2 to 3% nitrogen, 95% carbon dioxide and 0.3% oxygen. These conditions suggest the possibility of existence of life on it but so far no traces have been detected.

          All other planets beyond Mars (Jupiter, Saturn, Uranus, Neptune and Pluto) are farther away from the Sun. The surface temperature of these planets is so low that no living organism can survive there. Besides, their atmospheres contain gases like methane and ammonia which are not favourable to the evolution of living organisms.

          Thus our Earth is the only known planet in the solar system where life exists. 

           The earth is round, and so are the sun and the moon. Even the planets, their satellites and the stars are also spherical. Do you know why all these heavenly bodies are spherical in shape?

           According to a law of science, every physical system in the universe tries to attain a state of minimum energy for maximum stability. It is an established fact that if a sphere is made from a given volume of any material it will have the least surface area compared to any other form. Consequently the spherical bodies have the minimum surface energy. Substances try to acquire a round form like a football in order to achieve the state of minimum energy, that is, the state of maximum stability. This is why the sun, the moon, the stars, the earth and all other heavenly bodies have a spherical shape.

           The rain drops are also spherical. It is so because the surface of the spherical drops has the minimum energy and maximum stability. Whatever be the process of producing the water drops, they immediately try to acquire a spherical shape.

           Atoms and molecules are also spherical in shape. Even the electrons, protons and neutrons present in the atoms are also spherical.

          You might have observed that every man tries to earn the most with the least effort. Every student tries to secure maximum marks after putting in the minimum labour. Thus we see that the tendency to come to the minimum energy state is found in all the living and non-living bodies. As soon as a substance acquires the minimum energy state, its shape becomes spherical.

 

 

          We see millions of stars twinkling in the sky during night. They radiate their light in all the directions. Even though they appear very small yet in reality they are very big. Most of the stars are many times bigger than our earth. They look smaller only because of their great distance from the earth. Do you know why the stars twinkle?

          Our earth is surrounded by a thick cover of air known as the atmosphere. There is vacuum beyond the atmosphere. The gases present in the atmosphere are in constant motion. Because of the movement of gases the density of air in the atmosphere is not uniform everywhere. As such the refractive index of air varies from place to place. When the light from a star enters our atmosphere, it gets deviated from its path several times before reaching our eyes because of the changing density and the consequent change in the refractive index of the air.

          The deviation in the path of light in its passage from one medium to another is called ‘refraction’. Because of this refraction, the light reaching our eyes from the star varies. Due to this variation the stars appear to be twinkling.

          Now the question arises: why don’t moon, sun and other planets twinkle like the stars? This is so because compared to the stars, sun, moon and the planets are very near to earth and as such they appear bigger than the stars. Hence the angles subtended by the moon, sun and the planets at our eyes are larger than the angles subtended by the stars. Because of the larger angles, our eyes are not able to detect the deviation in the path of light from the sun, moon and the planets and hence they do not appear to be twinkling. 

          On the full moon day, the moon appears like a shining disc but it goes on waning till it disappears on the new moon night. And then it again starts increasing in size and becomes a full bright disc, on the full moon day. Changes in the moon’s size are called the ‘phases’ of the moon. Do you know why it occurs? 

          The fact is that the moon neither increases nor decreases in size. Its size simply appears to be changing because it changes its position with respect to the sun. We all know that the moon is the only satellite of the earth and revolves round the earth. It travels around the earth about 384,400 km away. This orbit takes 27 days and eight hours. It is illuminated by the sunlight. Only one side of the moon faces the earth. We do not see its other side. When the moon comes in between the earth and the sun, its bright side is not visible to people on the earth and only its dark side faces the earth. As a result the moon cannot be seen. This is the new moon day. As the position of the moon changes due to its motion from east to west with respect to earth, some illuminated part of its surface is visible to us. A week after the new moon, almost half of the illuminated lunar disc becomes visible to us. The bright part of the moon’s surface goes on increasing gradually and on the full moon day the whole disc, seen from the earth, is fully illuminated. On this day the earth is in between the sun and the moon. In the next fifteen days the bright portion of the lunar surface goes on decreasing and on the fifteenth day it disappears completely. In short, the changes in the apparent shape of the moon depend upon how much of its illuminated surface is visible to us.

          The cycle continues like this and we see the different phases of the moon. On the full moon day it appears just after the sunset and disappears only at the sunrise. The time between new moon phases is called the synodic month.

 

          The heavenly bodies that revolve round the sun are called planets. There are nine planets in our solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. The bodies revolving round these planets are called their ‘satellites’ or ‘moons’.

          Scientific investigations made so far have revealed that all planets do not have satellites. For example, Mercury and Venus do not have any satellite. Earth has 1 satellite - the moon. Mars has 2 satellites and the Jupiter has 16. The number of the moons revolving round Saturn is 24. The Uranus and Neptune have 15 and 6 satellites respectively. Pluto has 1 satellite.

          The size of different satellites is different. There are some satellites which are bigger than moon. The diameter of two satellites of the Mars, Deimos and Phobos, and the outer satellites of the Jupiter, Ganymede and Callisto are as big as Mercury and Mars. The diameters of Titan and Triton - the satellites of Saturn and Neptune are 5150 kms and 2700 kms respectively and more than the diameter of our moon.

          Except Titan, all the satellites have small force of gravity. As such none of them has any atmosphere. Because of low temperature at Titan, it has an atmosphere consisting of methane and hydrogen. But there is no life on this satellite.

          As yet we have not come across any satisfactory theory regarding the origin of the satellites. However, it is believed that their origin is similar to that of our solar system.

              The question whether there is life on planets other than the Earth has often perplexed scientists. If it is true that there is life on planets other than the Earth, what could be the possible ways of establishing contact with the living beings of other planets?

               It is felt that only radio can help to solve this problem. In 1960 American scientists received radio signals from some stars that created a hope about the existence of life on some other heavenly bodies. However, they could not get any concrete evidence to prove this.

                     A few years ago, people talked of having seen some unidentified flying objects (UFO’S) in the sky at many places. It was observed that the flying objects were shaped like saucers. People called them flying saucers. This led scientists to think of the possibility of life on other planets as well. It was felt that these saucers were extra-terrestrial attempts at scientific experiments. These saucers had different shapes. Some of them were round while others were cigar shaped. Many stories were heard about their colours. Their speed was reported to be very high.

                  These flying saucers have also been photographed. They were even chased by aeroplanes. A thorough study of these objects revealed that the so-called flying saucers were either balloons launched for weather studies or some meteoric bodies. Now scientists are of the view that the Earth is the only planet which has favourable conditions for the existence of life. Scientists have not found any other planet in the universe where there is life.

 

              The universe includes the Sun, Earth, solar system, galaxies and everything else that exists. Even with the most powerful telescopes, astronomers can not see the limit of the universe. Most scientists believe, however, the universe is mostly a vast empty space.

              Now the question that arises is how did this universe come into existence? There are many theories regarding its origin. According to one theory, the entire matter in the beginning of the universe was like a fire-ball. At a certain time, a big explosion took place and the matter of this giant fire ball got scattered in all directions. On cooling down these scattered parts gave birth to the galaxies. The matter of these galaxies has been expanding continuously.

            This incident took place around 20 billion years ago. Obviously, if this expansion continues, it will make the universe empty.

            However, according to another theory, because of the force of gravitation, a time will come when the expansion of the galaxies will stop, and they will start contracting. This suggests a pulsating universe. Again, according to a third theory, new galaxies are constantly under formation and the matter of the older galaxies goes on scattering. This is the theory of the balanced universe. It is difficult to say which of the three theories is authentic. However, in order to propound a correct theory regarding the origin of the universe and its existence, researches are still in process.