Subject - General Science
Chapter :Universe

Universe:

The universe is a huge wide-open space that holds everything from the smallest particle to the biggest galaxy.

Scientists believe that about 13.7 billion years ago, a powerful explosion called the Big Bang happened. This powerful explosion set the universe into motion and this motion continues today. Scientists are not yet sure if the motion will stop, change direction, or keep going forever.

Evolution of Universe:

The three main theories put forward to explain the origin and evolution of the universe are:

1. The Big Bang Theory
2. The Steady State Theory
3. The Pulsating Theory

• (i) The Big Bang Theory: Le Maitre and Gammow proposed this theory. According to this theory, at the beginning of the universe, the whole matter of the universe was once concentrated in an extremely dense and hot (~10 12K) fireball. Then about 20 billion years ago a vast explosion (big bang) occurred. The matter was broken into pieces, which were thrown out with high speed in all directions forming stars and galaxies; which are still moving way from one another. According to Hubble’s law, the velocity of recession of a galaxy becomes equal to the velocity of light at a distance equal of 20 billing light years. It means, the light rays from stars and galaxies, which are situated at a distance of 20 billion light years or more, can never reach us. Thus this distance becomes the boundary of observable universe. On account of continuous recession, more and more galaxies will go beyond this boundary and they will be lost. As a result of this, the number of galaxies per unit volume will go on decreasing and ultimately a time may come when we may have empty universe.

• (ii) Steady State Theory: Bondi, Gold and Fred Hoyle developed this theory. According to this theory, the number of galaxies in the observable universe is constant and new galaxies are continuously being created out of empty space, which fill up the gaps caused by those galaxies, which have crossed the boundary of the observable universe. As a result of it, the overall size of mass of the observable universe remains constant. Thus a steady state of the universe is not disturbed at all.

• (iii) Pulsating Theory: According to this theory, the universe is supposed to be expanding and contracting alternately i.e. pulsating. At present, the universe is expanding. According to pulsating theory, it is possible that at a certain time, the expansion of the universe may be stopped by the gravitational pull and they may contract again. After it has been contracted to a certain size, explosion again occurs and the universe will start expanding. The alternate expansion and contraction of the universe give rise to pulsating universe.

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Galaxies:

A galaxy contains stars, gas, and dust which are held together as a group by gravity. There may be millions, or even billions, of stars in one galaxy. There are billions of galaxies in the universe. Galaxies are labeled according to their shape. Some galaxies are called “spiral”, because they look like giant pinwheels in the sky. The galaxy we live in, the Milky Way, is a spiral galaxy. Some galaxies are called “elliptical”, because they look like flat balls. A galaxy may be called “irregular” if it doesn’t really have a shape. A new type of galaxy was discovered recently, called a “starburst” galaxy. In this type of galaxy, new stars just seem to ‘burst out’ very quickly.

(i) The Milky Way

• The Milky Way is over 100,000 light-years wide. It is called a spiral galaxy because it has long arms which spin around like a giant pinwheel. Our Sun is a star in one of the arms. When you look up at the night sky, most of the stars you see are in one of the Milky Way arms.

• A light-year is the distance light travels in one year. It is 9.5 trillion (9,500,000,000,000) kilometers. The size of a galaxy may be as little as a thousand light-years across or as much as a million light-years across.

(ii) Quasars

• Quasars are farther away from Earth than any other known object in the universe. Because they are so far away from us, it takes billions of years for the light they give off to reach Earth. The light stays the same, it just has to travel a long time to get to us. When we look at a quasar, it is like we are looking back in time. The light we see today is what the quasar looked like billions of years ago. Some scientists think that when they study quasars they are studying the beginning of the universe.

• Quasars give off huge amounts of energy. They can be a trillion times brighter than the Sun! Astronomers think that quasars are located in galaxies which have black holes at their centers. The black holes may provide quasars with their energy. Quasars are so bright that they drown out the light from all other stars in the same galaxy. The word quasar is short for quasi-stellar radio source. Quasars give off radio waves, X-rays, gamma-rays, ultraviolet rays, and visible light. Most of them are larger than our solar system.

(iii) Dark Matter

• Matter is anything that takes up space and has mass. We are used to matter which we will call visible matter. Visible matter can be seen because it gives off light or reflects light given off by another object. Dark matter cannot be seen. It does not give off light or reflect light.

• Scientists believe that over ninety-percent of the matter in the universe is dark matter. They also believe that by studying dark matter they will gain new information about the universe. Some of the information they hope to discover is the size, shape and future of the universe. Scientists also hope to learn about how galaxies formed by studying dark matter.

• Dark matter was once called “missing matter”. It was called this because scientists looking at the sky could not find it.

STARS

• A star is a huge, shining ball in space that produces a tremendous amount of light and other forms of energy. The sun is a star, and it supplies Earth with light and heat energy. The stars look like twinkling points of light — except for the sun. The sun looks like a ball because it is much closer to Earth than any other stars. Stars are formed initially from gas and dust. They are composed mainly of the hydrogen gas. Gas are very hot and give off huge amounts of energy in the form of heat and light. Our Sun is a medium sized star.

• Stars have a life-span of about 10 billion years, after which they will cease to exist.
• Stars are very far away from Earth. The closest Star is about 23.5 trillion miles away.

What Is The Closest Star?

• The closest star to us is the sun! Other than that, the closest star is Proxima Centauri, aka Alpha Centauri C (the dimmest star in the Alpha Centauri system). Proxima Centauri is 4.3 light-years from the Sun.

Why Do Stars Twinkle

• The scientific name for the twinkling of stars is stellar scintillation (or astronomical scintillation). Stars twinkle when we see them from the Earth’s surface because we are viewing them through thick layers of turbulent (moving) air in the Earth’s atmosphere.

The Solar System

• The word “solar” refers to the sun; the sun is one of the 150 billion stars of the Milky Way. It moves through space taking with it a larger family of objects. The whole group is called the solar system. Our solar system is elliptical in shape. The sun is the center of the solar system. Solar system is always in motion. Its largest and most important members are the nine known planets and their moons, along with smaller objects called comets, asteroids, and meteoroids that orbit the sun. The sun is the biggest object in our solar system. It contains 99.8% of the solar system’s mass. Many scientists believe that our Solar System is over 4.6 billion years old.

• The planets Mercury, Venus, Earth, and Mars, evolved as globes of rock that are present near the Sun. They were too small and their gravitational fields too weak to capture. However, far from the sun, the massive planets Jupiter and Saturn, with powerful gravitational fields, did attract and hold thick gaseous atmospheres of Hydrogen and Helium.

• The Sun is our closest star. It is a member of the Milky Way galaxy. The diameter of the Sun is 1,392,000 kilometers. It is believed to be over 4 billion years old. The Sun is a medium sized star known as a yellow dwarf. The Sun spins slowly on its axis as it revolves around the galaxy. The Sun is a large ball of gas consisting mostly of hydrogen and helium. The Sun is about 109 times larger than Earth.

Other Objects in the Solar System

• Asteroids: Asteroids are rocky and metallic objects that orbit the Sun but are too small to be considered as planets. They are known as minor planets. Most of the asteroids in our solar system can be found orbiting the Sun between the orbits of Mars and Jupiter. This area is sometimes called the “asteroid belt”. A few asteroids approach the Sun more closely.

• Asteroid belt: The asteroid belt is a doughnut shaped concentration of asteroids orbiting the Sun between the orbits of Mars and Jupiter, closer to the orbit of Mars.

• Comets: A comet is made of dirty ice, dust, and gas. Scientists believe that comets are made up of material left over when the Sun and the planets were formed. When a comet gets close to the Sun, part of the ice starts to melt. Scientists think- about 100,000 million comets orbit the Sun. There are some comets orbiting the Sun like planets. Their orbits take them very close to and very far away from the Sun. Comet can be seen only when it comes close to the Sun. The Sun’s heat melts the comet’s ice to form glowing gases. The gases stream out into a long tail that can extend to millions of kilometers.

• Meteorites: Besides asteroids some smaller pieces of rocks and dust also orbit the Sun. These pieces of rock or dust enter the Earth’s atmosphere. As they pass they encounter great friction, which causes them to heat up and burn out. These burning pieces of rock or dust are called as meteors. Although they are not stars, people call them as shooting stars, because they flash light across the sky. Most of the meteors burn up before they reach the Earth. Some are so large that a part of it reaches the ground as a meteorite. Any leftover part that does strike the Earth is called a meteorite. A meteorite can make a hole or crater in the ground when it hits it. The larger the meteorite, the bigger the hole.

Planets:

• By the current count of astronomers, our solar system includes 8 planets and 5 dwarf planets. The planets were formed during the process of solar system formation, when clumps began to form in the disk of gas and dusk rotating about our young Sun. Eventually, only the planets and other small bodies in the solar system remained. The four rocky planets at the center of the solar system Mercury, Venus, Earth, Mars, are known as the inner planets. Jupiter, Saturn, Uranus, and Neptune are all composed primarily of gas and are known as the outer planets.

Earth:

• The Earth is the third planet from the Sun in our Solar System. It is the planet, we evolved on and the only planet in our Solar System that is known to support life. It is 149.6 million kilometers away from the Sun. It has one moon. The Earth is the fifth-largest planet in our Solar System (after Jupiter, Saturn, Uranus, and Neptune).

• Each day on the Earth takes 23.93 hours (that is, it takes the Earth 23.93 hours to rotate around its axis once .). Each year on the Earth takes 365.26 Earth days (that is, it takes the Earth 365.26 days to orbit the Sun once).

The Moon:

• The moon is a satellite that revolves around the Earth in an oval shaped orbit. Earth’s moon is smaller than the Earth. It is smaller than the sun. It looks bigger because it is much closer to the Earth. It is 240,000 miles (384,000 kilometers) away. Moon appears in the nighttime. The Moon turns so slowly that a moon’s day is two Earth-weeks long. Then it is night for the next two weeks. The moon is very hot during daytime and gets very cold at night. It appears hotter than boiling water because there is no air to protect the moon from the hot sunlight. It gets very, very cold during night because there is no air to hold heat on the moon. The moon has no atmosphere, air or water, to support life. Moon can be seen clearly with your eyes, binoculars, or a telescope.

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