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Our Universe
ASTRONOMY
On a clear night, you can see thousands of stars in the sky. You can also see some planets, like Venus, Mars, Jupiter and Saturn. These are known as heavenly or celestial
objects. The scientific study of celestial objects developed greatly after the invention of the telescope in the 17th century.
The scientific study of celestial objects is called astronomy. Scientists who pursue astronomy are called astronomers.
objects. The scientific study of celestial objects developed greatly after the invention of the telescope in the 17th century.
The scientific study of celestial objects is called astronomy. Scientists who pursue astronomy are called astronomers.
GALAXIES AND THE UNIVERSE
A cluster of stars is called a galaxy. A galaxy contains billions of stars.
Galaxies may have different shapes, such as elliptical and spiral. Some galaxies do not have a regular shape and are called irregular galaxies.
Look at the sky from north to south on a clear, moonless night. You can see a long, faint strip of light across the sky. This is the Milky Way, also known as the Akashganga in India. Our planet Earth belongs to this spiral galaxy. It consists of 100-400 billion stars. There are millions of galaxies like the Milky Way.
As billions of stars form a galaxy, billions of galaxies together form the universe. Thus, the universe consists of everything that exists.
Objects in the universe are in continuous motion. Galaxies are also in motion. The Sun, which is in the Milky Way, moves at a speed of about 220 km/s.
The distance between stars and other
heavenly bodies is millions of kilometres. To make it easier to express such distances, we use some special units of measure.
• One light year is the distance travelled
by light in 1 year, and is equal to about
9,460,000,000,000 km (9.46 x 10^12 km).
• The astronomical unit (AU) is
approximately the distance between
the Sun and the Earth. (1 AU is about
149,600,000 km or 1.496 x 10^8 km.)
• The parsec is used to measure larger
distances, like the distances to stars.
(1 parsec is about 3.26 light years.)
Galaxies may have different shapes, such as elliptical and spiral. Some galaxies do not have a regular shape and are called irregular galaxies.
heavenly bodies is millions of kilometres. To make it easier to express such distances, we use some special units of measure.
• One light year is the distance travelled
by light in 1 year, and is equal to about
9,460,000,000,000 km (9.46 x 10^12 km).
• The astronomical unit (AU) is
approximately the distance between
the Sun and the Earth. (1 AU is about
149,600,000 km or 1.496 x 10^8 km.)
distances, like the distances to stars.
STARS
Stars are of different sizes and brightness. They can be seen with the help of powerful telescopes. Most of them are millions of times farther away from us than the Sun. Like the Sun, all stars emit light of their own.
Movement of Stars
The stars and the Sun appear to move
across the sky from east to west because the Earth rotates on its axis from west to east. However, there is one star that does not appear to move-this is the Pole Star (Dhruva Tara).
Constellation
Most of the stars in the night sky, when
viewed from Earth, appear to be in groups. Groups of stars forming a recognisable shape when seen from the Earth are known as constellations.
viewed from Earth, appear to be in groups. Groups of stars forming a recognisable shape when seen from the Earth are known as constellations.
The constellations appear to move in the sky from east to west. But the shape of the constellation remains the same because the stars in a constellation retain their position
with respect to each other. However, the stars need not be close to one another. Astronomers have recognised and named 88 constellations. The Great Bear, Orion, Scorpius and Cassiopeia are examples.
with respect to each other. However, the stars need not be close to one another. Astronomers have recognised and named 88 constellations. The Great Bear, Orion, Scorpius and Cassiopeia are examples.
Ursa Major
It is also known as the Big Dipper, the Great Bear or Saptarishi. It forms the shape of a large spoon-like utensil known as a dipper. There are three stars in the handle of the dipper and four in the bowl. The position of the Pole Star can be located with the help of Ursa Major.
Ursa Minor
It is also known as Little Dipper or
Little Bear. The stars are arranged in
a pattern similar to that of Ursa Major, but they are not as bright and are closer together than in Ursa Major. The last star in the handle of the Little Dipper is the Pole Star. Ursa Major and Ursa Minor are visible in summer in the northern hemisphere.
Orion
The arrangement of stars in Orion
resembles a hunter with a belt and a sword. It is visible during winter in the northern hemisphere.
Scorpius
This is one of the brightest constellations. The stars form the image of a scorpion, with a long, curving tail. The bright red supergiant star Antares marks the heart of the scorpion. The constellation is easily seen in summer in the northern hemisphere.
Cassiopeia
It has five prominent stars that form a W or M pattern. The stars in the constellation together are supposed to form an image of the mythological African queen Cassiopeia. It is visible in the northern sky in winter.
THE SOLAR SYSTEM
The part of the Milky Way galaxy in which we live is called the solar system. It is made up of the Sun and its eight planets, the moons of the planets, other heavenly bodies such as asteroids, comets, meteors and meteorites, and vast regions of empty space between the heavenly bodies.
The Sun is at the centre of the solar system. All the planets revolve around the Sun in definiteelliptical (oval) paths called orbits. They take different times to complete one revolution around the Sun.
Sun
The Sun is a star. It is the star nearest to Earth. Compared to other stars, it is of average size, mass and brightness. It looks bright and big only because it is closer to us than any other star.
• The distance of the Sun from Earth is
149 million kilometres, that is, 8.3 light
minutes (the distance travelled by light in 1 minute.)
• The diameter of the Sun is 1.39 million kilometres, approximately 109 times the diameter of the Earth; it is approximately 333,000 times heavier than the Earth.
• The surface temperature of the Sun is
approximately 6000 K; the temperature
of the Sun's core is 15.7 million K. (Note: To convert from degrees Celsius (°C) to kelvin (K), add 273.) The present composition of Sun is about 71% hydrogen, 27% helium and 2% other
elements by mass. The composition changes slowly over time due to reactions called nuclear reactions. These reactions convert hydrogen to helium. This is the source of the energy
released by the Sun, which is the source of almost all the energy on the Earth. The Sun is about 4.6 billion years old. It has used up about half of the hydrogen in its core. It will continue to radiate heat and light for another 5 billion years. Eventually, it will run out of hydrogen and die. Though it is an ordinary phenomenon in the universe for a star to die, the Sun's death will
result in the total destruction of the Earth.
Planets
There are eight planets in our solar
system. These are, in order of increasing distance from the Sun, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The planets Mercury, Venus, Earth and Mars are called inner planets as they are closer to the Sun than the other planets. They are rocky in composition.
Jupiter, Saturn, Uranus and Neptune are called outer planets as they are much farther away from the Sun. They are mostly gaseous in composition. The inner planets have few moons, whereas the outer planets have several moons. The outer planets each have at least one ring around them.
Planets have no light of their own. They shine because they reflect sunlight that falls on them. They rotate about their own axes as they revolve around the Sun.
Mercury
Mercury is the planet closest to the Sun. It is, therefore, very hot during the day. As Mercury is a small planet, its gravitational force is not enough to permanently hold an atmosphere. Because of the absence of an atmosphere, the heat is quickly lost
at night, and it becomes freezing cold.
Mercury has extreme temperatures ranging from 100 K to 700 K. It is covered with bowl-shaped holes called craters. Mercury is visible to naked eye just before sunrise and just after sunset, near the horizon.
Venus
Venus is the planet closest to the Earth.
It is about the same size as the Earth.
It is the hottest planet, though it is
further away from the Sun than Mercury. This is because it has a high percentage of carbon dioxide in its atmosphere, which traps heat due to the greenhouse effect. This raises the surface temperature of Venus to
above 740 K.
Venus is the brightest of all the planets. It appears in the eastern sky before sunrise and in the western sky after sunset. So it is called both the Morning Star and the Evening Star (though it is not a star). Venus spins from east to west. Most other planets spin from west to east. Venus also is visible to us in phases like the Moon.
Earth
The Earth takes 365+1/4 days, that is, aquarter of a day more than a year, to revolve around the Sun. It takes 24 hours or one day to rotate on its axis. It is the only planet known to have water in liquid form, and oxygen in gaseous form, and the only planet on which life is known to exist. The Earth is at the most favourable distance from the Sun and also has an atmosphere. These factors help to maintain the right temperature on the Earth for living things to survive. The axis of rotation of the Earth is tilted to the plane of its orbit and is not perpendicular
to it. This tilt is responsible for the change in seasons during the year.
Mars
Mars is seen in the sky as a red planet. The red colour is due to the presence of iron oxides in the soil. It is less than one-fifth the size of the Earth by volume. Its surface is covered with craters and mountains. Mars has a thin atmosphere with traces of carbon dioxide, nitrogen, argon and other
gases. It is a cold planet. Mars has two moons-Deimos and Phobos.
Jupiter
Jupiter is the largest. planet Being far away from the Sun, it is very cold. Its size is about 1300 times that of Earth, and its mass is about 318 times
that of the Earth. It rotates very rapidly on its axis. Jupiter has
rings that are much fainter, smaller and darker than those of Saturn. It has 69 moons, four of which are large and can be seen easily with the help of a telescope.
Saturn
Saturn is the second largest planet. It is
very cold. Its distinctive feature is the ring made of ice and dust that surround the planet. It is the least dense of all planets, its density being less than that of water. Saturn has 62 moons.
Uranus
Uranus is the coldest planet and is much bigger than the Earth. It can be seen only through a telescope. Uranus has rings and bands of clouds that blow around the planet rapidly. It appears to roll on its side as it has a highly tilted axis of rotation. It has 27
known moons.
Neptune
Being very far away from the Sun, Neptune is very cold. It can only be seen through a telescope. Neptune has 14 moons.