Galaxy and Stars

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Science > Physics > Our UniverseYou are Here

A Galaxy:

  • The stars, the planets, the moon and many other objects in the sky are called celestial or heavenly bodies.
  • The universe includes everything that exists. The branch of science that deals with the study of the universe is called astronomy.
  • The Big Bang Theory is the most widely accepted theory about the origin of the universe. According to this theory, the universe emerged from extremely dense and hot state. This was followed by a huge explosion, which gave birth to our universe. and it is assumed to be ever expanding.
  • A cluster of stars is called Galaxy. A galaxy contains billions of stars. Galaxies exist in different shapes. Two most common shapes are elliptical and spiral. Our galaxy is a spiral galaxy. The galaxies having no specific shapes are called irregular galaxies.
  • Our galaxy is called Milky Way (Akashganga). It consists of 100 billion stars including our sun. There are millions of galaxies like the Milky Way.

Our Galaxy
Far galaxy
  • Earlier our ancestors learned about the celestial objects through a direct and constant observation. At present we use three advanced techniques and methods to study the celestial objects. These three techniques are
    a) Use of telescope.
    b) Use of observatories with powerful telescopes
    c) By sending spaceships to distant planets.

Stars Observation

  • Hubble telescope is the most powerful telescope in the space.
  • The distance between two celestial objects is very large. Hence it can not be measured by using our practical terrestrial unit kilometre. The distance between celestial bodies is measured in a unit called a light year. Though it contains the word year, it is not the unit of time. It is the unit of distance.
  • Light moves at a velocity of about 300,000 kilometres per second. The distance that light can travel in one year is called the light year.
  • So in one year, light can travel about 10 trillion km. More precisely, one light-year is equal to 9,500,000,000,000 kilometers.

Our Galaxy, The Milky Way:

  • It measures some 120,000–180,000 light-years in diameter.
  • Our Solar System resides roughly 27,000 light-years away from the Galactic Center, on the inner edge of one of the spiral-shaped concentrations of gas and dust particles called the Orion Arm.

Galaxy 01

  • Our galaxy is not of perfectly disc-shaped but slightly warped.

Galaxy 02

  • Scientists believe that 90% of our galaxy’s mass consists of dark matter, which gives it a mysterious halo.
  • The Milky Way has between 100 to 400 billion stars.
  • The Milky Way is full of dust and gas.
  • The Milky Way wasn’t always as it is today. It acquired its current size and shape by eating up other galaxies and is still doing so today.
  • There is a black hole at the centre of the galaxy.
  • Our Milky Way has been around for about 13.6 billion years. Its age is almost equal to that of the universe.
  • The Milky Way, along with everything else in the Universe, is moving through space.

The Milky Way


  • The stars forming a group that has a recognizable shape is called constellation.
  • The shape of constellation remains the same.
  • The constellations appear to move in the sky from the east to west. Actually, the Earth rotates about its axis from the west to the east, hence we see the apparent movement of the constellations from the east to the west.
  • The constellations are named after observing their shapes and according to the prevailing beliefs or events.
  • A constellation does not have only 5 to 10 stars. It has a large number of stars. However, we see only the bright stars in the constellation with our naked eye.
  • All the stars which make up a constellation are not at the same distance. They are just in the same line of the sight in the sky.

Constellation The Great Bear or Ursa Major or Great Dipper or Saptarishi:

Constellation the Great Bear

  • The constellation we can see during summertime in the early part of the night is Great Bear. The Great Bear is also known as Ursa Major or Great Dipper or Saptarishi
  • There are seven prominent stars in this constellation. It appears like a big ladle or a question mark. Three stars of this constellation are in the handle of the ladle and four in its bowl.

Constellation Saptarishi

  • A pole star can be located using this constellation.

Constellation the Orion:

Constellation Orion

  • The constellation Orion can be easily seen on winter late nights. The Orion is also called the Hunter.
  • The three stars in the middle are said to be Orion’s belt. The Orion has seven to eight bright stars.
  • If the line on the stars from the belt is stretched Eastwards it leads to the star called Sirius. Sirius is the brightest star in the sky.

Constellation Cassiopeia:

Constellation Cassiopeia

  • Cassiopeia is a prominent constellation in the northern sky which is visible during winter in the early part of the night.
  • Cassiopeia constellation looks like the distorted letter W or M.

Constellation Scorpius:

Constellation Scorpius

  • It is one of the brighter constellations. Stars in this constellation form the image of a scorpion, with a long curving tail.
  • From the northern hemisphere, Scorpius can be seen in the southern sky, close to the horizon.
  • The Antares is the brightest star in the constellation Scorpius It marks as the heart of the scorpion.

International Astronomical Committee:

  • The whole sky is divided into 88 parts for convenience by International Astronomical Committee / Union (IAC or IAU).
  • There are 37 constellations belonging to the northern sky and 51 constellations belonging to the southern sky.

Indian Contribution:

Ancient Indian astronomers had imagined 27 constellations which they called nakshatras.

Ashwini Bharani Kritika
Rohini Mriga Ardra
Punarvasu Pushya Ashlesha
Magha Poorva Uttara
Hasta Chitra Swati
Vishakha Anuradha Jyeshtha
Tool Poorvashadha Uttarashadha
Shravan Dhanishtha Shattaraka
Poorvabhadrapada Uttarabhadrapada Revati


  • A star is a huge sphere of glowing gases. Star emits light of their own. The colour of a star depends on the temperature of the surface of the star. Blue stars are the hottest with the surface temperature of 28000 °C. Yellow white star (Our Sun) have the surface temperature of 6000 °C. Red stars have the surface temperature of 3000 °C.
  • The Sun is the nearest star to the Earth. The next nearest star to the Earth is Alpha Centauri which is at a distance of 40,000,000,000,000 km (40 billion km) from the Earth.
  • The stars are present in the sky during the daytime also. However, they are not visible then because of bright sunlight.
  • The Sun is a quiet smaller star than other stars but these stars are millions of times farther away than the sun to the Earth. Therefore the stars appear to us like points.
  • Stars appear to move from the east to the west. Actually, the Earth rotates about its axis from the west to the east, hence we see the apparent movement of the stars from the east to the west.
  • Polestar appears to be stationary throughout because it lies almost along the axis of rotation of the Earth.
  • The stars appear to be permanent because the physical characteristics like brightness, temperature, colour and size change very slowly with time as compared to our lifespan. Actually, stars are slowly dying. It takes millions of years for noticeable change to occur in a star while average life span of a human being is 70 years. Hence stars appear to be permanent to us even though they are born, grow and then die.
  • The stars are very far away from the Earth and appear as dots or points. The point position of the stars vibrate due to disturbances by air currents and hence they appear to twinkle.
  • The distances between stars is very large. To measure such large distances, the unit kilometre is very small. Hence astronomical unit light year is used to measure the distances between the stars.


Stars Sun

  • The Sun is the nearest star to the Earth. The sun is nearly 150,000,000 km (150 million km) away from the Earth.
  • The diameter of the sun is about 1,392,000 km. The Sun is so big that it can accommodate 1.3 million Earth in it.
  • The temperature on the surface of the Sun is about 6000 °C while the temperature at the centre of the Sun is about 1,400,000 °C.
  • Sun emits a huge amount of energy continuously which is produced due to the nuclear fusion reaction. In this process, four nuclei of hydrogen combine to form one nucleus of helium. During this process, a large amount of energy is produced.
  • The sun is the source of almost all energy on the Earth.
  • Sun is a star because it has its own source of energy. It continuously emits a huge amount of energy in the form of heat and light. It has a life period. It was born 5 billion years ago and is expected to glow for another 5 billion years.
  • Light takes 8 minutes to travel from the Sun to the Earth, hence the distance between the sun and the Earth is about 8 light minutes.
  • The distance of the nearest star Alpha Centauri from the Earth is 4.3 light years.

Pole Star:

  • Polestar can be located with the help of the constellation ‘The Great Bear’. Imagine a straight line passing through the stars at the end of the constellation ‘The Great Bear’ as shown. Extend this imaginary line towards the north (about 5 times the distance between the two stars). This line will lead to a star which is not bright. This is the Pole star.

Locating Pole Star

  • The pole star has unique importance. The Pole star lies almost along the axis of rotation of the Earth. Hence it appears to be practically stationary from the northern sphere. Thus we can locate the North direction using Pole star at night. Thus other directions can be located using Pole star.

Next Topic : Solar System

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