ICSE Class 8 Physics Chapter 01 The Universe PDF Download

The Universe

Learning Outcomes

Galaxy

Constellation

Solar system: sun and eight planets

Moon; phases, tides, and eclipses

Other solar-system bodies

Artificial satellites

Harry is writing his school address on a chart he is making for his school bulletin board. Have a look at the address. It is so interesting to read. Can you tell what is so special about it?

Hally Public School, 13 New Street, Mumbai, India, Asia, Earth, Solar system, Milky way Universe

You will notice that the address ends at the universe. This means that our final home is the big universe. Can you tell what universe is? The sun, the planets, the stars, moon, and all matter, energy, and space that exist constitute this big universe.

Every time we gaze at the sky, we wonder how the big universe came into existence. Scientists say that it is almost impossible to tell how old the universe is, but they think that there was a time when all matter, energy, and space was condensed into the size of a pinpoint. This was followed by a huge explosion because of which matter got scattered in all directions. This led to the formation of the universe. Scientists refer to this theory as the Big Bang theory.

Teacher's Note

When you write your own address, start from your house number and go all the way up to the universe - it shows how we are all connected as part of this vast cosmos.

Galaxy

The branch of science that deals with the study of the universe is called astronomy.

The universe is considered to be made of galaxies, which are the building blocks of the universe. A galaxy is a vast collection of stars, nebulae, and interstellar material held together by gravity. (Nebula is a cloud of dust and gas inside a galaxy, in which a star is born.)

As per an estimate, there are probably more than a hundred billion galaxies in our universe! The smallest galaxies contain about 100,000 stars, while the largest contain upto 3,000 billion stars. On the basis of shape, galaxies are classified as irregular, spiral, and elliptical. Figure 1.1 shows the shape of different galaxies.

Figure 1.1 (a) Irregular galaxy; (b) Spiral galaxy; and (c) Elliptical galaxy

Milky Way Galaxy

Do you know to which galaxy does our solar system belong? We belong to a spiral-shaped galaxy called the Milky Way galaxy. When you look up at the night sky, you can see a broad band of light in the sky. This is formed by millions of faint stars that together form the Milky Way galaxy. Have you ever wondered why this galaxy is called the Milky Way? Because it looks like a trail of milk split across the sky [Fig. 1.1(b)].

This spiral galaxy is vast, having a diameter of 100,000 light years.

Light Year

You might have measured distances in your day-to-day life in metres or kilometres. But, these units cannot be used to measure distances in space because the distances are extremely large. Therefore, astronomers often measure distances in space in light years. A light year is the distance travelled by light in one year.

We know that,

Distance travelled by light in one second = 3 x 10^8 m,

So, distance travelled by light in one minute = 3 x 10^8 x 60 m,

distance travelled by light in one hour = 3 x 10^8 x 60 x 60 m,

distance travelled by light in one day = 3 x 10^8 x 60 x 60 x 24 m,

distance travelled by light in one year = 3 x 10^8 x 60 x 60 x 24 x 365 m = 9.46 x 10^15 m or 9.46 x 10^12 km.

Teacher's Note

The immense distances in space, measured in light-years, help us appreciate just how vast the universe is - when we see a star's light, we're literally looking into the past.

Constellation

We know that a vast collection of stars is called a galaxy. When you look at stars on a clear night, you can see some groups of stars forming recognizable patterns. Such groups of stars forming recognizable patterns in the sky are called constellations. These are named after animals and mythological characters.

There are 88 constellations known so far. When any new star is discovered, it is considered to be a part of the constellation it is closest to.

Some constellations that you can easily identify with the naked eye are Ursa Major, Ursa Minor, and Orion.

Ursa Major

This constellation consists of many stars arranged in a pattern resembling a bear [Fig. 1.2 (a)]. Out of these stars, a group of seven stars constitute the brightest and the most recognizable part of the Ursa Major. This group of seven stars is called the Big Dipper or Saptarishi. If these seven stars are joined with imaginary lines, the figure obtained looks like a spoon with a long handle [Fig. 1.2 (b)].

Ursa Minor

Just like Ursa Major, Ursa Minor also consists of many stars, but seven stars called Little Dipper or Laglhu Saptarshi form its most recognizable part (Fig. 1.3). However, the stars here are closer and smaller. At the tail of Ursa Minor is a star, of average brightness, called the Pole star or Dhruv tara. It appears to be stationary in the sky, and all other constellations appear to revolve around it.

Orion

It is often called the Hunter and is one of the most common and easily identifiable constellations in the sky (Fig. 1.4). Orion dominates the winter sky, and its name in Indian Astronomy is Mriya.

Teacher's Note

Next time you look at the night sky, try to spot these constellations - they've guided travelers and storytellers for thousands of years across many cultures.

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