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(a) Our galaxy is called.........

(b) For measuring large distances........is used as a unit

(c) The speed of light is............km/s

(d) There are about..........stars in our galaxy

(e) The end stage of the Sun will be......

(f) Stars are born out of.....clouds

(g) Milky Way is a...........galaxy

(h) Stars are spheres of...........gas

(i) The masses of other stars are measured relative to the mass of the.............

(j) Light takes ................ to reach us from the Sun while it takes...........to reach us from the moon

(k) The larger the mass of a star the faster is it’s.......

(l) The number of fuels used in the life of a star depends on it’s............

(a) Milky Way Galaxy

Explanation:

Our galaxy has got its name due to its appearance in the sky. It looks like a spiral road of milk. Milky Way looks like a milky patch of sky.

(b) Light Year

Explanation:

1 Light Year = Distance travelled by light in one year

We know that,

Distance = Speed × Time …..1

Now, Speed of Light = ms^-1

Also, 1 km = 1000 m

So, Speed of light = kms^-1 …..2

Now, Time of consideration = 1 year

1 Year = 365 days

1 Day = 24 hours

1 Hour = 60 minutes

1 Minute = 60 seconds

So, 1 year = 365 × 24 × 60 × 60 seconds

1 year = 31536000 seconds …..3

Using equations 1, 2 and 3 we get,

Distance travelled by light in one year:

= km

= km

So, approximately

1 Light Year = km

Hence, one light year is very big unit and is also used to measure distances.

(c) The speed of light is 299792458 ms^-1

Explanation:

Speed of light is 299792458 ms^-1

This could be approximated to ms^-1.

Note: This is the speed of light in vacuum.

Speed of light is the maximum speed that can be attained.

(d) There are about stars in our galaxy

Explanation:

Our galaxy has about stars. The Sun is also a star.

Sun is called an ordinary star, since there are many stars that have higher and lower mass, size and temperature than Sun.

Sun appears to be largest because it is nearest to us.

(e) White Dwarf Star

Explanation:

We know that Sun is a star and that star undergoes evolution. This evolution of stars depends on their mass. The higher the mass of star, faster is the evolution.

Sun is an ordinary star.

White Dwarfs have large mass. White Dwarf are called so because of their very small size comparable to size of earth. Due to large mass and small size, they have high density.

(f) Interstellar Clouds

Explanation:

Interstellar clouds are huge clouds of dust and gas present between the stars and the galaxy. Size of interstellar clouds are about a few light years where one light year is equal to km. These interstellar clouds contract due to disturbances and due to these contraction density and temperature increases and a dense sphere of hot gases are formed.

A sufficient temperature and density nuclear energy generation starts, due to this gas sphere becomes self-luminous and a star is formed or in other words a star is born.

(g) Milky Way is a Spiral galaxy

Explanation:

Galaxies are divided into 3 types:

1. Spiral Galaxies

2. Elliptical Galaxies

3. Irregular Galaxies

This categorization is based on the appearance, structure and shape.

Milky Way is categorized under spiral galaxy. Its shape is like a disc with bulge in the centre.

Diameter of our galaxy is km

The galaxy is rotating around an axis passing through its centre and perpendicular to the disc.

(h) Stars are spheres of hot gas

Explanation:

This is because stars are made from contraction of interstellar clouds that are made of gas and dust.

Stars are a gas sphere at a very high temperature and density (hot gas) such that due to nuclear reaction they are self-luminous.

(i) Sun

Explanation:

Mass of Sun =

is used as a unit of mass

A unit is a quantity adopted as a standard of measurement.

For Example: Let mass of star be 10 times the mass of sun. So, its mass is written as = 10

(j) Light takes about 1 second to reach us from the Sun while it takes 8 minutes to reach us from the moon

Explanation:

We know that,

…..1

Distance between Earth and Moon = 384400 km

= 384400000 m ….2

Since, 1 km = 1000 m

Distance between Earth and Sun = km

= m

…..3

Since, 1 km = 1000 m

Speed of Light = ms^-1 …..4

Time taken for light to reach earth from moon

= seconds

= 1.281 seconds 1 second

Time taken for light to reach earth from Sun

= = 498.67 seconds 8 minutes

Where 1 second = minute

(k) rate of evolution

Explanation:

Evolution of star is the change in the properties of a star.

Rate of evolution is the change in properties with time.

The evolution process of a star depends on the mass of the star (that is due to the hot gases). The higher is the mass of star the higher is the rate of evolution.

(i) mass

Explanation:

The higher the mass of star the higher is the number of fuels used.

We know that stars are continuously emitting energy. Hence energy is decreasing so as to maintain a balance in temperature of fuels that is gases inside the star burn to give the energy.