Class 8 General Science Notes: - Unit-12: Our Universe: Solved Exercise (Long Questions and Answers)

(National Book Foundation - New Book/New Syllabus Based on National Curriculum Pakistan 2023-2024 and Onwards)

(Class 8 General Science Notes for 8th Centralized Exam 2023-2024 and onward, FDE, Islamabad)

C. Long Questions

Q1. What are galaxies? Explain different types of galaxies?

Ans:

Galaxies:

A galaxy is a huge collection of gases, dust, billions of stars, and their solar system.

Galaxies are classified into three main types.

1. Spiral galaxies.

Spiral galaxies are large rotating disks of stars and the central bright region at the core of a galaxy is called the galactic bulge. Example: Our milky way is a spiral galaxy.

2. Elliptical galaxies

Elliptical galaxies are roughly egg-shaped and have no galactic bulge at their centers. These galaxies contain old stars as there are few new stars formation occurring in them. They can range in size from a few thousand light-years across to more than a few hundred thousand.

3. Irregular Galaxies

Irregular galaxies are irregular in shape. They are in constant motion as a group.

Q.2- How do stars form? What stages does a star pass through? Why do some stars end up as white dwarfs and other as neutron stars or black holes?

Ans:

Stars are formed when gaseous clouds contract due to the pull of gravity.

As the particles merge, their kinetic energy increases. A nuclear fusion reaction starts. These reaction change light elements into heavier ones and release energy in the process and stars appear to glow. The energy released during this process stops further contraction due to gravity and stabilizes the size of the star.

Star end up or star death:

Death of stars occurs when the fuel for the nuclear reaction is used up. A dying star collapses under its gravity as the energy flow from the core of the star stops. Nuclear reactions outside the core cause the dying star to expand outward in the red giant phase before its collapse.

After red giant formation, the core of the star ends up as one of the following three types:

1. White dwarf

2. Neutron star (pulsar)

3. Black hole

Formation of white dwarf:

If the initial mass of the star is less than or equal to the mass of the sun, the red giant becomes a white dwarf.

Formation of Neutron star:

If the initial mass of a star is between one to five times of sun's mass, the red giant becomes a neutron star.

Formation of a black hole:

If the initial mass of the star is more than five times the mass of the solar mass, the red giant becomes a black hole.

Unit-12: Our Universe: Solved Short Questions and Answers

Class 8 General Science Notes

Source: General Science Book (National Book Foundation, Pakistan)

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