CBSE Notes Class 8 Science - Stars and The Solar System

The universe is home to countless celestial objects, with stars and the solar system essential for understanding space. Stars are massive balls of gas, mainly hydrogen and helium, that emit light and heat through nuclear fusion. The Sun, our nearest star, is crucial for life on Earth, providing warmth and energy. The solar system includes the Sun and all orbiting objects, such as eight planets, their moons, dwarf planets, asteroids, and comets. 

1.0Moon And Its Phases

• The Moon is the most luminous object in the night sky, but it does not emit its own light; instead, it reflects sunlight. It rotates on its axis while revolving around Earth. With no air or water, the Moon cannot support life.
• Its surface is dusty and barren, marked by craters and steep mountains. The Moon undergoes daily shape changes over a month, known as the "Phases of the Moon. "Despite appearances, the Moon is always round; it transitions from a full circle to a "no moon" phase (new moon) over about 15 days (waning) and then back to a full moon (waxing) over another 15 days.
• Only half of the Moon is illuminated by the Sun at any time. On a full moon, Earth is between the Sun and the Moon, fully exposing the illuminated side. During a new moon, the Moon is located between the Sun and Earth, causing it to be dark and only slightly illuminated by sunlight bouncing off Earth. The entire cycle takes about 30 days.

2.0The Star

• A star is a small, twinkling luminous object in the sky, with the Sun being the largest among them. Stars emit their own light, but because they are so far away-much farther than the Sun-they appear tiny to the naked eye.
• While stars are present during the day, they are hidden by bright sunlight. They seem to move from east to west due to Earth's rotation, rising in the east and setting in the west. The Pole Star, located on Earth's axis, appears stationary and does not move.

3.0Constellation

• A group of stars that forms an identifiable shape is called a constellation. Ancient people created constellations to help identify patterns in the night sky, often naming them after familiar objects and figures.

1. Great Bear-Also known as Ursa Major, the Big Dipper, or Saptarshi, this constellation is typically visible in early summer. It resembles a large ladle, consisting of seven prominent stars: three in the handle and four in the bowl. The constellation appears to move from east to west across the sky.
2. Pole Star-The Pole Star is located near Ursa Major and remains stationary in the sky.
3. Orion-Also known as the Hunter, this constellation is visible in the evenings during winter. It consists of 7 or 8 bright stars, with Sirius, the brightest star, located nearby.
4. Cassiopeia-This constellation is visible in the early part of winter nights and has a shape that resembles a vague "M" or "W."

4.0The Sun

The Sun is the closest star to us and serves as the primary source of heat, light, and energy for all planets, including Earth. Together with its family of eight planets, their satellites, and other celestial objects, the Sun forms what is known as the Solar System.

5.0Planets And Their Features

There are eight planets in the Solar System:

1. Mercury

2. Venus

3. Earth

4. Mars

5. Jupiter

6. Saturn

7. Uranus

8. Neptune

• Mercury, Venus, Earth, and Mars are known as the Inner Planets, while Jupiter, Saturn, Uranus, and Neptune are mentioned as the Outer Planets. 
• Although planets resemble stars, they do not twinkle because they do not produce their own light; instead, they reflect sunlight. The planets continuously rotate and revolve, causing their positions to change. The fixed path they follow around the Sun is called an orbit. 
• The time it takes for a planet to complete one revolution around the Sun is known as its period of revolution, which increases with distance from the Sun. The time it takes for a planet to finish one rotation on its axis is called its period of rotation.
• Some planets have moons, or natural satellites, that orbit them. For example, the Moon is Earth's natural satellite. There are also many man-made satellites orbiting Earth, known as artificial satellites. Generally, Inner Planets have fewer satellites, while Outer Planets have more.

1. Mercury (Budh)-It is the nearest planet to the Sun and the minutest planet in the Solar System. Its proximity to the Sun often causes it to be obscured by sunlight, making it visible only early in the morning or late at night in open areas free from trees and tall buildings. Notably, Mercury has no satellites.
2. Venus (Shukra)-It is Earth's nearest neighbor and the brightest planet in the sky, making it easily observable. It often appears in the eastern sky before sunrise and can also be seen in the western sky just after sunset, earning it the titles of the "morning star" or "evening star." Venus exhibits various phases, similar to the Moon, and notably has no satellites of its own. Interestingly, it rotates from east to west.
3. Earth-It is the only planet known to support life, thanks to several key factors: the right temperature, the presence of water, an atmosphere, and an ozone layer. In space, Earth appears bluish-green due to the reflection of light from its water bodies and landmasses. The Moon is Earth’s only natural satellite, and the slight 

The tilt of Earth’s axis is responsible for the changing seasons.

4. Mars (Mangal)-It is known as the "Red Planet" due to its reddish appearance. It is accompanied by two small natural satellites, which add to its intriguing characteristics in the Solar System.
5. Jupiter (Brihaspathi)-It is the largest planet in the Solar System, approximately 1,300 times bigger and 318 times heavier than Earth. It rotates rapidly on its axis and boasts numerous satellites. Jupiter is quite bright, making it easily visible in the night sky, along with its four largest moons.
6. Saturn (Shani)-It appears yellowish and is renowned for its stunning rings, which, while beautiful, are not visible to the bare eye and require a telescope for observation. Notably, Saturn is less dense than water, making it the least dense of all the planets in the Solar System.
7. Uranus-It has a highly tilted axis, giving it the unique appearance of rolling around the Sun. This planet can only be observed with a telescope and moves from east to west in its orbit.
8. Neptune-It is the farthest planet in the Solar System and can only be observed with a telescope. It is known to have a large number of satellites orbiting it.

6.0Asteroids

Asteroids are rocky-metallic objects that vary in size from tiny to quite large, orbiting the Sun but not large enough to be classified as planets. A significant number of these small celestial bodies are found in the vast gap between the orbits of Mars and Jupiter, known as the asteroid belt. Asteroids can only be observed with telescopes.

7.0Comets

Comets are celestial objects that orbit the Sun in elliptical paths, taking a long time to complete their revolution. They typically appear as bright heads with long tails, which always point away from the Sun. As a comet approaches the Sun, its tail lengthens significantly. Comets are periodic in nature; for example, Halley's Comet appears approximately every 76 years, with its last appearance in 1986.

8.0Meteors And Meteorites

Meteors, often referred to as "shooting stars," appear as bright streaks of light falling from the sky. When a meteor enters Earth's atmosphere at high speeds, the friction generated heats it up, causing it to glow momentarily. This intense heat also leads to its rapid evaporation.

9.0Artificial Satellite

Artificial satellites are man-made objects that orbit a planet in a manner similar to natural satellites. While the Moon serves as Earth's natural satellite, there are numerous artificial satellites in orbit around our planet. These satellites are used for various purposes, including weather forecasting and the transmission of radio signals. Aryabhatta was the first Indian satellite, launched in 1975, followed by others such as Bhaskara in 1979, Rohini in 1980, INSAT-4C in 2006, and Chandrayaan in 2008.