Class 6 Science Chapter-13 Question Answer

1. Stars and Celestial Motion

• Definition of Stars: Stars are luminous celestial bodies that generate their own heat and light. The Sun is the star closest to Earth.   
• Twinkling Effect: Stars twinkle due to atmospheric instability, which continually refracts and disturbs the light arriving from vast distances.   
• Apparent Motion: Because Earth rotates on its axis from west to east, the Sun, stars, and constellations appear to move across the sky from east to west.   
• The Pole Star (ध्रुब हाथरखि / Polaris): Situated directly above Earth’s rotational axis, the Pole Star appears stationary in the northern sky and does not shift position over time, making it an essential reference point for navigation. It is not visible from the Southern Hemisphere.   

2. Major Constellations (हाथरखि थुबुर)

• Ursa Major (दाउस्रिगोबा / Saptarshi / The Great Bear): A group of seven bright stars configured like a ladle or dipper. An imaginary line extended through its two pointer stars leads directly to the Pole Star.   
• Ursa Minor (उन्दै दाउस्रिगोबा / Little Dipper): A smaller group of seven stars resembling a small ladle, with the Pole Star located at the end of its handle.   
• Orion (गुरखिया बिबान / कालपुरुष / The Hunter): Prominent in the winter evening sky, distinguished by three belt stars. Extending a line eastward through these three stars leads directly to Sirius (लुब्धक) in Canis Major, the brightest star in the night sky.   
• Cassiopeia (काश्यपी / Sharmishtha): Composed of five prominent stars forming a recognizable distorted “W” or “M” pattern in the northern sky.   

3. The Solar System (सानखरां)

• The Sun: A huge ball of glowing gas approximately 4.6 billion years old, with a surface temperature around 5,500°C. Earth is about 150 million km (1 Astronomical Unit / au) away.   
• Proxima Centauri: The second closest star to Earth after the Sun, located at a distance of about 269,000 au. 

Classification of the 8 Planets:

• Inner / Terrestrial Planets (इसिं-ग्रह): Mercury, Venus, Earth, and Mars. They are relatively small with rocky surfaces.   
• Outer / Jovian Planets (बायजो-ग्रह / Gas Giants): Jupiter, Saturn, Uranus, and Neptune. They are massive, gaseous, and encircled by ring systems.   

Planetary Facts:

• Mercury (बुध): The closest and smallest planet; takes around 88 days to complete one orbit around the Sun; lacks an atmosphere and satellites.   
• Venus (शुक्र): The brightest planet (reflecting ~75% of sunlight), commonly referred to as the “Morning Star” or “Evening Star”. Due to a heat-trapping atmosphere, it is hotter than Mercury despite being farther away. Rotates from east to west.   
• Earth (बुहुम): Known as the “Blue Planet” because water covers ~70% of its surface. It takes 365 days to orbit the Sun and 24 hours to rotate on its axis.   
• Mars (मंगल): Known as the “Red Planet” due to its red soil and rocks. It has a thin atmosphere and two natural satellites.   
• Jupiter (बृहस्पति): The largest and heaviest planet (mass is roughly 318 times that of Earth).   
• Saturn (शनि): Known for its prominent, reflective rings composed of ice and rocky debris, first observed by Galileo in 1610. Its moon Titan is larger than Mercury.   
• Uranus (इउरेनास / अरुन) & Neptune (नेपसुन / बरुन): Very cold, distant outer planets with thick gaseous atmospheres. Uranus also exhibits an east-to-west axial rotation.   

4. Other Celestial Bodies

• The Moon (अखाफोर): Earth’s only natural satellite, orbiting Earth in roughly 27 days. It has no air or water, and its surface is marked with deep circular depressions called impact craters (अरगें खुगा).   
• Asteroids (ग्रह सेरेफ): Irregular rocky fragments orbiting the Sun primarily in the Asteroid Belt between the orbits of Mars and Jupiter.   
• Comets (धुमकेतु): Small bodies composed of ice, dust, and rock. When approaching the Sun, their frozen material vaporizes into a luminous tail that always points away from the Sun. Halley’s Comet appears roughly every 76 years (last seen in 1986; expected next in 2061).   
• Meteors and Meteorites: Small space debris that heats up and produces streaks of light due to atmospheric friction are called meteors (हाथरखि फाग्ला / shooting stars). Unburned fragments that successfully strike the Earth’s surface are called meteorites (दावबायखार हाथरखि).   

5. Indian Space Exploration Milestones

Mangalyaan (Mars Orbiter Mission): Launched on November 5, 2013, and successfully entered Martian orbit on September 24, 2014, making India the first nation to succeed on its maiden attempt.   

Chandrayaan Missions:

• Chandrayaan-1: Launched on October 22, 2008.   
• Chandrayaan-2: Launched on July 22, 2019, carrying an Orbiter, the Vikram lander, and the Pragyan rover.   
• Chandrayaan-3: Launched on July 14, 2023. On August 23, 2023, the lander Vikram made a historic soft landing near the lunar south pole, making India the first country to land in that region. August 23 is now commemorated annually as National Space Day (हायुंआरि अख्रांमा सान).   

Dr. Jitendra Nath Goswami: A scientist from Jorhat, Assam, who served as the Chief Scientist and developer for Chandrayaan-1, held leadership roles at PRL, and contributed to Chandrayaan-2 and Mangalyaan.   

6. The Galaxy and the Universe

• Milky Way (मैदेर लामा / अखां गंगा): The vast galaxy containing hundreds of billions of stars, including our Solar System. It would take roughly 100,000 years to cross it traveling at the speed of light.   
• Other Observable Galaxies: The Andromeda Galaxy (larger than the Milky Way) is visible from the Northern Hemisphere, while the Large and Small Magellanic Clouds are visible from the Southern Hemisphere.