Exercise Solutions (From Screenshot Page 134)

Q1: Fill in the blanks.

1. Stars are massive, luminous spheres of plasma.
2. The primary energy source of stars is nuclear fusion.
3. Stars are born in dense regions of gas and dust called nebulae.
4. A collapsing cloud of gas and dust that is not yet a star is called a protostar.
5. Our Sun is currently a main sequence star.
6. When a low-mass star runs out of hydrogen fuel, it becomes a red giant.
7. The expanding shell of gas ejected by a dying low-mass star is a planetary nebula.
8. The dense, hot core remaining after a low-mass star sheds its outer layers is a white dwarf.
9. A very massive star after exhausting its fuel becomes a red supergiant.
10. A catastrophic explosion of a massive star is called a supernova.
11. If the remnant core mass after a supernova is between 1.4 and 3 solar masses, it forms a neutron star.
12. If the remnant core mass after a supernova is greater than 3 solar masses, it forms a black hole.
13. A group of stars forming an apparent pattern in the night sky is a constellation.
14. Ursa Major is an example of a constellation.

Q2: Match the pairs.

(Note: As an AI, I cannot create interactive matching. I will provide the correct pairs.)

• Nebula - Birthplace of stars
• Main sequence star - Stable phase (e.g., Sun)
• Red Giant - Expanded low-mass star
• Planetary Nebula - Ejected outer layers of low-mass star
• White Dwarf - Remnant of low-mass star
• Red Supergiant - Expanded high-mass star
• Supernova - Explosion of massive star
• Neutron Star - Dense remnant of massive star
• Black Hole - Region of immense gravity
• Orion - Constellation

Q3: Give scientific reasons.

1. Stars appear as tiny points of light in the night sky.
   Reason: Stars are incredibly distant from Earth. Even though many stars are much larger and brighter than our Sun, their immense distances make them appear as tiny points of light to our naked eye. The light from them travels vast distances, and by the time it reaches Earth, it appears as a mere point.
2. The life cycle of a star depends on its mass.
   Reason: The initial mass of a star is the most critical factor determining its entire life cycle. More massive stars have stronger gravitational forces, which lead to higher core temperatures and pressures. This causes them to burn their nuclear fuel (hydrogen) much more rapidly than less massive stars. Consequently, massive stars have shorter lifespans and end their lives in more dramatic ways (e.g., supernovae, black holes), while less massive stars have longer lifespans and evolve into white dwarfs.
3. Black holes cannot be seen directly.
   Reason: Black holes are regions of spacetime where gravity is so incredibly strong that nothing, not even light or any other form of electromagnetic radiation, can escape from within a certain boundary called the event horizon. Since light cannot escape, black holes do not emit or reflect any light, making them invisible to telescopes and thus impossible to observe directly. Their presence is inferred by their gravitational effects on nearby matter.
4. Supernovae are important for the formation of new stars and planets.
   Reason: Supernovae are the catastrophic explosions of massive stars at the end of their lives. These explosions are incredibly powerful and are responsible for creating and dispersing most of the heavy elements (elements heavier than iron) into the interstellar medium. These heavy elements, along with hydrogen and helium, then become the raw material for the formation of new generations of stars and planetary systems. Without supernovae, the universe would primarily consist of hydrogen and helium, and the complex elements necessary for planets and life would not exist.

Q4: Answer the following questions.

1. Explain the birth of a star.
   Answer: Stars are born in vast, cold, and dense clouds of gas and dust known as nebulae. These nebulae are primarily composed of hydrogen and helium, along with trace amounts of heavier elements and dust particles. The process of star birth begins when a region within the nebula becomes dense enough, perhaps due to gravitational instabilities or shockwaves from nearby supernovae. Under its own gravity, this dense region begins to contract and collapse. As it collapses, the material in the center becomes increasingly compressed and heats up due to the conversion of gravitational potential energy into thermal energy. This hot, dense, and still-collapsing core is called a protostar. The protostar continues to gather mass from the surrounding nebula and its core temperature and pressure steadily rise. When the core temperature reaches a critical point (millions of degrees Celsius), nuclear fusion reactions begin, primarily converting hydrogen into helium. At this point, the outward pressure from the fusion reactions balances the inward pull of gravity, and the protostar becomes a stable, luminous main sequence star.
2. Explain the life cycle of a low-mass star (like our Sun).
   Answer: The life cycle of a low-mass star, such as our Sun, proceeds through several distinct stages:
   1. Nebula and Protostar: The star begins its life as a protostar formed from the gravitational collapse of a nebula.
   2. Main Sequence Star: Once nuclear fusion of hydrogen into helium begins in its core, the star enters the main sequence phase. It remains stable for billions of years, with a balance between gravitational collapse and outward pressure from fusion. (Our Sun is in this phase).
   3. Red Giant: After billions of years, the star exhausts the hydrogen fuel in its core. The core then contracts and heats up, causing the outer layers of the star to expand dramatically and cool, turning red. The star becomes a Red Giant, becoming much larger and cooler than its main sequence stage.
   4. Planetary Nebula: The outer layers of the Red Giant eventually become unstable and are gently ejected into space, forming an expanding, glowing shell of gas and dust called a Planetary Nebula.
   5. White Dwarf: The hot, dense, and very small core that remains after the planetary nebula dissipates is called a White Dwarf. It no longer undergoes nuclear fusion but slowly radiates away its residual heat.
   6. Black Dwarf: Over an extremely long period (trillions of years), a white dwarf is theorized to cool down completely, becoming a cold, dark, and non-luminous object called a Black Dwarf. (None have been observed yet).
3. Explain the life cycle of a high-mass star.
   Answer: High-mass stars (those with initial masses greater than about 8 times the Sun's mass) have a much shorter and more dramatic life cycle:
   1. Nebula and Protostar: Similar to low-mass stars, they begin as protostars forming from nebulae.
   2. Main Sequence Star: They enter the main sequence phase, burning hydrogen into helium at a much faster rate due to their immense gravity and higher core temperatures. This phase is much shorter than for low-mass stars.
   3. Red Supergiant: After exhausting hydrogen, these massive stars expand into enormous Red Supergiants. They continue to fuse heavier elements (like carbon, oxygen, silicon, up to iron) in their core.
   4. Supernova: Once the core forms iron (which cannot release energy through fusion), nuclear fusion stops. The core rapidly collapses under its own immense gravity, leading to a catastrophic and incredibly luminous explosion called a Supernova. This explosion briefly outshines an entire galaxy and disperses heavy elements into space.
   5. Remnants: The remnant left after a supernova depends on the mass of the remaining core:
      • Neutron Star: If the core's mass is between 1.4 and 3 times the Sun's mass, it collapses into an extremely dense object composed almost entirely of neutrons.
      • Black Hole: If the core's mass is greater than about 3 times the Sun's mass, the gravitational collapse continues indefinitely, forming a Black Hole – a region of spacetime with gravity so strong that nothing, not even light, can escape.
4. What are constellations? Give some examples.
   Answer: A constellation is a group of stars that appear to form a recognizable pattern or outline in the night sky when viewed from Earth. These patterns are purely apparent and are formed by our perspective; the stars within a constellation are often at vastly different distances from Earth and are not physically related. Historically, constellations were used by ancient civilizations for:
   • Navigation: Sailors and travelers used them to find their way.
   • Timekeeping: Farmers used them to track seasons for planting and harvesting.
   • Storytelling and Mythology: Many cultures associated constellations with myths, legends, and deities.
   Examples of constellations include:
   • Ursa Major (The Great Bear): Also known as the 'Big Dipper' or 'Sapta Rishi' in India.
   • Orion (The Hunter): A prominent winter constellation.
   • Leo (The Lion): A spring constellation.
   • Scorpius (The Scorpion): A summer constellation.
   There are 88 officially recognized constellations by the International Astronomical Union (IAU).

Q5: Differentiate between.

1. Red Giant and Red Supergiant
   

   Feature	Red Giant	Red Supergiant
   Precursor Star	Forms from a low-mass main sequence star (like the Sun)	Forms from a high-mass main sequence star
   Size	Large, but smaller than a supergiant	Enormous, one of the largest types of stars
   Fate	Evolves into a planetary nebula and then a white dwarf	Ends its life in a spectacular supernova explosion
   Core Fusion	Hydrogen fusion in a shell around a helium core	Fuses heavier elements (carbon, oxygen, etc.) in layers

2. Neutron Star and Black Hole
   

   Feature	Neutron Star	Black Hole
   Formation	Remnant of a massive star's supernova (core mass 1.4-3 solar masses)	Remnant of a very massive star's supernova (core mass > 3 solar masses)
   Composition	Composed almost entirely of neutrons	A region of spacetime, not composed of matter in the usual sense
   Density	Extremely dense, but has a physical surface	Infinitely dense singularity at its center
   Light Escape	Light can escape from its surface	Gravity is so strong that nothing, not even light, can escape