📑 Topics on this page (Jump to Topic):
📌 1. Universe & Galaxies
- Big Bang theory (Georges Lemaître): the universe began about 13.8 billion years ago from an extremely dense point; Edwin Hubble gave evidence of the expanding universe.
- The Milky Way is our spiral galaxy. The nearest large galaxy is Andromeda.
- A light year = distance travelled by light in one year (≈ 9.46 × 10¹² km) — a unit of distance, not time.
- Nearest star after the Sun: Proxima Centauri (≈ 4.24 light years); brightest star in the night sky: Sirius (Dog Star).
- Life cycle of a star: nebula → main-sequence star → red giant → white dwarf / neutron star / black hole. Chandrasekhar limit = 1.44 solar masses.
📌 2. The Sun
| Feature | Fact |
|---|---|
| Age | ≈ 4.6 billion years |
| Composition | Hydrogen ≈ 71–73%, Helium ≈ 25–27% |
| Energy source | Nuclear fusion (H → He) |
| Surface (photosphere) temp. | ≈ 5,500 °C (≈ 5,800 K) |
| Light takes to reach Earth | ≈ 8 minutes 20 seconds |
| Outer layer | Corona — visible only in a total solar eclipse |
- Sunspots are cooler, dark patches on the Sun; their cycle is about 11 years.
- India's solar mission Aditya-L1 (2023) is placed at the Sun-Earth Lagrange point L1.
📌 3. The Eight Planets
| Planet | Key fact | Moons (approx.) |
|---|---|---|
| Mercury | Smallest & closest to the Sun; fastest orbit (88 days) | 0 |
| Venus | Hottest & brightest planet; "Morning/Evening Star"; rotates east to west; Earth's "twin" | 0 |
| Earth | "Blue Planet"; densest planet; only known planet with life | 1 |
| Mars | "Red Planet" (iron oxide); Olympus Mons — tallest mountain in the solar system; moons Phobos, Deimos | 2 |
| Jupiter | Largest planet; fastest rotation (≈ 10 h); "Great Red Spot"; Ganymede — largest moon | 95+ |
| Saturn | Beautiful rings; lowest density (less than water); Titan — moon with a thick atmosphere | 140+ |
| Uranus | Tilted ≈ 98° — "planet on its side"; rotates east to west; looks green (methane) | 28 |
| Neptune | Farthest from the Sun; coldest; orbit ≈ 165 years | 16 |
- Inner (terrestrial) planets: Mercury, Venus, Earth, Mars — rocky. Outer (Jovian/gas) planets: Jupiter, Saturn, Uranus, Neptune.
- Order of size (largest to smallest): Jupiter > Saturn > Uranus > Neptune > Earth > Venus > Mars > Mercury.
- Pluto was reclassified as a dwarf planet by the IAU in 2006; other dwarf planets — Ceres, Eris, Haumea, Makemake.
💡 Memory Tip: Order of planets: "My Very Educated Mother Just Served Us Noodles".
📌 4. Moon & Other Bodies
- The Moon is Earth's only natural satellite; distance ≈ 3,84,400 km; we always see the same face because its rotation and revolution periods are equal (≈ 27.3 days). Its light reaches us in ≈ 1.3 s.
- Highest mountain on the Moon — Leibnitz; first person on the Moon — Neil Armstrong (Apollo-11, 1969).
- Chandrayaan-3 (2023): India became the first country to soft-land near the Moon's south pole; landing site "Shiv Shakti point".
- Asteroids: belt between Mars and Jupiter; largest body Ceres. Comets: ice-dust bodies whose tail always points away from the Sun; Halley's Comet appears every ≈ 76 years (next in 2061).
- Meteor = body burning in the atmosphere ("shooting star"); one that reaches the ground is a meteorite.
📌 5. Eclipses & Phases of the Moon
| Eclipse | Alignment | When |
|---|---|---|
| Solar eclipse | Sun – Moon – Earth (Moon in middle) | New moon |
| Lunar eclipse | Sun – Earth – Moon (Earth in middle) | Full moon |
- An annular solar eclipse happens when the Moon is far from Earth (near apogee) and cannot fully cover the Sun — the "ring of fire".
- Not every new moon gives an eclipse because the Moon's orbit is tilted ≈ 5° to Earth's orbit.
- Blue moon = second full moon in a month; Supermoon = full moon near perigee.
⚠️ Exam Traps:
- The hottest planet is Venus, not Mercury (Venus's thick CO₂ atmosphere causes a greenhouse effect).
- The largest moon is Ganymede (Jupiter); Titan is second.
- Venus and Uranus rotate east to west (retrograde).
- A light year is a unit of distance; a parsec is also distance (≈ 3.26 light years).
📌 6. Practice MCQs
📝 Practice MCQs (prepared by SF Prep — not PYQs)
Q1. Which is the largest planet of the solar system? Practice
Correct answer: (B)
Jupiter is the largest planet; its diameter is about 11 times that of Earth.
Jupiter is the largest planet; its diameter is about 11 times that of Earth.
Q2. Which is the hottest planet of the solar system? Practice
Correct answer: (C)
Venus's thick CO₂ atmosphere creates a strong greenhouse effect, making it hotter than Mercury.
Venus's thick CO₂ atmosphere creates a strong greenhouse effect, making it hotter than Mercury.
Q3. Which planet is called the "Red Planet"? Practice
Correct answer: (D)
Iron oxide on its surface makes Mars look red.
Iron oxide on its surface makes Mars look red.
Q4. Which is the largest natural satellite in the solar system? Practice
Correct answer: (A)
Jupiter's Ganymede is the largest moon; it is even bigger than Mercury.
Jupiter's Ganymede is the largest moon; it is even bigger than Mercury.
Q5. Which planet has a density lower than water? Practice
Correct answer: (B)
Saturn's average density is ≈ 0.69 g/cm³, less than water (1 g/cm³).
Saturn's average density is ≈ 0.69 g/cm³, less than water (1 g/cm³).
Q6. What is the main source of the Sun's energy? Practice
Correct answer: (C)
In the Sun's core hydrogen nuclei fuse into helium — nuclear fusion.
In the Sun's core hydrogen nuclei fuse into helium — nuclear fusion.
Q7. About how long does sunlight take to reach Earth? Practice
Correct answer: (D)
Mean distance ≈ 150 million km ÷ 3 lakh km/s ≈ 500 s ≈ 8 min 20 s.
Mean distance ≈ 150 million km ÷ 3 lakh km/s ≈ 500 s ≈ 8 min 20 s.
Q8. Which body was reclassified as a dwarf planet in 2006? Practice
Correct answer: (A)
In 2006 the IAU reclassified Pluto as a dwarf planet because it has not cleared its orbit.
In 2006 the IAU reclassified Pluto as a dwarf planet because it has not cleared its orbit.
Q9. A solar eclipse occurs when — Practice
Correct answer: (B)
A solar eclipse occurs on a new moon when the Moon is between the Sun and Earth.
A solar eclipse occurs on a new moon when the Moon is between the Sun and Earth.
Q10. Why do we always see the same side of the Moon? Practice
Correct answer: (C)
The Moon completes one rotation and one revolution in ≈ 27.3 days (tidal locking).
The Moon completes one rotation and one revolution in ≈ 27.3 days (tidal locking).
Q11. Which planet is called the "Morning Star" and "Evening Star"? Practice
Correct answer: (D)
Venus shines in the east before sunrise and in the west after sunset.
Venus shines in the east before sunrise and in the west after sunset.
Q12. Which planet is called the planet "on its side" due to a tilt of about 98°? Practice
Correct answer: (A)
Uranus has an axial tilt of ≈ 98°, so it orbits as if rolling on its side.
Uranus has an axial tilt of ≈ 98°, so it orbits as if rolling on its side.
Q13. The asteroid belt lies between which two planets? Practice
Correct answer: (B)
The main asteroid belt lies between the orbits of Mars and Jupiter; Ceres is its largest body.
The main asteroid belt lies between the orbits of Mars and Jupiter; Ceres is its largest body.
Q14. "Light year" is a unit of — Practice
Correct answer: (C)
Light year = distance light travels in one year (≈ 9.46 × 10¹² km).
Light year = distance light travels in one year (≈ 9.46 × 10¹² km).
Q15. A comet's tail always points — Practice
Correct answer: (D)
Solar wind and radiation pressure push the tail away from the Sun.
Solar wind and radiation pressure push the tail away from the Sun.
Q16. Which is the nearest star to Earth after the Sun? Practice
Correct answer: (A)
Proxima Centauri is ≈ 4.24 light years away; Sirius is the brightest star, not the nearest.
Proxima Centauri is ≈ 4.24 light years away; Sirius is the brightest star, not the nearest.
Q17. Olympus Mons, the tallest known mountain in the solar system, is on — Practice
Correct answer: (B)
Olympus Mons is a giant volcano on Mars (≈ 22 km high).
Olympus Mons is a giant volcano on Mars (≈ 22 km high).
Q18. Which was the first mission to soft-land near the Moon's south pole? Practice
Correct answer: (C)
ISRO's Chandrayaan-3 (23 Aug 2023) landed near the south pole; Luna-25 crashed.
ISRO's Chandrayaan-3 (23 Aug 2023) landed near the south pole; Luna-25 crashed.
Q19. The maximum mass limit (1.44 solar masses) for a white dwarf is named after — Practice
Correct answer: (D)
It is the Chandrasekhar limit; he received the Nobel Prize in 1983.
It is the Chandrasekhar limit; he received the Nobel Prize in 1983.
Q20. Why does an annular solar eclipse occur? Practice
Correct answer: (A)
Near apogee the Moon looks smaller, leaving a "ring of fire" of the Sun visible.
Near apogee the Moon looks smaller, leaving a "ring of fire" of the Sun visible.