General Science · Space Science

🌌 Chapter 13: Our Universe

BPSC / BSSC · Exam Fusion Prep  |  Big Bang · Galaxies · Stars · Solar System · Planets · India's Space Missions

🌌 Universe — At a Glance
📘 Definition

Collective form of all celestial bodies — Galaxies, Stars, Planets, Satellites — present in sky and space.

13.8 Billion
Age of Universe (years)
10²²
Total Stars in Universe
8 min 20 sec
Sunlight Reaching Earth
1.44 M☉
Chandrasekhar Limit
  • Universe origin: Big Bang Theory — George Lemaitre (1930)
  • Age: 13.8 billion years | Hubble constant H = 67 km/s/Mpc
  • Nearest star to Earth (after Sun): Proxima Centauri
  • Brightest star in the night sky: Sirius
  • Our galaxy: Milky Way (Spiral type)
📜 Theories of Universe — Timeline
Ancient
Geocentric Theory
Ptolemy — Earth is the centre of the universe; all celestial bodies revolve around Earth
1543 AD
Heliocentric Theory
Copernicus — Sun is the centre; planets revolve around the Sun
1785
Herschel Theory
Herschel — described shape and structure of the Milky Way galaxy
1925
Hubble Theory
Edwin Hubble — universe is expanding; galaxies are moving away
1930
Big Bang Theory
George Lemaitre — universe originated ~13.8 billion years ago from a singularity explosion
⚠️ Exam Trap — Theory Propounders
  • Geocentric → Ptolemy | Heliocentric → Copernicus (1543)
  • Big Bang → George Lemaitre (1930) — NOT Einstein or Hawking
  • Hubble gave the law of expansion (1925), NOT Big Bang
🔴 Red Shift & Hubble Law
Red Shift
Galaxy Moving AWAY from Earth

Light from galaxy shifts toward red end (longer wavelength). Indicates the galaxy is receding.

Blue Shift
Galaxy Coming CLOSER to Earth

Light shifts toward blue end (shorter wavelength). Indicates the galaxy is approaching.

V = HdHubble Law | V = recession velocity, H = Hubble constant (67 km/s/Mpc), d = distance
Hubble Constant (H)
67 km/s/Mpc
Age of Universe
13.8 Billion yrs
⚠️ Exam Trap — Red & Blue Shift
  • Red shift = galaxy moving away (most galaxies show red shift — universe is expanding)
  • Blue shift = galaxy moving toward us (Andromeda shows blue shift — approaching Milky Way)
🌌 Galaxy
📘 Definition

A huge group of stars, dust and gases gathered in large quantities by gravity. Total stars in universe ≈ 10²².

📂 Classification of Galaxies
Property Elliptical Spiral Irregular
Shape Elliptical / oval Spiral arms No regular shape
Star Age Old, small stars New stars in arms; Old stars in centre bulge Mix of new & old stars
Colour Yellow-Red Arms: Blue; Centre: Red Unpredictable variation
Star Formation Minimum Maximum Intermediate
Example Maffei-1 Milky Way Sextans A
⚠️ Exam Trap — Galaxy Types
  • Milky Way = Spiral galaxy (our galaxy)
  • Spiral galaxies have maximum star formation activity
  • Elliptical galaxies have minimum star formation (old stars, very little gas/dust)
  • Spiral arms are Blue (new hot stars); centre bulge is Red (old stars)
⭐ Stars
📘 Definition

Celestial bodies that shine with their own light and energy (due to nuclear fusion in their core).

Proxima Centauri
Nearest Star (after Sun)
Sirius
Brightest Star (night sky)
Pole Star
Always in Northern direction
🧪 Composition of Gases in Stars
H — 70%
He — 28%
C 1.5%
N+Ne
  • Hydrogen (H) = 70%
  • Helium (He) = 28%
  • Carbon (C) = 1.5%
  • Nitrogen (N) + Neon (Ne) = 0.5%
💀 Black Holes & Chandrasekhar Limit
📘 Black Hole

Formed due to energy loss in stars. Gravity so strong that not even light can escape.

M_star ≤ 1.44 M_sun → White Dwarf (stable)Chandrasekhar Limit
  • If star mass ≤ 1.44 solar masses → becomes White Dwarf (stable end state)
  • If star mass > Chandrasekhar limit → Supernova explosion → Neutron star or Black hole
  • Chandrasekhar limit named after Subrahmanyan Chandrasekhar (Indian astrophysicist)
Types of Stars
Classified by colour (temperature)
  • Red stars — coolest (3000–4000 K)
  • Yellow stars — medium (5000–6000 K); our Sun is a yellow star
  • Blue stars — hottest (above 10,000 K)
  • White dwarf — dense remnant of dead star
📘 Constellation

Group of stars forming organised and beautiful shapes. Examples: Saptarishi (Ursa Major), Orion (points to Sirius), Cassiopeia.

⚠️ Exam Trap — Stars
  • Nearest star (after Sun) = Proxima Centauri
  • Brightest star in night sky = Sirius (Dog Star)
  • Chandrasekhar limit = 1.44 solar masses — above this → star explodes as Supernova
  • Pole Star is always in the North — used for navigation
☀️ The Sun — Key Facts
109×
Diameter (vs Earth)
28×
Gravity (vs Earth)
8 min 20 s
Light Travel to Earth
Photosphere
External Surface
H — 71%
He — 26.5%
Others 2.5%
  • H = 71% | He = 26.5% | Other elements = 2.5%
  • External surface (visible layer) = Photosphere
  • Sun is a Star of the Milky Way galaxy
  • Energy produced by nuclear fusion (H → He)
⚠️ Exam Trap — Sun Facts
  • Sunlight takes 8 minutes 20 seconds to reach Earth (not 8 min or 8.5 min)
  • External surface of Sun = Photosphere (not chromosphere or corona — those are above)
  • Sun's gravity = 28 times Earth's gravity
  • Sun's diameter = 109 times Earth's diameter
🪐 Planets of Solar System
☿
Mercury
→
♀
Venus
→
🌍
Earth
→
♂
Mars
→
♃
Jupiter
→
♄
Saturn
→
⛢
Uranus
→
♆
Neptune
Inner Planets (Terrestrial)
Mercury, Venus, Earth, Mars
  • Closer to Sun
  • Rocky, dense surface
  • Smaller in size
  • Fewer moons
Outer Planets (Jovian)
Jupiter, Saturn, Uranus, Neptune
  • Farther from Sun
  • Gaseous (gas giants)
  • Much larger in size
  • Many moons & rings
Planet Key Fact
Mercury Closest to Sun; Smallest planet; No atmosphere; Extreme temp variation
Venus Hottest planet (greenhouse effect); Brightest planet; Rotates east to west (retrograde)
Earth Only planet with life; One natural satellite (Moon); Blue planet
Mars Red planet (iron oxide); 2 moons (Phobos, Deimos); India's MOM mission
Jupiter Largest planet; Great Red Spot (giant storm); 95 known moons
Saturn Least dense; Prominent rings; 146 moons
Uranus Rotates on its side (axial tilt ~98°); Coldest planet
Neptune Farthest from Sun; Strongest winds in solar system
⚠️ Exam Trap — Planets
  • Hottest planet = Venus (NOT Mercury, even though Mercury is closer to Sun)
  • Largest planet = Jupiter | Smallest = Mercury
  • Red planet = Mars | Blue planet = Earth
  • Pluto is a dwarf planet (demoted in 2006) — NOT a full planet
  • Inner planets = Mercury, Venus, Earth, Mars (4); Outer = Jupiter, Saturn, Uranus, Neptune (4)
🇮🇳 India's Important Space Missions & Satellites
Mission / Satellite Full Name / Details Significance
MOM Mars Orbiter Mission Famous Mars mission by ISRO
Mangalyaan Mars Orbiter Mission (2013) India's first Interplanetary Mission
GISAT-1 Geo Imaging Satellite-1 India's first satellite in Geostationary Orbit
APPLE Ariane Passenger Payload Experiment First Geostationary Telecommunication Satellite (India)
Chandrayaan-1 India's first Moon mission (2008) Discovered water molecules on Moon
Chandrayaan-3 India's Moon lander (2023) Soft-landed on Moon's South Pole (first country)
Aditya-L1 Solar observation mission (2023) India's first solar observatory mission
⚠️ Exam Trap — India's Firsts in Space
  • First Interplanetary Mission = Mangalyaan (Mars, 2013)
  • First Geostationary Satellite = GISAT-1
  • First Geostationary Telecom Satellite = APPLE (Ariane Passenger Payload Experiment)
  • APPLE full form is very frequently asked — memorize all 4 words
  • MOM = Mars Orbiter Mission = Mangalyaan (same mission, two names)
🎯 High-Frequency BPSC/BSSC Exam Points
  • Big Bang Theory → George Lemaitre (1930)
  • Heliocentric Theory → Copernicus (1543)
  • Geocentric Theory → Ptolemy
  • Red shift = galaxy moving away; Blue shift = coming closer
  • Hubble Law: V = Hd | H = 67 km/s/Mpc
  • Age of Universe = 13.8 billion years
  • Our galaxy = Milky Way (Spiral type)
  • Galaxy examples: Elliptical = Maffei-1 | Spiral = Milky Way | Irregular = Sextans A
  • Total stars in universe ≈ 10²²
  • Nearest star (after Sun) = Proxima Centauri
  • Brightest star (night sky) = Sirius
  • Pole Star = always in North
  • Chandrasekhar limit = 1.44 solar masses
  • Star composition: H=70%, He=28%, C=1.5%, N+Ne=0.5%
  • Sun: Diameter=109×Earth | Gravity=28×Earth | Light=8 min 20 sec
  • Sun surface = Photosphere | H=71%, He=26.5%
  • Hottest planet = Venus | Largest = Jupiter | Smallest = Mercury
  • Red planet = Mars | Blue planet = Earth
  • Inner planets: Mercury, Venus, Earth, Mars (rocky)
  • Outer planets: Jupiter, Saturn, Uranus, Neptune (gaseous)
  • India's first Interplanetary Mission = Mangalyaan
  • APPLE = Ariane Passenger Payload Experiment (first geostationary telecom satellite)
  • GISAT-1 = India's first satellite in geostationary orbit
📋 Quick Reference — Theories
Theory Propounder Year
Geocentric Ptolemy Ancient
Heliocentric Copernicus 1543
Herschel Theory Herschel 1785
Hubble Theory Edwin Hubble 1925
Big Bang George Lemaitre 1930
⚠️ Most Common Exam Traps
  • Big Bang = Lemaitre (NOT Hubble or Einstein)
  • Hottest planet = Venus (NOT Mercury)
  • Pluto = Dwarf planet (not a full planet since 2006)
  • Sun surface = Photosphere (not chromosphere)
  • Sunlight = 8 min 20 sec (not 8 min)
  • Nearest star = Proxima Centauri (Sun is NOT counted as "nearest star" in these questions)
  • APPLE full form: Ariane Passenger Payload Experiment
  • Milky Way = Spiral galaxy (not elliptical)
  • Chandrasekhar = 1.44 solar masses
  • Red shift = moving away; Blue shift = coming closer
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