This page covers the universe and solar system for OPSC OAS prelims and mains: the Big Bang and expanding universe, galaxies, stars and constellations, the Sun and solar activity, planets and small bodies, the Moon, eclipses and tides, plus Aditya-L1, Chandrayaan-3 and Odisha’s astronomical tradition. It ends with OAS-pattern MCQs drawn from previous year questions.
Big Bang Theory and the Expanding Universe
The Big Bang Theory is the standard cosmological model: the universe began from an extremely hot, dense singularity about 13.8 billion years ago and has been expanding and cooling since. Georges Lemaitre proposed the ‘primeval atom’ idea in the 1920s; Edwin Hubble provided observational support through the redshift–distance relation (Hubble’s Law) in 1929.
Cosmology is the study of the universe as a whole: its origin, evolution, large-scale structure and fate. Astronomy studies individual celestial objects; cosmography maps the universe.
Two observations are cited as the principal evidence for expansion:
- Redshift of distant galaxies: their spectral lines are shifted toward longer wavelengths, showing they are receding. The farther a galaxy, the faster it recedes (Hubble’s Law).
- Cosmic Microwave Background (CMB) radiation: relic radiation from about 3,80,000 years after the Big Bang, discovered accidentally by Penzias and Wilson in 1965. It has a nearly perfect blackbody spectrum at about 2.725 K and is observed at a redshift of about z ≈ 1100.
OPSC OAS Prelims 2018 asked who first proposed that the universe is expanding — Edwin Hubble.
Expansion evidence = redshift + CMB. The CMB was not discovered by Lemaitre; Lemaitre proposed the primeval atom, while Penzias and Wilson detected the CMB.
IMAGE (Timeline): A horizontal timeline of the universe from the Big Bang to the present.
Labels: Big Bang (13.8 billion years ago), first stars (~200 million years), formation of the Solar System (4.6 billion years ago), present day; mark CMB at 3,80,000 years after the Big Bang.
Caption: Major stages in the history of the universe.
Find it: NASA WMAP ‘Timeline of the Universe’ image or Wikimedia Commons ‘Big Bang timeline’.
Galaxies and the Milky Way
A galaxy is a gravitationally bound system of stars, stellar remnants, gas, dust and dark matter. Galaxies are classified as spiral, elliptical or irregular. The Milky Way is a barred spiral galaxy; its nearest large neighbour is the Andromeda Galaxy (M31), about 2.5 million light-years away.
| Type | Features | Example |
|---|---|---|
| Spiral | Flat rotating disk with arms; lots of gas and dust; active star formation | Milky Way, Andromeda |
| Elliptical | Ellipsoidal shape; old stars; little gas and dust; little new star formation | Messier 87 |
| Irregular | No regular shape; gas- and dust-rich; often small | Large Magellanic Cloud |
The Sun is about 26,000 light-years from the Galactic Centre and completes one revolution (a galactic year) in roughly 25 crore (225–250 million) years. BPSC (Pre) 1995; IAS (Pre) 1994
Stars and Constellations
A star is a luminous sphere of plasma held together by gravity, producing energy by nuclear fusion. The colour of a star indicates its surface temperature: blue stars are the hottest, red stars the coolest. The nearest star to Earth after the Sun is Proxima Centauri, about 4.24 light-years away.
A light-year is the distance light travels in one year in vacuum, about 9.46 trillion km. A parsec is about 3.26 light-years and is the distance at which 1 AU subtends an angle of 1 arcsecond. Both are distance units, not time units.
A constellation is a recognisable pattern of stars as seen from Earth; the stars in a constellation may lie at very different distances and are not physically bound. The International Astronomical Union recognises 88 constellations. Saptarishi (Ursa Major) is used to locate Polaris, the Pole Star, which indicates north.
- Constellations are not groups of equidistant stars; they are apparent patterns.
- A light-year measures distance, not time.
- Twinkling of stars is due to atmospheric refraction, not changes in the star itself.
The Sun and Solar Activity
The Sun is a G-type main-sequence star composed mainly of hydrogen (about 70%) and helium (about 26.5%). Its diameter is about 109 times Earth’s, and it contains about 99.8% of the Solar System’s mass. Sunlight takes about 8 minutes 20 seconds to reach Earth.
| Layer | Approximate temperature | Key feature |
|---|---|---|
| Core | 15 million °C | Nuclear fusion of hydrogen into helium |
| Convection zone | Drops to about 2 million °C | Energy transported by convection |
| Photosphere | About 5,500–6,000 °C | Visible surface; sunspots occur here |
| Chromosphere | Thousands of °C | Layer above the photosphere |
| Corona | Millions of °C | Outer atmosphere; visible during total solar eclipse |
Sunspots are cooler, darker regions on the photosphere where magnetic fields are strong (about 2,500 times Earth’s magnetic field). They follow an 11-year solar cycle. Solar flares are sudden releases of magnetic energy; Coronal Mass Ejections (CMEs) are huge clouds of plasma thrown into space. Both can cause geomagnetic storms and auroras (Aurora Borealis in the north, Aurora Australis in the south).
OPSC OAS Prelims 2021 asked the outermost layer of the Sun’s atmosphere — Corona.
India’s first dedicated solar mission, Aditya-L1, was launched on 2 September 2023 by PSLV-C57 and placed in a halo orbit around the Sun–Earth Lagrange point L1 on 6 January 2024, about 1.5 million km from Earth. It observes the Sun continuously without eclipse and samples the solar wind outside Earth’s magnetosphere.
The Solar System: Planets, Dwarf Planets and Small Bodies
The Solar System formed about 4.6 billion years ago from a collapsing interstellar cloud. It has eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The inner planets (Mercury to Mars) are terrestrial and rocky; the outer planets (Jupiter to Neptune) are giant planets, with Jupiter and Saturn called gas giants and Uranus and Neptune ice giants.
| Planet | Key facts |
|---|---|
| Mercury | Smallest planet; least massive; no moon; closest to the Sun |
| Venus | Hottest planet (greenhouse effect); Earth’s twin; Morning/Evening Star; no moon; retrograde rotation |
| Earth | Densest planet; one moon; astronomical unit defined by Earth–Sun distance |
| Mars | Red Planet (iron oxide); Olympus Mons, largest volcano; two moons (Phobos, Deimos) |
| Jupiter | Largest and most massive planet; Great Red Spot; many moons; faint rings |
| Saturn | Prominent ring system; Titan, largest moon; currently most confirmed moons |
| Uranus | Axial tilt about 98°, appears to roll; methane gives blue-green colour |
| Neptune | Farthest planet; coldest average temperature; methane gives blue colour; Triton, largest moon |
Pluto was reclassified as a dwarf planet by the International Astronomical Union in 2006 because it has not cleared its orbital neighbourhood. Ceres, in the asteroid belt, is also a dwarf planet.
Asteroids are rocky bodies, most in the asteroid belt between Mars and Jupiter. Comets are icy bodies that develop tails when near the Sun; short-period comets come from the Kuiper Belt beyond Neptune, and long-period comets from the Oort Cloud. Halley’s Comet returns about every 76 years. A meteoroid is a small space fragment; a meteor is the light streak when it burns in the atmosphere; a meteorite is the fragment that reaches the ground.
IMAGE (Diagram): The Solar System not to scale, showing the Sun, eight planets in order, asteroid belt and Kuiper Belt.
Labels: Sun, Mercury, Venus, Earth, Mars, asteroid belt, Jupiter, Saturn, Uranus, Neptune, Kuiper Belt; mark inner and outer planets.
Caption: Order of planets and position of the asteroid and Kuiper belts.
Find it: NASA Solar System diagram or NCERT Class 6 Geography Fig. 1.2.
The Moon, Eclipses and Tides
The Moon is Earth’s only natural satellite, about 3,84,400 km away. It has a diameter about one-quarter of Earth’s and is in synchronous rotation: its rotation period equals its orbital period (about 27.3 days), so the same face always points toward Earth. The Moon’s gravity is about one-sixth of Earth’s; an object’s weight decreases on the Moon but does not become zero. The Moon has no appreciable atmosphere, so sound cannot travel there.
OPSC OAS Prelims 2024 asked which planet has a day longer than its year — Venus.
Solar eclipse occurs at new Moon (Amavasya) when the Moon comes between the Sun and Earth. Lunar eclipse occurs at full Moon (Poornima) when Earth comes between the Sun and the Moon. The Diamond Ring effect is seen near the edges of the path of totality, at the start or end of totality. Spring tides occur at full Moon and new Moon; neap tides at the quarters.
On 23 August 2023, India’s Chandrayaan-3 soft-landed the Vikram lander near the lunar south pole (about 69°S). The site is named Shiv Shakti Point. India became the first country to soft-land in the lunar south polar region and the fourth to soft-land on the Moon.
IMAGE (Diagram): Solar and lunar eclipse geometry, showing the umbra and penumbra.
Labels: Sun, Earth, Moon; umbra, penumbra; solar eclipse at new Moon, lunar eclipse at full Moon.
Caption: Why solar eclipses occur at new Moon and lunar eclipses at full Moon.
Find it: NCERT Class 6 Geography eclipse diagram or Wikimedia Commons ‘Solar eclipse geometry’.
Odisha Angle: Pathani Samanta and Odisha’s Astronomical Tradition
Odisha has a significant astronomical tradition. Pathani Samanta (Samanta Chandra Sekhar, 1835–1904), born in Khandapada, was the last great naked-eye astronomer in the classical Indian tradition. He authored the Siddhanta Darpana, a Sanskrit treatise of about 2,500 verses completed in 1869, and designed precise instruments such as the Svayambhu-yantra and Chapa-yantra. He corrected errors in the lunar calendar through sustained observation. The Pathani Samanta Planetarium in Bhubaneswar is named after him, and his birth anniversary is observed on 13 December.
An OAS question can link Odisha’s heritage to astronomy: Pathani Samanta is associated with the Siddhanta Darpana and naked-eye astronomy, not with the telescope or the Big Bang.
Practice Questions
The ‘Big Bang Theory’ is related to which of the following?
- Continental drift
- Origin of the universe
- Origin of the Himalayas
- Eruption of volcanoes
Show answer
(b) – The Big Bang explains the origin and evolution of the universe. RAS/RTS (Pre) 2007
According to the Big Bang Theory, who proposed the idea of the universe beginning from a ‘primeval atom’ or singularity?
- Georges Lemaitre
- Galileo Galilei
- Euclid
- Aristotle
Show answer
(a) – Georges Lemaitre proposed the primeval-atom hypothesis; Hubble later gave observational evidence. UPSC 2019-style fact
Which of the following is/are cited by scientists as evidence for the continued expansion of the universe?
- Detection of microwaves in space
- Observation of redshift in space
- Movement of asteroids in space
- Occurrence of supernova explosions in space
Show answer
(a) – CMB (microwaves) and redshift are the two standard evidences; asteroid movement is not. IAS (Pre) 2012
The terms ‘Event Horizon’, ‘Singularity’, ‘String Theory’ and ‘Standard Model’ are sometimes seen in the news in the context of:
- Observation and understanding of the universe
- Study of solar and lunar eclipses
- Placing satellites in Earth orbit
- Origin and evolution of living organisms
Show answer
(a) – These are cosmology and fundamental-physics terms used to understand the universe. IAS (Pre) 2017
One astronomical unit (AU) is the average distance between:
- Earth and Sun
- Earth and Moon
- Jupiter and Sun
- Pluto and Sun
Show answer
(a) – 1 AU ≈ 149.6 million km, the mean Earth–Sun distance. CDS 2008; JPSC (Pre) 2011
Which planet currently has the maximum number of officially recognised moons?
- Jupiter
- Mars
- Saturn
- Venus
Show answer
(c) – Saturn has the largest confirmed moon count; Mars has two and Venus none. OPSC OAS Prelims 2019
The ‘Diamond Ring’ is a phenomenon observed:
- At the start of a total solar eclipse
- At the end of a total solar eclipse
- Only along the peripheral regions of the totality trail
- Only in the central regions of the totality trail
Show answer
(c) – The Diamond Ring is most prominent along the edges of the path of totality, at the beginning or end of totality. IAS (Pre) 1996
Which of the following statements about the Moon is/are correct?
- The Moon rotates on its axis in the same period as it revolves around Earth.
- The Moon has a thick atmosphere that transmits sound.
- An object’s weight becomes zero on the Moon’s surface.
- The Moon’s gravity is about one-sixth of Earth’s gravity.
Show answer
(a) and (d) – Synchronous rotation keeps the same face toward Earth; lunar gravity is about 1/6th of Earth’s, so weight decreases but is not zero. OPSC OAS-style statement question