Space is part of Physics. Below are notes on 12 sub-topics, followed by practice MCQs from State PSC and UPSC papers.

Astronomical Instrumentation (Telescopes, Spectrographs, Detectors across EM spectrum)

  • Astronomical Instrumentation: Tools for collecting, analyzing, and detecting electromagnetic radiation from celestial objects across the EM spectrum.
  • Key Components:
    • Telescopes: Gather and focus radiation (light-gathering power, angular resolution).
    • Spectrographs: Disperse light into constituent wavelengths for spectral analysis (composition, temperature, velocity).
    • Detectors: Convert collected radiation into measurable signals (e.g., ChargeCoupled Devices (CCDs) for optical, radio receivers, X-ray scintillators).
  • Types of Telescopes:
    • Optical Telescopes:
      • Refracting: Use lenses.
      • Reflecting: Use mirrors (dominant for large-scale astronomy).
    • Radio Telescopes: Large parabolic dishes to collect radio waves; often arrays for higher resolution (e.g., VLBI).
    • Infrared (IR), Ultraviolet (UV), X-ray, Gamma-ray Telescopes: Primarily space-based due to atmospheric absorption of these wavelengths.
  • Measurement of Distance between Earth and Planets:
    • Echo Method (Radar Ranging):
      • Used for closer solar system objects like the Moon, Venus, Mars.
      • Involves transmitting radio waves towards the object and measuring the time taken for the reflected echo to return to Earth.
      • Distance calculated as `d = (c t) / 2`, where `c` is the speed of light and `t` is the round-trip time.
    • Note: Parallax method is primarily for nearby stars.

Black Holes, Neutron Stars, and White Dwarfs (Physics of Compact Objects)

  • Black Holes:
    • Region of spacetime with gravity so strong no particles or electromagnetic radiation (light) can escape.
    • Formed from the gravitational collapse of very massive stars after a supernova.
    • Possess extremely high density.
    • Governed by strong gravitational force.
    • Key components:
      • Event Horizon: Boundary around a black hole beyond which escape is impossible.
      • Singularity: Infinitely dense point at the center of a black hole where all mass is concentrated.
    • First image of a black hole (M87) was captured by the Event Horizon Telescope (EHT) in 2019.
    • Foundational concepts related to stellar collapse leading to such objects were significantly contributed to by S. Chandrashekar. John Wheeler coined the term "black hole."
  • Neutron Stars:
    • Formed from the core collapse of massive stars (typically 8-20 solar masses) after a supernova, if the core

masses (above Chandrashekar limit).

    • Extremely dense, composed primarily of neutrons.
    • Very small, typically 10-20 km in radius.
    • Pulsars: Rapidly rotating, highly magnetized neutron stars that emit beams of electromagnetic radiation (often radio waves) from their magnetic poles. Observed as regular pulses of radiation as they rotate.
  • White Dwarfs:
    • Final evolutionary stage for low-tomedium mass stars (up to ~8 solar masses), like our Sun, after exhausting nuclear fuel.
    • Supported against gravitational collapse by electron degeneracy pressure.
    • Mass cannot exceed the Chandrashekar Limit (~1.4 solar masses).
    • Primarily composed of carbon and oxygen.
    • Very dense, but less so than neutron stars or black holes.
    • Gradually cool down over billions of years.
    • Studied in the field of Astronomy.
  • Contextual Terms (Observation and Understanding of the Universe):
    • Event Horizon: Boundary of a black hole.
    • Singularity: Center of a black hole.
    • String Theory: A theoretical framework in physics that proposes one-dimensional strings as fundamental constituents of the universe, aiming to reconcile general relativity and quantum mechanics.
    • Standard Model: A theory describing the fundamental particles and three of the four fundamental forces (strong, weak, electromagnetic) but does not include gravity.

Cosmic Microwave Background Radiation (CMBR) and Expansion of the Universe

  • The Big Bang theory explains the origin and evolution of the Universe.
  • The age of the Universe is estimated at approximately 13.8 billion years.
  • The concept of the expanding Universe, described by Hubble's Law, is based on observations that galaxies are moving away from each other.
  • Evidence for the expanding Universe stems from the Redshift of light from distant galaxies, which is explained by the Doppler effect.
  • Cosmic Microwave Background Radiation (CMBR) is relic electromagnetic radiation from the early Universe, roughly 380,000 years after the Big Bang (the recombination epoch).
  • CMBR uniformly fills all space and exhibits a near-perfect blackbody spectrum corresponding to a temperature of about 2.725 Kelvin.
  • CMBR is considered one of the strongest pieces of evidence supporting the Big Bang theory.

Cosmic Rays and Space Radiation Environment

  • Cosmic rays are highly energetic charged particles, primarily atomic nuclei (protons, alpha particles, heavier nuclei) and electrons. They are not electromagnetic waves.
  • Main sources of cosmic rays include galactic (e.g., supernovae remnants) and extragalactic sources. The Sun also emits energetic charged particles, known as Solar Energetic Particles (SEPs).

Dark Matter and Dark Energy (Physical Evidence and Properties)

  • Dark Matter: Invisible, non-baryonic (doesn't interact with light), accounts for ~27% of universe's mass-energy.
  • Evidence for Dark Matter:
    • Galaxy Rotation Curves: Galaxies rotate faster than visible matter suggests, requiring extra gravitational pull.
    • Gravitational Lensing: Bending of light around massive objects (galaxy clusters) indicates more mass than visually present.
    • Cosmic Microwave Background (CMB) Anisotropies: Patterns in CMB reveal influence of dark matter on early universe structure.
    • Large-Scale Structure Formation: Dark matter provides the gravitational scaffolding for galaxies and clusters to form.
  • Dark Matter Candidates: Weakly Interacting Massive Particles (WIMPs) are primary theoretical candidates.
  • Dark Matter Detection:
    • Direct Detection: Experiments search for WIMPs colliding with atomic nuclei in terrestrial detectors.
    • Indirect Detection: Search for products (e.g., neutrinos, gamma rays, positrons) from WIMP annihilation or decay in space.
    • Collider Searches: Attempt to produce WIMPs in high-energy particle accelerators.
  • Dark Energy: Mysterious repulsive force, accounts for ~68% of universe's mass-energy.
  • Evidence for Dark Energy:
    • Accelerated Expansion of the Universe: Observations of distant Type Ia supernovae show the universe's expansion is speeding up, not slowing down.
    • CMB Anisotropies: Consistent with a universe dominated by dark energy and dark matter.
    • Large-Scale Structure: Distribution of galaxies aligns with predictions incorporating dark energy.
  • Properties of Dark Energy: Uniformly distributed throughout space, possesses negative pressure, acts as a cosmic anti-gravity.
  • IceCube Neutrino Observatory:
    • Located deep under the ice at the South Pole.
    • World's largest neutrino detector, encompassing a cubic kilometer volume.
    • Detects high-energy neutrinos from cosmic sources (e.g., blazars, supernovae) by observing Cherenkov radiation.
    • Its operation requires it to be buried deep in the ice to shield from background cosmic rays and use the clear ice as a detection medium.
    • It also serves as a "telescope" to indirectly search for dark matter, particularly WIMPs, by looking for neutrinos produced from their annihilation or decay in the galactic center or Sun.

Gravitational Fields and Potentials in Space

  • Gravitational effects propagate at the speed of light.
  • Light from the Sun takes approximately 8 minutes to reach Earth.
  • If the Sun were to suddenly disappear, Earth would experience the lack of both sunlight and gravitational influence after about eight minutes.
  • The ratio of the Moon's diameter to the Earth's diameter is approximately 1:4.
  • The Moon has no atmosphere because its escape velocity is less than the root mean square (RMS) velocity of gas molecules, allowing them to escape into space.
  • The Moon's relatively small mass results in a weak gravitational pull and a low escape velocity.

Orbital Mechanics and Kepler's Laws

  • Kepler's First Law (Law of Orbits): Planets orbit the Sun in ellipses, with the Sun at one focus.
  • Kepler's Second Law (Law of Areas): A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time.
  • Implication of Kepler's Second Law: Planets/ satellites move faster when closer to the central body (perihelion/perigee) and slower when farther (aphelion/apogee).
  • Kinetic energy of an orbiting body is maximum at its closest approach (perihelion/perigee) and minimum at its farthest point (aphelion/apogee).
  • Kepler's Third Law (Law of Periods): The square of the orbital period ($T$) of a planet is directly proportional to the cube of the semi-major axis ($a$) of its orbit ($T^2 \propto a^3$). For circular orbits, $T^2 \propto R^3$.
  • When a satellite is brought nearer to Earth, its speed increases.
  • To calculate distance from signal travel time: Distance = (Speed of signal × Total Time) / 2. (e.g., Lunar orbit radius approx. $3.84 \times 10^8$ m).
  • An object released from an orbiting spaceship will continue to move along with the spaceship in the same orbit at the same speed (due to inertia and shared orbital velocity).
  • Artificial satellites are launched eastward from Earth to utilize Earth's rotational velocity, providing an initial speed boost that reduces fuel required to reach orbital velocity.
  • A satellite moving with uniform speed in a circular orbit around Earth always experiences acceleration (centripetal acceleration) because its direction of velocity constantly changes.
  • The time period of a satellite depends on its orbital radius and the central body's mass, not significantly on the satellite's own mass.
  • The same side of the Moon always faces Earth because its period of rotation on its axis is nearly equal to its period of revolution around the Earth (Tidal Locking).

Principles of Satellite Communication and Remote Sensing

  • Terrestrial TV/Radio transmission primarily relies on Line-of-Sight (LOS) propagation.
  • The maximum Line-of-Sight distance (d) from a transmitting antenna of height (h) to the horizon, considering Earth's curvature, is calculated using the formula: d = √(2Rh), where R is the radius of the Earth.
  • The area (A) covered by such a transmission is given by: A = πd², which can also be expressed as A = π(2Rh).
  • Given antenna height (h) = 75 m and Earth's radius (R) = 6.4 × 10⁶ m:
  • Maximum distance (d) calculation: d = √(2 ×
  • Area (A) covered calculation: A = π × (30.98 × 10³ m)² ≈ 3015.93 km² ≈ 3018 km² (using d=31km for precision: A = π (31000m)^2 ≈ 3018 km²).

Relativistic Astrophysics and Gravitational Waves (Theory and Detection)

  • LIGO stands for Laser Interferometer Gravitational-wave Observatory.
  • The LIGO experiment confirmed predictions of Einstein's General Theory of Relativity.
  • The first direct detection of gravitational waves occurred on September 14, 2015, by the LIGO collaboration.
  • This detection resulted from the merger of two black holes (a binary black hole merger), marking the first observation of such an event.
  • The detection of gravitational waves and binary black hole mergers provides strong evidence for Einstein's General Theory of Relativity.
  • Key predictions of Albert Einstein's General Theory of Relativity include:
    • Light is affected by gravity.
    • Matter warps its surrounding spacetime.
    • The Universe is constantly expanding (as described by cosmological models based on GR).
  • The observation of giant 'blackhole' mergers led to the detection of 'gravitational waves'.
  • The proposed gravitational-wave observation project in India is known by the acronym IndIGO.
  • Black holes were predicted by Einstein.

Space Plasma Physics (Solar Wind, Magnetosphere, Auroras)

  • Comet tails are directed away from the Sun due to solar radiation pressure.
  • In space (vacuum), the sky appears jet black as there is no atmosphere to scatter light.
  • Stars do not twinkle in space due to the absence of atmospheric turbulence.
  • In micro-gravity, candle flames become smaller, rounder, and dimmer due to the absence of convection (oxygen supply by diffusion).
  • The Troposphere (क्षोभमंडल) is the lowest layer of Earth's atmosphere.
  • Sunspot number is a measure of solar activity.
  • The frequency of solar flares is closely related to sunspot number.
  • The Photosphere is the visible surface of the Sun and the primary source of light and heat reaching Earth.
  • Magnetic fields leading to solar flares primarily originate in the Sun's Convective Zone.
  • Solar flares are sudden, intense bursts of electromagnetic radiation (X-rays, UV) and energetic particles, occurring in the chromosphere and corona, often above sunspot regions.
  • Coronal Mass Ejections (CMEs) are large expulsions of plasma and magnetic field from the Sun's corona into space.
  • Solar flares primarily affect Earth's ionosphere by increasing electron density, leading to disruption of radio communication (e.g., shortwave).
  • Potential effects of solar flares and CMEs on Earth include:
    • Auroras (Northern/Southern Lights)
    • Disruption of radio communication
    • Damage to satellites (electronics, solar panels)
  • Solar flares and CMEs are not directly linked to global climate change.
  • Distinction between Solar Flare and CME:
    • Solar Flare: Sudden burst of energy (electromagnetic radiation and energetic particles).
    • CME: Large expulsion of plasma (charged particles) and magnetic field.

Spacecraft Propulsion and Rocketry Principles

  • Jet engines require atmospheric oxygen for combustion, making them unsuitable for operation in space.
  • Rockets carry their own supply of oxidizer (containing oxygen) along with fuel, enabling them to operate in vacuum/space independent of atmospheric oxygen.

Stellar

Structure,

Evolution,

and

Nucleosynthesis

  • The Sun is the chief heavenly body of our solar system, comprising approximately 99.8% of its total mass.
  • A star's size is determined by the balance between the outward pressure from nuclear fusion and the inward force of gravity.
  • The apparent brightness of a star depends on its intrinsic luminosity (which relates to its size and temperature) and its distance from Earth.
  • The color of a star is a direct indicator of its surface temperature (e.g., blue/white stars are hotter, red stars are cooler).
  • Light from the Sun takes approximately 8 minutes to reach Earth.
  • Giant stars typically have shorter lifespans than dwarf stars due to their significantly greater rate of nuclear reactions, consuming their fuel much faster.
  • Earth is the densest planet in our solar system.
  • The Sun is the closest star to Earth.
  • The 'Diamond Ring' effect is a phenomenon observed at the beginning or end of a total solar eclipse.
  • The Sun's surface temperature is around 5778 K, which is significantly higher than Earth's average surface temperature.
  • The photosphere is the visible surface layer of the Sun.
  • A Supernova is a powerful stellar explosion marking the end of a massive star's life.

15. Defence Technology

Practice MCQs: Space

Astronomical Instrumentation (Telescopes, Spectrographs, Detectors across EM spectrum)

  1. Which of the following methods is used for measurement of distance between Earth and planets?

    Asked in: MPSC CS Prelims 2022

    1. Parallax method
    2. Direct distance measurement
    3. Slope tapping method
    4. Echo method
    Show answer

    Answer: A. Parallax method

    Parallax is the apparent displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. This geometric principle is used to calculate distance through triangulation.

Black Holes, Neutron Stars, and White Dwarfs (Physics of Compact Objects)

  1. In the universe, what are pulsars?

    Asked in: BPSC Prelims Exam 69th

    1. A group of stars
    2. Rotating neutron stars
    3. Explosion of a star
    4. Radio waves emitted by a star
    Show answer

    Answer: B. Rotating neutron stars

    These beams are observed as pulses as the star rotates, similar to a lighthouse beam. are temperature-sensitive resistors used in temperature measurement. Diodes are primarily for rectification. Transistors are mainly for amplification and switching. Capacitors store charge.

  2. Which of the following statement(s) is/are correct about Black holes? 1. A black hole is a region of spacetime having so strong gravitation that no particles or electromagnetic radiation can escape from it. 2. The first image of black hole was captured by the Event Horizon Telescope. 3. The ideas of black hole was first given by Isaac Newton.

    Asked in: GPSC Prelims CSP 2 2019

    1. 1 and 3 only
    2. 2 and 3 only
    3. 1 and 2 only
    4. 1, 2 and 3
    Show answer

    Answer: C. 1 and 2 only

    - Statement 1 is correct: A black hole is indeed a region of spacetime where gravity is so strong that nothing, not even light, can escape. This boundary is called the event horizon. - Statement 2 is correct: The first direct visual evidence and image of a black hole was captured by the Event Horizon Telescope (EHT) collaboration, released in April 2019, showing the supermassive black hole at the center of the galaxy M87. - Statement 3 is incorrect: The idea of a body massive enough to trap light was first speculated upon by John Michell in 1783 and Pierre-Simon Laplace in 1796 based on Newtonian mechanics. The modern concept of a black hole is based on Albert Einstein's theory of General Relativity (1915), and the first solution describing a black hole was found by Karl Schwarzschild in

  3. Isaac Newton (died 1727) predates these developments and did not conceive of black holes. 4. The terms "Event Horizon", "Singularity" "String Theory" and Standard Model" are seen in the news sometimes in the context of ______.

    Asked in: GPSC Prelims CSP 2 2019 · UPSC IAS 2017

    1. Observation and Understanding of the Uni‐ verse
    2. Missile Technology
    3. Observation of Ocean
    4. Communication
    Show answer

    Answer: A. Observation and Understanding of the Uni‐ verse

    - Event Horizon A boundary in spacetime beyond which events cannot affect an outside observer, commonly associated with black holes. - Singularity A point where a property is infinite, often used to describe the center of a black hole or the state of the universe at the Big Bang. - String Theory A theoretical framework in physics that postulates that the fundamental constituents of the universe are one-dimensional "strings" rather than point-like particles. - Standard Model The theory describing the fundamental particles and forces (excluding gravity) that make up the universe.

  4. 'Black hole' is a

    Asked in: UPSC NDA 1 2019

    1. huge black star which has zero acceleration due to gravity on its surface
    2. star which has moderate acceleration due to gravity on its surface
    3. star which has collapsed into itself and has large acceleration due to gravity on its surface
    4. star which has collapsed into itself and has zero acceleration due to gravity on its surface
    Show answer

    Answer: C. star which has collapsed into itself and has large acceleration due to gravity on its surface

    - A black hole is not a regular star; it is the result of the gravitational collapse of a very massive star after it exhausts its nuclear fuel. - This collapse compresses the mass into an extremely small volume, resulting in incredibly dense matter. - The gravitational pull becomes so strong that nothing, not even light, can escape from within a certain boundary called the event horizon. - The "surface" (event horizon) of a black hole is characterized by an extremely large acceleration due to gravity (often conceptualized by the escape velocity exceeding the speed of light). - Options A, B, and D are incorrect because a black hole is not a typical star, and it has extremely large, not zero or moderate, acceleration due to gravity at its boundary.

  5. The 'Black Hole' was first made known by

    Asked in: UPPCS Prelims 2015

    1. Herman Bandy
    2. Megh Nath Saha
    3. S. Chandrashekhar
    4. J.V. Narlikar
    Show answer

    Answer: C. S. Chandrashekhar

    S. Chandrasekhar's theoretical work on the gravitational collapse of stars provided foundational insights that led to the understanding of what we now call black holes. He received the Nobel Prize in Physics in 1983 for this work.

  6. A ‘black hole’ is a body in space which does not allow any radiation to come out. This property is due to its

    Asked in: UPPCS Prelims 2000

    1. very small size
    2. very large size
    3. very high density
    4. very low density
    Show answer

    Answer: C. very high density

    - Black holes have extremely strong gravity. - This gravity is due to a large mass compressed into a very small volume. - Mass in a small volume results in very high density. - The escape velocity at the event horizon exceeds the speed of light due to the extreme gravity. - Therefore, nothing, including radiation, can escape.

  7. In which field of science will you learn about “white dwarf”

    Asked in: RPSC RAS Prelims 2003

    1. Astronomy
    2. Agriculture
    3. Genetics
    4. Anthropology
    Show answer

    Answer: A. Astronomy

    A white dwarf is a stellar remnant composed mostly of electron-degenerate matter. It is the final evolutionary stage of low-to-medium mass stars that have exhausted their nuclear fuel.

  8. Black holes are celestial bodies governed by strong

    Asked in: lower devision clerk 2025

    1. Electric Force
    2. Gravitational Force
    3. Cosmic Force
    4. All of the above
    Show answer

    Answer: B. Gravitational Force

    Black holes are a consequence of Einstein's theory of general relativity, which describes gravity as a curvature of spacetime caused by mass and energy.

Cosmic Microwave Background Radiation (CMBR) and Expansion of the Universe

  1. According to the Big Bang theory of the origin of the Universe, the age of the Universe is estimated at about :

    Asked in: UPSC CISF AC EXE 2023

    1. 0-3 million years
    2. 4 - 6 billion years
    3. 7 - 10 billion years
    4. 13 - 18 billion years
    Show answer

    Answer: D. 13 - 18 billion years

    The standard cosmological model, which is based on the Big Bang theory, estimates the age of the Universe by observing the expansion rate (using the Hubble constant) and the composition of the Universe (from Cosmic Microwave Background data, e.g., from Planck or WMAP satellites). Current estimates based on these observations place the age of the Universe at approximately 13.8 billion years.

  2. ‘Big-Bang theory’ explains the origin of

    Asked in: UPPCS Prelims 2016

    1. Mammals
    2. Ice-age
    3. Universe
    4. Ocean
    Show answer

    Answer: C. Universe

    The Big Bang theory is the prevailing cosmological model that describes the early development of the Universe. It states that the Universe originated from an extremely hot and dense state that expanded rapidly. This theory explains the origin and large- scale structure of the entire Universe.

  3. The concept of expanding universe is based on

    Asked in: UPPSC Upper Mains 2010

    1. Doppler effect
    2. Stark effect
    3. Zeeman effect
    4. Raman effect
    Show answer

    Answer: A. Doppler effect

    The concept of the expanding universe is based on observations of the redshift of light from distant galaxies. Redshift is a phenomenon where the wavelength of light from a receding source is increased (shifted towards the red end of the spectrum).

  4. The age of our universe is closest to

    Asked in: WBCS Main Paper 4 2021

    1. 13.8 years
    2. 13.8 thousand years
    3. 13.8 million years
    4. 13.8 billion years
    Show answer

    Answer: D. 13.8 billion years

    - The age of the universe is determined by cosmological measurements. - The current widely accepted age is around 13.8 billion years.

Cosmic Rays and Space Radiation Environment

  1. Which one of the following statements is not true about cosmic rays?

    Asked in: UPPCS Prelims 2005

    1. They are electromagnetic waves.
    2. They have very short wavelength.
    3. They are made of highly energetic charged particles.
    4. They originate from the Sun.
    Show answer

    Answer: A. They are electromagnetic waves.

    Cosmic rays are high-energy charged particles (primarily protons and atomic nuclei, but also electrons and positrons) originating from outer space. They are not electromagnetic waves. Electromagnetic waves (like radio waves, visible light, X- rays, gamma rays) are composed of photons and have no electric charge. Statement A is therefore false.

Dark Matter and Dark Energy (Physical Evidence and Properties)

  1. In the context of modern scientific research, consider the following statements about 'IceCube', a particle detector located at South Pole, which was recently in the news :

    Asked in: UPSC IAS 2015

    1. 1 only
    2. 2 and 3 only
    3. 1 and 3 only
    4. 1, 2 and 3
    Show answer

    Answer: C. 1 and 3 only

    It is the world's largest neutrino detector, encompassing a cubic kilometre of ice. It is a powerful telescope to search for dark matter. It is buried deep in the ice. Which of the statements given above is/are correct? - Statement 1: IceCube is indeed the world's largest neutrino detector. It consists of thousands of sensors embedded in a cubic kilometre of ice at the South Pole, designed to detect high-energy neutrinos. - Statement 2: While neutrinos are fundamental particles relevant to various areas of physics, including cosmology and potential dark matter interactions, IceCube is primarily designed as a neutrino telescope to study astrophysical sources of high-energy neutrinos (like supernovae, gamma-ray bursts, active galactic nuclei). Searching for dark matter is not its primary or stated main purpose. - Statement 3: The detector sensors (Digital Optical Modules or DOMs) are lowered into holes drilled deep into the Antarctic ice, at depths between 1450 and 2450 meters, making this statement correct.

Gravitational Fields and Potentials in Space

  1. What would happen to earth if the sun suddenly disappears ? A. The earth immediately experiences the lack of sun light. B. The earth experiences the lack of sun light after about eight minutes. C. The earth immediately experiences the lack of gravitation. D. The earth experiences the lack of gravitation after about eight minutes. Choose the correct answer :

    Asked in: TSPSC Group 1 Prelims 2022

    1. A & C only
    2. A & D only
    3. B & C only
    4. B & D only
    Show answer

    Answer: D. B & D only

    Both light and gravity propagate at the speed of light (approximately 299,792 kilometers per second). The average distance between the Sun and the Earth is about 150 million kilometers. The time it takes for light (and gravitational influence) to travel from the Sun to the Earth is approximately: Time = Distance / Speed = (150 x 10^6 km) / (300,000 km/s) = 500 seconds = 8 minutes and 20 seconds (approximately 8 minutes). Therefore, if the Sun were to disappear suddenly, we would continue to see light from it and feel its gravitational pull for about eight minutes before noticing its absence.

  2. The ratio of the diameters of moon and earth is nearly

    Asked in: WBPSC CS Prelims 2021

    1. 1 : 2
    2. 1 : 5
    3. 1 : 4
    4. 1 : 8
    Show answer

    Answer: C. 1 : 4

    The approximate diameter of the Moon is 3,474 km. The approximate diameter of the Earth is 12,742 km. The ratio of the Moon's diameter to the Earth's diameter is $\frac{3474}{12742} \approx 0.2726$. is the closest value among the given options to the actual ratio (0.2726).

  3. There is no atmosphere on the moon because:

    Asked in: UPPSC Upper Mains 2012

    1. it is near to the earth
    2. it receives light from the sun.
    3. it revolves around the earth.
    4. escape velocity of gas molecules on it, is less than the root mean square velocity.
    Show answer

    Answer: D. escape velocity of gas molecules on it, is less than the root mean square velocity.

    The ability of a celestial body to retain an atmosphere depends primarily on its gravitational strength (related to mass) and the temperature of the gas (related to molecular velocity). If the escape velocity is less than the thermal velocity of gas molecules, the atmosphere escapes.

Orbital Mechanics and Kepler's Laws

  1. According to Kepler's laws of planetary motion, the speed of satellite, when progressively brought nearer to earth _______ .

    Asked in: GPSC Prelims CSP 2 2017

    1. Increases
    2. Decreases
    3. Remains same
    4. Cannot be estimated
    Show answer

    Answer: A. Increases

    The orbital velocity of a satellite is inversely proportional to the square root of its distance from the center of the Earth ($v \propto 1/\sqrt{r}$). Closer distance means higher speed.

  2. A laser light beamed at the Moon takes 2.56 s to return to earth after reflection at the Moon's surface. How much is the radius of the lunar orbit around the Earth ?

    Asked in: MPSC CS Prelims 2021

    1. $7.68 \times 10^8$ m
    2. $76.8 \times 10^5$ km
    3. $38.4 \times 10^5$ km
    4. $3.84 \times 10^8$ m
    Show answer

    Answer: D. $3.84 \times 10^8$ m

    The distance to an object can be calculated by measuring the time it takes for a signal (like laser light) to travel to the object and return after reflection, using the known speed of the signal (speed of light).

  3. Which of the following statement is correct ?

    Asked in: MPSC CS Prelims 2023

    1. The speed of Planet is faster when they are nearer to the sun and slower when they are farther away from the sun.
    2. The speed of Planet is faster when they are farther from the sun and slower when they are nearer to the sun.
    3. The speed of Planet is same when they are nearer to the sun or farther away from the sun.
    4. The speed of Planet is faster when they are nearer to the sun or farther away from the sun.
    Show answer

    Answer: A. The speed of Planet is faster when they are nearer to the sun and slower when they are farther away from the sun.

    Planets move faster when they are closer to the star they orbit and slower when they are farther away, a consequence of Kepler's Second Law (conservation of angular momentum).

  4. Two planets orbit the Sun in circular orbits, with their radius of orbit as R₁ = R and R₂ = 4R. Ratio of their periods (T₁/T₂) around the Sun will be

    Asked in: UPSC NDA 2 2020

    1. 1/16
    2. 1/8
    3. 1/4
    4. 1/2
    Show answer

    Answer: B. 1/8

    Kepler's Third Law relates the orbital period and orbital radius of planets orbiting the same central body: T² ∝ R³.

  5. If an apple is releasted from an orbiting spaceship, it will

    Asked in: UPPCS Prelims 2010

    1. fall towards the Earth
    2. move at a lower speed
    3. move along with the spaceship at the same speed
    4. move at a higher speed
    Show answer

    Answer: C. move along with the spaceship at the same speed

    An object in orbit around the Earth is in a state of continuous free fall, moving at a high velocity parallel to the Earth's surface. When an apple is released from an orbiting spaceship, it is released with the same velocity (speed and direction) as the spaceship at that moment, neglecting any relative velocity imparted by the release itself.

  6. A Ball thrown outside from an artificial satellite revolving round the Earth will:

    Asked in: UPPCS Prelims 1995

    1. Go on the Sun
    2. Go on the Moon
    3. Fall on the Earth
    4. Revolve round the Earth in the same orbit with the same time period as satellite
    Show answer

    Answer: D. Revolve round the Earth in the same orbit with the same time period as satellite

    If a ball is thrown from the satellite (assuming a small velocity relative to the satellite), its initial velocity relative to Earth will be very close to the satellite's orbital velocity. This new velocity vector will place the ball into its own orbit around the Earth. For a small throw velocity, this orbit will be very similar to the satellite's original orbit, including a similar time period.

  7. Assertion (A): Artificial satellites are always launched from the earth in the eastward direction. Reason (R): The earth rotates from west to east and so the satellite attains the escape velocity.

    Asked in: UPPCS Prelims 2002

    1. Both A and R are individually true and R is the correct explanation of A
    2. Both A and R are individually true but R is not a correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Show answer

    Answer: C. A is true but R is false

    - Assertion (A) states that artificial satellites are always launched from the earth in the eastward direction. This is largely true for launches aiming for prograde orbits (orbiting in the same direction as Earth's rotation). Launching eastward takes advantage of the Earth's rotational velocity, giving the rocket an initial speed boost and thus requiring less fuel to reach the desired orbital velocity. - Reason (R) states that the Earth rotates from west to east and so the satellite attains the escape velocity. While it is true that the Earth rotates from west to east and this rotation contributes an initial velocity, it is incorrect to say that the satellite "attains the escape velocity" because of this rotation in the context of a typical launch into Earth orbit. Most artificial satellites are launched into orbits around the Earth, not sent on trajectories to escape Earth's gravity. Escape velocity is significantly higher than typical orbital velocities. The Earth's rotation assists in achieving orbital velocity more efficiently, not necessarily escape velocity, and certainly doesn't cause the satellite to attain escape velocity on its own.

  8. [Figure shown in original text] The planet Mercury is revolving in an elliptical orbit around the sun as shown in the given figure. The kinetic energy of mercury is greatest at the point labelled

    Asked in: UPPCS Prelims 1997

    1. A
    2. B
    3. C
    4. D
    Show answer

    Answer: A. A

    According to Kepler's laws of planetary motion and the conservation of energy, a planet in an elliptical orbit moves faster when it is closer to the sun and slower when it is farther away. Kinetic energy is directly proportional to the square of the speed ($KE = \frac{1}{2}mv^2$).

Principles of Satellite Communication and Remote Sensing

  1. A TV tower has a height of 75 m. What is the maximum distance and area upto which this TV transmission can be received ? Take the radius of the earth as 6.4 × 10⁶ m.

    Asked in: Co operative Supervisor & Environment Supervisor (Group C) Exam 2023 · (Group C) Exam 2023

    1. 25 km and 3000 km²
    2. 30 km and 3010 km²
    3. 31 km and 3018 km²
    4. 32 km and 3020 km²
    Show answer

    Answer: C. 31 km and 3018 km²

    Co operative Supervisor & Environment Supervisor The range of radio wave propagation from a transmitting antenna is limited by the curvature of the Earth. The maximum line-of-sight distance from an antenna of height 'h' above the Earth's surface to the horizon is given by $d = \sqrt{2Rh}$, where R is the radius of the Earth. The area covered is a circle with this radius, $A = \pi d^2$.

Relativistic Astrophysics and Gravitational Waves (Theory and Detection)

  1. LIGO stands for

    Asked in: UPSC NDA 1 2019

    1. Laser Interferometer Gravitational wave Ob‐ servatory
    2. Light Interferometer Gravitational wave Ob‐ servatory
    3. Light Induced Gravity Observer
    4. Laser Induced Gascous Optics
    Show answer

    Answer: A. Laser Interferometer Gravitational wave Ob‐ servatory

    - LIGO stands for Laser Interferometer Gravitational-Wave Observatory. - It is a large-scale physics experiment and observatory designed to detect cosmic gravitational waves and to develop gravitational-wave science as an astronomical tool. - LIGO consists of two large observatories in the United States (Livingston, Louisiana, and Hanford, Washington) that work in tandem.

  2. LIGO experiment confirmed one of the predictions of :

    Asked in: UPSC NDA 1 2024

    1. String theory
    2. Special theory of relativity
    3. Quantum mechanics
    4. General theory of relativity
    Show answer

    Answer: D. General theory of relativity

    - LIGO detects gravitational waves. - Gravitational waves are a prediction of Einstein's General Theory of Relativity. - The first direct detection of gravitational waves by LIGO provided strong evidence for General Relativity.

  3. The detection of gravitational wave on 14th September, 2015 by the LIGO collaboration from the merger of two black holes, is the first direct detection of gravitational waves and the first observation of a binary black hole merger.

    Asked in: OPSC CS Prelims 2017

    1. Arrhenius theory of electrolytic dissociation
    2. Einstein's general theory of relativity
    3. Newton's law of gravity
    4. Theory of black body radiation
    Show answer

    Answer: B. Einstein's general theory of relativity

    This provides evidence for: Gravitational waves are ripples in spacetime predicted by Albert Einstein's general theory of relativity. Their detection by the LIGO collaboration in 2015 provided direct evidence for this theory and opened a new window for observing the universe.

  4. Consider the following phenomena : 1. Light is affected by gravity. 2. The Universe is constantly expanding. 3. Matter warps its surrounding space- time. Which of the above is/are the prediction/predictions of Albert Einstein's General Theory of Relativity, often discussed in media ?

    Asked in: UPSC IAS 2018

    1. 1 and 2 only
    2. 3 only
    3. 1 and 3 only
    4. 1, 2 and 3
    Show answer

    Answer: D. 1, 2 and 3

    Statement 1 is correct. A prediction of General Relativity is that light is affected by gravity. Massive objects curve spacetime, and light follows this curvature. This phenomenon, known as gravitational lensing or the bending of light by gravity, was famously confirmed during a solar eclipse in 1919. Statement 2 is correct. While Einstein initially favored a static universe, his field equations in General Relativity allow for dynamic solutions, including an expanding or contracting universe. The observation of the redshift of distant galaxies by Edwin Hubble in the 1920s provided strong evidence for an expanding universe, a phenomenon that is well-described by cosmological models based on General Relativity (like the Friedmann equations). Thus, the theory predicted the possibility of a non- static universe, which aligns with the observed expansion. Statement 3 is correct. This is the fundamental principle of General Relativity: mass and energy warp the fabric of spacetime around them, and this warping is what we perceive as gravity.

  5. Recently, scientists observed the merger of giant 'blackholes' billions of lightyears away from the Earth. What is the significance of this observation?

    Asked in: UPSC IAS 2019

    1. Higgs boson particles' were detected.
    2. 'Gravitational waves' were detected.
    3. Possibility of inter-galactic space travel through 'wormhole' was confirmed.
    4. It enabled the scientists to understand 'singu‐ larity'.
    Show answer

    Answer: B. 'Gravitational waves' were detected.

    Gravitational waves are ripples in the fabric of spacetime caused by accelerating massive objects. Highly energetic events like the merger of black holes or neutron stars are strong sources of gravitational waves.

  6. The proposed gravitational-wave observation project in India would be run by the consortium of research institutions known under the English acronym

    Asked in: APPSC Group II Prelims 2016

    1. IndIGO
    2. ILIGO
    3. IndLIGO
    4. InIGO
    Show answer

    Answer: A. IndIGO

    IndIGO (Indian Initiative in Gravitational-wave Observations) is a consortium of Indian researchers who are working on the gravitational-wave astronomy project in India, commonly referred to as LIGO-India.

  7. Black holes were predicted by

    Asked in: WBCS Main Paper 4 2019

    1. Copernicus
    2. Einstein
    3. Rutherford
    4. Satyendra Bose
    Show answer

    Answer: B. Einstein

    While earlier ideas of "dark stars" existed (e.g., John Michell, 1783) based on Newtonian gravity, the rigorous prediction and understanding of black holes as we know them today stem from General Relativity.

Space Plasma Physics (Solar Wind, Magnetosphere, Auroras)

  1. The tail of a comet is directed away from the sun because

    Asked in: UPPCS Prelims 1997

    1. as the comet rotates around the sun, the lighter mass of the comet is pushed away due to the centrifugal force alone
    2. as the comet rotates, the lighter mass of the comet is attracted by some star situated in the direction of its tail
    3. the radiation emitted by the sun exerts a radial pressure on the comet throwing its tail away from the sun
    4. the tail of the comet always exists in the same orientation
    Show answer

    Answer: C. the radiation emitted by the sun exerts a radial pressure on the comet throwing its tail away from the sun

    A comet's tail is pushed away from the sun by the combined effects of solar wind and radiation pressure from sunlight.

  2. Consider the following statements: A person in a spaceship located half way between the earth and the sun will notice that the I. Sky is jet black. II. Stars do not twinkle. III. Temperature outside the spaceship is much higher than that on the surface of the earth. Of these statements

    Asked in: UPPCS Prelims 1996

    1. Only III is correct
    2. I and II are correct
    3. I and III are correct
    4. I, II and III are correct
    Show answer

    Answer: D. I, II and III are correct

    I. The sky appears jet black in space because there is no atmosphere to scatter sunlight. II. Stars do not twinkle in space because twinkling is caused by the refraction of light by the turbulent Earth's atmosphere. III. Halfway between the Earth and the Sun, the intensity of solar radiation is significantly higher than on the surface of the Earth (which is protected and receives scattered light filtered by the atmosphere). While space itself has no temperature, an object exposed to direct sunlight would reach a very high equilibrium temperature as it absorbs this intense radiation. The vacuum of space prevents convection, leaving only radiation and conduction for heat transfer. The potential temperature an object reaches in direct sunlight at that distance would be much higher than typical Earth surface temperatures.

  3. Candle flame burn at Mirco-Gravity:

    Asked in: RPSC RAS Prelims 2007

    1. Becomes Longer
    2. Becomes Small
    3. Round Shape
    4. Remains same
    Show answer

    Answer: C. Round Shape

    In micro-gravity, the absence of convection causes a candle flame to burn with a round, spherical shape instead of the usual elongated teardrop.

  4. वा™ुमण्डल की सिबसिे Ǔनचली परत है - (1) क्षोभमं - (2) सिमतापमं

    Asked in: JUNIOR ACCOUNTANT AND TRA 2013

    1. क्षोभमंडलि
    2. सिमतिापमंडलि
    3. मध्यमंडलि
    4. स्थिमर्मोlÖफयर
    Show answer

    Answer: A. क्षोभमंडलि

    The troposphere is the lowest layer of the Earth's atmosphere.

  5. Consider the following statements: Statement-I: The sunspot number is a measure of the activity on the solar surface. Statement-II: The frequency of solar flares is closely related to the sunspot number.

    Asked in: GS 32 2023 IS DEEO 1505 2024

    1. Both Statement-I and Statement-II are indi‐ vidually true and Statement-II is the correct explanation of Statement-I.
    2. Both Statement-I and Statement-II are indi‐ vidually true but Statement-II is not the correct explanation of Statement-I.
    3. Statement-I is true but Statement-II is false.
    4. Statement-I is false but Statement-II is true.
    Show answer

    Answer: A. Both Statement-I and Statement-II are indi‐ vidually true and Statement-II is the correct explanation of Statement-I.

    Sunspots are indicators of solar activity, and their number is directly related to the frequency of solar flares.

  6. Which of the following regions of the Sun is most directly associated with solar flares?

    Asked in: GS 32 2023 IS DEEO 1505 2024

    1. Photosphere
    2. Chromosphere
    3. Corona
    4. Sunspot
    Show answer

    Answer: D. Sunspot

    Sunspots are areas of strong magnetic fields on the Sun's surface where solar flares originate.

  7. Which part of the Sun is characterized by magnetic fields that can lead to solar flares?

    Asked in: GS 32 2023 IS DEEO 1505 2024

    1. Convective Zone
    2. Radiative Zone
    3. Photosphere
    4. Core
    Show answer

    Answer: C. Photosphere

    The photosphere, due to its intense magnetic activity, is the region of the Sun most directly associated with the generation of solar flares.

  8. What is the primary effect of a solar flare on Earth's ionosphere?

    Asked in: GS 32 2023 IS DEEO 1505 2024

    1. It causes a decrease in electron density.
    2. It causes an increase in electron density.
    3. It causes a disruption of radio communication.
    4. It causes an increase in magnetic field strength.
    Show answer

    Answer: B. It causes an increase in electron density.

    Solar flares enhance ionization in the Earth's ionosphere, leading to an increase in electron density. This increased ionization is the direct physical impact, which then causes secondary effects like radio communication disruptions.

Spacecraft Propulsion and Rocketry Principles

  1. Consider the following statements in respect of a jet engine and a rocket: 1. A jet engine uses the surrounding air for its oxygen supply and so is unsuitable for motion in space 2. A rocket carries its own supply of oxygen in the gas from as fuel. Which of the above statements are correct ?

    Asked in: UPPSC Upper Mains 2007

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Show answer

    Answer: C. Both 1 and 2

    The fundamental difference enabling rockets to work in space, while jet engines cannot, is the rocket's ability to carry its own oxidizer (oxygen source) for combustion, whereas jet engines rely on drawing oxygen from the ambient air.

Stellar Structure, Evolution, and Nucleosynthesis

  1. Which is the chief heavenly body of solar system?

    Asked in: BPSC Prelims Exam 67th Re Exam

    1. Jupiter
    2. Saturn
    3. Sun
    4. Earth
    Show answer

    Answer: C. Sun

    The Sun is the central and most massive body in the solar system.

  2. Which two competing effects determine the size of a star?

    Asked in: UPSC Geoscientist 2022

    1. Nuclear fusion and electrostatic effects
    2. Nuclear fusion and magnetic effects
    3. Nuclear fusion and gravitational effects
    4. Gravitational and electromagnetic effects
    Show answer

    Answer: C. Nuclear fusion and gravitational effects

    Gravity constantly pulls the star's mass inward, tending to collapse it. Nuclear fusion reactions in the core, primarily converting hydrogen to helium, release enormous amounts of energy in the form of photons (radiation) and particles. This energy creates a strong outward pressure (radiation pressure and thermal pressure) that counteracts gravity. A star remains stable, maintaining a relatively constant size, when these two opposing forces are in equilibrium, known as hydrostatic equilibrium.

  3. The Brightness of a star depends on ______.

    Asked in: GPSC Prelims CSP 2 2019

    1. Size and temperature only
    2. Size and distance from the earth
    3. Size, Temperature and mass
    4. Size, temperature and distance from earth
    Show answer

    Answer: D. Size, temperature and distance from earth

    - A star's intrinsic luminosity (actual power output) is primarily determined by its size (radius) and surface temperature. Larger and hotter stars are more luminous. - The apparent brightness (how bright it appears to us) is inversely proportional to the square of its distance from the observer. A very luminous star can appear dim if it is far away, and a less luminous star can appear bright if it is close. - Therefore, apparent brightness depends on factors determining luminosity (size, temperature) and the distance.

  4. The brightness of a star depends on its

    Asked in: UPSC NDA 1 2016

    1. size and temperature only
    2. size and distance from the earth
    3. size, temperature and mass
    4. size, temperature and distance from the earth
    Show answer

    Answer: D. size, temperature and distance from the earth

    The apparent brightness of a star as observed from Earth depends on two primary factors: its intrinsic luminosity and its distance from the observer. The intrinsic luminosity of a star, which is the total energy it radiates per unit time, is determined by its size (radius) and its surface temperature. Larger and hotter stars are more luminous. Therefore, the brightness we perceive (apparent brightness) is a function of the star's size, temperature (which together determine luminosity), and the distance the light travels to reach us. The inverse square law dictates that brightness decreases with the square of the distance.

  5. How long does light take to reach the Earth from the Sun ?

    Asked in: UPSC NDA 2 2017

    1. About 4 minutes
    2. About 8 minutes
    3. About 24 minutes
    4. About 24 hours
    Show answer

    Answer: B. About 8 minutes

    - Distance from Sun to Earth is approximately 1.5 x 10^11 m. - Speed of light is approximately 3 x 10^8 m/s. - Time = Distance / Speed.

  6. Consider the following statements : Statement-I : Giant stars live much longer than dwarf stars. Statement-II : Compared to dwarf stars, giant stars have a greater rate of nuclear reactions. Which one of the following is correct in respect of the above statements ?

    Asked in: UPSC IAS 2024

    1. Both Statement-I and Statement-II are correct and Statement-II explains Statement-I .
    2. Both Statement-I and Statement-II are correct, but Statement-II does not explain Statement-I
    3. Statement-I is correct, but Statement-II is in‐ correct
    4. Statement-I is incorrect, but Statement-II is correct
    Show answer

    Answer: D. Statement-I is incorrect, but Statement-II is correct

    A star's lifespan is primarily determined by its mass and luminosity (rate of energy output, which is proportional to the rate of nuclear reactions). More massive stars have more fuel but burn it much faster due to higher core temperatures and pressures, resulting in shorter lives. Less massive stars burn their fuel slowly and live much longer. Giant stars are typically more massive and much more luminous than dwarf stars.

  7. The colour of the Star is an indication of its

    Asked in: UPPCS Prelims 2005

    1. distance from earth
    2. temperature
    3. luminosity
    4. distance from the Sun
    Show answer

    Answer: B. temperature

    The colour of a star is a direct indicator of its surface temperature. Cooler stars appear red (around 3,000 K), while hotter stars appear blue or blue-white (above 10,000 K). Stars with intermediate temperatures, like our Sun, appear yellowish- white (around 5,800 K). This relationship is described by Wien's Displacement Law, which relates temperature to the peak wavelength of emitted radiation.

  8. Light from the Sun reaches Earth in about:

    Asked in: UPPSC Upper Mains 2006

    1. 2 minutes
    2. 4 minutes
    3. 8 minutes
    4. 16 minutes
    Show answer

    Answer: C. 8 minutes

    The speed of light is approximately 300,000 kilometers per second, and the average distance between the Sun and the Earth is about 150 million kilometers.