UPSC Prelims 2023 Q56: Cepheids, Nebulae and Pulsars — Explained

Consider the following pairs:

(Objects in space : Description)

1. Cepheids : Giant clouds of dust and gas in space

2. Nebulae : Stars which brighten and dim periodically

3. Pulsars : Neutron stars that are formed when massive stars run out of fuel and collapse

How many of the above pairs are correctly matched?

  1. Only one
  2. Only two
  3. All three
  4. None

Answer: (a) Only one

Statement-wise explanation

  • 1: incorrect — Cepheids are not clouds. They are pulsating giant stars whose brightness rises and falls in a regular cycle, usually lasting from about a day to several weeks. In 1912 Henrietta Leavitt showed that a Cepheid's period is linked to its true brightness. 'Giant clouds of dust and gas in space' describes a nebula, so this pair has been swapped with pair 2.
  • 2: incorrect — A nebula is a huge cloud of gas, mostly hydrogen, and dust in space. New stars form in nebulae, such as the Orion Nebula. 'Stars which brighten and dim periodically' describes Cepheid variables, not nebulae. So this pair is also swapped.
  • 3: correct — When a massive star runs out of nuclear fuel, its core collapses, usually in a supernova, and leaves behind a neutron star. This is a city-sized object denser than an atomic nucleus. A pulsar is a fast-spinning, strongly magnetised neutron star whose beams of radiation sweep past Earth like a lighthouse, so we see regular pulses. Jocelyn Bell and Antony Hewish discovered the first pulsar in 1967; it pulsed every 1.33 seconds.

How to eliminate

This question can be solved by spotting the swap. Pairs 1 and 2 have their descriptions exchanged: 'clouds of dust and gas' belongs to nebulae, and 'stars that brighten and dim periodically' belongs to Cepheids. The word itself helps. 'Nebula' is Latin for mist or cloud, so a nebula cannot be a star that pulses. Once you see that Cepheids and nebulae have each other's descriptions, both pairs are wrong. That leaves only the pulsar pair to judge. A pulsar is a neutron star left behind when a massive star collapses, which is standard textbook content. So exactly one pair is correct. Fact to remember: Cepheid = pulsating variable star used to measure distance; nebula = gas-and-dust cloud where stars are born; pulsar = spinning neutron star.

Know more

Objects in space and the stellar life cycle

Stars are born in nebulae, which are cold clouds of hydrogen, helium and dust. Gravity pulls clumps of gas together until nuclear fusion starts. How a star ends depends on its mass:

  • Sun-like stars swell into red giants, shed their outer layers as a planetary nebula, and leave behind a white dwarf.
  • Massive stars (more than about 8 times the Sun's mass) explode as a supernova. The core collapses into a neutron star, or into a black hole if it is heavy enough.
Object What it is Why it matters
Nebula Cloud of gas and dust Stars are born here; the Kant–Laplace nebular hypothesis says the solar system formed from one
Cepheid variable Giant star that pulsates with a regular period Its period shows its true brightness, so it works as a 'standard candle' for measuring distance
Pulsar Fast-spinning, magnetised neutron star Very regular pulses make it a precise cosmic clock
Neutron star Collapsed core, about 20 km across, with more mass than the Sun End state of massive stars
Black hole Collapse so strong that not even light can escape End state of the heaviest stars

Cepheids and cosmic distances. In 1912 Henrietta Leavitt found that brighter Cepheids pulse more slowly. If you measure the period, you know the star's true brightness. Comparing that with how bright it looks from Earth gives its distance. In the 1920s Edwin Hubble used Cepheids to show that the Andromeda 'nebula' is a separate galaxy outside the Milky Way. That work led to the idea of an expanding universe.

Pulsars and India. Pulsar searches are a major use of the Giant Metrewave Radio Telescope (GMRT), run by NCRA-TIFR near Pune. Arrays of pulsars timed together (pulsar timing arrays) are used to search for very-low-frequency gravitational waves.

Exam tip: UPSC often tests these terms by swapping their descriptions. Learn one defining phrase for each object.

NCERT Concepts to Revise

  • Nebula: Two pairs turn on what a nebula is: a cloud of gas and dust, or a star that changes brightness.

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