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?

(a) Both Statement-I and Statement-II are correct and Statement-II explains Statement-I

(b) Both Statement-I and Statement-II are correct, but Statement-II does not explain Statement-I

(c) Statement-I is correct, but Statement-II is incorrect

Prelims · Science & technology · 22 questions

Space technology

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Prelims 2024 · Q31

Medium

With reference to radioisotope thermoelectric generators (RTGs), consider the following statements:

1.RTGs are miniature fission reactors.
2.RTGs are used for powering the onboard systems of spacecrafts.
3.RTGs can use Plutonium-238, which is a by-product of weapons development.

Which of the statements given above are correct?

Answer & explanation

Answer: (b) 2 and 3 only

An RTG is not a reactor: nothing is split in a chain reaction. It simply turns the steady heat given off by the natural decay of plutonium-238 into electricity using thermocouples, which is why it powers deep-space probes and rovers such as Voyager, New Horizons, Curiosity and Perseverance. The US made its plutonium-238 at the Savannah River nuclear-materials plant, part of its weapons complex.

✗ 1. RTGs have no fission or chain reaction. Heat from the spontaneous radioactive decay of plutonium-238 is converted to power by thermocouples, with no moving parts.
✓ 2. They supply electricity and warmth to spacecraft where sunlight is weak or dust blocks solar panels; the current US model, the MMRTG, gives about 110 watts at launch.
✓ 3. Plutonium-238 is not weapons-grade plutonium, but the US produced it at its Savannah River plant, a site built to make nuclear-weapons materials, until the late 1980s; it is the standard RTG fuel.

Remember · RTG = heat from natural decay of Pu-238 → electricity via thermocouples (Seebeck effect). No fission. Powers Voyager, Cassini, New Horizons, Curiosity, Perseverance.

Sources

US Department of Energy, Office of Nuclear Energy: What is a Radioisotope Power System? ↗ “Radioisotope power systems (RPS) convert heat generated by the natural decay of plutonium-238 —a radioactive isotope—into electrical power. They have powered more than two dozen U.S. space missions”
NASA Radioisotope Power Systems programme ↗ “a type of nuclear energy technology that uses heat to produce electric power for operating spacecraft systems in the darkest, dustiest, and most distant places in our solar system.”
NASA: About Plutonium-238 ↗ “U.S. production of plutonium oxide for space exploration at the Department of Energy’s Savannah River Plant in South Carolina ceased in the late 1980s, and interim purchases of heat source material from Russia have ended.”
US Department of Energy: Savannah River Site History 1950-1989 ↗ “The Savannah River Site was constructed during the early 1950s to produce the basic materials used in the fabrication of nuclear weapons, primarily tritium and plutonium-239, in support of our nation's defense programs.”

Question and answer: UPSC's official GS Paper I (2024, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 1 Oct 2026 (how we verify). Permalink ·

Prelims 2024 · Q32

Medium

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?

Answer & explanation

Answer: (d) Statement-I is incorrect, but Statement-II is correct

A star's mass decides how fast it burns. Big, massive stars need very high rates of nuclear fusion in their cores to hold themselves up against their own gravity, so they use up their hydrogen in a few million years, while small dwarf stars burn slowly and can shine for billions or even trillions of years. So the fusion-rate statement is true, and the longer-life statement is the reverse of the truth.

✗ Statement-I Massive stars are short-lived: some last only a few million years, whereas low-mass red dwarfs can last trillions of years; the Sun will spend about 10 billion years on the main sequence.
✓ Statement-II More massive stars must fuse hydrogen at a much higher rate to generate the outward pressure that balances their stronger gravity, which also makes them hotter and brighter.

Remember · Bigger star → faster fusion → shorter life. Massive stars: millions of years, end as supernovae; red dwarfs: trillions of years. Sun: about 10 billion years on the main sequence.

Sources

NASA Science: Stars ↗ “More massive stars must burn fuel at a higher rate to generate the energy that keeps them from collapsing under their own weight. Some low-mass stars will shine for trillions of years – longer than the universe has currently existed – while some massive stars will live for only a few million years.”
NASA Imagine the Universe: Stars ↗ “Our sun will spend about 10 billion years on the main sequence. However, a more massive star uses its fuel faster, and may only be on the main sequence for millions of years.”

Question and answer: UPSC's official GS Paper I (2024, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). Permalink ·

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