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11. ‘Black hole’ is a

‘Black hole’ is a

huge black star which has zero acceleration due to gravity on its surface

star which has moderate acceleration due to gravity on its surface

star which has collapsed into itself and has large acceleration due to gravity on its surface

star which has collapsed into itself and has zero acceleration due to gravity on its surface

This question was previously asked in

UPSC NDA-1 – 2019

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The correct option is C) star which has collapsed into itself and has large acceleration due to gravity on its surface. This describes the nature of a black hole.

– 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.

Black holes are predicted by Einstein’s theory of general relativity. They are regions of spacetime exhibiting such strong gravitational effects that nothing—not even particles and electromagnetic radiation such as light—can escape from inside their event horizon. The point of infinite density at the center of a black hole is called the singularity.

12. The brightness of a star depends on its

The brightness of a star depends on its

size and temperature only

size and distance from the earth

size, temperature and mass

size, temperature and distance from the earth

This question was previously asked in

UPSC NDA-1 – 2016

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The correct option is D.

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.

While mass is a fundamental property of a star and influences its temperature, size, and lifespan, the most direct factors determining luminosity are size and temperature. Apparent magnitude is a measure of apparent brightness, while absolute magnitude measures intrinsic luminosity (brightness at a standard distance of 10 parsecs).

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

Which two competing effects determine the size of a star?

Nuclear fusion and electrostatic effects

Nuclear fusion and magnetic effects

Nuclear fusion and gravitational effects

Gravitational and electromagnetic effects

This question was previously asked in

UPSC Geoscientist – 2022

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The size of a stable star is determined by the balance between the inward force of gravity and the outward pressure generated by nuclear fusion in its core.

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.

While other effects like magnetic fields can influence stellar activity and structure in specific regions or phases, the primary factors determining the overall size and stability of a star throughout its main sequence life are the balance between self-gravity and the pressure from nuclear fusion.