In stellar evolution, when a very massive star much larger than the Sun ends its life and collapses, the compact rapidly rotating remnant that emits regular pulses of radiation is called what?

Introduction / Context:
Stars evolve through different stages depending on their initial mass. Very massive stars end their lives in dramatic ways that can produce exotic remnants. One such remnant is a highly compact object that rotates rapidly and emits beams of radiation, which may be detected as regular pulses. This question examines your understanding of stellar evolution and the specific term used for such a rotating neutron star remnant.

Given Data / Assumptions:

- The star in question is very massive compared with the Sun.
- At the end of its life it collapses and leaves behind a compact rotating remnant.
- This remnant emits regular pulses of radiation that can be detected by observers.
- Options are Red giant, White dwarf, Pulsar and Black hole.
- We assume standard astrophysical terminology for stellar remnants.

Concept / Approach:
Massive stars can undergo a core collapse supernova at the end of their nuclear burning phases. If the core remaining after the explosion is between certain mass limits, it can contract into a neutron star. When such a neutron star has a strong magnetic field and rotates rapidly, it emits beams of electromagnetic radiation from its magnetic poles. As these beams sweep past Earth, they are observed as periodic pulses, and such an object is called a pulsar. Red giants and white dwarfs represent other stellar stages, while a black hole is an even more collapsed object with no direct radiation from a solid surface.

Step-by-Step Solution:
Step 1: Recognise that the description refers to a compact remnant of a massive star that emits regular pulses.Step 2: Recall that pulsars are rotating neutron stars with strong magnetic fields that beam radiation in narrow cones.Step 3: Red giant refers to a swollen, cool phase in a star life but not to a compact remnant.Step 4: White dwarfs are compact remnants of lower mass stars and usually do not produce strong periodic pulses of radiation in the same way.Step 5: A black hole does not let light escape from within its event horizon, so regular external pulses from its surface are not observed in this manner.

Verification / Alternative check:
To verify, think of the word pulsar itself, which suggests pulsing or periodic signals. Radio astronomers originally discovered pulsars as regular repeating pulses of radio waves arriving from deep space. Later studies showed that these pulses arise from rotating neutron stars, confirming that pulsar is the appropriate term for such an object. Cross checking with textbook diagrams of stellar evolution also supports that massive stars can end as neutron stars and pulsars under suitable conditions.

Why Other Options Are Wrong:
Red giant: This is a late stage in the life of a star where the outer layers expand and cool. It is not the final compact remnant described in the question.
White dwarf: This is a compact remnant of a medium mass star such as a Sun like star, but it typically radiates steadily rather than sending strong regular pulses in the same way as a pulsar.
Black hole: A black hole is an even more extreme end product where matter is so compressed that escape velocity exceeds the speed of light. Although black holes influence their surroundings, the term pulsar refers specifically to rotating neutron stars with pulsed emissions.

Common Pitfalls:
Learners may confuse different end stages of stars or assume that any compact remnant is automatically a black hole. Another confusion arises from mixing up red giants and white dwarfs, which are very different stages of stellar evolution. The key clue here is the mention of regular pulses of radiation, which directly points to the term pulsar. Connecting the name pulsar with pulsing signals is a useful memory aid for future questions.

Final Answer:
The compact rapidly rotating remnant that emits regular pulses is called a Pulsar.