Question

Which of these has an event horizon? A) giant C) black hole B) white dwarf D) neutron star

Short answer

C) A black hole.

Step-by-step solution

Identify Concepts

An event horizon is a boundary beyond which events cannot affect an outside observer. It is typically associated with black holes.

Evaluate Each Option

Consider each option: A) A giant star: While very large, it does not have an event horizon. B) A white dwarf: These are remnants of stars without sufficient gravity to form event horizons. C) A black hole: These objects certainly have event horizons. D) A neutron star: These are very dense stars but do not have event horizons unless they become black holes.

Identify Correct Answer

From the evaluation of each option, the black hole (C) is clearly the only one of these objects that inherently has an event horizon.

Key concepts

Event Horizon

The event horizon is a fascinating concept primarily connected to black holes. Imagine it as an invisible boundary that wraps around a black hole.
Once anything crosses this boundary, it can never escape the gravitational pull of the black hole. Not even light can escape.
This is why black holes appear "black" – they do not emit light beyond this point.

• It marks the point of no return.
• It is defined by the gravitational pull, which is so immense that escape velocity surpasses the speed of light.
• The event horizon is not a physical surface but a mathematical boundary in spacetime.

Understanding the event horizon helps us comprehend why black holes are such powerful cosmic phenomena.

Giant Star

Giant stars are an incredible stage in the life cycle of stars, characterized by massive size and brightness. As a star exhausts its hydrogen fuel, it expands and becomes a giant.
Here's what you need to know:

• Giant stars are much larger than our Sun, sometimes hundreds of times bigger.
• They have extended atmospheres which can make them appear puffed up.
• They do not have an event horizon, as their gravity isn't enough to condense into such a dense object like a black hole.

Studying giants helps astronomers understand star evolution and the mechanics behind their transformations.

White Dwarf

White dwarfs are intriguing relics of medium to small-sized stars, marking the end of a star's life cycle. When a star like our Sun exhausts its fuel, it sheds its outer layers, leaving behind a hot, dense core – a white dwarf.

• They have no fusion reactions occurring in their core.
• Typical white dwarf size is similar to Earth, but it contains the mass of the Sun, due to which it's incredibly dense.
• These stars do not have enough gravitational pull to form an event horizon and become a black hole.

White dwarfs serve as a glimpse into stellar life and death, acting as cosmic markers of star evolution.

Neutron Star

Neutron stars are remnants of massive stars that have undergone a supernova explosion. They are fascinating due to their incredible density and unique properties.

• They are composed entirely of neutrons, the result of protons and electrons combining under immense pressure.
• Neutron stars are incredibly dense; a sugar-cube-sized piece of neutron-star material would weigh as much as humanity combined.
• Like white dwarfs, they don't have an event horizon unless they further collapse into a black hole.

Exploring neutron stars tells us about extreme states of matter and the limits of physical laws in the universe.