Question

Census of Stellar Corpses. Which kind of object do you think is most common in our galaxy: white dwarfs, neutron stars, or black holes? Explain your reasoning.

Short answer

White dwarfs are the most common stellar corpses in our galaxy due to the abundance of smaller stars compared to more massive ones.

Step-by-step solution

Understanding the Options

To decide which type of stellar corpse is most common in our galaxy, we need to understand the nature of each: white dwarfs, neutron stars, and black holes. We know that these objects are remnants of stars that have completed their life cycle.

Formation of White Dwarfs

White dwarfs form from stars that were less than about 8 times the mass of our Sun. Such stars are very common and end their lives by shedding outer layers, leaving behind a dense core.

Formation of Neutron Stars

Neutron stars are the remnants of supernova explosions of massive stars, typically those between 8 to 20 times the mass of the Sun. While massive stars are less common, these remnants are still significant in number.

Formation of Black Holes

Black holes result from the gravitational collapse of very massive stars, generally more than 20 times the mass of the Sun. These stars are relatively rare compared to those forming white dwarfs and neutron stars.

Estimating Commonality

Since the stars that become white dwarfs are far more numerous than those forming neutron stars and black holes, we conclude that white dwarfs are likely the most common type of stellar corpse in our galaxy.

Key concepts

White Dwarfs

White dwarfs are the most common type of stellar remnant in our galaxy. These celestial objects are what remains after a star, similar in size to our Sun, exhausts its fuel and sheds its outer layers. What is left behind is a dense, hot core, which gradually cools over billions of years.
The process leading to the formation of a white dwarf involves a star undergoing a phase called the red giant stage. During this stage, the star expands enormously and loses mass through stellar winds. Finally, the core that remains shines faintly as a white dwarf, emitting faint light due to stored thermal energy.

• Origin: Stars less than 8 times the mass of the Sun.
• Composition: Carbon and oxygen under a thin hydrogen or helium atmosphere.
• Density: Extremely high, with masses around the Sun's range compressed into volumes similar to Earth.

White dwarfs illustrate the final stage in the life cycle of most stars, distinct by their commonality in the galaxy.

Neutron Stars

Neutron stars are the remnants of massive stars that have undergone a supernova, which is an explosive death. When a star between 8 to 20 times the mass of the Sun exhausts its nuclear fuel, its core collapses under gravity, causing a supernova. The outer layers explode into space, and the core compresses into an incredibly dense object called a neutron star.
These stars are fascinating due to their extreme properties.

• Diameter: Just about 20 kilometers, yet more massive than the Sun.
• Density: So high that a teaspoon of neutron star material would weigh billions of tons on Earth.
• Magnetic Field: Exceptionally strong, making neutron stars among the most magnetic objects in the universe.

Neutron stars can also spin at incredibly high speeds, sometimes emitting beams of radiation, observed as pulsars from Earth.

Black Holes

Black holes are mysterious objects formed from the remnants of the most massive stars, typically over 20 times the mass of the Sun. When these stars die, their core collapses to a point of infinite density called a singularity. The gravity around this point is so strong that not even light can escape, defining the black hole's event horizon.
Black holes are generally more rare than white dwarfs and neutron stars due to the specific conditions required for their formation.

• Structure: Feature an event horizon, singularity, and no surface.
• Detection: Indirectly observed by their effect on nearby stars and gas.
• Types: Ranges from stellar-mass black holes to supermassive black holes in galactic centers.

Though elusive, black holes play a crucial role in shaping galaxies and represent the ultimate endpoint for the heaviest stars.

Supernova

A supernova marks the explosive death of a massive star. It is one of the most powerful forces in the universe, briefly outshining entire galaxies. When a star runs out of nuclear fuel, its core collapses, and the outer layers are violently expelled.
Supernovae serve a crucial role in the synthesis and distribution of elements.

• Types: Typically categorized into Type I and Type II, depending on their causes.
• Element Formation: Produce heavy elements like iron and nickel, crucial for planet formation and life.
• Impact: Shockwaves can trigger the formation of new stars.

Supernovae, therefore, not only mark a violent end for certain stars but also foster the birth of new ones, contributing to the cosmic cycle of matter.

Star Life Cycle

The life cycle of a star is a journey from birth to death, influenced by its initial mass. Stars form from clouds of gas and dust, known as nebulae, which collapse under gravity to ignite nuclear fusion.
The mass of a star determines its fate. Smaller stars evolve into red giants and ultimately become white dwarfs. Larger stars go through a more tumultuous path, ending in supernovae and leaving behind neutron stars or black holes.

• Stellar Formation: Begins with protostar development.
• Main Sequence: Stars spend most of their lives fusing hydrogen into helium.
• Final Stages: Varies by mass, leading to red giants/supergiants.

Understanding the star life cycle is fundamental to studying the universe, as it dictates the creation and destruction of celestial objects, influencing cosmic evolution.