Question

Decide whether the statement makes sense (or is clearly true) or does not make sense (or is clearly false). Explain clearly; not all these have definitive answers, so your explanation is more important than your chosen answer. The radii of white dwarf stars in close binary systems gradually increase as they accrete matter.

Short answer

The statement does not make sense, as white dwarf radii decrease with accretion.

Step-by-step solution

Understanding White Dwarf Stars

White dwarf stars are the remnants of Sun-like stars after they have exhausted their nuclear fuel. They are typically very dense and composed mostly of electron-degenerate matter. This means that adding mass to a white dwarf doesn't lead to large increases in size, unlike what happens in non-degenerate matter.

Concept of Accretion in Binary Systems

In close binary systems, one star can gain mass by accreting material from its companion star. This process is common when a white dwarf pulls material from a nearby companion star, often leading to observable changes.

Effects of Accretion on Radius

A unique property of electron-degenerate matter in white dwarfs is that increasing their mass results in a smaller radius due to increased gravitational compression. Thus, as a white dwarf accretes mass, it becomes denser and its radius decreases rather than increases.

Evaluating the Statement

Given the properties of white dwarfs, the statement claiming the radius increases as they accrete matter contradicts the known behavior of these stars. Therefore, the statement does not make sense according to current astrophysical understanding.

Key concepts

Electron-Degenerate Matter

White dwarf stars are fascinating celestial objects composed mostly of electron-degenerate matter. But what exactly does that mean? In simple terms, this is a type of matter where the electrons are squeezed together so tightly that their quantum mechanical properties become important. These properties prevent the electrons from being compressed further due to the Pauli exclusion principle, which states that no two electrons can occupy the same quantum state. This creates a state where additional mass does not lead to a proportional increase in volume. Instead, adding more mass to a white dwarf can even result in a smaller radius. This is contrary to most materials, where adding mass usually increases size. Electron-degenerate matter is key to understanding why white dwarfs behave so differently compared to regular, non-degenerate stars.

Binary Star Systems

Binary star systems are one of the captivating phenomena in the universe, involving two stars orbiting a common center of mass. These systems can include different combinations of stars at various life stages, from young stars to remnants like white dwarfs, neutron stars, or black holes. Understanding binary systems is crucial because they are quite common in the universe, possibly making up 30% or more of all stellar systems. Binary systems come in various types, such as:

• Visual binaries: Where both stars can be separately observed through a telescope.
• Spectroscopic binaries: Detected through their spectral lines because they are too close to be resolved by a telescope.
• Eclipsing binaries: Change in brightness as one star passes in front of the other.

Binary systems often experience interesting dynamics, especially when one of the stars expands and sheds its outer layers to its companion, affecting both stars' evolution.

Accretion in Astronomy

Accretion is a fundamental process in astronomy, especially significant in binary star systems. It occurs when a celestial body accumulates matter from its surroundings, which in binary systems typically comes from a stellar companion. When we refer to accretion in white dwarfs, it commonly involves a white dwarf drawing material from a nearby companion star, a process often leading to fascinating phenomena like novae or type Ia supernovae. The accretion process is influenced by several factors, including:

• Mass transfer rate: The amount of mass being transferred.
• Roche lobe overflow: Occurs when a star's outer layers overflow into the white dwarf's gravitational influence.
• Angular momentum: Influences the accretion disk's formation around the accreting object.

In the context of white dwarfs, the accreted material adds to the star's mass, increasing its density, resulting in a decrease in radius due to the unique properties of electron-degenerate matter.

Stellar Evolution

Stellar evolution describes the life cycle of stars, including their birth, evolution, and eventual demise. White dwarfs represent one of the final stages in the evolution of stars similar to our sun. As a star exhausts its nuclear fuel, it sheds its outer layers and the core left behind cools into a white dwarf. Stellar evolution is influenced by a star's initial mass:

• Low to intermediate-mass stars, like our sun, become red giants and subsequently shrink into white dwarfs.
• Massive stars go through supernovae explosions, leaving behind either neutron stars or black holes.

In binary systems, stellar evolution can be even more complex due to mass exchange between stars, often resulting in unique paths, such as a white dwarf gaining enough mass to explode in a type Ia supernova. Studying stellar evolution in binary systems helps astronomers uncover the underlying processes that govern the universe's lifecycle.