Question

Choose the best answer to each of the following. Explain your reasoning with one or more complete sentences. What would happen to a neutron star with an accretion disk orbiting in a direction opposite to the neutron star's \(\operatorname{spin}\\{(a)\) Its spin would speed up. (b) Its spin would slow down. (c) Its spin would stay the same.

Short answer

The neutron star's spin would slow down due to opposite angular momentum from the accretion disk.

Step-by-step solution

Understand the Interaction

A neutron star with an accretion disk has mass transferring onto its surface from the disk. The direction of this mass flow can affect the neutron star's spin rate due to angular momentum exchange.

Determine the Effect of Opposite Momentum

If the accretion disk is rotating in the opposite direction to the neutron star's spin, the angular momentum transferred to the star will oppose its existing spin. This opposition will slow down the star's rotation.

Analyze the Options

Based on the understanding of angular momentum, option (b) is correct: the neutron star's spin would slow down because the accreting material is transferring opposite angular momentum to the star.

Key concepts

Accretion Disk

An accretion disk is a structure formed by diffused material in orbital motion around a central body like a neutron star. Imagine a swirling disk of gas and dust moving in spiral fashion towards the neutron star. This disk is created because of the gravitational pull of the star on nearby material.
As this material spirals inward, it gains speed and forms a disk. The inner regions of the disk experience intense gravitational pull and heat up, sometimes reaching millions of degrees. This heat makes the disk brightly luminous, often seen in X-rays. Here's why understanding accretion disks is crucial:

• Accretion disks play a vital role in transferring mass to neutron stars.
• They influence the spin and energy output of the neutron star.
• The process often leads to high-energy emissions, crucial for astronomical observations.

By absorbing material from the accretion disk, a neutron star can undergo changes in its spin, exemplifying the complex nature of these fascinating cosmic structures.

Angular Momentum

Angular momentum is a fundamental concept in physics, especially when discussing celestial objects like neutron stars. It's a measure of the amount of rotation an object has, taking both its mass and velocity into account. When mass is transferred from one object to another, like from an accretion disk to a neutron star, angular momentum is transferred as well. Here's a simple breakdown:

• Imagine a spinning ice skater: pulling in their arms increases their spin rate due to conservation of angular momentum.
• Conversely, if an accretion disk is spinning opposite to a neutron star's rotation, it acts as a drag, slowing down the neutron star.

When we consider the interaction between a neutron star and an oppositely spinning accretion disk, the angular momentum from the disk counters that of the star, slowing its spin. Understanding this exchange helps explain the dynamic behavior of neutron stars.

Neutron Star Spin

Neutron stars are remnants of supernova explosions, containing the mass of the Sun within a sphere only a few kilometers across. Due to their massive density, they can have incredibly fast spin rates, sometimes rotating several hundred times per second. The spin of a neutron star is critical for understanding its properties and behavior.
Here's what affects a neutron star's spin:

• The birth conditions post-supernova, such as the star's initial momentum.
• Interaction with accretion disks, which can increase or decrease its rotational speed.
• Magnetic forces, which can also play a significant role.

When an accretion disk spins opposite to the star's spin direction, it generally slows down the neutron star by transferring opposite angular momentum. This interplay affects the star's magnetic field, energy emission, and overall evolution. The spin rate is crucial not only for understanding the dynamics of neutron stars but also for predicting phenomena like gamma-ray bursts and gravitational waves.