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 spin? (a) Its spin would speed up. (b) Its spin would slow down. (c) Its spin would stay the same.

Short answer

Its spin would slow down due to subtracting angular momentum.

Step-by-step solution

Understanding the Concepts

A neutron star with an accretion disk has material orbiting around it that can transfer angular momentum to the star. The star's spin is affected by the angular momentum of the accreted material, which can increase or decrease the star's rotation speed.

Evaluating the Accretion Scenario

When the accretion disk orbits in the same direction as the spin of the neutron star, it adds angular momentum, speeding up the spin. Conversely, an accretion disk orbiting in the opposite direction will subtract angular momentum from the neutron star, affecting its spin.

Applying the Concept to the Given Problem

In this problem, the neutron star's accretion disk orbits in a direction opposite to its spin. This means that the material's angular momentum works against the current spin of the neutron star.

Determining the Effect on Spin

Since the accretion disk is orbiting opposite to the spin, it will slow down the spin of the neutron star by subtracting angular momentum. Hence, option (b) is correct: 'Its spin would slow down.'

Key concepts

Accretion Disk

In the world of astrophysics, an accretion disk is a fascinating structure that forms when material accumulates around a massive object. Imagine a swirling whirlpool made of gas and dust spiraling into the center. This "pool" is the accretion disk. It's found around bodies like black holes, young stars, and, as in our exercise, neutron stars.

• Accretion disks are powered by the gravitational pull of the central massive object.
• Material in the disk gradually spirals inward, getting hotter and faster as it does so.

In the context of our neutron star, the accretion disk is made of matter captured by the star's strong gravity. As this material moves closer to the neutron star, it can affect the star's spin due to a physics principle called angular momentum. Understanding this dynamic is key to grasping how accretion affects a neutron star's rotational speed.

Angular Momentum

Angular momentum is a crucial concept that explains how rotating objects behave. It's a measure of the 'amount of rotation' a body possesses due to its motion and where its mass is distributed in space. Angular momentum consistently plays a part in both everyday objects and astronomical phenomena. Consider a figure skater spinning with arms extended. As they pull their arms in, they spin faster. This conservation of angular momentum shows that if no external torques (twists or forces) are acting, the total angular momentum will remain constant. In our neutron star exercise:

• When the accretion disk orbits in the same direction as the neutron star's spin, it adds angular momentum to the star, causing it to spin faster.
• Conversely, if the disk orbits in the opposite direction, it subtracts angular momentum, resulting in a slower spin.

Understanding this principle helps determine how the direction and velocity of an accretion disk’s rotation can increase or decrease the spin of a neutron star.

Neutron Star Physics

Neutron stars are some of the most exotic and dense objects in the universe. Imagine a stellar core collapsing in on itself, creating an object only about 20 kilometers in diameter, yet with a mass greater than that of our sun. This is the essence of a neutron star.

• Composed mostly of closely packed neutrons, these stars have incredibly strong gravitational fields.
• Their surfaces can rotate rapidly, sometimes spinning around dozens of times per second.

The connection with an accretion disk comes from the tremendous gravitational forces exerted by these stars, capable of attracting material forming the disk. The interplay of these powerful forces with angular momentum can influence not only the spin but also the overall evolution of the neutron star itself. Neutron star physics helps explain why these interactions lead to changes in their spin direction when influenced by surrounding matter. Grasping these dynamics offers insights into how matter behaves under extreme gravitational conditions.