Problem 5 - Astronomy Journey to the Cosmic Frontier

Question

How can the stars in an elliptical galaxy be orbiting the center of the galaxy if the galaxy has essentially no angular momentum?

Step-by-Step Solution

Answer

Stars follow individual orbits, canceling out net angular momentum.

Step 1: Understand Angular Momentum in Galaxies

Angular momentum in a galaxy refers to the rotational motion of stars, gas, and other matter around the galaxy's center. An elliptical galaxy having essentially no angular momentum suggests that it does not have a significant overall rotation like spiral galaxies do.

Step 2: Consider Individual Star Orbits

Each star within an elliptical galaxy can have its own orbit, which contributes small amounts of angular momentum. These orbits can be diverse in direction and shape, such as elongated, circular, or inclined, and they can be randomly oriented.

Step 3: Aggregate Result of Individual Orbits

Although each star contributes individual angular momentum, the overall angular momentum of the galaxy is negligible because these orbits are randomly oriented. This means that individual angular momenta cancel out, resulting in a net effect where the stars still orbit but the galaxy itself shows little overall rotation.

Key Concepts

Angular Momentum

Imagine a spinning top. It rotates around its axis, and this spinning motion is a simple way to understand angular momentum. In the universe, angular momentum is a major player in the dynamics of galaxies. It refers to the rotation aspect of any object. This is important for galaxies because it affects their shape and behavior.

For elliptical galaxies, the situation is unique. These types of galaxies have very little angular momentum. Unlike their spiral cousins, they do not spin obvious rotations. Instead, the stars in elliptical galaxies have their own motions that don't add up to create a large, unified spinning movement. However, each star still moves in its path around the galaxy's center. These movements contribute tiny amounts of angular momentum. Yet, because of the random orientations of these movements, they cancel out overall, leaving the elliptical galaxy with essentially no net angular momentum.

Star Orbits

Stars in elliptical galaxies each have their own unique pathways, or orbits, which contribute to the galaxy's behavior. These orbits are comparable to the planets orbiting our solar system, but they differ because they can vary greatly in direction and shape.

• Some stars follow elongated paths, also known as elliptical orbits.
• Others might have more circular routes.
• The paths can be inclined, adding more variety to their motion.

This diversity creates a complex dance of stars within the galaxy. Despite their individual angular momentum contributions, the randomness of these orbits means they offset each other. This is why the galaxy seems like it’s almost stationary when viewed as a whole. Every star still interacts with the gravitational center of the galaxy, ensuring that these stars remain confined within the galaxy’s boundaries.

Galaxy Rotation

In a typical case of galaxy rotation, like with spiral galaxies, you would observe a distinct spinning motion. This is not the case for elliptical galaxies, which appear to lack obvious rotation. This absence of unified rotation does not mean that these galaxies are completely motionless.

While the stars themselves are in motion, the randomness and variability in star orbits mean that their movements do not lead to the galaxy spinning as a single entity. It’s almost as if, while each part of a machine is moving, the whole machine does not roll forward.

For elliptical galaxies, the lack of cohesive galaxy-wide angular momentum and rotation means they are shaped more by stellar interactions and mergers rather than by smooth spins. This results in the unique, more rounded appearance of elliptical galaxies. Ultimately, the stars’ individual motions maintain the galaxy’s stability without contributing to an overall rotation.