Sketch the rotation curve you would obtain if the Galaxy were rotating like a rigid body.

When teaching the concept of rigid body rotation, an analogy often used is that of a spinning record or merry-go-round. All points on these objects, no matter the distance from the center, complete a full revolution in the same amount of time. This idea is crucial when we imagine celestial bodies like galaxies and how they might rotate. If a galaxy rotated like a rigid body, the stars and other materials at various distances from the galactic center would maintain a constant angular speed.

This means that the further away you move from the center, the faster you would have to travel in a straight line (linear velocity) to keep up with this rotation. Mathematically, we express this as a simple proportional relationship: \( v = \( \omega \) r \), where \( v \) is the linear velocity, \( r \) is the distance from the axis of rotation, and \( \( \omega \) \) is the angular speed, which remains the same for all parts of the galaxy under rigid body rotation. Hence, if we were to sketch the rotation curve, we would draw a straight line that starts at the origin and increases consistently—showing that linear velocity is a linear function of radial distance.

By understanding this concept, students can better grasp why the actual rotational behavior of galaxies differs significantly from rigid body rotation, which is a simplified, more comprehensible model that sets a foundation for studying more complex rotational dynamics.

Angular speed is a measure of how fast an object rotates or revolves around an axis. It is an essential concept in both physics and astronomy, particularly when discussing rotational motion. If an ice skater pulls her arms in, she spins faster—her angular speed increases because the radius of her rotation (distance from her rotational axis) decreases.

To quantify the angular speed, we use the formula \( \( \omega \) = \frac{v}{r} \), where \( \( \omega \) \) is the angular speed, \( v \) is the linear velocity, and \( r \) is the radius of the circular path. In units, angular speed is measured in radians per second (rad/s) when using the International System of Units (SI). It's important to recognize that while linear speed and angular speed are related, they are not the same. Linear speed refers to the path length traveled per unit of time, whereas angular speed measures the angle swept per unit of time.

In the context of the galactic rotation exercise, we assume a constant angular speed to illustrate rigid body rotation. However, the actual angular speed of stars orbiting a galaxy's center can vary, reflecting the intricate gravitational dynamics within galaxies.

Differential rotation diverges from the concept of rigid body rotation. Here, unlike a merry-go-round or spinning record, not all parts of an object rotate at the same angular speed. The Earth, the Sun, and indeed our Galaxy, the Milky Way, exhibit differential rotation. On the Sun, for example, the equator rotates faster than the polar regions.

In the grand structure of a galaxy, differential rotation arises due to the gravitational pull exerted by the varying mass distributions within the galaxy. As a result, stars closer to the galactic center orbit faster than those at the edge. The rotation curve reflecting differential rotation starts to flatten out or even decline at the outer edges, which is a stark contrast to the linearly increasing rotation curve expected from rigid body rotation.

Students, upon grasping the concept of differential rotation, understand that the actual galactic rotation curve acts as evidence against galaxies rotating as rigid bodies. The discrepancy between the expected straight-line (rigid body) graph and the observed curve leads to interesting further discussions, particularly regarding the distribution of mass in galaxies and the existence of dark matter, which can help explain the flatness of the outer portion of the rotation curve.