Lesson Objectives:- The Milky Way Galaxy
- The orbit of stars in our galaxy
- The mass of the galaxy
The Milky Way Galaxy holds more than 100 billion stars and is a spiral galaxy. It is called a spiral galaxy because it has spiral arms that make up the flat disk surrounding the central bulge. The halo of a galaxy is the roughly spherical component of a galaxy which surrounds the entire disk. Most of the brightest stars are found in the disk, but there are prominent stars found in the globular clusters which reside in the halo.
The interstellar medium, which refers to clouds of interstellar gas and dust, fills the galactic disk and obscures our view when we use visible light to try to study most parts of our galaxy.
Every star in the galaxy orbits around the center of the galaxy. Stars in the disk of the galaxy orbit the galactic center in roughly circular paths going in the same direction in nearly the same plane, just like our planets orbit the Sun. Stars closer to the center complete each orbit in less time than stars farther out. Our Sun takes around 230 million years to complete one orbit.
Stars in the halo are not as organized. They orbit the galactic center, but with randomly oriented orbits, swooping high above and below the disk.
Inside the bulge of the galaxy, orbits are more complex with some stars orbiting like halo stars and others with orbits similar to disk stars.
Using Newton's version of Kepler's third law allows us to determine the mass of an object when we know the orbital period and average distance of a much smaller object in orbit around it. That means, using the Sun's orbital velocity and distance from the center of the galaxy allows us to calculate the mass of most of our galaxy -- the part that lies within the Sun's orbit.
Everything is not as simple and clear-cut as would be expected, however. Since the bulge of the galaxy appears to be where most of the galaxy's mass is concentrated, stars that are closer to the bulge should move faster -- just like planets closest to our Sun move faster. Studies of different stars, however, show that orbital velocities near the center of the galaxy and near the edge are similar.
This has led scientists to believe that most of the galaxy's mass resides far from the center but does not give off any observable light. This invisible matter distributed throughout the halo is called dark matter.