What are spiral arms caused by?

Astronomers believe that galaxies have spiral arms because galaxies rotate – or spin around a central axis – and because of something called “density waves.” Galactic density waves are like water waves. Water itself doesn’t move across a pond – instead, wave energy moves and affects the water as it passes.

How are spiral arms explained with density waves?

Originally, astronomers had the idea that the arms of a spiral galaxy were material. More specifically, the density wave theory argues that the “gravitational attraction between stars at different radii” prevents the so-called winding problem, and actually maintains the spiral pattern.

How spiral galaxies are formed?

In the process of merging, a disk is formed and stars are born in this disk. As small galaxies pass around the periphery of this disk, the effects of their gravity create a spiral structure in the disk. Spiral galaxies like our Milky Way Galaxy are thought to be formed in this manner.

What does the spiral density wave theory explain?

The Spiral Density Wave Theory Basically, this theory states that the spiral arms of a disk galaxy are regions of the galaxy that are of higher density; and so we call them density waves. They are also areas that are moving more slowly than the galaxy’s stars and gas.

What is the average separation between spiral arms?

We show for a typical arm cross-cut, a separation of 400 pc between the mid-arm and the dust lane (at the inner edge of the arm, toward the Galactic center).

What exists between the spiral arms of the galaxy?

The stars and gas and dust clouds in the disk congregate in a spiral pattern. There are many stars that are also in-between the spiral arms, but they tend to be the dimmer stars (G, K, M-type stars). Long-lived stars will move in and out of the spiral arms as they orbit the galaxy.

How many stars are in a spiral galaxy?

It is the dominant member of a small group of about half a dozen galaxies, the M74 galaxy group. In its entirety, it is estimated that M74 is home to about 100 billion stars, making it slightly smaller than our Milky Way.

Do spiral arms move?

The space between the spiral arms is not empty, and stars can move in and out of the arms as they orbit through the galaxy.

What causes density waves in galaxies?

If the center of the galaxy has a bar shaped and rotating mass distribution, it could cause enough gravitational disturbance in the disk to produce density waves.

Do elliptical galaxies have spiral arms?

Elliptical galaxies lack the swirling arms of their more well-known siblings, spiral galaxies. Instead, they bear the rounded shape of an ellipse, a stretched-out circle.

Why do galaxies appear to have spiral arms?

The reason we can see this spiral pattern is because as it passes through the galaxy the density wave compresses gas clouds, triggering star formation. The youngest, brightest stars will thus be nearest the wave and trace out an arm.

How are density waves produced in a spiral arm?

The density variations travel round the galaxy, much like a sound wave through the air; therefore a spiral arm is not a simply a concentration of co-orbiting stars and gas. “The problem of the origin of density waves is difficult because the billions of stars in a galaxy all exert gravitational forces on each other.

How are stars packed together in a spiral arm?

“A density wave is shorthand to describe the way stars in a galaxy are packed a little more closely together in the arms and spread more thinly in between the arms. The density variations travel round the galaxy, much like a sound wave through the air; therefore a spiral arm is not a simply a concentration of co-orbiting stars and gas.

How are spiral arms different from globular clusters?

In what way are they different? Spiral arms contain young stars, dust, and gas within which star formation continues, whereas globular clusters contain older star populations, with no dust, gas or on-going star formation. The disk of the Milky Way appears bluish because of the light from the many O and B type stars found there.