Why Is Our Galaxy Shaped like a Spiral?
A galaxy is a huge number of stars grouped together. Our galaxy contains about 1,00,000 million stars. It looks like a disc with a flat centre and spiral arms. From the front it looks like a slowly spinning Catherine Wheel. From the side it looks like a convex lens.
The sun is about two-thirds of the way from the centre of the galaxy. The sun takes 225 million years to travel around the galaxy. Because of its shape, our galaxy is classed as a spiral galaxy. Other galaxies have different shapes and sizes. The nearest galaxies to our galaxy are the Magellanic clouds.
Astronomers believe that galaxies have spiral arms because galaxies rotate – or spin around a central axis – and because of something called “density waves.” A spiral galaxy’s rotation, or spin, bends the waves into spirals. Stars pass through the wave as they orbit the galaxy center. The wave causes the stars to slow slightly and temporarily clump together.
Astronomers have long wondered why the spiral arms of a galaxy don’t wind up and vanish after a few rotations. Many astronomers think that there are multiple processes that contribute to creating the different kinds of spiral galaxies we see.
Today, galaxies are divided into four main groups: spiral, barred spiral, elliptical, and irregular.Together with irregular galaxies, spiral galaxies make up approximately 60% of galaxies in today’s universe. They are mostly found in low-density regions and are rare in the centers of galaxy clusters.