The Sun takes about 200 million years to complete one orbit around the centre. About 30 percent of all galaxies have spiral arms. Some have arms that spiral directly from the nucleus while others have a linear feature, called a bar, from whose ends the arms originate. Another beautiful example of a spiral galaxy is M83 . Presumably our own galaxy would resemble these galaxies in appearance if we could view it from the outside.
Spiral galaxies are rich in gas and dust.
These show the presence of dust lanes which obscure the starlight coming from near the midline of the disk. We see this in our galaxy where the Milky Way is divided into two portions for much of its length. Indeed, as we have noted above, the centre of the Milky Way galaxy is invisible in ordinary light because the interstellar dust in that direction is so thick. Elliptical galaxies show only small evidence for young stars, dust or gas. They are very different in size ranging from giant ellipticals with masses of about 1 million million times that of the Sun to dwarf ellipticals with masses closer to those of the globular clusters.