Spiral galaxies Our galaxy has arms of younger stars and gas that appear to spiral out from the centre. In fact the objects in these spiral arms are in almost circular orbits about the centre of the Galaxy. 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.
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 the centre of the Milky Way galaxy is invisible in ordinary light because the interstellar dust in that direction is so thick. They 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.