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. Some galaxies are neither ellipsoidal nor are they spirals. Another example of an irregular galaxy is the Messier object M82. Some galaxies show evidence for the generation of enormous amounts of energy from the vicinity of their nucleus. The right side shows a Hubble Space Telescope image of the central region of the galaxy where an enormous black hole is suspected to exist.