an E0 galaxy is pretty much round, while an E7 galaxy is extremely elongated, like a cigar. In this classification scheme (see figure to the left) a number is used to describe the roundness of ellipticals, e.g. The first to order galaxies not only by elliptical (E) or spiral (S) type was the astronomer Edwin Hubble. The spiral arms of spiral galaxies can be tightly wound or rather loosely wound, its bulge can be larger or smaller. For example, an elliptical galaxy can be nearly spherical or very elongated. However, even among the elliptical and spiral galaxies appearances vary. There are a few such dwarf galaxies around the Milky Way which are likely to be swallowed by our own galaxy over time. The smallest ellipticals are also the faintest galaxies they are called dwarf ellipticals. In comparison, the light from one end of the Milky Way needs about 100,000 light years to travel to the opposite end. The light in the largest ellipticals takes more than 2 million years to travel from one end of the galaxy to the other. Elliptical galaxies are in many ways very similar to the bulges of spiral galaxies.īoth the biggest and smallest galaxies in the local Universe are elliptical galaxies. It is also believed that they contain very little to no dust and gas, unlike spiral galaxies.
Most ellipticals are red in colour because they consist of mostly old stars and they do not form new stars. Because they are of elliptical shape, we call them elliptical galaxies. In contrast to the structured spiral galaxies, there exist galaxies that are very smooth, in which the stars are evenly distributed and that have pretty much no structure at all (like the galaxy on the left). While the spiral arms are best seen in galaxies that face us (we say the galaxy is face-on, like the one to the top right), we know they are very flat disks from the ones that are edge-on (like the one on the bottom right). The halo contains only a small number (compared to the many stars in the disk) of old stars which often reside in clusters.
This gives the bulge its distinct red colour. The bulge in the center of the galaxy on the other hand consists of mainly old stars which are much cooler and thus red. The stars, dust, and gas in the disk orbit around the galaxy's center. Star-forming regions, the birthplaces of new generations of stars, are located mainly along the spiral arms.
The disk contains mainly gas, dust and young stars (and young means a few million years) which shine blue due to their hot temperatures (much hotter than our Sun). Spiral galaxies have a disk, a bulge, and a halo. Because of their structure they are called spiral galaxies. Most galaxies in the Universe have a very structured appearance, with spiral arms, clumpy star-forming regions, dark dust lanes, and sometimes bars in their centers. Edge-on spiral galaxy with prominent dust lanes
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