|The Milky Way, credit:B. Fugate (FASORtronics)/ESO|
|Face-on spiral galaxy|
|Edge-on spiral galaxy with prominent dust lanes|
Our own galaxy, the Milky Way, is such a spiral galaxy. You might ask, how do we know that seeing that we are sitting right in the galaxy and can’t look at it from the outside. Well, if the Milky Way were not a flat, disky galaxy you would not see that white-ish band across the night sky because the stars would not be concentrated in a thin disk but be more evenly distributed across the sky. From the picture of the Milky Way (first image), you can recognize dark structures in the white-ish band of stars. These are created by dust and gas between us and the stars which block out the light of these stars.
Both the biggest and smallest galaxies in the local Universe are elliptical galaxies. The light in the largest ellipticals takes more than 2 million years to travel from one end of the galaxy to the other. In comparison, the light from one end of the Milky Way needs about 100,000 light years to travel to the opposite end. The smallest ellipticals are also the faintest galaxies; they are called dwarf ellipticals. There are a few such dwarf galaxies around the Milky Way which are likely to be swallowed by our own galaxy over time.
|Small (left) and Large Magellanic Cloud (right)|
credit: ESO/S. Brunier
As you might have expected there are also galaxies in the local Universe that don’t really fit in either of the above categories. Their shape is usually irregular and they are forming a lot of stars. We call them irregular galaxies. The most prominent examples are the two Magellanic Clouds, our intergalactic neighbours.
|Classification scheme for galaxies|
Galaxy SurveysAn increasingly popular method in astronomy to study the galaxy population in the Universe is to take a large survey, i.e. to look at many galaxies instead of just a few. This helps us to make more general statements about the galaxy population by averaging the properties of a large number of galaxies. Thanks to the Sloan Digital Sky Survey, which mapped more than a quarter of the sky, we know a lot about the galaxies in the local Universe. However, many astronomers are interested in the formation and evolution of galaxies over time which in part means to answer the question: Why do galaxies in the local Universe look the way they do? To answer this question we need to study galaxies that are much further away and which existed when the Universe was only a few billion years old (in comparison, the Universe today is about 13.5 billion years old). With the Hubble Space Telescope we are able to find far away galaxies which appear much fainter than close-by ones and map the distant Universe. From this we already learned that distant galaxies can look much different compared to the ones we see nearby (see for example the image of the Hubble Deep Field). However, the light of distant galaxies gets stretched and shifted to the red on its way to us (this is called redshift) and thus what we can observe of distant galaxies by analyzing images is difficult to compare to what we observe locally. With CANDELS we are able to observe far away galaxies in the same light as local galaxies. This will allow us to directly compare galaxies and to understand how distant galaxies evolved.
In future posts we will tell you more about the distant Universe, the shape of galaxies, interactions between galaxies, and the properties of galaxies. So stay tuned!