Galaxies and Galaxy groups
A Galaxy is a massive conglomeration of stars and gas that is bound together by the collective gravity of all the objects. It has a center of mass so strong that it can preside over thousands, millions, billions, or even trillions of stars.
Our home galaxy the Milky Way is a barred spiral galaxy, about 100,000 light-years across. If you could look down on it from the top, you would see a central bulge surrounded by four large spiral arms that wrap around it. Our Solar System is about 270,000 light-years away from the center of the Milky Way.
The Milky Way does not sit still, but is constantly rotating. As such, the arms are moving through space. The sun and our solar system travel with them.
Our solar system travels at an average speed of 515,000 mph (828,000 km/h). Even at this rapid speed, our solar system would take about 230 million years to travel all the way around the Milky Way.
So what does our Milky Way look like "from the side"? Unfortunately, we can't take a real photo of our Milky Way because we ourselves are located within this galaxy; see the position of our solar system above.
Recently, however, such a "side view" was created using the precise positions of more than a billion stars. The data comes from the ESA Gaia mission.
"Side view" of our Milky Way
The image shows our Milky Way as a very thin central disk, like many other spiral galaxies. This disk contains our sun and all the stars we see at night.
The colors of the curved central band come primarily from dark dust, bright blue stars, and red emission nebulae.
There are different types of galaxies
