Glossary

If you go to a dark place, away from city lights, you might notice a hazy luminous band as shown in Figure 2 in the sky. Because of its whitish appearance, it is called the Milky Way.

Figure 2: Milky Way as seen from the Earth
(Credit: By Steve Jurvetson - Flickr, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=23906915)

Since ancient times, people have wondered and speculated about this luminous band. In the 16th century Galileo Galilei looked at the Milky Way through his telescope and for the first time saw it as a collection of individual stars. Another thinker, Immanuel Kant correctly proposed that the Milky Way might be a rotating body of a huge number of stars, held together by gravitational forces akin to the Solar System but on much larger scales. All the stars we see are in our Milky Way. Scientists have calculated distances of many of the stars and have prepared a map of our Milky Way (Figure 3). The shape of the Milky Way is like a disk with a bulge at the centre and spiral arms in the outer parts. It so happens that our Sun is quite far from the centre of the Milky Way (Notice its position in the map). The  diameter of the Milky Way varies between 100,000 and 180,000 light-years. Since the Earth is inside the disk, the disk appears as a band. Also, since we are away from the centre, the Milky Way appears thicker on one side and thinner on the other. That means that when the Earth is towards the centre of the Milky Way in its orbit, we see the thick band of the Milky Way, but after six months, when the Earth moves to the other side, we see a thin band of the Milky Way.
 

Figure 3: Map of the Milky Way

Figure 3a: From above (Top view)⁵
(Credit: By NASA/JPL-Caltech/Federal University of Rio Grande do Sul - http://photojournal.jpl.nasa.gov/jpeg/PIA19341.jpg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=40704119)

Figure 3b: Profile (Side view)⁶
(Credit: By RJHall at English Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=52696960)

For several years, people thought that there is nothing outside the Milky way. But in the early 1920s, a scientist named Edwin Hubble could see other galaxies through a powerful telescope at Mount Wilson (USA). The neighboring galaxy, Andromeda, has a diameter of 2,20,000 light years, and it  is about 2.5 million light years from the Earth (Figure 4). This is the only galaxy we can see with our naked eyes, but it appears just as a medium size star.

Figure 4: The closest galaxy, Andromeda is about 2.5 million light years from the Earth
(Credit: By Adam Evans - M31, the Andromeda Galaxy (now with h-alpha) Uploaded by NotFromUtrecht, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=12654493)

So far scientists have observed Many galaxies. Galaxies have different shapes and sizes.

 To learn more about galaxies, watch the following video

and write you thoughts on the video in Notebook
(Click on icon to write)
 

How many galaxies are there?
(Video is taken from- https://www.spacetelescope.org/videos/heic1620a/)

⁵This is not a photo of the Milky Way. We cannot take a photo of the Milky Way because we cannot travel outside it. This is a combination of picture and map.
⁶All the directions are same in space; so there is no inherent top or side perspective to the Milky Way. For convenience, we use terms like top or side perspective as we would for a plate or a disk.