We are inside the disc of this galaxy, so we cannot see it from outside. But we can see different parts of the disk during different months of a year.

In this video, you see an animation of how Milky Way looks like from Dhaka during different months. Historically, only this shadowy band has been called the Milky Way, in Bangla আকাশগঙ্গা or ছায়াপথ, but the whole galaxy is also called the Milky Way now. Sometimes ‘Galaxy’ with an uppercase ‘G’ also refers to our galaxy, the Milky Way.

How did we first realize that we are not at the center of our galaxy? In 1917, Harlow Shapley located a lot of globular clusters (tightly bound clusters of thousands of stars). And he saw that the globular clusters are distributed in a halo around a point which is almost 25,000 light years away from us. You can see the positions of these clusters in the side view below.

The Milky Way has three main parts: a disk, a bulge and a halo. The disk is almost 100 kly (kilo/thousand light years) in diameter. The bulge is located at the center of the disk and the spherical halo surrounds the whole disk. The bulge is a little bit elongated like a flattened football and, hence, the Milky Way is a barred spiral galaxy. Spiral because the Galaxy has multiple spiral arms in the disk that spiral toward the Galactic Center (GC). The halo contains the globular clusters.

Gravity would make the whole Galaxy collapse onto itself if not for the rotation of the stars and gas around the Galactic Center. Rotation opposes gravity and makes the galaxy flat similar to the solar system. The sun and its neighbors orbit the Galactic Center at a speed of 220 km per second. At this speed it would take 220 million years for the Sun to complete one orbit around the center of the Milky Way.

The Sun is 26 kly away from the center, so it revolves around the center comparatively slowly. If you go closer to the center, your orbital speed will increase. As you go nearer, both the number of stars per cubic light-year and their average orbital speed increase. In the neighborhood of the Sun on average there are only 3 stars in 1000 cubic light-years, but near the core of our galaxy there are 10 million stars within just 1 cubic light-year.

Nebula is a cloud of gas and dust bright enough to emit radiation. But their mechanism of radiation is very different from the radiation of a star. Let us talk about the Orion Nebula and try to understand what exactly we are seeing in the picture.

The position of the Orion Nebula is at lower part of the Orion Constellation in this picture taken using an ordinary camera.

Here is the image of the Orion Nebula taken by our telescope, Ashvin II.

And here is the Orion Nebula through the eye of the great Hubble Space Telescope. Why is it so red? Yes, the colors are not real, and this is actually a composite image that combines visible-light and infrared radiations. But, it is still true that many nebulas look red. Why?

Moons shine by reflecting the light of their parent stars. But nebulas shine by not reflecting but absorbing the light of their native and neighboring stars. Let me make the point more clear.

Massive stars emit ultraviolet radiation and the atoms in a nebula absorb that light. Let us see what happens to a hydrogen atom after such an absorption.

Simplistically speaking, a hydrogen atom has a proton at the center and an electron around the proton. The electron has some specific allowed energy levels where it can stay. After absorbing a packet of ultraviolet light, it can jump from the 1st level ($n=1$) to the 3rd energy level ($n=3$). After a while, this electron can jump back to either the 2nd level ($n=2$) or its original home, the 1st level.

If the electron jumps to the second level, it emits a packet of light called the H-alpha photon and the wavelength of this light makes it red. Obviously then, inside a nebula there are a lot of 1-3-2 transitions go on giving rise to red H-alpha light. Do not ask me why it is called H-alpha.

Nebulas that emit light via transitions of electrons are called emission nebula. They are pretty bright. Orion Nebula can be seen with naked eye as well if you go to a very dark location. Emission nebula can be red or green. Red light comes from hydrogen atoms and green light from oxygen atoms.

But what’s up with the Horsehead Nebula? Why is there a dark region shaped liked a horsehead there?

Because a region shaped like a horsehead is obstructing starlight from reaching us. Instead of emitting its own light the atoms in this region are absorbing the light coming from the stars behind it. So the horsehead nebula has both emission and absorption features. The lower part only absorbs and obstructs. The horsehead is protruding out of that dark lower part. This lower part is an absorption nebula.

The stars that emit the ultraviolet light needed for the nebula to shine are also clearly seen in this picture taken by amateur instruments.

There are reflection nebulas as well that reflect light instead of emitting or absorbing, almost like the moon. These nebulas look blue because the starlight they reflect is blue and also dust is very efficient at reflecting blue light.