How did we find the shape of our Milky Way Galaxy? - VastlyCurious

     How can we take the picture of our Milkyway galaxy if we are present inside it, do you really think we can take go out of the Milkyway galaxy and take the whole picture of the galaxy in a single frame? Capturing the photo of our own galaxy would require us to send a space probe to 3 lakh trillion kilometres, which is not possible and will not be feasible in the near future. So yes! The images we saw of the Milkyway galaxy were not the actual photograph of the Milkway whereas those are the representations of our galaxy. So how did we find the shape and arrangement of our galaxy?

The Illustration of the spiral structure of the Milky Way

Herschel's map

    The British scientist, Herschel was the first person to observe the band of light running across the sky and started observing them. He split the observable space into 600 different zones and started mapping the star in different zones. He want to map out the galaxy and he did find the structure of the Milkyway as Spiral which was later found out to be true, he did that by assuming that if the stars are dimmer, they are further away and if the star is luminescent, then it must be closer to our planet. Based on this assumption, we found that the structure of our galaxy is a spiral and based on Herschel's map it was believed that we were in the center of the galaxy. 

The View of the Milky way from Earth

    But the thing he didn't know was that our galaxy is mainly composed of stardust and those dust will block the view of the stars in the galaxy. So in order to not allow the dust to block our view of the galaxy, modern astronomers used infrared telescopes to view the sky and there were able to piece together the structure of the galaxy. Now with the help of the Hubble space telescope, we can observe the universe and now the latest telescope, which is to be launched into space, the James Webb Telescope uses Infrared cameras and sensors to picture the galaxy.

The Structure of the Milky Way Galaxy

    We have taken photographs of our neighbouring galaxies, but finding out the true shape of our own galaxy is difficult. Taking the photograph of our galaxy is not possible as the width of our galaxy is approximately 1 lakh light-years. The highest distance covered by a spacecraft which is Voyager one is 21.2 billion kilometres (13.7 billion miles). Even if the Voyager one continues its journey at the same speed it would take about 562 million years to escape our galaxy and take a picture of it. It is not actually possible to take the photograph of the Milkway and so how did we find the shape and structure of the galaxy?

    Astronomers and scientists have predicted the shape of our galaxy by observing the shape of the neighbouring galaxies. There are multiple shaped galaxies found in the universe and the major ones are Elliptical Galaxies, Spiral Galaxies and Irregular Galaxies. Scientists also have observed our sky and found a large disc running across the cosmos. The band across the sky is the Milkyway galaxy itself and since the band appears to be flat, we might be present in the thin side of the galaxy. The galaxy encircles our planet, so our solar system must be present in the galaxy and not on the outside edge of the Milkyway.

Mapping the Spiral Structure of the Milky Way. Source: NASA

        By observing several stars in our galaxy and calculating the distance between the sun and the other stars in the galaxy, Scientists were able to map a 3d model of the Milky Way galaxy. Our Milky Way galaxy is a spiral galaxy with a rod-shaped core enclosed by Stars and specks of stardust. Our galaxy is named as the Milky Way galaxy because of it appears as an milky band of light in the visible sky. Our Galaxy stretches to about a diameter of 1,00,000 light years and our solar system is 25,000 light years from the center of our galaxy.

        The Milky Way galaxy has five spiral arms around the center: Nomra Arm, Scutum-Centaurus Arm, Sagittarius Arm, Perseus Arm and Outer Arm. Although most of the space in our galaxy is either empty or filled with dust clouds. Astronomers have recently found that typically a supermassive black hole will be present at the center of the galaxies and so there will a supermassive black hole at the center of the band of the Milky Way, but the black hole cannot be viewed. It is estimated that there are around 10 million stars at the bar core which spans to a diameter 1 light year and that is why the bar core appears luminescent in the night sky. Since most of the space in the Milky Way is just empty, when the Andromeda galaxy collides with the Milky Way galaxy in the next billion years, it won't cause any destruction of stars. Astronomers predict that this collision could happen in the next billion years and we might witness the collision for several billions of years.

Artistic Rendering of the collision between the Milky Way and the Andromeda Galaxy

    Scutum-Centaurus Arm and Perseus Arm are the dominant arms that arise from the core. The Nomra Arm, Sagittarius Arm and Outer Arm are placed between the two dominant arms. Orion spur is a mini branching of the Sagittarius Arm which is 10,000-20,000 light years wide and our solar system is present in the center of the Orion spur. Our Solar system is titled at an 60 degree angle with respect to the Milky Way and so we get tilted view of the arms. The Arms appear more luminescent not because of the presence of more stars in the arm, but because of the presence of nebulas in the arms. The Arms act as a nursery for the birth of new stars in the galaxy.