In the books of science and astronomy in schools, public books, articles on the internet, to scientific journals write that the Milky Way galaxy has a spiral shape. But, where do we know our galaxy form when we have never seen it from outside?
In fact, there are some clues that will enable us to easily know the shape of our galaxy, without having to assemble a spacecraft at great cost to glide out of the galaxy in order to simply know its shape.
First, we simply look toward the center of the galaxy with our eyes. In certain months, the Milky Way galaxy stretches across the sky, we can see this expanse of the Milky Way like clouds and long, thin dust.
In galaxy morphology, the major forms of galaxies in the universe are divided into three; spiral galaxies, elliptical galaxies, and irregular galaxies (shapeless).
This long and thin shape of the Milky Way stretch shows that we are looking at the Milky Way from its rim. Since there is no elliptical form that we see in the night sky, then from this alone we can declare that our galaxy is spiral-shaped.
Another evidence is that astronomers have now discovered a bulge in the central region of the Milky Way galaxy. In other spiral galaxies in the universe, astronomers also see this bulge. Therefore, the Milky Way means to have the same shape with other spiral galaxies with its bulge.
Secondly, when astronomers measure the velocity of stars and gas in our galaxy, they find that the rotation of the star's overall motion is different and has a random motion. This is one of the characteristics of a spiral galaxy type.
Thirdly, we can know the Milky Way galaxy in the form of a spiral of material possessed. The gas, color, and dust material fractions of our galaxy turn out to resemble other spiral galaxies in the universe, especially our neighboring spiral galaxy, Andromeda.
So, overall, the Milky Way galaxy is a pretty convincing argument with some of the evidence above. Even so, so far we still do not have the image of the Milky Way in its entirety, so that there is only supercomputer imagery fantasy from the results of star motion research.
ref : https://spacenesia.blogspot.co.id/2018/01/bimasakti-berbentuk-spiral.html