How does our Milky Way galaxy get its spiral form? 3 important parts of every galaxy

How does our Milky Way galaxy get its spiral form? What gives the Milky Way its shape? In fact, How is a spiral galaxy formed? Why is the galaxy spiral? What are the three or four main parts of a spiral galaxy? To know these answers keep reading…

In the universe, about 90% of galaxies including the Milky way have a spiral structure with two or more disks rotating with respect to their central bulge. Though we know that the milky way is a spiral galaxy but, how was it formed?

How does our Milky Way galaxy get its spiral form?
source: rawpixel.com

To know How does our Milky Way galaxy get its spiral form we will discuss it in 4 points :

1. Stellar population 

2. Stellar motions

3. Differential rotation

4. Winding Process


1. Stellar Populations:

If we draw a schematic drawing of milky way we will get three parts:

three parts of a galaxy (spiral galaxy)

1) Central bulge (nucleus):

It can be called the nucleus or the brain of the Milky way contains a mixture of the population I and population II stars. If you did not know, population I stars are stars with a high abundance of heavy elements and population II stars are with a low abundance of heavy elements.

2) Disk:

These are the arms of the Milky Way galaxy. In one arm named Orian, there is our Sun IN solar system at about 30,000 light-years distance from the galactical center but What process creates and maintains the beautiful spiral arms around spiral galaxies? and why this is called a disk to know the reason you have to look at the stellar motion. These arms rotate very fast and this speed makes it flattened like our ceiling fan however this disk section contains a population I star.

3) Halo: 

Halo is the best part of the Milky Way galaxy bounding it; it contains population II stars spreading randomly.

It’s believed that the halo may contain a very large and very massive invisible component (Scientists believe almost 10 times the mass we have till observed). These invisible components are called dark matter, anti-matter, etc. And this leads to the missing mass in the universe, an unresolved problem.


2. stellar motions:

we all know everything is moving in our universe including the universe itself. Virtually if we see anything like a star moving randomly, that is wrong, it is obvious that it is the part of any large scale elements in the universe rotating with respect to any center. However, our Milky Way is a bound stellar system, where it has to balance the gravitational force with centrifugal force (because the stars are in a rotation motion with respect to the center of the galaxy). There are also random motions of these stars which till now is not understandable. However, here are 3 important questions answered.

1-Why is the disk flattened?

The stars in the disk are in more rotating motion than moving randomly, so due to the higher rotation speeds of the stars, as compared to the small random motions, the disk becomes flattened.

2-Why is the central bulge spherical?

the stars in the central bulge prefer to move randomly. Their rotational motion is not so impactable. Due to this random motion, the stars form the central bulge spherical.

3-Why is Halo just a distribution and does not have any shape?

In the galactic halo, the stars have very high random velocities and they are less tightly bound together with gravitational force as compared to the central bulge and disk. This is why the disk is nothing but a roughly spherical distribution.

However, From the flattened shape of the disk, we have a clue that stars and interstellar gas clouds in the disk rotate about the center of the galaxy. The random motion of the stars is superimposed by the strong circular motion. However, we can not know if the rotation motion is uniform or non-uniform.


3. Differential rotation:

We know that in our solar system all planets are rotating in Keplar’s law of motion and with the same principle the moons are moving their respective planets. According to this law Venus, the closest planet moves at the fastest speed, and similarly due to the same reason earth rotates faster than Mars does. But, in the case of a galaxy, especially Milky Way this does not happen. It has different parts with different types of matter. Missing mass, dark matter, etc. problems lead it to have a fluctuating Center of Mass. Previously I said that there’s no clue to know whether the motions of a galaxy’s elements are uniform or not. The motion is more complicated called differential rotation, i.e. does not rotate like a solid body. In simple terms, the motion of different parts of the galaxy is different. Our sun being part of the disk also rotates around the center of the Milky Way. However, the mean motion of the material in the neighborhood of the sun is defined as the local standard of rest (LSR). Though the sun has a relative motion with respect to LSR, it’s negligible due to being a comparatively small star. However, the large stars with high-velocity move in a random manner, and their motion is considerable.


4. Winding process in Milky Way:

winding process in a galaxy

In the spiral arms of the Milky Way, there are young stars looking like beads in a string. The beads are actually HII regions with embedded OB stars which cause the HII regions to shine. Wait there is confusion- 

If the lifetime of OB stars is about a few million years and the life of the galaxy is about 10 billion years, then, how and why are the OB stars only found in the spiral arms?

How is this possible? This is possible only when the process of formation of OB stars is going on regularly. When the new stars form, old ones disappear and produce dark matter.

But, why are new stars born in a grand spiral pattern?

where the arms can be very narrow and very long (an HII region can be 100 ly from one end to another). What is the mechanism?

To understand this we have to bookmark differential rotation again, according to which normally the inner layers in the Milky Way rotate faster than outer ones.

let, at time t1, the newborn stars are in AB line. Suppose, at time t2, when the inner layer completes the one-half circle, the line forms the trailing spiral arc. By the time t3, when the innermost layer completes one full circle, the outermost layer completes only a fraction of a circle.

This is how the spiral structure of the Milky Way forms. The time period of rotation of the innermost layer is generally about 10^8 years and the age of a galaxy typically is about 10^10 years. So, after calculating we can say an innermost layer completes about 100 rounds. But, in most cases, we don’t have such a large number of turns. Then density-wave theory takes place of winding process theory.

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