Our Milky Way galaxy

Many questions arise in our mind related to astronomy but out of many questions one question will definitely arise viz. how the structure of universe is. At present time, our assumption is that the collection of crore stars systematically arrange among themselves and this system is called Galaxy.

• On the basis of Observation and mathematical calculation we got that The Sun completes one revolution around the center of our Milky Way galaxy in 2000 lakh year’s and also our Milky Way galaxy is in motion itself. The Shape of Our Milky Way galaxy is almost flat or disk’s shape.
• It has approximately 2*(10^11) stars and other objects like interstellar gas, stellar remnants and dark matter.
• At Night, we can see a wide white bar in the clean sky. The picture below seems to be a winter month in northern hemisphere. In reality, crore stars lives in this bar which is very close to each other. That is the reason why we cannot see them separately. This bar is a part of our own Milky Way galaxy. 

• Length of our Milky Way galaxy is about 1 lakh light years which means that if we travel with speed of light from one end to other end, then time taken will be 1 lakh years. 
• Its radius will be approximately 15000 pc.
• Distance between center of Milky Way galaxy and our solar system is 8.5kpc.
• Estimates mass of our galaxy is 2*(10^11) solar mass.
• In our Milky Way galaxy,  we estimate that there are 3 large and 30 small galaxies present. 

 

To study the Milky Way galaxy, generally we divide our Milky Way galaxy into 3 parts.

 1. A Central Bulge: A Central Bulge of Milky Way galaxy is the a spherical clouds of stars. We cannot see the Central Bulge because it is in the center and it is surrounded by gases or dust which absorb the visible light.  The mass of A Central Bulge is estimated to be10^10 Solar mass. We suppose that the Central of Central Bulge or central part is massive Black Hole.

 2. The Flattened galactic disk: Radius of Disk is about 15000 pc. But it has much less thickness. As we move far from the plane of disk, the density of stars will decrease. 

• Mostly Stars lies in the plane of disk.
• Disks have spiral arms.
• Birth place of mostly new stars and mostly stars clusters are observed in these arms. 
• Our Solar system is located on a Orion arm. 
• The center of the flattened galactic disk component is Galactic Center.

3.  Halo: The Central bulge and disk component are surrounded by Halo component. We don’t have much knowledge about halo component. as estimated that old stars and gases matter are present. And these stars are mostly in the form of denser clusters.

• We believe that mass of halo component is equal amount of mass of present in the disk.
• Approximately 10^5 or 10^6 stars can present in each clusters. These clusters are called globular clusters. We cannot see separately these stars because density of these stars is much high in these clusters. These clusters  appear in the form of circular spherical light.
• The nature of galactic rotation suggests that there must be many times more matter than what is visible in it. But we cannot see theses matter using visible wavelength. These matters are called dark matter.

The stars of Milky Way galaxy are a part of gravitational system. The rotational motion of Milky Way galaxy is very slow as compare to that of objects in our solar system. For example, our Sun revolves around the center of Milky Way galaxy and completed one rotation in 2000 lakh years. Rotational motion of Milky Way galaxy gives more stability. It is because centrifugal force is balancing the gravitational force.

• A hypothesis of origin of Milky Way galaxy is that approximately in 10 billion years, spherical distribution of matter in our galaxy converts into a flat galaxy.

 

Spiral Arms: The Spiral arms are also bright in infrared due to the presence of dust. The spiral arms appear bright in visible light from newly formed, massive stars.

The Prominent arms of spiral arms:

1. Norma arms,
2. Scutum-crux arm,
3. Sagittarius arm,
4. Orion arm(Our Sun’s location),
5. Perseus arm and
6. Cygnus arms.

Spiral arms are not present from the origin of the milky way galaxy. Formation of spiral arms due to dynamical rotation of galaxy or dynamical interaction of two galaxies.

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