THE MILKY WAY GALAXY
What is Milky Way?
The Earth, The moon, the Sun and billions of other stars belong to the Milky Way Galaxy. The Milky Way galaxy exhibits a distinct barred spiral shape. The galaxy gets its name from the band of stars that appear as milky white light against the dark night sky.
The Milky Way galaxy houses hundreds of billions of stars and enormous amounts of dust and gas, held together by the extremely strong gravitational field of the central black hole. The Milky Way Galaxy is known to range over 100,000 light-years.
Our solar system is located approximately 25,000 light-years away from the galactic center. Just like our Earth revolves around the Sun, the entire solar system revolves around the galactic center of the Milky Way. Our solar system takes nearly 250 million years to complete one revolution around the center.
Structure of Milky Way
It has been established that the Milky Way is a barred spiral galaxy. But, the question is how we can determine the shape of Milky Way while residing inside it?
The image of the Milky Way that we see is based on several clues, estimations and theories.
- The first indication of the shape of the Milky Way comes from the bright white band of stars that expands across the sky. This band of stars suggests that they are arranged in the form of a flat disk, as found in the spiral galaxies.
- Based on the images taken by several telescopes, both on the ground and in space, astronomers concluded the spiral shape from numerous directions. The pattern of stars present in the band also supported the conclusion.
- The mapping of bright stars, dust, and clouds of ionized gases in the Milky Way’s disk also gave a clue about the galaxy’s spiral nature.
- Further evidence related to the spiral nature of the Milky Way comes from the measurement of the quantities of dust present in the band and the dominant colors of light that reaches us.
*** Over the years, scientists and researchers have debated the number of spiral arms of the Milky Way. Some theories suggest that there are two spiral arms, whereas other ideas point towards the possibility of four arms. The latest information found indicates that our galaxy has four arms.
Formation of Milky Way
The formation of the Milky Way began along with a number of other overdensities in the Universe a while after the occurrence of the Big Bang. Few of these overdensities are said to be the buds of several globular clusters in which the oldest residual stars in the Milky Way galaxy formed.
Around half the substance present in the Milky Way came from other distant galaxies through collisions. These globular clusters and stars now cover the stellar halo of the Milky Way galaxy. It took the Milky Way some billion years from the birth of the first stars to reach the mass that was sufficient to spin rapidly (compared to other galaxies). The conservation of angular momentum caused the large gaseous interstellar matter to collapse from an approximately spheroidal shape to a spiral disk. Therefore, the succeeding generations of stars (including our Sun) formed in the Milky Way resided in this spiral disk.
Contents of the Milky Way Galaxy
The Milky Way galaxy houses about 100-400 billion stars and almost that many planets (as the Milky Way contains minimum one planet per star). The exact number is difficult to count as the distant, dim, low-mass stars are barely visible. The central part of the Milky way comprises of a massive stellar black hole.
As the distance from Milky Way’s center decreases, the concentration of stars also decreases. The Galactic disk of dust and gases (that fills the space between the stars) is surrounded by a spherical Galactic Halo of globular clusters and stars.
What lies beyond?
The Universe houses billions of other galaxies. Out of all these, we can see only three other galaxies with our naked eye. These galaxies appear as fuzzy spots in the sky. There lies a dark matter halo surrounding the Milky Way that spans about 2 million light-years.
The Universe houses billions of planetary systems like ours. Our planetary system is established around the star ‘Sun’ formed about 4.5 billion years ago.
The solar system is situated in the outer region of the Milky Way’s spiral disk. We know about the eight planets that revolve around the Sun. But, the Solar system extends way beyond that. The Kuiper Belt that lies beyond Neptune’s orbit also falls under the solar system. This belt is a ring of icy bodies.
Past the Kuiper Belt is the Oort Cloud that forms a giant shell of icy space debris surrounding the solar system. The Oort Cloud has never been witnessed directly, and its existence is based on mathematical models and observations of comets, asteroid, etc. formed there. The Oort Cloud marks the periphery of the Sun’s gravitational influence.
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