The Milky way galaxy

 The place where we stay: Milky way

Whenever 'galaxy' as a topic is spoken of, Milky Way is the first thing that comes to everyone's mind. As you are standing under a completely dark, starry sky, away from light pollution, the Milky Way appears like a cloud across the cosmos. This cloud actually resolves into millions of stars, whose distance and close proximity to each other do not permit us to pick them out individually with just our eyes. Milky Way is the galaxy that includes our solar system, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky. From Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within. Milky Way is a barred spiral galaxy with an estimated visible diameter of 100,000-200,000 light-years. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610, most astronomers thought that the Milky Way contained all the stars in the Universe. The Milky Way has several satellite galaxies and is a part of the local group of galaxies, which form the part of the Virgo Supercluster, which in turn is a part of the Laniakea Supercluster. The Milky way is the second-largest in the local group (after the Andromeda Galaxy). It is estimated to contain 100-400 billion stars and at least that number of planets (it contains at least one planet per star, resulting in 100-400 billion planets, according to a January 2013 study of the five-planet star system Kepler-32 by the Kepler Space observatory). A different January 2013 study analysis of Kepler data estimated that at least 17 billion Earth-sized exoplanets reside in the Milky Way. 11 billion of them may be orbiting sun-like stars. The nearest exoplanet maybe 4.2 light-years away, orbiting the red dwarf Proxima Centauri, according to a 2016 study. Such Earth-sized planets might be more numerous than gas giants.

It may also contain ten billion white dwarfs, a billion neutron stars, and a hundred million stellar black holes. Filling the space between the stars is a disk of gas and dust called the interstellar medium. From our calculations, the Solar system is located at a radius of about 27,000 light-years from the Galactic center, on the inner edge of one of the spiral-shaped arms called the Orion Arm. The stars in the innermost 10,000 light-years form a bulge and one or more bars that radiate from that bulge. The galactic center is an intense radio source known as Sagittarius A, a supermassive black hole. Stars and gases at a wide range of distances from the Galactic center orbit at approximately 220 kilometers per second. The Milky Way as a whole is moving at a velocity of approximately 600 km per second with respect to the extragalactic frames of reference. The oldest stars in the Milky Way are nearly as old as the Universe itself and thus probably formed shortly after the Dark ages of the Big Bang.

The galaxy's two major arms (Scutum-Centaurus and Perseus) can be seen attached to the ends of a thick central bar, while the two now-demoted minor arms (Norma and Sagittarius) are less distinct and located between the major arms. The major arm consists of the highest densities of both young and old stars; the minor arms are primarily filled with gas and pockets of star-forming activity. When we look to the edge, we see a spiral arm of the Milky Way known as the Orion-Cygnus Arm. Our sun lies near a small, partial arm called the Orion Arm, or Orion Spur, located between the Sagittarius and Perseus arms. If we look in the other direction, one would naturally expect to be able to see the center of the galaxy, which is located in the constellation of Sagittarius. But unfortunately, we cannot see it. The galactic center is hidden from us behind vast clouds of dark gas that telescopes operating in visible light cannot see through. 

Our Milky Way galaxy is one of the billions in the universe. We don't know exactly how many galaxies exist, a modern estimate says that there are almost 2 trillion of them. The Milky Way is assumed to be formed close to the timeline of the Big Bang. The unaided eye only see three galaxies: Andromeda galaxy and Magellanic Clouds (two amorphous dwarf galaxies orbiting our own).