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By comparison, the total mass of all the stars in the Milky Way is estimated to be between 4.6 × 10 10 M ☉ and 6.43 × 10 10 M ☉.
Galaxies are categorized according to their visual morphology as elliptical, [5] spiral, or irregular. [6] The Milky Way is an example of a spiral galaxy. It is estimated that there are between 200 billion [7] ( 2 × 1011) to 2 trillion [8] galaxies in the observable universe.
By 1781 the final published list grows to 103 objects, 34 of which turn out to be galaxies. 1785 — William Herschel carried the first attempt to describe the shape of the Milky Way and the position of the Sun in it by carefully counting the number of stars in different regions of the sky.
While the Andromeda Galaxy contains about 1 trillion (10 12) stars and the Milky Way contains about 300 billion (3 × 10 11 ), the chance of even two stars colliding is negligible because of the huge distances between the stars.
In November 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of sun-like stars and red dwarf stars within the Milky Way Galaxy. 11 billion of these estimated planets may be orbiting sun-like stars.
The rate of star formation in the Milky Way is much higher, with the Andromeda Galaxy producing only about one solar mass per year compared to 3–5 solar masses for the Milky Way. The rate of novae in the Milky Way is also double that of the Andromeda Galaxy.
In November 2013, it was estimated that 22±8% of Sun-like stars in the Milky Way galaxy may have an Earth-sized planet in the habitable zone. Assuming 200 billion stars in the Milky Way, that would be 11 billion potentially habitable Earths, rising to 40 billion if red dwarfs are included.
Ceers-2112 formed soon after the big bang created the universe (which is estimated to be 13.8 billion years old), and the galaxy’s distinct structure was already in place 2.1 billion years later.
The word galaxy is derived from the Greek galaxias (γαλαξίας), literally "milky", a reference to the Milky Way. Galaxies range in size from dwarfs with just a few billion (109) stars to giants with one hundred trillion (1014) stars, each orbiting its galaxy's center of mass.
Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. The observable universe contains an estimated 1022 to 1024 stars. Only about 4,000 of these stars are visible to the naked eye—all within the Milky Way galaxy.