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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.
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.
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.
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.
The first aspect of the Fermi paradox is a function of the scale or the large numbers involved: there are an estimated 200–400 billion stars in the Milky Way (2–4 × 10 11) and 70 sextillion (7×10 22) in the observable universe.
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 galaxies in the Local Group, the cluster of galaxies which includes the Milky Way and the Andromeda Galaxy, are gravitationally bound to each other. It is expected that between 10 11 (100 billion) and 10 12 (1 trillion) years from now, their orbits will decay and the entire Local Group will merge into one large galaxy.
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.