Galaxies are massive systems of stars, gas, dust, and dark matter bound together by gravity. There are an estimated 100 billion to 2 trillion galaxies in the observable universe, which range in size from dwarf galaxies with a few thousand stars to giants containing trillions.Astronomers generally classify all galaxies into three main shapes:Spiral Galaxies: Giant, rotating pinwheels like our Milky Way. They feature bright, winding arms of gas and dust where active star formation occurs.Elliptical Galaxies: Nearly spherical to egg-shaped galaxies composed mostly of older stars. They contain very little gas and dust, meaning star formation has largely stopped.Irregular Galaxies: Shapeless, chaotic galaxies lacking ordered structure. They are typically smaller but rich in bright, young stars.Galaxies do not sit in isolation; they are gravitationally grouped into clusters and vast superclusters. For example, the Milky Way belongs to the Local Group, which contains about 51 galaxies.Astrophysicists estimate there are about 2 trillion galaxies in the observable universe. Because light has a finite speed, we can only see a portion of the universe; the true number across the entire, possibly infinite universe is unknown.