What is a galaxy? How many stars in a galaxy? How many stars/galaxies in the Universe?

What is a galaxy?

NGC 3603, a giant HII region in the Milky WayNGC 3603, a giant HII region in the Milky Way. Ultraviolet light from the young star cluster in the centre of the nebula is heating and ionising the surrounding hydrogen gas. © European Southern Observatory, Very Large TelescopeGalaxies are large systems of stars, dust and gas bound together by gravitational force. They are elliptical, spiral or irregular in shape. Our Sun is part of a spiral galaxy called the Milky Way, and lies about two-thirds of the way out from the centre to the edge.

How many stars in a galaxy?

The best estimates suggest that the Milky Way contains about 500 thousand million stars and a total mass equivalent to 1.9 million million Suns.

Galaxies vary in size considerably. For example, giant elliptical galaxies may have up to 100 times the mass of the Milky Way (or the equivalent of 1900 million million Suns) whereas the recently discovered Ultra-Compact Dwarf galaxies may have just a few tens of millions of stars.

How many stars and galaxies in the universe?

The best estimates suggest that there are at least 70 thousand million million million (70 sextillion or 7 × 1022) stars in the Universe. The Universe probably contains more than 100 thousand million (100 billion or 1011) galaxies.

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