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Galaxies, explained

Some galaxies are similar to the Milky Way, but some are quite different.

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A composite image of the Messier 81 (M81) galaxy shows what astronomers call a "grand design" spiral galaxy, where each of its arms curls all the way down into its center. Located about 12 million light-years away in the Ursa Major constellation, M81 is among the brightest of the galaxies visible by telescope from Earth.

Galaxies are sprawling systems of dust, gas, dark matter, and anywhere from a million to a trillion stars that are held together by gravity. Nearly all large galaxies are thought to also contain supermassive black holes at their centers. In our own galaxy, the Milky Way, the sun is just one of about 100 to 400 billion starsthat spin around Sagittarius A*, a supermassive black hole that contains as much mass as four million suns.

The deeper we look into the cosmos, the more galaxies we see. One 2016 study estimated that the observable universe contains two trillion—or two million million—galaxies. Some of those distant systems are similar to our own Milky Way galaxy, while others are quite different.

Types of galaxies

Before the 20th century, we didn't know that galaxies other than the Milky Way existed; earlier astronomers had classified them as as “nebulae,” since they looked like fuzzy clouds. But in the 1920s, astronomer Edwin Hubble showed that the Andromeda “nebula” was a galaxy in its own right. Since it is so far from us, it takes light from Andromeda more than 2.5 million years to bridge the gap. Despite the immense distance, Andromeda is the closest large galaxy to our Milky Way, and it's bright enough in the night sky that it's visible to the naked eye in the Northern Hemisphere.

In 1936, Hubble debuted a way to classify galaxies, grouping them into four main types: spiral galaxies, lenticular galaxies, elliptical galaxies, and irregular galaxies.

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