How big is Whirlpool Galaxy? | Scale of the Universe

The Whirlpool Galaxy is a spiral galaxy about 28 million light-years from Earth, in the constellation Canes Venatici. Its disk is about 60,000 light-years across. A light-year is the distance light travels in one year, about 9.46 trillion kilometers. Earth sees the Whirlpool face-on, and a smaller galaxy, NGC 5195, sits at the tip of one of its spiral arms.

Size

A galaxy has no sharp edge, so its width depends on how faint a glow is counted. Measured out to a standard faint level of brightness, the Whirlpool's disk is about 7.7 arcminutes across on the sky, about a quarter of the width of the full Moon. An arcminute is one-sixtieth of a degree. At the galaxy's distance, that is about 60,000 light-years, the same figure NASA gives.

Catalogs do not all agree on where the faint edge lies. Another standard catalog of bright galaxies gives the Whirlpool a width of 11.2 arcminutes, which would make it about 90,000 light-years across.

The distance was measured in 2016 using the Hubble Space Telescope. The brightest red giant stars in a galaxy all reach about the same true brightness, so how bright they look reveals how far away they are. By this method, the Whirlpool is about 28 million light-years away. Earlier estimates ranged from about 16 million to 40 million light-years, and the same angle on the sky means a wider galaxy if it is farther away.

Size comparisons

The Whirlpool Galaxy is about three-fifths the width of the Milky Way's disk, which is about 100,000 light-years across.
It is about the same width as the Triangulum Galaxy, which is about 60,000 light-years across.
In one catalog that measures both galaxies the same way, its companion NGC 5195 is about two-fifths as wide as the Whirlpool.
If the Whirlpool Galaxy were shrunk to the size of a basketball, Earth would be about 110 meters (370 feet) away.

A galaxy and its companion

The Whirlpool is a grand-design spiral, a galaxy with prominent, well-formed spiral arms. Some astronomers think its arms stand out so clearly because of a close encounter with NGC 5195. The smaller galaxy's gravity tugs on the arm it sits beside, and that pull sets off the birth of new stars.

Images from Hubble show that NGC 5195 is passing behind the Whirlpool, as seen from Earth. It has been gliding past the larger galaxy for hundreds of millions of years.

Where stars are born

The Whirlpool's arms are long lanes of stars and gas laced with dust. They squeeze hydrogen gas together, forming clusters of new stars. Infrared images from Hubble found hundreds of small clumps of stars hidden in the dust, each about 65 light-years wide.

In 2023, the James Webb Space Telescope showed dark bubbles in the Whirlpool's gas, carved by energy pouring out of its stars. The images are part of a survey of how young star clusters emerge from the clouds of gas and dust that formed them.

Seeing the Whirlpool

Charles Messier discovered the Whirlpool in 1773, and it is also known as Messier 51, or M51. It is one of the brightest galaxies in the sky, and it can be seen with a good pair of binoculars.