How big is Pinwheel Galaxy? | Scale of the Universe

The Pinwheel Galaxy, also called Messier 101 or M101, is a large spiral galaxy in the constellation Ursa Major, the Great Bear. Its disk of stars, dust and gas is about 170,000 to 190,000 light-years across, where a light-year is the distance light travels in one year, about 9.46 trillion kilometers. The galaxy is about 21 million light-years from Earth.

Size

M101 faces Earth almost head-on, so its whole disk and all its spiral arms are in view. NASA gives its width as 170,000 light-years, from a detailed portrait by the Hubble Space Telescope released in 2006.

A galaxy has no sharp edge, so astronomers often measure its width out to where its glow fades to a set level of faintness. At that level, the Third Reference Catalogue of Bright Galaxies gives M101 a width of about 29 arcminutes, nearly the width of the full Moon. An arcminute is a sixtieth of a degree. Turning that angle into a width depends on the galaxy's distance.

A 2019 study measured the brightest old red giant stars in M101's outskirts and put the galaxy 6.52 million parsecs away, where a parsec is about 3.26 light-years. That is about 21 million light-years, which makes the catalog width about 180,000 light-years. Studies of pulsating stars called Cepheids give a slightly larger distance, about 22 million light-years, and a width near 190,000. NASA's pages use an older distance of 25 million light-years.

Size comparisons

M101 is nearly twice as wide as the disk of the Milky Way, which is about 100,000 light-years across.
If the Milky Way's disk were shrunk to the width of a basketball, 24 centimeters (9.4 inches), M101 would be about 43 centimeters (17 inches) across and about 51 meters (170 feet) away.
M101 is about 1.3 times as wide as Virgo A, another giant galaxy in the explorer, when both are measured to the same faintness.
Measured the same way, M101 is also about 1.3 times as wide as the bright disk of the Andromeda Galaxy, the nearest large galaxy to the Milky Way.

Stars and star birth

Astronomers estimate that M101 contains at least one trillion stars. About 100 billion of them could be like the Sun in temperature and lifetime. The spiral arms are dotted with large nebulae, clouds of gas where new stars are forming. Clusters of hot, young, blue stars trace out the arms.

The disk is so thin that Hubble can see more distant galaxies through it. Pierre Méchain, a colleague of the astronomer Charles Messier, discovered M101 in 1781. It can be seen through a small telescope.

Supernovae in the Pinwheel

Astronomers have recorded several supernovae, explosions of stars, in M101. One of them, SN 2011fe, was the nearest recent Type Ia supernova as of 2024. That kind of supernova reaches a known peak brightness, so astronomers use it to measure distances to faraway galaxies and how fast the universe is expanding.

Koichi Itagaki discovered another, SN 2023ixf, on May 19, 2023. It was a Type II supernova, the explosion of a massive star, and models fit a star about 15 times the mass of the Sun. Because M101 is photographed so often, amateur and professional astronomers later found the supernova in many pictures taken before it was reported.