The Cartwheel Galaxy is a ring-shaped galaxy in the constellation Sculptor, roughly 400 to 500 million light-years away. A light-year is the distance light travels in one year, about 9.46 trillion kilometers. The galaxy's outer ring of young stars is about 140,000 to 150,000 light-years across at its widest, roughly one and a half times the width of the Milky Way's disk. The ring formed after a smaller galaxy crashed through the Cartwheel.

Size

The Cartwheel has two rings: a bright inner ring around its core, and a colorful outer ring about four times as wide. The outer ring is not a perfect circle. A 2020 study describes it as an oval about 150,000 light-years long and 110,000 light-years wide, with its center about 21,000 light-years to one side of the galaxy's core.

Seen from Earth, the outer ring is only about 72 arcseconds across, about one twenty-sixth the width of the full Moon. An arcsecond is 1/3,600 of a degree. Turning that angle into a true size needs the galaxy's distance, which astronomers work out from how fast the expanding universe is carrying it away from Earth. Studies in 2020 and 2023, using slightly different distances, put the ring at about 150,000 and 144,000 light-years across at its widest, respectively.

Size comparisons

The outer ring is about 1.5 times as wide as the disk of the Milky Way, which is about 100,000 light-years across.
The Pinwheel Galaxy, one of the next larger objects in the explorer, is about 1.2 times as wide as the Cartwheel's outer ring.
If the outer ring were shrunk to the width of a basketball, 24 centimeters (9.4 inches), the Milky Way's disk would be about 16 centimeters (6.3 inches) across.
At that scale, the inner ring would be about 6 centimeters (2.3 inches) across.

How the ring formed

The Cartwheel was probably once a normal spiral galaxy like the Milky Way. Then a smaller galaxy plunged through it at high speed. The collision sent two rings spreading outward from the center, like ripples on a pond after a stone is tossed in. Ring galaxies like this are less common than spiral galaxies.

The outer ring has been expanding for about 440 million years. As it moves, it plows into the gas around it and sets off the birth of new stars. A 2020 study found a bar of stars in the Cartwheel's center that survived the collision, a sign that the galaxy had that bar before the crash.

Stars and X-rays in the ring

A 2024 study found that the outer ring is made mostly of stars that formed within the last 400 million years. It holds millions of bright young stars. The most massive of them explode as supernovae, explosions that mark the death of a massive star, and leave behind black holes and neutron stars, the collapsed cores that remain. Some of these pull matter from a companion star and shine brightly in X-rays.

The Cartwheel has an unusually large number of these X-ray sources, because so many massive stars formed in its ring. Infrared images from the James Webb Space Telescope also show regions rich in silicate dust, like much of the dust on Earth, and in hydrocarbons, which are compounds of hydrogen and carbon. These regions form spokes between the rings.

Sources

Webb Captures Stellar Gymnastics in the Cartwheel Galaxy. NASA, 2022.
Cartwheel Galaxy: Astronomers Do Flips Over Cartwheel Galaxy. NASA Chandra X-ray Center, 2006.
Discovery of a near-infrared bar and a pseudobulge in the collisional ring galaxy Cartwheel. Barway et al., Monthly Notices of the Royal Astronomical Society, 2020.
Star formation history of the post-collisional Cartwheel galaxy using Astrosat/UVIT FUV images. Mayya et al., Monthly Notices of the Royal Astronomical Society, 2023.
Understanding the unusual life of the Cartwheel galaxy using stellar populations. Ditrani et al., Astronomy & Astrophysics, 2024.
Our Milky Way Galaxy: How Big is Space?. NASA, 2019.
Third Reference Catalogue of Bright Galaxies (RC3). de Vaucouleurs et al., Springer, 1991, via CDS VizieR.
The Carnegie-Chicago Hubble Program. VII. The Distance to M101 via the Optical Tip of the Red Giant Branch Method. Beaton et al., The Astrophysical Journal, 2019.
Moon Fact Sheet. NASA, 2024.
Measuring the Universe. International Astronomical Union.
Official Basketball Rules 2026: Basketball Equipment. FIBA, 2026.

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