The Canis Major Dwarf Galaxy is a crowded patch of stars toward the constellation Canis Major, about 23,000 light-years from the Sun. A light-year is the distance light travels in one year, about 9.46 trillion kilometers. Astronomers announced it in 2003 as a small galaxy being torn apart by the Milky Way, but later studies suggest it is part of the Milky Way's own disk. The size shown above, 1 zettameter or about 106,000 light-years, is not a measurement of it: the measured crowd of stars is about 14,000 light-years deep.

Size

The Canis Major overdensity, as astronomers also call it, has no clear edge. It is a region where stars are more crowded than in the matching part of the sky on the other side of the Milky Way's disk. At its core there are up to eight times as many stars as in that matching region.

The team that found it measured its shape using red giant stars. They measured the region where stars are at least half as crowded as at its center. By that measure it is about 5,200 light-years thick, at right angles to the Milky Way's disk, and about 14,000 light-years deep along the line of sight from Earth.

A 2007 survey found that on the sky it is at least five times as long as it is tall, stretched along the Milky Way's disk. The galaxy continued past the edge of the area surveyed, so it spans more than 27 degrees of sky, or more than about 11,000 light-years.

Size comparisons

Its longest measured dimension, about 14,000 light-years, is about one-seventh the width of the Milky Way's disk, which is about 100,000 light-years across.
The Triangulum Galaxy, a whole spiral galaxy about 60,000 light-years across, is more than four times as wide.
On the sky, the overdensity covers about 100 square degrees, roughly 460 times the area of the full Moon.
If the Milky Way's disk were shrunk to the width of a basketball, the overdensity would be a smudge about 3 centimeters (1.3 inches) long. It would sit about 10 centimeters (4 inches) from the center of the disk and 5.5 centimeters (2.2 inches) from the Sun's place in it.

How it was found

In 2003, a team of astronomers searched the Two Micron All Sky Survey, a map of the whole sky in infrared light. Infrared light gives a better view than visible light through the Milky Way's disk. The team counted M giants, a kind of red giant star, and compared the sky below the Milky Way's disk with the sky above it. The strongest difference was an oval crowd of stars toward Canis Major.

It held about as many M giants as the core of the Sagittarius Dwarf Galaxy, a small galaxy that the Milky Way is tearing apart. From that, the team estimated a mass of 100 million to 1 billion times the Sun's. Four star clusters lie around it and move with it.

The team proposed that it is what remains of a dwarf galaxy. Stars pulled from it, they suggested, form a ring around the edge of the Milky Way's disk. That made it the closest galaxy known at the time, nearer than the Sagittarius Dwarf Galaxy.

Galaxy or part of the Milky Way

Far from its center, the Milky Way's disk is not flat. It is warped, bending below its usual plane toward Canis Major, and it grows thicker toward its edge. A 2006 study found that the overdensity lies where this downward bend is strongest. Its stars, the study found, match those of the Milky Way's outer disk.

A 2021 study, using star measurements from the Gaia space telescope, reached a similar conclusion. It found that the overdensity comes more from the warp in the Milky Way's disk than from a swallowed galaxy. As of 2026, whether it is a galaxy at all is disputed.

Sources

A dwarf galaxy remnant in Canis Major: the fossil of an in-plane accretion onto the Milky Way. Martin et al., Monthly Notices of the Royal Astronomical Society, 2004.
A Canis Major over-density imaging survey. I. Stellar content and star-count maps: a distinctly elongated body of main sequence stars. Butler et al., The Astronomical Journal, 2007.
Outer structure of the Galactic warp and flare: explaining the Canis Major over-density. Momany et al., Astronomy & Astrophysics, 2006.
A revised view of the Canis Major stellar overdensity with DECam and Gaia: new evidence of a stellar warp of blue stars. Carballo-Bello et al., Monthly Notices of the Royal Astronomical Society, 2021.
The Closest Galaxy: Canis Major Dwarf. Astronomy Picture of the Day, NASA, 2007.
Measuring the Universe. International Astronomical Union.
Our Milky Way Galaxy: How Big is Space?. NASA, 2019.
Hubble takes gigantic image of the Triangulum Galaxy. ESA/Hubble, 2019.
Moon Fact Sheet. NASA, 2024.
Official Basketball Rules 2026: Basketball Equipment. FIBA, 2026.

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