How big is Sagittarius Dwarf Galaxy?

The Sagittarius Dwarf Galaxy is a small galaxy that the Milky Way's gravity is tearing apart. Its main body is about 10,000 light-years long, where a light-year is the distance light travels in one year, about 9.46 trillion kilometers. It lies about 86,000 light-years from Earth, on the far side of the center of the Milky Way. Streams of stars pulled out of it wrap around the Milky Way.

Size

The galaxy is coming apart, so it has no clear edge. Its size depends on where it is taken to end.

A 2021 study used the Gaia spacecraft to map the positions and motions of its stars in three dimensions. It found that the streams leave the main body about 1.5 kiloparsecs from the center. A kiloparsec is 1,000 parsecs, about 3,260 light-years.

That makes the main body about 3 kiloparsecs, or 10,000 light-years, long. At its center is a bar-shaped crowd of stars about 2.5 kiloparsecs (8,000 light-years) long. The streams leave from the ends of this bar.

The main body is stretched out: a 2003 survey in infrared light found it only about a third as wide as it is long. Fainter stars reach farther. A 2020 study found that the galaxy's remains extend up to about 5 kiloparsecs (16,000 light-years) from the center before they merge into the streams.

A 2012 catalog that includes these outer stars gives a half-light radius of 2.6 kiloparsecs. That is the distance from the center within which half of the galaxy's light is found.

Size comparisons

The disk of the Milky Way, about 100,000 light-years across, is about 10 times as long as the galaxy's main body.
If the Milky Way's disk were shrunk to the width of a basketball, the main body would be about 2.3 centimeters (0.9 inches) long. It would lie about 14 centimeters (5.6 inches) from the center of the ball, a little beyond its edge.
The main body is about one and a half times as long as the whole Canes Venatici I Dwarf Galaxy, whose stars spread over about 2 kiloparsecs (6,500 light-years).
It is less than half as far from Earth as the Small Magellanic Cloud, which is about 200,000 light-years away.

A galaxy being torn apart

As the galaxy orbits the Milky Way, the Milky Way's gravity pulls stars off it. They form two long streams, one ahead of the galaxy and one behind. A 2003 survey traced the stream behind it at 20–40 kiloparsecs from the Sun, and the stream ahead of it out to about 45 kiloparsecs. The Sun lies close to the plane of the streams' orbit, so some stars near the Sun may once have belonged to the Sagittarius galaxy.

What remains has a mass of about 400 million Suns, and only about a quarter of it is in stars. A 2020 study found that even its center is not dense enough to hold together against the Milky Way's pull. The galaxy will probably be torn apart completely within the next billion years.

Hidden behind the Milky Way's center

The galaxy lies behind the bulge, the dense crowd of stars around the center of the Milky Way, so it was discovered only in 1994. Astronomers found it from the speeds of its stars, which they measured in a survey of stars in the direction of the Milky Way's center. At the galaxy's center sits Messier 54, a dense cluster of old stars. The Sagittarius Dwarf Galaxy is not the same as the Sagittarius Dwarf Irregular Galaxy, a different galaxy about 3.5 million light-years away.