How big is Triangulum Galaxy? | Scale of the Universe

The Triangulum Galaxy is a spiral galaxy about 2.7 million light-years from Earth, and it is about 60,000 light-years across. A light-year is the distance light travels in one year, about 9.46 trillion kilometers. It is the third-largest galaxy in the Local Group, the family of galaxies that includes the Milky Way. On a very dark night it can be seen without a telescope, as a faint, blurry patch in the constellation Triangulum, which gives the galaxy its name.

Size

ESA gives the Triangulum Galaxy's width as about 60,000 light-years, and NASA describes it as about half the size of the Milky Way. Like every galaxy, it has no sharp edge, so its width depends on how faint a glow is counted.

The Updated Nearby Galaxy Catalog measures galaxies out to a very faint level of brightness. It gives the Triangulum Galaxy a width of 69 arcminutes on the sky, more than twice the width of the full Moon. An arcminute is one-sixtieth of a degree. At the distance the catalog uses for the galaxy, that angle converts to about 57,000 light-years.

Turning an angle into a width needs the distance. The catalog's distance for the Triangulum Galaxy was measured using variable stars, stars that regularly brighten and dim.

Size comparisons

The Triangulum Galaxy is about three-fifths the width of the Milky Way's disk, which is about 100,000 light-years across. It is less than a third the width of the Andromeda Galaxy, which ESA puts at about 200,000 light-years.
It is about 1.2 times as wide as the Sombrero Galaxy, which is about 50,000 light-years across.
NGC 604, a huge cloud of glowing gas in one of its spiral arms, is about 1,300 light-years across. About 46 of these clouds side by side would span the galaxy.
If the Triangulum Galaxy were shrunk to the size of a basketball, the Milky Way would be a disk about 40 centimeters (16 inches) across, about 11 meters (36 feet) away.

The smallest spiral in the Local Group

The Local Group has more than 50 galaxies. The largest is the Andromeda Galaxy, followed by the Milky Way and then the Triangulum Galaxy, and the rest are dwarf galaxies. The Triangulum Galaxy is the smallest of the three spirals. It has about 40 billion stars. The Milky Way has at least 10 times as many, and Andromeda about 100 times as many.

Unlike the two larger spirals, the Triangulum Galaxy has no bright bulge of stars at its center and no bar linking its arms to the center. It holds a great deal of gas and dust, and it forms new stars with a total mass of about one Sun every two years.

A giant star nursery

NGC 604 is one of the largest regions of star formation in the Local Group. It holds more than 200 young stars of the hottest and most massive kinds, which can have more than 100 times the mass of the Sun. The Milky Way has no region like it. The region is estimated to be about 3.5 million years old.

Discovery and future

Charles Messier was the first to catalog the galaxy, in August 1764, and it is also known as Messier 33, or M33. In the 1920s, the astronomer Edwin Hubble studied variable stars in it to estimate its distance. That showed it is a separate galaxy and not, as had been suspected, a nebula: a glowing cloud inside the Milky Way.

The Triangulum Galaxy lies only slightly farther away than the Andromeda Galaxy and may be held by Andromeda's gravity. Both are moving toward the Milky Way. The Triangulum Galaxy could become part of the expected collision between Andromeda and the Milky Way, more than 4 billion years from now.