How big is Canes Venatici I Dwarf Galaxy?

The Canes Venatici I Dwarf Galaxy is a faint dwarf galaxy that orbits the Milky Way, about 710,000 light-years from Earth in the constellation Canes Venatici. A light-year is the distance light travels in one year, about 9.46 trillion kilometers. Half of the galaxy's starlight comes from an oval region about 3,600 light-years long. Its faintest stars spread over about 6,500 light-years.

Size

A dwarf galaxy has no sharp edge, so astronomers often give its half-light radius: the distance from the center within which half of its light is found. The team that discovered the galaxy in 2006 measured a half-light radius of about 8.5 arcminutes on the sky, where an arcminute is a sixtieth of a degree.

At a distance of about 220,000 parsecs, where a parsec is about 3.26 light-years, that is about 550 parsecs. Half of the galaxy's light therefore comes from a region about 1,100 parsecs, or 3,600 light-years, long. A 2012 catalog of nearby dwarf galaxies gives a similar radius, 564 parsecs.

The galaxy is not round. Its half-light region is an oval about three-fifths as wide as it is long, about 2,200 light-years across.

The discoverers traced its stars over about 2,000 parsecs (6,500 light-years) along its long axis. That made it one of the largest dwarf spheroidal galaxies known around the Milky Way at the time. A 2008 study measured its distance more precisely, at 218,000 parsecs, about 710,000 light-years.

Size comparisons

Measured the same way, by the radius that holds half its light, the galaxy is about three times as wide as the Leo II Dwarf Galaxy. Yet Leo II gives off about three times as much light.
The disk of the Milky Way, about 100,000 light-years across, is about 15 times as long as the galaxy's full extent.
If the Milky Way's disk were shrunk to the width of a basketball, the galaxy would be a faint smudge about 1.6 centimeters (0.6 inches) long. It would be about 1.7 meters (5 feet 7 inches) away, the height of a person.

Found in a survey of the sky

The galaxy's stars are spread so thinly, and it is so far away, that it went unnoticed until 2006. Astronomers then found it as a slight excess of stars in images from the Sloan Digital Sky Survey.

At the time, only ten dwarf spheroidal galaxies were known to orbit the Milky Way. A dwarf spheroidal is a small, rounded galaxy with a very faint glow. The discoverers noted that finding such a faint galaxy suggested that more remained to be found.

Two generations of stars

Most of the galaxy's stars are more than 10 billion years old. They contain very little iron: about a hundredth as much, compared with their hydrogen, as the Sun.

A 2008 study with the Large Binocular Telescope found a second, much younger group of stars, about 1.4–2 billion years old. These stars have about three times as much iron as the old ones and are packed more closely toward the center. They make up only 3–5 percent of the mass of the stars inside the half-light radius.