How big is NGC 4889?

NGC 4889 is a giant elliptical galaxy, the brightest galaxy in the Coma Cluster of galaxies. It is about 250,000 light-years across along its longest axis, measured out to a standard level of faintness that astronomers use to compare galaxies. A light-year is the distance light travels in one year, about 9.46 trillion kilometers.

Size

A galaxy like NGC 4889 has no sharp edge. Its stars thin out gradually, so its glow fades slowly into the dark. To compare galaxies fairly, astronomers measure each one out to the same faint brightness, a level called D25. At that level, NGC 4889 is about 2.6 arcminutes long on the sky, where an arcminute is one-sixtieth of a degree.

Turning that angle into a width needs the galaxy's distance. A team led by Nicholas McConnell put it at about 336 million light-years, which makes the width about 250,000 light-years. NASA gives a rounder distance of about 300 million light-years. Seen from Earth, the galaxy is an oval about one and a half times as long as it is wide.

Deeper images show its glow reaching farther. The Siena Galaxy Atlas, which traces galaxies to a fainter level in red light, measures NGC 4889 at about 4.9 arcminutes. At the same distance, that is about 480,000 light-years. The two figures measure to different edges, and neither is wrong.

Size comparisons

NGC 4889 is about two and a half times as wide as the disk of the Milky Way, which is about 100,000 light-years across.
If the Milky Way were shrunk to the size of a basketball, NGC 4889 would be about 60 centimeters (2 feet) across and about 800 meters (half a mile) away.
The long tail of stars behind the Tadpole Galaxy, about 245,000 light-years long, is about as long as NGC 4889 is wide.
IC 1101, the giant galaxy at the center of the cluster Abell 2029, is about 1.4 times as wide when both are measured to the D25 level.

The brightest galaxy in the Coma Cluster

NGC 4889 sits near the center of the Coma Cluster, one of the richest clusters of galaxies near the Milky Way. It lies in the constellation Coma Berenices. The astronomer William Herschel discovered it, and it is also known as Caldwell 35.

The most massive galaxies are usually giant ellipticals like this one, near the centers of galaxy clusters. They are smooth, oval collections of mostly old stars, and they form few new ones. NGC 4889 has an apparent magnitude of 11.5, a measure of how bright it looks from Earth in which larger numbers mean fainter objects. NASA says it is best seen through a large telescope under dark skies.

A giant black hole

At the center of NGC 4889 is a supermassive black hole. In 2011, McConnell's team estimated its mass at about 21 billion times the mass of the Sun. The estimate is uncertain: their likely range runs from 6 billion to 37 billion. The black hole at the center of the Milky Way is about 4 million times the mass of the Sun, so NGC 4889's is about 5,000 times as massive.

A black hole cannot be seen directly. The team used the Gemini North and Keck II telescopes in Hawaii to measure how fast stars move near the galaxy's center. Computer models of the stars' orbits then showed how much mass was needed to explain those speeds.