IC 1101

What is IC 1101?

Galaxies· Distance ~1 billion ly. A supergiant elliptical among the largest galaxies known. It sits at the heart of the cluster Abell 2029, having grown by devouring its neighbours, and counting its faint halo it spans dozens of Milky Ways. Compare it with our galaxy on the size ladder.

Its place in the sky

A galaxy, in the direction of Serpens, spanning about 1 arcminutes of sky — 0 times the width of the full Moon.

Distance and scale

Figures are representative approximations. Source: compiled from NASA and IAU public data.

Frequently asked

How far is IC 1101 from Earth?

~1 billion ly.

How big is IC 1101?

~4 million ly or more, halo included.

Where in the sky is IC 1101?

In the direction of Serpens.

The trouble with "four million light-years" — where does a galaxy end?

The four to six million light-years usually quoted for IC 1101's diameter is not a true edge but the point where the halo's light falls below detection. Giant ellipticals have no sharp boundary — star density thins gradually — so deeper exposures make the diameter larger. The Milky Way, halo included, also runs to hundreds of thousands of light-years, and comparing "diameters" is meaningful only on the same standard.

The standard measures of galaxy size are therefore the radius at which surface brightness drops to a set level, or the radius enclosing half the light. By those measures IC 1101 still ranks among the largest known galaxies, but its position changes against recently found "ultra-diffuse" galaxies and other cluster-central galaxies measured in radio. In the size ladder IC 1101 is drawn at its large halo-inclusive value, a number to be read with the caveat "as far as the light reaches".

The galaxy in depth

IC 1101 is a supergiant elliptical galaxy about a billion light-years away in Serpens, the central galaxy of the cluster Abell 2029. It holds around 100 trillion stars — hundreds of times the Milky Way's count — and including its faint outer halo it spans more than four million light-years, which is why it is so often called the largest galaxy known. Where a galaxy 'ends' is a matter of definition, though, so the ranking remains debated.

William Herschel found it in 1790, noting only a faint nebulous patch. Galaxies like this grow over billions of years at the centre of a cluster by swallowing their neighbours, and IC 1101 is thought to host a central black hole of tens of billions of solar masses. Almost no new stars form, so it glows with the yellow light of an old population, while the hot gas that fed its growth fills the whole cluster with X-rays.

At 15th magnitude and a billion light-years out, it is barely a smudge even in amateur telescopes, so no Sky-view link is offered. Instead, pick IC 1101 on the size ladder and set it beside the Milky Way and Andromeda to see how extreme the size range of galaxies can be.

Related

Figures are representative approximations. Simulated positions use J2000 mean elements.