Astronomy

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Umbrella Galaxy. While visible in even a 4-inch
telescope, this object should excite CCD imagers
because it displays a crescent-shaped structure
extending laterally from an enormous jet that
seems to emanate from the galaxy’s heart; we now
know it to be a tidal tail that formed when NGC
4651 ripped apart a smaller companion during a
series of repeated encounters.
Let’s now follow the western lock of Berenice’s
Hair southwest — from 12 to 7 Com, and then
about 3¾ ̊ farther to the southwest. Here we arrive
at a splendid (though often overlooked) double
star, 2 Comae Berenices. The 6th-magnitude
primary has a magnitude 7.5 secondary 3.6" to the
southwest. In the 19th century, Admiral William
Henry Smyth called it a “beautiful object ... two
jewels fixed in the field,” with a “pearly white”
primary and a “lilac” secondary; the lilac being a
common color contrast phenomenon.

Extragalactic pandemonium
We end our journey by returning to the wishbone
and looking about 10 ̊ east for a bright pairing of
stars: 4th-magnitude Beta (β) and 5th-magnitude
41 Com, which itself is a naked-eye double. Center
41 Com in your telescope, move about 1½° west-
northwest, and let your gaze relax — you have
arrived at the heart of the Coma Cluster of

Galaxies (Abell 1656). As it is close to the north pole of

the Milky Way, its members are not dimmed by inter-

vening dust.

A veritable blizzard, the cluster contains more than

650 galaxies, making it one of the densest collections in

the universe. The members spread across 1½ ̊ of sky. In

the grander scheme, the Coma Cluster is a vast jungle of

galaxies, with some 30,000 of them down to magnitude

19 lying within 6° of the cluster’s core. Despite its great

distance (300 million light-years), about a dozen of its

members are within reach of a 4-inch telescope.

The two brightest — NGC 4889 and NGC 4874 —

are both 11th-magnitude giant ellipticals. NGC 4889

appears as a small, faint, slightly out-of-round glow with

a smooth outer halo that gradually brightens to a sharp

core. NGC 4874 is merely a swollen spot of haze. Once,

without knowing it, I sketched two 13th-magnitude

companion galaxies to NGC 4889 — NGC 4886 and

NGC 4898 — believing at first that they were details

belonging to NGC 4889. Unlike the Virgo Cluster,

which is rich in spirals, the Coma Cluster is rich in

ellipticals.

As you survey the region for fainter members, also

keep in mind that astronomers have recently discovered

within this cluster more than 800 galaxies that could

contain as much as 100 times more dark matter than

visible matter. These are “failed” galaxies, which

stopped producing stars between 7 billion and 10 billion

years ago.

When probing this extragalactic graveyard, you

can also use your imagination to sense the 47 ghost

galaxies discovered in 2015 by Dragonf ly — an array

in New Mexico composed of eight Canon telephoto

lenses. These “ultra-diffuse” galaxies are as large as

our Milky Way but contain only about one-thousandth

as many stars, making them appear as large spheroidal

phantoms.

Stephen James O’Meara is a contributing editor

of Astronomy and author of numerous popular books on

astronomical observing.

The Coma Cluster of

Galaxies (Abell 1656)

lies some 300 million

light-years away and

contains more than 650

galaxies. The cluster’s

center is dominated by

two bright ellipticals,

NGC 4889 and NGC

4874, visible as the oval

smudges in the middle of

this image. BERNHARD HUBL

In the

Coma Cluster,

astronomers

have recently

discovered

more than 800

galaxies that

could contain

as much as

100 times more

dark matter than

visible matter.