Messier 77: Cetus A

Messier 77 looks like an ordinary small galaxy in the eyepiece, but it holds one of the most studied active galactic centers in the sky. It lies in Cetus, the sea monster, almost exactly on the celestial equator.

What it is

M77 is a spiral galaxy about 45 million light-years away. Its bright inner part covers about 7 by 6 arcminutes, which at that distance is roughly 90,000 light-years across, and faint outer arms extend farther. It is the nearest and brightest example of a Seyfert galaxy, a spiral whose core shines with gas racing outward at hundreds of kilometers per second. The power source is a supermassive black hole, about 10 million times the mass of the Sun according to SEDS, pulling in gas from its surroundings. The core is also a strong radio source, which gave the galaxy its other name, Cetus A.

In 2022 the IceCube neutrino observatory at the South Pole reported evidence that M77 is a source of high-energy neutrinos, the first such signal tied to a galaxy like this.

When to see it

M77 is an autumn object. It crosses the meridian around midnight in early November and around 10 p.m. in early December. From 40°N it reaches about 50° above the southern horizon. Sitting on the celestial equator, it is visible from everywhere on Earth except the far polar regions.

How to find it

Find the head of Cetus, a small loop of stars south of Aries, with Menkar (Alpha Ceti) as its brightest star. Gamma Ceti, at the southwest corner of the head, lies about 5° west of Menkar. About 3° southwest of Gamma is Delta Ceti, a 4th-magnitude star. M77 lies under a degree east-southeast of Delta Ceti, close enough to share a low-power field with it.

What you will see

In binoculars M77 is a small, faint spot near Delta Ceti, visible under dark skies. A 70–100 mm telescope shows a compact, bright, almost star-like center surrounded by a small round halo; it is brighter per square arcminute than many galaxies, so it survives moderate light pollution. In a 200 mm telescope the halo grows into an oval a few arcminutes across, and on a very good night you may see uneven brightness hinting at the inner spiral arms.

History

Pierre Méchain discovered M77 on October 29, 1780. Charles Messier added it to his catalog on December 17, 1780, but described it as a cluster of stars with nebulosity. Lord Rosse later saw it as a spiral. In 1908 Edward Fath recorded unusual bright emission lines in its spectrum, confirmed by Vesto Slipher in 1917. Carl Seyfert described the class of galaxies with such bright cores in 1943, and M77 became its prototype.