Messier and other deep-sky objects

In August 1758 Charles Messier, a French astronomer who spent his nights searching for comets, came across a faint misty patch near the tip of the southern horn of Taurus. It looked like a comet, but it did not move against the stars from one night to the next. So that he would not be fooled by it again, Messier began a list of such objects, and by 1781 the list held 103 of them. Seven more that he or his colleague Pierre Méchain had seen were added in the twentieth century, which brings the list to 110. The patch in Taurus is M1, the Crab Nebula, the remains of a star whose explosion Chinese astronomers recorded in 1054. Messier's list of things to ignore has become the list that every observer learns first, and the observation rounds of the olympiad often ask us to find these objects in the sky or mark them on a chart.

O1.8.1The Messier catalogue

Objects outside the solar system that look like more than a point of light, such as star clusters, nebulae and galaxies, are called deep-sky objects. The table sorts the Messier objects by kind, and the next part of this section explains what each kind is.

The three others are M24, a bright patch of the Milky Way in Sagittarius called the Sagittarius Star Cloud, M40, a pair of faint stars in Ursa Major, and M73, a small group of four stars in Aquarius. M102 is uncertain. Méchain later wrote that it was a second sighting of M101, but its description fits the Spindle Galaxy, NGC 5866, in Draco, and most lists today take it to be that galaxy.

The brightest Messier object is the Pleiades, M45, of magnitude 1.2, and the faintest are galaxies of about magnitude 11. Of the 110, 29 have a total magnitude of 6.0 or brighter, although, as we shall see at the end of this section, the total magnitude of a large object says little about how easy it is to see. Messier observed from Paris, at latitude 48.8° N, so his list stops in the south at about declination −35°. Its southernmost object is the open cluster M7 in the tail of Scorpius, at declination −34.8°, which never rose more than about 6° above his horizon.

Far more objects are listed in the New General Catalogue, or NGC, which John Dreyer published in 1888 with 7840 entries, and in its two supplements, the Index Catalogues, or IC, with 5386 more. Most Messier objects have an NGC number as well. M31 is NGC 224 and M42 is NGC 1976. The far southern sky, which Messier never saw, is known by NGC numbers. Omega Centauri is NGC 5139, 47 Tucanae is NGC 104, the Carina Nebula is NGC 3372 and the Jewel Box is NGC 4755 (Section O1.6). The Double Cluster in Perseus, NGC 869 and NGC 884 (Section O1.5), is one of the finest sights of the northern sky, yet Messier did not list it.

O1.8.2Kinds of deep-sky object

An open cluster is a group of a few dozen to a few thousand stars that formed together from one cloud of gas and are still loosely held together by their gravity. Open clusters are young by the standards of stars, the Pleiades being about 100 million years old, and they belong to the disc of our Galaxy. On the sky they therefore lie along the Milky Way, and 20 of the 26 Messier open clusters are within 5° of the galactic equator, the middle line of the Milky Way (Section 1.6).

A globular cluster is a tight ball of a hundred thousand to a few million stars, about 12 billion years old, which orbits the centre of our Galaxy far above and below the disc. Globular clusters swarm round the centre of the Galaxy, which lies about 26 000 light-years away in Sagittarius, so most of them are seen in that direction. Of the 29 Messier globular clusters, 28 lie in the half of the sky centred on the galactic centre, and Sagittarius and Ophiuchus hold seven each. The one exception is M79 in Lepus, south of Orion.

A diffuse nebula is a cloud of gas and dust. An emission nebula, such as the Orion Nebula, M42, glows because hot young stars inside it make its hydrogen shine, and a reflection nebula, such as M78 in Orion, shines by the starlight that its dust scatters. Most of them are places where stars are forming now, so they lie in or near the band of the Milky Way.

A planetary nebula is a shell of gas thrown off by a dying star like the Sun and lit by the hot core left in the middle. It has nothing to do with planets. The name comes from William Herschel, who thought they looked like the small round discs of planets in his telescope. The Ring Nebula, M57, in Lyra (Section O1.4) is the best known. A supernova remnant is the wreck of a star that exploded, and the Crab Nebula is the only one in Messier's list.

A galaxy is a whole system of stars like our own Galaxy, and all but the closest are millions of light-years away. The Andromeda Galaxy, M31, about 2.5 million light-years away, is the nearest large one. Galaxies lie in every direction, but the dust in the disc of our own Galaxy hides those that lie behind the Milky Way, so every Messier galaxy is more than 21° from the galactic equator. Many crowd into Virgo and Coma Berenices, which between them hold 18 of the 40, because in that direction we look into the Virgo cluster, a great cluster of galaxies about 54 million light-years away.

O1.8.3Finding the brightest

Most of the bright Messier objects lie close to stars we already know, and each can be reached by a short hop. In the winter sky of the northern hemisphere, the Orion Nebula, M42, is the middle star of Orion's sword (Section O1.2), which looks hazy even to the naked eye, and the Pleiades, M45, need no finding. The Crab Nebula, M1, lies 1.1° north-west of ζ Tau, the tip of the southern horn of Taurus, and M35 lies 2.3° north-west of Propus at the feet of Gemini. M41 is 4.0° south of Sirius, and the Beehive, M44, is in the middle of Cancer (Section O1.3).

In spring the Big Dipper leads to several galaxies. The line from Phecda through Dubhe, continued about as far again, reaches the pair M81 and M82. The Whirlpool Galaxy, M51, is 3.6° south-west of Alkaid, and the Pinwheel Galaxy, M101, 5.7° east of Mizar. Further south, the heart of the Virgo cluster lies about halfway along the line from Denebola to Vindemiatrix (ε Vir), where M84, M86 and M87 lie within 0.3° of the line.

In summer the Milky Way is full of them. M13, the Hercules Globular Cluster, lies on the western side of the Keystone of Hercules (Section O1.4), a third of the way from η Her to ζ Her. The Ring Nebula, M57, is between Sheliak and Sulafat in Lyra, 0.8° from Sheliak, and the Dumbbell Nebula, M27, lies in Vulpecula, 3.2° north of γ Sge at the tip of the Arrow. The globular cluster M4 is 1.3° west of Antares. The open clusters M6 and M7 lie just beyond the sting of Scorpius, M6 5.0° north of Shaula and M7 4.7° north-east of it. The Lagoon Nebula, M8, is 6.1° north of Alnasl, the tip of the spout of the Teapot, and the globular cluster M22 is 2.4° north-east of Kaus Borealis, the top of its lid. In Scutum, a small constellation between Aquila and Sagittarius, lies M11, the Wild Duck Cluster, one of the richest open clusters in the sky.

In autumn the Andromeda Galaxy, M31, and the Triangulum Galaxy, M33, are found from Mirach (Section O1.5), and the globular cluster M15 lies 4.2° north-west of Enif, the nose of Pegasus.

O1.8.4Seeing faint objects

The magnitude of a star measures light that arrives from a single point. The total magnitude of a galaxy or a nebula measures all of its light as if it came from one point, although that light is spread over the whole area of the object, and what the eye notices is how bright each part of that area is. This is measured by the surface brightness, the magnitude of the light from one square arcsecond. If the light of an object of total magnitude $m$ is spread evenly over an area $A$ in square arcseconds, each square arcsecond gets a fraction $1/A$ of it. Since five magnitudes are a factor of 100 in brightness (Section O1.1), a factor $1/A$ in light adds $2.5{\log }_{10}A$ to the magnitude, so the mean surface brightness is

in magnitudes per square arcsecond. Chapter 4 returns to surface brightness and shows why it does not depend on distance.

The night sky itself has a surface brightness, about 21 to 22 magnitudes per square arcsecond at a dark site and several magnitudes brighter in a city, and an object is seen by the light it adds to the sky's own glow, so what matters is the contrast between the two. A large object whose mean surface brightness is close to that of the sky is very hard to see, although its centre, which is brighter than the mean, can still show at a dark site, as M33 does to the naked eye. From a town, where the sky is several magnitudes brighter, M33 and M101 disappear while the small planetary nebulae and globular clusters are still easy.

Two habits help with faint objects. The eye needs 20 to 30 minutes in the dark to reach its full sensitivity, and one look at a bright white screen undoes much of it, which is why observers use dim red light. Faint light is also seen better a little to the side of where we look, because the cells of the eye that work in dim light lie away from the centre of its field of view. Looking slightly away from a faint object, called averted vision, often shows it when looking straight at it does not.