M66 is a superb bright intermediate spiral galaxy located in the constellation of Leo. It's the brightest of a trio of galaxies that form - along with M65 and NGC 3628 - the well known and popular "Leo Triplet" or "M66 group". All three objects can be observed with small telescopes in the same low power field of view. With the exception of the M81/M82 pair in Ursa Major, the Leo Triplet is arguably the most sought after galaxy grouping for amateur astronomers.

Charles Messier discovered both M66 (mag. +8.9) and M65 (mag. +9.6) on March 1, 1780. The third member of the triplet, NGC 3628, has a debatable apparent magnitude. Some texts record it as the brightest member of the three while others the faintest! For our purposes we estimate NGC 3628 to be brighter than M65 and almost as bright as M66. However, what is clear is that NGC 3628 suffers from low surface brightness and therefore is the most difficult member of the trio to spot. Messier missed it completely and it was not until April 8, 1784 when it was finally discovered by William Herschel.

To find the triplet, look to the eastern part of Leo. This zodiac constellation is relatively large and bright and somewhat looks like the Lion it represents. The brightest star in Leo and its only first magnitude star is Regulus (α Leo – mag +1.4). Positioned approximately 16 degrees northeast of Regulus is Chertan (θ Leo - mag. +3.3). Along with the Zosma (δ Leo - mag. +2.6) to the north and Denebola (β Leo - mag. +2.1) to the east, Chertan forms a prominent right-angled triangle. Located 2 degrees south of Chertan is 73 Leo (mag. +5.3). M65 lies 0.75 degrees east of this star with M66 a further 0.33 degrees southeast of M65. Located 0.5 degrees north of the Messier pair is NGC 3628.

The galaxies are best seen during the months of March, April and May.

M66 Intermediate spiral galaxy by the Hubble Space Telescope (NASA/ESA/The Hubble Heritage Team STScI/AURA)

Finder Chart for M66 (also shown M65, M95, M96 and M105)

Finder Chart for M66 (also shown M65, M95, M96 and M105) - pdf format

Finder Chart for M87 (also shown M49, M53, M58->M60, M64->M66, M84->M86, M88->M91 and M98->M100)

Finder Chart for M87 (also shown M49, M53, M58->M60, M64->M66, M84->M86, M88->M91 and M98->M100) - pdf format

From a dark site, M66 (and M65) are faintly visible as fuzzy patches of light with 10x50 binoculars. Through larger 20x80mm binoculars or a small 80mm (3.1-inch) telescope, M66 appears as a thin oval smudge of light with a brighter centre. Since M66 and M65 are separated by only 20 arc minutes of apparent sky, both galaxies fit neatly in the same low magnification field of view.

A medium size telescope of 150mm (6-inch) aperture shows M66 as a slightly edge on silvery disk of light with a bright stellar like nucleus. A 250mm (10-inch) scope hints at a spiral structure with the brighter nebulosity wrapped in a fainter halo. On the other hand, M65 appears fainter and more edge on when compared to M66.

In total, M66 measures 9.1 x 4.1 arc minutes in apparent size. At a distance of 36 million light-years from Earth, this corresponds to an actual diameter of 95,000 light-years. The galaxy is estimated to contain about 200 billion stars.

So far, four supernovae have been recorded in M66 (1973R, 1989B, 1997bs and 2009hd). The brightest of the four was 1989B which reached magnitude +12.2 on February 1, 1989.

M66 Data Table

Object TypeIntermediate spiral galaxy
ClassificationSAB (s)
Distance (kly)36,000
Apparent Mag.8.9
RA (J2000)11h 20m 15s
DEC (J2000)12d 59m 28s
Apparent Size (arc mins)9.1 x 4.1
Radius (light-years)47,500
Number of Stars200 Billion
Notable FeatureMember of the Leo Triplet along with M65 and NGC 3628

Sky Highlights - April 2017

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