Galaxy Tour: The Leo Triplet

The Leo Triplet includes edge-on spiral galaxy NGC 3628 (below left), spiral galaxy M65 (top), and spiral galaxy M66 (below right). Image credit: Terry Hancock at Downunderobservatory.com

The Leo Triplet includes edge-on spiral galaxy NGC 3628 (below left), spiral galaxy M65 (top), and spiral galaxy M66 (below right). Image credit: Terry Hancock at Downunderobservatory.com

The constellation Leo is stuffed with galaxies that are visible in a backyard telescope. Three of the brightest and best known are M65, M66, and NGC 3628, also known as the “Leo Triplet”.  You can see all three galaxies in virtually any telescope and even in a good pair of binoculars. If you can see all three galaxies at once, keep in mind you’re seeing at one time the collected light of more than half a trillion stars!

Since it’s on the ecliptic, the constellation Leo and its trove of galaxies is visible from most parts of the northern and southern hemispheres. Leo is visible nearly overhead in April and May in the northern hemisphere. It’s easily distinguished by a large sickle-shaped group of stars which marks the head and neck of the celestial lion, as well as a collection of bright stars to the east that mark the body and haunches of the great beast. In the southern hemisphere, Leo is visible over the northern horizon and its lion-like shape appears upside down.

Location of the Leo Triplet south of the star Chertan. In this map, M65=N3623, M66=N3627, and of course NGC3628=N3628).

Location of the Leo Triplet south of the star Chertan. In this map, M65=N3623, M66=N3627, and of course NGC3628=N3628). Created with SkyX by Software Bisque.

The Leo Triplet lies about 2.5o south-southeast of the star Chertan in the haunches of the celestial lion. The southwestern galaxy of this trio is Messier 65. It’s visible as a dim 9th-magnitude smudge in binoculars or a finderscope in dark sky. In low-power in a 4” scope, M65 appears a bright oval with a clearly defined core and with faint extensions visible with averted vision that give the galaxy a distinctive “S” shape. At 100x or more, the galaxy appears clearly elongated north-south. The spiral arms will be tenuously visible in dark sky. In long-exposure images, the arms of this galaxy appear clearly distorted by gravitational interaction with the neighboring galaxies.

M66 lies just 1/3 of a degree east-southeast of M65. The total brightness of M66 is slightly greater than M65, but the former appears fainter to most observers because it’s more spread out. At 50x in a 4” scope, M66 has a star-like nucleus and an oval core and a fainter oval halo. This galaxy has a length:width ratio of 2:1, whereas M65 is about 5:1. The S-shaped spiral arms of M66 are also distorted by gravitational interaction with its neighbors.

[The Leo Triplet.  NGC 3628 is at upper left, M65 is upper right, and M66 is lower right]

NGC 3628, the third member of the triplet, has magnitude of 9.5, just a tiny bit fainter than M65 but it’s far more spread out; the low surface brightness makes it a challenging sight. In city skies, it may be totally washed out. But in suburban and darker skies, this edge-on spiral galaxy is a striking sight. It appears long and thin in a 4” scope at 50-75x, somewhat needle-shaped with only a slight brightening at the core. The galaxy runs east-southeast to west-northwest.

The Leo Triplet lies at an average distance of some 30 million light years. The three galaxies are gravitationally bound to each other. Close-up images of NGC 3628 show a disk distorted by gravitational interaction with M65 and M66. This interaction has pushed and pulled the gas and dust within the galaxy to create a rapid and widespread star formation, and NGC 3628 is classified as a “starburst galaxy”. Gas is consumed in star formation at such a rapid rate that there will be little left in just 5-10 million years. Supernovae of young stars will clear out the remaining gas, leaving NGC 3628 with little fuel left to create new stars for the rest of its days. The galaxy will settle as a quiet and nearly featureless object that appears far less interesting to Earthbound observers in the distant future.

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