Leo hangs clearly in the sky at sunset – look south to see the two patterns of stars that the brain puts together to make a lion. A backwards question mark represents the head and mane and a triangle of stars to the lower left forms the lion’s hindquarters and tail. Leo is best known for its stars – particularly Regulus, its brilliant heart. But the constellation also contains quite a few bright galaxies. Top of the pile for beauty are three galaxies that are known as the Leo Triplet: M65, M66, and NGC 3628. The M in the names of the first two refers to Charles Messier – the great cataloguer of faint fuzzies from the late 1700s. Galaxies M65 and M66 are respectively the 65th and 66th objects in his catalogue of objects that are not comets. NGC 3628 was not seen by Messier – hence it has no messier designation. Its discovery was left to William Herschel in 1784.
All three of these galaxies are spirals – like our own Milky Way: pinwheels of stars and gas, the largest stretching out about one hundred thousand light years. All three galaxies show signs of distortion due to the proximity of the others. In the case of NGC 3628, we see it from the side – so it looks like a streak of light with lanes of dark dust running down the middle. Exact edge on galaxies are rare – and this one is unusual even given that – its dust lane is distinctly off centre and the disk fans out near the galaxy’s edge – making it rather boxy in appearance. There are many new stars being formed in NGC 3628 – making it a “starburst galaxy” – compared to our Milky Way where only about one solar mass of material forms a new star each year. And the larger companions to this galaxy have pulled out a tail of gas that spans a quarter of a million light-years – and contains enough gas to make half a billion stars as massive as the Sun.
In comparison, M65 is an elegant spiral galaxy which is highly tilted to our line of sight, showing us a prominent and bright central bulge. Besides the bulge, M65also has a dust lane, surrounding the galaxy and seen where it hides a bright background.
The final one of the three – M66 has asymmetric spiral arms. Usually spiral arms wind about a galaxy’s core in a symmetric way – but these are uneven – due to the influence of its neighbours. M66 boasts a remarkable record of supernovae explosions. The galaxy has hosted three supernovae since 1989, the latest one occurring in 2009. (Our galaxy last had a supernova in 1604). A supernova is a stellar explosion that may momentarily outshine its entire host galaxy. It then fades away over a period of a few weeks or months.
Both Hubble and the European Southern Observatory have released high resolution images of this beautiful set of triplets – look for them if you don’t have access to a massive telescope of your own.
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