What is Leo’s trio?. Well, that’s what you see in the image, a group of three galaxies called M66 about 40 million light years from here, that is, inside the local cluster . Formed by the M65 above, M66 (below left, from whom the triplet is named) and NGC 3628 to the left (a nebula).
Although the three are typical spiral galaxies, the NGC 3628 has more definition of nebula even being a cluster of stars. That is, it is within the definition according to the new general catalog (hence its name).
The proximity of these three galaxies makes them have strong gravitational interactions between them which is causing the M66 and the M65, their spirals, to be opening more than they should according to their own gravitational interactions. Come on, your spirals are opening. Meanwhile, NGC 3628, due to its composition, that is, the orientation, is “swelling”.
The process of decomposition of NGC 3628 by the severity of its two neighbors is what is conferring that aspect of nebula rather than galaxy and is what was thought to be a nebula. Something very curious.
The importance of the triplet of Leo is not so much what is happening to them (that also) but it is helping to calculate the gravitational effects of “something else” exists, that is, dark matter and its interactions as the force exerted between the three galaxies is not so much for the phenomena that are happening.