![]() M51 displays a star-forming activity much stronger than isolated galaxies like our own. This contrasts with the yellowish colours in the satellite galaxy, coming from lighter, older and colder stars. Particularly, the spiral galaxy exhibits bluish hues due to the massive, young and hot stars that populate its disk. This way, the contrast among the different stellar populations in the field is more obvious. ![]() The final colour balance has been done in such a way that the light coming from both galaxies, put together, is considered as white. The true-colour image has been obtained gathering light through a set of three filters that mimic the chromatic sensitivity of the human eye. For this reason, we offer the M51 image in two versions: true colour and H-alpha enhanced. Images taken in the so-called H-alpha colour reveal where the gas is ionised by newborn stars. ![]() Star forming regions can be detected thanks to the pinkish glow of ionised hydrogen. The gaseous content of colliding galaxies gets compressed and this triggers violent bursts of star formation (starbursts). Many of the worlds in that area are doomed to get lost into the emptiness of space, as they move far away from their parent galaxies. This has happened to many stars from the secondary galaxy: streams of stars stripped of from NGC 5195 extend over the left (north) part of the image as a fuzzy fog. Instead, they suffer strong alterations in their trajectories, what usually implies that many of them are expelled into the intergalactic space. When galaxies collide, their stars do not crash ones against the others. The interaction of both galaxies induces a bunch of collateral effects, being the spiral structure of the main galaxy only one of them. Now, the small galaxy is located slightly behind the disc of the Whirlpool, and moving away from us. M51 and its companion are performing a cosmic dance that, during the last 500 million years has made NGC 5195 pass twice through the disk of M51. At a distance of 23 millions of light-years, the apparent dimensions of M51 mean that that galaxy has to be quite similar to our own, yet somewhat smaller. Just by chance, we see the disk of M51 face on from Earth, what allows studying it in detail. Now it is clear that these two stellar systems are colliding and that the outstanding spiral shape of M51 is due, mainly, to the tidal forces unleashed during this process. A large telescope is needed to see its intricate shape, but even small amateur telescopes reveal that this galaxy is not isolated, but has a small companion, the dwarf irregular galaxy NGC 5195. ![]() Galaxy M51 was discovered by Charles Messier in 1773, but its outstanding spiral structure was first perceived by William Parsons (Earl of Rosse) in 1845, using his huge reflecting telescope, the Leviathan of Parsonstown. Every relevant collection of astronomical photographs has to include this object at least once, if not more times, and every new look to this magnificent object reveals different aspects of this grand-design spiral galaxy. The same could be said about M51, the Whirlpool Galaxy, in the context of astronomical imaging. The Spanish poet José Hierro, when presenting his book Cuaderno de Nueva York ("New York Notebook"), said that any major poet has to write about both New York and spring, at least once in a lifetime.
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