Spring and fall are some of the best times to take a peek at our galactic neighbors while the obscuring band of our own Milky Way lies near the horizon in the evening. Galaxies come in a wide variety of shapes and apparent sizes and many are not alone. Now is a prime time to view one of the brighter and most photogenic spiral galaxies in the northern sky – the Whirlpool Galaxy.
In 1845, William Parsons, 3rd Earl of Rosse, observed the Whirlpool with a 72″ reflecting telescope and first detected its spiral shape, making it the first known “spiral nebula.” This descriptor would continue to be used up into the early 20th century for objects like the Whirlpool, especially as astrophotography began to reveal previously unseen structures. Nearly 150 years after this galaxy’s discovery would we finally have evidence that these types of objects reside outside of the Milky Way, changing them from “spiral nebulae” to “spiral galaxies.”
Though 31 million light-years away, the Whirlpool is still one of our Milky Way’s nearby neighboring spiral galaxies. It is also one of the closest Seyfert galaxies to the Milky Way. Named after Dyer Observatory’s founding director Dr. Carl K. Seyfert, galaxies of this type have actively accreting supermassive black holes at their centers that produce core emission not seen in other galaxies.
The Whirlpool Galaxy (NGC 5194) is often referred to as “M51” as it is entry 51 on 18th-century comet sleuth Charles Messier’s list of comet lookalikes. (Messier’s list contains 110 galaxies, nebulae, and star clusters that tend to be some of the brightest and easiest to spot.)
The Whirlpool Galaxy is also categorized as a “grand design” spiral because of the two very prominent, symmetrical arms, and it is thought that the definition of the spiral arms may have been enhanced by gravitational interactions with its companion galaxy, a bright dwarf galaxy known as NGC 5195. Long-exposure images will reveal a stream of dark dust stretching from M51 in front of NGC 5195. This “tidal bridge” is a result of gravitational interactions between the two galaxies. Even deeper exposures taken under very dark skies will reveal a faint cloud of stars emanating out from NGC 5195. In the featured image above, the brighter portions of this star cloud appear as three points protruding out that are often referred to as “the crown.”
M51’s spiral arms are noticeably blue due to the concentration of hot, young stars present in the galaxy. Spiral galaxies like the Whirlpool often have lots of the raw star-forming material (gas and dust) within their spiral arms, and since hot, luminous blue stars do not live long lives, their presence signals to astronomers that star formation must have been fairly recent. NGC 5195 does not appear to have undergone recent star formation due to its relatively dull color.
Some spiral galaxies exhibit multiple prominent arms, some exhibit a couple, and other “peculiar” spiral galaxies may only exhibit one spiral arm due to factors such as interactions with other galaxies. Other spiral galaxies don’t really show any well-defined arms and are known as”flocculent” spirals. NGC 5195 may be one such spiral.
Though it is detectable with binoculars under dark skies, a moderately sized telescope is needed to be able to note anything besides a faint smudge in M51. Despite the pair’s proximity to the Big Dipper (part of the constellation Ursa Major), they are technically in the adjacent constellation Canes Venatici, the hunting dogs. M51 is placed conveniently well in the sky, making it one of the easiest spiral galaxies to find. Just locate the naked-eye star Alkaid at the end of the Big Dipper’s handle and head a smidge towards the brighter stars of Canes Venatici.
