What later became known as the Whirlpool Galaxy was discovered on this date in 1773 by Charles Messier while hunting for objects that could confuse comet hunters (because they were diffuse). He designated it in his catalogue (still used by some people) as M51. Its companion galaxy, NGC 5195, was discovered in 1781 by Pierre Méchain, although at the time it was not known that it was interacting with M51. In 1845, William Parsons, 3rd Earl of Rosse, employing a 72-inch (1.8 m) reflecting telescope at Birr Castle in Ireland, found that the Whirlpool Galaxy possessed a spiral structure: the first of what were called “nebulae” at the time to be known to have one. These “spiral nebulae” were not recognized as galaxies until Edwin Hubble was able to observe Cepheid variables, (pulsing stars that provide benchmarks of distance), in some of these “spiral nebulae,” which provided evidence that they were so far away that they must be entirely separate galaxies.
The advent of radio astronomy and subsequent radio images of M51 unequivocally demonstrated that the Whirlpool and its companion galaxy are indeed interacting. Sometimes the designation M51 is used to refer to the pair of galaxies, in which case the individual galaxies may be referred to as M51A (NGC 5194) and M51B (NGC 5195).
With the recent SN 2005cs derived estimate of 23 million light year’s distance, and an angular diameter of roughly 11.2′, it can be inferred that M51’s bright circular disk has a radius of about 43,000 light-years. Overall the galaxy is about 35% the size of the Milky Way. Its mass is estimated to be 160 billion solar masses. A black hole, surrounded by a ring of dust, is thought to exist at the heart of the spiral. The dust ring stands almost perpendicular to the relatively flat spiral nebula. A secondary ring crosses the primary ring on a different axis, a phenomenon that is contrary to expectations. A pair of ionization cones extend from the axis of the main dust ring.
The very pronounced spiral structure of the Whirlpool Galaxy is believed to be the result of the close interaction between it and its companion galaxy NGC 5195; specifically, it passed through the main disk of M51 about 500 to 600 million years ago. In this model, NGC 5195 came from behind M51 through the disk towards us, and made another disk crossing as recently as 50 to 100 million years ago until it is where we observe it to be now, slightly behind M51.
Stars are usually formed in the center of the galaxy. The center part of M51 appears to be undergoing a period of enhanced star formation. The present efficiency of star formation, defined as the ratio of mass of new stars to the mass of star-forming gas, is only ~1%, quite comparable to the global value for the Milky Way and other galaxies. It is estimated that the current high rate of star formation can last no more than another 100 million years or so.
Significant compression of hydrogen gas occurs within the galaxy that leads to the development of star-birth regions. In pictures of M51 these show up as the bright blue ‘knots’ throughout the spiral arms. Hydrogen is the most common physical component of the interstellar medium (the vast space between stars and planetary systems in galaxies). It forms huge, very diffuse, clouds throughout the entire galaxy. When large sources of gravitational pull pass nearby, such as other galaxies, gravitational interactions produce compression waves that sweep through these hydrogen clouds. This causes some regions of the previously diffuse gas to compress into tight pockets of opaque and dense gas; these are dust lanes often seen in the spiral arms. In regions where the concentration and density of gas reaches a critical value, further collapse under its own gravitational pull occurs, and stars are born at the center of the collapse, where the gas is compressed so strongly that fusion initiates.
When this happens, these new-born stars consume huge amounts of gas causing them to expand, shine even hotter, and finally sweep away the surrounding layers of dust and gas by increasing efflux of the stellar wind. The gigantic proportions of the clouds out of which they are born means stars seldom, if ever, are created in isolation.
Talking about star formation, galaxies, interstellar distance and so forth always leaves me in awe. Just writing that the Whirlpool Galaxy contains 160 billion solar masses is staggering enough. Talking about galaxies intersecting every 100 million years or so is simply incomprehensible. It’s not something I can really get my mind around. I have enough trouble thinking about the size of the earth relative to our own star, which, in itself, is a small fry in comparison with other stars in the universe. Then there’s light years. What a phenomenal distance: the distance it takes light to travel in a YEAR!! Astronomers such as Carl Sagan were driven by the marvels of the universe towards atheism. I’m driven in the opposite direction, but not in ways you might think. These observations lead me to reject hopelessly naïve conceptions of creation and whatnot, and instead to ponder not only the vastness of the universe but also the vastness of our ignorance of it, and the pitiful inadequacy of the tools we are using to investigate it. We know just a little bit more than NOTHING, yet pride ourselves on how much we think we know. Hopeless arrogance. Let’s cook instead. That’s more manageable.
If you search online you’ll find hundreds of galaxy-themed desserts, most of them swirly colorful things reminiscent of the Whirlpool Galaxy. This site is typical:
Failing your own imagination (and squirt bottles full of colored icing and chocolate, there’s this video: