This is another of my favorite Hubble images, taken in January of 2005. It's an image of the Whirlpool Galaxy, NGC 1594, better known as M51, and, just to its right, its much smaller companion galaxy, NGC 5195. Sometimes the two galaxies are referred to as M51a and b.Technically, the image to the right is a four-color mosaic, composed of 96 slightly overlapping images taken with four different filters (hence the lovely colors). The Whirlpool galaxy, as its name and image suggests, is a spiral galaxy, like Earth's own Milky Way.
The smaller "companion" galaxy has enough of a gravitational pull on the gasses of the larger galaxy, that the gravitational forces are triggering the formation of new stars, appearing as bright red clusters in the larger image, the red filter highlighting the consequent hydrogen emissions from the newly born stars. The resolution of the 2005 Hubble images is such that astronomers can study the very detailed, intricate structures embedded in the spiral arms of the larger galaxy, in the form of "spurs" of dust spinning off the two arms, as well as a large central dust "disk" in the nucleus. The way the disk is formed, and some of the data from images using non-visible light waves suggest that there may be a black hole at its center.
When astronomers first began to study M51, they thought that the two galaxies were joined; Hubble's extraordinary detail, and the evidence from radio astronomy, makes it very clear that the two galaxies are separate. NGC 5195, the smaller companion galaxy, is passing behind the Whirlpool, making measured progress over hundreds of millions of years. As the smaller galaxy passes, however, its gravitational pull is enough to tug at the larger Whirlpool, creating gravitational ripples or waves in its central disk. When the waves pass through nearby dense clouds of gas, they exert pressure on the gassy matter near the inner edges of the Whirlpool's spiral arms, collapsing the clouds and generating new stars, visible in the reddish-pink areas along the edge. Eventually, mature star clusters emerge, the bright blue clusters along the arms.
About four years before the Hubble images were generated, in 2001, Cal Tech's Spitzer telescope team used their own infra red data, plus x-ray data from NASA's Chandra X-ray Observatory with earlier Hubble data to create this composite image of M51. The purple point-like objects are black holes and neutron stars in binary star systems, revealed by Chandra; Chandra also detects the presence of hot gasses in the spaces between stars, which appears as a diffuse glow in the image. The earlier Hubble optical data (green) and the infra red data from Spitzer (red in the image) show long streams in the spirals arms, filled with stars and gas, intermingled with dust. The blue points are young, and still very hot stars, that are still emitting lots of ultraviolet, captured with Cal Tech's GALEX telescope. There's another gorgeous image, taken with the Earth-based telescope National Science Foundation's 0.9-meter telescope located at Kitt Peak National Observatory near Tucson, Arizona, and using a mosaic CCD camera here, from 2001.
In this pan-and-zoom version of the 2005 Hubble image of the heart of M52, the blue areas are fairly young, very hot stars, while the clusters of red are hydrogen regions, the nurseries of new stars. Look closely at the edges of the two spiral arms, and you can see streams of dust and dense matter being pulled away from them. The spiral structure of M52 was most likely caused by the smaller NGC 5195/M51b passing through the central disk of the larger galaxy, 500 or 600 million years ago, from behind, and then crossing again 50 to 100 million years ago, and ending up where it is today, slightly behind M51. Those younger stars are one reason that the Whirlpool is so very bright compared to the surrounding objects in the night sky.
M51 is roughly 30 million light years from Earth, yet it is also one of the brightest spirals in the night sky. One of the interesting things about M51 is that it can be seen quite well from Earth, using fairly basic telescopes (on a dark night, M51 is even bright enough to see with binoculars). To find it, follow the handle of the Big Dipper's dipper away from the cup, all the way to the last, eastern-most bright star in the handle. Then, look just a bit to the south-west, and the brightest object is likely to be M51, the Whirlpool galaxy, and its companion. The Whirlpool galaxy is so bright in fact that in 1845 in Ireland William Parsons the Third Lord of Rosse, turned his then state-of-the art seventy-two inch telescope to one of the brightest spots in the night sky, in the constellation Canes Venatici. Parsons observed the Whirlpool galaxy well enough to create the drawing you see here. You'll note that Rosse's drawing very clearly shows the dark center of M51, the spiral arms, and the smaller companion galaxy, streaming away from the center.
Both Albert Boime, a UCLA art historian, and Simon Singh in his book Big Bang: The Origins of the Universe have both pointed out that there are remarkable similarities between the sketch Lord Rosse made of M51 in 1845, and Vincent Van Gogh's painting of the night sky in Arles, France, "The Starry Night" 44 years later. If you look at the image of Van Gogh's painting, you can see the larger, central spiral, with a smaller spiral spinning off of it. You can even see the dark center of M51. Compare Van Gogh's picture, to the drawing Lord Rosse made, embedded above. We'll never know, of course, but it is tempting to speculate about Van Gogh seeing Rosse's drawing, or possibly, catching a glimpse of the Whirlpool and its companion through a friendly astronomer's telescope.
