The stars of the Big Dipper are a wonderful guide to a handful of splendid galaxies and other deep-sky sights. Above and below the handle of the Dipper, most vividly, lie two stately face-on spiral galaxies that are visible, at least to some degree, in a small telescope. In dark skies, these two nearby galaxies display clear hints of a striking and naturally ubiquitous pinwheel shape that also reveals itself in the clouds of a hurricane or the seed arrangement in a sunflower.
Let’s begin with the most famous galaxy around Dipper’s handle, the Whirlpool Galaxy, which lies over the border in the constellation Canes Venatici. This showpiece object is just 3º southwest of the star Alkaid. An elegant face-on spiral, which is about 1/3 the diameter of our own galaxy, lies at a distance of about 23 million light years. And it’s not alone: the galaxy interacts with its much smaller neighbor NGC 5195, and the interaction has triggered an intense round of star formation, especially in the rich spiral arms of M51. In images of the galaxy, you can see evidence of this activity in the numerous pink emission nebulae and blue-white clots of new stars (see above). Images also show a bridge of stars that appears to connect the two galaxies.
As with most spirals, the core of M51 outshines the fainter spiral arms. It’s the core you can see in a pair of binoculars or a telescope. In light-polluted skies, you will only see the core, no matter how big your telescope, because the fainter spiral arms are overwhelmed by the artificially brightened sky. In a 3-inch telescope or larger, the core of NGC 5195 is also visible.
Urban stargazers often look to M51, see a couple of dim smudges, and wonder what all the fuss is about. But in dark sky, with a careful gaze and averted vision, the galaxy is transformed into a glorious– if faint– spiral swirl in an 8″ or larger telescope. Keen-eyed observers can see spiral structure in smaller scopes in dry and very dark sky.
The first to note the spiral structure of the Whirlpool Galaxy was the wealthy Irish amateur astronomer Lord Rosse. He used a homemade reflecting telescope, an immense instrument with a mirror some 6 feet in diameter which for decades was the largest telescope in the world. Rosse went on to carefully sketch about a dozen of these “spiral nebulae” in these days before photography.
From the heyday of Rosse through the 1920s, these “spiral nebulae” were fuel for a vigorous debate about the nature of the universe. Some believed these objects were relatively nearby and represented new solar systems forming from the cosmic ether. This view was backed up by the theories of Immanuel Kant who first suggested that solar systems might form as swirling spirals. Kant died in 1804, well before the spiral nebulae were sketched by Rosse.
Others believed that spiral nebulae were systems of millions of stars in their own right, well outside our own galaxy, which itself was just one “island” of stars amidst millions. This “island universe” theory was also proposed by Kant without evidence but with his characteristically uncommon insight. It was left to Edwin Hubble and others in the late 1920s to prove the island universe concept was more or less correct.
Now look above the handle of the Dipper to find the galaxy Messier 101, the second galaxy on this tour. It’s sometimes called the “Pinwheel Galaxy”, which is confusing because M33 in Triangulum and sometimes M99 in Coma Berenices take the same name. All these galaxies are beautiful face-on spirals, at least in photographs. But all are challenging objects to see visually because of their low surface brightness.
M101 is easy enough to find. It forms a nearly equilateral triangle with Mizar and Alkaid, the last star in the handle of the Dipper. You can follow four 5th-magnitude stars from Mizar on a line about 2º east. The last of these stars should share the same low-power field of view as M101.
Messier 101 is a respectable magnitude 7.9, but its light is spread over a diameter as large as the full Moon. So you need fairly dark sky to see it at all, and extremely dark sky before you have a chance of seeing spiral structure. In urban locations, the sky itself is sometimes brighter than the galaxy, which makes it invisible even in large scopes.
This is not an object that gives up detail with a quick glance. In a 3” or 4” scope at 50x, if the sky is dark enough to see the galaxy at all, M101 will appear as a round, featureless glow about 15’ across (1/4 degree). Increase to 100-125x to bring out scant structure in the halo and reveal the oval shape of the core. A larger telescope brings out more detail, naturally. In any scope, look carefully for structure using averted vision and a great deal of patience. If your sky is dark and clear, you may begin to trace out the patterns of the spiral arms.
Once you’ve seen M101 you’ve also seen M102. Charles Messier, according to many astronomers, mistook M102 for M101. So they are one and the same.
M101 is about 26 million light years away. It is a large galaxy, with a diameter 70% larger than the Milky Way, and it encompasses the mass of 100 billion suns.