Two spirals shine in the northern sky tonight: M51 and M101. Although technically the same magnitude, one is more challenging than the other. Looking for a sky event this week? Check out our full  Sky This Week  column.  May 7: Io crosses Jupiter Let’s take advantage of the moonless early-evening sky to enjoy two lovely face-on spiral galaxies. We’ll start at the brighter, easier option: M51, also known as the Whirlpool Galaxy. And it’s easy to find — it lies just 3.5° southwest of Alkaid (Eta Ursae Majoris), the star at the very end of the Big Dipper’s handle. At magnitude 8.4, this 11’-wide spiral is visible as a circular fuzz even in small scopes, though the larger your aperture, the better. Scopes 6 inches or larger will start to show more detail. Look for a small, brighter center surrounded by a faint halo — that’s the galaxy’s nucleus and spiral arms, respectively. You may also notice a compact, bright spot nearby.…