No, it’s pretty light.
Explanation: This joke is a pun that plays on the double meaning of “light.” The Aurora Borealis, also known as the Northern Lights, is a natural light display in the sky. The word “light” can refer to both the visual phenomenon and something that isn’t heavy. The punchline “pretty light” humorously suggests both the weightlessness of light and the beauty of the Aurora Borealis.