Stargazing in Seattle
Mars, the only major planet in the evening sky this month, is visible high in the West on clear nights after sunset. Last month, Mars was near the star Aldebaran but has moved nearly 20 degrees east along the ecliptic over the past few weeks, toward the stars of the constellation Gemini. Currently, Mars forms an equilateral triangle with two red giant stars. One is Aldebaran (which marks the eye of the bull in Taurus). The other is Betelgeuse (one of Orion’s shoulders). Mars glows with a similar red color as these two stars, but for a different reason. Mars appears red because it reflects sunlight from the red rocks that form its surface. Betelgeuse and Aldebaran shine because they are glowing balls of hot, mostly hydrogen gas. A star’s color is determined by how hot this gas is. Most of the stars we see in the sky are very hot and radiate white or even blue-white light. Red giant stars like Aldebaran and Betelgeuse, however, are nearing the end of their life as a main sequence star. They have become larger (and brighter) but their surface is cooler and glows with a soft red color, like an ember in a fireplace. Star colors are subtle but most of the other bright stars in Orion are hot blue-white class O stars (the hottest class) or white class B (almost as hot), and provide a good color contrast to these two red giant stars.
On the evenings of April 16 and 17, Mars will be joined by a crescent moon, as Luna makes her monthly trip around the Earth. The best time to look is toward the west in the evening, shortly after it gets dark.