If we could travel back in time several thousand years to observe the night sky, we would notice two main differences compared to the night skies of today.
The most obvious would be the lack of a sky glow produced by a myriad of outdoor lights. This glow washes out the richness and beauty of the starry sky, except for those few that live far away from light polluted cities.
The second difference is more subtle. While most of the stars and constellations would appear the same, a few stars would be out of place. One of the most noticeable is Arcturus, fourth brightest star in the night sky. Arcturus, a red giant, is a prominent star on clear spring evenings. It can be found by following the curve made by the handle of the Big Dipper. As you observe it, can you detect a hint of red or orange color?
What makes Arcturus different? It is a galactic invader, a star that was once part of a small galaxy that was ripped apart and absorbed by our Milky Way Galaxy in the distant past.
Our Sun is in motion around the center of our galaxy. Most of the stars around us are in similar orbits, so we all move more or less together, like a large flock of migrating birds. Because of this, stars appear motionless over time spans as short as several thousand years. In contrast, Arcturus has a very different orbit. It is currently plunging down through the galactic plane, moving sideways to the local stellar “traffic stream,” That is why this bright star shifts its position noticeably with time. It takes Arcturus slightly less than 1600 years to move one degree in the sky, twice the apparent width of the full moon.
So, as the weather begins to warm, look off to the east and greet this galactic visitor.