The Sun is the star at the center of our Solar System. The Sun governs our daily lives, and energy from the Sun has been essential for nearly all life on Earth for more than 3 billion years. But what if there were two, three, four, five, or even six Suns in our sky? That question might seem ridiculous, but it turns out that more than half of the stars in our galaxy are either in double star or multiple star systems that are held together by gravity.
Many prominent stars are gravitationally bound double stars, also called binary stars. These include Polaris (also called the North Star), and Sirius, the brightest star in the night sky.
It’s easy to resolve another famous double star using just your unaided eyes or binoculars. On these spring evenings, the Big Dipper rises high in the northern sky. Look carefully at the middle star in the “handle” of the Big Dipper. If you have excellent vision, you’ll notice that this star has a nearby companion. The brighter of the two stars is called Mizar, and the fainter star is called Alcor. If you can’t resolve this pair with your eyes alone, try using binoculars. A small telescope will reveal that Mizar itself consists of a binary pair, called Mizar A and Mizar B. Astronomers using advanced techniques have discovered that Mizar A, Mizar B, and Alcor are each binary stars. Amazingly, astronomers have concluded that Mizar/Alcor is not just a binary system, but a gravitationally bound six-star system!
Beyond the Sun, the very nearest star, 4.2 light years or about 25 trillion miles distant, is Proxima Centauri, a dim red dwarf star that is part of the Alpha Centauri triple star system. Proxima Centauri orbits two brighter, Sun-like stars, called Rigel Kentaurus and Toliman. Astronomers recently discovered and confirmed two exoplanets that orbit Proxima Centauri. One of these exoplanets, Proxima Centauri b, is similar to Earth in size and orbits within that star’s “habitable zone.” This means that temperatures on the planet’s surface could be Earth-like.
Is there life on planets in such multiple star systems? What could that life be like? If you saw the movie, “Star Wars”, then you know that the fictional planet “Tatooine” was in a double star system. The day/night and annual cycles for such a planet would be more complex than those in our single star system. How would organisms adapt to radically changing cycles of brilliant starlight and darkness? Such fascinating questions will challenge scientists for decades to come.