One of the most often questions asked about astronomy is “how do we know?” Unlike physics and chemistry, which allow for direct, hands-on experimentation, astronomy is a science that must be done remotely. The other challenge is that processes in space typically play out over time periods far longer than a human lifetime. Therefore we can’t actually “witness” stellar evolution from beginning to end; we can only observe various stages and infer that one leads to another. The skies of February allow even amateur astronomers to witness the steps of stellar evolution, all within a relatively small area of sky.
Star birth begins with a nebula. A nebula is a gigantic cloud of dust and gas, mostly hydrogen. In dense enough regions gravity causes the cloud to collapse. As the gas compresses it heats up to the point where nuclear fusion can begin, and a proto-star is formed. To see this stage of stellar evolution, turn your telescope to the three points of light below the belt of Orion the Hunter. The center point looks a bit fuzzy to the naked eye but the telescope reveals it to be the Great Orion Nebula. This is a classic example of a stellar nursery and the four stars in the center, known as the trapezium, are newly formed stars.
The next stage of stellar evolution is main sequence. This is where 90 percent of all stars spend 90 percent of their lives. Sirius, in the constellation Canis Major is an example of a main sequence star. It is a white star, which means it is very hot. Stars like Rigel in Orion are even hotter. However, Rigel is not a main sequence star. It is a Blue Supergiant, which means it is far larger and more massive than other stars. It also means that Rigel will not live as long as stars like the sun. Stars that burn hot have very short lives.
When a star like Rigel reaches the end of its life cycle, it swells and cools becoming a red super giant, like Betelgeuse. Red super-giants like Betelgeuse will experience a spectacular end. As the outward pressure from nuclear fusion wanes, the crushing pull of gravity causes the star to collapse. The collapsing outer layers rebound off the core and are blasted out into space as a super nova. The remains of a super nova are characterized by M1, the Crab Nebula, just off the tip of one horn of Taurus the Bull.
Smaller stars, like our sun, do not meet such violent ends. Instead, their outer layers are gently puffed off into space, more like an expanding soap bubble than an explosion. This forms what is known as a planetary nebula. A beautiful example of a planetary nebula lies just 8 degrees southwest of the star Pollux in Gemini the Twins. This planetary nebula, known as the Eskimo Nebula, appears as a fairly bright star. By using averted vision the faint nebulosity can be detected.
No human has ever observed the complete life cycle of any one star. The process of stellar evolution takes millions or billions of years. However, by careful observation of the various stages, we can piece together how the process occurs. The bright constellations of February offer us a wonderful chance to see each of these stages without having to move our telescopes very far.