The stellar life cycle, part one: A star is born
Every so often, I like to go into more detail about the life and times of stars we see night after night over Butler, provided clouds don’t get in the way. All of the stars we gaze upon are going through different stages of life. It’s not like the carbon-based life we thrive on, but like all living things, stars are born, live and die. The common thread to all three stages is gravity. When it comes to astronomy, you can almost say that gravity is everything. This week I want to take you to the maternity ward of stellar birth.
Since stars, including our sun, are basically huge balls of mainly hydrogen gas. It only makes sense that stars are born out of loose clouds of hydrogen. As it turns out, there are gigantic hydrogen gas clouds, trillions of miles in diameter, all around our home Milky Way galaxy and the billions and billions of other galaxies that make up our known universe. These nebulae are the birthplace of hundreds and thousands of stars, many of which are born at nearly the same time in these giant stellar nurseries.
There are many great examples of stellar birth in the night sky throughout the year. The easiest one to find and see is the Orion Nebula in the constellation Orion, best seen in late autumn, winter and early spring. It’s a little below the three bright stars in a row that outline the hunter's belt. Below Orion’s belt, there’s a line of three fainter stars in a shorter row that depict Orion's sword. Even to the naked eye, the middle star of the sword seems fuzzy. That's because it’s not a star, but a vast cloud of hydrogen gas over 80 trillion miles in diameter, and more than 1,400 light-years away, with just one light-year equaling nearly six trillion miles.
The best nebula in the summer isn’t quite as attractive, but it’s still well worth your time to check it out. It’s the Lagoon Nebula in the constellation Sagittarius the Archer, in the low south-southeast early evening sky. Sagittarius is supposed to outline a half-man, half-horse shooting an arrow. Good luck seeing that. Although, its brightest stars clearly outline a teapot, and that’s the constellation’s nickname. The Lagoon Nebula is not far above the teapot’s spout. It’s called the Lagoon Nebula because, when viewed through an optical telescope, it has a greenish tinge to it.
Speaking of telescopes for backyard observing, there are now relatively less expensive digital photographic telescopes that are easy to use. The images you get in just a few minutes show you much more color and detail. The most expensive ones are around $700. The one I recommend is the ZWO SeeStar 30S Pro.
So, how do these gigantic gas clouds, otherwise known as nebula, become the birthplace of stars? Here’s what happens. The gravitational influence from a passing star or group of stars, or the shock waves from an old exploding star in the distance, stirs up the nebulae. As this happens, random pockets of denser gas begin developing within the nebulae, and stellar birth gets cooking. Since these denser balls of hydrogen are more massive than the surrounding looser nebulae, they start acquiring a gravitational force that draws in more and more of the surrounding hydrogen. As this happens, these protostars become even more massive, giving them a stronger gravitational pull that allows them to pull in more and more of the surrounding gas.
Like a snowball rolling down a hill, these balls of hydrogen gas grow and grow like crazy until they become massive enough to become stars. They get “lit up” and start shining like stars when their nuclear fusion furnaces ignite deep in their interiors. That can’t happen until the giant hydrogen gas ball becomes so massive that its own gravitational force squeezes it so hard, causing tremendous pressure to build up in the center of the protostar. We’re talking to the tune of billions of pounds per square inch! Just like a giant pressure cooker, this drives the temperature up millions of degrees inside the core. When a critical level of heat is built up in the center of the protostar, the nuclear fusion process begins, and the star starts its life of shining. In the center of these nebulae, new gravitationally born stars light up the surrounding hydrogen gas like a fluorescent light by the tremendous radiation pouring out of these baby stars.
Next week in Starwatch, I’ll have more on the inner workings of stars. The dynamics are mind-blowing and controlled mainly by gravity.
Mike Lynch is an amateur astronomer and professional broadcast meteorologist for WCCO Radio in Minneapolis/St. Paul. He is also the author of “Stars: a Month by Month Tour of the Constellations,” published by Adventure Publications and available at bookstores and at adventurepublications.net. Contact him at mikewlynch@comcast.net.
