A star in the night sky is a wonder to behold. But where does it come from? And how is it born? During the evolution of the universe, in places where matter was slightly denser, atoms began to clump together under the force of gravity, forming huge clouds of dust and gas. Gradually, parts of these clouds condensed further under gravity until they became extremely dense and hot. The scene was now set for star formation, the culmination of a process of gravitational contraction lasting 10 million to 15 million years. When the temperature at the center of such dense matter reaches several million degrees Centigrade, nuclear reactions begin, fusing atomic nuclei and forming heavier elements. The tremendous energy released in this reaction exerts an outward pressure that stops the mass from contracting further, holding it up against its own gravity. And voila! A new star is born.
Over their lifetimes, stars form heavy elements that are spewed out as gas and dust into interstellar space through stellar winds and the death of stars in supernova explosions. Concentrations of this gas and dust produce the right conditions for new star formation, and are known as stellar nurseries. So as stars die, they allow for new star formation, recycling the required raw materials. In addition, the shock waves from supernova explosions provide energy that helps trigger the contraction of gas clouds to form stars.
Under the influence of gravity, the first stars gathered together to form galaxies, and galaxies have continued to form and merge with one another in an ongoing process. Given that our own galaxy has about 400 billion stars, and there are about 170 billion galaxies in the observable universe, it’s hard to even begin to imagine how many stars there must be overall.
So what exactly is the fertility rate of galaxies, the rate at which the stars within them are born? Because stars vary in size, the rate is standardized so that it is measured in solar masses, or the mass of our Sun. Here in our own Milky Way, the rate is still uncertain, with estimates varying between about one and eight stars the size of our Sun per year. The current estimates of star formation may seem low, considering the huge number of stars in our galaxy. To create all those stars the rate must have been much greater in the past.
Recent research has shown that there is tremendous variation in the rate of star formation in different galaxies, let alone in different time periods. Using the Smithsonian Astrophysical Observatory’s Submillimeter Telescope Array on Mauna Kea in Hawaii researchers discovered a galaxy known as GOODS 850-5 that is generating more than 4,000 new stars a year. This galaxy is about 12 billion light-years from Earth which means that astronomers are seeing the light from it as it was when the Universe was only 1.7 billion years old.
Astronomers using NASA’s Hubble and Spitzer space telescopes have recently discovered that one of the most distant galaxies known, GN-108036, is generating about 100 Sun-sized stars a year. This seems surprising because the galaxy is about five times smaller and 100 times less massive than our Milky Way, yet it produces far more stars. Bear in mind, however, that the galaxy is 12.9 billion light-years from Earth, so we are seeing it as it existed in the very distant past. The Milky Way itself was formed very early in our universe’s history, so the findings for GN-108036 may give us some insights into the way our own and other early galaxies looked and functioned. The early universe would likely have had a much higher rate of star formation than that of today because galaxies would then have been smaller and denser, as well as closer together, with collisions between them more common. It is in fact known that bursts of star formation occur after galaxies collide or have close encounters with one another.
So what of the future? As the expansion of the universe continues to accelerate, with stars and galaxies moving farther apart at an increasing rate, it seems likely that the rate of star formation will continue to slow down, though no one knows for sure how it will all end. We’re talking billions of years into the future though, so for now, the brilliant stars of the night sky are ours to enjoy, and hold the promise of joy for generations to come.
Copyright © Amanda Jermyn