See Inside February 2010

Mysteries of How a Star Is Born

Making a star is no easy thing

Courtesy of NASA, ESA and the Hubble Heritage Team (STScI/AURA)

If there is anything you think astronomers would have figured out by now, it is how stars form. The basic idea for how stars form goes back to Immanuel Kant and Pierre-Simon Laplace in the 18th century, and the details of how they shine and evolve were worked out by physicists in the first half of the 20th century. Today the principles that govern stars are taught in middle school, and exotica such as dark matter dominate the headlines. It might seem that star formation is a problem that has been solved. But nothing could be further from the truth. The birth of stars remains one of the most vibrant topics in astrophysics today.

In the simplest terms, the process represents the victory of gravity over pressure. It starts with a vast cloud of gas and dust floating in interstellar space. If the cloud—or, more often, a dense part of such a cloud called a core—is cool and dense enough, the inward pull of its gravity overpowers the outward push of gaseous pressure, and it begins to collapse under its own weight. The cloud or core becomes ever denser and hotter, eventually sparking nuclear fusion. The heat generated by fusion increases the internal pressure and halts the collapse. The newborn star settles into a dynamic equilibrium that can last millions to trillions of years.

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