When is a star born?

As with humans and animals, stars have a long period in the "womb" before being born. For humans, this period is only nine months. For stars, it can be several million years.

Before a star is born it is called a protostar. There are three main stages of growth. In the beginning the protostar is a loose, spherical cloud of very cold gas and dust. How cold? About a minus 400 degrees Fahrenheit (or -250 Celsius). This is called the Class 0 protostar stage. Inside the cloud a dense core is beginning to form. It takes about 10,000 years for a protostar to mature from an amorphous cloud into a somewhat spherical one.

Gravity is the main force powering this. Gas is simply flowing toward the core, like water down a hill. For the next 100,000 years, the cloud condenses and becomes more spherical and the core gets denser. The cloud and core begin to heat up. A new structure begins to develop -- something called a stellar disk. This is a rotating, flat plane of dust and gas that forms like a belt around the center of the spherical protostar. Picture Saturn and its rings, and you'll get a good idea of what a stellar disk looks like. This is the Class I protostar stage.

After nearly a million years for growth, the Class I protostar becomes a... you guessed it... Class II protostar. (There's a Class III stage, too; scientists often lump II and III together with the name pre-main-sequence star.) At this point, the protostar "looks" like a star: It is very spherical and hot. That stellar disk of gas and dust may develop into a protoplanetary disk, which would give rise to gaseous planets like Jupiter and rocky planets like Earth.

Over the next million years, the protostar core grows denser and hotter. Get ready for star birth.

The signature of star birth is nuclear fusion. That's when the star begins burning (fusing) hydrogen atoms. Consider our Sun: Great heat and pressure in the Sun's core enables the Sun to fuse four hydrogen atoms into one helium atom, giving off much energy which heats the Earth. Once the protostar core gets hot and dense enough (density is related the pressure, helping to squeeze atoms together), a nuclear reaction takes place. The star turns on.

If the new star is the size of our Sun, it will have a long life. The star will burn for another 10 billion years. Larger stars burn faster, though. Very massive stars -- over 10 times more massive than the Sun -- use up all their nuclear fuel in only about 100 million years.

Class 0 protostars made the news this month. Scientists at NASA Goddard detected X-ray light for the first time from a nearby Class 0 protostar. The mystery is how such a cold cloud can generate X rays, which is a high-energy form of light associated with hot objects that are millions of degrees. It seems that the scientists are seeing activity in the protostar core, which is much hotter and more energetic than many had assumed. In a way, the scientists captured the first "ultrasound" of an embryonic star deep in its protostellar womb.

This week's question is provided by Christopher Wanjek. Mr. Wanjek is a science writer supporting the Beyond Einstein initiative, a roadmap to understand the forces of nature beyond General Relativity and Quantum Mechanics through the study of the Universe from the Big Bang to black holes.