A Star is Born

Horsehead nebula seen by Hubble Space Telescope Horsehead nebula seen by Hubble Space Telescope
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November 26, 2001


Vast clouds of gas and dust are swirling throughout our Milky Way galaxy. Some of these clouds are stellar nurseries, places where thousands of stars like our Sun are being born right now. These clouds are huge, about 100,000 times the size of our solar system and many thousands of times the mass of our Sun.

Life Rises from a Death Spiral

So just how does a run-of-the-mill, dark cloud of gas and dust become a star-forming region? Astronomers know that the cloud needs to be stirred up. They think a nearby exploding star, called a supernova, can trigger this motion. A supernova is the fiery death of a massive star, which causes gas to move up to 10,000 or 20,000 kilometers (approximately 6,000 to 12,000 miles) per second. Once this occurs, the moving gas and dust in the cloud collect more in some areas than others. The gravitational attraction of this slight clump attracts more gas and dust, which then creates stronger gravity, which in turn attracts more gas and dust. Pretty soon, this clump of matter is on its way to becoming a star!

These clumps are the very youngest stars. At this point, they are 300 to 500 times the size of our solar system, yet these stars are difficult to see. For one thing, they are still surrounded by a dark cocoon of matter, deep inside the larger cloud of gas and dust. Out of the billions of stars in our galaxy, we currently know of about only 50 stars at this stage of development. However, as we develop better detectors, we're finding more baby stars. Upcoming missions, such as the JPL-managed Space Infrared Telescope Facility, will aid in understanding this and later stages of star formation.

Gravity Moves into Action

In the next stage of star development, gravity pulls more and more matter from the surrounding nebula, a concentration of gas and dust, into these clumps. Over a period of 10,000 to 100,000 years, the central `knot' of matter in each mass concentration gets denser and warms up. The clump glows simply from gravitational contraction; energy literally comes from matter falling into the core of the clump. Eventually, the cocoon of matter disperses and the teenage star is more easily seen.

After this long birthing process, the toddler star is able to break out of its natal cocoon of matter. This occurs first through the north and south poles, in part because streams of matter move away from the poles of the star at about 100 to 300 kilometers (approximately 50 to 200 miles) per second. Exactly how and why these jets of matter form is not well-understood, but they are seen in nearly every star in this age range. These jets can create dramatic structures and motions as they literally smash into the rest of the nebula. In fact, many protostars, or forming stars which have not yet begun to shine, are found not from observations of the protostar itself, but inferred from observations of the matter pushed around by the jets.

Over a period of about 100,000 to 3,000,000 years, the disk thins out, and the jets slow down and turn off. Sometime in this stage, the protostar stops accumulating significant amounts of matter from the disk and nebula. Although much smaller than when it started out, the system is still pretty large. The disk is about twice as large as the solar system, and the protostar is about twice as large as the Sun.

Planets Begin to Form

Over the next 40 million years, the protostar contracts and warms up, and the disk steadily thins out and cools off. The disk cools off enough for clumps of matter to form in the disk. These clumps push the matter in the disk around, smashing into other clumps, and eventually forming planets. This is why planets in the same solar system are all essentially in the same plane and going the same way around the Sun -- it is the same orientation and direction of rotation of the original disk from which the planets formed. Understanding this stage is crucial to understanding exactly how and where planets form in the disk, and many scientists at JPL spend a lot of time thinking about how this might work.

After about 50 million years, the star finally stops contracting. The relentless push of gravity has compressed the core, heating it up until the star turns hydrogen into helium in its core. This process releases energy, causing the star to glow brightly. The pressure of all this energy trying to get out of the star stops the contraction caused by gravity. The forming planets still need more time to cool off and even combine into larger planets, more like what we see in our own solar system.

Depending on how far away from the parent star the planets have formed, there is a chance that life could begin to form on some of them. Then things can get REALLY interesting ...