Seeing through the problem
But an international team of researchers has figured out a way to peer inside interstellar clouds by looking through them. The technique involves focusing on light from more distant stars.
Looking at one relatively nearby cloud in this way, they measured the amount of starlight absorbed by various regions, and then calculated density and temperature differences from the core to the outskirts. The results show an object on the verge of collapse.
A star is about to be born.
"These measurements constitute a major breakthrough in the understanding of dark clouds," said João Alves of the European Southern Observatory. "For the first time, the internal structure of a dark cloud has been specified with a detail approaching that which characterizes our knowledge of stellar interiors."
Alves, along with Charles Lada of the Harvard-Smithsonian Center for Astrophysics and Elizabeth Lada of the University of Florida, discuss the work in the Jan. 11 issue of the journal Nature.
Star birth -- What's known
Interstellar space is a misnomer. In fact, the area between stars is laced with gas and small dust particles. In places, this stuff collects into clouds, called nebulae. A typical nebula is about 74-percent hydrogen, 25-percent helium, and 1-percent heavier elements.
The temperature and density of a nebula determines whether it can become a stellar nursery or not. By nature, gravity works to pull the gas and dust together. But, by virtue of its activity, the material also exerts an outward pressure. The warmer the material, the more active it is, and the more it pushes outward.
In certain types of nebulae, where the temperature is cool enough, the force of gravity overcomes the outward pressure. The gas and dust becomes more tightly packed, and eventually a gravitational collapse occurs.
In this way, one or more clumps form. Scientists call them protostars.
And if all goes well, the battle between thermal pressure and gravity transforms a protostar into what scientists call a main-sequence star. For this to happen, the temperature has to rise enough at the core of the object to jump-start thermonuclear fusion, which converts hydrogen to helium and powers all stars, including our Sun.
The heat from this burning creates thermal pressure, which keeps the bulky mass from collapsing under its own weight. Balance is created, and the star can shine for billions of years.
Humble beginnings: A Bok globule
The Alves team studied a
, a type of nebula or dark cloud known for its small size and compact, nearly spherical shape. The object, called Barnard 68, was a good candidate for study, having a particularly simple shape and well-defined edges. It is also relatively nearby, just 410 light-years away.The cloud contains about twice as much mass as our Sun, but of course this mass of gas and dust is widely distributed: Barnard 68 covers an area about 12,500 times bigger than the distance from Earth to the Sun. The cloud, therefore, is many billions of times less dense than the air you breathe.
Still, the stars behind Barnard 68 are mostly obscured, their light scattered by dust. The effect is similar to what makes a sunset on Earth appear red. Blue light, having a short wavelength, is the most scattered and doesn't get through while longer-wavelength red light passes through more easily.
Barnard 68 is dense enough, in fact, that if you could reshape it and slip it between us and the Sun, Earth would be plunged into eternal darkness.
Turning on the lights
Using large telescopes with sensitive instruments at the La Silla and Paranal observatories in Chile, operated by the European Southern Observatory, the research team made long exposures that revealed 3,708 background stars. The light of surrounding stars -- those not obscured by the cloud -- provided a basis for estimating what the light should have looked like from the obscured stars.
By measuring the actual colors of these stars, after their light had been scattered by dust, the scientists calculated the amount of dust in various parts of the cloud, which also tells them how much gas is present, because gas and dust are known to stick together.
They found that the density of Barnard 68 increases steadily from the edges to the center, which indicates a fine balance between gravity and thermal pressure. The balanced physics, they note, is much like what's found in a main-sequence star.
But this balance is tenuous, they said.
Barnard 68 appears to be at the beginning of a collapse, expected to take place in about 100,000 years. If things work out, a new equilibrium will be achieved and a new star -- one very similar to our Sun at its birth several billion years ago -- will grace the heavens.
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