Star formation is the process by which a cold, dark cloud of gas and dust is transformed into a brilliant star with a surface temperature anywhere from 3,000 to 50,000 K (4,900 to 90,000°F; 2,700 to 50,000°C). Many regions of the Milky Way galaxy are filled with cold clouds of gas that begin to contract, under certain conditions, as a result of their own gravitational attraction. As one of these clouds contracts, it heats up and tends to become spherical. The heating, however, produces pressure in the gas that counteracts the contraction, and eventually the contraction may s if the gravity and gas pressure balance one another. If the cloud has become hot enough to begin thermonuclear fusion reactions at its center, it can then sustain itself against its own gravity for a long time. Such a cloud is then called a star.
When one looks up on a clear night, stars are seen—thousands of them—glittering against the seemingly empty backdrop of space. However, there is something else out there; vast clouds of cold, dark gas and dust, visible only by the dimming effect they have on starlight shining through them. This is the interstellar medium, and it is the birthplace of the stars.
In most places the interstellar medium is almost a vacuum, a million trillion times less dense than air. In other places, however, there are much greater concentrations of clouds, sometimes so thick and dense that one cannot see through them at all. Such a cloud is the famous Horsehead Nebula in the constellation Orion. Often these clouds are enormous, thousands of times as massive as the sun.
Unlike the sun, however, these interstellar clouds have relatively weak gravity. The gravitational attraction between two particles decreases as the separation between them increases, and even in a huge cloud like the Horsehead Nebula, the matter is much more thinly distributed than in the sun. Therefore, the matter in the cloud tends not to condense. It remains roughly the same size, slowly changing its shape over the course of millennia.
Imagine a cloud, drifting along through the inter-stellar medium. The cloud is unthinkably cold, in excess of–400°F (–240°C). It is not very dense, but it is so large that it renders the stars behind it either invisible or as dim, red points of light. It is made mostly of hydrogen, and has had its present shape and size for thousands of years.
Then, something happens. A hundred parsecs away (about 190 trillion miles), a star explodes. It is a supernova, the violent end to a massive star’s life. An expanding, devastating blast races outward, forming a shock wave. It sweeps everything before it, clearing the space through which it passes of the interstellar medium. And eventually, it encounters the cloud.
The shock wave slams into the cloud. The cold gas and dust is violently compressed byles are squeezed together, their mutual gravitational attraction grows. So tightly are they now packed that they begin to coalesce under their own gravity. The shock has transformed the cloud: many parts are still thin and diffuse, but now there are multitudes of condensing blobs of gas. They did not contract by themselves before, but now they have been given the necessary impetus.
When a blob of gas condenses, energy is released, and one of the beautiful theorems of physics shows scientists that half the energy goes into heating the gas. So as the blobs in the disrupted cloud condense, they get progressively hotter. Eventually they begin to glow a dull red, much as an electric burner on a stove begins to glow when it becomes sufficiently hot.
This process of contraction cannot continue indefinitely. As the temperature in a contracting blob of gas becomes higher, the gas exerts a pressure that counteracts the inward force of gravity. At this point, perhaps millions of years after the shock wave slammed into the dark cloud, the contraction s"top">s. If the blob of gas has become hot enough at its center to begin thermonuclear fusion of hydrogen into helium, it will remain in this stable configuration for millions or billions of years. It has become a star.
Nature is filled with symmetries, and this is one of the most enchanting symmetries. The death of one star triggers the birth of new stars. Moreover, what of the rest of the dead star, the expanding blast of gas and dust that encountered no interstellar clouds? Eventually it comes to a halt, cooling and fading into darkness, where it becomes part of the interstellar medium. Perhaps, millions of years in the future, a shock wave will plow into it.
The scenario described above leads to a situation like that shown in the Great Orion Nebula. Brilliant, newly born stars blaze in the foreground, while the great cloud surrounding them glows in the background. This nebula glows because the intense radiation from the massive young stars near it is heating it. Contrast this with the Horsehead Nebula, which has no such sources of heat and therefore is dark.
These newly formed stars can themselves trigger star formation. Radiation—that is, light—exerts pressure on surrounding matter. The young stars in the Orion Nebula are huge by stellar standards, and their radiation is intense. Many of them lose mass continuously in a stellar wind that streams out into the cloud. After a few million years, the most massive of them will explode as supernovae. These effects can cause other parts of the neighboring cloud to begin contracting. Therefore, star formation might be able to bootstrap its way through an entire cloud, even if only part of the cloud is disrupted by a shock wave.
An interstellar cloud does not always have to be disrupted by a shock wave to form stars, however. Sometimes a cloud may collapse spontaneously, and the process describing this phenomenon was discovered by English astronomer James Jeans (1877–1947). Above the so-called Jeans mass, which depends on the temperature and density of the cloud, a cloud will break up and contract spontaneously under its own gravity. Large clouds can break up into numerous cloudlets this way, and this process leads to the formation of star clusters such as the Pleiades. Often, two stars will form very close to one another, sometimes separated by a distance less than that from the Earth to the Sun. These binary systems, as well as multiple systems containing three to six stars, are quite common. They are more common, in fact, than single stars: most of the stars seen at night are actually binaries.
An important avenue of research involves studying the cycle of star births and deaths in the galaxy. Formation of stars depletes the interstellar medium, since some of its gas goes into making the stars. Then, as a star shines, a small part of its matter escapes its gravity and returns to the interstellar medium. More importantly, massive stars return a large fraction of their matter to the interstellar medium when they explode and die. This cycle of depletion and replenishment is critically important in understanding the types of stars seen in the galaxy, and the evolution of the galactic system as a whole.
The advent of powerful new telescopes like the Hubble Space Telescope (HST) has opened astronomer’s eyes to new stars that may require new theories of formation. In 1997, the brightest star ever seen was discovered (by HST) at the core of the Milky Way galaxy. Named the Pistol star, it has the energy of 10 million suns and would fill the distance of the Earth’s orbit around the Sun. In addition, its stellar wind is ten billions times stronger than the sun’s solar wind. Astronomers conjecture that it was created from one to three million years ago. The Pistol star is about 25,000 light-years from the Earth; it is so turbulent that its eruptions create a gas cloud (the Pistol Nebula) that is four light-years across. It had been thought that a star so big could not have formed without blowing itself apart. So, in the 2000s, astronomers are re-examining their ideas about stellar formation partially based on the Pistol star, especially of supermassive stars near the centers of galaxies.
While some astronomers study the galactic or the interstellar medium, others study newly forming protostars. Protostars are hot, condensing blobs of gas that have not quite yet achieved starhood, and they are hard to observe for two reasons. First, the phase of star formation is quite short by astronomical standards, so there are not nearly as many protostars as there are fully formed stars. Second, protostars are often thickly shrouded by the remnants of the cloud from which they are forming. This makes them appear much dimmer, and so much harder to observe and study.
Fortunately, newly forming stars do have some observable characteristics. A disk of dust and gas may girdle a protostar. An exciting possibility is that these disks are pro"top">lanetary systems. Earth’s own solar system is thought to have formed from such a disk that surrounded the newly forming Sun, and disks around other stars such as Beta Pictoris may be current sites of planetary formation. Additionally, a protostar with a disk may produce two beams of gas that stream outward from its poles along the lines of magnetic field associated with the disk. These so-called bipolar out-flows are classic signatures of a protostar with a disk.
It is not necessary to observe only the Milky Way galaxy to find newly forming stars. Modern telescopes, including the currently operational Hubble Space Telescope and the James Webb Space Telescope (scheduled to be operational after 2013), are used to study star-forming regions in other galaxies. High-resolution observations can detect individual stars in the Milky Way’s satellite galaxies and in some other nearby galaxies. In distant galaxies, the regions of heated gas produced by new stars are visible. Observations of star formation in other parts of the Universe help confirm and give astronomers a broader perspective on the theories regarding star formation in the Sun’s own celestial neighborhood.
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Jeffrey C. Hall