As the huge interstellar cloud collapses into many fragments, it is useful to consider the processes inside one of the individual cloud fragments as it continues to develop into a star. Most of these fragments are about one to two solar masses but can be about 100 times the size of the Earth's present solar system. The temperature of the cloud is about the same as when it started to condense, but it would have an increased density of about 1012 particles per cubic meter in the center of the cloud fragment. The temperature has not changed because the density of the cloud is still too low to capture photons emitted from the gas, and the energy of the photons escapes to space instead of being absorbed by the cloud. However the very center of the cloud may experience significant warming by this stage, perhaps to 100 K, as the gas is denser there and can absorb more of the radiation produced in the gas. As the cloud continues to shrink, it becomes denser so that eventually the cloud begins to trap the radiation across large regions, and the temperature of the whole cloud increases. This causes an increase in the internal pressure (equated with temperature and speed of particle movement), which grows strong enough to overcome the force of gravity that was pulling the cloud together. At this stage, the contraction of the cloud stops and the fragmentation of the original cloud stops. The orion nebula, in the constellation orion, provides beautiful examples of cloud fragments that are lit by the absorption of radiation produced in the cloud.
The time from initial contraction to the end of fragmentation of the interstellar cloud may take only
10,000 6,000 Temperature (Kelvin)
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10,000 6,000 Temperature (Kelvin)
Hertzsprung-Russell diagram showing stellar evolution. The diagram plots luminosity on the vertical axis and surface temperature or spectral class on the horizontal axis.
a few tens of thousands of years, and by this stage the size of the cloud fragment is roughly the same as Earth's present solar system, but the central temperature of the cloud has now reached about 10,000 K, while the peripheral temperatures at the edge of the cloud are still close to their starting temperatures. since the edges of the cloud are cooler than the denser interior, they are also thinner, and the cloud takes on the shape of a thick ball in the center surrounded by a flattened and outward thinning disk.
The central density in this stage may be 1018 particles per cubic meter.
The center of the collapsed fragment is now roughly spherical, dense, and hot and begins to resemble an embryonic protostar, as it continues to grow in mass as gravity attracts more material into its core, although the size of the internal embryonic protostar continues to shrink since the force of gravity in the core remains greater than the internal pressure generated by the gas temperature. At this stage
Infrared and visible light composite of Orion Nebula, 1,500 light-years from Earth, taken by Spitzer and Hubble Space Telescopes, November 7, 2006. The image shows a region of star birth, including four massive stars at the center of the cloud, occupying the region that resembles a yellow smudge near the center of the image. Swirls of green reveal hydrogen and sulfur gas that is heated and ionized by the intense ultraviolet radiation from the massive young stars at the center of the cloud. The wisps of red and orange are carbon-rich organic molecules in the cloud. The orange-yellow dots scattered throughout the cloud are infant stars embedded in a cocoon of dust and gas. The ridges and cavities in the cloud were formed by winds emanating from the four super massive stars at the center of the cloud. (NASA Jet Propulsion Laboratory)
the embryonic protostar develops a protosphere, a surface below which the material is opaque to the radiation it emits.
The protostar in the center of the collapsed disk continues to shrink and grow in density, its internal temperature increases, and the surface temperature on its protosphere continues to rise, generating higher pressures. About 100,000 years after the initial fragment formed from the interstellar cloud, the center of the protostar reaches about 100,000,000 K, and free electrons and protons swirl around at hundreds of miles (km) per second, but temperatures remain below the critical value (107 K) necessary to start nuclear fusion to burn hydrogen into helium. The protosphere has a temperature of a few thousand K, and the radius of the protosphere places it at about the distance of Mercury from the sun.
At this stage the protostar can be plotted on an H-R diagram, where it would have a radius of 100 or more times that of the present sun, a temperature of about half of the sun's present temperature, and a luminosity that is several thousand times that of the sun. The luminosity is so high because of the large size of the protostar, even though the temperature is lower than the present temperature of the sun. The energy source for the luminosity and elevated temperatures in this protostar is from the release of gravitational potential energy during collapse of the interstellar cloud.
Pressures build up inside the protostar at this stage from the elevated temperatures, but the gravitational force is still stronger than the thermal pressures, so contraction continues albeit at a slower rate. Heat diffuses from the core of the protostar to the surface, where it is radiated into space, limiting the rise in temperature but allowing contraction to continue. If this did not happen then the temperatures would rise in the star and it would not contract enough to reach densities at which nuclear fusion would begin, and it would not form into a true star, but would remain a dim protostar.
The protostar continues to move down on the evolutionary H-R diagram toward higher temperature and lower luminosity as the surface area shrinks. Internal densities and temperatures increase while the surface temperature remains about the same,
Horsehead Nebula in the Orion molecular cloud complex. This image was produced from three images obtained with multimode FORS2 instrument at the second VLT Unit Telescope. (European Southern Observatory)
but the surface can be intensely active and be associated with intense solar winds such as those that characterize T-Tauri stars. A possible example of a protostar in this stage of evolution in the orion molecular cloud is known as the Kleinmann-Low Nebula, which emits strong infrared electromagnetic radiation at about 1,000 times the solar luminosity. some protostars are surrounded by dense dark dust clouds that absorb most of the ultraviolet radiation emitted by the protostars. This dust then reemits the radiation at infrared wavelengths, where it appears as bright objects. since the source of the radiation is cloaked in a blanket of dust, these types of structures have become known as cocoon nebula.
By the time the protostar has shrunk to about 10 times the size of the sun, the surface temperature is about 4,000 K and its luminosity is now about 10 times that of the sun. However, the central temperature has risen to about 5,000,000 K, which is enough to completely ionize all the gas in the core but not high enough to start nuclear fusion. The high pressures cause the gravitational contraction to slow, with the rate of continued contraction controlled by the rate at which the heat can be transported to the surface and radiated away from the protosphere. strong presolar winds at this stage blow hydrogen and carbon monoxide molecules away from the protostar at velocities of 60 miles per second (100 km/sec). These winds encounter less resistance in a direction that is perpendicular to the plane of the disk formed from the flattened interstellar cloud fragment, since there is less dust in these directions. In this stage, therefore, some protostar nebulae exhibit a strong bipolar flow structure, in which strong winds blow jets of matter out in the two directions perpendicular to the plane of the disk. This strange-looking structure eventually decays, though, as the strong winds blow the dust cloud away in all directions.
Approximately 10 million years after becoming a protostar, when the temperature reaches about 10,000,000 K and the radius about 6,200 miles (10,000 km), nuclear fusion begins in the center of the protostar. At this stage, a true star is born and is typically a little larger than our present sun, the surface temperature is about 4,500 K, and the luminosity is about two-thirds that of the present sun.
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