Figure 7: The elemental composition of the solar system. The abundance of hydrogen is arbitrarily set to 1012 so that the smallest abundance in the graph is about 1. (Click on image for larger version.)|
III. Composition of the Universe
A. What It is, and How do we Know?
Astronomers seek to understand what the universe is made of. In practical terms, this means determining the relative abundances of the different elements. However, there is no direct way to measure the composition of the Universe as a whole. This is because different objects in the universe have different compositions. The different compositions reflect the different environments in which these objects are formed and their different histories. Matter clumps together in the form of stars, gas clouds, planets, comets, asteroids and meteors. So astronomers determine the composition of those objects, and ultimately attempt to deduce the overall make- up of the universe.
The composition of the universe is not static. We've seen that the universe started with just hydrogen and helium, and that heavier elements are made through fusion in stars and the explosive power of supernovae. So the first generation of stars was made only of hydrogen and helium. Thus, this first generation of stars could not be accompanied by rocky planets. As the universe aged, more light elements were turned into heavy elements. After the first generation of massive stars went supernova, they enriched space with heavy elements, allowing later generations of stars to have rocky planets.
The change in the elemental composition also happens at different rates in different places. Star formation is much faster in the dense cores of galaxies, so there will be more heavy elements there than in the slower-paced outskirts of the galaxies.
Composition is usually determined via spectroscopy. Each element gives off a unique signature of specific wavelengths of light, which are observed as bright lines in its spectrum. By measuring the relative intensities of these "emission lines" from different elements, it is possible to determine the relative abundances of the elements. When interstellar gas absorbs light, we see absorption lines, evidenced as dark lines in the spectrum. These lines can be used to tell us the composition of the gas cloud. The spectrum of a star shows the elements in the star's outer layers. The spectrum of a planet shows the elements on its surface and in its atmosphere. Meteorites found on earth, lunar samples, and cosmic rays are the few pieces of the Universe in which the elements can be directly separated and measured chemically.
Recommended activities: What's Out There?, Nickel-odeon, and Cosmic Abundances
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