Begin with water, because it is a familiar substance and a useful metric for density. Table 5 lists various estimates for densities throughout the universe. To gain a better understanding of these densities, students can compare the number of water molecules in a cubic meter (or centimeter) for each density.

Table V. Average Densities of Objects in the Universe.

 Density Number of Water Molecules per cubic metric unit Water 1 gram/cm3 3.345 x 1022 molecules per cm3 Rock 2.67 gram/cm3 8.9 x 1022 molecules per cm3 Earth 5.5 gram/cm3 1.8 x 1023 molecules per cm3 Sun 1.4 gram/cm3 4.683 x 1022 molecules per cm3 Jupiter 1.33 gram/cm3 4.45 x 1022 molecules per cm3 Solar System 2 x 10-12 gram/cm3 7 x 1010 molecules per cm3 Molecular Clouds 2 x 10-22 gram/cm3 10 molecules per cm3 Milky Way Galaxy 6 x 10-23 gram/cm3 2 molecules per cm3 Local Group 7 x 10-37 gram/cm3 24 molecules per km3 Critical Density of the Universe 1 x 10-29 gram/cm3 1/3 molecule per m3

Certain calculations have been made using only lower limits for mass calculation, making the density calculations also lower limits. Other evidence has suggested that this is a lower limit for the mass calculation for most of the universe. However, astronomers to this day are uncertain of the explanation for this evidence that suggests the mass of the universe is ten times greater than the mass we can see in the form of stars and dust. For more information on this subject, called dark matter, see http://map.gsfc.nasa.gov/m_uni/uni_101matter.html.

Overall, students should begin to understand although we live in a place that is particularly dense, most of the universe is not like Earth. Matter tends to clump together, even on the largest scales of the universe that we know today. This leaves quite a bit of empty space in the universe. This empty space becomes notable once the jump is made from the solar system to molecular clouds. The molecular clouds are simply dust clouds which eventually break up and condense to form stars. The last line in the table is the Critical density of the Universe. What this means will become more clear after studying the Big Bang and expanding universe, but in a nut shell, the critical density is the density that is needed in the universe to make it stop expanding and collapse on itself due to gravity. With current data, it does not appear that the universe is dense enough on the whole to collapse. Astronomers, however, will still actively study the possibility of collapse until enough data is collected to accurately determine the density of the universe.

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