RUSSELL KIGHTLEY / SCIENCE PHOTO LIBRARY RUSSELL KIGHTLEY / SCIENCE PHOTO LIBRARY
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Measuring a galaxy. Conceptual artwork of a spiral galaxy being weighed on a balance, and measured with compasses and a ruler. This represents the ability of astronomers to estimate the mass and size of galaxies and other astronomical objects. The figures shown are similar to estimates for our own Milky Way galaxy. The mass shown, 10 to the power of 42 kilograms (kg), is equivalent to 2000 billion times the mass of our Sun. The diameter of about 34 kiloparsecs is roughly equivalent to 10,000 light years. The spiral galaxy is orbited by dwarf galaxies (small white dots on arcs), as is the Milky Way.
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