![]() Mass is a measure of how much stuff something is made of. If we want to measure how heavy an asteroid is, we could do it with camels – but in space we’re more interested in mass than in weight. So if we want to talk about how big Betelgeuse is, it’s much more convenient to use the radius of the Sun as our unit, instead of the radius of Earth (or to describe it as 632 billion Astros). Astro the whippet contemplating the wonders of the Universe (probably). Laura DriessenĪt an even larger scale, consider the star Betelguese: its radius is 83,000 Earths, or 764 times the radius of the Sun. That number of Astros is a bit ridiculous, which is why we adjust our unit choice to one that makes more sense. Jupiter’s radius is 11.2 Earths, or 85 million Astros. ![]() Earth’s radius is about 638 million cm, or 7.5 million Astros. The basic problem is that lots of things in space are way too big for our familiar units. Stefan Kraft / Wikimedia, CC BY-SA Why do astronomers use such strange units? A platypus that is approximately 1/18th of the size of Asteroid 2023 FH7. But what about a pulsar-wind nebula with a brightness of a few milliCrab? That’s where things get weird. The idea of a planet that’s 85% the mass of Earth seems straightforward. ![]() These outlandish comparisons are the invention of Jerusalem Post journalist Aaron Reich (who bills himself as “creator of the giraffe metric”), but real astronomers sometimes measure celestial objects with units that are just as strange. You may have heard about an asteroid set to fly near Earth that is the size of 18 platypus, or maybe the one that’s the size of 33 armadillos, or even one the size of 22 tuna fish.
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