There’s no use using an armadillo, or even your local whippet, unless you’ve made sure the definition is clear.Parsec pär'sÄ-c, in astronomy, basic unit of length for measuring interstellar and intergalactic distances, equal to 206,265 times the distance from the earth to the sun, 3.26 light-years, or 3.08 Ã- 1013 km (about 19 million million mi).ĭistances to the closest stars can be determined through measurement of their trigonometric parallax. Whether we use lions or tigers or Crabs, astronomers make sure to define the units we’re using. And if we’re talking about a particular object, say a black hole binary system called GX339-4, and it’s only five thousandths as bright as the Crab, we say it’s 5 milliCrab bright.īut buyer beware! The brightness of the Crab is different depending on what energy of X-ray light you’re looking at, and it also changes over time. So every X-ray astronomer knows how bright a Crab is. VLA/NRAO/AUI/NSF Chandra/CXC Spitzer/JPL-Caltech XMM-Newton/ESA and Hubble/STScI Dubner (IAFE, CONICET-University of Buenos Aires) et al. Image of the Crab Nebula where red is radio from the Very Large Array, yellow is infra-red from the Spitzer Space Telescope, green is visible from the Hubble Space Telescope, and blue and purple are X-ray from the XMM-Newton and Chandra X-ray Observatories respectively. The Sun has an apparent magnitude of –26! We call these brightness values “magnitudes”. Notice here that a brighter star has a lower number. In the second century BC, the ancient Greek astronomer Hipparchus looked up at space and gave the brightest stars a value of 1 and the faintest stars a value of 6. If we want to measure how bright something is, astronomical units of measurement get even weirder. For example, the centre of our very own galaxy, the Milky Way, is about 8,000 parsecs away from Earth, or 1.6 million AU. Parsecs are handy for measuring even bigger distances because 1 parsec = 206,265 AU. Parsec is short for “parallax second”, and if you remember your trigonometry, this is the length of the hypotenuse of a right-angle triangle when the angle is 1 arcsecond (1/3,600 degrees) and the “opposite” side of the triangle is 1 AU. The Sun and Earth are 149 million kilometres apart, and we give this distance a name: an astronomical unit (AU).įor an even twistier unit of distance, we use the parsec (insert Han Solo Kessel run joke here). Astronomical units and parsecsĪstronomers also use comparisons to measure how far apart things are. For example, the Andromeda galaxy is approximately three trillion times the mass of the Sun (or 3×10 41 – that’s a 3 followed by 41 zeros – Astros). On the other hand, Astro’s mass is how much stuff he’s made of – and it’s the same no matter which planet he’s on.Īstronomers use Earth and the Sun as handy units to measure mass. This would be easy to do on Earth, even easier somewhere with lower gravity like the Moon, and much harder somewhere with higher gravity like Jupiter. We can think of weight in terms of how hard it is to lift an 18kg Astro off the ground. On Earth the weight of an object, like Astro, depends on the mass of Astro and the gravitational force pulling him down to the ground. 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). Laura DriessenĪt an even larger scale, consider the star Betelguese: its radius is 83,000 Earths, or 764 times the radius of the Sun. Astro the whippet contemplating the wonders of the Universe (probably).
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