You may have heard about an asteroid 18 times the size of a platypus approaching Earth, or an asteroid the size of 33 armadillos, or even 22 tuna fish.
These strange comparisons are the invention of Jerusalem Post journalist Aaron Reich (who describes himself as “the inventor of the giraffe scale”), but real astronomers sometimes measure celestial bodies in completely strange units.
The idea of a planet with 85% the mass of Earth is straightforward. But what about a pulsating wind nebula with a brightness of just a few milligrammes? This is where things get weird.
Why do astronomers use such strange units?
The fundamental problem is that many things in space are too large for our familiar units.
Consider the star Betelgeuse: its radius is 83,000 Earth radii, or 764 times the radius of the Sun. Therefore, if we want to talk about the size of Betelgeuse, it is more appropriate to use the radius of the Sun as a unit rather than the radius of the Earth (or describe it as 632 billion Astros).
If we want to measure how heavy an asteroid is, we can do so with beauty—but in space we’re more interested in mass than weight. Mass is a measure of how much matter an object is made of.
On Earth, the weight of an object like an astro depends on the astro’s mass and the gravitational force pulling it toward Earth.
We can think of weight in terms of how hard it was to lift the 18 kilogram Astros off the ground. This would be easier to do on Earth, easier somewhere with low gravity like the Moon, and more difficult somewhere with high gravity like Jupiter.
Astro’s mass, on the other hand, is how much material it’s made of—it’s the same no matter which planet it’s on.
Astronomers use the Earth and the Sun as manual units of mass. For example, the Andromeda galaxy has three trillion times the mass of the Sun (or 3 x 1041 – that’s 3 followed by 41 zeros – Astros).
Astronomers also use comparisons to measure how far away objects are. 149 million kilometers separate the Sun and Earth, a distance we call an astronomical unit (AU).
For a more standard unit of distance, we use the parsec, short for “parallax second”, and if you recall trigonometry, the angle is 1 arcsecond (1/3600 degree) and the “opposite” side of the triangle is 1AU.
Since 1 parsec = 206265 AU (astronomical unit), a parsec is handy for measuring large distances. For example, the center of our galaxy, the Milky Way, is about 8,000 parsecs or 1.6 million astronomical units (AU) from Earth.
If we want to measure how bright something is, astronomical units are even more different. In the second century BC, the ancient Greek astronomer Hipparchus looked at space and assigned a value of 1 to the brightest stars and 6 to the faintest stars.
Note here that the brighter star has a lower number. We call these brightness values ”magnitudes”. The apparent magnitude of the Sun is -26!. Each step of the scale is a 2.512-fold difference in brightness.
The light we see with our eyes is, for obvious reasons, called “visible” light. The light we use to take pictures of your bones is called X-ray light.
When astronomers use X-ray light to observe the sky, we sometimes measure the brightness as a “crab,” a fast-spinning neutron star (or pulsar) in the remnant of a supernova that is so bright as we see it through our X-rays. – Ray telescopes. It is so bright in X-ray light that astronomers have used it to calibrate their telescopes since the 1970s.
So every X-ray astronomer knows how bright a lobster can be. If we talk about a specific object, let’s say a black hole binary system called GX339-4, which is only five thousand “grabs” bright, let’s say 5 “milligrabs” bright.
But buyer beware! The brightness of the “crab” varies depending on the energy of the X-ray light you see, and it changes over time.
Whether we use lions, tigers or crabs, astronomers make sure to specify which units we are using. There is no point in using “armor” until you make sure the definition is clear.
The report was prepared by Laura Nicole Dressen, postdoctoral researcher in the University of Sydney’s Department of Radio Astronomy.
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