You may have heard about an asteroid that will fly near Earth the size of 18 platypusesor maybe the one that is the size of 33 armadillosor even one the size of 22 tuna fish.
These bizarre comparisons are the invention of Jerusalem Post journalist Aaron Reich (d bills itself as “creator of the giraffe metric”), but real astronomers sometimes measure celestial bodies with equally strange units.
The idea of a planet that is 85% of the Earth’s mass seems simple. But what about a pulsar wind nebula with a brightness of a few milliKrab? That’s where things get weird.
Why do astronomers use such strange units?
The fundamental problem is that many things in space are much too big for our familiar units.
Take my whippet Astro, who is 94 cm tall. The radius of the Earth is about 638 million cm, or 7.5 million Astros.
The radius of Jupiter is 11.2 Earths or 85 million astros. That number of Astros is a bit ridiculous, so we’re adjusting our unit choice to one that makes more sense.
On an even larger scale, consider the star Betelguese: its radius is 83,000 Earths, or 764 times the radius of the Sun. So if we want to talk about how big Betelgeuse is, it’s much more convenient to use the Sun’s radius as our unit, rather than the Earth’s radius (or to describe it as 632 billion Astros).
If we wanted to measure how massive an asteroid is, we could do it with camels – but in space we are more interested in mass than weight. Mass is a measure of how much material something is made of.
On Earth, the weight of an object, such as Astro, depends on Astro’s mass and the gravitational pull that pulls it toward the ground.
Read more: Explainer: what is mass?
We can think of weight in terms of how difficult it is to lift an 18 kg Astro off the ground. 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.
On the other hand, Astro’s mass is the amount of material it’s made of – and it’s the same no matter what planet it’s on.
Astronomers use the Earth and Sun as convenient units to measure mass. For example, the Andromeda galaxy is approx three trillion times the mass of the sun (or 3×1041 – that is a 3 followed by 41 zeros – Astros).
Astronomical units and parsecs
Astronomers also use equations to measure how far things are from each other. The sun and the earth are 149 million kilometers apart and we give this distance a name: an astronomical unit (AU).
For an even sharper distance unit, we’ll use the parsec (insert Han Solo Kessel run kidding here). Parsec is short for “parallax second,” and if you remember trigonometry, it’s the length of the hypotenuse of a right triangle when the angle is 1 arcsecond (1/3,600 degrees) and the “opposite” side of the triangle is is 1AU.
Parsecs are useful for measuring even greater distances, because 1 parsec = 206,265 AU. For example, the center of our own galaxy, the Milky Way, is about 8,000 parsecs from Earth, or 1.6 million AU.
When we want to measure how bright something is, astronomical units of measurement get even weirder. In the second century BC, the ancient Greek astronomer Hipparchus looked up into space and gave the brightest stars a value of 1 and the faintest stars a value of 6.
Note that a brighter star has a lower number. We call these brightness values ”magnitudes”. The sun has an apparent magnitude of –26!
Even more confusing than a negative brightness, every single step in magnitude is a difference of 2,512 times in brightness. The star Vega has an apparent magnitude of 0, which is two and a few times brighter than the star Antares with an apparent magnitude of 1.
Finally, the milliCrab
The light we see with our eyes is called “visible” light for obvious reasons. 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 in “Crabs”.
The Crab is a rapidly spinning neutron star (or pulsar) in the remnants of an exploded star that is extremely bright when viewed with our X-ray telescopes. It is so bright in X-ray light that astronomers have used it calibrate their telescopes since the 1970s.
So any X-ray astronomer knows how bright a crab is. And if we are talking about a particular object, say a black hole binary system called GX339-4and it’s only five thousandths as bright as the Crab, we say it’s 5 milliCrab bright.
But buyer beware! The brightness of the Crab is different depending on the energy of the X-ray light you are looking at, and also changes over time.
Whether we use lions or tigers or crabs, astronomers make sure they define the units we use. There’s no point in using an armadillo, or even your local whippet, unless you’ve made sure the definition is clear.
Read more: Beating Heart of the Crab Nebula