A Side of Science: How the Cow got its name

Name tag with the words: hello my name is AT2018cow

In my latest video, I talked about a mysterious object that astronomers have been studying. This object, called AT2018cow has been nicknamed the Cow – but how did this puzzling object get such an unusual name?

Discovery image of AT2018cow.
Credit: Credit: Stephen Smartt/ATLAS

When a transient object is spotted – that is, an object that suddenly brightens in the sky – it is assigned a name of the form ATyyyyx – the AT stands for “Astronomical Transient”, the yyyy is the year, and x is a combination of letters. For the first 26 transient events reported in a year, the x is simply an uppercase letter of the alphabet. The next 676 are lower-case two-letter combinations “aa”, “ab”, “ac”,… through “zz.” For example, the 4th transient of this year was called AT2019D while the 42nd was called AT2019ap.

After the two-letter combinations are exhausted, they move on to three-letters starting with “aaa.” In the case of AT2019cow, it was the 2441st transient detected in 2019*. So it was just a coincidence that this unusual object happened at just the right time to have such a fun name!

Supernova 1994D was the 4th transient object spotted in 1994.
Credit: NASA/ESA

The number of transients discovered each year has been growing as our ability to keep a continuous eye on more and more of the sky has improved. Up until the late 1980s, astronomers usually only needed single-letter designations, with the occasional need for the two-letter ones. However, since the 1990s, we’ve needed the two-letter names every year. We didn’t need the three-letter designations at all until the mid-2010s.

Some of the names of these transients will change as they are classified. For example, once AT2015cn was found to be a supernova, it’s name was officially changed to SN2015cn – where the “SN” is for supernova.

The International Astronomical Union – the IAU for short – is the only source of officially recognized names for cosmic object. Sometimes there are fun names given to something – like the Cow – but these are not official designations.**

And, in case you missed my Space Cow video, be sure to check it out.

Sources and additional resources:

* The first 26 were A-Z, the next 676 were aa-zz, then another 676 were aaa-azz, another 676 were baa-bzz, and then finally 387 were caa-cow.

** Unfortunately, none of those name-a-star services are assigning names to stars that astronomers will recognize. There are a number of conventions for naming astronomical objects, and typically stars and objects outside our solar system are named by their position in in the sky. While this convention creates a phone-number-type name, it makes it easy to know exactly which source is being discussed.

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