The names of the five stars closest to the Sun exemplify how confusing (or historically rich) astronomical nomenclature can be.
Proxima Centauri is the closest star. The second and third closest form a binary and are known collectively as α Centauri (or Rigel Kentaurus) and individually as α Centauri A and α Centauri B. The fourth closest is Barnard’s Star. The fifth is WISE 1049−5319, which is also known as Luhman 16.
The brown dwarf binary WISE 1049−5319 appears as a yellow disk in the center of this mid-wavelength IR image. CREDIT: NASA/IPAC Infrared Science Archive
Just how the five stars got their names depends, in part, on when they were first observed. Alpha Centauri is the third brightest star in the night sky. As such, it has been named by several cultures. Its Chinese name, 南門 (Nán Mén), means "Southern Gate." Arab astronomers called it رجل القنطورس (Rijl Qanṭūris), "Centaur’s Foot."
The name α Centauri originates in the first systematic stellar naming convention, which was devised in 1603 by the Bavarian astronomer Johann Bayer. "Centauri" ("of Centaurus") indicates the constellation that the star belongs to; the Greek letter indicates the star’s brightness rank in the constellation.
The other stars in the top five are not visible to the naked eye and weren’t, therefore, cataloged by Bayer, whose naming convention predated the invention of the telescope by eight years. Proxima Centauri and Barnard’s Star are both red dwarfs. Proxima was named by the astronomer who discovered it in 1915, Robert Innes. Barnard’s Star was named after Edward Barnard, who was the first to measure the star’s velocity across the sky in 1916.
Remarkably, the discovery of WISE 1049−5319 was published on arXiv just last month. Kevin Luhman of Penn State University and his collaborators identified the star—which is, in fact, a brown dwarf binary—in observations made by NASA’s Wide-field Infrared Survey Explorer spacecraft. The numbers after the spacecraft’s abbreviated name are the binary’s celestial coordinates.
Names like WISE 1049−5319 are now the norm in astronomy. Space-based and ground-based observatories whose sensitivity exceeds their predecessors tend to find many new objects. Although you can’t tell from its name that WISE 1049−5319 is a brown dwarf binary, you can presume from the WISE part of the name that it’s an IR source and that it’s faint (because if it were bright, it would have been discovered and named earlier). Using the coordinates to identify sources might seem long-winded compared to a serial number, but the coordinates are astronomically meaningful, whereas a serial number wouldn’t be.
Rakhat, α Centauri Bb, or both?
I was reminded of the quirkiness of astronomical names earlier this week, when I read a news story in New Scientist entitled "Closest exoplanet sparks international naming fight."
The dispute pits Uwingu, a startup company whose mission is to fund projects that inform the public about space science, against the International Astronomical Union, the world’s official arbiter of astronomical naming conventions and planetary nomenclature for planetary bodies.
By IAU-sanctioned convention, exoplanets are named after the the stars they orbit, with the addition of a lower-case letter: "a" designates the star; "b," the first planet discovered; "c," the second; and so on. Officially, the closest exoplanet to the Sun is called α Centauri Bb, but if you paid Uwingu $4.99, you could suggest a name. And if you paid $0.99, you could vote on the suggestions. Currently, the leading name for α Centauri Bb is Rakhat, which is what Mary Doria Russell called a planet that orbits the star in her 1996 science fiction novel, The Sparrow .
I think it’s great that a real planet is named after a fictional one. Granted, Rakhat conveys less astronomical information than α Centauri Bb does, but I don’t see why the two names can’t coexist. I doubt anyone would be confused.
What’s more, the IAU’s exoplanet naming convention, though simple and straightforward, does not necessarily yield neat, rational names. That’s because the stars that harbor exoplanets, like the five stars closest to the Sun, follow a mix of naming conventions. Exoplanet examples include 51 Pegasi b (the first one discovered), KIC 12557548b (the evaporating exoplanet), and HD 85512 b (a super-Earth).
Despite astronomy’s modest technological payoffs, the general public continues to fund astronomical research—thanks, in part, to the time and energy astronomers devote to engaging the public. By giving people the opportunity to name exoplanets, Uwingu is making them partners in a scientific enterprise. The IAU should support, not fight, such deep public engagement.
