Castor: The Sextuplet Set of Stars

Castor is a bright star in the constellation Gemini that, along with Pollux, is one of the two main guideposts for the asterism that is sometimes nicknamed "The Twins."

At magnitude 1.58, Castor is the 20th brightest star in Earth's night sky. It is also relatively close to the planet, at an estimated distance of 51 light-years from Earth.

A closer examination of the star with a telescope actually reveals it is made up of many. What naked-eye observers see as Castor is actually the combined light of six stars, ranging from main sequence stars to dwarfs. [The Brightest Stars in the Sky: A Starry Countdown]

Past observations with the European Space Agency's XMM-Newton satellite of this system revealed a variable set of X-ray flares, suggesting that the "schizophrenic" system may see most of its X-ray radiation from huge eruptions, NASA stated.

Locating Castor

Gemini is the most northerly constellation of the Zodiac. It appears to lie on its side as a long rectangle with the stars Castor (alpha Gemini) and Pollux (beta Gemini) on the left. Castor's location is:

  • Right ascension: 07 hours 34 minutes 36.0 seconds
  • Declination: +31 degrees 53 minutes 18 seconds

Castor and Gemini in mythology and history

The name "Castor" flows from Greek and Latin mythology (the name is Latin, while the mythology is from both cultures).

According to legend, Castor and his brother, Pollux, were both believed to be twin gods who assisted sailors who were shipwrecked, and who also were willing to entertain gifts to bestow sailors with good sailing breezes, according to Encylopedia Britannica.

The twins were sometimes referred to as the Dioscuri in Greek mythology. The typical storyline said that Castor was the son of Tyndareus (a mortal) and Pollux a son of Zeus (a god), but both had the same mother, Leda.

The term "Gemini" is what we now use for the constellation to which Castor belongs. Gemini is perhaps most famous now as the name for a 1960s-era NASA spacecraft that carried two men into orbit. The program was an important precursor to Apollo as it tested space activities such as docking and rendezvous, or the act of reliably bringing two spacecraft together in space.

What we see as Castor is actually the combined light of six stars. Castor AB is a pair of A-type stars that orbit each other. Around each of these stars is an invisible dwarf star. South of this pair is YY Gem, another binary composed of two dwarf stars. (Image credit: European Space Agency/EPIC team)

Castor the sextuplet

Telescopic observations of Castor began to reveal a more complicated story of the single star. Today, NASA says the system is made up of six stars:

  • A pair of main-sequence A stars, Castor AB, that orbit each other every 467 years
  • An "invisible" dwarf that orbits Castor A
  • Another dwarf that orbits Castor B
  • A pair of stars just south of the Castor AB complex, called YY Gem. This is also a binary system with two dwarfs that whip in their respective orbits around each other every 19 hours.

In 2000, the European Space Agency's XMM-Newton orbiting telescope's EPIC camera revealed that these binary stars are also large sources of X-rays with temperatures in the millions of degrees, NASA stated.

"Throughout the 25-hour observation of the sextuplet, the EPIC images reveal that all three X-ray sources are variable, indicating giant releases of energy that can evolve in a few minutes or over several hours," NASA wrote in 2000.

"But the frequency of the flaring on Castor is quite surprising. At no period during XMM-Newton's observation was the emission constant, perhaps indicating that almost all of the observed X-ray radiation stems from giant eruptions."

— Elizabeth Howell, Contributor

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Elizabeth Howell
Staff Writer, Spaceflight

Elizabeth Howell (she/her), Ph.D., is a staff writer in the spaceflight channel since 2022 covering diversity, education and gaming as well. She was contributing writer for for 10 years before joining full-time. Elizabeth's reporting includes multiple exclusives with the White House and Office of the Vice-President of the United States, an exclusive conversation with aspiring space tourist (and NSYNC bassist) Lance Bass, speaking several times with the International Space Station, witnessing five human spaceflight launches on two continents, flying parabolic, working inside a spacesuit, and participating in a simulated Mars mission. Her latest book, "Why Am I Taller?", is co-written with astronaut Dave Williams. Elizabeth holds a Ph.D. and M.Sc. in Space Studies from the University of North Dakota, a Bachelor of Journalism from Canada's Carleton University and a Bachelor of History from Canada's Athabasca University. Elizabeth is also a post-secondary instructor in communications and science at several institutions since 2015; her experience includes developing and teaching an astronomy course at Canada's Algonquin College (with Indigenous content as well) to more than 1,000 students since 2020. Elizabeth first got interested in space after watching the movie Apollo 13 in 1996, and still wants to be an astronaut someday. Mastodon: