365DaysDate: March 13, 2009

Title: Unlikely Twins in Gemini

Podcaster: Christopher Crockett

Organization: Lowell Observatory

Description: High over head in winter skies, the constellation Gemini represents the twins Castor and Pollux and contains the two bright stars of the same names. While ancient astronomers may have considered this pair to be “twin stars”, modern astrophysics has revealed that these two stars couldn’t be more different. In this podcast, I will introduce you to Gemini and we will explore the so-called “twins” whose lives have taken very different paths.

Bio: Christopher Crockett is a University of California, Los Angeles graduate student currently working as a predoctoral fellow at Lowell Observatory. His research involves searching for planets and brown dwarfs around very young stars (“only” a few million years old). It is hoped that the results from this research will help constrain models of planet formation and lead to a better understanding of where, when, and how often planets form. Chris is also passionate about astronomy outreach and education and will talk for hours about the Universe if you let him.

Today’s Sponsor: This episode of “365 Days of Astronomy” is sponsored by Edmund Featherstone who uses podcasts to help maintain an interest in astronomy when light pollution hides the starry sky–I have no particular sponsorship to suggest. I just enjoy listening to these podcasts (plus the Astronomy Cast ones). Since I moved to London over 30 years ago I haven’t had much opportunity to observe the night sky (seeing any stars is something which happens a couple of times a year!) so I find that these productions fill a gap and help to maintain an interest.


Hello.  This is Christopher Crockett from Lowell Observatory.

In my February 9 podcast, I introduced you to some of the wonders contained within the constellation Orion.  This month, we’re going to explore a close companion of the Great Hunter.  Standing above and to the left of Orion (or below and to the right for our south-of-the-equator listeners) is the constellation Gemini, the Twins.  The northernmost of the zodiac constellations, Gemini is well placed shortly after sunset in mid-March high and to the south when viewed in the Northern Hemisphere.  I usually see Gemini as two stick figures with their arms interlinked.  They stand in the night with their feet to the south and their faces high overhead.  The most prominent features of Gemini, and the target of this podcast, are the two bright stars marking the heads of the twins: Castor and Pollux.

The names for the twin stars come from the Greek myth of the twins Castor and Polydeuces (Pollux is the Latin name).  The Greeks tell us that Zeus, in the form of a swan, seduced the Queen of Sheba, Leda.  The very same night, Leda slept with her husband Tyndareus.  Both couplings resulted in Leda giving birth to quadruplets.  Zeus fathered Polydeuces and the infamous Helen of Troy, while Tyndareus laid claim to Castor and Clytemnestra. Polydeuces and Helen, being children of Zeus, were immortal.  The other two siblings were not.  The myth describes Castor and Polydeuces growing to be very close and even coming to dress and look alike.  They were both accomplished warriors and accompanied Jason and his Argonauts on their quest for the Golden Fleece. When the mortal Castor eventually died, the distraught Polydeuces pleaded with Zeus to place them together in the sky so that could remain with one another for eternity.  They now watch over the winter sky as the constellation Gemini.

Modern observations of the twins reveal they could not be more different.  Castor and Pollux, like most stars within a constellation, are not physically related.  The two stars are roughly 18 light years apart; Castor sits at about 50 light years from Earth and Pollux, 34 light years.

The closer star of the pair, Pollux, is the brightest star in Gemini and the 17th brightest star in Earth’s skies.  Astronomers classify Pollux as an “orange giant”.  The star’s orange color is indicative of it’s surface temperature: roughly 4500 °C  (or 8200 °F) which makes it about 1500 degrees cooler than our Sun.  The “giant” classification tells astronomers something about what’s going on deep in the core of the star.  In last month’s Orion podcast, we discussed how a star is born when the core temperature and density become high enough to start fusing hydrogen nuclei to form helium.   I also mentioned that as stars age they fuse progressively heavier elements in their cores.   Pollux is a good example of what happens in between.  Stars spend about 90% of their lives creating helium out of the abundant hydrogen that makes up a star’s bulk.  The process of turning hydrogen into helium provides the energy required to produce the outward gas pressure necessary for the star to hold up its own mass against self-collapse.  But, like any fuel source, there is only a finite amount of hydrogen in the star’s central regions and it is eventually depleted.  When the hydrogen in the core runs out the star, for a time, loses the energy source needed to hold itself up.  The star reacts by beginning to collapse.  As the outer layers fall inward, the temperatures and densities in the core begin to climb.  At core temperatures approaching 100 million degrees, conditions are now ripe for the newly formed helium nuclei to begin fusing to form the elements carbon, oxygen, and neon.  With the onset of helium fusion, the star’s collapse is once again halted for the time being.  The stars outer layers actually “puff up” in response to the new energy source driving the star to become larger – hence the classification as a “giant”.  This is where we find Pollux.  It has entered the helium-burning phase of its life.  Interestingly, the star continues to fuse hydrogen only the hydrogen burning is no longer taking place at the star’s center but in a shell surrounding the core.

Helium burning is a relatively short period of a star’s lifetime, lasting only about 10-100 million years.  If 10 million years still sounds like a long time, remember that these stars typically live for several billion years.  If the Sun’s eventual 10 billion year lifetime were condensed to a typical human lifetime of 80 years, that 10 million years would only be about a month long.  Think of the helium phase as one Christmas shopping season in your life.

In 2006, astronomers confirmed the presence of a planet in orbit around Pollux!  This is one of only a handful of stars that you can see with your naked eye and also has a planetary companion.  This planet, creatively named “Pollux b”, is a gas giant like our own Jupiter or Saturn but weighing in at about two and a half times the mass of Jupiter.   The orbit of Pollux b places it a hair further from Pollux than Mars is from our Sun.  Despite that distance, the radiation incident on the planet’s surface is roughly 12 times greater than what we receive on Earth from the Sun!  This is because Pollux burns with the intensity of 32 Suns.  And because Pollux is considerably larger than our Sun, it would appear roughly five times larger in the skies of Pollux b than the Sun appears to us on Earth.

We next turn our gaze towards Castor and find a very different star system.  I emphasize “system”, because it turns out Castor is not a single star but something more akin to a small stellar cocktail party.  The Italian astronomer Giovanni Cassini discovered in 1678 that Castor consists of two stars, Castor A and Castor B, locked in orbit around one another.  This may have been the first time anyone had seen gravitationally bound objects outside of our Solar System.  Today, these two companions can be seen with any modest backyard telescope.

A and B are pretty similar.  Unlike Pollux, they are still burning hydrogen in their cores but at about twice the mass of the Sun, they are burning at a higher temperature, about 9000 °C or 16,000 °F!  They are, quite literally, “white” hot!

But that’s not all.  In the 19th century, both Castor A and B were found to have companions of their own.  That is, Castor A and B aren’t just two stars orbiting one another, they are actually two binary star systems!  A quadruple system!  The smaller, fainter companions to A and B can not be seen directly because they are much dimmer than and very close to their parent stars.  In fact, the distance between each parent and its companion is so small that trying to see them as separate stars is roughly equivalent to discerning a person lying down on the Moon from here on Earth, a feat no current telescope could manage.  Astronomers learned of their presence using indirect methods similar to how we find extrasolar planets today.

The nature of these two dim companions is difficult to discern.  They are most likely very cool (“only” a couple thousand degrees), dim, red stars known as “red dwarfs”.  Though much dimmer than their brilliant cousins, red dwarfs may actually be the most common type of star in our Galaxy, accounting for more than half the stars in the Milky Way!  Because of their tight orbits, both of these dwarfs orbit their brighter companions relatively quickly.  Castor A’s companion orbits in just 9 days while Castor B’s partner swings around once every 3 days!

To make things more complicated, the two bright stars of Castor have a third star in orbit around both of them named, not surprisingly, Castor C.  What’s more, and I really hope you’re writing all of this down at this point, Castor C is itself a tightly packed binary system!  This third binary consists of another pair of very dim red dwarfs – the combined light of the pair is only about 5% that of our Sun.  They travel around Castor A and B on a very wide, slow orbit which takes about 10,000 years to complete.  10,000 years is considered by many historians to be the length of human civilization.  The last time this pair was at this point in its orbit around Castor A and B, your ancestors were most likely discovering agriculture.  The two members of this red dwarf binary system orbit each other much more quickly, however.  They are locked in a very tight orbit that takes only 20 hours to complete.   For perspective, the planet Mercury, the closest planet to our Sun, takes 88 days to complete its orbit.  Our Moon can make it once around the Earth in about 29 days.  Imagine two stars, each containing about 60% the mass of our Sun, whipping around one another in a mere 20 hours!!

When all is said and done, Castor is actually six gravitationally bound stars – a triple binary system!

Castor and Pollux, the twins from Greek legend, are far more different than the Greeks could have ever imagined.  One is a helium-burning orange giant and home to a Jupiter-class planet; the other is a young, sextuple star system.  On these cool March evenings, the twins shine brilliantly appearing as nothing more than two adjacent, unassuming points of light while modern astronomy has revealed that they are nothing alike, and anything but ordinary. One can only imagine what stories the other hundred billion stars in our Galaxy have yet to reveal to us!

End of podcast:
The 365 Days of Astronomy Podcast is produced by the New Media Working Group of the International Year of Astronomy 2009. Audio post-production by Preston Gibson. Bandwidth donated by and wizzard media. Web design by Clockwork Active Media Systems. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at or email us at Until tomorrow…goodbye.

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