Looking Up: Zub A Dub Dub, 3 Stars In A...

May 7, 2018

"Zubenelgenubi and Friends" Moderately bright Zubenelgenubi is the star just off the upper right hand limb of an eclipsed Moon in this telescopic view from Port Elizabeth, South Africa. Its fainter companion star, is seen here on the far right. Astronomer Francois du Toit reports that both stars were visible to the unaided eye on the night of May 4th, 2004 during the Moon's total eclipse phase.
Credit Francois du Toit / nasa.gov

This week on Looking Up we learn about an interesting star system, and also how to pronounce Zubenelgenubi.

There is a very interesting star to see in the southern Colorado sky right now, and it’s pretty easy to find this month. Why? Because it has a giant ball of gas nearby as a marker. That’s right, the massive planet Jupiter, the brightest thing in our sky after the Moon and Venus is just below the difficult to pronounce star that is in the constellation of Libra.

The star’s name? Glad you asked, it’s Zubenelgenubi, try saying that three times fast. Zub, as I’ll call this star for the rest of this episode may not look like much, but it represents what you might call the ying and yang of double stars. You see, Zub is actually a triple star system. The two bigger components, about 77 ly from Earth, are both very bright, relatively speaking. But something else is even more remarkable. As it turns out, these two stars orbit each other, but they are not very close together at all. To give you a sense of the distances involved, Pluto is about 39 times farther out from the Sun than the Earth. We call that 39 Astronomical Units. These two Zub stars are 5500 AUs apart. That means they are over half a trillion miles apart. They take roughly 200,000 years to complete one orbit.

And here it gets even weirder. One of the two Zub stars is itself a double star. But these two stars are not far from each other at all! In fact, they are only a few tenths of an AU apart. Far out and squeezed together all in one star system.

If you’d like to take a closer look at Zubenelgenubi or any of the other wonderful and amazing things in the sky, please visit csastro.org for a link to information on our monthly meetings and our free public star parties.