George's Random Astronomical Object

Object 2: HS Hydrae

Podcast release date: 26 August 2019

Right ascension: 10:24:36.8

Declination: -19:05:33

Epoch: J2000

Constellation: Hydra

Corresponding Earth location: Approximately the centre of the Coral Sea off the northeast coast of Australia

HS Hydrae is a binary system. Binary star systems are actually quite common, and in fact the Alpha Centauri system, which is the closest star system to Earth, is a triple star system. Unless the binary stars are particularly close to Earth or has some type of peculiar properties, they generally are not worth talking about. However, since I am talking about this binary system, it has to be interesting for some reason.

HS Hydrae is a case where the orbits of the stars are arranged in such a way that, as seen from Earth, they eclipse each other. This causes the brightness of the binary star system to change over time. Situations where this occurs are very useful generally, as the orientation of the stars in their orbit around each other is known instantaneously. This along with distance measurements to the starsand measurements of the Doppler shifting cause by the stars orbiting each other can be used to accurately measure the masses of the stars themselves. However, many binary stars are in eclipsing binary systems, and this is also a method for identifying exoplanets, so the fact that HS Hydrae is another eclipsing binary system just does not seem interesting.

The orbital time period is a relatively short 1.56 days [1], so it seems like the stars are very close to each other. This may seem short, but since we know it is possible to find binary stars systems with orbital periods as short as a few hours [2], the orbital period of HS Hydrae does not seem very interesting.

What is interesting, though, is that the axis of the stars' orbits in HS Hydrae is precessing. Normally, the axis of a rotating object in space or two objects orbiting each other in space will always point in one direction. This phenomenon is called angular momentum. However, external forces can cause the axis of rotation of a single object or the axis of orbit of two objects to move, which is called precession. For example, the Earth's axis of rotation is precessing in a way that the position of the North Celestial Pole loops through a circular path in the sky once every 26000 years. Right now, the Earth's North Pole points at Polaris in Ursa Minor, but about 5000 years ago, the Earth's North Pole pointed at Thuban in the constellation Draco, and in 12000 years, it will be Vega in the constellation Lyra [3].

The axis of the orbits of the two stars in HS Hydrae also appear to be precessing, as identified by Petr Zasche and Anton Paschke in a paper published in 2012 [1]. The variations in the brightness of HS Hydrae have, well, varied over time, if that makes sense. Observations from 1975 [4] showed that, when the stars eclipsed each other, the brightnesses varied by about half a magnitude. In 30 years, this decreases to less than 0.1 magnitudes. This could happen if the stars' orbits changed from more completely eclipsing each other in the past to only partially eclipse each other now. The people who identified this precession indicated that the stars could potentially stop eclipsing each other very soon if they have not done so already.

HS Hydrae is one of only a very few binary star systems where this precession has been seen, but the implications of this are that the orientations of binary stars' orbits can potentially change in a relatively short amount of time. If this type of precession occurs very regularly, it could make it more difficult to reliably measure the masses of stars in binary systems. On the other hand, it could also mean that more binary stars become eclipsing binary systems in the future. So, expect more variation in the variability of variable stars.


[1] Zasche, P. and Paschke, A., HS Hydrae about to turn off its eclipses, 2012, Astronomy and Astrophysics, 542, L23

[2] Jones, David et al., The short orbital period binary star at the heart of the planetary nebula M 3-1, 2019, Monthly Notices of the Royal Astronomical Society, 482, L75

[3] Schuler, Rich, What exactly is the North Star?, 2006, Scientific American

[4] Gyldenkerne, K. et al., Four-colour photometry of eclipsing binaries. I. HS Hya, light curves, photometric elements, and determination of helium content., 1975, Astronomy and Astrophysics, 42, 303

Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende, which is distributed by under a Creative Commons 4.0 Attribution License

Sound Effects: dronemachine, fons, Hedmarking, ivolipa, jameswrowles, quadraslayer, shoba, and Xulie at The Freesound Project

Image Viewer: Aladin Sky Atlas (developed at CDS, Strasbourg Observatory, France)


© George Bendo 2019. See the acknowledgments page for additional information.

Last update: 25 April 2020