Object 46: LX Serpentis

Podcast release date: 03 May 2021

Right ascension: 15:38:00.1


Epoch: ICRS

Constellation: Serpens

Corresponding Earth location: Slightly less than 1700 km west-southwest of the tip of Baja California

LX Serpentis is a cataclysmic binary star system located at a distance of 1608 light years (493 pc) [1,2] in the constellation Serpens. A cataclysmic binary star system consists of a white dwarf and another star (usually a Sun-like star) that are close together [3]. Originally, LX Serpentis would have started out as a relatively normal binary star system consisting of two Sun-like stars that were separated by a relatively large distance. However, one of the stars would have run out of hydrogen to fuse into helium in its core, and that would have caused the star to form a red giant that would have engulf the other star. This would have caused the centers of the two stars to begin to merge together, but before that happened, the red giant would have evolved further and expelled its outer layers of gas. This left behind a white dwarf and a Sun-like star that are so close to each other that the white dwarf can gravitationally strip away the outer layers of gas from the other star. As this gas falls into the white dwarf, it forms a disk around the dwarf, and this disk can become so hot that it produces X-rays.

Binary star systems like LX Serpentis are called cataclysmic because they can suddenly vary in brightness. These brightness variations are caused by changes in the rate in which gas falls into the white dwarfs in these systems. In the case of LX Serpentis specifically, it was first identified as a cataclysmic binary in 1979. An Armenian-Soviet astronomer named Jivan A. Stepanian had been making comparisons between photographic images and found that LX Serpentis was 40 times brighter in 1978 than it was in 1950 [4]. He was unable to identify what it was at the time, but subsequent observations and analyses proved that it was a cataclysmic variable star [5,6]. At this time, astronomers knew of less than a dozen cataclysmic variables, so Jivan Stepanian's discovery was very exciting, and people even started calling the star Stepanian's Star. (As a side note, this episode is now the second in a row to feature an astronomical object with a name based on the name of an Soviet-era Armenian astronomer.)

One of the interesting things about this system is that it is oriented in such a way that astronomers can see the two stars eclipse each other. This doesn't necessarily always happen with binary star systems generally or with cataclysmic binary star systems specifically. By observing how the stars changed in brightness over time, astronomers were able to determine that the two stars orbit each other once every 3 hours and 48 minutes [6].

So, LX Serpentis, or Stepanian's Star, was very exciting for a little while in the early 1980s at the same time that people were listening to ABBA, Queen, and Blondie. However, people eventually lost interest in both LX Serpentis and these music bands until there was a revival much later. In the case of LX Serpentis, it was in 1999 when the measurements of slight variations in the orbital period over time were used to create a musical about a wedding on a Greek island. No, wait, that's the revival of interest in ABBA's music.

The revival of interest in LX Serpentis was in 2017 when a group of astronomers led by Li Kai at Shandong University used the measurements of slight variations in the orbital period over time to search for exoplanets in this star system [7]. The idea was that, as an exoplanet orbits the two stars, the gravitational pull of the planet on the stars would cause the stars to move slightly, and the eclipses would seem slightly shorter as both stars moved towards us and slightly longer as the two stars moved away from us. The astronomers indeed found that the eclipses varied this way very slightly over a period of 22.8 years, demonstrating that at least one exoplanet was present in the system [7]. They were even able to estimate that the exoplanet has a mass of 7.5 times the mass of Jupiter and that it orbits the two stars at a distance of 9.12 Astronomical Units [7], which in our Solar System would be equivalent to slightly less than the distance from the Sun to Saturn. If you go back to Episode 44, you can hear me discuss how a different group of astronomers found an object that was 7 times the mass of Jupiter in orbit around a completely different white dwarf and how they decided to call that new object a really really small brown dwarf (which is a type of failed star), possibly because they were too nervous to say that they had found an exoplanet. In contrast, the people who found the exoplanet in orbit around LX Serpentis were clearly much more confident about calling their discovery an exoplanet.

What's rather amazing is that the exoplanet, which is a gas giant, was able to survive a situation where one of the stars expanded into a red giant and then lost its outer layers of gas in the form of a nebula. When astronomers talk about our Sun dying, they mention how the Sun will expand and engulf the Earth and several other planets, and the same may have happened to the exoplanet orbiting LX Serpentis if it was closer to the two stars. However, it was apparently far enough away that it could avoid being consumed by the red giant [7], which I find really impressive.


[1] Gaia Collaboration et al., The Gaia mission, 2016, Astronomy & Astrophysics, 595, A1

[2] Gaia Collaboration et al., Gaia Early Data Release 3: Summary of the contents and survey properties, 2020, arXiv e-prints, arXiv:2012.01533

[3] Szkody, P. and Gaensicke, B. T., Cataclysmic Variables, 2012, Journal of the American Association of Variable Star Observers (JAAVSO), 40, 563

[4] Stepanian, J. A., A Noteworthy Star in Serpens, 1979, Information Bulletin on Variable Stars, 1630, 1

[5] Wenzel, W., Object Stepanyan 153519 Possibly of U Geminorum Type, 1979, Information Bulletin on Variable Stars, 1720, 1

[6] Horne, K., Stepanyan's star - A new eclipsing cataclysmic variable., 1980, Astrophysical Journal Letters, 242, L167

[7] Li, Kai et al., A possible giant planet orbiting the cataclysmic variable LX Ser, 2017, Publications of the Astronomical Society of Japan, 69, 28


Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

Sound Effects: Dalibor, Department64, furbyguy, Harha, ivolipa, jameswrowles, metrostock99, and mikeriley131 at The Freesound Project

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