Object 63: UX Ari

Podcast release date: 27 December 2021

Right ascension: 03:26:35.4


Epoch: ICRS

Constellation: Aries

Corresponding Earth location: The Zagros Mountains within Bushehr Province in Iran

UX Arietis (which I am going to call UX Ari for short) is a triple star system at a distance of 164.9 light years (50.56 pc) from Earth [1,2].

In the center of the system are a red subgiant star and a yellowish Sun-like star [3]. While the Sun-like star is still fusing hydrogen into helium in its core, the red subgiant's core filled up with helium a little while ago, which has triggered the evolution of the star from a Sun-like star into a red giant. These two central stars are separated by a distance of about 0.09 Astronomical Units (AU) [3]. 1 AU is the distance from the Sun to Earth, while the distance from the Sun to Mercury is 0.39 AU, so the two stars at the center of UX Ari are actually quite close to each other. The two stars complete an orbit around each other once every 6.44 days, which is quite rapid [3].

As for the third star, it seems to be something a bit smaller than the Sun, possibly a red dwarf, and it orbits the central two stars once every 111 years or so according to the last measurements that I could find [4]. It's presence has only been inferred by how it affects the motion of the other two stars in the system [4,5]. It's not really that important unless you need to precisely calculate the velocities of everything else in the system, so I'm not going to mention it again.

Anyway, the red subgiant and the Sun-like star at the center of UX Ari both comprise a type of variable star system called an RS Canum Venaticorum variable star (or RS CVn star for short because pronouncing "Canum Venaticorum" is a little tricky). This classification is used because UX Ari varies in brightness in the same way and for the same reasons as RS CVn, but since RS CVn was the first star system discovered to vary in brightness in this specific way, all other star systems that vary this way are called RS CVn star systems.

(I don't understand why variable star observers like creating classifications for variable stars named after one of the stars with that type of variability. It would be like, for example, if every president of the United States was called a George Washington because George Washington was the first president of the United States or if every astronaut was called a Yuri Gagarin because Yuri Gagarin was the first astronaut in space. On the other hand, many of the Roman emperors were called Caesars after Julius Caesar, and the United States Navy likes to name classes of ships after other ships, which means you have things like the Independence-class littoral combat ship USS Independence, so it's not just astronomers who name categories of things after things in those categories. I suppose if astronomers who study variable stars want to say that UX Ari is an RS CVn type variable star system, we're all supposed to understand that.)

Anyway, all of these RS CVn type variable star systems, including UX Ari, have a Sun-like star and either a red subgiant or a red giant star that are very close together. These stars do not necessarily eclipse each other as seen from Earth, which means that they do not vary in brightness because one star passes behind the other. Instead, UX Ari and other star systems like it vary in brightness because the two stars have really complex and strong magnetic fields, and because the stars are so close together, these magnetic fields end up interacting with each other and producing all sorts of effects that cause the binary star system to vary in brightness.

First of all, any star's magnetic field will create starspots, which are relatively cool patches on the star's surface and are analogous to the sunspots on our Sun. However, a star with really strong and complex magnetic fields, like the red subgiant in UX Ari, is going to be covered in lots and lots of starspots. In fact, more than three fifths of the surface of UX Ari's red subgiant star is covered in starspots [3]. As a result, the star has kind of a mottled appearance. This means that, as the star rotates, we could end up seeing fewer or more starspots on the side facing Earth, which will make it look like it's varying in brightness. These starspots also change slowly over time, so the brightness of the star and the exact way in which it varies in brightness will also change.

However, this is not the only effect of those weird and wild magnetic fields. Strong magnetic fields on any star are associated with stellar flares. Flares are explosions on the surface of a star that produce large amounts of emission across the electromagnetic spectrum. In the case of UX Ari, those magnetic fields have produced multiple flares on the surface of the red subgiant that have been detected in X-rays, ultraviolet light, visible light, and radio waves [6,7,8,9,10,11,12,13,14]. In fact, what makes UX Ari particularly special is that it tends to produce a relatively high number of flares compared to any other RS CVn star [3]. Also, some of these flares, and least in radio waves, are associated with loops of gas that extend very far from the surface of the red subgiant [11]. These flares provide interesting insights into the stars in this star system, and it's these flares that make UX Ari one of the more interesting binary variable star systems in astronomy research as well as one of the more unusual star systems close to our own.


[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] Hummel, C. A. et al., Orbital Elements and Stellar Parameters of the Active Binary UX Arietis, 2017, Astrophysical Journal, 844, 115

[4] Peterson, W. M. et al., Radio Astrometry of the Triple Systems Algol and UX Arietis, 2011, Astrophysical Journal, 737, 104

[5] Aarum Ulv\ras, V. and Engvold, O., Separation of high resolution spectra of the multiple star system UX Ari, 2003, Astronomy & Astrophysics, 402, 1043

[6] Simon, T. et al., IUE spectra of a flare in the RS Canum Venaticorum-type system UX Arietis., 1980, Astrophysical Journal, 239, 911

[7] Tsuru, T. et al., X-ray and radio observations of flares from the RS Canum Venaticorum system UX Arietis., 1989, Publications of the Astronomical Society of Japan, 41, 679

[8] Montes, D. et al., Simultaneous Hα, Na I D1, D2, and He I D3 observations of a flare on the RS CVn system UX Ari., 1996, Astronomy & Astrophysics, 310, L29

[9] Torricelli-Ciamponi, G. et al., Radio flares from the active binary system UX ARIETIS, 1998, Astronomy & Astrophysics, 333, 970

[10] Trigilio, C. et al., Strong radio flaring period in UXArietis, 1998, Astronomy & Astrophysics, 330, 1060

[11] Franciosini, E. et al., Flaring loop structures at VLBI scale in UX ARIETIS, 1999, Astronomy & Astrophysics, 341, 595

[12] Güdel, Manuel et al., Flaring and Quiescent Coronae of UX Arietis: Results from ASCA and EUVE Campaigns, 1999, Astrophysical Journal, 511, 405

[13] Franciosini, E. et al., BeppoSAX observation of a large long-duration X-ray flare from UX Arietis, 2001, Astronomy & Astrophysics, 375, 196

[14] Cao, Dong-Tao and Gu, Sheng-Hong, Optical flare events on the RS Canum Venaticorum star UX Arietis, 2017, Research in Astronomy and Astrophysics, 17, 055


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