Object 125: HR 1099

Podcast release date: 27 May 2024

Right ascension: 03:36:47.3


Epoch: ICRS

Constellation: Taurus

Corresponding Earth location: Slightly less than 500 km north of the Seychelles in the Indian Ocean

This star system goes by two different names. Actually, this star system has over 70 names, but most astronomers use either HR 1099 or V711 Tauri to identify the object. HR 1099 sounds like it should be a business form, but the HR actually stands for "Havard Revised", and it refers to a revised catalog of stars published by Harvard Observatory in 1908 [1]. Ironically, later versions of this revised catalog were published by Yale University [2]. Anyway, HR 1099 was the 1099th star system in this catalog. The second name for this system, V711 Tauri, is a modern convention used for variable star systems. The name consists of the letter V for variable, a number, and the constellation name. I will use HR 1099 for the rest of this podcast, mainly because it seems like most of the recent scientific papers published about this object have used HR 1099 for the name.

So, HR 1099 is a variable star system, and if you notice, I have been referring to it as a system and not a star. That is because it is a binary star system. More specifically, it is a type of variable star system called an RS Canum Venaticorum variable. These class of variable stars are named after RS Canum Venaticorum, the first star identified in this class of stars, and if you listen to episode 63, you can hear me rant about how it's confusing to name a class of stars after one of the stars in the class. I'll skip that rant in this episode.

Anyway, this system consists of two stars. The brighter star is a red subgiant that was once a Sun-like star but that has relatively recently filled up its core with helium and is beginning to evolve into a red giant. The other is actually a very Sun-like star, as in it looks like it's the same size and spectral type and everything. These two stars are really close together; they are separated by around 0.045 AU, or 0.045 times the distance between the Earth and the Sun [3]. These two stars are closer to each other than Mercury is to our Sun, and the stars are in fact so close that the magnetic fields from the two stars are able to interact, producing all sorts of weird effects, including potential stellar flares and starspots. It is all of this stuff associated with the magnetic field interactions that makes this star system vary in brightness. The most notable variations in brightness occur as the stars orbit each other. Areas with lots of starspots will either come into view, making the star system seem darker, or go out of view, making the star system seem brighter.

HR 1099 played a very special role in proving that RS Canum Venaticorum stars vary because of the interactions between the magnetic fields of the two stars in each of these systems. Spectroscopic observations at Kitt Peak in 1979 revealed the presence of titanium oxide [4], which absorbs light at a wavelength of 8860 Angstrom. One form of titanium oxide is used in sunscreen, which seems like it should be important to this discussion but is actually completely irrelevant [5]. Titanium oxide has also been seen in spectra of sunspots on our Sun, so the detection of titanium oxide in the spectrum of HR 1099 was a clear indication that at least one of the stars in the system had starspots. In addition, the light absorbed by the titanium oxide seemed to vary periodically in a way that was consistent with the starspots being on only one side of one of the stars, probably the bigger star, which we would expect if the magnetic fields of the two stars were interacting [4]. Hence, HR 1099 has been very important for advancing our understanding of this class of variable stars.

As a very bright and nearby RS Canum Venaticorum star, HR 1099 continues to be included in many variable star surveys, and the system is often targeted specifically for detailed observational analyses. From what I can tell, it is a farily notable variable star system that will be studied for decades to come.


[1] Pickering, Edward Charles, Revised Harvard Photometry: a catalogue of the positions, photometric magnitudes and spectra of 9110 stars, mainly of the magnitude 6.50, and brighter observed with the 2 and 4 inch meridian photometers, 1908, Annals of Harvard College Observatory, 50, 1

[2] Hoffleit, Dorrit, Catalogue of Bright Stars, 1964

[3] Golay, Walter W. et al., Time-lapse Very Long Baseline Interferometry Imaging of the Close Active Binary HR 1099, 2024, Astrophysical Journal, 965, 86

[4] Ramsey, L. W. and Nations, H. L., HR 1099 and the starspot hypothesis for RS CVn binaries., 1980, Astrophysical Journal Letters, 239, L121

[5] TDMA, How titanium dioxide in sunscreens protects us from the sun, 2024, TDMA


Podcast and Website: George J. Bendo

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