Object 5: RV Octantis

Podcast release date: 07 October 2019

Right ascension: 13:46:31.7


Epoch: ICRS

Constellation: Octans

Corresponding Earth location: An area of Marie Byrd Land in Antarctica near the Ross Ice Shelf.

RV Octantis is an RR Lyrae variable star. This class of variable stars are old versions of stars like the Sun. The Sun is about 5 billion years old but will continue to fuse hydrogen into helium in its core for another 5 billion years. In RR Lyrae stars, which are more than 10 billion years old, the cores filled up with helium a long time ago and the stars transformed into red giant stars. After this, the internal pressure got large enough for the helium to fuse into carbon in the stars' cores with hydrogen fusing into helium in the outer shells.

The atmospheres of RR Lyrae stars are unstable and pulse. Typically, the outer atmosphere of one of these stars cools and contracts periodically, but when the atmosphere contracts, the gas increases in temperature, and the star gets brighter. This increase in brightness causes the outer atmosphere to heat up and expand, but then, after expanding, the atmosphere cools. The star becomes fainter, the atmosphere contracts, and this cycle repeats itself. The typical pulsation period is between 4 and 20 hours [1]. The stars are all about the same size and have the same luminosities [1]. If astronomers compare the expected luminosity of an RR Lyrae variable to the brightness of the star as measured from Earth, they can determine the distance to the star.

The oldest paper that I could find on RV Octantis is a science bulletin from 1963 entitled "Three 'poorly Known' Variable Stars" written by W. Strohmeier and H. Ott [2]. It's hard for me to tell whether this was when RV Octantis was first identified as variable or just the first time that anyone actually documented that it was variable. The star has a period of 13 hours and 42 minutes, or about 0.571 days [3]. Its distance has been measured as 3225 light years or 989 parsecs [4,5].

Since its variability was first observed or documented in 1963, RV Octantis has been treated as a relatively typical and unexciting RR Lyrae variable. However, in 2009, George W. Preston reported the presence of helium in 10 RR Lyrae stars as determined by looking at their specrta, and the astronomer used RV Octantis as the best example of this phenomenon [3]. Like other elements or molecules in space, helium absorbs or emits light at specific wavelengths that appear as lines in spectra, but typically, the light is either absorbed or emitted but not both. In the case of RV Octantis, both emission and absorption of light by helium is seen in the star's spectrum. The light absorption comes from gas that is moving slightly relative to the hotter gas that is emitting light, so because of Doppler shifting, the absorption appears at a slightly different wavelength than the emission. This combination of emission and absorption by a spectral line is referred to as a P-Cygni line because such spectral lines were first observed in the spectra of P Cygni (although RV Octantis is very different from P Cygni).

This specific combination of emission and absorption, which appears just before the star becomes brightest in its pulsation period, implies that the gas is shocked as the outer atmosphere of the star is contracting [6]. These shocks were expected based on similar observations of hydrogen lines in the spectra of stars. However, producing these types of spectra lines typically involves ionizing the gas, and helium requires more energy than hydrogen to ionize. This observational result has forced astronomers to re-examine their models of how the outer atmospheres of RR Lyrae variables work and specifically to adjust their models of how shock waves propagate through the outer atmospheres of these stars [6].


[1] Lub, J., RR Lyrae stars., 1978, The Messenger, 13, 15

[2] Strohmeier, W. and Ott, H., Three "poorly Known'' Variable Stars, 1963, Information Bulletin on Variable Stars, 38, 2

[3] Preston, G. W., Helium lines in RR Lyrae spectra, 2009, Astronomy & Astrophysics, 507, 1621

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

[5] Gaia Collaboration et al., Gaia Data Release 2. Summary of the contents and survey properties, 2018, Astronomy & Astrophysics, 616, A1

[6] Gillet, D. et al., A helium P-Cygni profile in RR Lyrae stars?, 2016, Astronomy & Astrophysics, 587, A134


Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

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