George's Random Astronomical Object

Object 59: VW Cephei

Podcast release date: 01 November 2021

Right ascension: 20:37:21.6

Declination: +75:36:02

Epoch: J2000

Constellation: Cepheus

Corresponding Earth location: An area in the interior of northwest Greenland

So, VW Cephei is a binary star system at a distance of 86.4 light years (26.5 pc) [1,2] that contains two yellow-orange stars (or orange-yellow stars). I've discussed a lot of binary star systems and multiple star systems in this podcast so far, but VW Cephei is actually a type of binary star system that I haven't covered yet. It was identified in 1926 as an example of what is called an eclipsing contact binary system [3]. Alternately, it is often referred to as a W Ursa Majoris type variable star after the first star system that was identified as an eclipsing contact binary.

So now, an explanation for what an eclipsing contact binary star system is. In most binary star systems, the stars are usually seaparated by a large enough distance that they generally exist as distinct individual stars within the star system even though the stars are still gravitationally bound to each other and still orbit around the center of gravity in the system. If the stars are relatively close to each other, though, they will tidally distort each other, so they will look kind of elliptical, and they could heat each other's surfaces. If the stars are really close, then the outer atmospheres of the stars will touch each other and merge together, and the two stars will end up enveloped in a single oblong layer of gas [4]. This is what is happening in VW Cephei and other contact binary star systems. In addition to this, VW Cephei happens to be oriented in such a way that, as seen from Earth, the stars eclipse each other, and this causes the star system to vary in brightness.

Based on the most recent models of this system, the two stars in VW Cephei have very different sizes. The bigger star is about the same size as the Sun, although it's slightly more massive, while the smaller star has a diameter that is close to half the size of the Sun (ignoring the parts that are distorted by the tidal forces in the star system) [5]. The stars are only separated by a distance equivalent to the diameter of the Sun [5], so you can imagine just how close they would look to each other if you were in a spaceship in the star system (or if you were just outside a spaceship in the star system). Because the stars in VW Cephei are so close, they not only share an outer layer of gas but orbit each other very rapidly. They complete an orbit around each other about once every six and a half hours [5], so during a typical night of observing the star with a telescope on Earth, astronomers can watch each star eclipse the other star at least once. These eclipses produce distinct drops in the brightness of the star system.

However, astronomers noticed that the variations in the brightness of these two stars was a little bit more complex than what could be explained by just one star eclipsing the other one. This turns out to be the result of starspots on the bigger star [5]. Starspots, which are the same as sunspots but on stars other than the Sun, are slightly cooler parts of the stars' surfaces that are usually associated with strong magnetic fields as well as bright flares that produce large amounts of energy across the electromagnetic spectrum. The magnetic fields in the Sun by itself are rather complicated and difficult for astronomers to describe, so you can imagine that the tidal distortion of the two stars in VW Cephei has made the magnetic fields extra weird. It's usually very difficult for astronomers to map the starspots on other stars because they are very far away, but the unusual light variations in VW Cephei allowed astronomers to figure out that the larger star had either a couple of large starspots or a couple of clusters of starspots on its surface [5]. These starspots produce other effects as well. I was particualrly impressed by the results of X-ray observations of this star system that not only detected the thin, hot outer corona of gas that envelops both of these stars but also detected flares associated with the magenetic fields around the starspots [6,7,8].

VW Cephei is not the only eclipsing contact binary star system in the sky. After all, I indicated that astronomers sometimes call it a W Ursa Majoris type star system because it is similar to W Ursa Majoris. However, VW Cephei is one of the brightest and well-studied of these eclipsing contact binary star systems. Therefore, what astronomer learn about VW Cephei is very useful for understanding these star systems in general, and this is why VW Cephei is so important in astronomy.


[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] Schilt, J., Two new variables stars of the type of W Ursae Majoris., 1926, Astrophysical Journal, 64, 215

[4] Malatesta, Kerri, W Ursae Majoris, 2021, American Association of Variable Star Observers

[5] Mitnyan, T. et al., The contact binary VW Cephei revisited: surface activity and period variation, 2018, Astronomy & Astrophysics, 612, A91

[6] Choi, C. S. and Dotani, T., ASCA Observation of a Long-Duration X-Ray Flare from the W UMa--Type Binary VW Cephei, 1998, Astrophysical Journal, 492, 761

[7] Gondoin, P., X-ray spectroscopy of the W UMa-type binary VW Cephei, 2004, Astronomy & Astrophysics, 415, 1113

[8] Huenemoerder, David P. et al., X-Ray Spectroscopy of the Contact Binary VW Cephei, 2006, Astrophysical Journal, 650, 1119

Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

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© George Bendo 2021. See the acknowledgments page for additional information.

Last update: 31 October 2021