Object 19: V505 Sagittarii

Podcast release date: 20 April 2020

Right ascension: 19:53:06.4


Epoch: ICRS

Constellation: Sagittarius

Corresponding Earth location: Tropical rainforests in eastern Bolivia

V505 Sagittarii, which is located in the constellation Sagittarius at a distance of somewhere between about 275 and 390 light years (about 85 and 120 pc) [1,2,3], is a rather complex star system, but it took a while for people to figure out how complex it actually is.

Initially, it was identified as an eclipsing binary star system in the year 1934 by Cuno Hoffmeister [4]. An eclipsing binary star system is a type of star system where we can see the two stars in the system pass in front of each other as they orbit each other. When they do this, the brightness of the star system decreases slightly. In V505 Sagittarii, the two stars orbiting each other once every 28 hours and 24 minutes [5]. The peak brightness of the system is 6.5 magnitudes. For reference, the faintest stars that the average person can see in the sky in a dark location are somewhere between magnitude 5 or 6, so V505 Sagittarii is slightly too faint to see without a telescope.

The binary star system was later specifically identified as an Algol-type binary star system, which is a class of stars named after Algol, the brightest star in the class. In these systems, the two stars are located very close together. One of the stars is still converting hydrogen to helium in its core like the Sun, but the other star has used up all of the hydrogen in its core and has begun to expand to become a red giant. Because the expanding star, which is called a subgiant star, is very close to its companion, the expanding outer atmosphere of the subgiant ends up being gravitationally stripped by the other star [6]. This makes V505 Sagittarii sound very cool, but our galaxy contains a lot of these types of stars, and some of them, like Algol, are located much closer to Earth and are therefore much more interesting.

In 1987, a third star was found orbiting the binary star system in V505 Sagittarii [7]. The third star is very similar to the Sun. It orbits the inner pair of stars on a elongated elliptical orbit with an orbital period somewhere between 38 and 105 Earth years [3,5]. This begins to make V505 Sagittarii sound cool again except for the fact that lots of other star systems contain three stars, including the Alpha Centauri star system, which is the closest to our Solar System, and Algol itself. You can also go back to episode 7, where I describe 30 Arietis, a star system containing four stars and an exoplanet.

People continued to make measurements of V505 Sagittarii, and it looked like a rather ordinary triple star system for a while, but then something happened sometime around the year 2000, when the orbits of the three stars changes slightly but rather abruptly. The timing of the two central stars eclipsing each other shifted by about 2 minutes, and the orbit of the third star changed in both size and shape [3,5].

People who remember the year 2000 will recall that a lot of software developers and computer support people spent a lot of time trying to fix the Y2K bug in all of the world's computer software. This was a bug where computers would not be able to recognize dates after the year 1999 as taking place in the year 2000 or later but would instead think that the dates referred to the year 1900 or some other earlier time in the 20th century. It would be tempting to think that the orbits of V505 Sagittarii were affected by a computer glitch because the star system's IT support did not patch their computers, but this is a rather silly idea, as any aliens that inhabit this star system probably developed better coding practices before we did. A slightly less silly idea is that the Y2K bug affected astronomical measurements for this object, but given that no other astronomical observations or analyses have been affected by such a glitch, this also seemed unlikely.

A group of astronomers in Prague who specialize in looking at weird multi-star systems decided to investigate V505 Sagittarii in more detail so that they could understand what affected to the orbits of all of the stars in the system. In their analysis, they eliminated the possibility that some of the older measurements were wildly inaccurate, that the third star somehow gravitationally interacted in an unusual way with the central two stars, that a gradual or sudden transfer of gas between the two eclipsing stars led to a sudden change in the orbit of the third star, or that variations in the magnetic fields around the central pair of stars somehow changed the orbits of all three stars [5]. They eventually concluded that a fourth star must have gravitationally interacted with three other stars in this system [5]. However, they haven't been able to figure much more out about this potential fourth star because they have not actually directly detected the fourth star but just inferred its presence based on models decribing the change in the orbits of the other three stars. It's possible that the fourth star is in a relatively stable orbit around the central pair of stars or that the fourth star is not at all associated with the other stars in V505 Sagittarii system and that it just happened to pass through the star system around the year 2000. I would be surprised if the second explanation is the correct one, and so would the people in Prague who suggested this idea, as such interactions between star systems are extremely rare. In any case, this could be a peculiar star system that astronomers will continue to investigate in more detail in the future.


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

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

[3] Cvetković, Z. et al., A New Recalculated Orbit for the Visual Binary CHR 90, 2010, Astronomical Journal, 140, 25

[4] Hoffmeister, Cuno, Neuer Bedeckungsveränderlicher 6. Größe 399.1933 Sagittarii, 1933, Astronomische Nachrichten, 250, 47

[5] Brož, M. et al., A Unified Solution for the Orbit and Light-time Effect in the V505 Sgr System, 2010, Astronomical Journal, 139, 2258

[6] Price, Aaron, Beta Persei (Algol), 2010, American Association of Variable Star Observers

[7] McAlister, Harold A. et al., ICCD Speckle Observations of Binary Stars. I. A Survey for Duplicity Among the Bright Stars, 1987, Astronomical Journal, 93, 183


Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

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