Object 29: Zeta Virginis

Podcast release date: 07 September 2020

Right ascension: 13:34:41.6


Epoch: ICRS

Constellation: Virgo

Corresponding Earth location: Approximately 270 km south-southeast of Kiritimati in the Pacific island nation of Kiribati

Zeta Virginis is the sixth-brightest star in the constellation Virgo, a constellation everyone know about because it is one of the twelve constellations in the zodiac. Zeta Virginis also has the name Heze. A lot of other stars have names like this that were originally created by Arabic astronomers, Greek astronomers, or other ancient astronomers, so it seems like Heze should be a name with some sort of ancient origin. However, I could not find anything that explains where that name came from or what exactly it was supposed to mean.

Among all of the objects covered in my podcast so far, I think that Zeta Virginis is the brightest in the visible part of the spectrum as seen from Earth. It has an apparent magnitude of 3.4, which makes it about 5 to 10 times brighter than the faintest stars that can be seen without a telescope [1]. Unfortunately, it's a little tricky to describe where it is with respect to the other stars in the constellation. While everyone knows about the constellation Virgo and while the constellation does contain a reasonable number of bright stars, it doesn't really have a terribly distinct shape. I recommend looking about 10 degrees north-northeast of Spica, which is the brightest star in Virgo.

The reason why Zeta Virginis is relatively bright as seen from Earth is because it is relatively nearby (as a distance of 73.5 light years or 22.5 pc [2,3]) and because it is a relatively bright bluish-white star. Technically, it's stellar classification is either A2V or A3V, which means that it's like the Sun in that it fuses hydrogen into helium in its core, but it's much brighter and much hotter. It's also about twice as massive as the Sun.

The close proximity of Zeta Virginis combined with its stellar classification is what attracted a group of astronomers to observe the star for a very specific scientific study. Astronomers looking at slightly smaller and slightly fainter stars had frequently found that those stars were orbited by relatively small companion objects like red dwarfs and brown dwarfs, but it was not immediately clear if bright stars like Zeta Virginis had similar companions.

The problem is that bright stars like Zeta Virginis tend to saturate any astronomical cameras that try to take photos of the stars. To get an idea of how this works, go out on a clear night to a relatively dark place where you can see the stars in the sky. Bring a flashlight with you (or, if you are in the UK, Australia, or New Zealand, bring a torch with you). Wait in the darkness so that your eyes adjust to dim light and so that you can see all of the stars in the sky. You may even be able to see the Milky Way. After a few minutes, point the flashlight or torch at your face and turn it on, and then try counting how many stars you can see. You probably can't see much of anything and you are probably in a little bit of pain because you are shining a very bright light in your face. This is the same type of problem that astronomers have with trying to find fainter things around really bright stars. Individual stars should look relatively point-like in an astronomical image, but if the star is too bright, the starlight will not only saturate some of the pixels in the center of the image but will also spread over a huge circular region, making it very difficult to see anything near the star itself.

So, astronomers set out to try to find any companion star in orbit around Zeta Virginis by observing it in near-infrared light using an interesting combination of a coronagraph and adaptive optics. In a coronagraph, a circular disk is inserted in front of the camera's digital detector to block out the light from the center of the star itself. Unfortunately, because of blurring by the Earth's atmosphere, the starlight will still appear to be smeared over a broad area outside the center of the image even when the center of the star itself is blocked out. Adaptive optics systems use a deformable mirror to compensate for how the starlight is blurred by the Earth's atmosphere, and this stops the starlight from very bright stars appearing spread out so much.

By combining these techniques, astronomers were able to find a very faint, very small red dwarf in orbit around the brighter star [4]. The difference in brightness between the two stars is about a factor of 600. The red dwarf has a mass about 0.17 times the mass of the Sun, or about 8% of the mass of the larger bluish-white star that it's orbiting. The red dwarf orbits the bluish-white star once every 124 years.

Now, this is just one star, but this has implications for how both stars and planets form. It's well known at this point that stars form out of giant clouds of gas in space, and when stars initially form, they will be surrounded by disks of material left over from their creation. Very often, astronomers expect these disks to form planets, but the observations of Zeta Virginis demonstrate that it may be possible for small stars to form out of the gas in these disks as well.


[1] Ducati, J. R., VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system., 2002, VizieR Online Data Catalog

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

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

[4] Hinkley, Sasha et al., Discovery and Characterization of a Faint Stellar Companion to the A3V Star ζ Virginis, 2010, Astrophysical Journal, 712, 421


Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

Sound Effects: Breviceps, Diicorp95, dronemachine, ivolipa, jameswrowles, newagesoup, subodh3012, ToomSter, and Xulie at The Freesound Project

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