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The Pleiades

September 24, 2014

The Pleiades (Πλειαδες) were the seven nymph companions of Artemis, the Greek goddess of the hunt and the protectress of young girls. The nymphs were depicted as nubile maidens, and I'm reminded of the nymphs when I see Sir Galahad's experience at a certain castle in Monty Python and the Holy Grail.[1]

Pleiades from mythologyModern artists enjoy painting nude nymphs, but Medieval artists preferred their nymphs to be chaste, as in this circa 850 illustration.

The Pleiades were Alcyone, Celaeno, Electra, Maia, Merope, Sterope, and Taygeta.

(From a folio, circa 830-840, at the Leiden University Library, via Wikimedia Commons.)

The Pleiades star cluster, also known as Messier 45, is named after the nymphs. The Pleiades are quite close to us by astronomical standards, and the stars are easily visible by the unaided eye. In fact, the ability to resolve the individual stars of the Pleiades was an ancient eye test.

Pleiades star clusterThe Pleiades star cluster in visible light.

Joining the seven nymphs are their parents, Atlas and Pleione.

(NOAO/AURA/NSF image.)

Galileo Galilei, who was the first to view the Pleiades through a telescope, saw that there were many more stars in that region than the few observed with the unaided eye. Galileo published a sketch of the Pleiades with 36 stars in the Sidereus Nuncius (1610). Armed with telescopes and an emergent understanding of astrometry, astronomers have measured the distance to the Pleiades. The following table shows some historical estimates of that distance.

 MethodLight Years
 Stellar Parallax440
 Hipparcos Satellite385
 Hubble Space Telescope 450
 Optical Interferometry440

As can be seen from the table, the Hipparcos Satellite value, which was a parallax measurement anticipated to be highly accurate, is quite a bit removed from the others. This has caused considerable puzzlement since it was published in 1997. The Hubble Space Telescope measurement was based on visual magnitudes as interpreted by a fit to the Hertzsprung–Russell diagram. The optical interferometry value was made using the orbital parameters of the double star, Atlas, in the Pleiades.

As I wrote in a previous article (Parallax and Stellar Aberration, August 29, 2012), parallax is a simple geometrical technique for distance measurement. As a young Boy Scout, I learned the parallax method for measuring such things as the width of a river, as shown in the following figure.

Distance measurement using similar trianglesDistance measurement of the width of a river using similar triangles.

Since triangles ABC and OPC are similar, we can calculate OC knowing OA, AB, and PO.

(Figure rendered by the author using Inkscape).

To measure stellar distances, you need to use trigonometry instead of this geometrical method. Using two extremes of the Earth in its orbit around the Sun, you can observe how the angular position of a star changes against the background of the "fixed stars" (see figure). In fact, the common stellar distance unit, parsec, is defined as the distance for which there is a parallax of one arcsecond. For those who like large numbers, a parsec is equal to 9,460,730,472,580 kilometers.

Stellar parallax diagram

Stellar parallax diagram. A parsec, defined as the distance for which there is exactly an arcsecond's parallax, serves as an astronomical measure of distance. Distance is computed as the reciprocal of the parallax angle, since these angles are small and the tangent of the angle is nearly equal to the angle itself. (Wikimedia Commons image, modified by the author using Inkscape.)

Radio astronomers, using a worldwide network of radio telescopes, has recently measured the distance to the Pleiades to a high precision by an absolute trigonometric parallax measurement using very long baseline interferometry.[2-6] The astronomers are from the University of California (San Diego, California), the Harvard-Smithsonian Center for Astrophysics (Cambridge, MA), the National Radio Astronomy Observatory (Socorro, NM), the California Institute of Technology (Pasadena, California), and the Academia Sinica Institute of Astronomy and Astrophysics (Hilo, HI). Their study revealed that the distance to the Pleiades is 136.2 ± 1.2 pc (444.25 ± 3.9 ly).[2] This result is in agreement with the more than a dozen previous measurements using multiple techniques.[4]

Figure captionThe Pleiades distance measurement.

Paired observations in August and February are shown.

(Image: Alexandra Angelich, NRAO/AUI/NSF, simplified for clarity.)

The Hipparcos distance measurement, which is out of line with all the others, would have been a problem for stellar theorists. The short distance meant that the hydrogen-burning stars of the Pleiades had some peculiarity, such as a high fraction of helium in their interiors.[3] Astronomers were worried that some long standing theories would need revision. Carl Melis, the first author of the study and an astronomer at the University of California in San Diego, is quoted by Nature as saying, "If we don't understand young stars, we're kind of hosed."[4]

Mellis and his colleagues created a huge, intercontinental array of radio telescopes for the Pleiades distance measurement. This network included the many telescopes of the US Very Long Baseline Array that extends from Hawaii to the Virgin Islands, the Robert C. Byrd Green Bank Telescope in West Virginia; the William E. Gordon Telescope of the Arecibo Observatory in Puerto Rico, and the Effelsberg Radio Telescope in Germany.[5-6] This large array of telescopes synthesized an Earth-sized virtual telescope that allowed a very accurate position measurement.[6] The Pleiades distance was measured through observations of five of its stars, two of which were a binary pair, over the course of eighteen months.[4]

This brings us to what might have happened with Hipparcos. The Hipparcos satellite, launched in 1989 by the European Space Agency, measured the parallax of 118,000 stars.[3,5] The Hipparcos data was used to generate a three-dimensional map of our locality in 1997.[4] Hipparcos, however, could not use the "fixed stars" as a reference, since it could not detect distant galaxies. Instead, the parallax distances were calculated on a self-consistent basis with respect to the entire 118,000 catalog of stars.[4]

Hipparcos measured 53 Pleiades stars, in contrast to the four of the present study, so you would think that it would be a more accurate measurement.[5] The satellite however, had technical problems. During launch, a booster rocket failed to fire, the satellite was placed in a highly elliptical orbit, and corrections had to applied to the data.[5] Not only that, but the Pleiades are in a region of sky for which such corrections are difficult.[5] Another possibility is that bright stars might have saturated the detector, although the Hipparcos team discounts that suggestion.[4]

It's important to address such sources of error, since another satellite, Gaia, was launched in 2013 to measure distances of about one billion stars, including hundreds in the Pleiades.[5-6] Gaia, however, can use distant galaxies as its fixed stars reference.[4] It's hoped that having such a reference point will prevent the type of systematic error that may have spoiled the Hipparcos measurement.[4]


  1. Monty Python and the Holy Grail (1975, Terry Gilliam and Terry Jones, Directors) on the Internet Movie Database.
  2. Carl Melis, Mark J. Reid, Amy J. Mioduszewski, John R. Stauffer, and Geoffrey C. Bower, "A VLBI resolution of the Pleiades distance controversy," Science, vol. 345, no. 6200 (August 29, 2014) pp. 1029-1032.
  3. Léo Girardi, "Perspective - One good cosmic measure," Science, vol. 345, no. 6200 (August 29, 2014) pp. 1001-1002.
  4. Mark Zastrow, "Row reignites over distance of Pleiades star cluster," Nature News, August 28, 2014.
  5. Ken Croswell, "Astronomers measure precise distance to controversial star cluster," Science, August 28, 2014.
  6. Radio Telescopes Settle Controversy Over Distance to Pleiades, National Radio Astronomy Observatory Press Release, August 28, 2014.

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