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The Pleiades
Many a night I saw the Pleiads, rising thro' the mellow shade,
Glitter like a swarm of fireflies tangled in a silver braid.
- Alfred, Lord Tennyson, 1837-8, Locksley Hall

The Pleiades

[AAO Photograph]
(C) Copyright Anglo-Australian Observatory / Royal Observatory, Edinburgh.
Permission required for use. Photography by David Malin.

The Pleiades star cluster, also known as the Seven Sisters and Messier 45, is a conspicuous object in the night sky with a prominent place in ancient mythology. The cluster contains hundreds of stars, of which only a handful are commonly visible to the unaided eye. The stars in the Pleiades are thought to have formed together around 100 million years ago, making them 1/50th the age of our sun, and they lie some 130 parsecs (425 light years) away. From our perspective they appear in the constellation of Taurus, with approximate celestial coordinates of 3 hours 47 minutes right ascension and +24 degrees declination. For northern hemisphere viewers, the cluster is above and to the right of Orion the Hunter as one faces south, and it transits -- reaches its highest point in the sky, midway between rising and setting -- around 4am in September, midnight in November, and 8pm in January.

[unlabeled chart] [labeled chart]
XEphem charts showing the cluster's position. Left: a 90-degree star field centered on the Pleiades;
north is up and east is to the left. Right: same field with the cluster marked (as M45), as well as
constellations and the celestial and Galactic equators (red solid and dashed lines, respectively).

The image at the top of the page shows the central part of the cluster, where the brightest stars are found. This color photograph was taken in three filters, each exposed for about a half-hour, by David Malin with the UK Schmidt Telescope. The image is roughly 1.5 degrees wide, or three times the angular diameter of the moon. North is up and east is to the left. The cluster distance of 130 parsecs makes the physical width of the picture about 3.4 parsecs (11 light years); the cluster itself has a width perhaps 10 times greater, but most of the bright stars are found within one or two degrees of the core.

Brightest Pleiads

[chart of brightest Pleiads] The figure at right, covering a somewhat larger area than the AAO photograph, shows the brightest stars in the vicinity, most of which are members of the Pleiades cluster (some appear in the same part of the sky but actually lie at a different distance). Data for these stars are listed in a table below. All stars selected have magnitudes of 6.5 or brighter, which is about the limit of human vision under ideal conditions -- how many you actually see depends on your own eyesight, local atmospheric transparency, and light pollution levels. While few people can see stars as faint as magnitude 6.5, this limit is interesting for a different reason: it includes all stars in the vicinity that are listed in the 1725 British Catalogue of John Flamsteed, the first British Astronomer Royal. In a crude fashion, Flamsteed's stars correspond to the best the eye can do. The Greek-letter designations in Johann Bayer's 1603 Uranometria are also commonly used to label visible stars, but Bayer's catalog doesn't go as faint as Flamsteed's, and consequently only the brightest Pleiad has a Bayer designation.

The following table lists the bright stars in order from west to east (right to left in the figures above), giving the name, Bayer and Flamsteed designations where applicable. More modern Henry Draper catalog numbers are also given, in addition to apparent visual magnitude, spectral type, and cluster membership status. Additional information on each star is available by selecting the appropriate link (Note: the linked data may not be accessible to all users; if you do not seem to have access, consult this SIMBAD page for more information).

(a plain-format table is available for those who can't parse HTML Tables)
Name Bayer / Flamsteed HD V mag Spectral Type Mem? More
Celæno 16 Tauri 23288 5.46 B7 IV variable Yes data
Electra 17 Tauri 23302 3.70 B6 IIIe emiss. line Yes data
  18 Tauri 23324 5.64 B8 V   Yes data
Taygeta 19 Tauri 23338 4.30 B6 IV variable Yes data
Maia 20 Tauri 23408 3.87 B8 III variable Yes data
Asterope 1 21 Tauri 23432 5.80 B8 V variable Yes data
Asterope 2 22 Tauri 23441 6.43 A0 Vn   Yes data
Merope 23 Tauri 23480 4.18 B6 IVe emiss. line Yes data
  24 Tauri 23629 6.29 A0   Yes data
Alcyone Eta / 25 Tauri 23630 2.90 B7 III emiss. line Yes data
    23712 6.49 K5 variable No data
    23753 5.44 B8 V variable Yes data
  26 Tauri 23822 6.47 F0   No data
Atlas 27 Tauri 23850 3.62 B8 III spect. binary Yes data
Pleione 28 Tauri 23862 5.09 B8 IVevar irreg. var Yes data
    23923 6.17 B8 V   Yes data
    23950 6.07 B8 III   Yes data
    23985 5.23 A2 V variable No data
    24368 6.34 A2 V variable No data
  33 Tauri 24769 6.05 B9.5 IV ellips. var. No data
    24802 6.19 K0   No data
These data were taken from the SIMBAD database.

As a matter of perspective, the faintest stars listed above are still 40 times brighter than our own sun would appear at a similar distance, and the brightest Pleiad, Alcyone, is 1000 times more luminous! Stars like our sun, of which there are a few in the cluster, appear as faint flecks of light in the AAO photograph at the top of this page, and are well below the sensitivity of the human eye. They are easy to confuse with the numerous stars behind the cluster that also appear in the picture and look very similar. Careful observation and analysis is required to determine which of these fainter stars are cluster members.

A separate membership issue is also worth mentioning. Mythologically speaking, Atlas and Pleione are not Pleiades, but rather the parents of the Seven Sisters. Why do they get two stars named after them? Atlas is already pretty busy holding the heavens up on his shoulders, and myths vary as to whether or not Pleione was placed in the sky with her daughters. So how did the current naming scheme come about? Beats me. If you know, send me email.

Reflection Nebula

Several Pleiads appear surrounded by intricate blue filaments of light. This nebulosity is the result of starlight scattering (reflecting) off minute grains of interstellar dust in the vicinity. The dust particles are inside a cloud of mostly hydrogen gas that the cluster seems to be plowing into.

Sensitive instruments show the nebula extending several degrees from the cluster center in optical, infrared, and radio emission. Much of it can also be seen in ulraviolet light. I studied the interstellar matter around the Pleiades for my PhD thesis.

Outside Links

A few links to external pages of interest:

This page is maintained by Steven Gibson.