The Pleiades in Taurus
November 2023 - Nebula and Cluster of the Month
November is dominated by the arrival of the brilliant winter constellations. Leading the pack is the beautiful open cluster, the Pleiades. This little shining web of stars is so familiar that deep-sky observers tend to ignore it. It’s too bright, it’s too easy, it won’t fit in the field of view, etc. BORING!
But no, it’s not boring. The Pleiades have been important for pretty much the whole of human history (and prehistory). In ancient Greece, the heliacal rising of the Pleiades (the first time they could be seen in the pre-sunrise eastern sky) signalled the start of the sailing season and also the agricultural year.
The Nebra Sky Disc, discovered in Germany in 1999 and dated to 1600BC shows a cluster of seven stars which is believed to represent the Pleiades.
The Pleiades appear in myths all around the world, and these myths are surprisingly similar. In the familiar Greek legend, the Pleiades were the seven nymph daughters of the Titan Atlas, whose job it was to carry the Earth on his shoulders. With this burden, he was unable to protect his daughters so Zeus (who sired children on more than one of the sisters), turned them into stars to protect them from the depredations of the hunter and woman-chaser, Orion.
Australian Aboriginal legends speak of the Pleiades as seven young girls, fleeing either a hunter (represented again by the constellation Orion) or fleeing several young men (also represented by Orion).
This and remarkably similar myths occur in nearly all cultures across the world. How could Australian Aboriginal legends be so similar to Greek ones? Was the Greek legend imported into Australia some time in the past 200 years? It would seem not. The Australian myth seems to be of very considerable antiquity.
The similarity of so many legends from disparate cultures is so remarkable that Australian astronomers Ray and Barnaby Norris have suggested1 that the legend originates in Africa, before the spread of modern humans across the globe. They suggest that the legend is at least 100,000 years old, making it the oldest of all humanity’s stories.
From an astronomical viewpoint, the Pleiades is one of the closest open clusters to us, being about 400 light-years away. It’s a young cluster, the bright stars all being B-type giants. Although the cluster is best known for its six (or seven) brightest stars, there are in reality about 100 stars visible in a small telescope. This number rises to over 1000 with proper motion and distance studies of the stars in the vicinity. Imagine a roughly conical cloud of ordinary stars (spectral types A to K) with a long chain of brilliant blue stars projecting out of its wide base, like an umbrella or a mushroom. We are lined up with this chain of bright stars, so we see them clumped together. This is what the Pleiades actually look like.
The individual brightest stars all have names, the names given to the seven sisters and their parents in Greek mythology.
| Star | Name | Magnitude | Spectral Type |
|---|---|---|---|
| η Tau | Alcyone | 2.8 | B5 |
| 16 Tau | Celaeno | 5.5 | B5 |
| 17 Tau | Electra | 3.7 | B5 |
| 19 Tau | Taygeta | 4.3 | B5 |
| 20 Tau | Maia | 3.9 | B5 |
| 21 Tau | Asterope (Sterope I) | 5.8 | B8 |
| 22 Tau | Sterope II | 6.4 | B9 |
| 23 Tau | Merope | 4.2 | B5 |
| 27 Tau | Atlas (the father) | 3.6 | B8 |
| 28 Tau | Pleione (the mother) | 5.0 | B8 |
21 and 22 Tau are separated by only 2’, and can’t really be distinguished from each other by the naked eye. Combined, they form the star called Asterope, but individually they have been named Sterope I and Sterope II.
Through a telescope (other than a wide-field instrument), the Pleiades can be a little underwhelming, being too large to fit in the field of view. At best you can get two or three of the bright stars in. In large binoculars or a wide-field telescope, however, they are breathtaking.
The Pleiades are also famous for the bright blue nebulosity in which they are wreathed. This is caused by light from the brilliant blue stars reflecting off dust particles. It was once thought that the nebulosity represented the remnants of the material from which the cluster was formed, but what we’re actually seeing here is a dusty region of the interstellar medium that the Pleiades are currently passing through. It is unrelated to the cluster.
The brightest part of this nebula is around the star 23 Tauri (Merope). It bears the designation NGC 1435 and given the right conditions is not difficult to see, even with a small telescope. The sky must be dark and this is the biggest stumbling block to seeing the nebula.
I have never seen the nebulosity from a suburban site, despite much looking. However, I was amazed by just how clear it was from a dark-sky site. The NELM was 6.0 (5 Aurigae at an altitude of 60°). I was using my 12” (300mm) Newtonian reflector with a 25mm eyepiece which gave a magnification of x60 and a field of view of 50’. I used no filters.
NGC 1435 was visible as a quite distinct fan-shaped nebula with Merope at its apex. Although nowhere hard to see, it was clearest at its edges. It fades away imperceptibly to the west. It does not look blue!
If you are fortunate enough to be in a dark-sky location with your telescope and the Pleiades are visible, do have a go at seeing the nebulosity. If the nebula around Merope is clear, scan over the whole cluster. You may get to see some of the fainter wisps.
The Pleiades can be enjoyed, therefore, with everything from your naked eye up to a large telescope.
It is a magical thing simply to look up at the Pleiades in wonder and to feel a distant, tenuous but real connection to our long-distant ancestors as they stared in awe into the dark African skies and, not understanding what they saw, first began to tell stories.
| Object | RA | Dec | Type | Magnitude |
|---|---|---|---|---|
| NGC 1435 | 03h 46m 20s | +23° 57’ 00” | Reflection nebula within the Pleiades |
References:
- Advancing Cultural Astronomy, Springer 2020.
