Taurus the Bull

“When the Pleiads, Atlas’ daughters start to rise

Begin your harvest; plough when they go down

For forty days and nights they hide themselves,

And as the year rolls around, appear again

When you begin to sharpen sickle-blades;

This law holds on the plains and by the sea,

And in the mountain valleys, fertile lands

Far from the swelling sea.”

Works and Days – Hesiod (8 BC)

One of the highlights of Taurus and indeed the whole sky is the glorious open cluster M45. I have often stood beneath the summer celestial sphere and gazed longingly at the small misty glow within the ‘bull’s’ body, my imagination captured by this cosmic ‘cloud’ of hot young stars who collectively form the stellar family ‘Pleiades’ or ‘Seven Sisters’. Even the immortal poet Tennyson was captured by their sheer beauty, writing:

“Many a night I saw the Pleiads,

rising thro’ the mellow shade,

Glitter like a swarm of fireflies

Tangled in a silver braid.”

This fine group has been revered by humankind since antiquity. Titles like ‘Virgins of Spring’, ‘Stars of Abundance’ and ‘Stars of the Season of Blossoms’ were enshrined with the Pleiades. Chinese women worshiped them as the ‘Seven Sisters of Industry’. The Maori of New Zealand revered it as Matariki and greeted its rising with laments for those who had recently died. An important task of Matariki was to keep moving in a cluster through the heavens so that it could predict lean and fat seasons and bring food supplies to those eagerly waiting.

There was a universal vision of a hen and her chicks amongst many, the Russians calling this group the ‘Sitting Hen’, the Danes referring to it as ‘Eve Hen’. The French and Italians called it the ‘Pullets’ and the Hungarians saw it as ‘Hen with Fledglings’.

Amongst Mediterranean peoples, the rising of the Pleiades heralded the beginning of the season for navigation. Manilius called this group ‘Narrow Cloudy Train of Female Stars.’ The Finns and Lithuanians saw it as a ‘Sieve’ whilst some French peasants thought it a ‘Mosquito Net.’

To the naked eye from a reasonably dark site, six Pleiads are easily seen though upwards of nineteen have been reported. I have, under a very dark sky and with good atmospheric seeing, seen 11 stars (admittedly when I was younger) but the closeness of the members themselves makes resolution something of a trial. With binoculars, the cluster comes alive and with greater optical power, literally hundreds of stars are seen.

Riccioli in 1651 caught the flavour of this cluster when he wrote:

“…a first rate field-glass, taking in 3 1/4° and magnifying seven diameters, shows 57; Hooke, in 1664 saw 78 with the best telescope of his day; Swift sees 300 in his 4 ½ inch and 600 with his 16 inch.”

Modern estimates put the membership of the Pleiades at around 300 to 500 stars. There are no bright red or yellow members within the cluster, the brighter stars being of earlier spectral types B through to F. In contrast though, there are a few faint red dwarf variable stars similar to the flare stars inside the Orion Nebula. They typically have a luminosity only 1/100th that of our Sun and shine between 13.3 and 16.9 magnitudes. In 1963, G. Haro studied 7 of these stars and found them to have light variations of around 0.8 to 3.7 magnitudes within a period of several minutes to 3 hours. It was in 1767 that Rev. John Mitchell calculated the probability of finding such a group of stars such as the Pleiades by chance alignment. His calculations showed a chance of 1/496,000. It was this calculation along with the commonality of such clusters in the sky that lead him to believe that these clusters are in fact physical groups.

The Pleiades group of stars lies around 380 light years from our Sun, as measured by the satellite Hipparcos. The members are thought to be around 100 million years old, young on a cosmological scale.

The brightest member star and the most centrally located is Alcyone (eta), a B-type subgiant of magnitude 2.86 with a luminosity of 2300 Suns. Its diameter is probably no more than 10 times our Sun and its mass around 5 Suns. Alcyone is the most massive of the member stars.

Archibald Lampman wrote of Alcyone:

“…the great and burning star,

Immeasurably old, immeasurably far,

Surging forth its silver flame

Through eternity, …Alcyone.”

The early Arabs called Alcyone the ‘Walnut’, the ‘Central One’ and the ‘Bright One’. A telescope centred on this beauty will show 3 faint stars forming a beautiful triangle close by. Maia is the first born and the most beautiful sister of all. Shining at magnitude 3.9, this B-type giant star has a connection with our current calendar. Electra is a magnitude 3.7 B-type giant and Celaeno is a B-type subgiant of magnitude 5.5. Taygeta is a similar B-type star shining at magnitude 4.3 and may be a spectroscopic binary. There is also a companion of magnitude 8.1 with a 68.8” separation first discovered by Sir John Herschel. Sterope is a pair of widely separated stars of magnitude 5.8 and 6.4, this pair sometimes known as Asterope. Merope is the only sister to marry a mortal and therefore hides her head in shame. She is immersed in a diffuse and wispy nebulosity, discovered by Tempel in 1859.

This nebula is part of a huge faint cloud (NGC 1435) that cloaks this whole region. Merope shines at magnitude 4.2, another B-type subgiant. This great cloud of illuminated dust and gas was thought to be the nursery from which the Pleiades were born. However, clusters the age of the Pleaides at 20 million years should have long ago dissipated their original prenatal gas. Obviously this is not so here so further study was needed to unlock the mystery.

Recent evidence shows that the Pleiades have burrowed through a non-related cloud of gas and dust on their collective journey around the galaxy. The stars are also not hot enough to ionize this gas, except for gas that lies very close by so what we see is in fact starlight reflected off the grains of dust, giving rise to the blue colour of this nebula in photographs. This finding is further evidenced by the fact that the Pleiades and the nebula NGC 1435 do not share a common motion across the sky.

NGC 1435 can be extremely difficult to discern in the telescope, sometimes missed in large mirrors and seen in smaller. Sky conditions and optics all play a part but once experienced, it is never forgotten. Walter Scott Houston, using a 20cm reflector in 1974 was surprised when he:

“…looked into the eyepiece, expecting to see a few faint wisps, the field was laced from edge to edge with bright wreaths of delicately structured nebulosity…”

The brightest region of NGC 1435 lies about Merope, discovered as mentioned previously by Ernst Tempel on October 19, 1859 with a 10cm refractor from Italy. A faint extension, NGC 1432 was discovered about Maya in 1875. The rest of this faint nebulosity surrounding Alcyone, Celaeno, Electra and Taygeta was discovered five years later.

Next to Merope lies IC 349, a reflection nebula first seen E.E. Barnard in 1890 using the Lick Observatory 36” telescope. Named Barnard’s Merope Nebula, it lies close to its illuminating star with a separation of 0.06 light years. This equates to 3,500 times the Earth/Sun separation for comparison.

Pleione and Atlas, parents of the Pleiads are indeed joined in wedlock, these two stars forming an actual binary system with an apparent separation of 5.0’, a marriage made in heaven. Atlas is a magnitude 3.8 B-type giant catalogued as a double by F. Struve in 1827. The very close separation of this pairing at 0.4” and a magnitude difference of 3 makes resolution difficult in even the largest of telescopes.

Pleione is a 5th magnitude B-type main sequence star with a manic rotation, around 100 times our Sun’s rotation. This causes it to eject a series of expanding gaseous shells in an outward direction from the equatorial region. Known as BU Tauri, this variable star’s brightness has only been slight in recent years.

Around 36.0” to the north northeast of Merope lies the fascinating variable star T Tauri. This very young star is immersed within a fascinating faint reflection nebula called Hind’s Variable Nebula. Discovered by Hind in the 1850s, this nebula changes its brightness over time, presumably because of passing dust obscuring then releasing the light of the nebula. A small nebula NGC 1554 was discovered nearby by Struve in 1868 but it has disappeared. Dreyer found no sight of it in 1877 whilst using Lord Rosse’s very large telescope. A 14th magnitude star lies at this position, 4.4’ west southwest of T Tauri.

A tiny nebula 4.0’ southeast of T Tauri has also disappeared, one only observation recorded by Bigourdan in 1890. This region deserves constant monitoring for further changes though NGC 1555 is currently rather small and faint.

There is another open cluster that resides within the boundaries of our celestial ‘bull’, the large v-shaped Hyades star cluster that outlines the bull’s head, it’s glaring eye represented by the fiery orange giant star Aldebaran (alpha Tauri). Derived from Al Dabaram the ‘Follower', it pursues the beautiful ‘Seven Sisters’ across the heavens.

The ancient desert dwellers, surrounded by oceans of sand, turned their curious eyes to the sky and delighted in the heavenly theatre. More than one had their eye drawn to bright Aldebaran whom they called the ‘Camel’.