The Secrets of the Seven Sisters

Directly overhead in mid-evening for the next several weeks, just to the west of Jupiter, you will find a patch of light that may at first glance appear as a cloud. Staring directly at it you will soon realize that this patch consists of a group of
stars very close to one another. Five stars are easily seen, forming a sort of stubby dipper. With slightly sharper eyesight, or darker skies, we can see the full set of the Seven Sisters, known from the times of the ancient Greeks as the Pleiades. 

Many ancient cultures refer to this distinct grouping of stars that are so easily spotted in winter skies throughout the northern hemisphere. The Greeks associated them with the seven daughters of the titan Atlas, whose fate it was to be punished for warring against Zeus by being condemned to hold the world on his shoulders.  The sisters were pursued by many suitors, and ultimately by Orion the hunter after Atlas was subjected to his fate.  Zeus first turned the sisters into doves, and then into stars to comfort their father, yet Orion himself became a constellation that now eternally pursues the Pleiades across our winter skies.

Under perfect conditions, as many as 11 members of the Pleiades cluster can be seen with the unaided eye.  The famed astronomer Kepler reported as many as 14 in an age pre-dating the telescope.  My favorite renaissance scientist Galileo was the first to observer the Pleiades with a telescope.  A pair of binoculars or a small telescope brings the number of visible stars to about 100.

The determination that the Pleiades are not merely close to each other as observed from Earth, but indeed form a physically connected cluster bound to one another by gravitation was suspected as early as the late 1700’s, based initially on just the odds of having this many bright stars located in so small a visual space (the odds were found to be 496,000 to 1). 

With modern photographic techniques that reveal a myriad of stars in every direction a telescope is pointed, the problem of determining which stars are actually in a given group becomes more complicated.  The accepted method is to determine each candidate star’s proper motion – how the star is moving very slowly through the sky year after year relative to the rest of the stars – and finding which stars in a group share a common motion.  Using this technique, professional astronomers using larger instruments estimate that there are 1000 stars in the Pleiades cluster.

This group of stars is one of the nearest open clusters to Earth, with a distance of approximately 440 light years.  An open cluster forms from a single gargantuan cloud of gas, which fragments and collapses under its own gravitational pull into dozens to hundreds of stars.  The Orion nebula, which was discussed in these pages recently, provides a view of a huge open cluster in its early stage of formation.  The Pleiades, on the other hand, is a completed cluster with no additional star formation in progress.  The stars of the cluster are very young, with ages of about 100 million years.

In an open cluster, the stars are all of nearly the same age, and all lie at approximately the same distance from Earth.  Because of these facts, open clusters are of particular interest to astrophysicists who seek to understand the evolution of stars and changes in their properties over time scales of millions to billions of years.  Specifically, differences in the output of energy, both in magnitude and spectral character (color) across the members of an open cluster must be explained either as variations in the mass or size of the stars, or more exotic properties.   

Surprisingly, for a cluster so near Earth and so intensely studied, the Pleiades have been the subject of a few astronomical intrigues in recent years. 

To measure the distance to nearby stars, the method of parallax can be used.  Precise measurements of the apparent position of a closer star against more distant stars reveals a very slight but repeatable oscillation as the star is observed over a year, as the Earth travels around the Sun in its orbit. 
Considering the Earth at two positions in its orbit, plus the star in its fixed position as forming a vast triangle, if we can measure the angle at the star’s position, and we know the distance Earth has travelled in its orbit between observations of the star, we can determine the height of the triangle, the distance to the star. 

The Pleiades lies sufficiently distant from the Earth to make traditional ground-based telescope measurements of its parallax unreliable due to the inherent blur of the atmosphere.  Astronomers had estimated the distance to the cluster by comparing the apparent brightness of the cluster’s stars to the brightness of similar stars closer to Earth with measurable distances, and arrived at a distance of 440 light years. 

However, in 1997, a space-based telescope designed to measure parallax more accurately than can be done from the ground determined the distance to be 400 light years.  This apparently slight difference challenged the existing theories of star evolution, and brought into question the estimated distances of thousands of stars.  This controversy was resolved seven years later by using the more accurate Hubble Space Telescope, which measured a parallax corresponding to the original distance estimate.

As far back as the 1800’s astronomical photography had revealed that the Pleiades are surrounded by a cloud of gas that glows dimly by reflecting the intense radiation released from the young stars of the cluster.  Although initially believed to be the remains of the gas cloud from which the stars were formed, later studies indicated an age of 100 million years for the cluster members, by which time the group of stars would have ejected the remaining gases from the original cloud.  In the 1980s, observations of the reflection nebula determined that the gas is not associated with the Pleiades themselves, but consists of two independent interstellar gas structures through which members of the Pleiades are currently passing.

Recent observations of the cluster have discovered a large population of “brown dwarves” within the population of stars.  A brown dwarf is a large gaseous body that lacks sufficient mass to ignite the thermonuclear reactions that power a true
star.  The gas giants of our own solar system can be considered to be very small versions of these objects.  With masses typically from 10-60 times that of Jupiter, these objects are now conjectured by some astrophysicists to outnumber the actual stars of our galaxy. The brown dwarves of the Pleiades were among the first of these objects to be directly observed.

To leave you with an unsolved mystery, very recent surveys have discovered the presence of several white dwarf stars within the cluster.  White dwarves are the remains of stars with masses similar to that of our Sun which have exhausted their thermonuclear fuel supply after several billion years of evolution.  Larger stars that have shorter lifetimes create more exotic remnants (neutron stars or black holes).  Finding white dwarves in a cluster with an age of merely 100 million years is therefore difficult to explain with current theories of star formation.  A variety of questionable explanations have been given, including capture of the white dwarves from outside the cluster as it moves through space, or rapid mass loss from large stars due to extremely high rotation rates or solar winds, but none of these explanations have been generally accepted by the astronomical community.

As our tour of the universe continues in these pages, we find that even the simplest of structures – a rather small cluster in the neighborhood of our Sun which we have observed for many thousand years – contains secrets and mysteries which fuel the opportunity of scientific discovery.