The Pleiades

The Stars of the Pleiades

The Pleiades are a dipper-shaped cluster of stars situated in the "shoulder" of the constellation Taurus. While roughly 400 light years from Earth, the Pleiades are one of the most visible clusters in the night sky. On a given night, between six and seven of the nearly 500 stars in the clusters are visible with the naked eye (Frommert, online). The Pleiades have been the subject of a plethora of myths and tales from a variety of cultures; they are best known, however, as the seven sisters from Greek mythology. The nine most prominent stars have individual Greek names and represent each of the seven sisters of Greek mythology and their parents. The brightest member in the Pleiades is Alcyone (25 Tau.), which has an apparent magnitude of about 2.9. The other stars in the cluster that have Greek names include Merope, Celaeno, Sterope (which is actually a double star), Taygeta, Maia, Electra, Atlas, and Pleione. Their apparent visual magnitudes range from 3.8 to 5.5 (Hipparcos, online).

 A closer look at the Pleiades reveals a reflection nebula surrounding the cluster. This nebulosity was first thought to be gas and dust that was left over from the formation of the stars within the cluster. Further investigation of the cluster proved the stars to be around 100 million years old, much older than originally estimated, and far too old to have left over gas and dust surrounding them (Barentine, online). Astronomers later discovered a shock wave or wake in the interstellar medium around the Pleiades caused by ultraviolet radiation reacting with dust and gas. The motion of this wake carries the dust or gas in the interstellar medium with it, and allows astronomers to detect the motion of this gas, and the source of the wake (White, p. 234). Using this technique, researchers have traced the movement of the nebulous gas surrounding the Pleiades back to Gould's belt, east of the Pleiades, between the constellations Taurus and Perseus. The source of this shock wave  is believed to be a star that exploded in that area around 15 million years ago (Barentine, online). The nebulous clouds are clearly visible in the photograph below.

Brown Dwarfs

Over the past few years, astronomers have made major breakthroughs in the understanding of star formation. One such breakthrough was the discovery of brown dwarfs. Brown dwarfs are small stars that are unable to shine. While they tend to be roughly the size of Jupiter, brown dwarfs contain 10 to 80 times more mass (Henry, p. 25).  A brown dwarf forms in a  manner similar to main sequence stars. In the case of a brown dwarf, however, the forming protostar does not develop an  adequate amount of mass, and cannot generate enough temperature and pressure to cause fusion (Kaufmann, 1999). After  its initial formation, a brown dwarf may flicker or emit a small amount of light, due to small amounts of burning deuterium (Henry, p. 25). Over time, however, a brown dwarf will continually fade, and its temperature and luminosity will drop, making brown dwarfs exceptionally difficult to find (Henry, p. 25). One tool used by astronomers to detect brown dwarfs is a lithium test. Lithium is a trace element that is incorporated into stars when they form. When a normal star ignites, and begins fusion, the lithium present is burned off. The temperature in a brown dwarf, however, is not high enough to burn lithium, and the element will appear in the spectra of the brown dwarf (Henry, p. 26). Brown dwarfs are more easily located when it is young, and its luminosity and temperature are the greatest. For this reason, astronomers look toward areas of recent star formation, such as star clusters, when attempting to detect brown dwarfs. Recently, astronomers were able to locate two possible brown dwarfs in the Pleiades, PPL 15 and Teide 1 (Henry, p. 27).  Hambly et. al. has discovered several more good candidates in the Pleiades as well, and astronomers across the world are looking towards the Pleiades for evidence of brown dwarfs. A photograph of a brown dwarf around star Gliese 229B, which lies between Lepus and Canis Major, is shown below (Henry, p. 27).

 

A brown dwarf. Courtesy of the Space Telescope Science Institute, operated for NASA by AURA. Available: http://oposite.stsci.edu/pubinfo/
 
 

The H-R Diagram

Astronomers use a variety of tools to analyze the properties of a star cluster. One such tool is the H-R diagram. Developed independently by Enjar Hertzsprung and Henry Norris Russell, the H-R diagram plots star luminosity against surface temperature with temperature on the x-axis increasing from right to left (Kaufmann, Freedman, p. 475). Because a star's surface temperature and luminosity will change as it ages, H-R diagrams are especially helpful in determining the age of a star or star cluster. When plotted on an H-R diagram, most stars (including our Sun) fall along a line called the main sequence that runs from the upper left corner to the lower right corner of the graph (Kaufmann, Freedman, p. 475). Stars that do not lie on the main sequence are classified as particular types of stars depending on their relative position on the graph. Stars that group in the lower left corner of the graph have a high surface temperature but low luminosity, and are referred to as white dwarfs. Stars that group in the upper right corner (with low surface temperature and high luminosity) are called giants or supergiants (Kaufmann, Freedman, p. 475). H-R diagrams are especially helpful in determining the age and evolution of a star cluster because most of the stars in the cluster will have been formed at around the same time, and  will follow a similar sequence of changes that can be seen on the diagram. An H-R diagram of the nine main stars of the Pleiades is shown below.