by Pat Browne
The Pleiades – a naked eye Open Star Cluster
(also known from Greek mythology as the Seven Sisters)
We first encountered the Pleiades setting in the West in the early springtime (see Early Spring Night Sky session – after Orion nebula). It was setting in the West because it is located in Taurus next to our familiar winter constellation Orion. In autumn, (beginning in October), it is visible rising in the East well after astronomical twilight. You can see the Pleiades naked eye above the emerging form of Orion the hunter (Northern Hemisphere).
This photograph of the Pleiades captures the 7 stars in the dipper star pattern and the nebulousity around these hot brilliant stars – courtesy Luc Bellavance Astro Rimouski
The “7 Sisters” is an Open Cluster
Open clusters lie within the plane of the galaxy, and contain from a dozen to 100 stars. These cluster members are ‘born’ from a common cloud of dust and gas. The distance to these objects is usually < 1000 light years. Open clusters can often be seen just with the naked eye if bright and close enough; binoculars and small telescopes reveal more detail. This open cluster is easy to view in binoculars or a small telescope, even in an urban environment.
The Pleiades cluster is dominated by hot blue and luminous stars that have formed within the last 100 million years. A faint reflection nebulosity around the brightest stars was thought at first to be left over from the formation of the cluster (hence the alternative name Maia Nebula after the star Maia), but is now likely an unrelated foreground dust cloud in the interstellar medium, through which the stars are currently passing. – courtesy https://en.wikipedia.org/wiki/Pleiades
The Distance Controversy…
It is interesting to note that this cluster – visible without any optical aid, also has a very accurate distance measurement thanks to the astrometric efforts of various science teams. Radio astronomers have gauged the distance to this cluster to be roughly 440 light years. The method used was trigonometric parallax to astronomical objects
Since 2014, the ground-based radio measurements have been considered the most accurate. See www.astronomy.com/news/2014/08/radio-telescopes-settle-controversy-over-distance-to-pleiades
The extraordinary resolution provided by Arecibo radio telescope revealed a controversy: The data provided from the Hipparcos satellite mission produced over a 10% smaller distance of 390 light years. The controversy had to wait for the next satellite mission to determine which distance is more accurate . As of 2018, we now have the data from the Gaia satellite which confirms the VLBI ground based observations!
The following excerpt is taken from the article Gaia Weighs in on the Pleiades Distance Controversy:
Results from various ground-based techniques all agreed that the distance was about 133 parsecs, making the Pleiades a solid rung on the lower end of the “Cosmic Distance Ladder.” This important role was called into question by results from the [ ESA parallax measuring] satellite, Hipparcos, the gold standard of distance measurements. The distance measured by Hipparcos is 120.2 ± 1.5 parsecs [390 ly], significantly and disturbingly different from traditional ground-based values and setting up the so-called “Pleiades distance controversy.” Although this amounts to only a 10% difference in the distance, the result propagates through the system and affects the size, age, and physics of the universe and objects in the universe. This disagreement led to significant shifts in the cosmic distance scale and controversial revisions of physical models required to obtain the Hipparcos result.
The VLBI result, which uses Arecibo (red),is consistent with previous ground-based measurements (black). The Hipparcos results (blue) set up the controversy, but the new measurements from Gaia (green) confirm those of Arecibo VLBI.To understand more about how Gaia continues the mission of extending precise measurement of the stellar distances see –Gaia mission – astrometry in space
Observing the Pleiades:
Seven of the brightest stars, potentially visible with the naked eye are:
The “Visual Observing controversy” or .. How many stars can you see naked eye?
Comment: “In fact, the number of stars you can see within the Pleiades cluster, using just your eye, varies depending on your own eyesight, local atmospheric transparency and light pollution levels. Some people simply see fainter stars than others. It’s possible that early skywatchers, whose skies were darker and clearer than our modern skies, more often saw more than six stars here. Even today, people with exceptional vision see seven, eight or more stars in the Pleiades with the unaided eye.” courtesy – earthsky.org
With a small telescope, you can observe the various multiple star systems and note the colour contrasts of the stars embedded in the cluster. You can also try to map the nebulousity surrounding various members, such as Merope Maia. and Alcyone. Pleione is a very interesting binary star with special spectral characteristics: see Pleone – a shell star with circumstellar disk