the "Seven Sisters"

the Pleiades

by Vichaya Sirisanthana

The Pleiades are a cluster of young stars imbedded in a reflection nebula. Located in the northern constellation of Taurus, the cluster contains hundreds of stars lying about 400 light year from us. Nevertheless, under normal conditions only seven of the brightest stars can easily be seen with naked eyes. Thus the Pleiades are also called the "Seven Sisters," which are the daughters of "father" Atlas and "mother" Pleione: Alcyone, Asterope (a double star), Electra, Maia, Merope, Taygeta and Celaeno.

The Pleiades are located at right ascention 3 hours 47 minutes and declination 24 degrees 7 minutes north. Their apperent magnitude, as a whole, is 1.6, and they spread out over 110 arc minutes in the sky.

This image is from http://www.ast.cam.ac.uk/AAO/AAO/local/www/dfm/uks018.html

During the Fall semester 1999, I chose to take images of the Pleiades as my final assignment in the Astronomical Technique class (Phys 20). Both of my images were taken on November 28th around 10.45pm, using the 16-inch telescope at the Macalester Observatory and the CCD imaging technology. Since our 16-inch telescope has a narrow field of view whereas the Pleiades spread almost 2 degrees on the sky, my images could capture only parts of the star cluster.

Exposure time:

0.1 second

Resolution:

250x242

Focus:

3170

Airmass:

1.068

65532

Stars:

Background:

110

Signal/Noise Ratio:

255

0.1 second

Exposure time:

Resotion:

250x242

3170

Focus:

Airmass:

1.071

6100

Stars:

Background:

141

Signal/Noise Ratio:

77.19

Since the Pleiades are pretty bright objects (apperent magnitude = 1.6), the exposure time for both images was short (0.1 seconds). The resolution of the images was 250x242 pixels per inch. The airmass was low (1.068 and 1.071) because at the time I took the images, the Pleiades were almost at the zenith.

Three color filters were used to image the Pleiades at different wavelengths. After setting the CCD imaging equipment, the first image was taken through the red filter. The red filter yeilds only red light to pass through it, and, thus, the CCD ship captured only red objects in the sky. The same field of the sky was then taken through the green and blue filters, respectively. Similarly, the green filter only yeilds green light and the blue filter only yeilds blue light. After completing the first image, the second image was also taken in three different wavelengths.

After taking all the images, each three images of the same field--but different wavelengths--were combined into one final image, using Photoshop. Since all colors can be represented by mixing three main light colors--red, green, blue--the final images are in real colors.

The final images, then, were tested for the Signal/Noise Ratios, which basically tells us the clearness of the images. First, we measured the value of the brightest star in the image. Then measured the value of the background. The result of the square root of the former over the latter is the Signal/Noise Ratio. Consistent with the low airmass, the Signal/Noise Ratios of my images are quite good (65532 and 6100).

This image is from http://www.ras.ucalgary.ca/~gibson/pleiades/

The stars in the Pleiades are blue. This may not be obvious in either of my images. However, if you look closely at the brighter star on my second image, you can see a little blue color on the outer layer. In fact, images from larger, better telescopes reveal that almost all of the brightest stars in the Pleiades are blue. The blue color tells us that these stars in the Pleiades are young and hot. Researchers discovered that all of the stars in the Pleiades were formed at roughly the same time, and the age of the cluster is about 50 million years (Kaufmann. Universe. 506.)

This image was brought from http://astronomer.net/messier/m/m045.html

The blue region that you see in the image above is a reflection nebula. A reflection nebula is basically interstellar gas and dust that lies between us and the stars behind. The nebula appears blue because blue light from the stars behind get scattered when trying to pass through those gas and dust. This is the same reason why our sky looks blue.

The fact that the Pleiades are imbedded in a reflection nubula makes them one of the most spectacular star clusters we have ever known.