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Physics Photo of the Week

May 6, 2005

Open Cluster M67

M67 is an old open cluster in the constellation Cancer in the high sky - about 10 degrees west of the zenith at 9:00 PM in early May.  This image was made on May 1, 2005 with a new camera called the "Deep Sky Imager".  The camera is attached to the prime focus of an 8-inch Celestron telescope.  Warren Wilson College Physics Department is developing a course activity for Astronomy and Physics in which the colors of the stars are analyzed.  Notice the subtle color variation in the stars in the image on the right.

The interesting feature of this open cluster is its rather large age.  Ordinary stars like our sun shine by nuclear fusion of hydrogen into helium.  The more massive stars burn hotter than less massive stars.  If the brightness of the stars in a cluster are plotted on the vertical axis while the color index is plotted on the horizontal axis, the stars of a young cluster follow a diagonal line called the "main sequence".  The plot is called the Hertsprung-Russell Diagram (H-R diagram).  The main sequence is illustrated well  with the H-R diagram below for the fairly young cluster Chi Persei.  Notice the predominance of bright blue stars in the image of Chi Persei, and how its H-R diagram slopes from upper left (bright, blue-hot stars) down to lower right (faint, red-cool) stars.

Chi Persei
Contrast the H-R diagram for M67 - this week's feature.  The photograph for M67 indicates that many of the brightest stars are red, the H-R diagram shows that the bright stars lie in the upper right of the plot indicating that they are red giants.  This is due to the large age of the cluster.  The bright stars exhaust their hydrogen fuel faster.  When the hydrogen is depleted, the core of the star collapses due to gravity, the resulting greater pressure ignites helium fusion.  The helium fusion in the core causes the outer envelope of the star to expand and become cooler.  The cooler temperature results in a redder star, and the expansion makes for a bright red star.

M67 - April 2004 - Digital Camera
At Warren Wilson College we are currently investigating the practicality of using the new Deep Sky Imager (available from Meade Instruments) with a consumer-grade digital camera.  The lower images were made from the digital camera, while the large image of this week's feature was produced with the new Deep Sky Imager.  The digital camera apparently shows better color contrast than the Deep Sky Imager, but the analysis algorithms are currently under development.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature an interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to

Click here to see all Physics Photo of the Week for 2005

Click here to see all Physics Photo of the Week for 2004.