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Astronomical Observatory: Cool Images

Astr112 Photography Projects, Fall 2007

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M35 and NGC 2158, Julia Vander Molen, Honors project

M35 and NGC 2158

M35 (upper left) and NGC 2158 (bottom right) are both open star clusters located near the “foot stars” of the Gemini constellation. Although many scientists agree that M35 was discovered in 1745 by Philippe Loys de Chéseaux, Charles Messier credits John Bevis for his independent discovery of it in 1750. M35 covers an area roughly the size of the full moon or 30 arc minutes. This photograph covers an area of 30 arc minutes but only shows a portion of M35. At a distance of 2800ly (860pc), the linear size of M35 is around 24 light years in diameter. There are various estimates of how many stars comprise this cluster but an average is about 250 stars.

M35 is comprised of several dozen young stars which give the cluster its blue hue. These stars are blue because of their high surface temperature around 10,000 K. The hottest star in the cluster is spectral class B3. Because these high-mass main-sequence stars have lifetimes of only a few hundred million years or so, the cluster itself can be no older than that. The cluster is in fact about 150 million years old. M35 also contains some yellow and orange giants of spectral class late G-early K, which are some of its most luminous stars. One yellow giant can be seen in M35 in the photograph above. These stars have exited the main sequence already and the rest of the blue stars will eventually follow this progression.

NGC 2158 is located 15 arc minutes southwest of M35 and is about 5 arc minutes in diameter. This makes its linear size 25 light years in diameter. It has no blue stars and is comprised of only low-mass main-sequence stars. This is why the cluster appears more yellow than M35. All of the high-mass stars have left the main sequence meaning that this cluster must be older than M35. It is in fact nearly 1.05 billion years old. Because only redder stars comprise NGC 2158, the stars in the cluster are much cooler than those in M35. The hottest star is spectral class F0. Because NGC 2168 is comprised of so many more stars and is much more compact than M35, it has previously been considered a globular cluster. However, scientists today consider it to be an open cluster.

M35 is large enough to be seen with the naked eye; however, a small telescope can reveal the distinct stars and features of the cluster.  A more powerful telescope is necessary to view NGC 2158, which is located approximately 17,000ly (5200pc) away.

In 2005, thirteen new variable stars were discovered in M35. Variable stars are stars whose luminosity increases and decreases in brightness over a short period of time. They can be used to determine distances to other stars with similar light curves located elsewhere in the universe. Three eclipsing binaries were found. These specific eclipsing binaries are stars that orbit so close to each other their surfaces almost touch. Eclipsing binaries typically have a period of less than a day. One percent of all stars belong to this group of stars and they are the most common variable star in the universe. Thus, they can be useful as standard candles for determining the distances to stars in nearby galaxies.

Bonanos, A.Z. “Eclipsing Binaries: Tools for Calibrating the Extragalactic Distance Scale.” 31 October 2006.

Freedman and Kaufmann, Universe: Stars and Galaxies. 3rd Ed. 508-510. 2007.

Frommert, Hartmut and Kronberg, Christine. “Messier 35.” 25 August 2007.

Hu, Juei-Hwa. "Discovery of 13 New Variable Stars in the Field of the Open Cluster NGC 2168 (M35)." Chinese Journal of Astronomy and Astrophysics 5.4 (2005): 356-62.

Right Ascension (J2000) 06:08:39
Declination (J2000) 24:10:19
Filters used blue(B), green(V), red(R)
Exposure time per filter 20x20 seconds in B, 10x20 seconds in VR
Date observed

November 12, 2007 (BVR)





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