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Astr110 Photography Projects, Fall 2007

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M73 Asterism (NGC 6994), SolJi Jeon



This object was first discovered in 1780 by Charles Messier. Although this is generally defined as a cluster according to Astronominal observation, its obscure appearance as a nebula and the debate over the four stars having a physical association with each other resulted in the characterization of the object as a "Group or Asterism of four stars". However, it was still named M73 next to an open cluster, M72, because of their close position. Recently, a research published by Odenkirchen and Soubiran in 2001 made it clear that M73 (NGC 6994) has no physical relations with each other by using three factors; radial velocity, proper motion and distance. First of all, the differences in radial velocity of these stars were "too large to interpret stars as a wide triple system or a long-lived unbound group". Second of all, this research also shows that these stars "do not share a common mean tangential motion" as their proper motion vectors were varied. This also proves an unrelated relations of the stars. Lastly, Odenkirchen and Soubiran measured spectroscophy*of the stars, and concluded that "absolute magnitudes and their apparent brightness do not agree with each other", which means that these stars are not clustered.

Above are two pictures of M73 taken on October 23, 2007 by the Calvin College Observatory. The first picture is a synthesis of four different colored filters; Blue, Visual(Green), Clear and Red. Ten pictures of each filter were combined first and those combinations were once more put together to make one final result. The second is the same copy, except its brightness is adjusted to observe the color differences of the stars. As shown above, the biggest and the smallest star are both brighter than any of the others. Although the exact color of the biggest and the smallest star is not shown clearly, the rim of them is more red than that of the other two stars. It is the temperature of the star that determines the color. Cooler stars are closer to red, and warmer stars are closer to blue in general. Therefore, the biggest and the smallest stars are low-termerature ones where the other two are high-temperature stars. The linear size of M73 is 0.15 light year.

* observing a spectrum of light or radiation


Frommert, Hartmut and Kronberg, Christine. "NGC 6994." Students for the Exploration and Development of Space. <http://seds.org/messier/m/m073.html>

Ibid, Messier Questions&Answers

Odenkirchen, M.& Soubiran, C. "NGC 6994 - clearly not a physical stellar ensemble." Astronomy&Astrophysics manuscript (2001): 1-6.

Right Ascension (J2000) 20h 59m 00sec
Declination (J2000) 12 38' 00"
Filters used blue(B), green(V), red(R), and clear(C)
Exposure time per filter

5 sec for (C), 10 sec for (V)(R), 20 sec for (B)

Date observed

October 23, 2007 for (B)(V)(R)(C)