Abstract
Star clusters are among the most intriguing, amazing, and beautiful objects in the night sky. They can be young or old, large or small, bright or faint, and so on. But what is important, as they relate to this guide, is that seen in a telescope (or binoculars, or even the naked eye), they can be glorious, with a dazzling array of colors, brightnesses, and even shapes with arcs and streams, wisps of nebulosity, and dark dust lanes, making them literally breathtaking.
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Notes
- 1.
The number of clusters that can be seen in very large telescopes is astounding. Literally hundreds more will be available. And if you live in the southern hemisphere, then the number of clusters is vast, due to the Large and Small Magellanic Clouds.
- 2.
For a detailed explanation of the classification of stars, and even more information about their life cycles, see the references listed at the end of this book.
- 3.
Not all of these are visible in the range of amateur telescopes as mentioned in this book.
- 4.
These are large groups of stars, so, can be thought of as clusters – with only a small distortion of the truth.
- 5.
Those of us familiar with the climate of northern Europe and the UK know that even in early and late summer, when the days may be warm, and sometimes hot, the nights can still be quite chilly.
- 6.
Believe it or not, many observers myself included, have reached this stage on several occasions, not wanting to waste a minute of a clear night!
- 7.
You may notice that some constellations are absent from the list. That is because they do not have any clusters in them.
- 8.
The material was presented this way after much consultation with many astronomers.
- 9.
If a constellation appears in 2 months, it is because it culminates on either the last, or first, day of a month.
- 10.
Any cluster or constellation can of course be observed earlier or later than this date as it rises about 4 min earlier each night, nearly ½ h each week, and thus about 2 h a month. To observe any cluster or constellation earlier than its culmination date, you will have to get up in (or stay up to) the early hours of the morning. To observe later than the culmination date will mean observing earlier in the evening, and vice versa.
- 11.
It can of course be seen earlier or later in the month given, but of course it may not be at its highest point in the sky at midnight.
- 12.
The quoted magnitude of a cluster is the integrated magnitude. This is the combined visual magnitude of all its components, as if the cluster’s light was combined into a star like point. It may be the result of only a few bright stars, or, on the other hand, may be the result of a large number of faint stars. But beware! The cluster is not a point of light but spread across a discernible diameter, so, as the light spreads out, its intensity will drop off rapidly, causing the cluster to become fainter. Thus, the quoted magnitude may appear considerably dimmer than a star of the same absolute magnitude. Different observers will see differing magnitudes. Treat the given value with a certain amount of caution.
- 13.
Also, the diameter of a cluster is often misleading, as in most cases it has been calculated from photographic plates, which, as experienced amateurs will know, can bear little resemblance to what is seen at the eyepiece.
- 14.
They also quite heavy, and so could probably be used as an emergency counterweight.
- 15.
It is often useful to be able to determine the night sky’s observing conditions (light pollution, haze, cloud cover, transparency) before starting an observing session, so as to determine what types of objects will be visible and even allow you to decide whether observing is viable at all. A good way to do this is to use a familiar constellation, which should be observable every night of the year, and estimate what stars in the constellation are visible. If only the brighter stars are visible, then this would limit you to only bright stellar objects, while if the fainter stars in the constellation can be seen, then conditions may be ideal to seek out the more elusive, and fainter objects. A favorite constellation used by many amateurs for just such a technique is Ursa Minor, the Little Bear. If, once outside, you can see υ UMi (magnitude 5.2) from an urban site, then the night is ideal for deep-sky observing. However, if υ UMi is not visible, then the sky conditions are not favorable for any serious deep-sky observing, but casual constellation observing may be possible. If the stars δ, ε, and ζ UMi, located in the “handle” of the Little Bear, are not visible (magnitudes 4.3, 4.2 and 4.3, respectively) then do not bother observing at all, but go back indoors and peruse this book.
- 16.
Compiling such an guide that includes a terrific amount of data was a difficult, but tremendously rewarding task. However, there may be times when I have inadvertently made a typographic error, such as in the size of a cluster, or its magnitude, etc. If this occurs then I apologise, as any such errors are mine and mine alone.
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Inglis, M. (2013). Introduction to Star Clusters. In: Observer’s Guide to Star Clusters. The Patrick Moore Practical Astronomy Series. Springer, New York, NY. https://doi.org/10.1007/978-1-4614-7567-5_1
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