Cosmic Perspectives

Here's what the night sky would look like if the Orion Nebula, the Pleiades, the Crab Nebula, and the Dumbbell Nebula (top row, left to right) and the Hercules Cluster (bottom) were all at the same distance as the Pleiades.
S&T Illustration
In the September 2007 issue of Sky & Telescope, we show what a few nebulae and star clusters would look like if they were all at the same distance as the Pleiades. There's also a graph showing how bright our Sun would look if it were moved to these objects' actual distances. But the magazine doesn't have enough room to list data for many such objects. And even if it did, your eyes would glaze over trying to read it. Far better to have the information on a computer, where you can sort and search it any way you want.

For people who run Excel on their computers, we've published the data as an Excel spreadsheet. It's also available as comma-separated values and as an HTML file.

Sources

In general, the distances to deep-sky objects are not known very accurately. This is particularly true of nebulae, where professional astronomers' distance estimates frequently vary by a factor of two or three, and occasionally much more.

Angular sizes are equally hard to pin down, but for a different reason. Few deep-sky objects have well-defined edges that can be measured objectively. For instance, deep photographs show that M42 and M78 are actually two bright areas within a huge luminous cloud that covers most of Orion, and it's a judgment call where to draw the boundaries within this super-nebula. So published diameters for M42 vary anywhere from 30' to 1.5° or more.

The primary data source for this article is the website of the Students for the Exploration and Development of Space (SEDS), which has lots of information on the Messier objects and a selection of interesting non-Messier objects. The SEDS data is generally quite good, but as with most amateur sources, there's some variation in quality. So everything was cross-checked with other sources whenever possible.

The Catalogue of Milky Way Globular Cluster Parameters, published and maintained online by William E. Harris (McMaster University), is generally considered to contain the most authoritative data on globular clusters. We used the Harris distances in the rare cases where they disagree with the SEDS distances.

WEBDA, by Jean-Claude Mermilliod and Ernst Paunzen, collects data from many professional sources on open clusters. The index page to each cluster's information gives a summary of the clusters vital statistics, including distance and visual magnitude. However, this does not always reflect the latest widely accepted value.

The NGC/IC Project database is an excellent source for historical and observational data but contains no distances.

SIMBAD is a general index to the papers in professional astronomy journals containing information on any given object. It often includes summary information on each object's primary page.

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