Compare the candidate with nearby stars in the same image: if it is broader than the image’s point-spread function (PSF), it is resolved or extended—but that alone does not make it a galaxy. A cluster can look like a tiny blur, and a distant galaxy can look like a point. The strongest identification combines apparent size, shape, resolved stars, color, and survey or catalog context.
First check whether the image can resolve the object
The PSF is the image’s representation of an unresolved point source. Nearby, unsaturated stars offer a practical reference: compare their apparent profiles with the candidate’s. If the candidate remains broader than those stars, that supports an extended-source interpretation. It does not by itself distinguish a galaxy from a star cluster, nebula, or blend of multiple stars. Observing conditions, especially ground-based atmospheric seeing, and image noise can affect the comparison. [SDSS photometric classification]
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A point-like candidate is not automatically a star or cluster. A distant globular cluster may be unresolved, while a faint galaxy may show no visible internal structure at the image’s resolution. If the source is unresolved, blended, or noisy, morphology alone may not settle the question.
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Compare the candidate’s shape and visible stars
Open clusters
Open clusters are relatively loose groupings. NASA describes their density as low enough that individual stars may be visible through a telescope and, in some cases, to the unaided eye. Their outlines are generally more irregular than spherical. A group of discrete stars that appears coherent rather than randomly scattered is consistent with an open cluster. [NASA Science, “Hubble’s Star Clusters”]
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Globular clusters
Globular clusters are dense and roughly spherical, with thousands to millions of stars. Their crowded centers can blur into a compact patch of light: NASA notes that individual stars in globular-cluster centers are difficult to distinguish even with powerful telescopes. A concentrated, nearly round appearance supports this interpretation, but a compact galaxy can look similar in a low-resolution image. [NASA Science, “Hubble’s Star Clusters”]
Background galaxies
An extended profile, asymmetry, or visible disk, bar, spiral pattern, dust lane, or irregular structure can support a galaxy identification. These features are clues, not requirements: a distant or faint galaxy may appear smooth or point-like. Conversely, blends and PSF effects can make a source look extended or asymmetric. [SDSS photometric classification]
Use color and apparent size together
Color is informative only in context. Globular clusters generally appear redder than open clusters because their stellar populations are older, but filters, dust, image processing, and the populations being compared all affect measured colors. A color in a processed image may encode filter bands rather than the object’s appearance to the eye. Check the caption for the filters and image provenance before interpreting a color difference. [NASA Science, “Hubble’s Star Clusters”] [IPAC/Caltech, 2MASS All-Sky Explanatory Supplement]
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ESA/Hubble’s 3 December 2018 account of a Hubble image of the Coma Cluster describes astronomers using color and size together to identify globular clusters and reject background galaxies in the same field. That is an example of a joint selection method, not a universal color cutoff or size threshold for other targets and instruments. The image depicts Coma about 300 million light-years away. [ESA/Hubble, “Clusters within clusters”]
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Check catalogs without treating labels as proof
Survey pipelines often distinguish point-like from extended sources by comparing how well different image models fit. For example, the SDSS Data Release 2-era pipeline classified a source as extended when psfMag − cmodelMag > 0.145. This is a documented threshold for that pipeline and dataset context, not a general astronomy rule. SDSS documentation notes exceptions, including close star pairs and Seyfert galaxies with bright nuclei, which can complicate classification. [SDSS photometric classification]
Catalog selection has other limits. IPAC/Caltech describes the 2MASS extended-source catalog as useful for finding extended sources, but its “g_score” does not tell one galaxy from another and can also select Galactic nebulae and young stellar objects. The supplement says catalog completeness was probably greater than 95%; its stated science requirement concerned sources brighter than Ks = 13.5 mag at Galactic latitude |glat| > 30°. Those figures describe 2MASS’s catalog and requirements, not the completeness of galaxy catalogs generally. [IPAC/Caltech, 2MASS All-Sky Explanatory Supplement]
A practical workflow for an astronomical image
- Read the caption and scale bar. Note the telescope or survey, filters, and angular scale. Apparent size depends on distance and resolution; processed colors may represent separate bands. [ESA/Hubble]
- Compare with nearby stars. Use unsaturated stars in the same image as a PSF reference. Ask whether the candidate is broader, asymmetric, or still extended beyond the stellar profile. Treat the result cautiously if seeing, noise, or crowding is significant. [SDSS]
- Look for resolved membership and structure. Discrete stars in a loose, irregular grouping favor an open cluster; a dense, nearly round concentration is consistent with a globular cluster. A visible galaxy-like structure supports a galaxy interpretation, but its absence does not rule one out. [NASA Science] [SDSS]
- Compare multi-band color with apparent size. Use colors appropriate to the image’s filters and target population; do not apply a threshold from a different survey or instrument. [ESA/Hubble]
- Consult catalog metadata. Check which survey version produced the label, how it defines extended sources, and whether quality flags or crowding could matter. A catalog classification is evidence, not an infallible verdict. [SDSS] [IPAC/Caltech]
- Choose stronger evidence if the distinction matters. A higher-resolution image, additional filters, catalog context, or spectroscopy may help resolve an ambiguous case. Spectroscopy is not necessary for every casual visual identification.
What one striking image can—and cannot—show
NASA’s Scientific Visualization Studio describes a Hubble mosaic of the Coma galaxy cluster that captured 22,426 globular star clusters across a field about 2.2 million light-years wide, at a distance of about 300 million light-years. The figures illustrate why crowded stellar systems and background galaxies can occupy the same image and why astronomers use more than a first-glance shape. [NASA Scientific Visualization Studio]
For a general introduction to the differences between open and globular clusters, see OpenStax Astronomy 2e, Chapter 22: Star Clusters.
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