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The Apparent Size of Pluto and the Stars

posted Sep 28, 2012, 7:23 PM by Abel Mendez   [ updated Sep 28, 2012, 7:28 PM ]
Dwarf planet Pluto is so small and far away that it only shows as a featureless dim dot in any ground or space telescope. Only NASA's New Horizon space mission will get close enough to Pluto on July 2015 to observe its surface details. Now consider the stars, many are even larger than the Sun but they are much farther away. Are the apparent size of the stars larger than Pluto? They seem bigger because we can see them with the naked-eye, but in fact the apparent size of most stars are much smaller than Pluto (see figure below). It is their brightness than make them look much larger and fuzzier to our eyes and our telescopes.

If we barely see surface features on Pluto, neither on any exoplanet, which are much smaller and dimmer than their parent stars. Only those few exoplanets bright enough, because they are young and hot, can be directly seem in telescopes as faint points. Most are to dim and out shined that and are only detected because they affect the star light in some subtle ways.


Caption: Best image of the Pluto-Charon system ever taken by a ground observatory. The image was taken by the Gemini North telescope on July 2012 with an average resolution of 20 milliarcseconds (mas). We added for comparison the apparent size of Pluto and Charon (dotted circles) with those of four known stars (not including the star glare which make them look much bigger). The size of Epsilon Eridani is comparable to detecting a 5 meter diameter object on the Moon from Earth, a nearly impossible activity. Exoplanets are much smaller. CREDIT: PHL @ UPR Arecibo, Gemini Observatory/NSF/NASA/AURA