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How to Identify a Planet and a Star

Physically, stars and planets are very different. Planets are made up of solids or gases, and are much smaller than stars. Planets we see are only visible because of reflected light from the Sun. Stars, on the other hand, are hundreds to thousands of times larger than planets, and are so massive that hydrogen nuclei in their cores are fused together by the force of gravity. This nuclear fusion produces enough light that we can see stars that are hundreds or thousands of light years away. As a result, stars maintain the same relative position in the sky from day to day and year to year -- but planets move significantly due to their orbit around the Sun.

Things You'll Need

Telescope or binoculars

Instructions

Distinguishing Between Stars and Planets
- 1
  Look closely at the object in question. Try to determine whether or not it is "twinkling." This may be very difficult to do with the naked eye, so use binoculars or a telescope if you can, and the difference will be much more obvious. Because stars are so far away, their light essentially comes from a single, tiny point. This light scatters as it comes through the Earth's atmosphere, creating a twinkling effect. Planets are much closer, so they have more of an actual shape, rather than a single point. The scattering effect from different parts of the planet cancels out so that the planet does not twinkle except under certain unusual atmospheric conditions.
- 2
  Note the location of a particular celestial object relative to the others around it. It may be helpful to sketch a picture showing the configuration of those nearby.
- 3
  Return to the same spot a few nights later, at around the same time. All of the stars in the sky will be at slightly different positions relative to the horizon, due to the Earth's rotation and the changing of the seasons -- but they will maintain the same position relative to each other. If the object in question has moved relative to the stars around it, the object is a planet.

Astronomy