Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

November 19, 1997

Diffraction Spikes: When Stars Look Like Crosses
Credit: B. Stecklum (TLS) et al., WFPC2, HST, NASA

Explanation: Unusual appendages around bright stars are commonplace, but never seem to be mentioned. What are they? First, a telescope brings starlight falling over a large area to a small area. To get at this small area, however, one must go inside a reflecting telescope, and this can only be done with support rods, which are right in the view of the telescope. The wave nature of light causes it to deflect when passing near these rods. Light scatters away from the original destination point ending up elsewhere and appearing as "diffraction spikes." These annoying spikes steal precious light from the central image and hide light from fainter, more interesting stars. Above, astronomers are more interested in the half-circled point near the image centre, than the cool-looking diffraction spikes from the bright star at the bottom. Apparently, that half-circle is a new stellar system forming in the Lagoon Nebula.

Tomorrow's picture: Escape From The Sun

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Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA)
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&: Michigan Tech. U.