What causes the horizontal and vertical lines coming out of pictures of stars, instead of them simply appearing as circles? For example, this picture from Wikipedia: 
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$\begingroup$ It should probably be noted that Fraunhofer diffraction is common in all kinds of photography, and is present in nearly every photo; though to greater degrees for photos with more closed apertures and far more obvious in high contrast situations provided by lights, like stars. Here is a great question in the Photography stack exchange on the topic, and another from Physics. $\endgroup$– Mitch GoshornJul 3, 2015 at 2:49
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3 Answers
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They are called diffraction spikes, and they're artifacts from a supporting structure inside a reflector-type telescope.
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3$\begingroup$ Maybe you could say how this leads to the pattern seen in this case. Might also be worth explaining why stars show diffraction spikes but galaxies do not. $\endgroup$– ProfRobJul 3, 2015 at 6:13
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sometimes you can get these spike artifacts from the microlenses over the CCD's sensor array as well.
also if your CCD does not have antiblooming logic, a very bright star can cause neighboring pixels to saturate as they are being read out, leading to a "spike" only along the readout axis. here's a good document with some common artifacts, from the hubble team: HandoutIIIc.
the spikes in the OP's image are likely caused by the spider vanes which hold the secondary mirror in place, as Russell says.
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(I'd make this a comment, but no reputation yet.)
Minute Physics has a good video on this! Filtering light through any lens causes spikes that identify what kind of aperture was used. That looks like a Hubble picture because of the diamond spikes.
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$\begingroup$ It is not to do with the circular aperture in the case shown. $\endgroup$– ProfRobJul 3, 2015 at 6:11