I took some photos on a nice sunny day with my old Pentax K-r. I used fixed aperture mode because I like depth of field in photos. Settings for this particular photo are f/5.6, 1/100s, 55mm and ISO200 according to the exif data.
As you can see in this about 2-3 times zoomed in version of my photo, the yellow surface that is lit up by the sun radiates into the shadowy part behind it.
How is this effect called? How can I prevent it?
It's bloom, and while we typically think of this as coming from objects casting illumination in the shot (like light fixtures) it can also be caused by objects in the shot being overexposed, like the yellow fabric in the foreground in your picture. Highly illuminated and overexposed objects will bleed out into surrounding areas which are most visible in portions of high contrast, and this cannot be fully escaped because lenses are physically incapable of being perfect and not all of the photos entering your camera will be kind enough to do so in ordered lines perfectly perpendicular to the front of the lens.
Quite a wide swath of that object turned out pure white, 255/255/255, which I discovered by stabbing it with my eyedropper tool. Since whatever-it-is is actually yellow in reality, your exposure settings were too bright for that particular object and how it was illuminated.
I'll point out that 1/100 is an incredibly slow shutter speed for a subject illuminated by sunlight, even for an old and crusty camera operating at low ISO.
Fixes:
Now that's a good explanation - but I'll add one possible tip: if you shoot in RAW format (which may not even be possible on that camera) you may be able to rescue over-exposed areas to some extent in RAW format processing tools. Massively over exposed areas are probably not going to work very well - the colour balance of the rescued image tends to go wrong, for instance - and you may need to use some moderately advanced editing facilities.
You can see bloom with your bare eyes. Ever tried to look inside an open window in a sunlit white wall? The inside is orders of magnitude darker and will be drowned from the bloom, not to mention your iris (aperture) will shrink. You have to make a "toy telescope" (rolled-up cardboard, fists or similar) and block the light parts to make the bloom disappear and see inside the room. So if we're judging cameras by how well they imitate eyesight, this is not a flaw.
Thank you a lot for the detailed explanation! I will keep your tips in mind.