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Arcturus is the third brightest star in the night sky. It is located slightly less than 37 light-years away, in the constellation Boötes. The easiest way for an amateur astronomer to locate Arcturus is to follow the arc made by the handle of the Big Dipper.
Arcturus is a red giant star with a mass between 1 and 1.5 times that of the Sun. It is notable for its particularly strong emission lines, meaning its atmosphere is dusty and low-temperature compared to that of a main sequence star. This is common among red giants, but particularly pronounced with Arcturus. Instead of having a clearly demarcated atmosphere like the Sun, the atmospheres of red giants such as Arcturus tends to just trail off into space.
Although Arcturus is similar to the mass of the Sun, it is much larger, measuring about 16 solar diameters. Having departed the main sequence, it no longer burns hydrogen in its core, but rather in a shell outside of an inert helium core. This helium is the product of hundreds of millions or billions of years of continuous fusion. Arcturus' mass is not great enough to create the necessary temperatures in its core to fuse helium into carbon, so the helium just sits there. Its powerful gravity compresses the hydrogen above it, causing it to fuse faster, making the atmosphere of the star expand and become more tenuous.
Because of its relative nearness, Arcturus has been a popular case study for the emerging science of astroseismology. Astronomers have observed that the star is variable at factor of 0.4% over eight day cycles. The atmosphere of the star also oscillates slightly. Red giants like Arcturus can be thought of as large gas clouds with a "true sun" located near their core.
Arcturus has been noted by astronomers since ancient times, and is mentioned twice in the Bible. It has also been a popular subject of science fiction.