Arcturus is a red giant star located in the constellation Bootes, visible by following the arc of the Big Dipper. It has a mass similar to the Sun but is much larger, with a dusty and cold atmosphere that disappears into space. Arcturus is notable for its strong emission lines and has been studied through astroseismology. It is mentioned in the Bible and has been a popular subject in science fiction.
Arcturus is the third brightest star in the night sky. It is located just under 37 light-years away, in the constellation Bootes. The easiest way for an amateur astronomer to locate Arcturus is to follow the arc formed 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 that its atmosphere is dusty and cold compared to that of a star in the sequence principal. 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 like Arcturus tend to disappear into space.
Although Arcturus is similar in mass to the Sun, it is much larger, measuring about 16 solar diameters. Having left the main sequence, it no longer burns hydrogen in its core, but rather in a shell outside an inert helium core. This helium is the product of hundreds of millions or billions of years of continuous fusion. Arcturus’ mass isn’t large enough to create the temperatures needed in its core to fuse helium into carbon, so the helium is there. Its powerful gravity compresses the hydrogen above it, causing it to melt faster, causing the star’s atmosphere to expand and become thinner.
Due to its relative proximity, Arcturus has been a popular case study for the emerging science of astroseismology. Astronomers have observed that the star is variable by a factor of 0.4% on eight-day cycles. The star’s atmosphere also fluctuates slightly. Red giants like Arcturus can be thought of as large clouds of gas 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.
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