Red Sirius

On this page: binary star | historically red | explanations

Binary Star

Sirius is a binary pair that is the brightest star in the night sky.

Sirius A is a bright A-type star with a surface temperature of 9,940 K. It has about twice the mass of the Sun and is 25 times as luminous.

Sirius B is a white dwarf about 0.98 solar masses and 10,000 times fainter in visible light than Sirius A, but outshines it in x-rays.

The distance between them varies between 8 and 32 AU, and they lie 8.6 light years from Earth. The pair have a controversial colour history.

Historically Red

In 1760 Thomas Barker became the first modern astronomer to note a discrepancy with historic records of Sirius' colour. Some of the records record Sirius as blue, some record it as red.

The Dogon tribe in Mali, Africa, knew that Sirius used to be red.
In ancient China it was the standard star for the colour white.

Historical observations change back and forth.

In 250BC the Greek poet Aratus recorded Sirius as red.
In 100BC Chinese astronomer Sim Qian recorded Sirius as blue.
In AD50 Roman philosopher Seneca (tutor to Nero) recorded Sirius as a deeper red colour than Mars.
In AD85 the Roman Flaccus recorded Sirius as "angry gold".
In AD100 poet Marcus Manilius described it as sea blue, and three Chinese astronomers Ban Gu, Ban Chao, and Ma Xu also all recorded Sirius as blue.
In AD150 Greek astronomer and mathematician Claudius Ptolemy recorded Sirius as red, along with five other stars, Betelgeuse, Antares, Aldebaran, Arcturus and Pollux, all of which are clearly of orange or red hue.
In AD200 Chinese astronomer Liu Biao recorded Sirius as blue.
In AD400 Latin writer Avienus recorded Sirius as blue.
In AD588 Gregory of Tours gave Sirius the nickname Rubeola ("reddish").
In AD646 Chinese astronomer Li Chun-feng recorded Sirius as blue.

(The family name of each of the Chinese astronomers is the first of their names listed.)

Explanations

Stellar evolution has been rejected as an answer because of the length of time it would take either star to change from one state to another and the lack of a nebula.

One explanation that has relatively recently been offered is that small amounts of hydrogen from the thin "atmosphere" of white dwarf Sirius B may have percolated down to the core, where it might start fusing into helium. The resulting burst of heat would expand the atmosphere hundreds of times, causing it to cool and glow red. After about 250 years the outer layers would collapse again and return to its dull and dim white dwarf state. However, observations in the historical records indicate Sirius took 100 years to change from blue to red to blue again, and did it several times in a row.

Another explanation is that many of the ancient observations of Sirius being red are claimed to actually be misrepresented recordings of observations of red flashes caused by it being low in the southern sky, and thus affected by atmospheric twinkling; and that Gregory of Tours was actually referring to Arcturus. Ptolemy's recording is not of red flashes, so is written off as being someone else's observations which he was recounting. (This is the "we can't explain it so we'll deny it" explanation. Of course, the question must be asked why Ptolemy would say something so ridiculous if it wasn't actually red.)

Probably coincidentally the system has an orbital period of 50.1 years.