Rasalgethi is a red supergiant at the head of Hercules, a vast, slowly varying star with a companion that looks green beside it in a telescope.
At a glance
| type | red giant (class M) |
|---|---|
| distance from the Sun | 360 light-years (110 pc) |
| its light left it | 360 years ago |
| luminosity | 7,490 × the Sun |
| radius | 310 × the Sun |
| surface temperature | 3,050 K |
| constellation | Hercules (Hercules) |
Read off the piece’s own catalogue, the same values it draws with.
About Rasalgethi
Rasalgethi, Alpha Herculis, is an enormous cool star, so large that, placed where the Sun is, it would reach beyond the orbit of the Earth. Its surface is cool enough to glow red, and its brightness drifts irregularly as its loose outer layers pulse.
A telescope shows a fainter companion close by, itself a binary, which by contrast with the red primary is often described as green. The whole system is wrapped in gas that the giant sheds into space.
The name is Arabic for the head of the kneeler, the old picture of Hercules kneeling. Hercules stands upside down in our sky, so his head is at the south of the figure, almost touching Rasalhague, the head of Ophiuchus.
What you see in the piece
In the piece it is one of the reddest stars in its part of the sky, because it is one of the coolest; its neighbour Rasalhague looks white beside it and lies far closer.
Measured or modelled
Its size is uncertain: a star this cool has no sharp edge, and its measured width changes with the wavelength used.
Questions
How big is Rasalgethi?
Its radius is 310 × the Sun, large enough to reach beyond the orbit of the Earth if it stood where the Sun is.
What does Rasalgethi mean?
It is Arabic for the head of the kneeler, the old picture of Hercules.