Rasalgethi is a cool, red M5Ib-II bright giant or supergiant perhaps 900 times as luminous as the sun and maybe 300 times the diameter of the sun.
Rasalgethi is a multiple star system. The companion Rasalgethi B is itself a binary star. The B star revolves at a distance of 4.8 arc sec from A, that is, about 570 AU with a period of 3600 years.
Rasalgethi A was discovered by Sir William Herschel to be a variable star in 1795. The star varies about 1 magnitude on the time scale of 50 to 150 days.
The star is surrounded by an expanding envelope of material stretching out to about 900 AU from the star. It is estimated that the rate of mass loss is 0.3% of the mass of the sun in 100,000 years.