S Persei

S Persei

Location of S Persei (circled) near the Double Cluster (north is left)
Observation data Epoch J2000      Equinox J2000
Constellation	Perseus
Right ascension	2h 22m 51.70928s[1]
Declination	58° 31′ 11.4476″[1]
Apparent magnitude (V)	+9.23[2] (7.9 - 12.0[3])
Characteristics
Spectral type	M3 Iae–M7[3]
B−V color index	+2.65[2]
Variable type	SRc[3]
Distance		7,900 ± 300 ly (2,420 ± 100 pc)
Absolute magnitude (MV)	–6.36 (at mV 9.23)[2]
Details
Mass	20[6] M☉
Radius	1,212[7] – 1,364[8] R☉
Luminosity	123,000[9] – 186,000[10] L☉
Surface gravity (log g)	0.0[11] cgs
Temperature	3,500[12] (3,000[13] – 3,600[10]) K
AAVSO 0215+58
Database references
SIMBAD	data

S Persei is a

S Persei varies slowly by several magnitudes, a factor of over 40 in brightness. It has a main period of somewhat over two years, but shows significant unpredictability. There is a strong variation in the amplitude from around one magnitude to about four magnitudes, and these have been interpreted as beats due to a second period of about 940 days.^[16] Other analyses find only the primary period of 813 ± 60 days.^[17]

S Persei is classified as a semiregular variable star of type SRc, indicating that it is a supergiant, and it has one of the largest visual amplitudes of any variable of this type.^[3] While the General Catalogue of Variable Stars lists it as varying between magnitudes 7.9 and 12.0, it has since been seen fainter.^[17]

The spectral type of S Persei also varies. Typically it is a red supergiant of spectral class M3 or M4, but particularly at deep visual minima it may show a much cooler spectral type of M7 or M8, highly unusual for a supergiant.^[18]

Properties

Many of the visually bright variable stars belong to this class of semiregulars, as these stars are extremely large and luminous, and hence visible across long distances. S Persei has been described as a hypergiant^[19] and has a radius over 700 times the sun's radius (R_☉).^[2] Its angular diameter has been measured directly and found to be somewhat elliptical. Modelled as a uniform disk, the radius corresponds to 1,212 ± 124 R_☉.^[7]

The temperature has been calculated from the spectrum using a DUSTY model,^[20] giving an effective photospheric temperature of 3,500 K and a temperature of 1,000 K for the surrounding dust torus.^[12] This is consistent with previous studies, but the derived luminosity from different authors varies from 86,000 L_☉ to 186,000 L_☉.^[10][21][22] Older studies frequently calculated higher luminosities, lower temperatures, and consequently larger values for the radius.^[23]

The mass of S Persei is also uncertain, but expected to be around 20 M_☉.^[6] Mass is being lost at 2.4 to 2.6×10⁻⁵ M_☉ per year,^[12] leading to an extensive and complex circumstellar environment of gas and dust.^[10]

Location

S Persei is surrounded by clouds containing water molecules which produce

S Persei is a double star. The red supergiant has an A0 11th magnitude companion at 69".[25] There are also several other 8th to 10th magnitude stars within half a degree of S Persei.^[26]

References