SY Equulei, also known as HD 203664, is a single variable star located in the equatorial constellation Equuleus. It has an average apparent magnitude of about 8.5, varying by a few hundredths of a magnitude, making it readily visible in binoculars and small telescopes, but not to the naked eye. The star is relatively far away at a distance of 8,000 light years[2] and is receding with a heliocentric radial velocity of 48 km/s.[7] At that distance, SY Equulei is dimmed by 0.19 magnitudes due to interstellar dust.[16]
| Observation data Epoch J2000.0 Equinox J2000.0 (ICRS) | |
|---|---|
| Constellation | Equuleus |
| Right ascension | 21h 23m 28.8086s[2] |
| Declination | +09° 55′ 54.9204″[2] |
| Apparent magnitude (V) | 8.52 - 8.58[3] |
| Characteristics | |
| Spectral type | B0.5 IIIn[4] |
| U−B color index | −1.00[5] |
| B−V color index | −0.20[5] |
| Variable type | β Cephei[6] |
| Astrometry | |
| Radial velocity (Rv) | 48±5.5[7] km/s |
| Proper motion (μ) | RA: −0.185 mas/yr[2] Dec.: −9.049 mas/yr[2] |
| Parallax (π) | 0.3869 ± 0.0558 mas[2] |
| Distance | approx. 8,000 ly (approx. 2,600 pc) |
| Absolute bolometric magnitude (Mbol) | −6.04[8] |
| Details | |
| Mass | 14[9] M☉ |
| Radius | 10.44[10] R☉ |
| Luminosity | 2,490[11] L☉ |
| Surface gravity (log g) | 3.8±0.2[12] cgs |
| Temperature | 28,184+1,328 −1,268[12] K |
| Metallicity [Fe/H] | 0.00[13] dex |
| Rotational velocity (v sin i) | 180[14] km/s |
| Age | ~7.5[12] Myr |
| Other designations | |
| Database references | |
| SIMBAD | data |
Throughout the late 20th century, HD 203664 was known to have a dust cloud surrounding it. Subsequent observations from Kenneth R. Sembach (1995) reveal it to contain high abundances of calcium as well as traces of magnesium, aluminum, and silicon.[17] The cloud probably came from outside the galactic plane and is moving towards the star at a rate of 70 km/s.[17] The star has a high galactic latitude, indicating its location in the galactic halo. HD 203664 was most likely ejected from its birthplace to its current distance.[9] However, its status as a Beta Cephei variable wasn't discovered until a survey of 2000 using Hipparcos data.[8] It was then given the designation SY Equulei.[18]
SY Equulei has a stellar classification of B0.5 IIIn,[4] indicating an evolved B-type star with nebulous (broad) absorption lines due to rapid rotation. Unlike most stars of its type, it spins rapidly with a projected rotational velocity of 180 km/s,[14] which is 40% of its break-up velocity.[12] It has 14 times the mass of the Sun[9] and a radius of 10.4 radius.[10] It radiates at 2,490 times the luminosity of the Sun[11] from its photosphere at an effective temperature of 28,184 K,[12] giving a whitish blue hue.
SY Equueli is a variable star with an amplitude of 0.07 magnitudes[6] and an average period of 3.98 hours.[19] In later observations, SY Equulei was found to have multiple periods.[12]