| 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 |
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]
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]
