SY Equulei
A distant Beta Cephei variable in Equuleus with rapid rotation.
SY Equulei, also known as HD 203664, is a single variable star in the equatorial constellation Equuleus. It is notable as a Beta Cephei variable discovered through Hipparcos data, with an average apparent magnitude of about 8.5 and a variation of a few hundredths of a magnitude, making it visible in binoculars and small telescopes but not to the naked eye. The star lies at a distance of 8,000 light years and is receding, with its light dimmed by 0.19 magnitudes due to interstellar dust.
- Apparent magnitude
- 8.5 (average)
- Variation amplitude
- 0.07 magnitudes
- Average period
- 3.98 hours
- Distance
- 8,000 light years
- Stellar classification
- B0.5 IIIn
- Mass
- 14 solar masses
- Luminosity
- 2,490 solar luminosities
Lore & Background
Throughout the late 20th century, HD 203664 was known to have a dust cloud surrounding it. Observations by Kenneth R. Sembach in 1995 revealed the cloud contains high abundances of calcium as well as traces of magnesium, aluminum, and silicon. The cloud probably originated outside the galactic plane and is moving toward the star. The star has a high galactic latitude, indicating its location in the galactic halo, and was most likely ejected from its birthplace to its current distance. Its status as a Beta Cephei variable was not discovered until a survey using Hipparcos data, after which it received the designation SY Equulei.
Reader's Guide
SY Equulei holds significance as a Beta Cephei variable discovered relatively late, through Hipparcos data, highlighting the value of space-based astrometry in identifying variable stars. Its stellar classification of B0.5 IIIn indicates an evolved B-type star with broad absorption lines due to rapid rotation, spinning at 40% of its break-up velocity—unusually fast for its type. The star's surrounding dust cloud, rich in calcium and other metals, and its high galactic latitude suggest an origin outside the galactic plane, with the cloud moving toward the star. The star's ejection from its birthplace adds to its context as a halo object. With a variation amplitude of 0.07 magnitudes and an average period of 3.98 hours, later observations found multiple periods, indicating complex pulsation behavior. Its distance of 8,000 light years and dimming by interstellar dust make it a challenging but rewarding target for amateur astronomers with binoculars or small telescopes.
Did You Know?
- SY Equulei has a projected rotational velocity that is 40% of its break-up velocity.
- The star's surrounding dust cloud contains high abundances of calcium, magnesium, aluminum, and silicon.
- It was originally known as HD 203664 before being designated SY Equulei after its variability was discovered.
- The star has a high galactic latitude, placing it in the galactic halo.
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