GY Andromedae
A binary variable star with a 23-year magnetic cycle and promethium.
GY Andromedae, also cataloged as GY And, is a binary variable star of the α² Canum Venaticorum type located in the northern constellation Andromeda. Its visual magnitude shifts between 6.27 and 6.41, which means it is difficult to see without a telescope, even under clear skies. The star’s magnetic activity follows an unusually slow cycle, completing one full variation roughly every 23 years. Based on parallax data, it lies about 460 light-years (140 parsecs) from Earth.
Classified as an Ap/Bp star, GY And exhibits a peculiar spectrum with strong lines of chromium and europium. These spectral lines change in intensity over the same 23-year cycle, though their pattern is opposite in phase to the brightness variations. Its most remarkable feature is the presence of promethium in its emission spectrum—a radioactive element with no stable isotopes. All promethium isotopes have half-lives of 17.7 years or less, so the element detected in the star’s outer layers likely comes from the spontaneous fission of heavier, transuranic elements.
The star is also a spectroscopic binary. Canadian astronomer William Edmund Harper first suspected this based on radial velocity measurements taken at the Dominion Astrophysical Observatory between 1927 and 1935. In 1958, American astronomer Horace W. Babcock confirmed the binary nature. The two components orbit each other every 273 days along a highly elliptical path with an eccentricity of 0.47. Their separation is at least 37.4 million kilometers, or about 0.25 astronomical units.
- Constellation
- Andromeda
- Visual magnitude range
- 6.27m – 6.41m
- Distance
- about 460 light-years (140 parsecs)
- Spectral type
- Ap/Bp star
- Orbital period
- 273 days
- Orbital eccentricity
- 0.47
- Minimum separation
- 37.4 million km (0.25 AU)
Lore & Background
Based on radial velocity measurements taken at the Dominion Astrophysical Observatory between 1927 and 1935, Canadian astronomer William Edmund Harper strongly suspected that this star was a spectroscopic binary. In 1958, American astronomer Horace W. Babcock confirmed the binary nature of the star. It has an orbital period of 273 days with a large eccentricity of 0.47, and the two components are separated by an estimated distance of at least 37.4 million km, or 0.25 astronomical units.
The star is classified as an Ap/Bp star, with a peculiar spectrum showing lines of chromium and europium that change in intensity over a period matching the variability cycle, although opposite in phase. Its most striking characteristic is the presence of the unstable element promethium in its emission spectrum. All isotopes of this element are radioactive with half-lives of 17.7 years or less. The promethium in the outer envelope may be generated by the spontaneous fission of higher-mass transuranic elements.
The magnetic activity on this star shows an unusually long period of variability, cycling about once every 23 years. Its brightness fluctuations in visual magnitude between 6.27m and 6.41m make it a challenge to view with the naked eye even in good seeing conditions.
Reader's Guide
GY Andromedae is significant as an α2 Canum Venaticorum-type variable star, a class known for strong magnetic fields and spectral peculiarities. Its 23-year magnetic cycle is unusually long, and the presence of promethium—a radioactive element with no stable isotopes—makes it a rare object of study. The promethium in its outer envelope may originate from spontaneous fission of higher-mass transuranic elements, offering clues to nucleosynthesis in stellar atmospheres. The star's binary nature, confirmed by Horace W. Babcock in 1958 after earlier suspicions by William Edmund Harper, adds complexity: the orbital period of 273 days and high eccentricity of 0.47 suggest a dynamically interesting system. The variability in chromium and europium lines, opposite in phase to the brightness cycle, ties spectral changes to magnetic activity. Located about 460 light-years away, GY Andromedae remains a challenge for naked-eye observation but a valuable target for studies of magnetic stars and exotic element production.
Did You Know?
- GY Andromedae has a magnetic activity cycle that lasts about 23 years.
- The star's spectrum contains promethium, a radioactive element with no stable isotopes.
- It is a spectroscopic binary with an orbital period of 273 days and an eccentricity of 0.47.
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