XZ Andromedae
An Algol-type eclipsing binary with suspected additional companions.
XZ Andromedae, also cataloged as XZ And, is a binary star system located in the constellation Andromeda. It has a peak apparent visual magnitude of 9.91, which dims to 12.45 every 1.357 days. This pattern of brightness change is characteristic of Algol-type variable stars.
The system's primary star has a mass 3.2 times that of the Sun and a spectral type of A4IV-V, placing it between a main-sequence star and a subgiant. Its companion is less massive, at 1.3 solar masses, but physically larger, making it an evolved subgiant with a spectral type of G5IV. This secondary star is expected to become a white dwarf before the primary leaves the main sequence. Since 2019, there has been suspicion that the eclipsing pair is accompanied by two additional, similar stars locked in a 1:3 mean-motion resonance, with orbital periods of 33.43 and 100.4 years.
The star's variability was first detected by Henrietta Levitt from photographs taken between 1916 and 1919. Confirmation came from Arville D. Walker and Priscilla Fairfield, with the discovery announced by Harlow Shapley in 1923. Originally designated BD+41 376, it received its variable-star name, XZ Andromedae, in 1924.
For Algol variables, the photometric period matches the orbital period of the system. However, XZ Andromedae shows slight period variations that can be explained by three distinct cycles lasting 137.5, 36.8, and 11.2 years. Each could be caused by a faint body orbiting the binary, though one of the shorter cycles might also result from magnetic interactions between the stars (the Applegate mechanism). Another interpretation holds that the long cycle is actually a steady increase in orbital period due to mass transfer from the secondary—which fills its Roche lobe—to the primary.
- Maximum apparent visual magnitude
- 9.91
- Minimum apparent visual magnitude
- 12.45
- Period of variability
- 1.357 days
- Primary star mass
- 3.2 M☉
- Primary spectral type
- A4IV-V
- Secondary star mass
- 1.3 M☉
- Secondary spectral type
- G5IV
Lore & Background
The variability of XZ Andromedae was discovered by Henrietta Levitt by examining photographs taken from 1916 to 1919. Variability was confirmed by Arville D. Walker and Priscilla Fairfield. The discovery was announced by Harlow Shapley in 1923. The star, originally known as BD+41 376, received the variable star designation XZ Andromedae in 1924.
The primary star of the system has a mass of 3.2 M☉ and has a spectral type A4IV-V, meaning that it has intermediate characteristics between a main sequence star and a subgiant one. The secondary is less massive (1.3 M☉) but larger than the primary, so it is an evolved subgiant star and its spectral type is G5IV. The secondary component will likely evolve into a white dwarf before the primary leaves the main sequence.
Since 2019, it is suspected that the eclipsing binary is orbited by an additional two similar stars in a 1:3 mean-motion resonance with periods 33.43 and 100.4 years. Photometric periods of Algol variables match the orbital period of the system. However, in XZ Andromedae slight period variations have been observed that can be reproduced with three different cycles of 137.5, 36.8 and 11.2 years, respectively. Each of them could be the effect of another faint body orbiting the binary system, but one of the two shorter cycles could also be an effect of magnetic interaction between stars (the Applegate mechanism). Other research states that the long cycle is instead a long-term period increase caused by mass transfer from the secondary (that fills its Roche lobe) to the primary component.
Reader's Guide
XZ Andromedae is notable as an Algol-type eclipsing binary whose light curve and period variations have been studied for over a century. Its discovery by Henrietta Levitt from early photographic plates, and confirmation by Walker and Fairfield, highlights the role of early 20th-century astronomical surveys. The system's primary and secondary stars have well-determined masses and spectral types, with the secondary being an evolved subgiant that will become a white dwarf before the primary leaves the main sequence. The observed period variations—cycles of 137.5, 36.8, and 11.2 years—are a key feature, as they may indicate the presence of additional faint bodies or magnetic interactions via the Applegate mechanism. The 2019 hypothesis of two additional stars in a 1:3 mean-motion resonance with periods of 33.43 and 100.4 years adds further complexity. The alternative explanation that the long cycle is due to mass transfer from the secondary filling its Roche lobe underscores the ongoing debate about the system's evolution. XZ Andromedae thus serves as a valuable case study for binary star evolution, mass transfer, and the search for circumbinary companions.
Did You Know?
- XZ Andromedae was discovered as a variable by Henrietta Levitt from photographs taken between 1916 and 1919.
- The secondary star is less massive than the primary but larger, and is an evolved subgiant of spectral type G5IV.
- The long period variation of 137.5 years may be caused by mass transfer from the secondary to the primary.
Frequently Asked Questions
What is XZ Andromedae?
XZ Andromedae, often shortened to XZ And, is an eclipsing binary star system situated in the constellation Andromeda. It belongs to the Algol-type family of variable stars, meaning its brightness dips periodically as one component passes in front of the other.
How does XZ Andromedae's brightness change?
The system brightens to a peak apparent visual magnitude of 9.91 and then fades to 12.45 on a repeating cycle of 1.357 days. That roughly 2.5-magnitude drop is what makes it visible to small telescopes at its brightest but very faint at minimum.
What are the two stars in the XZ Andromedae pair like?
The primary is a hotter A4IV-V star with 3.2 solar masses, sitting right at the boundary between main-sequence and subgiant phases. Its companion is a cooler G5IV subgiant that, despite having only 1.3 solar masses, is physically larger than the primary.
Why is XZ Andromedae classified as an Algol-type variable?
The classification comes from the fact that the less massive star is the physically larger, more evolved one, so it partially covers the brighter primary during eclipse. This 'backwards' mass-radius relationship, combined with the short 1.357-day period, is the hallmark signature of Algol-type systems.
Does XZ Andromedae have more than two stars?
Astronomers suspect the system may harbor additional companions beyond the two known components, though those extras have not yet been firmly confirmed. If verified, XZ Andromedae would be a multiple-star system rather than a simple binary.
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