AB Andromedae
A contact binary with variable brightness and a possible third body.
AB Andromedae (AB And) is a binary star in the constellation Andromeda, discovered to be an eclipsing binary by Paul Guthnick and Richard Prager in 1927. It is notable as a W Ursae Majoris variable, a contact binary where the two stars share their envelopes, and for its complex period changes that may indicate a third body or other dynamical processes.
- Discoverers
- Paul Guthnick and Richard Prager
- Discovery year
- 1927
- Maximum apparent visual magnitude
- 9.49
- Minimum apparent visual magnitude
- 10.46
- Period
- roughly 8 hours
- Spectral type
- G5 (both stars)
- Third body orbital period
- 19,046 days
- Third body minimum mass
- 0.007 M☉
- Third body eccentricity
- 0.22
Lore & Background
AB Andromedae was identified as an eclipsing binary by Paul Guthnick and Richard Prager in 1927. Its maximum apparent visual magnitude is 9.49, and it dims to magnitude 10.46 in a periodic cycle of roughly 8 hours. The variability is typical of W Ursae Majoris stars, meaning the two stars form a contact binary. Both stars have a spectral type of G5, and one is a main sequence star very similar to the Sun. They orbit so closely that their envelopes touch, a dynamically stable phase that should last until one star leaves the main sequence. The system may host a third body with an orbital period of 19,046 days, a minimum mass of 0.007 M☉, and an eccentricity of 0.22, but not all collected data are consistent with this hypothesis. The two stars eclipse each other during their orbit, but because they are elongated, they show a constant variation instead of discrete eclipses. The periodicity is clear but changes with time, showing a long-term trend and a periodic modulation of 7,000 days. Proposed causes include a third body, magnetic interaction, mass transfer, mass loss, and an internal mechanism in the touching envelopes.
Reader's Guide
AB Andromedae serves as a key example of a W Ursae Majoris contact binary, where the two stars share their outer envelopes and exhibit continuous brightness variation rather than distinct eclipses. Its discovery by Guthnick and Prager in 1927 established it as an early recognized system of this type. The system's period changes—a long-term trend plus a 7,000-day modulation—make it a valuable laboratory for studying stellar interactions. The possible third body, with a 19,046-day orbit and minimum mass of 0.007 M☉, remains unconfirmed due to inconsistent data. The star's significance lies in the multiple competing explanations for its period variation: third-body gravitational effects, magnetic activity, mass transfer, mass loss, or internal envelope dynamics. This uncertainty highlights the complexity of contact binary evolution. The system's G5 spectral type and one Sun-like component allow comparisons with solar physics. Its dynamically stable contact phase, lasting until one star leaves the main sequence, provides a snapshot of binary evolution. The 8-hour orbital period and magnitude range of 9.49 to 10.46 make it accessible for amateur and professional monitoring, contributing to long-term studies of period changes.
Frequently Asked Questions
What is AB Andromedae?
AB Andromedae is a contact binary in the constellation Andromeda, meaning its two stars are so tightly bound that they share a single gaseous envelope. It is classified as a W Ursae Majoris-type eclipsing variable.
Who discovered AB Andromedae and when?
Paul Guthnick and Richard Prager identified the system as an eclipsing binary in 1927. Their observations placed it firmly within the W UMa family of short-period contact variables.
How bright does AB Andromedae appear and how long is its eclipse cycle?
Its visual magnitude swings between roughly 9.49 at maximum and 10.46 at minimum, so it requires at least a small telescope to see. The full eclipse cycle repeats on a timescale of about eight hours.
What spectral type do the two stars in AB Andromedae have?
Both components are classified as G5 stars, indicating they share a similar surface temperature and spectral energy distribution despite being locked inside a common envelope.
Is there evidence for a third body orbiting AB Andromedae?
The system exhibits complex, irregular period changes that go beyond what mass transfer between the two contact stars would predict. This has led astronomers to suspect an unseen third object is gravitationally perturbing the binary.
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