DS Andromedae
Eclipsing binary in NGC 752 with total secondary eclipse.
DS Andromedae (DS And) belongs to the open cluster NGC 752 and is an eclipsing binary in the constellation Andromeda. Its brightness varies from a maximum apparent visual magnitude of 10.44 down to 10.93 during the primary eclipse and 10.71 during the secondary eclipse. The system is viewed almost exactly edge-on, so the secondary eclipse is total—an orientation that reveals the spectral type of the fainter component.
The primary star is classified as spectral type F3IV-V, indicating it is evolving off the main sequence and expanding. The secondary, estimated from the brightness difference between the two stars during eclipses, is thought to be a main-sequence star of spectral type G0. It does not appear in the system’s spectrum. The modeled apparent magnitudes are 10.62 for the primary and 12.47 for the secondary.
DS Andromedae’s age is very close to the hook at the end of the main sequence, having just entered the subgiant branch. Based on the main sequence turnoff of NGC 752, its age is 2.0 ± 0.2 billion years, which allows accurate calculation of its physical properties. Neither star fills its Roche lobe, and no mass transfer has occurred; they evolve as isolated stars.
The star’s variability was discovered in 1957 by A. Alksnis, who examined 60 photographic plates. The light curve shows a main eclipse when the secondary passes in front of the primary and a secondary eclipse when the opposite happens, repeating with a period slightly longer than one day. The primary eclipse is annular because the smaller, cooler secondary crosses the hotter, brighter primary. In the General Catalogue of Variable Stars, DS And is classified as an Algol variable (detached), though it is sometimes considered a β Lyrae variable (contact).
- Constellation
- Andromeda
- Member of
- NGC 752
- Maximum apparent visual magnitude
- 10.44
- Magnitude during main eclipse
- 10.93
- Magnitude during secondary eclipse
- 10.71
- Primary spectral classification
- F3IV-V
- Secondary spectral type
- G0
- Age
- 2.0±0.2 Gyr
- Apparent magnitudes of components
- 10.62 and 12.47
Lore & Background
The primary star has a spectral classification F3IV-V, matching the evidence for a star that is evolving off the main sequence and is expanding its radius. The secondary is thought to be a main sequence star with spectral type G0. It is not visible in the spectrum of DS Andromedae, but the temperature and spectral type can be estimated from the difference in brightness of the two components, determined from the eclipses. The two components are modelled to have apparent magnitudes of 10.62 and 12.47 respectively.
The age of DS Andromedae appears to be very close to the hook at the end of main sequence phase of evolution and has just reached the subgiant branch. Its age can be determined at 2.0±0.2 Gyr from the NGC 752 main sequence turnoff and this allows its physical properties to be accurately calculated. It appears to be not quite filling its roche lobe and no mass transfer has taken place between the two stars; they are evolving as isolated stars.
A. Alksnis discovered the variability of DS Andromedae's brightness in 1957, by measuring 60 photographic plates. The light curve of the star shows a main eclipse when the secondary star passes in front of the primary, and a secondary eclipse when the opposite occurs. This cycle repeats with a periodicity slightly over one day. Since the system is almost exactly edge-on the secondary eclipse is total, and allows the determination of the spectral type of the secondary component. The primary eclipse is annular as the smaller secondary passes in front of the hotter brighter primary. It is classified as an Algol variable (detached) star in the General Catalogue of Variable Stars, but is sometimes considered to be a β Lyrae variable (contact).
Reader's Guide
DS Andromedae is significant as an eclipsing binary system that provides a rare opportunity to directly determine the spectral type of its secondary component, thanks to the total secondary eclipse. Its membership in the open cluster NGC 752 allows its age to be precisely constrained to 2.0±0.2 Gyr from the cluster's main sequence turnoff, making it a valuable benchmark for stellar evolution models. The system is in a critical evolutionary state, just reaching the subgiant branch and not yet filling its Roche lobe, so no mass transfer has occurred. The classification uncertainty—listed as an Algol variable (detached) in the General Catalogue of Variable Stars but sometimes considered a β Lyrae variable (contact)—reflects ongoing debate about the degree of interaction between the components. The discovery by A. Alksnis in 1957 using photographic plates underscores the historical importance of archival plate surveys in identifying variable stars. The system's nearly edge-on orientation and the annular nature of the primary eclipse further contribute to its value for testing binary star models.
Did You Know?
- DS Andromedae was discovered to be variable by A. Alksnis in 1957 from 60 photographic plates.
- The secondary eclipse is total because the system is almost exactly edge-on.
- The system is a member of the open cluster NGC 752, with an age of 2.0±0.2 Gyr.
Frequently Asked Questions
What is DS Andromedae?
DS Andromedae is an eclipsing binary star system in the constellation Andromeda. It belongs to the open cluster NGC 772 and is visible only with small telescopes or binoculars under dark skies.
How much does DS Andromedae's brightness change between eclipses?
The pair peaks at a visual magnitude of 10.44, drops to 10.93 during the primary eclipse, and fades to 10.71 during the secondary eclipse. That gives a total amplitude of roughly half a magnitude.
What is unusual about the geometry of DS Andromedae's orbit?
We see the system almost exactly edge-on, which makes the secondary eclipse total. This orientation allows the spectral type of the fainter companion to be read directly from the depth and shape of the light curve.
What is the spectral classification of DS Andromedae's primary star?
The brighter component is classified F3IV-V, indicating it is a subgiant-to-dwarf star in the early stages of leaving the main sequence and beginning to expand.
Why do amateur variable-star observers like to track DS Andromedae?
Its well-defined, repeatable eclipses and membership in the nearby NGC 772 cluster make it a straightforward photometry target. The total secondary eclipse also gives a clean spectral signature for the fainter star, adding a nice analytical twist to the observation.
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