6 Cassiopeiae
White hypergiant with disputed classification and a close O-class companion.
6 Cassiopeiae (6 Cas) is a white hypergiant star in the constellation Cassiopeia, also classified as a small-amplitude variable. Under good skies, it can be seen without binoculars at 5th magnitude.
The primary star, 6 Cassiopeiae A, is a white A2.5-type hypergiant with roughly 25 times the Sun’s mass and 200,000 times its luminosity. It varies slightly and irregularly, making it an Alpha Cygni variable. Its status as a hypergiant is debated: an early spectral classification of A3 Ia+ may have used the plus sign to indicate extra spectral lines from a possible companion, not necessarily a hypergiant luminosity. Later studies, however, have assigned it more conventional hypergiant types like B9Ia+ and A3Ia+.
6 Cas A has several close companions, the most prominent being an 8th-magnitude O-class bright giant just 1.5 arcseconds away. This companion is spectral type O9.75 with an absolute magnitude of −5.8. Both stars are thought to belong to the Cassiopeia OB5 stellar association, located roughly 8,000 light-years away. Gaia parallaxes of nearby stars suggest the association’s average distance is about 9,000 light-years.
The star’s brightness variation was discovered by Manali Kallat Vainu Bappu in 1958. As an Alpha Cygni variable, 6 Cas A pulsates erratically between magnitudes 5.34 and 5.45. One study found a strongest period of 37 days. It also carries the variable star designation V566 Cassiopeiae.
- Spectral type
- A2.5 (hypergiant); also given as B9Ia+ and A3Ia+
- Mass
- ~25 solar masses
- Luminosity
- ~200,000 solar luminosities
- Apparent magnitude
- 5.34 to 5.45
- Variable star designation
- V566 Cassiopeiae
- Distance
- ~8,000 light-years (Cassiopeia OB5 association); Gaia suggests ~9,000 light-years
- Companion spectral type
- O9.75 bright giant
Lore & Background
6 Cassiopeiae A is a white A2.5 type hypergiant, about 25 times as massive as the Sun and 200,000 times as luminous. The star is slightly and erratically variable, classified as an Alpha Cygni variable. Not all sources consider 6 Cas to be a hypergiant; it is thought that the '+' in an early A3 Ia+ spectral classification referred to indications of additional spectral features from a possible companion rather than the more modern indication of a hypergiant luminosity class. However, later publications have given more conventional hypergiant spectral types such as B9Ia+ and A3Ia+.
6 Cas A has a number of close companions, most notably an 8th magnitude O class bright giant at only 1.5″. Its spectral type is O9.75 and its absolute magnitude is −5.8. Both are considered to be members of the Cassiopeia OB5 stellar association at a distance of around 8,000 light-years, along with several other nearby stars. Gaia parallaxes of the nearby stars suggest a mean distance to the association of about 9,000 light years.
Manali Kallat Vainu Bappu discovered that the star's brightness varies, in 1958. 6 Cas A is an α Cyg variable, pulsating erratically between 5.34 and 5.45. The strongest period detected in one study was 37 days. It has the variable star designation V566 Cassiopeiae.
Reader's Guide
6 Cassiopeiae is significant as a rare white hypergiant, though its classification remains disputed—some sources interpret the '+' in its spectral type as indicating a companion rather than hypergiant status. Its variability, discovered by Manali Kallat Vainu Bappu in 1958, places it among the Alpha Cygni variables, which are erratically pulsating supergiants. The star's close companion, an O9.75 bright giant at 1.5″, makes it a valuable binary system for studying massive star evolution. Both stars are members of the Cassiopeia OB5 association, with distances estimated around 8,000 to 9,000 light-years, providing context for the region's stellar population. The star's variable star designation V566 Cassiopeiae is used in catalogs. Its legacy lies in the ongoing debate over hypergiant classification and the insights it offers into massive binary systems and stellar pulsation.
Did You Know?
- 6 Cassiopeiae is also designated V566 Cassiopeiae as a variable star.
- The star has a close companion of spectral type O9.75 at only 1.5″ separation.
More in Binary and Multiple Stars, Part 3 1-24
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