Also known as a metallic-line star, a type of A star whose spectrum has strong and often variable absorption lines of some metals (hence the "m"), such as zinc, strontium, zirconium, and barium, more typical of an F star, and deficiencies of others, such as calcium and/or scandium. These abundance anomalies are due to some elements being pushed to the surface because they are better light-absorbers, while other elements sink to lower levels under gravity – an effect that requires slow stellar rotation.
Normal A stars spin quickly, but the vast majority of Am stars are known to be members of close binary systems, with orbital periods of less than 1,000 days, in which the two stars slow each other down by tidal action. Familiar examples include Sirius and Acubens (Alpha Cancri). An example of an Am star which is single is HD 8801. This star is also unusual for an Am star in that it pulsates, and it is unique among known stars in that it pulsates intrinsically with both gamma Doradus and delta Scuti frequencies.
Related entries• Ap star
• manganese star
• variable stars
Related categories• TYPES OF STAR
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