Class A — A-type (White)

A-type stars are white or blue-white stars with surface temperatures of 7,500 to 10,000 K. They comprise about 0.6% of main-sequence stars and are known for having the strongest hydrogen absorption lines in their spectra. Many of the best-known stars are A-type, including Sirius (A1V), the brightest star in the night sky, and Vega (A0V), which was historically the reference star for the magnitude system. A-type stars live for roughly 1-2 billion years.

A-type stars are white to blue-white stars with surface temperatures between 7,500 and 10,000 K. They represent about 0.6% of main-sequence stars and include many of the most culturally important stars in the sky. A-type stars are particularly significant in astronomical history: Vega, the prototypical A0V star, served as the original definition of apparent magnitude 0.0 and was the first star (other than the Sun) to be photographed (1850) and to have its spectrum recorded (1872).

Characteristics

A-type spectra are dominated by the hydrogen Balmer series, which reaches maximum strength at A0. These stars are massive enough (1.4-2.1 solar masses) to have radiative envelopes rather than convective ones, which allows chemical peculiarities to develop. The Am (metallic-line) and Ap (peculiar) stars show unusual abundances of certain elements concentrated by diffusion in their stable atmospheres. A-type main-sequence stars live for roughly 1-3 billion years. The debris disks detected around many A stars (notably Vega and Fomalhaut) hint at planetary systems in formation.

Notable Examples

Sirius (Alpha Canis Majoris, A1V) is the brightest star in the night sky at magnitude -1.46, largely due to its proximity at 8.6 light-years. Its white dwarf companion Sirius B was the first white dwarf discovered. Vega (Alpha Lyrae, A0V) and Altair (Alpha Aquilae, A7V) join Deneb to form the Summer Triangle. Fomalhaut (Alpha Piscis Austrini, A3V) hosts a dramatic debris ring and was the first star with a directly imaged exoplanet candidate. Castor (Alpha Geminorum) is a spectacular sextuple star system dominated by A-type components.

HIP 25445 HIP 25447 HIP 25448 HIP 25453 HIP 25454 HIP 25456 HIP 25459 HIP 25460 HIP 25462 HIP 25463 HIP 25464 HIP 25467 HIP 25471 HIP 25472 HIP 25474 HIP 25478 HIP 25483 HIP 25487 HIP 25488 HIP 25495 HIP 25502 HIP 25507 HIP 25511 HIP 25513 HIP 25517 HIP 25520 HIP 25522 HIP 25525 HIP 2553 HIP 25530 HIP 25535 HIP 25536 HIP 25542 HIP 25543 HIP 25546 HIP 25551 HIP 25574 HIP 25577 HIP 25584 HIP 25585 HIP 2559 HIP 25591 HIP 25593 HIP 25594 HIP 25597 HIP 25600 HIP 25602 HIP 25603

Frequently Asked Questions

What is a class A star?

Class A (A-type (White)) stars are white stars. A-type stars are white or blue-white stars with surface temperatures of 7,500 to 10,000 K. They comprise about 0.6% of main-sequence stars and are known for having the strongest hydrogen absorption li

How hot are class A stars?

Class A stars have surface temperatures between 7,500 K and 10,000 K.

What color are class A stars?

Class A stars appear white.

How many class A stars are in the StarFYI database?

StarFYI currently catalogs 18,693 class A stars.

How luminous are class A stars?

Luminous, 5-25 solar luminosities