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 48951 HIP 48958 HIP 48960 HIP 48971 HIP 48972 HIP 48977 HIP 48993 HIP 48994 HIP 49032 HIP 49036 HIP 49043 HIP 49047 HIP 49050 HIP 49062 HIP 49068 HIP 49072 HIP 49078 HIP 49084 HIP 49088 HIP 49093 HIP 49099 HIP 4911 HIP 49113 HIP 49120 HIP 49125 HIP 4913 HIP 49146 HIP 4916 HIP 49164 HIP 49165 HIP 49169 HIP 49198 HIP 49199 HIP 49200 HIP 49204 HIP 49207 HIP 49209 HIP 49218 HIP 49225 HIP 49233 HIP 49252 HIP 49259 HIP 49263 HIP 49267 HIP 49275 HIP 4928 HIP 49292 HIP 49294

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