Altair is one of the brightest and most closely studied stars in the night sky, known for its striking white color and proximity to Earth. Classified as an A-type main-sequence star, it is hotter and more luminous than our Sun. Its unique properties, especially its extreme rotation, make it a valuable natural laboratory for astrophysicists.
Stellar Classification and Luminosity
The specific stellar classification for Altair is A7Vn. The “A” in its spectral type indicates that it is an A-type star, meaning it is hotter than the Sun, giving it a brilliant white or bluish-white appearance. The number “7” places it toward the cooler end of the A-type range, with a surface temperature between 7,500 and 8,000 Kelvin.
The Roman numeral “V” denotes its luminosity class, identifying it as a main-sequence star. This classification means Altair is currently in the stable, hydrogen-fusing phase of its life, similar to the Sun. The “n” suffix stands for “nebulous” or “fuzzy” lines in its spectrum, which is a direct consequence of its rapid rotation blurring the light. Altair’s total luminosity is approximately 10 to 11 times that of the Sun.
Physical Characteristics and Scale
Altair possesses physical dimensions notably larger than the Sun. The star has a mass that is approximately 1.8 times the mass of the Sun. This greater mass means its core processes hydrogen fusion at a much faster rate.
Altair’s radius is about 1.8 times the Sun’s radius, although this measurement is complicated by its non-spherical shape. Its effective surface temperature is estimated to be around 7,500 Kelvin, compared to the Sun’s temperature of about 5,778 Kelvin. This combination of greater mass, size, and temperature results in the star’s high intrinsic brightness.
The Effect of Rapid Rotation
Altair’s defining characteristic is its fast rotation, which profoundly affects its shape and surface temperature. The star completes one full rotation in approximately 9 to 10 hours, compared to the Sun’s rotation period of about 25 days. This high rotational speed causes a significant centrifugal force at the star’s equator.
This centrifugal force flattens the star, transforming its shape from a sphere into an oblate spheroid. Observations show that Altair’s equatorial diameter is over 20 percent greater than its polar diameter. This distortion means the star’s poles are closer to the center of mass than its equator.
The distortion leads to gravity darkening, where the poles are hotter and brighter than the equator. Because the poles are closer to the star’s core, they experience a stronger gravitational pull, leading to a higher temperature and greater light emission. Conversely, the equatorial regions bulge outward, lowering the surface gravity and temperature, making them appear dimmer. Altair spins at nearly 90 percent of the speed that would cause it to break apart.
Location and Astronomical Context
Altair is located close to Earth, at a distance of approximately 16.7 light-years. This proximity is one reason it ranks as the twelfth brightest star. It is the brightest object in the constellation Aquila, which represents the Eagle.
Altair is one of the three bright points forming the Summer Triangle asterism. This easily recognizable pattern consists of Altair along with the stars Deneb and Vega. Its brightness and position make Altair a useful reference point for sky-gazers and astronomers studying stellar properties.