Altair

Physical Characteristics

Altair is classified as an A7 V (or A7 Vn) main-sequence star, placing it in the category of hot, blue-white hydrogen-burning stars more massive and more luminous than the Sun. Its effective surface temperature is approximately 7,400–7,800 K — well above the Sun's ~5,780 K — and its bolometric luminosity is roughly 10 to 11 times solar. With a mass of about 1.7–1.8 solar masses and a mean radius in the range of 1.6–2.0 solar radii (varying significantly by latitude due to its rotation), Altair is a modestly larger and hotter version of our star, still firmly on the main sequence fusing hydrogen into helium in its core.

Its most physically remarkable attribute is its rotation. Altair completes one full rotation in approximately 9 hours — some analyses place this at under 8 hours, others at 9–10 hours — compared with the Sun's roughly 25-day equatorial rotation period. The resulting equatorial centrifugal acceleration is so large that the star is significantly oblate: interferometric measurements find an equatorial radius of approximately 2.03 solar radii and a polar radius of approximately 1.63 solar radii, a difference of about 25%. This is a more extreme flattening than that of any planet in the Solar System; for comparison, Saturn — the most oblate planet — has an equatorial radius only about 11% larger than its polar radius, and Jupiter about 9%.

The oblateness is directly tied to surface gravity. Effective gravity (the vector sum of gravitational and centrifugal accelerations) is highest at the poles, where centrifugal force is absent, and lowest at the equator. This latitudinal variation in surface gravity drives the phenomenon known as gravity darkening (described by the von Zeipel theorem for radiative envelopes): regions of lower effective gravity are cooler and dimmer, while higher-gravity regions are hotter and brighter. In Altair's case, the polar regions are visibly brighter and hotter, and the equatorial belt is cooler and darker — the equatorial near-infrared flux is only about 60–70% of the polar flux, as measured by infrared interferometry.

Altair is also classified as a Delta Scuti variable. Satellite photometry from the Wide-Field Infrared Explorer (WIRE) satellite in 1999 revealed that its brightness fluctuates by a few thousandths of a magnitude with periods between roughly 0.8 and 1.5 hours, consistent with low-amplitude radial and non-radial pulsations typical of this class. The Delta Scuti classification was formally established in 2005. In addition, Altair is a weak coronal X-ray emitter, with the strongest X-ray emission concentrated near the equatorial region. The star's spectral lines are noticeably broadened (hence the "n" suffix in its classification), a direct consequence of the Doppler smearing caused by its rapidly spinning surface.

Stellar Evolution and Age

Altair is a main-sequence star currently fusing hydrogen into helium in its core. Its age is uncertain: some analyses favor approximately 100 million years, placing Altair as a comparatively young star near the zero-age main sequence, while at least one stellar catalog assigns a value of approximately 1.2 billion years. Recent astrophysical discussions tend to favor the younger estimate, noting that the presence of circumstellar dust around Altair is more consistent with a younger system, and that Altair's mass and luminosity place it early in its hydrogen-burning phase.

For a star of 1.7–1.8 solar masses, total main-sequence lifetime is on the order of roughly one billion years. Altair is already somewhat bright for its spectral type, which has led some authors to propose that it may be in the early stages of evolving off the main sequence toward subgiant status — reflected in the alternative classification A7 V–IVn. As core hydrogen is progressively converted to helium, the core contracts and heats, the outer layers expand slightly, and the star becomes incrementally more luminous over time.

On longer timescales, once Altair's core hydrogen is exhausted it is expected to leave the main sequence, expand into a red giant (or possibly pass through a pulsating giant phase, given its mass range), and ultimately shed its outer envelope to become a white dwarf. This endpoint — a slow-cooling stellar remnant — is the fate of most stars in Altair's mass range. Spectroscopic analysis of Altair's atmosphere indicates it is metal-rich, with roughly twice the solar iron abundance, implying that it formed from interstellar material already enriched by earlier stellar generations. Altair currently resides within the G-cloud, a region of interstellar gas and dust through which the Sun and several neighboring stars are traveling.

Observational Status and Companions

As of the mid-2020s, Altair is treated as a single star with no confirmed planetary, brown-dwarf, or stellar companions. Radial-velocity surveys, transit searches, and direct-imaging campaigns have not detected any companion signal. The star's rapid rotation significantly broadens its spectral lines, making high-precision radial-velocity work more challenging than for slow-rotating solar-type stars, which may partly explain the absence of strong constraints on close companions. No candidate planets or substellar companions have appeared in the refereed literature.

Interferometric work with CHARA and other arrays continues to refine Altair's fundamental parameters and surface map. Broader companion-detection programs in the 2020s — including Gaia-informed VLTI/GRAVITY studies that have directly imaged dim companions to other bright nearby stars — have not yet targeted Altair for dedicated companion searches. The techniques required to detect Jupiter-class companions around bright A-type stars are maturing, but as of mid-2026 no such detections around Altair have been reported.

Mythology and Cultural History

Altair's name derives from the Arabic phrase al-naṣr al-ṭā'ir, meaning "the flying eagle" or "flying vulture." In medieval Arabic astronomy this term referred not to Altair alone but to the asterism formed by Altair together with its two flanking stars β Aquilae (Alshain) and γ Aquilae (Tarazed), envisioned as a bird in flight. The same three-star grouping appears in al-Achsasi al-Mouakket's catalogue under the name that was later translated into Latin as Vultur Volans, "the flying vulture."

The eagle identity is considerably older than Arabic astronomy. Historical and philological analysis suggests that Babylonian and Sumerian observers already called Altair's region of the sky "the eagle star," making the eagle association one of the better-documented cases of a specific stellar image surviving continuously across multiple civilizations — from ancient Mesopotamia through the Arabic tradition and into the modern constellation Aquila, whose name is simply Latin for "eagle."

In Greek and Roman mythology, Aquila is the eagle of Zeus (Jupiter in Roman tradition), the divine bird entrusted with carrying the god's thunderbolts and retrieving them — earning it the epithet "thunderbird of the Greeks." Two prominent myths attach to the constellation: in one, Aquila is the eagle that abducted the Trojan youth Ganymede on Zeus's orders, bearing him to Olympus to become cup-bearer of the gods; in the other, it is the eagle that perpetually tormented Prometheus by eating his liver each day, until Heracles killed the bird with an arrow. As the brightest star in Aquila, Altair has traditionally been understood to mark the eye or heart of this legendary bird.

Indian astronomical tradition assigns Altair and its two companion stars a sacred role as the footprints of Vishnu, the preserver god of the Hindu trinity, transforming the same three-star group from a bird into a cosmic marker of divine presence.

In Chinese sky lore, Altair carries two distinct but equally rich identities. As Niú Láng Xīng ("Cowherd Star" or Herdboy Star), it is the male protagonist of one of East Asia's most beloved romantic legends. In the story, Niulang (Altair) and Zhinü (Vega) fall in love, marry, and are eventually separated by the Heavenly Emperor, who places the Milky Way between them as punishment for neglecting their duties. The two companion stars flanking Altair, β and γ Aquilae, are identified in some versions as their children. Once a year, on the seventh day of the seventh lunar month, magpies form a bridge across the Milky Way so the lovers can reunite; if it rains that night, the drops are said to be the weaver girl's tears at being unable to cross. This legend underlies the Qixi Festival in China, the Tanabata festival in Japan — where Altair is Hikoboshi and Vega is Orihime — and cognate celebrations in Korea. Separately, the same asterism of Altair, β and γ Aquilae was called Hégǔ ("River Drum" or "Battle Drum") in Chinese military astronomy, symbolizing the command to attack.

Among the Koori people of Victoria, Australia, Altair is known as Bunjil, the wedge-tailed eagle, with β and γ Aquilae interpreted as his two wives, represented by black swans. The pattern of Altair accompanied by its two flanking stars forming a meaningful cultural triad — powerful central figure with two attendants — thus appears independently in Indigenous Australian, Chinese, Arabic, and other traditions, a striking cross-cultural convergence.