Alpha Pictoris
Brightest star in the southern constellation of Pictor
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Alpha Pictoris (α Pic, α Pictoris) is the brightest star in the southern constellation of Pictor. It has an apparent visual magnitude of 3.27,[2] which is bright enough to be viewed from urban areas in the southern hemisphere. This is actually a binary star system, whose components complete an orbit every three years and seven months.[7] It is close enough for its distance to be measured using parallax shifts, which yields a value of roughly 97 light-years (30 parsecs) from the Sun, with a 5% margin of error.[1] Alpha Pictoris has the distinction of being the south pole star of the planet Mercury.[14]
Properties
The primary component is classified as a Lambda Boötis star.[15] The stellar classification of A8 Vn kA6[4] shows this peculiarity, with the kA6 notation indicating weaker than normal calcium K-lines in the spectrum. The 'n' following the main sequence luminosity class of V indicates the absorption lines in the spectrum are broad and nebulous. This is caused by the rapid spin of the star, which has a high projected rotational velocity of 206 km/s.[10] Spectroscopy shows narrow, time-varying absorption features being caused by circumstellar gas moving toward the star. This is not the result of interstellar matter, but a shell of gas along the orbital plane. Alpha Pictoris is categorized as a rapidly rotating shell star that may have recently ejected mass from its outer atmosphere.[16][17]
Alpha Pictoris A is larger than the Sun, with a 60% greater mass[7] and a 355% greater radius.[8] Its estimated age is roughly 660 million years,[11][12] even though it is already nearing its main sequence lifetime, having gone through 94% of the expected timespan on this stage of evolution.[3] It is radiating 40 times as much luminosity as the Sun from its outer atmosphere at an effective temperature of 7,451 K.[9] At this heat, the star glows with the white hue of an A-type star.[18] It is also one of the brightest known Delta Scuti variables.[19] The space velocity components of this star in the galactic coordinate system are U = -22, V = -20 and W = -9 km/s.[20]
Data from the Hipparcos mission uncovered the presence of a binary companion[15] with a companion orbiting at a semimajor axis of around 3.25 AU, or three times the distance from Earth to the Sun.[7][a] Alpha Pictoris is an X-ray source, which is unusual for an A-type star since stellar models don't predict them to have magnetic dynamos. This emission may instead be originating from the companion.[16][21]
Notes
- Calculated using a3 = (M1 + M2) • P2,
where a is the semi-major axis in astronomical units, M1 and M2 are the primary and secondary's mass respectively, in M☉, and P is the period in years.