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| Alioth | |
|---|---|
| Name | Alioth |
| Designation | Epsilon Ursae Majoris (Epsilon UMa, ε UMa) |
| Epoch | J2000 |
| Constellation | Ursa Major |
| Apparent magnitude | 1.76 |
| Spectral type | A1p (chemically peculiar) |
| Distance | 81 ly (≈24.9 pc) |
| Radial velocity | −12 km/s |
| Proper motion | 1.5″/yr |
| Luminosity | ~80 L☉ |
| Temperature | ~9,000 K |
| Mass | ~2.9 M☉ |
| Radius | ~4.1 R☉ |
Alioth is a bright chemically peculiar A-type star in the northern constellation Ursa Major, notable for its role in the Big Dipper asterism and for being one of the brightest stars visible from northern latitudes. It is an oblique rotator with pronounced magnetic phenomena, spectral peculiarities, and low-amplitude photometric variability. Alioth has long served as a navigational and cultural marker and as a target in studies of stellar magnetism, diffusion, and circumstellar matter.
The traditional name originates in medieval Arabic astronomical tradition, preserved through translations into Latin and European star catalogues. Historical transmitters include observers associated with the Islamic Golden Age and later cataloguers such as those in the work of Johannes Hevelius, John Flamsteed, and Bayer. The International Astronomical Union standardized proper names for bright stars in the 21st century, following conventions developed by committees influenced by historical usage from sources like the Almagest tradition and Renaissance atlases. The star also appears under catalog designations used by institutions such as the Henry Draper Catalogue, the Hipparcos Catalogue, and observatories maintaining the Bright Star Catalogue.
Alioth is classified spectrally as an A-type chemically peculiar (Ap) star, exhibiting abnormal abundances of elements such as chromium, strontium, and europium in its photosphere—features that link it to the class studied in the context of atomic diffusion and radiative levitation by researchers at institutions like Harvard College Observatory and Royal Greenwich Observatory. Its effective temperature, inferred from model atmospheres used by teams at Max Planck Institute for Astronomy and University of Cambridge stellar groups, is roughly 9,000 K, placing it hotter than the Sun and consistent with its white color recorded by observers from Royal Astronomical Society meetings. Mass and radius estimates derive from evolutionary tracks computed with codes developed at facilities like Padova Observatory and Geneva Observatory, yielding values near 2.5–3.0 solar masses and radii of a few solar radii. Its bolometric luminosity situates it among the more luminous members of the Ursa Major moving group documented by investigators at European Southern Observatory.
Alioth sits in the Ursa Major constellation, forming the tail of the Big Dipper asterism alongside stars catalogued by early surveys such as those by Ptolemy and later mapped in atlases by Hevelius and Flamsteed. Parallax measurements from the Hipparcos mission and refined astrometry from the Gaia mission place it at approximately 24.9 parsecs. Proper motion and radial-velocity vectors have been measured by spectroscopic programs at institutions like MKO and Geneva Observatory, revealing motion consistent with membership in or dynamical relation to moving groups identified by kinematic studies such as those led by Eggen and later refined by teams analyzing Gaia DR2. Historical photographic astrometry from observatories like Mount Wilson Observatory contributed early constraints on its motion.
Alioth exhibits low-amplitude photometric variability and strong, organized magnetic fields characteristic of Ap stars, phenomena investigated by research groups at observatories including Kitt Peak National Observatory and Calar Alto Observatory. Rotational modulation due to surface abundance patches of elements such as chromium and europium causes periodic spectral and flux changes, a behavior modeled using techniques developed by teams at University of Vienna and University of Leicester. Magnetic field measurements using spectropolarimeters installed at facilities like Canada–France–Hawaii Telescope and ESPaDOnS at CFHT have revealed field strengths that influence diffusion processes discussed in theoretical work from Max Planck Institute for Solar System Research and Institut d'Astrophysique de Paris. Period analyses referencing data from space missions such as Hipparcos and ground-based photometry trace rotational periods and secular changes.
Searches for infrared excess and circumstellar material around Alioth have been undertaken with instruments aboard missions and facilities like IRAS, Spitzer Space Telescope, WISE, and ground-based submillimeter arrays operated by ALMA partners; results largely indicate no substantial, warm debris disk typical of some Vega-like systems catalogued by teams at NASA and ESA. Radial-velocity monitoring by programs using spectrographs at Keck Observatory and European Southern Observatory has not produced confirmed exoplanet detections, though the presence of chemically stratified atmospheres complicates high-precision velocity work similar to challenges faced in searches around other early-type stars studied by groups at Lick Observatory and Anglo-Australian Observatory. Upper limits from direct imaging campaigns conducted with adaptive optics systems at Gemini Observatory place constraints on massive, wide-orbit companions.
Alioth’s prominence in the Big Dipper made it a navigational aid for seafaring cultures, noted in records associated with Vikings, Polynesians, and medieval European mariners; its role in folklore intersects with mythic traditions archived by scholars at institutions like British Museum and Smithsonian Institution. Astronomers including Tycho Brahe, Edmond Halley, and later observers at Greenwich Observatory and Harvard Observatory documented its position and properties across centuries, contributing to stellar cartography efforts that culminated in modern catalogues compiled by IAU-affiliated committees. Modern public outreach about Alioth features in planetarium programs run by organizations such as American Museum of Natural History and Royal Observatory, Greenwich, reflecting its enduring place in both professional astronomy and popular stargazing.
Category:Stars