This article was accepted into the corpus but its outbound wikilinks were never NER-processed — typical at the deepest BFS hop or when the run's entity cap was reached. No expansion funnel to show.
| Megrez (Delta Ursae Majoris) | |
|---|---|
| Name | Megrez (Delta Ursae Majoris) |
| Epoch | J2000 |
| Constellation | Ursa Major |
| Ra | 13h 03m 33.35s |
| Dec | +54° 59′ 19.5″ |
| App mag v | 3.31 |
| Spectral type | A3V |
| B v | +0.10 |
| Radial velocity | +6.9 |
| Parallax | 40.32 |
| Mass | 2.0 |
| Radius | 1.4 |
| Luminosity | 14 |
| Temperature | 8,300 |
| Age | 400 million |
| Names | Delta Ursae Majoris, HD 103287, HR 4554, HIP 58001 |
Megrez (Delta Ursae Majoris) is the faintest of the seven principal stars of the Big Dipper asterism in the constellation Ursa Major. It is a white main-sequence star visible to the naked eye and notable for its role in the outline of the Plough and as a nearby example of an early-type A-class dwarf. Megrez lies well within the scope of modern astrometric surveys and has been the subject of spectroscopic and photometric studies by observatories and space missions.
The traditional name "Megrez" derives from Arabic through medieval European sources, and the Bayer designation "Delta Ursae Majoris" follows the system created by Johann Bayer. Catalog identifiers include entries in the Henry Draper Catalogue as HD 103287, the Bright Star Catalogue as HR 4554, and the Hipparcos Catalogue as HIP 58001, reflecting inclusion in surveys by institutions such as the Harvard College Observatory and the European Space Agency. Modern databases maintained by organizations like the International Astronomical Union standardize these designations for cross-referencing between missions such as Gaia (spacecraft) and observatories like Mount Wilson Observatory.
Megrez is classified as an A-type main-sequence star with a spectral type around A3V, exhibiting a white hue characteristic of stars with effective temperatures near 8,300 K. Its mass and radius, estimated from stellar models calibrated by data from Hipparcos and Gaia DR2, are approximately twice the mass of the Sun and about 1.4 solar radii, yielding a bolometric luminosity on the order of 14 times solar. Photometric colors recorded in the Johnson–Morgan photometric system and spectral line profiles measured at facilities like Palomar Observatory and Kitt Peak National Observatory inform determinations of metallicity and surface gravity. Rotational broadening in high-resolution spectra indicates moderate projected rotational velocity, consistent with measurements from instruments operated by the European Southern Observatory.
As an A3V dwarf, Megrez is on the hydrogen-burning main sequence and is modeled by stellar evolution codes used by research groups at institutions such as the Max Planck Institute for Astronomy and Princeton University. Isochrone fitting based on photometry and parallax suggests an age of a few hundred million years, placing it younger than the Sun but older than members of very young moving groups like the TW Hydrae association. Evolutionary tracks computed with physics from collaborations including the Geneva Observatory predict that Megrez will exhaust core hydrogen and evolve toward the subgiant branch on timescales of several hundred million to a few billion years, similar to other A-type stars studied in surveys by the Kepler Mission and the Transiting Exoplanet Survey Satellite.
Precise parallax measurements from Hipparcos and refined by Gaia yield a distance of about 81 light-years (≈25 parsecs), placing Megrez in the local solar neighborhood cataloged by projects at the Harvard-Smithsonian Center for Astrophysics. Radial velocity and proper motion data tie it to kinematic analyses performed by researchers at institutes like the European Southern Observatory and the Harvard College Observatory. Space motion vectors computed in the Galactic coordinate frame indicate that Megrez is not presently a member of prominent nearby moving groups such as the Ursa Major moving group despite its proximity to that asterism; studies comparing its motion to membership lists assembled by teams at the University of Vienna and University of Groningen help clarify its dynamical status.
High-resolution imaging and spectroscopic monitoring from facilities including Palomar Observatory and adaptive optics systems at observatories like Keck Observatory have not revealed a close stellar companion, placing limits on binary companions down to certain contrast ratios and separations. Radial velocity surveys and speckle interferometry performed by groups at the US Naval Observatory and the Cerro Tololo Inter-American Observatory constrain possible low-mass companions. Infrared observations by missions such as Infrared Astronomical Satellite and the Spitzer Space Telescope have been examined for excess emission indicative of a debris disk; while some A-type stars exhibit prominent circumstellar disks (e.g., Vega, Fomalhaut), Megrez shows at most marginal or absent infrared excess in published catalog comparisons compiled by the NASA/IPAC Infrared Science Archive.
Megrez figures prominently in human star lore through its role in the Big Dipper and the Plough, motifs present in the mythologies of cultures including the Greeks, Native American peoples, and Chinese astronomy. Historical star catalogs such as those by Ptolemy and medieval Islamic astronomers mention the stars of Ursa Major, and European navigational charts used during the Age of Discovery exploited the asterism for orientation. In modern times, Megrez has appeared in outreach materials produced by institutions including the Royal Astronomical Society and educational programming at observatories like Griffith Observatory, serving as a reference point in amateur astronomy guides published by organizations such as the Royal Greenwich Observatory and the American Astronomical Society.
Category:Ursa Major Category:Main-sequence stars Category:A-type stars