Delta Ursae Majoris (δ Ursae Majoris, abbreviated Delta UMa, δ UMa), formally named Megrez , is a star in the northern constellation of

Ursa Major Ursa Major (; also known as the Great Bear) is a constellation in the northern sky, whose associated mythology likely dates back into prehistory. Its Latin name means "greater (or larger) bear," referring to and contrasting it with nearby Ursa ...

. With an

apparent magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's ...

of +3.3, it is the dimmest of the seven stars in the

Big Dipper The Big Dipper ( US, Canada) or the Plough ( UK, Ireland) is a large asterism consisting of seven bright stars of the constellation Ursa Major; six of them are of second magnitude and one, Megrez (δ), of third magnitude. Four define a "bowl" ...

asterism. Parallax measurements yield a distance estimate of from the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

Stellar properties

Delta Ursae Majoris has 63% more mass than the Sun and is about 1.4 times its radius. It has a stellar classification of A3 V, which means it is an

A-type main sequence star An A-type main-sequence star (A V) or A dwarf star is a main-sequence (hydrogen-burning) star of spectral type A and luminosity class V (five). These stars have spectra defined by strong hydrogen Balmer absorption lines. They measure between 1 ...

that is generating energy at its core through the

nuclear fusion Nuclear fusion is a reaction in which two or more atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles ( neutrons or protons). The difference in mass between the reactants and products is manife ...

of hydrogen. It shines at 14 times the luminosity of the Sun, with this energy being emitted from its outer envelope at an effective temperature of 9,480 K. This gives it the white hue typical of an A-type star. This star has an excess emission of infrared radiation, indicating the presence of circumstellar matter. This forms a

debris disk A debris disk (American English), or debris disc (Commonwealth English), is a circumstellar disk of dust and debris in orbit around a star. Sometimes these disks contain prominent rings, as seen in the image of Fomalhaut on the right. Debris di ...

around an orbital radius of 16

astronomical unit The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbits ...

s from the star. This radius is unusually small for the estimated age of the disk, which may be explained by drag from the Poynting–Robertson effect causing the dust to spiral inward. It has two faint companions, a 10th magnitude star and an 11th magnitude star, both at an angular separation of two

arcminute A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of one degree. Since one degree is of a turn (or complete rotation), one minute of arc is of a turn. The n ...

s from the primary. Delta Ursae Majoris is a member of the Ursa Major moving group, an

association Association may refer to: *Club (organization), an association of two or more people united by a common interest or goal *Trade association, an organization founded and funded by businesses that operate in a specific industry *Voluntary associatio ...

of stars that share a common motion through space and likely formed in the same

molecular cloud A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit absorption nebulae, the formation of molecules (most commonly molecular hydroge ...

. The space velocity components of Delta Ursae Majoris in the

galactic coordinate system The galactic coordinate system is a celestial coordinate system in spherical coordinates, with the Sun as its center, the primary direction aligned with the approximate center of the Milky Way Galaxy, and the fundamental plane parallel to an a ...

are

, V, W The comma is a punctuation mark that appears in several variants in different languages. It has the same shape as an apostrophe or single closing quotation mark () in many typefaces, but it differs from them in being placed on the baseline (t ...

= 15.35, +1.17, –11.52

Nomenclature

Sidney Hall - Urania's Mirror - Ursa Major

''δ Ursae Majoris'' ( Latinised to ''Delta Ursae Majoris'') is the star's Bayer designation. It bore the traditional name ''Megrez'' and the historical name ''Kaffa''. ''Megrez'' comes from the ''al-maghriz'' 'the base f the bear's tail. Professor Paul Kunitzch has been unable to find any clues as to the origin of the name ''Kaffa'', which appeared in a 1951 publication, ''Atlas Coeli'' (

Skalnate Pleso Atlas of the Heavens The ''Skalnaté Pleso Atlas of the Heavens'' (''Atlas Coeli Skalnaté Pleso 1950.0'') is a set of 16 celestial charts covering the entire sky. It is named after the Skalnaté Pleso Observatory in Slovakia where it was produced. The first versi ...

) by Czech astronomer

Antonín Bečvář Antonín Bečvář (; 10 June 1901 – 10 January 1965) was a Czech astronomer who was active in Slovakia. He was born (and died) in Stará Boleslav. Among his chief achievements is the foundation of the Skalnaté Pleso Observatory and the ...

. The

Hindu Hindus (; ) are people who religiously adhere to Hinduism. Jeffery D. Long (2007), A Vision for Hinduism, IB Tauris, , pages 35–37 Historically, the term has also been used as a geographical, cultural, and later religious identifier for ...

s knew this star as ''

Atri Atri ( sa, अत्रि) or Attri is a Vedic sage, who is credited with composing numerous hymns to Agni, Indra, and other Vedic deities of Hinduism. Atri is one of the Saptarishi (seven great Vedic sages) in the Hindu tradition, and the on ...

'', one of the Seven Rishis. In Chinese, (), meaning '' Northern Dipper'', refers to an asterism equivalent to the Big Dipper. Consequently, the

Chinese name Chinese names or Chinese personal names are names used by individuals from Greater China and other parts of the Chinese-speaking world throughout East and Southeast Asia (ESEA). In addition, many names used in Japan, Korea and Vietnam are ofte ...

for Delta Ursae Majoris itself is (, en, the Fourth Star of Northern Dipper) and (, en, Star of Celestial Balance).AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 6 月 15 日
/ref>

Namesakes

USS ''Megrez'' (AK-126) was a

United States Navy The United States Navy (USN) is the maritime service branch of the United States Armed Forces and one of the eight uniformed services of the United States. It is the largest and most powerful navy in the world, with the estimated tonnage ...

Crater class cargo ship The ''Crater''-class cargo ship were converted EC2-S-C1 type, Liberty cargo ships, constructed by the United States Maritime Commission (USMC) for use by the US Navy during World War II. The designation 'EC2-S-C1': 'EC' for Emergency Cargo, '2' fo ...

named after the star.

References

{{Stars of Ursa Major A-type main-sequence stars Triple star systems Ursa Major Moving Group Megrez Ursae Majoris, Delta Big Dipper Ursa Major (constellation) Durchmusterung objects Ursae Majoris, 69 106591 059774 4660