Ursa Minor

FIELDS OF STUDY: Astronomy; Observational Astronomy

ABSTRACT: Ursa Minor is a circumpolar constellation in the Northern Hemisphere. The constellation resembles a ladle and is often referred to as the Little Dipper. Its counterpart is Ursa Major, which contains the Big Dipper. Ursa Minor contains Polaris, which is also known as the North Star because of its proximity to the north celestial pole. Various myths exist about Ursa Minor, particularly Greek myths. Ursa Minor has several valuable uses in navigation and in the field of astronomy.

The Little Dipper

Ursa Minor is one of the eighty-eight constellations that are officially recognized by the International Astronomical Union (IAU). Ursa Minor is a circumpolar constellation, because from the perspective of a viewer in the Northern Hemisphere, it never sets and is visible at all times in the night sky. It is more precisely classified as a northern circumpolar constellation, as compared to a southern circumpolar constellation, which is visible at all times in the Southern Hemisphere night sky. Besides Ursa Minor, other northern circumpolar constellations include Ursa Major, Draco, Perseus, Auriga, Lynx, Cepheus, Cassiopeia, and Camelopardalis. Ursa Minor is bordered by the constellations Draco, Chamaeleon, and Cepheus.

Ursa Minor means "little bear" in Latin. The larger constellation Ursa Major, also known as the Great Bear, is Ursa Minor’s counterpart. Ursa Minor resembles a ladle, which is why it is often referred to as the Little Dipper. Ursa Major also contains a ladle-shaped pattern of stars, known as the Big Dipper. According to Tom Kerss, an astronomer at the Royal Observatory Greenwich, Ursa Minor’s Little Dipper got its name from the Big Dipper.

At one time, Ursa Minor was part of the constellation Draco, forming an asterism called the Dragon’s Wing. An asterism is a pattern of stars that is generally simpler than a constellation and is not recognized by the IAU. It may form part of an official constellation, or it may include stars from more than one constellation,

Key Attributes

Ursa Minor is located in the Northern Hemisphere. It takes up an area of 256 square degrees and is the fifty-sixth largest of the eighty-eight constellations. Ursa Minor is visible between the latitudes of 90 degrees north (the North Pole) and 10 degrees south. It is best seen in June around 9:00 p.m. The constellation resembles a ladle, complete with a "bowl" and "handle." There are seven stars in the constellation that form this ladle: alpha Ursae Minoris (better known as Polaris), beta Ursae Minoris (Kochab), gamma Ursae Minoris (Pherkad), delta Ursae Minoris (Yildun), epsilon Ursae Minoris (Urodelus), zeta Ursae Minoris (Akhfa al Farkadain), and eta Ursae Minoris (Anwar al Farkdain). Polaris, Yildun, and Urodelus form the handle of the ladle, and Kochab, Pherkad, Akhfa al Farkadain, and Anwar al Farkdain make up the bowl.

Polaris is one of the principal features of Ursa Minor. Also known as the North Star or the Pole Star, Polaris is a yellow supergiant star and is the brightest star in Ursa Minor. Polaris is less than 1 degree from the north celestial pole, making it a navigational aid in determining true north. It has a right ascension of 2 hours, 31 minutes, and 48.7 seconds. Right ascension is measured eastward from the vernal equinox, one of two points at which the celestial equator intersects with the ecliptic, or the apparent path of the sun through the sky. Polaris has a declination of about +89 degrees. It aligns with Earth’s axis, which is also called the celestial pole.

Polaris is part of a multiple star system, meaning that it has several companion stars. One of the companion stars is hard to see because it is so close to Polaris. It was first observed in 2006 with the Hubble Space Telescope. Another of Polaris’s companion stars was first discovered in 1780 by Sir William Herschel (1738–1822). Located at the end of Ursa Minor’s handle, Polaris is about 430 light-years from Earth.

Kochab, the second-brightest star in Ursa Minor and the brightest in the bowl of the ladle, is an orange giant about 126 light-years from Earth. Pherkad is the third-brightest star in the constellation. It is about 480 light-years from Earth and about 1,100 times brighter than the sun. Kochab and Pherkad are commonly called the "Guardians of the Pole" because they were twin pole stars from about 1500 BCE to 500 CE, before Polaris became the pole star. Yildun, a white dwarf, is about 183 light-years away. Urodelus is a triple star system that is approximately 347 light-years from Earth. Its principal star is a yellow giant. Akhfa al Farkadain is a white main-sequence dwarf that will eventually become a giant star. It is about 380 light-years away. Anwar al Farkdain is a yellow-white main-sequence dwarf that lies about 97 light-years away.

One of the distinguishing features of Ursa Minor is the annual Ursid meteor shower. As seen from Earth’s northern hemisphere, the Ursids radiate from near Ursa Minor and Ursa Major. The meteor shower begins around December 17 and continues for the remainder of the month. During the period of greatest activity, observers can see as many as ten meteors per hour shooting across the sky over a twelve-hour period.

Another key feature of Ursa Minor is the Ursa Minor Dwarf, which is a dwarf elliptical galaxy. Discovered in 1954 by Albert George Wilson (1918–2012) at the Lowell Observatory, the Ursa Minor Dwarf galaxy is a satellite galaxy to the Milky Way galaxy. Its stars began forming stars about eleven billion years ago. As of 2022, astronomers have discovered nine exoplanets orbiting stars in Ursa Minor.

History

Although the discovery of Ursa Minor cannot be pinpointed for certain, the description and naming of the constellation is thought to have originated with the Greek philosopher Thales of Miletus (ca. 625 BCE–545 BCE), although he may have been describing a constellation already used by the Phoenicians, from whom his family descended. Regardless, Ursa Minor was catalogued by the Greek astronomer Ptolemy (ca. 100–ca. 170) in the second century CE. Ptolemy included Ursa Minor in his list of forty-eight constellations.

Numerous myths surround Ursa Minor. In ancient Greece, the constellation was known as Cynosura, Greek for "dog’s tail." It was seen as part of the constellation Canis Major, "the great dog." The constellation was later described in relation to a Greek myth about the nymphs Ida and Adrasteia, who nursed Zeus on Mount Ida in Crete when he was young. According to the myth, Zeus honored his nymph nurses by placing them in the sky as Ursa Major and Ursa Minor. A different Greek myth contends that the seven stars of the Little Dipper represent the Hesperides, who were the seven daughters of Atlas.

Navigation and Astronomy

Ursa Minor has played an important role in navigation for thousands of years. The Phoenicians often used Ursa Minor for navigation because it was a good guide to true north, leading the ancient Greeks to often refer to the constellation as the Phoenician. Polaris is especially significant, as it aligns with Earth’s axis and stays stationary in the night sky over the North Pole, while the other stars in the Northern Hemisphere appear to rotate around it. This fact, along with its brightness, has long made Polaris a key star for navigation.

Polaris is a pulsating variable star, which means it has defined periods of brightness and pulsation. This attribute makes it a valuable star for measuring distances between objects in space. In 2014 astronomers discovered that Polaris is brighter than it was in the past. A team of astronomers determined that the star is approximately two and a half times brighter than it was about two hundred years ago. In fact, since astronomers began studying Polaris, the star has brightened by 15 percent. It began dimming in the 1990s but began to brighten again in the first decades of the twenty-first century.

PRINCIPAL TERM

• celestial equator: the imaginary projection of Earth’s equator onto the night sky.
• circumpolar constellation: a pattern of stars that, from the perspective of the observer, never sets and is always visible in the night sky, despite Earth’s rotation.
• constellation: a pattern of stars that can be seen in the night sky from Earth.
• declination: a space object’s angular distance north or south of the celestial equator.
• International Astronomical Union (IAU): a group founded in 1919 to foster cooperation between astronomers from around the world and promote the science of astronomy.
• right ascension: a space object’s east-west position along the celestial equator, measured eastward from the vernal equinox.

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