The asterism is often confused for the constellation itself, much like the Big Dipper is sometimes confused for Ursa Major, the Great Bear, but it is only the brightest part of the constellation.
The Little Dipper is formed by the prominent stars in Ursa Minor constellation, but they are not the only stars in the constellation. The handle of the Dipper is formed by the stars of the Bear’s tail, while the Dipper’s cup is formed by the bright stars forming the Bear’s flank.
The Little Dipper is important in navigation as its brightest star, Polaris, also known as the North Star, reveals the location of the North Celestial Pole. Polaris is the nearest bright star to the pole. The star’s angle above the horizon can also be used to find your latitude on Earth, which used to make the North Star exceptionally useful to sailors. For observers near the equator, Polaris appears near the horizon.
For those observing the night sky from the North Pole, the North Star appears directly overhead. From mid-northern latitudes, Polaris appears halfway between the horizon and the zenith. Polaris is relatively easy to find in the night sky because two bright stars at one end of the Big Dipper point in its direction.
The Little Dipper is visible between latitudes 90 and -10, which means that anyone trying to observe it south of 10°S won’t have much luck because the asterism (and the constellation itself) can’t be seen from most locations in the southern hemisphere.
The best time of year to observe the Little Dipper is June at around 9 PM.
To see the whole asterism, one needs good viewing conditions and very dark skies because the four stars lying between the North Star on one side and Kochab and Pherkad marking the outer bowl on the other, are relatively dim.
The Little Dipper asterism is formed by six named stars – Polaris (Alpha Ursae Minoris), Kochab (Beta Ursae Minoris), Yildun (Delta Ursae Minoris), Pherkad (Gamma Ursae Minoris), Ahfa al Farkadain (Zeta Ursae Minoris), and Anwar al Farkadain (Eta Ursae Minoris) – and Epsilon Ursae Minoris.
α Ursae Minoris (Alpha Ursae Minoris), better known as Polaris or the North Star, is the brightest star in the constellation Ursa Minor. The North Star marks the end of the Little Dipper’s handle, or the tip of the Little Bear’s tail. One of the star’s ancient names, Cynosūra, is derived from the Greek phrase meaning “the dog’s tail.” In Greek times, the constellation Ursa Minor was taken to represent a dog. The star has been known by many other names, including Alruccabah, Navigatoria, Mismar, Yilduz, and Star of Arcady.
Polaris is a yellow-white supergiant belonging to the spectral class F7. The star has an apparent magnitude of 2.02. Its distance is estimated to be between 325 and 425 light years from Earth. Alpha Ursae Minoris is classified as a Cepheid variable. It is the nearest Cepheid variable to Earth. The star is believed to be much brighter today than it was when Ptolemy observed it, when it was a third magnitude star.
Polaris is really a multiple star system, consisting of the main star, two smaller companion stars, and two more distant components. The main component in the system is an F7 yellow supergiant with a mass 4.5 times that of the Sun. The two smaller stars are main sequence stars belonging to the spectral classes F3 and F6.
Polaris was not always the North Star, nor will it stay the northernmost star forever. In approximately 14,000 years, the bright star Vega in the constellation Lyra will take over, until Polaris takes over again in another 14,000 years. This happens as a result of precession of the Earth’s axis, which is caused by the gravitational attraction of the Sun and the Moon. After the 21st century, the celestial pole will move away from Polaris and, by the 41st century, it will come near Gamma Cephei in Cepheus constellation.
β Ursae Minoris (Beta Ursae Minoris), or Kochab, is an orange giant with a visual magnitude of 2.08, which makes it only a little less bright than Polaris. It lies at a distance of 130.9 light years from the solar system. With a surface temperature of 4,030 K, Kochab is 390 times more luminous than the Sun. It is the brightest star in the Little Dipper’s bowl. It lies about 16° from Polaris in the sky. Kochab has the stellar classification of K4 III and a radius about 42 times that of the Sun.
Kochab and Pherkad served as twin pole stars from 1500 BC to 500 AD, but neither of them was as close to the celestial pole as Polaris is today.
γ Ursae Minoris (Gamma Ursae Minoris), or Pherkad, has an apparent magnitude of 3.05 and is about 487 light years distant from Earth. The star has an old Arabian name, Pherkad, which is derived from a phrase meaning “the dim one of the two calves.” Pherkad is indeed not as bright as Kochab, which is close to Polaris in brightness.
Pherkad belongs to the spectral class A3 Iab, with the ‘Iab’ indicating that the star is an intermediate luminosity supergiant. The white supergiant is a rapid rotator, with a projected rotational velocity of 180 km/s. Gamma Ursae Minoris is also classified as a shell star, which means that it has a circumstellar disk of gas around the equator. The star is 1100 times more luminous than the Sun and has a radius 15 times solar.
Yildun, δ Ursae Minoris (Delta Ursae Minoris), lies at an approximate distance of 172 light years from Earth and has a visual magnitude of 4.35. It is a white main sequence dwarf belonging to the spectral class A1Vn. It has a radius 2.8 times that of the Sun and is 47 times more luminous.
ζ Ursae Minoris (Zeta Ursae Minoris) is also known as Ahfa al Farkadain. The star has a visual magnitude of 4.32 and is approximately 380 light years distant from the solar system. It is another white main sequence star in the Little Dipper. It has the stellar classification A3Vn. The star’s traditional name is derived from the Arabic phrase aḫfa al-farqadayn, which means “the dimmer of the two calves.” The traditional names of Zeta and Eta Ursae Minoris were originally applied to the brighter Pherkad and Kochab.
η Ursae Minoris (Eta Ursae Minoris) is known as Anwar al Farkadain (“the brighter of the two calves”) or Alasco. It has an apparent magnitude of 4.95 and lies at a distance of 97 light years from Earth. Eta Ursae Minoris belongs to the spectral class F5 V, which means that it is a main sequence dwarf. It has a visual magnitude of 4.95, making it barely visible to the naked eye. The star lies abour 97.3 light years from the solar system.
ε Ursae Minoris (Epsilon Ursae Minoris) is a triple star system located about 347 light years from Earth. The primary component in the system is an eclipsing spectroscopic binary star belonging to the spectral class G5III, with a mean visual magnitude of 4.21. The star system’s light changes from magnitude 4.19 to 4.23 with a period of 39.48 days as a result of the eclipses. The system is classified as an RS Canum Venaticorum type variable, which is to say it is a close binary pair of stars with active chromospheres that cause large stellar spots which in turn cause variations in brightness. The third component in the Epsilon Ursae Minoris system is an 11th magnitude star, located 77 arcseconds away.
In the early myths, the seven stars that form the Little Dipper represented the Hesperides, the nymphs who were tasked with guarding Hera’s orchard where immortality-giving apples grew. The nymphs occasionally plucked from the grove and Hera placed Ladon, a never-sleeping dragon with a hundred heads, to keep an eye on them. Ladon is represented by the neighbouring constellation Draco. Heracles killed the dragon and stole the golden apples as part of his eleventh labour.
The constellation Ursa Minor was created by Thales of Miletus around the year 600 BC from stars that previously marked the wings of Draco, the Dragon. Draco constellation is still looping around Ursa Minor in the sky. Thales created the new constellation after Phoenician sailors had showed him how to use the stars of the Little Dipper to find north. In his days, the direction of the North Celestial Pole was marked by the stars Kochab and Pherkad, not by Polaris. This is why these two bright stars are still sometimes referred to as the Guardians of the Pole.
Because the seven bright stars of the Little Dipper point the way north, the Latin word for “north” is septentrio, derived from “septem triones,” or seven oxen.
By following the line between these two stars upwards, out of the cup, you will come across Polaris, which is the next bright star along that line.
The distance from the Big Dipper to Polaris is about five time the distance between Merak and Dubhe, which are also known as the Pointer stars as they point the way to the North Celestial Pole.
Aside from Polaris, the only stars in the Little Dipper that are bright enough to be readily seen from urban areas on a clear night are Kochab and Pherkad.
These two stars, the Guardians of the Pole, appear to march around the North Star and are the nearest bright stars to the pole except Polaris.