Ursa Minor - Constellation Guide

Ursa Minor

Ursa Minoris

UMi

The Little Bear



Introduction

Ursa Minor is a medium size constellation located in the far northern reaches of the sky. Also known as the Little Bear, its main group of stars resemble a smaller version of the Great Bear of Ursa Major. Greek astronomer Thales first mentioned the constellation, around 600 BC, after realising it could be used as a better guide to finding true north than Ursa Major. Over the years it has been visualised as many things, including a dog's tail revolving its tip and a bunch of jewels.

Ursa Minor has been important for navigation since it contains the North Celestial Pole (NCP). Currently, the constellation's brightest star Polaris (mag. +1.97) is only three quarters of a degree from the NCP, thereby providing a convenient marker. Polaris is slowly edging nearer and on March 24, 2100 it will be less than half a degree removed. After that it gradually moves away.

Like Ursa Major, the main seven stars of Ursa Minor form the handle of a ladle. The bowl contains second magnitude Kochab (β UMi) and third magnitude Pherkad (γ UMi), which are collectively known as the guardians of the pole. The remaining members of the group are fainter, down to fifth magnitude, but do provide a useful sky darkness check scale. On good nights all seven stars can be seen with the naked eye.

In terms of size, Ursa Minor covers 256 square degrees and is the 56th largest constellation in the sky. It's devoid of bright deep sky objects but contains a few interesting galaxies within amateur range, including the Ursa Minor Dwarf galaxy, a satellite of the Milky Way. Ursa Minor also contains some variable stars and a few double stars of interest.

From locations north of latitude 25N, the constellation is circumpolar and therefore never sets. For those living south of 25S, it always remains below the horizon and can never be seen.

Ursa Minor Star Chart - pdf format (credit:- freestarcharts)

Interesting Stars

Bright Star, Multiple Star, Variable Star

Polaris (alpha Ursa Minoris - α UMi) - is a multiple star system, combined mag. +1.97, that's 434 light-years from Earth. The main component is a F7Ib yellow-white Cepheid type supergiant that varies slightly by 0.05 magnitudes over a period of 4 days; too small a change to be noticed visually. A small 80mm (3.1 inch) refractor at 75x easily reveals a mag. +8.7 yellow-white dwarf companion, which was discovered by William Herschel in 1780. Separation is 18 arc seconds.

The system contains another 3 dwarf stars, although none can be seen with amateur scopes.

Bright Star

Kochab (beta Ursa Minoris - β UMi) - mag. +2.07, is an orange giant star, spectral type K4III, that's 131 light-years away. It's an evolved star meaning its exhausted its core hydrogen, swollen and cooled to become a giant.

Kochab is estimated to be 3 million years old. It's 450x more luminous and 42x larger than the Sun. A planet 6.1x larger than Jupiter has been discovered orbiting the star.

Double Star, Variable Star

Pherkad (gamma Ursa Minoris - γ UMi) - mag. +3.0, is a blue white supergiant star approx. 487 light-years distant. This fast spinning shell star is 15x as large as the Sun and over 1,000x more luminous. Its rotational velocity causes a pulsating disk of gas to surround the equator, resulting in small brightness variations of 0.05 magnitudes every 3.4 hours.

The fifth magnitude star 11 UMi appears near Pherkad and forms a nice unrelated naked eye / binocular double.

Multiple Star, Variable Star

Epsilon Ursae Minoris (ε UMi) - is a multiple star system, combined mag. +4.2. The main star is a RS Canum Venaticorum type eclipsing spectroscopic binary. It fluctuates from mag. +4.19 to +4.23 over a period of 39.48 days. The brightness change is not discernable to the naked eye.

The third system component shines at eleventh magnitude and is separated by 77 arc seconds from the main pair. It can be seen in an 80mm (3.1-inch) scope at medium to high powers.

The epsilon UMi system is 303 light-years from Earth.

Other Stars

Zeta Ursae Minoris (ζ UMi) - mag. +4.3, is a white A type main sequence star 380 light-years distant. Classified as a dwarf, it's nevertheless 3.4x the size of the Sun and 200x more luminous. It's possibly slightly variable of the Delta Scuti type.

Delta Ursae Minoris (δ UMi) - is a white A1V type main sequence dwarf with an apparent mag. +4.35. It's 172 light-years from Earth, 2.8x larger than the Sun and 47x more luminous.

Delta UMi has the traditional name Yildun, derived from the Turkish word for star.

Eta Ursae Minoris (η UMi) - mag. +5.0, is the faintest of the seven stars making up the Little Dipper asterism. The star is often used as a test of sky darkness. If you can spot all seven stars of the Little Dipper, in particular eta UMi, then you have at least a reasonably dark sky.

Eta UMi is a yellow-white F-type giant star 97 light-years distant. It's 1.4x more massive than the Sun.

Variable Stars

RR UMi - is a semiregular variable M5III red giant star that changes between magnitudes +4.4 and +4.8, over a period of 43 days. It's probably the same star as Tau Ursae Minoris (τ UMi).

U UMi - is a Mira type long period variable with a magnitude range between +7.1 and +13.0, over a period of 330.92 days. At brightest, it's easily seen with binoculars.

S UMi - is a Mira type variable for binocular and small telescope observers with a similar brightness range and period to U UMi. It peaks at mag. +7.5 and dims down to mag. +13.2. The period is 331 days.

T UMi - is another Mira star visible with binoculars and small telescopes. Magnitude range is from +7.8 to +15.0. Period 301 days.

V UMi - is a semi-regular variable star that can be followed with binoculars through the full duration of its range. It varies between magnitudes +7.1 and +8.7 over 73 days.

Nova

RW Ursae Minoris (Nova Ursae Minoris 1956) - is a cataclysmic variable star that exploded as a nova in 1956. At brightest it reached sixth magnitude and therefore at the edge of naked eye visibility. It was easily seen with binoculars and small scopes.

The star has now faded significantly to magnitude +19. It's estimated to be 16,300 light-years distant.

Deep Sky

Galaxies

Ursa Minor Dwarf (PGC 54074, UGC 9749) - is a dwarf spheroidal satellite galaxy of the Milky Way. It shines at mag. +11.9 and covers 30 x 19 arc minutes of apparent sky. Although within the range of amateur telescopes, the Ursa Minor Dwarf galaxy is a difficult target due to its low surface brightness. In fact, it wasn't discovered until 1955 by A.G. Wilson of the Lowell Observatory from photographs taken during the Palomar Sky Survey.

Under dark skies the Ursa Minor Dwarf can be glimpsed with a 150mm (6-inch) reflector but a larger instrument is recommended. Averted vision and low magnifications assist, but the real key to spotting this galaxy is dark transparent skies. At best, it appears as a faint large elongated smudge of light that's slightly brighter towards the centre. In large amateur scopes, the much more distant and fainter edge-on galaxy IC 1110 can also be seen at high magnifications. It's located at the edge of the Ursa Minor Dwarf galaxy and shines at mag. +14, apparent size 1.4 x 0.4 arc minutes.

The Ursa Minor Dwarf galaxy is probably as old as the Milky Way itself. Since star formation has long ceased, it contains mostly older stars. It's 225,000 light-years away.

NGC 6217 - mag. +11.2, is a barred spiral galaxy located 67 million light-years distant. It's characterized as a starburst galaxy, therefore exhibits a high rate of star formation compared to a typical galaxy, and contains many young stars.

NGC 6217 is positioned approx. 2.5 degrees east of Zeta Ursae Minoris and a similar distance north of Eta Ursae Minoris. Through a 150mm (6-inch) telescope it appears unspectacular but is better in larger scopes. For example with a 350mm (14-inch) reflector, NGC 6217 appears as a rectangular patch of wispy light with a brighter nucleus. A magnification of about 200x is recommended. In total it spans 3.3 arc minutes of apparent sky.

NGC 6217 was catalogued as Arp 185 by Halton Arp in his "Atlas of Peculiar Galaxies" that was complied in the 1960's.

NGC 6251 - mag. +13.5, is an active Seyfert type elliptical supergiant galaxy. It's an intrinsically large galaxy but appears faint and small from our perspective, simply due to its vast distance. Large amateur telescopes of the order of 300mm (12-inch) or greater reveal a small smudge of light, similar to a de-focused star. It spans 1.8 x 1.6 arc minutes.

NGC 6251 emits strongly in gamma rays and radio waves and is one of the most extreme examples of a Seyfert galaxy. It's noted for its one-sided radio jet that was discovered in 1977.

The galaxy is 340 million years distant and has a spatial diameter of 180,000 light-years. It's estimated to contain a trillion stars.

Meteor shower

The Ursids is an annual meteor shower active between December 17th and 26th. Normally it produces a ZHR (Zenithal Hourly Rate) of about 10, although there have been at least two major outbursts in the past 75 years (1945 and 1986). During outburst more than 50 meteors per hour have been observed, propelling it into the realms of the better annual showers of the year. Its parent body is comet 8P/Tuttle.

The radiant is positioned just a few degrees northwest of Kochab. Peak activity usually occurs on the night of December 22nd / 23rd.

Ursa Minor Data Table

Henry Draper Catalogue (HD) Hipparcos Catalogue (HIP) Bayer Flamsteed Struve Name RA (J2000) DEC (J2000) Visual Mag. Var. Var. Mag. Range Period (days) Double Sep. (arc secs) PA (deg.) Mag. Primary. Sec 8890 11767 Alpha UMi 1 93 Polaris 02h 31m 47.08s +89d 15h 50.9s 1.97 Y 1.94 -> 1.99 4.0 Y 18 233 A 2.0 / B 8.7 131873 72607 Beta UMi 7 --- Kochab 14h 50m 42.40s +74d 09h 19.7s 2.07 --- --- --- --- --- --- --- 137422 75097 Gamma UMi 13 --- Pherkad 15h 20m 43.75s +71d 50h 02.3s 3.00 Y 2.97 -> 3.02 0.142 Y 1020 90 A 3.0 / B 5.0 153751 82080 Epsilon UMi 22 --- --- 16h 45m 58.16s +82d 02h 14.1s 4.21 Y 4.19 -> 4.23 39.48 Y 77 1 A 4.2 / B 11.2 142105 77055 Zeta UMi 16 --- --- 15h 44m 03.46s +77d 47h 40.2s 4.29 --- --- --- --- --- --- --- 166205 85822 Delta UMi 23 --- Yildun 17h 32m 12.90s +86d 35h 10.8s 4.35 --- --- --- --- --- --- --- 132813 73199 RR UMi --- --- --- 14h 57m 35.12s +65d 55h 56.6s 4.63 Y 4.44 -> 4.85 43.3 --- --- --- --- 148048 79822 Eta UMi 21 --- --- 16h 17m 30.50s +75d 45h 16.9s 4.95 --- --- --- --- --- --- --- 125556 69816 U UMi --- --- U UMi 14h 17m 19.90s +66d 47h 39.2s 7.1 --- 7.1 -> 13.0 330.92 --- --- --- --- --- --- V UMi --- --- V UMi 13h 38m 41.07s +74d 18h 36.3s 7.1 --- 7.1 -> 8.7 73 --- --- --- --- 139492 75847 S UMi --- --- S UMi 15h 29m 34.57s +78d 38h 00.3s 7.5 --- 7.5 > 13.2 331 --- --- --- --- 118556 --- T UMi --- --- T UMi 13h 34m 41.12s +73d 25h 53.0s 7.8 --- 7.8 -> 15.0 43.3 --- --- --- --- --- --- RW UMi --- --- RW UMi 16h 47m 54.75s +77d 02h 12.2s 19 --- 6.0 -> 19.0 --- --- --- --- ---

Ursa Minor Deep Sky Data Table