Ursa Major – the Great Bear

 

 

Ursa Major is probably the most well-known constellation in the northern sky, although not many people know that it is made up of more than just the stars we know as the Plough. The Plough, or the Big Dipper, is an asterism, or shape within a constellation, making up the tail and part of the back of the bear. The name "the great bear" was probably assigned because of the constellation's northern location, and bears were associated with the north.

A couple of notes about individual stars within Ursa Major: xi Ursa Majoris in 1828 became the first binary star system to have its orbit calculated, and the name of the star zeta Ursa Majoris, Mizar, comes from the Arabic word for 'groin,' as the Arabs interpreted it as being in that area of the bear's body (although we see it as being in its tail). Mizar is in fact a double star, with a partner Alcor, and the Arabs also termed them the 'horse and rider.' The Romans used them as a test of vision when selecting archers for the army.

In Greek mythology, the bear was considered to be the nymph Callisto in her bear form. Callisto was a companion of the virgin goddess Artemis (known by the Romans as Diana), protector of babies and animal young and patroness of childbirth. The god Zeus (aka Jupiter) seduced and impregnated her. Artemis banished her and after she gave birth Zeus' wife Hera (aka Juno) turned her into a bear. Years later her son Arcas encountered her and, not knowing she was his mother, tried to kill her. To save Callisto Zeus put the two of them in the sky. As some strange kind of revenge Hera made sure that the bear Callisto would never touch the water, and in the latitudes in which the storytellers lived the constellation never goes below the horizon.

Callisto is also the name of one of Jupiter's Galilean satellites.

The Great Bear was also identified by the Greeks as the tree nymph Adrasteia.

Many North American aboriginal tribes (at least Algonquin, Iroquois, Illinois, and Narragansett) associate the constellation with a bear. According to some legends, the "bowl" shape in the plough was a bear and three stars of the handle three hunters pursuing it. When the constellation was low in the sky in the autumn, it was said that the bear had been injured and its blood stained the leaves on the trees red.

The Plough, as well as being known as the 'Big Dipper' in the USA, is identified by some cultures as a wagon or cart or a bull's thigh. The Chinese associate it with the government!

The Plough may be used to navigate at night. The two stars at the end of the hook-shape point towards Polaris, the North Star, and hence the direction of the north. Before and during the American civil war, songs were spread to slaves in the south (who were mostly illiterate) to help them escape to the north using the Plough to navigate, telling them to follow the "drinking vessel" to a better life.

The seven stars of the Plough however are not moving in the same direction and so over time this familiar asterism will dissolve, in fact it has only been over the last 50,000 years or so that a discernible dipper shape has formed. As the stars move away from each other the system will become more and more plough like, as alpha UMa moves southwards and in front of the rest.

 

The stars of Ursa Major are fairly bright, and widely dispersed. The Big Dipper itself only covers half of the width. The constellation also extends quite a distance south, with its most southerly star, xi Ursae Majoris (Alula Australis), as far south as Leo and Cancer.

 

 

Stars

 

The most famous multiple star systems is Mizar and Alcor. Zeta UMa called Mizar - from the Arabian word for girdle or apron, it is about 2.4 mag in brightness; therefore it is possible for the naked eye to see the 4th mag. companion, called Alcor (80 UMa). This star system is 78 light years away according to its parallax.

 

The star that is moving so that the Big Dipper becomes more Plough like is, Dubhe (alpha UMa). It is a yellow giant, about 25 times the size of the Sun, and 86 light years away. It is a close visual binary as well.

 

Looking at the shape of the Bear you have: Merak (beta UMa), or "loin"; Phecda (gamma UMa): thigh, and finally Megrez (delta UMa): root (or base) of the tail. These three are all white (A-type) stars, and all within 100 light years distance. In the tail, first encountered is epsilon UMa, an alpha-CV type variable. Alpha-Canum Venaticorum type stars are rotating variables which typically have very little change in visual magnitude. These are generally A-type stars but they have an unusual abundance of heavy metals and a corresponding lack in the more common elements.  These stars are divided into three groups: those with predominantly silicon spectral lines, those with manganese, and those with chromium-strontium lines. Epsilon UMA shows a strong chromium line.

 

Also in the tail is another A-type white star, called Alioth the star is one of the brightest in the constellation, although one of the more distant stars. Finally we have eta Ursae Majoris, called either Benetnasch or Alkaid, both of which mean "chief of the mourners". This is a blue-white star, a bit further than the rest at about 95 light years. 

 

Other objects in Ursa Major

 

  • Planetary nebula M 97.
  • Galaxies: M 81, M 82, M 101, M 108 and M 109.

 

M-97 (NGC 3587), the Owl Nebula:

 

Visual Brightness

9.9 (mag)

Apparent Dimension

3.4x3.3 (arc min)

This is a type 3a  Planetary Nebula in Ursa Major. It appears as a gray puff of light which is slightly bright in the centres, at times (especially with averted vision), the ‘eyes’ of the owl can be seen.

 

 

 

 

 

 

 

M-81 (NGC 3031), Bode’s Galaxy:

 

Visual Brightness

6.9 (mag)

Apparent Dimension

21x10 (arc min)

 

M81 is the first of the four objects originally discovered by Johann Elert Bode, who found it, together with its neighbour M82, on December 31, 1774. Bode described it as a "nebulous patch", about 0.75 deg away from M82.

 

 

M-82 (NGC 3034), the Cigar Galaxy:

 

Visual Brightness

8.4 (mag)

Apparent Dimension

9x4 (arc min)

[M82, HST]

This is an irregular galaxy type Ir-II. A great amount of mottling across its length can be seen. The southern edge appears flatter, and it seems to be 'pinched' near the center on this side. M82 was discovered on December 31, 1774 by Johann Elert Bode together with M81. In the infrared light, it is the brightest galaxy in the sky; it exhibits a so-called infrared excess (it is much brighter at infrared wavelengths than in the visible part of the spectrum).

 

 

 

 

 

 

 

 

M-101 (NGC 5457), the Pinwheel Galaxy:

 

Visual Brightness

7.9 (mag)

Apparent Dimension

22.0(arc min)

This is a type Sc spiral galaxy and was discovered by Pierre Méchain in 1781. Visually it is quite symmetric, it has a brighter core surrounded by an envelope which sometimes can be seen to be spiral arms. On photographs it is revealed as one of the most prominent Grand Design spirals in the sky, it is remarkably unsymmetrical, its core is being considerably displaced from the centre of the disk.

 

 

 

 

 

 

 

 

 

 

 

M-108 (NGC 3556):

 

Visual Brightness

10.0 (mag)

Apparent Dimension

8x1 (arc min)

 

This is a type Sc Spiral galaxy. This was discovered by Pierre Méchain in 1781.  As the discovery of M-108 had not been published, William Herschel independently rediscovered this object on April 17, 1789, and catalogued it as H V.46.

 

The nearly edge-on galaxy M108 appears to have no bulge and no pronounced core at all, it is just a detail-rich mottled disk with heavy obscuration along the major axis, with few H II regions and young star clusters exposed against the chaotic background. There's little evidence for a well-defined spiral pattern in this Sc galaxy, which is receding at 772 km/sec.

 

 

 

 

M-109 (NGC 3992):

 

Visual Brightness

9.8 (mag)

Apparent Dimension

7x4 (arc min)

This is a type SBc Spiral Galaxy and was discovered by Pierre Méchain in 1781. M109 is one of the "Theta"-like barred spirals, which appears as a "hazy spot" situated just 40' SE of the mag 2.44 star Gamma UMa (Phad, or Phecda).

 

 

 

 

 

 

 

 

Images

 

Cover Image: http://www.classicalmythology.org/images/ursa-major.gif

Map: http://www.seds.org/Maps/Pics/ursamajor.gif

M97: http://hea-www.harvard.edu/~swolk/pretty.html

M81: by Ray Gralak - http://www.gralak.com/Astro/ Astro.html

M82: http://www.seds.org/messier/ more/m082_hst.htm HST Image

M101: http://www.diana.dti.ne.jp/~show-g/ galaxies.htm

M108: http://www.astr.ua.edu/gifimages/ m108.html, Kitt Peak National Observatory

M109: by Alan Chen - www.astrovid.com/ starlight_xpress_mx7c_images_by_.htm

 

 

Contributors

 

Samuel George

Alexandra Yannacopoulou