The constellation Ursa Major is not only the third biggest of all the 88 recognized constellations, taking up an area of 1279.66 square degrees, or 3.10% of the night sky, but it is also one of the oldest recorded, with a history that dates back to pre-historic times. This means that it was already old and well-known to most ancient cultures by the time Ptolemy included it in his astronomical treatise Almagest in the early 2nd century AD, which he listed under the Greek name “Arktos Megale”, whose subsequent Latin name Ursa Major means the “greater she-bear”.
For observers in the northern hemisphere, Ursa Major is a fixed, north circumpolar constellation that is easily found by virtue of its two brightest stars which form the Big Dipper asterism, two stars of which always point directly to Polaris, the North Star. However, the asterism forms just the tail and hindquarters part of the ‘Great Bear’, with the constellation as a whole actually containing a total of eighteen stars which host twenty-four planets between them, as well as seven Messier objects. Below are some details of a few of the principal deep sky objects in Ursa Major:
Bode’s Galaxy (M81, NGC 3031)
• Coordinates: RA 09h 55m 33.2s |Dec. +69° 3′ 55″
• Magnitude: +6.94
• Distance: 11.74 million light years
• Galactocentric velocity: 73 ± 6 km/sec
• Apparent Dimensions: 90,000 light years wide
Being relatively close to Earth, M81 is among the most studied spiral galaxies by professional astronomers due both to the 70-million solar mass black hole situated at its core, and the fact that M81 and its neighbors, M82 and NGC 3077, are involved in a gravitational tug of war that is stripping dust and gas from all three galaxies. This material is continually being “recycled” between all three galaxies in a process that causes a high rate of star formation in both M82 and NGC 3077.
Look for M81 about 10 degrees to the north-westward of the star Dubhe (Alpha Ursae Majoris), where several other members of the M81 Group can also be seen. While M81 itself is an easy target for both binoculars and modest amateur telescopes, instruments of 8-inch aperture and larger are required to discern any structure within the galaxy. Note that M81 is only visible for southern hemisphere observers that are immediately south of the equator.
Cigar Galaxy (M82, NGC 3034)
• Magnitude: +8.41
• Distance to Earth: 11.4-12.4 million light years
• Galactocentric velocity: Undetermined
• Apparent Dimensions: 40,000 light-years wide
Located about 12 million light years away, this edge-on (hence its apparent shape) spiral galaxy is a close neighbor of M81, and the prototypical example of I0 galaxies, which are a chaotic in form type of Irregular Galaxy. In terms of its luminosity, this starburst galaxy is more than five times brighter than the Milky Way, and its core is at least one hundred times brighter than the core of our own galaxy. In 2014, investigations into M82 led to the discovery of the pulsar now designated M82 X-2, which with a luminosity of about ten million Suns, is the most luminous pulsar yet discovered anywhere in the Universe.
M82 is an easy target for small to medium amateur telescopes. Look for the galaxy close to M81, about 10 degrees to the north-westward of the star Alpha Ursae Majoris. When viewed from Earth, the cores of M81 and M82 appear to be separated by about 130,000 light years, but the actual separation is 300,000 light years, give or take about 200-300 light years.
Pinwheel Galaxy (M101, NGC 5457)
• Magnitude: +7.86
• Distance to Earth: 20.9 ± 1.8 million light years
• Galactocentric velocity: Undetermined
• Apparent Dimensions: 170,000 light-years wide
First discovered by French astronomer Pierre Méchain in 1781, M101 is perhaps the most iconic of all known grand design spiral galaxies. With a diameter of 170,000 light years, M101 is slightly bigger than the Milky Way, and contains at least 100 billion disc stars, and a further estimated 3 billion stars in its central bulge.
However, M101 is better known for its many H II regions, most of which are unusually big and bright, where large numbers of stars are forming. In fact, a study during 1990 showed that M101 has a total of 1,264 star forming regions, three of which are so bright and prominent that they have their own NGC designations included in the New General Catalogue of Nebulae and Clusters of Stars (NGC), i.e., NGC 5461, NGC 5462, and NGC 5471.
Two further distinguishing features of M101 are that it has 5 companion galaxies (NGC 5204, NGC 5474, NGC 5477, NGC 5585) whose combined gravitational effects are distorting M101, and that it contains a suspected low-mass binary black hole, designated M101 ULX-1. This suspected black hole may only weigh in at about 20-30 or so solar masses, but the high-energy X-rays emitted by the system shows that it is consuming material at a rate that is several times higher than what standard models predict, or allow for.
Although M101 is relatively bright at magnitude +7.86, it has a low surface brightness, which means that it is not an easy target for small telescopes. In fact, seeing this galaxy at all requires exceptionally dark skies, and telescopes with at least 8 inches of clear aperture. Use low-power eye pieces for the best views.
The Owl Nebula (M97)
• Magnitude: +9.9
• Apparent dimensions: 3 light-years wide
Discovered by Pierre Méchain in 1781, this planetary nebula derives its name after a sketch made of it in 1848 by the Earl of Rosse who exaggerated it to resemble the face of an owl. Located about 2,000 light years away, this nebula is about 8,000 years old, and was molded by the solar wind of its progenitor star as it evolved along the asymptotic branch of the Hertzsprung-Russell diagram.
Look for the Owl Nebula about 2.5 degrees to the south-eastward of the star Merak (Beta Ursae Majoris) at the south-western corner of the Big Dipper’s bowl. A line drawn from Merak through the star Dubhe (Alpha Ursae Majoris) at the north-western corner of the bowl, points directly to Polaris. However, while the nebula is visible as a faint patch of light in large binoculars and small telescopes, larger instrument of 8-inch aperture will show the “eyes” of the owl if seeing conditions are exceptionally good, and the sky is completely dark.
Other noteworthy objects in Ursa Major
Of the deep-sky objects in Ursa Major, the three galaxies and one nebula listed above are the most easily found and viewed. However, Ursa Major contains a number of other galaxies, most notably the barred spiral galaxies M108 and M109, and a few double stars although these objects may be difficult to spot from urban areas where light pollution is a limiting factor.