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Ursa Major is a large sprawling constellation, the third largest in fact. It's mainly known as the home of the Big Dipper (UK: the Plough), certainly the best known asterism in all the heavens. The constellation offers a number of objects, some well known, others rather obscure, and one star that has recently been in the news as having at least one "temperate" planet circling about it.
The stars are fairly bright, and widely
dispersed. The Big Dipper/Plough covers only half of the breadth, and the
constellation extends much farther south, with its most southerly star, xi
Ursae Majoris (Alula Australis), as far south as Leo and Cancer.
The name "The Great Bear" seems to have been assigned to the constellation
in antiquity, due to its northern latitudes. Only a prodigious bear could
live in such a northerly clime. Interestingly, a number of North American
tribes (Algonquin, Iroquois, Illinois, and Narragansett, among possibly others)
also associated the constellation with a gigantic bear.
In Greek mythology Callisto, daughter of King Lycaon, was chosen as a young
child to be one of Artemis's companions. Now Artemis was Apollo's sister,
patroness of childbirth and protector of babies and of suckling animals. The
one thing she prized above all was her chastity; she even asked Zeus for
eternal virginity, which he granted.
Zeus had the habit of seducing young maidens, and eventually he got around
to Callisto. When Artemis discovered that Callisto was pregnant, she took
her revenge. Artemis loved to hunt; she would take her revenge in the
chase. So she changed Callisto into a bear. (Remember that Artemis is
the same goddess that caught Actaeon watching her bath. She turned him
into a stag then set his own hounds on him. They ripped him to pieces.)
So Artemis's plan was to have Callisto, as a bear, hunted down and killed.
But Zeus took pity, and sent Callisto to the heavens, keeping the same
form of a bear. Her son Arcas would grow up to be the ancestral founder
of the Arcadians, before he too joined his mother in the heavens as
Ursa Minor.
This particular asterism has also a long history, seen in many cultures
as a chariot or wagon. (Burnham, as one would expect, has a thorough
discussion on this aspect of the constellation.)
The seven are not moving in the same direction, and over time the asterism
will dissolve. In fact, it is only the last 50,000 years or so that a
discernible "dipper" has formed. As the stars move their separate ways, the
shape will more and more become plough-like, with the pointer star
(alpha Ursae Majoris) moving in front of the rest, and somewhat
south of its present position.
Beta Ursae Majoris is named Merak, or "loin"; gamma is
Phecda: thigh, and delta is called Megrez: root (or base) of the
tail. These three are similar stars, all white (A-type) stars, and all
within 100 light years distance.
When we go out onto the tail, we first encounter epsilon UMa, an
alpha-CV type variable (see below), and another A-type white star. Called
Alioth (which no one has adequately translated) the star is one of the
brightest in the constellation, although one of the more distant stars
(if we go by its parallax). The star tables show a distance of only 64
light years. This distance is disputed and may be too small; the parallax
indicates a distance of 360 light years.
Then comes zeta Ursae Majoris (Mizar: girdle or apron). This star forms a fine binary (perhaps optical?) with Alcor (80 UMa) (see below). The star is 78 light years away according to its parallax.
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.
Nearby, marking the bear's ear, is a small group of stars made up of
sigma1, sigma2, and rho. Our
constellation map marks the spot simply with a lower-case sigma. The
visual binary sigma2 is discussed below.
Xi Ursae Majoris is far to the south, marking one of the bear's
feet. This star is not only an interesting binary, but also an historic
one, as it was the first binary to have its orbit calculated (in 1828).
[Note: See the Binocular Section link, below, for updated figures.]
Several have just been mentioned. But there are other binaries in Ursa
Major worthy of investigating.
Phi UMa is even closer these days [PA
243º, separation 0.23"] but the distance is gradually widening.
It has a period of 105.5 years. The two stars are similar in magnitude: 5.3,
5.4, resulting in a combined magnitude of 4.6.
Sigma2 UMa is a much easier binary to resolve; presently the
separation is 3.8" at PA 355º. The companion, a rather dim 8.2 visual magnitude,
describes a leisurely 1067 year orbit. As with
many slowly orbiting binaries, this one has had a variety of calculated
periods, although Burnham's "best modern computation [of] 706 years" is
now considered out of date.
Xi UMa is an attractive binary [4.3, 4.8] with a fast
orbit. This star shouldn't cause too many problems
to resolve; its closest point came in 1993 and it too is widening, presently
sitting at PA 302º and separation 1.3". The star was designated an
RS CVn type variable in 1993.
Zeta UMa, Mizar, is the best of the bunch and probably the easiest to find as well. A multiply system with Alcor, AB form a fixed binary at PA 152º, separation 14.4". Alcor (component C) is a distant 12 minutes east (709").
Both A and B are also spectroscopic binaries (that is, each one has a
companion too faint for observation but which shows up when studied
spectroscopically). The presence of such a companion is deduced from
changes in the doppler shift in the spectral lines of the primary.
This is a 3.99 visual magnitude star, 81 light years away. Alcor serves as a good jumping off point to study M101, a spectacular face-on spiral galaxy (see below).
Ursa Major has no notable variables, but there are a number which might be of
some interest.
These stars are divided into three groups: those with predominantly silicon
spectral lines, those with manganese, and those with chromium-strontium lines.
Epsilon Ursae Majoris shows a strong chromium line.
Ursa Major has five Messier objects: M40, M81, M82,
M97, and M101.
M101 (NGC 5457) is a vast galaxy, one of the largest known, with
open spirals. Although seen face on, it's fairly dim; it takes a large
scope and an exceptionally good night to see this nebula at its best.
Groombridge 1830 and Lalande 21185, both of which require a
finder's chart, and 47 Ursae Majoris, which has recently been found
to have a planet which could theoretically support water.
To locate Groombridge 1830 first draw a line between phi Ursae
Majoris and xi Ursae Majoris. Move up this line to its midpoint
then look to the east roughly at the same distance. You are now in the
vicinity. There are several 6m stars in the region, but only one just north
of a small but bright galaxy (NGC 3941). This is Groombridge 1830.
In the late summer of 2001 a second planet was announced as being associated with 47 UMa.
These planets go under the labels "47 UMa b" and "47 UMa c".
47 UMa "c" has a mass of 0.76 Jupiters and an orbit of 2594 days. These two planets orbit 47 UMa at a comparable distance between Mars and Jupiter in our solar system, with the newly discovered "c" planet being the more distant of the two. There may be more, smaller, planets in the system.
The planets' "sun", 47 UMa, is remarkably similar to ours: it's spectral type is G0V (the Sun's is G2V), it has a surface temperature of 5882 kelvin (the Sun's is 5780), and an absolute magnitude of 4.40 (Sun: 4.79).
You can arrive at 47 Ursae Majoris from a number of directions.
Perhaps the easiest is to start at phi UMa and drop down in a
south-south-west direction five degrees. You'll find three bright stars
in a crescent. The most northerly star of this trio is 47 UMa
Alternatively, locate the triangle formed by phi UMa, mu UMa,
and 46 LMi. At the centre of this triangle, move slightly to the east,
where you will find this trio of stars.
For a little more on Ursa Major visit the Binocular Section.
Or go to
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