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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.
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Artemis gathered about her a number of young nymphs. Reflecting her own
vows on chastity, she also required complete fidelity from these young women
(girls actually, who would grow into womanhood). One of these was Callisto.
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.
The constellation itself doesn't have a particularly memorable shape to it;
few of us bother to discover the form of a bear in the heavens. Rather it
is the asterism known as the Big Dipper (or Plough if you live in the UK) that
is most noticeable.
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The stars that make up the Big Dipper are seven in number, and follow the
Greek alphabet, making them easy to remember. Thus, apart from alpha Ursae
Majoris, the Dipper's bowl is made up of beta, gamma, and
delta, then epsilon, zeta, and eta finish the
asterism.
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.
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This star, alpha UMa (Dubhe: the Bear), is a yellow giant, about
25 times the size of the Sun, and 86 light years away. It is a close
visual binary, discussed below.
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.
Other stars in Ursa Major.
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Omicron Ursae Majoris is called Muscida, meaning "muzzle", and
indeed this star marks the bear's nose. Muscida has a very faint
(15m) companion, a dwarf star at PA 191 degrees and separation of 7".
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).
Double stars in Ursa Major:
Several have just been mentioned. But there are other binaries in Ursa
Major worthy of investigating.
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Dubhe (Alpha Ursae Majoris) is a well-known binary, with a close 4.8m companion which orbits every 44.66 years. In 2000 the values are: 1.9, 4.8; PA 214º, separation 0.6".
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").
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Mizar was the first binary system to be discovered (in 1650), and is
usually the first binary to be found and studied by amateur astronomers. No matter how long you study the stars, coming back to Mizar is always a treat.
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.
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Although at a great distance from Mizar (perhaps three light years away), Alcor (80 UMa) may be gravitationally bound to this star as it shares the same proper motion. However, most authorities believe the stars only form an optical binary.
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).
Variable stars in Ursa Major:
Ursa Major has no notable variables, but there are a number which might be of
some interest.
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Epsilon Ursae Majoris is an alpha-CVn type variable: 1.76-1.78 every five
days and two hours.
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Alpha-Canum Venaticorum type stars are rotating variables which typically
evince very little change in visual magnitude. These stars are generally
A-type (that is, they have a spectrum range of B9-A5) but curiously enough
they show an unusual abundance of a number 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 Ursae Majoris shows a strong chromium line.
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R Ursae Majoris is a Mira-type variable with period of 301.62 days, and a magnitude change from 6.5 to 13.7. Curiously, it is actually a brighter red when at its minimum; at maximum it loses much of its colour. The 2000 maximum was expected in the latter half of March.
Deep Sky Objects in Ursa Major:
Ursa Major has five Messier objects: M40, M81, M82,
M97, and M101.
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M40 is the Messier object that really isn't one. In 1764 Messier
went looking for an object that had been catalogued as a nebulosity in this
area. What he found was two ninth-magnitude binary stars, very close together,
which he assumed had been mistakenly catalogued as the nebulosity. However
instead of leaving the matter there, he proceeded to catalogue the stars as
his No. 40.
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A hundred years later the stars were catalogued by Winnecke as binaries
called "Winnecke 4"; they still go by this name. The binary (9.9, 9.3;
PA 83 degrees, and separation 49") is found one and a half degrees north-east
of delta UMa. The easiest way to find the binary is to draw a line
from delta to 70 UMa, then another half a degree beyond this point.
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M81 (NGC 3031) is a superb spiral galaxy, and with M82 in
the same field, half a degree to the north, forms a splendid pair.
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The distance is approximately seven to nine million light years and, as
Burnham reports, the galaxy is considered one of the most dense galaxies
known, with a total mass of 250 billions suns. A large scope is needed
to catch the fine detail in the spiral's arms.
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M82 (NGC 3034) floats above M81 like an ethereal UFO; any minute
you think it's going to zip away in the night sky.
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The galaxy isn't, as one might think, a spiral on edge, but is usually
described as spindle shaped. The galaxy is rather mysterious; it's
thought that an explosion at its centre one and a half million years
ago created the odd shape, which is still expanding at a rate of 950 km/second.
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M97 (NGC 3587) often called the "Owl Nebula" for its two dark central
areas (revealed only in the largest telescopes) resemble an owl's eyes.
The nebula is formed by the still expanding shell of its central star,
which is very small and compact, with a surface temperature as much as
85,000 kelvin.
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.
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Located five and a half degrees east of zeta UMa, the usual method
given to find M101 is to star hop. From zeta UMa (Mizar) proceed
to Alcor, then over and slightly north to 81 UMa, and now down to the
southeast to 83, then 84. Now locate 86 UMa, to the southeast. This star
forms the bottom point of a wide-v shape with 84 and M101.
Some stars of note:
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.
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Groombridge 1830 is famous for having one of the largest proper motions,
(7.050 arc seconds) third after Barnard's Star and Kapteyn's Star. In only
511 years it shifts its position by one degree. It's 28.8 light years away,
and has a space velocity of 312 km/s. Its Epoch 2000 values are: right
ascension 11h, 52m, 58.7s; declination +37 degrees, 43', 07".
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Called a subdwarf, the star has about half the diameter of the Sun and
only one-fifth of its luminosity; yet because it is so close, its visual
magnitude is a fairly bright 6.45.
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.
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Lalande 21185 is a red dwarf of very small mass (about one third
of the Sun's). Quite close at 8.2 light years, its proper motion is also
very fast, at 4.777 arc seconds, and it has a space velocity of 187 km/s.
Its Epoch 2000 values are: right ascension 11h, 03m, 20.2s; declination
+35 degrees, 58', 13".
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Lalande 21185 has a visual magnitude of only 7.49, and an absolute
magnitude of 10.49. Finding the star could be an adventure; Burnham's
finder's chart (on his page 1981) is useful.
First start from xi UMa. Star-hop to nu UMa, just to the
north two degrees, then find the brightest star lying to the west (about
six degrees). This is 46 Leonis Minoris; the fainter star just to the east
is in Ursa Major, but is called 47 Leonis Minoris. Now move to the north
one and three-quarter degrees. This is a rather bleak part of the sky. Two
degrees to the east you're in the field; now you have to rely on the finder's
chart.
On 17 January, 1996, Geoffrey Marcy and Paul Butler announced they
had found two new Jupiter-sized planets circling visible stars: 70
Virginis and 47 Ursae Majoris.
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".
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It took eight years of observation from the Lick Observatory, outside
San Jose, California, to verify the first planet orbiting 47 Ursae Majoris. It has a period of 1089 days, a mass of 2.54 Jupiters, an orbital radius approximately twice that of the Earth's, and according to the official press release it "probably has a region in its atmosphere where the temperature would allow liquid water."
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.
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