Meet Alkaid, the Big Dipper’s Handle

Alkaid's Vitals

Official name	Alkaid
Other designations	Eta Ursae Majoris, HIP 67301, HD 120315, HR 5191
Nicknames	-
Apparent magnitude	1.86
Distance from Earth	104 light-years
Type	B3 V
Color	Blue
Mass	6.1 solar masses
Radius	2.9 solar radii
Constellation	Ursa Major
Right ascension	13h 47m 32s
Declination	+49° 18’ 47”
Multiple system?	No
Variable star?	No
Exoplanets status	None known
Probable fate	White dwarf

Physical Characteristics

If you’re new to astronomy, you may have heard, “The Sun is just an ordinary/average star!” So when you walk out under a night sky and look up, you might imagine the brightest stars as solar duplicates seen from a distance.

It’s a pleasant thought, but inaccurate. Many of the brightest stars are actually much larger than the Sun. Stars like Betelgeuse, Deneb, Antares, Aldebaran, Canopus, Rigel, Polaris — the list goes on — are all gargantuan objects that make the Sun seem rather small in comparison.

What the phrase really means is that the Sun is a main sequence star — basically, a star that is in the “normal” process of fusing hydrogen to produce energy. Such stars do indeed make up 90% of the stellar population, so the Sun is “average” in this sense. But stars are further categorized by spectral type — letter codes that indicate a star’s temperature, size, and behavior.

It can be informative to compare other star types to the Sun. For example, the Sun is a G class main-sequence star — quite similar to Alpha Centauri A, which is only slightly larger. Procyon is an F-class star which starts to look slightly more yellow. Nearby, Sirius represents an A-type star, a bit hotter, a bit brighter, and white. And then there are B-type stars like Alkaid, the endpoint of the Big Dipper’s handle — and the star that we’re exploring here.

To be sure, Alkaid is different from our star: It far outweighs the Sun and has a far higher temperature. It’s also much bluer in color. It releases energy in the form of light and radiation at a greater rate. In terms of diameter, Alkaid is only about three times bigger than our star, much closer to the Sun in size than many famous bright stars.

Alkaid has the mass of about six Suns, so it’s not large enough to trigger an eventual supernova. Instead, Alkaid’s lifecycle will mimic the Sun’s — eventually becoming a red giant and then slowly fading into a white dwarf as its fuel dwindles. This fate will come sooner for Alkaid than for the Sun, because its higher energy output means that it will expend the available nuclear fuel more quickly.

Alkaid has high proper motion, which means that when careful telescopic measurements are taken, we can see the star moving in relation to more distant background stars. Because Alkaid is a hefty 100 light-years distant, this movement appears to be very slow, and it takes time to see any appreciable change, but Alkaid is definitely on the move. How fast? About 0.122 arcseconds per year. Remember that the full Moon is about 1,800 arcseconds (1/2 a degree) wide, so at its current rate, it would take Alkaid about 14,754 years to cross the Moon’s angular width.

Origin / Mythology

A bear…a wagon…a ladle…in various cultures and at various times, Ursa Major and the Big Dipper have been all of these things. In the U.S. and Canada, the seven stars known as the Big Dipper are generally seen as a colossal cosmic spoon — a ladle, or “dipper.” Cross the Atlantic however, and you find that many Europeans see the grouping as a wagon or cart of some sort.

How a culture imagines the Big Dipper/Ursa Major plays a role in how they perceive Alkaid. For the Mi'kmaq and Iroquois of eastern North America, Alkaid was one of three hunters pursuing the bear. The Greeks saw Alkaid as the tip of the bear’s long tail, which was stretched out unnaturally long after Jupiter tossed the bear into the sky. Chinese astronomers referred to Alkaid as a “revolving light.”

Ancient Arabic astronomers also knew about the bear concept; however, they saw the Big Dipper itself as a funeral cart. In this case, Alkaid represented one of three daughters, or “mourning maidens,” who were grieving the loss of their father, laid to rest in the cart. Alkaid means “leader” in Arabic, so that daughter might have overseen the procession.

How to See Alkaid 

The Big Dipper asterism is in the northern part of the sky, and stars in that area behave a bit uniquely. Rather than rising in the east and making a long arc across the sky to the west (the way the Sun does), northern stars rotate throughout the night in a circle centered on Polaris and don’t set at all, a type of movement called circumpolar. For much of North America, the Big Dipper stars fall into this category. This means that at mid-northern latitudes, you can locate Alkaid no matter what time of night it is and no matter what season. (Since it’s on the end of the Dipper’s handle, it may briefly duck below the horizon for some locations at certain times of the year.)

Finding Alkaid is quite easy. Look to the north in the evening to find the Big Dipper — high in the sky and tipped upside down in the spring, low and upright in the fall. Alkaid is the last star in the Dipper’s handle, away from the bowl. Or is it the bear’s tail? Or a mourning daughter? Or is it perhaps a large blue sun, waving back to us across the light-years?

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Daniel Johnson is a Wisconsin-based freelance writer and professional photographer and the co-author of over a dozen books. He’s a longtime amateur astronomer and fortunate enough to live in a rural region with excellent seeing conditions. You can view some of Dan’s photography (he does a lot of animals!) at www.foxhillphoto.com