For live links, click to: riverhouses.org/2019-aldebaran
December is the first month of Orion Term in the River Houses, and as our monthly star calendar will tell you, December’s Great Star is Aldebaran, the brightest star in the constellation Taurus the Bull. Its formal designation is α Tauri — “alpha of Taurus.” Aldebaran is easily recognized in the small V-shaped cluster of stars called the Hyades, which outlines the face of the Bull. Taurus and Aldebaran are high in the east in the early evening now — washed out a bit by the bright moon this week, but as the month proceeds the moon will dim and slide out of the way and they’ll come into clearer view.
If you want to introduce your students to Aldebaran and Taurus, you can start with some basic astronomy and astronomical mythology from your backyard star guide (riverhouses.org/books):
“Taurus is a constellation of the zodiac at its northern peak in winter. It is easy to locate because Orion’s belt points right toward Aldebaran, the red eye of the bull. Farther to the northeast, the 444-light-years-distant Pleiades star cluster marks the animal’s shoulder. Also known as the Seven Sisters, it is one of the more easily identified deep-sky objects. A line traced from Betelgeuse (in Orion) [our February star] through Aldebaran will bring the dazzling group into view….
“Numerous myths accompany this constellation. The ancient Egyptians associate the Bull with Osiris, god of life and fertility, while in Greek mythology the Bull is one of Zeus’s disguises, used to capture Europa and bring her across the sea to the continent that bears her name. To some, Taurus represents the golden calf formed by the idling followers of Moses while he received the Ten Commandments.“ (Backyard Guide to the Night Sky, pages 266–267)
That’s plenty for beginning students — your little lesson is done. If you want to get more advanced, the Wikipedia page on Aldebaran is packed with additional information on everything from astrometry to cultural history.
Aldebaran is a classic red giant, more than 40 times the diameter of our sun and only about 65 light-years distant — quite close in astronomical terms. It is somewhat older than the sun: a little more than six billion years old against the sun’s five billion.
Because Aldebaran is so prominent and colorful in the northern hemisphere night sky, it has attracted the attention of careful observers from earliest times. Detailed studies of Aldebaran by Edmund Halley (of comet fame) helped to establish that the so-called fixed stars are in fact not fixed but rather exhibit “proper motion” — individual motion through space along different paths. (The term proper motion often confuses beginners. It doesn’t mean “correct” motion — the modern sense of the word “proper.” It means personal, individual motion — motion that is a property of the object itself.) From our point of view here on earth, Aldebaran has shifted its position against the distant background by about seven minutes of arc over the past 2000 years. The diameter of the moon is about thirty minutes of arc (half a degree), so over the past two millennia Aldebaran has moved by about a quarter of the diameter of the full moon.
The name Aldebaran means “the follower” in Arabic. (Astronomy and mathematics are the two fields that have borrowed the greatest number of words from Arabic.) The name was initially applied to the whole Hyades cluster and it refers to the way these stars trail along behind the Pleiades as they cross the winter sky.
Sometime this month, take your homeschool students out at dusk and introduce them to this giant ancient sun, and teach them its name, and so give them a new friend for life.
What stellar observations have you made in your homeschool lately? 😊
❡ Alpha and beta and gamma, oh my: Most of the principal stars within each constellation have both old vernacular names — Vega, Sirius, Arcturus, and so on — as well as more formal scientific designations. The German astronomer Johann Bayer (1572–1625) devised the formal system of star designations that is still in common use today. In Bayer’s system, the stars in each constellation, from brightest to dimmest, are assigned a lowercase letter of the Greek alphabet: α (alpha, brightest), β (beta, second brightest), γ (gamma, third brightest), δ (delta, fourth brightest), and so on. This letter designation is combined with the Latin name of the constellation in its possessive (genitive) form: Lyra becomes Lyrae (“of Lyra”), Canis Major becomes Canis Majoris (“of Canis Major”), and so on. The brightest star in the constellation Lyra (the star Vega) thus becomes α Lyrae (“alpha of Lyra”), the brightest star in Canis Major (the star Sirius) becomes α Canis Majoris (“alpha of Canis Major”), and so on, through all 24 Greek letters and all 88 constellations. How bright would you expect, say, the σ (sigma) star of Orion to be? Not very bright — it’s far down the alphabet — but σ Orionis happens to mark the top of Orion’s sword, so even though it’s not very bright it’s still notable and easy to locate on a dark night. ✨
❡ Star bright: The brightness of a star as we see it in our night sky is its magnitude — or more properly, its apparent magnitude. The scale of star magnitudes was developed long before modern measuring instruments were invented, so it can be a little bit confusing for beginners. Originally, the brightest stars in the sky were called “first magnitude” and the less-bright stars “second magnitude,” “third magnitude,” and so on, down to the dimmest stars visible to the naked eye, which were called “sixth magnitude.” In the nineteenth century the star Vega (our August star) was chosen as the standard brightness reference and its value on the magnitude scale was defined to be zero (0.0). Five steps in magnitude (from 0.0 to 5.0 or from 1.0 to 6.0) was defined to be a change in brightness of 100 times: a star 100 times dimmer than Vega (0.0) was defined to be a magnitude 5.0 star. Vega is not quite the brightest star is the sky, however, so the scale also had to be extended into negative numbers: Sirius (our March star), for example, is magnitude –1.5, about three times brighter than Vega (at 0.0). The planet Venus at its brightest is about magnitude –4.2; the full moon is about magnitude –12.9; the sun is magnitude –26.7. By contrast, the dimmest stars visible to the naked eye in a populated, light-polluted area are about magnitude 3.0; the dimmest stars visible under very dark conditions are about magnitude 6.5. The Hubble Space Telescope in orbit around the earth has photographed distant stars and galaxies below magnitude 30, the dimmest celestial objects humans have seen so far. 🔭
❡ Watchers of the skies: Teaching your students to recognize the constellations is one of the simplest and most enduring gifts you can give them. We recommend the handy Backyard Guide to the Night Sky (riverhouses.org/books) as a general family reference — it will help you identify all the northern hemisphere constellations and will point out many highlights, including the names and characteristics of the brightest stars. Your recommended world atlas also has beautiful maps of the whole northern and southern hemisphere night skies on plates 121–122. Why not find a dark-sky spot near you this month and spend some quality homeschool time beneath the starry vault. 🌌
❡ First star I see tonight: This is one of our regular Homeschool Astronomy posts featuring twelve of the most notable stars of the northern hemisphere night sky. Download and print your own copy of our River Houses Star Calendar (riverhouses.org/calendars) and follow along with us as we visit a different Great Star each month — and make each one of them a homeschool friend for life. 🌟