For live links, click to: riverhouses.org/2019-arcturus
July is the middle month of Hercules Term in the River Houses, and as our monthly star calendar will tell you, July’s great star is Arcturus, the brightest star in the constellation Boötes the Herdsman. Its formal designation is α Boötis — “alpha of Boötes.” Boötes and Arcturus are already high overhead in the early evening this month, passing off to the west as the night goes on. With Regulus (our April star) and Spica (our June star), Arcturus forms “the Spring Triangle” (even though we’re now into summer).
If you want to introduce your students to Arcturus, you can start with some basic astronomy from your homeschool star atlas (riverhouses.org/books):
“Boötes is an easily recognized constellation resembling a huge kite. Arcturus, its brightest star, has a magnitude of –0.1 and is the 4th brightest star visible from Northern latitudes. Arcturus can be located by following backwards the curve of the Big Dipper handle; the curved line connecting these stars extended about 30º will lead your eye to Arcturus. It is a giant star about 80 times as bright as the sun and is 37 light years away.“ (Celestron Sky Maps, page 8)
The planisphere on the front of your star atlas will also help you locate Arcturus, and that’s plenty for beginning students — your little lesson is done. If you want to get more advanced, the Wikipedia page on Arcturus is packed with additional information on everything from astrometry to cultural history.
Arcturus is an ancient orange-yellow giant about seven billion years old. It has roughly the same mass as our sun, but it has expanded to enormous size as its supply of hydrogen has become depleted. It’s also relatively close to us — current estimates of its distance put it at about 37 light years. It’s generally thought to be a single star, without any companions.
The name Arcturus comes from the Greek and means “watcher of the bear” or “guardian of the bear” — a reference to its association with the constellation Ursa Major (the Great Bear, including the Big Dipper), which Boötes and Arcturus follow around the pole. Arcturus and the other stars of Boötes were well known to the Greeks, and as with any ancient celestial configuration they are surrounded by many different and sometimes contradictory mythological accounts of their origins.
This month, take your homeschool students out at dusk and introduce them to this great nearby 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 be the top star 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 speaking, its apparent magnitude. The scale of star magnitudes was developed centuries ago, long before modern measuring instruments were invented, so it can be a little bit confusing for beginners. Originally, the brightest stars 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 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, 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 has imaged distant stars and galaxies below magnitude 30 — these are the dimmest objects 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. The planisphere on the front of your River Houses star atlas (riverhouses.org/books) will let you dial up the northern hemisphere sky for any night of the year, and the descriptions and maps of each constellation will point out the highlights, including the names and characteristics of the brightest stars. Your general 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 monthly “great star” 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. 🌟