Click to: riverhouses.org/2020-alpheratz
October is the middle month of Cygnus Term in the River Houses, and as our monthly star calendar will tell you, October’s Great Star is Alpheratz, the brightest star in the constellation Andromeda the Chained Maiden. Its formal designation is α Andromedae — “alpha of Andromeda.” Andromeda and Alpheratz are high in the east at sunset now, passing directly overhead around midnight and then off to the west as the night goes on.
If you want to introduce your students to Alpheratz and Andromeda you can start with some basic astronomy and astronomical mythology from your backyard star guide (riverhouses.org/books):
“In ancient Greek myth, Princess Andromeda of Ethiopia was chained to a rock as a sacrifice to the gods. In the sky, Andromeda is a faint V-shaped constellation that is most often seen upside down. To locate her on fall evenings, trace a line northeast from [the 2.1-magnitude star Alpheratz,] the northeast corner of the Great Square of Pegasus….
“Andromeda’s mother, Cassiopeia, claimed that her daughter’s beauty surpassed that of the daughters of Nereus, a god of the sea and father-in-law of Poseidon. Angered by her boasting, Poseidon sent the monster Cetus to destroy the kingdom of Ethiopia unless Cassiopeia and her husband Cepheus sacrificed their daughter. The princess was rescued by Perseus, who used the head of Medusa to turn Cetus to stone. The whole cast of the tale — her father Cepheus, Cassiopeia, Cetus, Perseus, and the hero’s winged horse Pegasus — are all nearby in the sky.“ (Backyard Guide to the Night Sky, pages 248–249)
That’s plenty for beginning students — your little lesson is done. If you want to get more advanced, the Wikipedia page on Alpheratz is packed with additional information on everything from astrometry to cultural history.
The blue-white star we see as Alpheratz is actually a close double star — two suns orbiting each other — but they are so close that they cannot be resolved even with a sizable telescope. The double nature of Alpheratz was first detected by close examination of the way the star’s light spectrum changes from week to week. These spectroscopic measurements suggest the two Alpheratz companions circle each other once every 97 days. (Compare this to the earth’s orbit around the sun, which takes 365 days.)
The Alpheratz system is fairly close to us in astronomical terms: a little less than 100 light-years away. (The photons of light you see from it tonight were emitted in the early twentieth century and have been traveling toward us for nearly a hundred years until the moment they strike your eye.) The larger of the two Alpheratz companion stars is almost three times the diameter of the sun, and the smaller is about one and a half times the sun’s diameter. The Alpheratz system is quite young: only about 60–70 million years old, in contrast to more than 4.5 billion for our old yellow sun.
Although Alpheratz is officially part of Andromeda, it lies on the border of the adjacent constellation Pegasus and can be most easily recognized as the top left star (when you’re facing south) of the asterism called the Great Square of Pegasus. Its name means “the navel [of the horse]” in Arabic, reflecting the star’s position in adjacent Pegasus.
Sometime this month, take your homeschool students out at dusk and introduce them to this pair of orbiting stars, and teach them its name, and so give them a new friend for life.
❡ 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 name of the constellation in its Latin 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. 🌃
❡ And all dishevelled wandering stars: How far away are the stars? Do they all occupy a single celestial “dome” that rotates through the heavens (as some ancient and medieval astronomers believed), or are they scattered through space at different individual distances? Astronomers had long suspected that the fixed stars existed at different distances from us, but early attempts to measure those distances failed. It was not until the early 1800s that instruments and measuring techniques became precise enough to allow the first stellar distances to be calculated using the technique of parallax. Parallax is the displacement in the apparent position of an object with respect to the background when an observer moves from side to side. It’s an ordinary phenomenon you experience every day — it’s how we judge distances as we move through the landscape. Stellar parallaxes are extremely small — fractions of an arc-second (one 3600th of a degree) — and they are calculated by measuring a star’s position against the background at opposite sides of the earth’s orbit, six months apart. (That’s the astronomical equivalent of taking one step to the side.) Vega, our August star, was one of the first stars to have its parallax measured; modern estimates put it at about 0.13 arc-seconds. Apply some trigonometry, and that yields a distance of about 25 light-years. 🔭
❡ 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. 🌟
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