Click to: riverhouses.org/2020-spica
June is the first month of Hercules Term in the River Houses, and as the River Houses Star Calendar will tell you, June’s Great Star is Spica, the brightest star in the constellation Virgo the Virgin. Its formal designation is α Virginis — “alpha of Virgo.” Spica (in Latin) is the ear of grain — the spike — in the left hand of the harvest maiden Proserpine (Persephone in Greek).
If you want to introduce your students to Spica and Virgo you can start with some basic astronomy and astronomical mythology from your backyard star guide (riverhouses.org/books):
“Virgo is another of the zodiacal constellations that straddles the ecliptic band and the only one representing a woman. It was first cataloged by the Greek astronomer Ptolemy in the second century. The magnitude-1 star Spica can be located by star hopping. From the end of the handle of the Big Dipper, move south in the “arc to Arcturus” [our July star] in the constellation Boötes, then “speed on to Spica” directly beneath it.
“Located some 263 light-years from Earth, brilliant blue-white Spica is the fifth brightest star in the entire heavens, a blue giant about 14 times the mass of our own sun and 2,000 times more luminous. While it may look like a single star, it is in fact a double. Both are hot blue giants that orbit each other only 11 million miles (18 million km) apart.“ (Backyard Guide to the Night Sky, page 202)
That’s plenty for beginning students — your little lesson is done. If you want to get more advanced, the Wikipedia page on Spica is packed with additional information on everything from astrometry to cultural history.
Blue-white Spica and its companion star are so close to each other — closer than the sun and Mercury — that they are deformed by gravity and are more egg-shaped than spherical. The primary star is about seven times the diameter of the sun; the companion is about four times the diameter of the sun. Spectroscopic analysis shows that they orbit each other with a period of about four days. The Spica system is also quite young: about 12.5 million years old.
As noted above, Spica is very close to the ecliptic, the path followed by the moon, sun, and planets as they cross the earth’s sky, and so on rare occasions it is briefly covered by them — an event called an occultation. The last occultation of Spica by Venus occurred in the year 1783, and the next will occur in 2197.
Measurements of Spica’s position are believed to have led the ancient astronomer Hipparchus to discover the precession of the equinoxes.
Sometime 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 will you be making in your homeschool this Hercules Term? 😊
❡ 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. 🔭
❡ 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. 🌟