April is the middle month of Leo Term in the River Houses, and as our monthly star calendar will tell you, April’s Great Star is Regulus, the brightest star in constellation Leo the Heavenly Lion. Its formal designation is α Leonis — “alpha of Leo.” Leo and Regulus are high in the southern sky in the early evening this month, passing over to the west as the night goes on.
If you want to introduce your students to Regulus and Leo you can start with some basic astronomy and astronomical mythology from your backyard star guide:
Leo is a large and easily recognized constellation that sits in a rather empty portion of the sky, just beyond Ursa Major and between its fellow zodiacal constellations Cancer and Virgo. A sickle-shaped asterism represents the lion’s head, mane, and chest. Shaped like a backward question mark, the period of this giant piece of celestial punctuation is marked by the brilliant star Regulus, while the stars trailing to the east mark Leo’s hindquarters. This is one of the easier constellations to construct mentally, as it resembles the classic image of the Sphinx.
The beacon-like, blue-white light of Regulus has long made it easy to see. Observation records of it date back to Babylonian tablets from around 2100 B.C. The sphinx, an icon of ancient Egyptian civilization, may have been modeled after this celestial beast. Better known in antiquity as Cor Leonis, the Lion’s Heart, its current name is actually taken from a Latin word meaning “little king,” and it is often known as the Royal star…. Regulus lies 79 light-years away, shining some 160 times brighter than our own sun and with a diameter five times larger. Binoculars and small telescopes show a very dim companion star. The fainter star’s real distance from Regulus is about 100 times the distance tiny Pluto orbits from our sun. (National Geographic Backyard Guide to the Night Sky, page 200)
That’s plenty for beginning students — your little lesson is done. If you want to get more advanced, the Wikipedia page on Regulus is packed with additional information on everything from astrometry to cultural history.
The Regulus we see with the naked eye is blue-white, but that Regulus is actually just the primary star (Regulus A) of a quadruple system that is arranged into two pairs: Regulus A and a suspected white dwarf that is detectable only spectroscopically, and Regulus B and C (a pair that is visible as a single star in binoculars and small telescopes). The B-C pair orbit each other, and together they orbit the primary star Regulus A and its own tiny companion.
The primary star in the Regulus system, Regulus A, is spinning so fast that it bulges out at the equator: it takes only 16 hours to complete one rotation, in contrast to about 25 days for the sun. The four-star Regulus system is estimated to be about one billion years old — quite old, but less than a quarter of the age of our own solar system. If you’d like to give Regulus an in-person inspection just bring your earth-based spaceship up to the speed of light and you’ll be able to get there in only 79 years. 🚀
Sometime this month, take your homeschool students out at dusk and introduce them to this great system of stars, and teach them its name, and so give them a new friend for life.
What stellar observations have you been making in your homeschool this Leo 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 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 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 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. 🌟