For live links, click to: riverhouses.org/blog/2019-sirius 😊
We’re going to start a new thing today: a monthly great star. March is the perfect month to begin this new tradition, because just as it’s getting dark each evening at this time of year the brightest star in earth’s night sky, Sirius, appears high in the south. Brilliant white Sirius will be our star for the month.
Sirius is the brightest star — the “alpha” star — in the constellation Canis Major, the Big Dog that trails behind Orion the Hunter as he crosses the winter and early spring sky to the west. Sirius is commonly called “the Dog Star,” marking as it does the Big Dog’s head. Its formal designation is α Canis Majoris — “alpha of Canis Major.”
If you want to introduce your students to Sirius, you can start with some basic astronomy from our recommended homeschool star atlas (riverhouses.org/books):
“Sirius, the brightest star in the sky (magnitude –1.44) is the most conspicuous star in this constellation [Canis Major, the Big Dog]. It is easily located by following downward the line connecting the three stars in the Belt of Orion. Sirius is the nearest star to the sun visible to the naked eye in the northern latitudes, being but 9 light years distant. It has a white dwarf companion, a star so dense that one cubic inch weighs a ton.“ (Celestron Sky Maps, page 18)
The planisphere on the front of your star atlas will help you locate Sirius, and that’s plenty for beginning students — your little lesson is done. If you want to get more advanced, the Wikipedia page on Sirius is packed with additional information on everything from astrometrics to cultural history.
Sirius is indeed a double star system, but the smaller star of the pair is only visible in the largest telescopes. The name Sirius means burning, shining, or scorching in Greek (σείριος), either from its brightness or from the timing of its dawn rising along with the sun in mid-summer. (Today, we tend to make seasonal associations with the stars and constellations according to their evening visibility, but in ancient times, it was the dates on which particular stars rose just ahead of the sun at dawn that was commonly used for calendrical purposes.)
Sirius is roughly twice the diameter of our sun, and it is much younger — less than 250 million years old, in contrast to the sun’s 4.6 billion year age. Sirius was one of the first stars whose “proper motion” against the distant background was confirmed. The astronomer Edmond Halley (1656–1742), by comparing contemporary measurements of the position of Sirius with those of the ancient astronomer Claudius Ptolemy (ca. 100–170), determined that Sirius had moved about 30 arc minutes across the sky since Ptolemy’s day — a positional change in 1500 years equivalent to the diameter of the moon. The “fixed stars” were not so fixed after all.
Just yesterday, as I was walking back from the grocery store around sunset, I turned to the south and there was Sirius emerging in the twilight, the first star of the evening. 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 easy to locate on a dark night.
❡ 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. Why not find a dark-sky spot near you this month and spend some quality homeschool time beneath the starry vault. 🌠 🔭 🌌 🌟