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You are here: Home > Homeschool Astronomy > Monthly Great Stars > This Post

๐ŸŒŸ STAR BRIGHT: Deneb for September and the New Homeschool Year

10 September 2022 by Bob O'Hara

(This is the first of our monthly “Star Bright” posts for the 2022โ€“2023 homeschool year. Print your own River Houses Star Calendar to follow along with us, and add your name to our free weekly mailing list to get great homeschool teaching ideas delivered right to your mailbox all through the year.)

September is the first month of Cygnus Term in the River Houses, and as our monthly star calendar will tell you, September’s Great Star is Deneb, the brightest star in the constellation Cygnus the Swan (naturally enough). Its formal designation is ฮฑย Cygniย โ€” “alpha of Cygnus.” Cygnus and Deneb are high in the east at dusk now, passing overhead in the evening and flying to the west as the night goes on.

Star chart of the constellation Cygnus and its alpha star, Deneb, which marks the tail of the Great Swan. The plane of the Milky Way passes right along the axis of Cygnus and this whole area is rich with stars. Our August star, Vega in Lyra, appears along the right edge of this chart. (Image: Wikimedia Commons.)

If you want to introduce your students to Deneb and Cygnus you can start with some basic astronomy and astronomical mythology from your backyard star guide:

Cygnus lies in what for observers is a dense and fascinating part of the sky. The bird’s wings span the Milky Way at a location packed with stars and an assortment of deep-sky objects. Owing to the positions of its bright stars it is sometimes called the Northern Cross โ€” the northern parallel to the brilliant Southern Cross constellation in the Southern Hemisphere. You will see Cygnus highest in the sky in late summer and early fall, with its head pointing south like a bird on its migratory path toward warmer climates.

Deneb, the alpha star and tail of the bird, is a brilliant magnitude 1 star that joins Altair and Vega to form the Summer Triangle. Deneb is 25 times more massive than our sun and 60,000 times more luminous. (Backyard Guide to the Night Sky, page 216)

That’s plenty for beginning studentsย โ€” your little lesson is done. If you want to get more advanced, the Wikipedia page on Deneb is packed with additional information on everything from astrometry to cultural history.

A magnificent annotated image of the whole region of Cygnus, with its alpha star Deneb in the lower left, and with the Swan “flying” diagonally up to the right. This image has been specially processed to reveal the vast areas of hydrogen gas, glowing here in red, all through this region of the skyย โ€” this is not how it appears to the naked eye. (Image: Rolf Geissinger via apod.nasa.gov.)

Deneb is an enormous blue-white supergiant, about 200 times the diameter of our sun. If it were placed at the center of our solar system it would extend out to engulf the orbits of Mercury, Venus, and the Earth. Deneb is estimated to be about 2,600 light-years away, so the photons striking your eye when you gaze upon it were actually emitted by the star 2,600 years agoย โ€” when the ancient Greek poet Sappho was composing her lyrics in the eastern Aegeanย โ€” and they have been traveling at the speed of light to greet us ever since. The name Deneb is Arabic for tailย โ€” it’s the tail of the swan for us, but for medieval Arabic astronomers it was usually imagined as the tail of aย heavenly hen.

The constellation Cygnus the Swan (along with Lacerta, Lyra, and Vulpecula), in an imaginative illustration from Urania’s Mirror (1824). Deneb, the alpha star of Cygnus, marks the swan’s belly in this drawing instead of its tail, as would be more conventional today. (Image:ย Wikimediaย Commons.)

Sometime this month, as the new homeschool year begins, take your students out at dusk and introduce them to this giant sun, and teach them its name, and so give them a new friend for life.

What astronomical observations and stellar sightings will you be making in your homeschool this Cygnus Term?ย ๐Ÿ˜Š

โกโ€…Alpha and beta and gamma, oh my: Most of the principal stars within each constellation have both old vernacular namesย โ€” Vega, Sirius, Arcturus, and so onย โ€” as well as more formal scientific designations. The German astronomer Johann Bayer (1572โ€“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: ฮฑโ€…(alpha, brightest), ฮฒโ€…(beta, second brightest), ฮณโ€…(gamma, third brightest), ฮดโ€…(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 ฮฑโ€…Lyrae (“alpha of Lyra”), the brightest star in Canis Major (the star Sirius) becomes ฮฑโ€…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 ฯƒย (sigma) star of Orion to be? Not very bright โ€” it’s far down the alphabet โ€” but ฯƒโ€…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.ย โœจ

โกโ€…Star bright: The brightness of aย star as we see it in our night sky is its magnitudeย โ€” or more properly, its apparent magnitude. The scale of star magnitudes was developed long before modern measuring instruments were invented, so it can be aย 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: aย 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 โ€“1.5, about three times brighter than Vega (at 0.0). The planet Venus at its brightest is about magnitude โ€“4.2; the full moon is about magnitude โ€“12.9; the sun is magnitude โ€“26.7. By contrast, the dimmest stars visible to the naked eye in aย 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.ย ๐ŸŒƒ

โกโ€…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 dayย โ€” it’s how we judge distances as we move through the landscape. Stellar parallaxes are extremely smallย โ€” fractions of an arc-second (one 3600th of a degree)ย โ€” 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.ย ๐Ÿ”ญ

โกโ€…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 referenceย โ€” 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 121โ€“122 (10th and 11th eds.). Why not find a dark-sky spot near you this month and spend some quality homeschool time beneath the starry vault.ย ๐ŸŒŒ

โกโ€…Hitch your wagon to a star: 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 monthย โ€” and make each one of them a homeschool friend for life.ย ๐ŸŒŸ

โกโ€…Print this little lesson: Down at the bottom of this post you’ll find a “Print” button and icon, along with several social-media share buttons. The Print button will let you create a neat and easy-to-read copy of this little lesson, and it will even let you edit and delete sections you don’t want or need (such as individual images or footnotes). Give it a try today!ย ๐Ÿ–จ

โกโ€…Become a Friend! If you enjoy the educational materials we distribute each week, please support our work and the noble cause of homeschooling by becoming a Friend of the River Houses! Your support keeps us going and growing!ย ๐Ÿ˜Š

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Related Posts:

  • ๐ŸŒŸ STAR BRIGHT: Arcturus for July๐ŸŒŸ STAR BRIGHT: Arcturus for July
  • ๐ŸŒŸ STAR BRIGHT: Alpheratz for October๐ŸŒŸ STAR BRIGHT: Alpheratz for October
  • ๐ŸŒŸ STAR BRIGHT: Algol for November๐ŸŒŸ STAR BRIGHT: Algol for November
  • ๐ŸŒŸ STAR BRIGHT: Spica for June๐ŸŒŸ STAR BRIGHT: Spica for June

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