deep-Sky obServing Target these crimson beauties, and you’ll be seeing red all year long. ⁄ ⁄ ⁄ BY michael e. Bakich Tour the sky’s the skY’s reddest stars, almost reddest stars all of which are variable stars, deposit ceti.” Later, Italian astronomer Giuseppe carbon compounds on their surfaces. Observers love red stars, and they’re Piazzi (1746–1826) called it Garnet Sidus in As the number of dark molecules cool in more ways than one. First, they have the lowest sur- a catalog of stars he compiled. Today, we builds up, the stars become redder and know it as Herschel’s Garnet Star. fainter. astronomy: roen kelly face temperatures of any stars. And second, they’re great fun As the prototype of a class of variable ι to look at because not many astronomical objects reveal their stars called Mu Cephei variables, this star period. Because Y doesn’t strictly adhere to ν N swings between magnitudes 3.6 and 5 dur- this schedule, astronomers classify it as a N Y colors to the eye. But what makes them so red? And where κ ing a period of roughly 2 years. These num- semi-regular variable. But what makes it λ do you find the reddest? This guide will and visual (V) filters. They then subtract bers, however, give only its apparent famous is its color. help you discover some of these under- the visual magnitude from the blue. This brightness. In reality, Mu ranks as one of Italian astronomer Father Angelo Secchi β 2 R observed jewels. leaves a number — designated B–V — the brightest and largest stars known. (1818–1878), who classified a large number C A N E S E LEPUS Few naked-eye stars show color. The called the color index. The more positive More than a billion Suns could fit inside of stars by their spectral types, was so VENATICI µ closest we come to seeing a bright red star the color index, the redder the star is. Mu Cep, and, if it occupied our solar sys- impressed with Y CVn that he called it “La 6 without optics is coppery Betelgeuse (Alpha For example, if a star has a blue magni- tem’s center, its outer atmosphere would lie Superba.” Scientists study a star’s spectrum α Orionis) and orange Antares (Alpha Scor- tude of 5 and a visual magnitude of 3, then beyond Jupiter’s orbit. It outstrips the Sun’s by measuring the intensity of its light at 3° 3° pii). And while some may quibble about the B–V = 2. Color indices range from about energy output by some 350,000 times. For different wavelengths. reality of green stars or if a star looks pur- –0.5 for the bluest stars to a bit more than 5 Mu, B–V = 2.26. The standard spectral classes assign let- ASY-VS1207_01 ASY-VS1207_03 LA SUPERBa (Y Canum Venaticorum) lies ple due to a contrast effect, nearly everyone for the reddest. A color index of 2 makes ters to stars based on their temperatures. HINd’s CRIMSON STAR (R Leporis) glows 4.5° north-northeast of magnitude 4.2 sees red well. our example a red star. The Sun’s color Superb! From hottest to coolest, the letters run O, blood-red 3.5° west-northwest of Mu (μ) Chara (Beta [β] CVn). La Superba is a vari- Leporis. For the reddest view, observe R index is 0.65. In the northern constellation Canes Venat- B, A, F, G, K, and M. Since that system able star. Its brightness changes from Lep through a 6-inch or larger telescope How red is red? Other color indices exist. Adding ultra- ici the Hunting Dogs, only 8° west of the debuted, however, astronomers have classi- magnitude 4.8 to 6.3. at its faintest, magnitude 11.7. Astronomers determine a star’s redness by violet, red, and infrared filters gives rise to famous Whirlpool Galaxy (M51), sits a fied even cooler stars. And the coolest of all observation and simple math. They mea- U–B (usually for hot objects), R–I (for cool variable star labeled Y. Its magnitude ranges are carbon stars, designated C. star’s surface. There, carbon molecules R Lep varies in brightness by more than sure the star’s magnitude through blue (B) objects), and other combinations. B–V, from about 4.8 to 6.3 during a 160-day Carbon stars are giants much larger than absorb short-wavelength light (green, blue, 6 magnitudes, from 5.5 to 11.7 during a however, is the color index astronomers use the Sun that have evolved past the point and violet), making the star abnormally 430-day period. Because the star’s atmo- most often. N where only hydrogen fusion provides their red. In the case of La Superba, B–V = 2.55. sphere absorbs the most short-wavelength CEPHEUS α energy. In these stars’ cores, helium fuses light at minimum brightness, R Lep looks Jewel in the king’s crown 19 into carbon and oxygen at much higher A drop of blood reddest when faintest. That, then, is the Mu (μ) Cephei sits 4.9° southeast of magni- ν temperatures. Because the energy output The star R Leporis in Lepus the Hare is time to observe its color. R Lep will reach tude 2.4 Alpha Cephei. In 1783, German- has increased, the outer layers of these stars another variable carbon star that appears minimum during autumn 2008. born English astronomer Sir William E λ swell and cool, thus becoming red. But a intensely red. With a B–V of 2.7, R Lep rose Herschel (1738–1822) described this µ standard red giant doesn’t appear all that to notoriety in October 1845, when British Tips for observing red stars star as having “a very fine deep garnet ζ red because its production of oxygen is astronomer John Russell Hind (1823–1895) First, use your largest telescope. Your eyes colour, such as the periodical star ο greater than that of carbon. discovered it. require a minimum brightness to trigger In carbon stars, however, carbon pro- He found its color, “of the most intense their color receptors. That’s why we see 3° IC 1396 duction outpaces that of oxygen, and car- crimson, resembling a blood-drop on the color during the daytime, but not at night through a telescope, Herschel’s bon compounds such as carbon monoxide background of the sky; as regards depth of without artificial lighting. Garnet Star (Mu [μ] Cephei) will appear ASY-VS1207_02HERSCHEL’s GARNET STAR (Mu [μ] redder than in this image. Cameras accu- Cephei) sits at the northern edge of (CO) and cyanogen (CN) migrate to the color, no other star visible in these latitudes Center the star in your eyepiece’s field of mulate and store light better than the emission nebula IC 1396. Mu lies 3.6° could be compared with it.” R Leporis now view, and ever-so-slightly defocus the human eye, so they increase the brightness west-northwest of Zeta (ζ) Cephei. michael e. Bakich is a senior editor of carries the moniker “Hind’s Crimson Star” image. Spreading out the star’s light into a All charts: aSTRONOMY: RICHARD TALCOTT AND ROEN KELLY of celestial images. Anthony AyiomAmiTiS Astronomy. after its discoverer. small disk will allow you to see its color © 2010 Kalmbach Publishing Co. This material may not be reproduced in any form without permission from the publisher. www.Astronomy.com www.Astronomy.com 83 Red filter Green filter Blue filter RGB + + = aN RGB IMAGE of R Leporis, taken with a CCD camera, shows the star appears brightest through a red filter; green and blue filters record it progressively fainter. Because R, G, and B images are monochromatic, they appear colorless. AdAm BLOCK/NOAo/AUrA/nSF ν N µ N N V γ CRATER α λ γ α β DY NGC 7000 Deneb δ λ CYGNUS ι ε ξ V CRUX 3° 3° θ α ν γ β 3° ASY-VS1207_04V cYgNi sits 2.9° due north of Deneb ASY-VS1207_05RUBY CRUCIS (DY Crucis) lies less than 1' ASY-VS1207_06PERHAPS THE SKY’s reddest star is V σ (Alpha [α] Cygni), but it’s not easy to find. northwest of magnitude 1.3 Mimosa (Beta Hydrae. It sits 3.5° south-southwest of At maximum brightness, it glows weakly [β] Crucis). To see 9th-magnitude DY Cru- Alkes (Alpha [α] Crateris). At maximum χ at magnitude 7.8, but it fades 6 magni- cis, insert an eyepiece that gives 100x or brightness, V Hya hovers near naked-eye tudes as it moves to minimum. It will more, and place brilliant Mimosa outside visibility at magnitude 6.5, but it drops to appear reddest near minimum brightness. the field of view. magnitude 12 during a 533-day period. better. Be aware that your scope’s optics from our atmosphere, like the Sun’s chang- For the best view, move magnitude 1.3 may not be perfectly aligned. First, adjust ing color at sunrise and sunset. Believe me, Mimosa out of your eyepiece’s field of view. the focus so the eyepiece moves inward. you won’t need the help. • V Hydrae — Observers consider this Then, move past the focus point an equal the reddest of all stars when its brightness amount in the other direction. View which- Author picks lies at minimum, and with a B–V of 5.5, I ever disk looks rounder. Along with the three famous stars above, can’t disagree. V Hydrae’s magnitude varies If possible, observe these targets when lots of other deep-red stars populate the between 6.5 and 12 during a 533-day they’re on or as close to the meridian (the sky.