Astronomy August 2022

2 2 The splendid lunar crater 4 The imposing Leibnitz 3 Clavius was imaged in 1953 with Mountains, located near the the 200-inch Hale telescope lunar south pole, are visible (left) and in 2005 with a behind the striking crater Celestron 14-inch Schmidt- Moretus in the foreground in this Cassegrain consumer scope and shot taken during a favorable an early model Nikon Coolpix libration in August 2016. 3.3-megapixel CCD camera (right). LEFT: PALOMAR OBSERVATORY 5 The remarkable system of rilles (long, narrow lunar valleys) 3 Showing some of the Moon’s running down the western major craters, including portion of Mare Tranquilitatis Schomberger at lower left, this (Sea of Tranquility) pop into detailed limb view of the lunar view in this shot taken along south pole taken July 2, 2017, the terminator (the dividing captures how it might appear line between lunar day and to passengers during a flyover. night) on May 27, 2020. 4 5 Aug. 23, 2016 Aug. 24, 2016 Aug. 25, 2016 Sept. 24, 2016 6 Progressing shadows on Tycho Crater provide patient observers with a variety of captivating views. The images in this series were all taken with a 10-inch Opticon Schmidt- Cassegrain telescope. 6 July 17, 2018 Feb. 13, 2019 May 31, 2020 WWW.ASTRONOMY.COM 51

78 7 Evolving illumination Straight Wall formation 11 Great seeing conditions reveals features within craters (Rupes Recta) in this image Oct. 28, 2021, enabled this (from top) Ptolemaeus, taken Feb. 23, 2018. The stunningly detailed view of Alphonsus, and Arzachel in 68-mile-long (110 km) feature Clavius Crater. these shots taken May 12, is surprisingly short, with a 2019; Feb. 12, 2019; and height of just 980 feet (300 m). 12 Craters Theophilus (left) March 24, 2018, respectively. To its right are craters (from and Cyrillus (right) appear top) Arzachel, Thebit, and crisply defined in this April 8 Thanks to grazing light Purbach. 2018 image processed for conditions Feb. 12, 2019, the maximum detail. shadows cast by crater 10 Imaged under excellent Alphonsus and its prominent seeing conditions Aug. 14, 13 Taken using a 14-inch central peak evolve over a 2017, Ptolemaeus Crater Celestron, the HDR- period of just 48 minutes. shows incredible detail. The processed image of smallest crater pits resolved Alphonsus (right) stacks up 9 Lunar shadow play unlocks in this view are less than well with the LRO’s shot a linear fault known as the 0.6 miles (1 km) wide. shown at left. LEFT: NASA 9 10 11 12 13 52 ASTRONOMY • AUGUST 202 2

lunar features. These can reveal unusual forms, as well as terrain features that are not readily appar- ent in other lighting conditions. At approximately 108 million years old, the prominent and dis- tinctive Tycho Crater is among the youngest impact features on the Moon. Its bright interior is particu- 14 larly evident when the Sun is over- head, and its distinctive pattern of ejecta rays extends up to 930 miles “The future holds (1,500 km) across the lunar surface. The well-known trio of craters better images for mePtolemaeus, Alphonsus, and Arzachel provides a good example and everyone who putsof relative dating of lunar features. Of the three, only Arzachel has a their heart into well-defined, walled rim and prom- pushing deeper into inent central peak. Alphonsus’ wall and central peak are comparatively the heavens.” less well defined, and the interior of Ptolemaeus is almost fully lava — Robert Reeves 15 filled and lacks any evidence of a 14 In 1967, Lunar Orbiter 4 captured this central peak. Based on these mor- then-impressive view of Walther Crater, shown at left. But many more intricate details phologies, it is clear that Ptolemaeus are revealed in the high-resolution, HDR- processed image at right, which was taken is the oldest and Arzachel the Pushing the envelope in 2016 using a 14-inch Celestron. LEFT: NASA youngest of the three. And digital Most astrophotographers like to 15 This spectacular image of craters Eudoxus (bottom) and Aristoteles (top), taken photography is helping amateurs experiment with their avocation in April 16, 2019, under exceptionally good atmospheric conditions, also reveals a wealth finally explore that lunar history an effort to squeeze every last bit of detail in the surrounding regions. Such details are typically evident only under low- for themselves. of information out of their digital angle illumination. data, providing as detailed a view Reeves so nicely put it in his com- pelling 2016 Astronomy.com arti- Ramping up as possible. We are no exception. cle, “Lunar landscapes love affair”: “So why do I keep imaging the the resolution One such method to get the Moon after half a century? Simply said, I haven’t done my best yet. I In the past, astrophotography using most out of your data is to use haven’t finished exploring. There is more to see. The future holds better film or photographic plates had processing techniques like high images for me and everyone who puts their heart into pushing major limitations. The silver halide dynamic range (HDR). However, deeper into the heavens, and I am going to enjoy the ride while crystals these methods used were this will only work effectively with I can!” inherently grainy and not very sen- top-quality stacked images contain- We could not agree more. sitive to light. To top it off, because ing several hundred frames. Leo Aerts and Klaus Brasch have both maintained a lifelong interest in exposures tended to require several At this point, one might reason- astronomy, especially when it comes to observing. seconds, they were also seriously ably ask: Are there no limits to affected by atmospheric turbulence what amateur lunar imagers can or seeing. As a result, it was almost hope to achieve in the future, or impossible to obtain diffraction- have we reached a pinnacle? limited images — those with resolu- Clearly, larger aperture telescopes tions limited only by optics — with will help you capture more impres- any telescope, large or small. sive results by having a higher However, most of these limita- angular resolution. But will con- tions do not apply to digital photog- tinuing improvements in digital raphy. Because only the very best cameras and data-processing appli- frames are combined and pro- cations expand those horizons even cessed, a final digital image can more? Only time will tell. approach the full resolution poten- But then again, does it really tial of the scope used to obtain it. matter? As expert imager Robert WWW.ASTRONOMY.COM 53

THE STAR ththaet cchoasmngoesd

An astroimager follows in Edwin Hubble’s footsteps to prove the utter vastness of our universe using a single star. BY ROD POMMIER OU KNOW THAT M31 (NGC 224) in Andromeda is another galaxy far outside our own Milky Way, don’t you? Of course you do! Everyone knows that. This colorful image shows a PARCHMENT SCROLL: FRENTA/DREAMSTIME But we haven’t always known Edwin Hubble poses in front of a model of closeup of a portion of the it. In fact, we’ve only known for Mount Wilson’s 100-inch Hooker Telescope in Andromeda Galaxy’s (M31) just under a century. Prior to this photograph shot circa 1948, years after disk. The Cepheid variable his groundbreaking 1929 paper calculating star M31-V1 is indicated that, astronomers referred to the distance to M31. While he was observing with an arrow. The shot is M31 and scores of other galaxies scat- M31 and the variable star M31-V1, the Hooker a composite of luminance tered throughout the sky as spiral reflector was the world’s largest telescope. data acquired on many of nebulae. They were visible in great the 57 nights over which numbers in a bewildering variety of HUB 1033 (11), EDWIN POWELL HUBBLE PAPERS, THE HUNTINGTON the author imaged, to sizes, shapes, and orientations. But no LIBRARY, SAN MARINO, CALIFORNIA which color data have one knew their distance. And their subsequently been added. true nature was a hotly debated issue. had to be another galaxy. The uni- verse suddenly got much bigger. In ROD POMMIER On April 26, 1920, astronomers fact, if the myriad spiral nebulae that Harlow Shapley of Mount Wilson appeared smaller than M31 were also Observatory and Heber Curtis of galaxies in their own right, they must Lick Observatory held a Great Debate be farther still. The universe had to at the Smithsonian Institution in be unbelievably enormous. Washington, D.C. The topic: the nature of spiral nebulae and the scale Hubble’s star, the first variable of the universe. Shapley had mea- found in M31, has since been dubbed sured the size of the Milky Way in M31-V1. It is the star that changed 1915 and found it far larger than most the cosmos. astronomers had imagined. He argued the Milky Way was the entire Emulating Hubble universe and the spiral nebulae were smaller objects within it. Perhaps As an avid astrophotographer, I they were swirling stellar nurseries or wanted to take my own image of this condensing solar systems. Curtis argued they were galaxies, each like the Milky Way, and therefore extremely large and at vast distances. The debate had no clear winner. Just a few years later, in 1923, Edwin Hubble settled it. Using the 100-inch Hooker Telescope and photographic glass plates at Mount Wilson Observatory, he discovered a variable star within M31. Hubble used that star to show M31 lies far outside the Milky Way, proving it WWW.ASTRONOMY.COM 55

star. But could I? I don’t have a 100-inch all, we are approaching the centennial of College Observatory, Henrietta Swan telescope. I have a 14-inch telescope, Hubble’s discovery of M31-V1. What bet- Leavitt discovered a relationship between which is magnitudes smaller. On the ter time to emulate his work? To do that, I a Cepheid’s period and true luminosity. other hand, I do have a cooled CCD would not only need to duplicate Hubble’s She noted that the longer a Cepheid vari- camera, which is much more sensitive to astrophotography; I would also need to able’s period, the brighter it appeared. In light than the photographic glass plates understand how he used his images of 1912, she published a graph showing a Hubble used. Could my smaller telescope M31-V1 to prove M31’s distance. strong positive linear correlation between with a more responsive detector possibly the logarithm of these stars’ periods and average apparent magnitudes. This is now A standard candlematch a larger scope with less sensitive plates when looking at this faint target? Hubble determined the distance to M31 known as the period-luminosity relation- With some research, I found that 11 by finding a so-called standard candle ship, or the Leavitt law. members of the American Association of within it. A standard candle is an object of Danish astronomer Ejnar Hertzsprung Variable Star Observers (AAVSO) had known luminosity, or intrinsic brightness. realized the tremendous significance of successfully imaged M31-V1 in 2010 at the If you know an object’s luminosity, you Leavitt’s discovery. Once calibrated, this request of the Space Telescope Science can compare that to how bright it appears relationship would allow astronomers to Institute. Researchers wanted to know from your vantage point and work out calculate the distance to any Cepheid when the star was brightest to best image how far away it must be. The standard from two pieces of data: its period and its it with the Hubble Space Telescope (HST) candle Hubble found in M31, the star average apparent magnitude. But for a public outreach program. The M31-V1, is a Cepheid variable star. Hertzsprung’s early attempts at calibration AAVSO report indicated they found Cepheids are pulsating stars whose were crude at best, yielding a distance to M31-V1 a challenging but achievable tar- brightness varies over timescales ranging the Small Magellanic Cloud of 30,000 get for modern CCD cameras and “larger” from one to more than 120 days. They light-years, compared to the currently telescopes. How large was not reported. exhibit a distinctive pattern on a graph accepted value of 200,000. Still, their success convinced me I could at of brightness versus time, called a light Shapley revised the calibration but least attempt to image M31-V1 for myself. curve, consisting of a sharp increase in his work was also incorrect, leading him Then a bigger question hit me. While brightness followed by a gradual dim- to estimate our galaxy’s diameter was imaging M31-V1, could I also use my ming. This pattern repeats at regular 300,000 light-years instead of the currently images to prove M31 is another galaxy? intervals, known as the period. accepted 100,000. His measurements did That would be a fantastic project. After While working for the Harvard correctly show we were at the outskirts of the Milky Way, rather than its center — the biggest demotion of our place in the universe since Copernicus put the Sun at BELOW: Delta (δ) Cephei is the prototype Cepheid variable. This light curve shows the center of the solar system. its apparent magnitude versus time. All Cepheids produce a characteristic sawtooth pattern, with a rapid increase in brightness followed by a slow dimming. The LEAVITT LAW timescale over which this pattern repeats is the period, and the difference between maximum and minimum brightness is the amplitude. Delta Cephei’s period is 12 5.4 days and its amplitude is 0.7 magnitude. ASTRONOMY: ROEN KELLY, AFTER R NAVE/HYPERPHYSICS RIGHT: Henrietta Swan Leavitt discovered a linear relationship between the logarithm of the periods of Cepheid variables, plotted on the x-axis, and their apparent magnitudes, plotted on the y-axis. This relationship was derived from 25 Cepheids in the Small Magellanic Cloud. The upper tracing and best-fit line show the stars’ maximum magnitudes; the bottom tracing and best-fit line, their minimum magnitudes. Leavitt suggested the relationship could best be expressed using the stars’ period and average apparent magnitude. This is now known as the period- luminosity relationship, or the Leavitt law. ASTRONOMY: ROEN KELLY, AFTER LEAVITT & PICKERING, 1912 13 Magnitude at maximum Apparent magnitude Magnitude CEPHEID LIGHT CURVE 14 3.5 Magnitude 15 at minimum 4.0 Period 16 4.5 2 4 6 8 10 12 0.0 0.2 0.4 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 Time (days) Logarithm of period 56 ASTRONOMY • AUGUST 202 2

However, a much bigger demotion was Absolute magnitudeCALIBRATED PERIOD-LUMINOSITY RELATIONSHIP to come. –8 How Hubble did it –7 In September 1923, Hubble began tak- ing serial exposures of M31 from Mount –6 Wilson. On the night of Oct. 5/6, he made a 45-minute exposure on plate –5 H335H. (The first H stands for Hooker, the last for Hubble.) Upon examination, –4 M = –3.57 he marked three stars in black with the letter N for novae, because they appeared –3 to be new compared to earlier plates. However, he subsequently noticed that –2 Log(4.76) = 0.68 one of those three was present on earlier plates, including H331H taken the previ- –1 ous night. In fact, it appeared on archival plates as far back as 1909, but fluctuated 0 0.5 1.0 1.5 2.0 in brightness. So, it couldn’t be a nova Logarithm of period and must be a variable star. This graph shows the period-luminosity relationship, calibrated by measuring the distances to Cepheid With a red pen, Hubble crossed out variables in the Milky Way. These distances can be used to calculate the stars’ true luminosities, or the letter N and wrote “VAR!” for vari- absolute magnitudes, to calibrate the y-axis of the graph. Calibration enables use of any Cepheid able. Why the exclamation point? Hubble variable as a standard candle, whose distance can be determined from only its period and average realized that if this star was a Cepheid, he apparent magnitude. In this case, an example Cepheid with a period of 4.76 days yields an absolute had struck astronomical gold. If he could magnitude of –3.57. ASTRONOMY: ROEN KELLY, AFTER AUSTRALIA TELESCOPE NATIONAL FACILITY determine the Cepheid’s period and its average apparent magnitude, then he produce a light curve to determine both Hubble’s glass photographic plate H335H, obtained could calculate the distance to M31 and M31-V1’s period and average apparent with the 100-inch Hooker Telescope Oct. 5/6, 1923, solve the mystery of the spiral nebulae. magnitude. Measuring magnitudes of shows the galaxy M31. Three “new” stars are marked stars on digital images is now done with in black with the letter N. However, Hubble later In early 1924, Hubble imaged this star photometry software, which determines noted that the star at the top right was present on as many successive nights as weather the magnitude of a target star by compar- on earlier plates but fluctuated in brightness. He permitted and determined its nightly ing its brightness to that of a comparison subsequently crossed out the N and marked it with magnitude. His data produced the char- star of known magnitude on the same “VAR!” in red. This is the Cepheid variable M31-V1, acteristic light curve of a Cepheid. He image. My imaging software, MaxIm DL which Hubble used to calculate the distance to measured its period as 31.415 days and Pro, includes a tool to do this. M31 and solve the mystery of the spiral nebulae. estimated its median apparent magni- tude at 18.5. From the period, Hubble The AAVSO website’s Star Plotter, COURTESY OF CARNEGIE INSTITUTION FOR SCIENCE derived an absolute magnitude of –5.0. which allows users to create star charts at He then calculated that for a Cepheid various scales and orientations to match reduce exposure time. M31 spans the this bright to exhibit an apparent magni- their images, includes comparison stars width of six Full Moons across the sky, tude of 18.5, it had to be nearly 1 million and their magnitudes. Fortunately for so only a portion of it would fit within light-years away. Therefore, M31 could me, the AAVSO established many com- my scope’s field of view. Therefore, my only be an enormous independent galaxy parison stars within 15' of M31-V1 in first tasks were to determine M31-V1’s outside the Milky Way. preparation for their 2010 project location within M31 and how best to frame it on my CCD chip. In 1929, Hubble published an Research indicated that to obtain the estimated distance to M31 of 900,000 most accurate magnitude measurements, light-years, calculated using additional I should bin my images to 1x1. Further, observations and Shapley’s revised my telescope is a Schmidt-Cassegrain, calibration of the period-luminosity which is subject to mirror flop — a shift relationship. The currently accepted in mirror position with actions such as distance to M31 is 2,537,000 light-years. focusing or parking the telescope. Mirror The same errors that caused Shapley to flop can significantly change the illumi- overestimate the diameter of the Milky nation of a CCD chip between imaging Way caused Hubble to underestimate sessions, affecting magnitude readings. the distance to M31. Therefore, I committed to shooting new flat field calibration frames each night. Planning the project I imaged through a clear filter to To reproduce Hubble’s work, I needed to capture as many photons as possible and used an f/7.5 focal reducer/corrector to WWW.ASTRONOMY.COM 57

MagnitudeLIGHT CURVE OF M31-V1 18.2 18.4 18.6 18.8 19 19.2 19.4 19.6 19.8 20 –10 0 10 20 30 40 Phase days Incorporating data from observations made over 57 nights, the author’s light curve exhibits the characteristic pattern of a Cepheid variable. From this light curve, the author obtained the star’s period of 31.91 days, peak apparent magnitude of 18.6, minimum apparent magnitude of 19.8, average apparent magnitude of 19.2, and amplitude of 1.2 magnitudes. ASTRONOMY: ROEN KELLY, AFTER ROD POMMIER M31-V1 is at R.A. 00h41m27.3s, Dec. his work, I knew it would require at least the Full phase coincided with the dim 41°10'10.4\", in the northeast quadrant of a month and likely longer. A Cepheid’s portion of the star’s cycle. M31. Using imaging-planning software, I period is most reliably measured determined turning my CCD camera to a between dates of maximum brightness. Then, there was weather. Hubble rotation angle of 135° and aiming at R.A. I didn’t know where M31-V1 would be worked in sunny southern California, 00h41.1m, Dec. 41°11' placed M31-V1 near in its cycle during my first observation. where he had only a few cloudy nights. the center of my chip. This orientation Therefore, I’d likely have to continue I live in rainy Portland, Oregon, where would also include the magnitude 9.27 imaging until it peaked in brightness even clear nights are often interrupted by foreground Milky Way star SAO 36590 as and carried out another full cycle until clouds rolling in. There was also a risk of a suitable guide star in my off-axis guider, it peaked again. That meant I’d have to heavy smoke obscuring the sky for long allowing for accurate tracking during image some nights with a nearly Full periods during the upcoming wildfire imaging sessions. Several AAVSO com- Moon, which unfortunately would be season. This would indeed be a challenge. parison stars were present on my images. in the vicinity of M31 during upcoming months. Bright moonlight flooding Imaging the star Hubble didn’t know how many nights down my telescope tube might com- he would have to image M3-V1 to deter- pletely wash out M31-V1, especially if I began imaging in early August, a time mine its period, but with the benefit of when M31 rises above 45° altitude by 1 A.M. I calibrated and stacked the first night’s sub-exposures into a single inte- grated image. Then I zoomed in on the region containing Hubble’s Cepheid for Hubble created this hand-drawn light curve for M31-V1 from images obtained in early 1924. From these data, he showed that the star had the characteristic light curve of a Cepheid with a period of 31.415 days and an average apparent magnitude of 18.5. Using those data, he calculated that M31 had to be nearly 1 million light-years from Earth — therefore, so far away it must be a separate galaxy. COURTESY OF THE HARVARD UNIVERSITY ARCHIVES 58 ASTRONOMY • AUGUST 202 2

INSET: A color image shows a closeup of M31-V1, marked with ticks, against the dusty background of M31’s spiral arms. It is a small portion of this larger mosaic of M31, made by shooting 11 frames vertically along the axis of the galaxy with a 14.5-inch f/8 Cassegrain, filling in the rest with a 6-inch refractor. Each sub-exposure was 20 minutes in length. TONY HALLAS inspection. M31-V1 appeared as a small, couple of weeks as it faded for a third definitely exhibited the characteristic faint dot right where it was supposed to time. At that point, having imaged over a pattern of a Cepheid. I could now derive be! I stopped and stared at that star for period of 57 nights, I knew I had collected the period and apparent magnitude to a long time. During my 35 years as an data for more than one full period. calculate the distance to M31. astrophotographer, I have captured and inspected countless stars in my shots. This With my imaging completed, I cali- The apparent magnitudes for the first star was as inconspicuous and seemingly brated and stacked all the sub-exposures and second maxima were 18.6 and both insignificant as any I had ever seen. Yet no from each night into an integrated image minima were 19.8. Therefore, the ampli- other star I have imaged has been more for that date. I was ready to make my tude of my light curve was 1.2 magnitudes important to cosmology and our under- own light curve of M31-V1. and the average apparent magnitude was standing of our place in the universe than 19.2. The difference in Julian dates for the this small, dim one. Seeing the star that Making light curves maxima yielded a period of 31.91 days. changed the universe on an image I had Although this is not the period of 31.415 made myself took my breath away. I calibrated my photometry software on days derived by Hubble, the small differ- a comparison star near the center of each ence had no appreciable impact on my Spurred on by my initial success, I image. This worked extremely well — my calculated absolute magnitude. This is eagerly returned to my backyard observa- magnitude readings on all other compar- because the period-luminosity relationship tory every clear night. I couldn’t wait ison stars read accurately to within a few uses the logarithm of the period to obtain to see if and when M31-V1 changed in hundredths of a magnitude. That gave absolute magnitude. The logarithms of brightness. As the Moon waxed toward me confidence my recorded magnitudes 31.91 and 31.415 both round to 1.5. Full, the star grew dim and I was con- for M31-V1 were also accurate. Although cerned it might soon disappear from my my software displays magnitude readings The distance to M31 images. My fears were allayed as it sud- to three decimal places, I could only jus- denly brightened, quickly reaching a peak. tify reading M31-V1’s brightness to the To compare my results for the distance I continued imaging for weeks afterward, nearest 0.1 magnitude because the mag- to M31 directly to Hubble’s, I needed to as it slowly faded again. As the Moon once nitudes of comparison stars in my chart use Shapley’s flawed calibration of the again approached Full, the star quickly were given only to one decimal place. period-luminosity relationship. Curiously, brightened, reaching a second peak. I Hubble adjusted this graph to yield observed every clear night for another After measuring M31-V1’s magnitude absolute magnitude at maximum, rather on all my images, I plotted these by date than the average absolute magnitude as on a graph. The resulting light curve WWW.ASTRONOMY.COM 59

ABOVE: This calibrated and stacked image — greatly zoomed in — from the author’s first night of in my backyard what Hubble did at Mount observation shows M31-V1, indicated by an arrow, as a faint dot, right where expected. ROD POMMIER Wilson, with precisely the same result. ABOVE RIGHT: The author measured the nightly apparent magnitude of M31-V1 using the photometry Still, that result is incorrect because tool in MaxIm DL Pro. After calibrating the tool on a non-variable star of known magnitude, he could it is based on an incorrect calibration of confidently read the magnitude of M31-V1. This image, in which the tool’s bull’s-eye is centered on the period-luminosity relationship. Since M31-V1, shows the star’s magnitude as 18.7 (to the nearest 0.1 magnitude). ROD POMMIER 1929, as technology has improved, the calibration has been revised. This has originally suggested by Leavitt and used Although Hubble did not show his values greatly increased the calculated distance in virtually all other calibrations. Hubble for maximum apparent and absolute to M31. With the advent of HST, that indicated he believed his maximum mag- magnitude, he gave their difference: number is now 2.537 million light-years. nitude readings were more reliable than m – M = 22.2. That was precisely the those obtained during dimmer portions value I had obtained! That could only A memorable endeavor of the cycle. Based on this graph, the mean Hubble had obtained exactly the logarithm of my 31.91-day period yielded same distance: 897,879 light-years. He With that, I declared the project a great an absolute magnitude for M31-V1’s simply rounded to 900,000 light-years in success. Following in Hubble’s footsteps maximum of –3.6. his paper. Now I was thrilled. I had done was an exhilarating experience I will cer- tainly remember for the rest of my life. Once you have an object’s absolute UP TO DATE Most of all, I am amazed that I, a mere magnitude and corresponding apparent amateur astronomer using equipment in magnitude, it is simple to calculate its One of HST’s chief goals was to precisely my backyard, was able to reproduce a feat distance using an equation called the determine distances to 10 Milky Way that less than a century ago was accom- distance modulus: m – M = 5[log10(d/10)], Cepheids by measuring their trigonometric plished by the world’s greatest astronomer where m is the apparent magnitude, M is stellar parallaxes — which can only be using the world’s largest telescope. the absolute magnitude, and d is the dis- done from space — to produce a calibra- tance in parsecs. (One parsec is equal to tion of unprecedented accuracy for the This is a testament to amateur astron- 3.26 light-years.) Solving this equation period-luminosity relationship. omy as a hobby. Want to be an amateur for d gives d = 10(m-M+5)/5. archaeologist or paleontologist? Good luck The equation for the period-luminosity accessing an Egyptian tomb or a T. rex My values for m and M yielded a relationship using the HST calibration is: fossil bed to conduct your own research distance of 275,423 parsecs, or 897,879 M = (–2.43 + 0.12)[log10(P)-1.0] – (4.05 + 0.02), projects. Such valuable materials are light-years — very close to Hubble’s pub- where M is absolute magnitude and P is reserved exclusively for professionals. lished value of 900,000. With this, I had the period. accomplished my goal. Despite the flawed Not so with amateur astronomy. All calibration, I had also proven M31 is so Using my data with the HST calibration, astronomers have unrestricted access to far away it must be a separate galaxy. how close could I come to the currently the same crucial resource: the entire sky accepted distance of M31? My period of above us. And with that, the sky is truly I was extremely pleased that my 31.91 days yields an absolute magnitude the limit of what we amateurs can do. result was so close to Hubble’s. The dif- of –5.27 using this calibration. Then the ference was only 2,121 light-years. Then, distance modulus gives a distance of Rod Pommier is a surgeon and longtime while reading Hubble’s 1929 publication 776,247 parsecs or 2.531 million light- deep-sky observer who has written many again, I noticed something remarkable. years. — R.P. articles for Astronomy. 60 ASTRONOMY • AUGUST 2022

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A S K A S T R O Astronomy’s experts from around the globe answer your cosmic questions. DAVINCI has an Skydiving It’s true that by cutting the parachute loose, the DS intricately planned onto Venus will spend less time in Venus’ lower atmosphere. But in descent stage to many ways, this is an advantage because the craft spends make the most of its Q IWHY WILL DAVINCI JETTISON ITS less time exposed to the harsh conditions there. If the hour-long trip through PARACHUTE SO QUICKLY INTO DS remained longer on the parachute, it would need to Venus’ atmosphere. VENUS’ ATMOSPHERE? WON’T THIS be designed to absorb more heat to keep the internal RESULT IN LESS TIME TO COLLECT DATA science instruments cool and have larger batteries to NASA GSFC VISUALIZATION AND AND IMAGES? keep the craft operating longer through the descent. This would increase the weight, which makes it even CI LABS MICHAEL LENTZ AND Steven Portalupi more challenging to build a parachute to support it! Newmarket, New Hampshire COLLEAGUES To ensure DAVINCI meets its science objectives, the A I Venus and its massive atmosphere present an DS makes use of state-of-the-art chemistry instruments incredibly challenging environment for any in situ probe mission. The planet’s surface temperature With a magnetic field over a thousand trillion times greater than is approximately 860 degrees Fahrenheit (460 degrees Earth’s, magnetars are the reigning magnetic kings of the Celsius) thanks to the dense CO2 atmosphere, which cosmos. ESO/L. CALÇADA also creates a surface pressure 90 times greater than Earth at sea level. Sulfuric acid clouds exist roughly 25 to 43 miles (40 to 70 kilometers) above the surface in a thick layer. When the DAVINCI descent sphere spacecraft (DS) jettisons its main parachute approximately 32 minutes into the descent — around 24 miles (39 km) above the surface — the temperature will already be 304 F (153 C). This truly hellish environment presents challenges that other planetary probes don’t have to face. Trying to build a parachute that could survive these conditions would be risky and expensive. Thus, DAVINCI employs fixed drag plates to slow its descent after jettisoning the parachute. The thick venusian atmosphere also helps because the descent is more like settling into a fluid than falling through air. 62 ASTRONOMY • AUGUST 202 2

and telecommunications systems that can sample the Apollo 11 astronauts captured this image of Earth rising above SEND US YOUR atmosphere every 500 feet (150 meters) of descent and the Moon’s horizon in July 1969. NASA QUESTIONS take near-infrared images every few seconds. The DS will transmit this data to its companion craft in Q I AT WHAT RATE IS THE MOON Send your venusian orbit, the Carrier Relay Imaging Spacecraft MOVING AWAY FROM EARTH? astronomy questions (CRIS), before reaching the surface. So, in the short WHAT KINDS OF CONSEQUENCES WILL via email to askastro@ hour that DAVINCI will spend in Venus’ atmosphere, OUR PLANET SEE AS OUR SATELLITE astronomy.com, or it will acquire groundbreaking measurements and MOVES FARTHER AWAY? write to Ask Astro, images far beyond what any previous mission has P.O. Box 1612, managed. Eliot H. Ginsberg Waukesha, WI 53187. Riverview, Florida Be sure to tell us Colby Goodloe your full name and DAVINCI Descent Sphere Lead Engineer, A I Let’s first look at why the Moon is moving away where you live. NASA Goddard Space Flight Center, Greenbelt, Maryland, from us. It boils down to one of Newton’s laws: Unfortunately, we conservation of angular momentum. As the Moon’s cannot answer all on behalf of the DAVINCI Project gravity pulls on Earth, it produces tidal forces that questions submitted. make the oceans bulge and cause Earth’s rotation to Q I I’VE READ THAT THE STRENGTH lose momentum. Slowing Earth’s rotation in turn speeds OF A NEUTRON STAR’S MAGNETIC up the Moon’s orbit, which must expand to conserve the FIELD IS GREATER THAN ANY OTHER total momentum of the Earth-Moon system. FOUND IN THE UNIVERSE. WOULDN’T A SUPERMASSIVE BLACK HOLE HAVE The Moon is moving away from Earth at about A STRONGER ONE? 1.49 inches (3.78 centimeters) per year. And as it moves away, its orbital period increases and Earth’s rotation Patrick Clough slows down. Looking at the average rate of retreat over Wichita, Kansas the last 4 billion years, it should take about 50 billion years before the Moon takes as long to complete one A I The answer to this question is quite compli- orbit as Earth takes to complete one rotation. cated. There is a so-called no-hair theorem, which basically states that only three observable At this point, Earth will be tidally locked to the Moon, parameters can be determined for each black hole: which will always sit above the same point on the planet. its mass, electric charge, and rotation. The hair here Only half of the planet will ever see the Moon. The is a metaphor for all other possible parameters, includ- Moon’s changing impact on our tides would also cease, ing magnetic fields, which disappear inside the black though there would still be some time-dependent tides, hole and become inaccessible to scientists. So, a black thanks to the Sun. The Sun-Earth tidal tug-of-war hole by itself does not have any measurable magnetic would eventually reverse the Earth-Moon process, field. bringing the Moon steadily closer to Earth until our planet’s gravity tore it apart. However, any matter that accretes onto a black hole could be magnetized. In this case, the magnetic field Of course, in 50 billion years, the Sun will have long will become stronger as the matter approaches the black since become a white dwarf. (This will happen in hole and is compressed. So, magnetic fields do exist 10 billion years.) And, in all likelihood, Earth and the around supermassive black holes, but their source is the Moon will not survive the Sun settling into its twilight accretion disk, not the black hole itself. years. For example, when the Event Horizon Telescope col- Caitlyn Buongiorno laboration imaged the supermassive black hole in M87, Associate Editor they observed radio waves that were polarized by the magnetic field in the surrounding accretion disk. The team estimated the magnetic field strength to be between two to 50 times stronger than Earth’s magnetic field. But that is a tiny magnetic field compared with the magnetic fields around pulsars and magnetars. In particular, magnetars retain the strongest magnetic fields in the universe, at a thousand trillion times stron- ger than Earth’s field. Andrei Igoshev Astronomy Research Fellow, University of Leeds, Leeds, United Kingdom WWW.ASTRONOMY.COM 63

READER GALLERY Cosmic portraits 1 2 3 1. COLORFUL CLOUD The emission nebula Sharpless 2–124 lies about 15,000 light-years away in Cygnus the Swan. This image consists of 15 hours of exposure time with a 4.2-inch scope in the Hubble palette. • Emil Andronic 2. OFF THE BEATEN PATH The object Kohoutek 2–1 (PK 173–05.1) in Auriga has been cataloged as an irregular galaxy, an HII region, and a reflection nebula. But its relative brightness through an OIII filter suggests that it is, as Czech astronomer Luboš Kohoutek originally categorized it in 1963, a planetary nebula. This image was taken over nearly 47 hours in OIII with a 4-inch scope. • Douglas J. Struble 3. THROWN FOR A LOOP The Helix Galaxy (NGC 2685) is an unusual polar ring galaxy, with tendrils of stars and dust coiled around the disk’s main plane. Roughly 40 million light-years distant in Ursa Major, it glows at magnitude 12.7. The imager used 6-inch scopes and 20.4 hours of exposure time in LRGB filters. • Peter Goodhew 64 ASTRONOMY • AUGUST 2022

4 4. MILK AND ROSES The Milky Way arches over Rose Lake, Idaho, in this panorama of sixteen 13-second exposures. The lit areas are not intentionally light-painted, but the result of light pollution from residences. • Ron Reeve 5. RIGHT ON TARGET A green flash occurs when, near sunset or sunrise, the atmosphere bends red, orange, and yellow light away from an observer. In this meticulously planned composite image, the photographer captured a green flash through the arches of Meloria Tower, an 18th-century landmark built on a tiny islet near Livorno, Italy. • Marco Meniero 6. CHURNING UP THE COSMOS The Propeller Nebula (DWB 111) in Cygnus would seem to have been produced by a rotating object flinging material into space. But, curiously, the 5 object has no central star. A 2021 study found the region is likely excited by the star WR 140, about 50’ away. This image taken with Hubble palette filters represents eight hours of exposure through an 8-inch scope. • Chuck Ayoub SEND YOUR IMAGES TO: Astronomy Reader Gallery, P.O. Box 1612, Waukesha, WI 53187. Please include the date and location of the image and complete photo data: telescope, camera, filters, and exposures. Submit images by email to [email protected]. 6 WWW.ASTRONOMY.COM 65

BREAKTHROUGH TOO CLOSE FOR COMFORT Although the recently deployed James Webb Space Telescope may represent the future of space-based astronomy, the venerable Hubble Space Telescope still rules the present. If you have doubts, explore this dramatic image of Hickson Compact Group 40. This eclectic collection holds three dust-laden spiral galaxies, one elliptical galaxy, and one lenticular (lens-shaped) galaxy. The five island universes crowd into a region less than twice the diameter of the Milky Way’s disk. Gravity is slowly pulling the galaxies together, and astronomers estimate that they will collide and merge into a single giant elliptical in a billion years or so. Hickson Compact Group 40 lies some 350 million light-years from Earth in the constellation Hydra the Water Snake. NASA/ESA/STSCI 66 ASTRONOMY • AUGUST 202 2

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SOUTHERN SKY BY MARTIN GEORGE October 2022 Giant planets rule the night As twilight fades to lives up to its reputation in Mercury lies in the morn- Mu1 (μ1) and Mu2 (μ2), followed darkness this month, October. The planet spans ing sky this month, reaching by Lambda (λ). Gamma pin- Saturn stands out from its 50\" to start the month and greatest western elongation points the bird’s head and lies perch high in the northern sky. 48\" as the calendar turns to October 8. Unfortunately, the near the constellation’s border The ringed planet shines at November. Small scopes reveal planet then stands only 18° with Piscis Austrinus the magnitude 0.5 against the a wealth of detail in Jupiter’s from the Sun. This small sepa- Southern Fish. relatively faint backdrop of massive atmosphere. Look for a ration combines with the shal- Capricornus the Sea Goat. The series of alternating dark belts low angle between the eastern In the opposite direction 4th-magnitude star Iota (ι) and brighter zones that run horizon and the ecliptic to from Beta, the triangle of stars Capricorni lies less than 1° parallel to one another. The make spotting the inner world Epsilon (ε), Eta, and Zeta (ζ) southwest of the solar system’s Great Red Spot should be obvi- nearly impossible from mid- marks Grus’ tail. The bird’s second-largest world. ous if it is on the hemisphere southern latitudes. wings run perpendicular to this Saturn’s high altitude makes facing Earth. And be sure to extended line through Beta. it the top choice for early eve- check out the planet’s four Venus passes on the far side The western wing ends at ning telescope viewing. Even bright Galilean satellites. of the Sun from our vantage Alpha (α) and the eastern wing the smallest scope shows the point October 22 and remains terminates with the Theta (θ) planet’s 18\"-diameter disk sur- The next bright planet out of sight all month. and Iota pair. rounded by a spectacular ring doesn’t put on a good show system that spans 40\" and tilts until after midnight. Once the The starry sky Grus first appeared in its 15° to our line of sight. During familiar shape of Orion the current form in Johann Bayer’s moments of good seeing, the Hunter fully clears the eastern As the winter Milky Way dips classic 1603 atlas, Uranometria, Cassini Division appears as a horizon, you’ll easily spot Mars low in the west on October eve- though he didn’t invent it. Long dark gap between the outer to the constellation’s lower left. nings, turn your attention away ago, celestial cartographers A ring and the brighter B ring. The Red Planet resides in east- from our galaxy’s plane. One of deemed the stars of Grus as an Small instruments also reveal ern Taurus the Bull, sharing my favorite sights at this time extension of Piscis Austrinus. 8th-magnitude Titan, Saturn’s this part of the sky with two of year is the constellation Grus Ptolemy considered the star we largest moon, along with a trio other ruddy objects: the 1st- the Crane, which lies nearly know as Gamma Gru as mark- of 10th-magnitude moons. magnitude stars Aldebaran in overhead in the evening sky. ing the tip of the Southern Jupiter hangs low in the Taurus and Betelgeuse in Fish’s tail, but Bayer’s chart east during twilight in early Orion. Mars dominates its I first became familiar with shows the bird and fish clearly October. Look for the giant neighbors, however, brighten- this bird in March 1970 when I separated. Not long after Bayer’s planet’s position to improve ing to magnitude –1.2 by was keenly following the prog- atlas appeared, a few astrono- dramatically, however, as both October’s close. ress of Comet Bennett as it mers tried to turn the Crane the evening and the month headed northwest through the into a flamingo and called the wear on. Jupiter gleams at Mars suffers from a low alti- constellation. At one point, the constellation Phoenicopterus. magnitude –2.9 among the tude this month as it travels comet passed close to the strik- (In zoology, the flamingo background stars of southern along the northernmost part of ing naked-eye pair Delta1 (δ1) belongs to the Phoenicopteridae Pisces the Fish. That is some the ecliptic. It reaches a peak and Delta2 (δ2) Gruis. family.) However, no one ever 400 times brighter than the altitude of just 30° to 40° as uses this name today. constellation’s luminary, mag- morning twilight commences. The Crane’s shape is rela- nitude 3.6 Eta (η) Piscium. Still, the view through a tele- tively easy to identify. First find Intriguingly, Comet Bennett Once Jupiter climbs high in scope proves worthwhile the lengthy line of stars that wasn’t the first well-known the north later in the evening, because the planet’s diameter starts with Beta (β) Gru and comet to pass through Grus. spend some time viewing it grows from 12\" to 15\" during stretches to the northwest, end- Ninety years earlier, the Great through your telescope. The October. This is big enough ing with Gamma (γ) Gru. Beta Southern Comet of 1880 visited solar system’s largest world that most scopes will show lies in the main body of the the Crane. Observers reported surface details under good bird, with the Delta1 and Delta2 that this dirty snowball from seeing conditions. pair marking the beginning of the outer solar system sported the Crane’s long neck. The neck a spectacular tail. continues with another pair,

STAR DOME S CRUX ANS 516 C E N TAU R U S MUSCA ` HOW TO USE THIS MAP CTIARRIUCASITNNRGUAUSLLE_U M CHAMAELEON This map portrays the sky as seenLUPUS A S N E M NGC near 30° south latitude. Located 2070 inside the border are the cardinalSW directions and their intermediate APUS CULUM HYDRUS points. To find stars, hold the map SCP overhead and orient it so one of NORMA the labels matches the direction NGC 6231 NGC 6397 SMC you’re facing. The stars above ARA O CTANS the map’s horizon now match PAVO what’s in the sky.M4 Antares TUCANA S NGC 104 Achernar The all-sky map shows how the sky looks at: C 10 P.M. October 1 O A UCSOTRROANLAI S 9 P.M. October 15 TELESCOPIUM 8 P.M. October 31 R Planets are shown at midmonth P MAP SYMBOLS I Open cluster U Globular cluster Diffuse nebula M6 Planetary nebula S Galaxy M7 INDUS STAR MAGNITUDES M8 M Fomalhaut S SCULPTOR Sirius IX 0.0 3.0 M20 AQUILA ICRO GRUS 1.0 4.0 M22 AGIT 2.0 5.0 OPHIUCHUS M16 S E R P E N S C AU DA SCUTUM SCOPIU A UPSI STCRIISN U S STAR COLORS M17 TA R I U S Saturn WA star’s color depends M on its surface temperature. M11 CAPRICORNUS The hottest stars shine blue A QUAR Ju •• Slightly cooler stars appear white U LEUS • Intermediate stars (like the Sun) glow yellow E Q I U S • Lower-temperature stars appear orange • The coolest stars glow red U • Fainter stars can’t excite our eyes’ color AltairS A G I T TA Enif receptors, so they appear white unless you VULPECULA use optical aid to gather more light DELPHINUS M15 BEGINNERS: WATCH A VIDEO ABOUT HOW TO READ A STAR CHART AT PEGASUS www.Astronomy.com/starchart. NW CYGNUS L ACERTA Deneb N

VO CARINA OCTOBER 2022 SAT. NGC SUN. MON. TUES. WED. THURS. FRI. PICTOR Canopus 1 RET DORADO COLUMBA 2345678 LMC SE 9 10 11 12 13 14 15 HOROLOGIUM CAELUM 16 17 18 19 20 21 22 ILLUSTRATIONS BY ASTRONOMY: ROEN KELLY PHOE ERIDANUS 23 24 25 26 27 28 29 30 31 Note: Moon phases in the calendar vary in size due to the distance from Earth and are shown at 0h Universal Time. NGC 25 E CALENDAR OF EVENTS SGP 1 Mercury is stationary, 15h UT upiter Mira 3 First Quarter Moon occurs at 0h14m UT CETUS 4 The Moon is at perigee (369,325 kilometers from Earth), 16h34m UT 5 The Moon passes 4° south of Saturn, 16h UT PISCES Path of the Sun (ecliptic) 7 Asteroid Vesta is stationary, 6h UT 8 The Moon passes 3° south of Neptune, 3h UT ARIES Uranus The Moon passes 2° south of Jupiter, 18h UT Pluto is stationary, 18h UT M33 TRIANGULUM NE Mercury is at greatest western elongation (18°), 21h UT 9 Full Moon occurs at 20h55m UT M31 12 The Moon passes 0.8° north of Uranus, 7h UT ANDROMEDA 15 The Moon passes 4° north of Mars, 5h UT 17 The Moon is at apogee (404,328 kilometers from Earth), 10h20m UT Last Quarter Moon occurs at 17h15m UT 19 Asteroid Juno is stationary, 0h UT 21 Orionid meteor shower peaks 22 Venus is in superior conjunction, 21h UT 23 Saturn is stationary, 9h UT 25 New Moon occurs at 10h49m UT; partial solar eclipse 29 The Moon is at perigee (368,291 kilometers from Earth), 14h36m UT 30 Mars is stationary, 11h UT