My first Magazine Sky & Telescope - 03.2019

slice of the H-R diagram (see page 18). Stars in the instability strip are variable, their light changing in predictable patterns. Almost all of them have moved out of the main sequence, the prime of life when stars fuse hydrogen in their cores, and have entered their golden years. This latter period splits into three sequential branches: red giants with inert helium cores surrounded by a shell of fusing hydrogen; horizontal-branch giants, when the helium core has reignited and is now trans- forming itself into carbon; and asymptotic giant branch (AGB) stars, those that have a carbon core surrounded by nested shells of fusing helium and hydrogen. The instability strip cuts a path through these stages. Perhaps the most famous denizens of the instability strip are Cepheids. Cepheids are yellow and orange giants and supergiants, with masses of 3 to 18 Suns and girths 35 to 80 times larger than our star. They used to be big, bluish B stars, but such massive stars live short, frenetic lives, and at 50 to 200 million years they have already entered middle age. Cepheids breathe in and out in a predictable pattern, controlled by a thin layer of ionized helium in their interi- ors. When this layer compresses, its temperature rises, and its helium atoms lose a second electron. The extra electrons prevent radiation from below from traveling freely through the layer. Heat that once passed through the helium is now trapped beneath it. The temperature and pressure below increase. As the pressure skyrockets, it shoves the layers above it outwards, and the helium expands and becomes more tenuous, permitting heat to pass through it again and lowering the interior pressure. But once the pressure below the layer drops, gravity takes over and the layer sinks and compresses again. This cycle repeats many times, causing the star to pulse. Polaris is the closest Cepheid, lying some 445 light-years away. For several years astronomers debated its precise dis- tance — which is ironic, given that Cepheids derive most of their fame from their role as rungs on the cosmic distance ladder. In 1912 Henrietta Swan Leavitt discovered that the brighter a Cepheid is intrinsically, the longer it takes to cycle through a pulsation period. Because Leavitt was studying stars in the Small Magellanic Cloud dwarf galaxy, all the stars had roughly the same distance. Once astronomers calculated how far away one Cepheid variable lay, they could deduce the distance to any other Cepheid in the universe simply by measuring how quickly the star cycles through its pattern and how bright it appears. And because Cepheids radiate some 10,000 times more energy than the Sun does, they can be seen to very great distances indeed. Depending on their mass, stars may cross the instability strip multiple times. A star of Polaris’s mass will cross the Cepheid region thrice. But due to the previous disagreement about the North Star’s distance, no one has been quite sure where Polaris is in its evolution. Part of the problem is, Polaris isn’t your typical Cepheid. It has a more complicated jiggle than the simple breath-in-and-out pattern that most Cephe- ids follow, so astronomers don’t use it in the distance ladder. It also may be brightening 50 to 100 times faster than expected, explains Edward Guinan (Villa- nova University). “It should not be doing this,” Guinan says. Cepheids can brighten with time, but Polaris has shot up more than 10% in the last century. “Doesn’t make any sense.” Guinan, Scott Engle (Villa- nova), and their colleagues discov- ered the change while gathering data for another Polaris project. They dug up measurements from previous centuries and traced the decline back to Johannes Hevelius in the 1600s. To be thorough, they decided to go even further back and check the data in two ancient star catalogs: Ptolemy’s Almagest from around the year 150, and al-S.ūfı̄’s 964 Kitāb al-Kawākib al-Thābitah (“Book of the Fixed Stars”). Many assume al-S.ūfı̄’s book is a copy of Ptolemy’s, but when Guinan looked at the u OUR STELLAR CAST In our three-part series we’ll discuss Polaris, the Alpha Centauri system, and Betelgeuse. These stars span a range of sizes, shown to scale. Listed diameters (“D”) are approxi- mate and given as multiples of the Sun’s diameter. We’ve also included the Sun for visual reference. Betelgeuse D = 887 q Sun D = 1 Alpha Centauri A D = 1.2 Alpha Centauri B D = 0.86 Proxima Centauri D = 0.15 Polaris D = 45 Massive stars live fast and die young, and Polaris is already a puffy, aging star that has devoured the supply of hydrogen fuel in its core. sk yandtele scope.com • M A RCH 2 019 17

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