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What Makes Stars Tick? Hydrogen Hydrogen Hydrogen Inside Most Stars Is a Chaotic and High-Energy Environment

illustrated -3 Hydrogen-2 Beryllium-7

Helium-3 Hydrogen Normal helium Hydrogen Boron-8 What makes stars tick? Hydrogen Hydrogen Hydrogen Inside most stars is a chaotic and high-energy environment. Hydrogen Radiation pressure How do changes inside stars coincide with what we see? Core Hydrogen Normal by Liz Kruesi; illustrations by Roen Kelly helium to helium Lithium-7 ook up at the night sky from a dark site, and you’ll see within them and how we, as observers, view each stage. Nuclear reactions that convert Hydrogen tens of thousands of burning orbs of gas. Just one of This research has shown that a ’s mass dictates hydrogen to helium are the most Beryllium-8 those twinkling dots we call stars could be a behemoth almost everything about the object, from the core temper- common reactions occurring L within a star. This with a mass 80 times that of our own . At it’s core sits a ature, to how long the star lives, to how it dies. While the conversion liberates energy from cauldron of nuclear reactions that power the star, allowing Sun and a star 10 times its mass may have similarities dur- hydrogen nuclei (protons). Three Normal us to see it glowing from hundreds of light-years away. ing the “adult” stages of their lives, they couldn’t be more hydrogen-to-helium conversion helium Normal What could hold such a massive object together? And different as they reach the later stages. chains are important in stars con- helium how does the pent-up energy not blow it apart? Why do some end their lives in taining less than about 1.5 solar A star’s important balance masses (M ). In addition to pro- Normal Normal helium Over the past century, astronomers have learned an spectacular blasts while others Gravity pulls a star’s gas toward the center. There, the ducing helium, the reactions spit helium immense amount about stars. They’ve pieced together the puff away their outer shells and pressure and are so high that out high-energy radiation life cycles of different stars to learn what’s happening fade slowly? occurs. In any star on the main sequence, protons (the (gamma rays) and neutral tiny- -13 Hydrogen cores of hydrogen ) fuse together to create helium. mass particles called neutrinos. The outward radiative pressure from fusion balances the inward gravitational force. Proton Neutrino Positron Nitrogen-13 Nitrogen-14 The Hertzsprung- Neutron Electron 10,000 1,000 Russell (H-R) A Sun-like star (0.8 to 4 solar masses) Hydrogen A low-mass star S diagram upergiants (less than 0.8 solar mass) The Hertzsprung-Russell (H-R) Hydrogen diagram is the essential manual when it comes to understanding s Carbon-12 -15 nt stars. Astronomers plot stars ia G according to their and Core 100 100 . A star’s temperature Core relates to its color — cooler stars Ma Normal in are red while the hottest ones are seq Helium ue blue. You’ll also notice that more nc luminous stars tend to be larger. Convection e A massive star Radiation Hydrogen This is because depends (more than 4 solar masses) Nitrogen-15 partly on surface area. During the bulk of their life- 1 10 times, stars fuse hydrogen into helium. Fusion is the mechanism Different The CNO cycle The Sun that powers the stellar orbs. While A different set of reactions Luminosity (solar units) stars undergo hydrogen fusion, convective zones becomes more important in they lie along the “main sequence” Do stars of different masses main sequence stars with of the H-R diagram. cores much hotter than the Radius (solar radii) differ while they’re on the main sequence? A low-mass star (less Sun’s core. Four hydrogen W 1 nuclei still convert to a 0.01 hit than 0.8 solar mass [M ]) doesn’t e d helium nucleus, but with the wa have a radiative zone — the rfs convective zone reaches from Core help of carbon, nitrogen, and the core to the outer layer. A oxygen isotopes. This CNO Sun-like star (between 0.8 and 4 cycle is the main reaction M ) has a convective outer layer chain in stars greater than surrounding a radiative zone about 1.5 M . Spectral class 0.1 0.0001 surrounding the core. In a high- O B A F G K M mass star (greater than 4 M ), Associate Editor Liz Kruesi loves the convective and radiative 30,000 10,000 6,000 3,000 high-energy astrophysics. zones are switched. Surface temperature (Kelvins)

© 2014 Kalmbach Publishing Co. This material may not be reproduced in any • 44 Astronomy June form2010 without permission from the publisher. www.Astronomy.com Stars illustrated The evolution of stars A massive star (greater than 4 solar masses) Stars more massive than 4 M Over time, the oxygen-- A low-mass star (less than 0.8 solar mass) 1 1 fuse hydrogen to helium along 7 core cools and M fades as a . a the main sequence for between 10 in s and 100 million years. Whereas Like other stars on the main After about 1 trillion years, the eq u e low-mass stars follow the proton- sequence, a star with 0.4 solar star has used up most of its n 1 2 c mass (M ) will fuse hydrogen to hydrogen. Nuclear fusion slows, e proton chains, these high-mass 8 helium at its core. However, convec- and the reactions create less 0.01 stars fuse hydrogen mostly via the tion runs throughout the entire star energy. Gravity pulls the outer lay- CNO cycle. and will ers toward the core. Unlike more 1 When hydrogen fusion stops, bring more 1 massive stars, the core can’t 0.4 hydrogen become hot enough to initiate 2 solar 2 the helium core contracts and

Luminosity (solar units) heats up. A hydrogen shell sur- into the core helium fusion. As the star shrinks, Spectral class mass rounds the core. The star’s outer during the it slowly fades and cools, eventu- O B A F G K M star’s long life. ally becoming a . layers puff out, and the star 30,000 10,000 6,000 3,000 becomes a red supergiant. Mostly helium Surface temperature (Kelvins) The core’s heat fuses 3 helium to carbon. Then car- A Sun-like star (0.8 to 4 solar masses) bon and helium can combine Hydrogen into oxygen. Hydrogen contin- core ues burning in the shell. The Helium 9 For some 10 billion years, a star helium nucleus is an , with a mass similar to the Sun’s astrophysicists call this type of star gets hotter and becomes a Oxygen-neon- Carbon 1 magnesium fuses hydrogen in its core via the fusion the triple-alpha process. 10,000 blue supergiant. proton-proton reactions. Helium 6 slowly builds up in the core, but it’s The star now fuses helium to 7 Eventually, the star’s helium Stars greater than 8 M can Iron is the most tightly bound not enough to begin helium fusion. 4 carbon in its core. It gets hotter 5 4 wanes at its core. The carbon- 8 get hot enough to fuse neon 9 nucleus. So iron is the end of The star is on the main sequence. and therefore becomes bluer. A 3 oxygen core collapses and heats and oxygen into additional ele- the road. The core begins to col- hydrogen shell still surrounds the up. The rising temperature initiates ments. The core rapidly (a few lapse under gravity and heats up. core. The star is now on the hori- 100 4 2 more fusion and therefore more decades) creates a silicon core, The star explodes as a , zontal branch. energy. This pushes the outer lay- which then fuses to a -iron leaving behind either a neutron 2 core. The star has shells of other star or a black hole. M ers farther out; as they expand, ain nuclei surrounding the heavy core. se they cool. The star is again a red qu en supergiant. 5 ce 1 1 5 One 8 Luminosity (solar units) solar 9 mass Helium Hydrogen Ten 0.01 masses 10,000 2 Helium has been collecting 8 1 6 5 in the core, and hydrogen is Carbon- Hydrogen 2 oxygen core Helium 4 mostly depleted in the inner 10 3 2 Spectral class percent or so. The core begins to collapse, increasing pressure and Carbon builds up at the core, O B A F G K M 100 1 temperature. A shell of hydrogen but it’s not hot enough to fuse. 5 30,000 10,000 6,000 3,000 moves outside the core. Fusion Occasionally, a carbon and helium Five Surface temperature (Kelvins) Oxygen-neon- Hydrogen solar continues around the core, and the nucleus will fuse to create oxygen. masses M magnesium core Helium ain star’s layers expand. As the star After between 0.1 and 1 billion se Carbon qu en grows bigger, it cools, and the star years, the star has a carbon-oxygen Eventually, the star becomes 7 ce enters the branch. core surrounded by a helium shell, 7 a Mira variable, pulsating with The temperature in the core 1 which is still surrounded by a hy- a period of about a year. Over an 5 is high enough to fuse carbon. Fusion continues in the sur- drogen shell. astronomically brief 100,000 years, This allows several reactions, which rounding hydrogen shell and the star loses all of the material 3 create oxygen, neon, and magne- Luminosity (solar units) builds the core’s mass. When the After the core converts much except its carbon-oxygen core. As sium. These nuclei build up in the inner half of the core’s mass has of its helium to carbon, it col- the layers separate, they create 6 star’s core. 0.01 become nearly pure helium, the lapses and heats up. Just as before, beautiful images in the sky. A plan- core reaches a critical temperature, the star’s outer layers grow larger etary nebula is the remnant outer Only about 1,000 years pass 7 which allows three helium nuclei to and cool. This time, it’s on the layers from a Sun-like star. 6 before carbon fusion ceases. In fuse into one carbon nucleus. This . A star a star less than 8 M , the oxygen- point is called the less than 4 M can’t get hot An Earth-sized, white-hot 8 neon-magnesium core collapses Spectral class because helium fusion starts sud- enough to begin carbon fusion. core of carbon-oxygen remains. 0.0001 8 and heats up, but not enough to O B A F G K M denly for stars less than 2.5 M . So the star’s layers — including For the next hundred billion years initiate new fusion. The star loses Stars above that threshold start the hydrogen and helium shells — or more, the white dwarf will all of its material (except its core) 30,000 10,000 6,000 3,000 fusion more gradually. Because a continue to expand. gradually cool and fade. and forms a . Surface temperature (Kelvins) White dwarf

46 Astronomy • June 2010

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