Sun - Solar Atmosphere, Radiation, Core

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The temperature at the visible surface is about 5,800 K but drops to a minimum about 4,000 K at approximately 500 kilometres above the photosphere. The density, about 10−7 gram per cubic centimetre (g/cm3), drops a factor of 2.7 every 150 kilometres. 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The temperature at the visible surface is about 5,800 K but drops to a minimum about 4,000 K at approximately 500 kilometres above the photosphere. The density, about 10−7 gram per cubic centimetre (g/cm3), drops a factor of 2.7 every 150 kilometres. 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border-right-sm border-left-sm open"> <div class="drawer d-flex flex-column open"> <div class="left-rail-section-content"> <div class="topic-left-rail-header text-truncate bg-gray-50 position-relative text-right d-flex align-items-center"> <div class="tlr-title px-20 py-15 text-left"> <em class="material-icons text-gray-400 d-lg-none" data-icon="toc"></em> <a class="font-serif font-weight-bold text-black link-blue" href="https://www.britannica.com/place/Sun">Sun</a> </div> <button aria-label="Close" class="js-sections-close-button btn-link btn-sm btn d-lg-none position-absolute top-0 p-10 right-0" > <em class="material-icons font-26" data-icon="close"></em> </button> </div> <div class="section-content pl-10 pr-20 pl-sm-50 pr-sm-60 pl-lg-5 pr-lg-10 pt-10 pt-lg-0 bg-gray-50 clear-catfish-ad"> <div class="toc mb-20"> <div class="font-serif font-14 font-weight-bold mx-15 mb-15 mt-20"> Table of Contents </div> <ul class="list-unstyled my-0" data-level="h1"><li data-target="#ref1"><div class="pl-25"><a class="link-gray-900 w-100" href="/place/Sun">Introduction</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li><li data-target="#ref54136"><div class="d-flex align-items-center"><div class="ml-25"></div><a class="w-100 link-gray-900" href="/place/Sun#ref54136">Physical properties</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li><li data-target="#ref54137"><div class="d-flex align-items-center"><button class="h1-link-drawer-button btn btn-xs btn-circle d-flex rounded" type="button" aria-label="Toggle Heading"><em class="material-icons font-18" data-icon="keyboard_arrow_right"></em></button><a class="w-100 link-gray-900" href="/place/Sun/Internal-structure">Internal structure</a></div><div class="ml-40 toc-drawer sub-toc-drawer"><ul class="list-unstyled" data-level="h2"><li data-target="#ref54138"><a class="w-100 link-gray-900" href="/place/Sun/Internal-structure#ref54138">Energy generation and transport</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref54139"><a class="w-100 link-gray-900" href="/place/Sun/Evolution">Evolution</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref54140"><a class="w-100 link-gray-900" href="/place/Sun/Evolution#ref54140">Helioseismology</a></li></ul></div></li><li data-target="#ref54141"><div class="d-flex align-items-center"><button class="h1-link-drawer-button btn btn-xs btn-circle d-flex rounded" type="button" aria-label="Toggle Heading"><em class="material-icons font-18" data-icon="keyboard_arrow_right"></em></button><a class="w-100 link-gray-900" href="/place/Sun/Solar-atmosphere">Solar atmosphere</a></div><div class="ml-40 toc-drawer sub-toc-drawer"><ul class="list-unstyled" data-level="h2"><li data-target="#ref54142"><a class="w-100 link-gray-900" href="/place/Sun/Solar-atmosphere#ref54142">Photosphere</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref54143"><a class="w-100 link-gray-900" href="/place/Sun/Chromosphere-and-corona">Chromosphere and corona</a><ul class="list-unstyled" data-level="h3"><li data-target="#ref54144"><a class="w-100 link-gray-900" href="/place/Sun/Chromosphere-and-corona#ref54144">Chromosphere</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref54145"><a class="w-100 link-gray-900" href="/place/Sun/Corona">Corona</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref54146"><a class="w-100 link-gray-900" href="/place/Sun/Corona#ref54146">Solar wind</a></li></ul></li></ul></div></li><li data-target="#ref54147"><div class="d-flex align-items-center"><button class="h1-link-drawer-button btn btn-xs btn-circle d-flex rounded" type="button" aria-label="Toggle Heading"><em class="material-icons font-18" data-icon="keyboard_arrow_right"></em></button><a class="w-100 link-gray-900" href="/place/Sun/Solar-activity">Solar activity</a></div><div class="ml-40 toc-drawer sub-toc-drawer"><ul class="list-unstyled" data-level="h2"><li data-target="#ref54148"><a class="w-100 link-gray-900" href="/place/Sun/Solar-activity#ref54148">Sunspots</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref54149"><a class="w-100 link-gray-900" href="/place/Sun/Prominences">Prominences</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref54150"><a class="w-100 link-gray-900" href="/place/Sun/Flares">Flares</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref54151"><a class="w-100 link-gray-900" href="/place/Sun/Solar-terrestrial-effects">Solar-terrestrial effects</a></li></ul></div></li><li data-target="#ref54152"><div class="d-flex align-items-center"><div class="ml-25"></div><a class="w-100 link-gray-900" href="/place/Sun/History-of-observation">History of observation</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li></ul> <a class="toc-extra-link link-gray-900" href="https://www.britannica.com/place/Sun/additional-info">References & Edit History</a> <a class="toc-extra-link link-gray-900" href="/facts/Sun">Related Topics</a> </div> <div class="tlr-media-slider pb-10 mb-30"> <a class="section-header link-gray-900 font-serif font-14 font-weight-bold mb-10 mx-10" href="https://www.britannica.com/place/Sun/images-videos">Images, Videos & Interactives</a> <div class="slider js-slider position-relative d-inline-flex align-items-center mw-100 "> <div class="slider-container js-slider-container overflow-hidden d-flex overflow-hidden text-nowrap ml-15"> <a href="https://cdn.britannica.com/64/94264-050-AAAB7996/Joshua-trees-Tree-National-Park-California.jpg" data-href="/media/1/573494/97735" class="media-overlay-link d-inline-block mr-5"> <img loading="lazy" src="https://cdn.britannica.com/64/94264-120-FD9463AB/Joshua-trees-Tree-National-Park-California.jpg" alt="Joshua Tree National Park" height="50" /> </a> <a href="https://cdn.britannica.com/01/81401-050-8EB87030/planets-solar-system-Pluto-montage-images-Sun.jpg" data-href="/media/1/573494/74702" class="media-overlay-link d-inline-block mr-5"> <img loading="lazy" src="https://cdn.britannica.com/01/81401-004-2B2364B6/planets-solar-system-Pluto-montage-images-Sun.jpg" alt="solar system to scale" height="50" /> </a> <a href="https://cdn.britannica.com/25/126025-050-0FB12805/foreground-Inland-Sea-Seto-Great-Bridge.jpg" data-href="/media/1/573494/120232" class="media-overlay-link d-inline-block mr-5"> <img loading="lazy" src="https://cdn.britannica.com/25/126025-159-60E79F2E/foreground-Inland-Sea-Seto-Great-Bridge.jpg" alt="Inland Sea" height="50" /> </a> <a href="https://cdn.britannica.com/91/118391-050-AB82752B/Photosphere-limb-image-Sun-Solar-and-Heliospheric-Oct-29-2003.jpg" data-href="/media/1/573494/120445" class="media-overlay-link d-inline-block mr-5"> <img loading="lazy" 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Author of <i>Astrophysics of the Sun </i>and others.</div> </a> <div data-popper-arrow></div> </div> <span class="btn btn-link editor-link p-0 qa-byline-link gtm-byline font-12 byline-contributor text-decoration-underline"> Harold Zirin</span>, <div class="editor-popover popover p-0"> <a class="d-block p-20 qa-editor-popup gtm-byline font-12 byline-contributor" href="/contributor/Kenneth-Lang/7799632" > <div class="editor-title font-16 font-weight-bold">Kenneth Lang</div> <div class="editor-description font-12 font-serif mt-5 clamp-description text-black">Professor of astrophysics and radio astronomy, Tufts University. Prof. Lang is a world renowned author of popular and scholarly books in astronomy and astrophysics.</div> </a> <div data-popper-arrow></div> </div> <span class="btn btn-link editor-link p-0 qa-byline-link gtm-byline font-12 byline-contributor text-decoration-underline"> Kenneth Lang</span><span class="text-gray-700 mx-5">•</span><a class="see-all border-gray-700 gtm-byline" rel="nofollow" href="https://www.britannica.com/place/Sun/additional-info#contributors">All</a> </div> <div class="font-serif font-12 text-gray-700"> <span class="qa-fact-checked-by">Fact-checked by</span> <div class="editor-popover popover p-0"> <a class="d-block p-20 qa-editor-popup font-12" href="/editor/The-Editors-of-Encyclopaedia-Britannica/4419" > <div class="editor-title font-16 font-weight-bold">The Editors of Encyclopaedia Britannica</div> <div class="editor-description font-12 font-serif mt-5 text-black">Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree. They write new content and verify and edit content received from contributors.</div> </a> <div data-popper-arrow></div> </div> <span class="btn btn-link editor-link p-0 qa-byline-link font-12 "> The Editors of Encyclopaedia Britannica</span></div> <div class="last-updated font-12 font-serif"> <span class="text-gray-700"> Last Updated: <time datetime="2025-02-24T00:00:00CST" >Feb 24, 2025</time> •</span> <a class="byline-edit-history" href="https://www.britannica.com/place/Sun/additional-info#history" rel="nofollow">Article History</a> </div></div> </div> <button class="d-flex d-lg-none btn btn-outline-blue border rounded-sm shadow-sm mobile-toc-button gtm-mobile-toc-inline-button d-none d-sm-block js-sections-inline-button module-spacing btn d-lg-none"> <em class="material-icons mr-5 ml-n10 my-n5 md-icon" data-icon="toc"></em> Table of Contents </button> <div class="d-flex d-sm-none flex-row"> <button class="d-flex d-lg-none btn btn-outline-blue border rounded-sm shadow-sm 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font-12 js-toggle-recent-news"> <span class="text-gray-500">...</span><span>(Show more)</span> </button> </span> </div> </div> <div class="rest-of-recent-news-items"> <div class="recent-news-item mt-5"> <a class="font-14 gtm-ap-news-link" href="https://www.nasa.gov/news-release/nasa-sets-launch-coverage-for-missions-studying-cosmic-origins-sun/" rel="nofollow">NASA Sets Launch Coverage for Missions Studying Cosmic Origins, Sun</a> <span class="font-14 text-gray-600"> <span>•</span> Feb. 19, 2025, 1:23 AM ET (NASA (.gov)) </span> </div> <div class="recent-news-item mt-5"> <a class="font-14 gtm-ap-news-link" href="https://www.smh.com.au/business/consumer-affairs/sun-sets-over-minimum-payments-for-victorians-with-rooftop-solar-20250215-p5lcdu.html" rel="nofollow">Sun sets over minimum payments for Victorians with rooftop solar</a> <span class="font-14 text-gray-600"> <span>•</span> Feb. 15, 2025, 7:07 AM ET (Sydney Morning Herald) </span> </div> <div class="recent-news-item mt-5"> <a class="font-14 gtm-ap-news-link" href="https://scitechdaily.com/solar-orbiter-just-solved-a-major-mystery-about-the-sun/" rel="nofollow">Solar Orbiter Just Solved a Major Mystery About the Sun</a> <span class="font-14 text-gray-600"> <span>•</span> Feb. 7, 2025, 5:48 AM ET (SciTechDaily) </span> </div> <button class="js-toggle-recent-news d-flex btn btn-unstyled font-14 pr-10 rounded-sm mt-10" aria-label="Toggle additional news items"> Show less <em class="material-icons" data-icon="expand_less"></em> </button> </div> </div><span class="marker before-article"></span><span class="marker h4"></span><section data-level="1" id="ref54141">  <section data-level="2" id="ref54142"> <h2 class="h2"><span id="ref1027838"></span><a href="https://www.britannica.com/science/photosphere" class="md-crosslink " data-show-preview="true">Photosphere</a></h2> <span class="marker PREMOD1 mod-inline"></span><div class="assemblies"><div class="w-100"><figure class="md-assembly m-0 mb-md-0 card card-borderless print-false" data-assembly-id="143499" data-asm-type="video"><div class="md-assembly-wrapper card-media" data-type="video" video-id="139433"><a data-id="139433" class="gtm-assembly-link d-flex justify-content-center" style="--aspect-ratio: 16/9" href="/video/dopplergram-Helioseismic-board-photosphere-material-satellite-line-March-29-2010/-143499"><img src="https://cdn.britannica.com/33/139433-138-75FFDF3D/dopplergram-Helioseismic-board-photosphere-material-satellite-line-March-29-2010.jpg?w=800&h=450&c=crop" alt="Observe a dopplergram showing the velocity of solar material in the photosphere captured by the Helioseismic and Magnetic Imager" loading="lazy"><script type="application/json"> { "sources": [ { "file" : "//content.jwplatform.com/manifests/hwbfOjQt.m3u8" } ], "image": "https://cdn.britannica.com/33/139433-138-75FFDF3D/dopplergram-Helioseismic-board-photosphere-material-satellite-line-March-29-2010.jpg" ,"tracks": [ { "file" : "//assets-jpcust.jwpsrv.com/tracks/R1GmGeve", "label": "English" } ] ,"adfile": "//content.jwplatform.com/manifests/RbtSVNis.m3u8" } </script><div class="btn btn-xl btn-white btn-circle position-absolute shadow" style="top: 50%; transform: translateY(-50%)"><em class="material-icons" data-icon="play_arrow"></em></div></a></div><figcaption class="card-body"><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><span class="md-assembly-title font-weight-bold mr-5 d-inline font-sans-serif md-video-caption" video-control="139433">Observe a dopplergram showing the velocity of solar material in the photosphere captured by the Helioseismic and Magnetic Imager</span><span>A dopplergram taken by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory satellite, showing the velocity of solar material in the photosphere on the line of sight, March 29, 2010. White pixels are moving away from the camera, and black pixels are moving toward the camera.</span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div><a class="font-14 mt-10 d-inline-block" href="/place/Sun/images-videos">See all videos for this article</a></figcaption></figure></div></div><p class="topic-paragraph">Although there are no fires on the surface of the Sun, the <a href="https://www.britannica.com/science/photosphere" class="md-crosslink " data-show-preview="true">photosphere</a> seethes and roils, displaying the effects of the underlying <a href="https://www.britannica.com/science/convection" class="md-crosslink " data-show-preview="true">convection</a>. <a href="https://www.britannica.com/science/photon" class="md-crosslink " data-show-preview="true">Photons</a> flowing from below, trapped by the underlying layers, finally escape. This produces a dramatic drop in <a href="https://www.britannica.com/science/temperature" class="md-crosslink " data-show-preview="true">temperature</a> and <a href="https://www.britannica.com/science/density" class="md-crosslink autoxref " data-show-preview="true">density</a>. The temperature at the visible surface is about 5,800 <a href="https://www.britannica.com/science/kelvin" class="md-crosslink autoxref " data-show-preview="true">K</a> but drops to a minimum about 4,000 K at approximately 500 kilometres above the photosphere. The <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="density" href="https://www.britannica.com/dictionary/density" data-type="EB">density</a>, about 10<sup>−7</sup> gram per cubic centimetre (g/cm<sup>3</sup>), drops a factor of 2.7 every 150 kilometres. The solar atmosphere is actually a <a href="https://www.britannica.com/science/vacuum-physics" class="md-crosslink " data-show-preview="true">vacuum</a> by most standards; the total density above any square centimetre is about 1 gram, about 1,000 times less than the comparable mass in the <a href="https://www.britannica.com/science/atmosphere" class="md-crosslink " data-show-preview="true">atmosphere</a> of <a href="https://www.britannica.com/place/Earth" class="md-crosslink " data-show-preview="true">Earth</a>. One can see through the atmosphere of Earth but not through that of the Sun because the former is shallow, and the <a href="https://www.britannica.com/science/molecule" class="md-crosslink " data-show-preview="true">molecules</a> absorb only <a href="https://www.britannica.com/science/radiation" class="md-crosslink autoxref " data-show-preview="true">radiation</a> that lies outside of the visible spectrum. The hot photosphere of the Sun, by contrast, contains an <a href="https://www.britannica.com/science/ion-physics" class="md-crosslink " data-show-preview="true">ion</a> called negative <a href="https://www.britannica.com/science/hydrogen" class="md-crosslink " data-show-preview="true">hydrogen</a>, H<sup>−</sup>, a hydrogen nucleus with two <a href="https://www.britannica.com/science/electron" class="md-crosslink " data-show-preview="true">electrons</a> attached. The H<sup>−</sup> <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="ion" href="https://www.britannica.com/dictionary/ion" data-type="EB">ion</a> absorbs radiation voraciously through most of the spectrum.</p><span class="marker MOD1 mod-inline"></span> <span class="marker PREMOD2 mod-inline"></span><div class="assemblies"><div class="w-100"><figure class="md-assembly m-0 mb-md-0 card card-borderless print-false" data-assembly-id="14613" data-asm-type="video"><div class="md-assembly-wrapper card-media" data-type="video" video-id="23178"><a data-id="23178" class="gtm-assembly-link d-flex justify-content-center" style="--aspect-ratio: 16/9" href="/video/film-updrafts-granules-dissolution-convection-cells-surface/-14613"><img src="https://cdn.britannica.com/78/23178-138-564B2CDB/film-updrafts-granules-dissolution-convection-cells-surface.jpg?w=800&h=450&c=crop" alt="Watch gas updrafts form convection cells on the Sun's surface before dissolving or exploding" loading="lazy"><script type="application/json"> { "sources": [ { "file" : "//content.jwplatform.com/manifests/VtxXAZlz.m3u8" } ], "image": "https://cdn.britannica.com/78/23178-138-564B2CDB/film-updrafts-granules-dissolution-convection-cells-surface.jpg" ,"tracks": [ { "file" : "//assets-jpcust.jwpsrv.com/tracks/Lxo1nXCW", "label": "English" } ] ,"adfile": "//content.jwplatform.com/manifests/0LuQY5cO.m3u8" } </script><div class="btn btn-xl btn-white btn-circle position-absolute shadow" style="top: 50%; transform: translateY(-50%)"><em class="material-icons" data-icon="play_arrow"></em></div></a></div><figcaption class="card-body"><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><span class="md-assembly-title font-weight-bold mr-5 d-inline font-sans-serif md-video-caption" video-control="23178">Watch gas updrafts form convection cells on the Sun's surface before dissolving or exploding</span><span>This time-lapse film shows the formation and dissolution of granules, updrafts of gas that form convection cells on the surface of the Sun. Each granule is about 1,500 kilometres wide and lives for about 20 minutes before either dissolving or exploding into other granules.</span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div><a class="font-14 mt-10 d-inline-block" href="/place/Sun/images-videos">See all videos for this article</a></figcaption></figure></div></div><p class="topic-paragraph">The photosphere is the portion of the Sun seen in ordinary <a href="https://www.britannica.com/science/light" class="md-crosslink " data-show-preview="true">light</a>. Its image reveals two dominant features, a darkening toward the outermost regions, called <a href="https://www.britannica.com/science/limb-darkening" class="md-crosslink autoxref " data-show-preview="true">limb darkening</a>, and a fine rice-grain-like structure called granulation. The darkening occurs simply because the temperature is falling; when one looks at the edge of the Sun, one sees light from higher, cooler, and darker layers. The granules are convective cells that bring <a href="https://www.britannica.com/science/energy" class="md-crosslink autoxref " data-show-preview="true">energy</a> up from below. Each cell measures about 1,500 kilometres across. Granules have a lifetime of about 25 minutes, during which hot gas rises within them at speeds of about 300 metres per second. They then break up, either by fading out or by exploding into an expanding ring of granules. The granules occur all over the Sun. It is believed that the explosion pattern shapes the surrounding granules in a pattern called mesogranulation, although the existence of that pattern is in <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="dispute" href="https://www.britannica.com/dictionary/dispute" data-type="EB">dispute</a>. A larger, undisputed pattern called supergranulation is a network of outward <a href="https://www.britannica.com/science/velocity" class="md-crosslink autoxref " data-show-preview="true">velocity</a> flows, each about 30,000 kilometres across, which is probably tied to the big convective zone rather than to the relatively small granules. The flow concentrates the surface <a href="https://www.britannica.com/science/magnetic-field" class="md-crosslink " data-show-preview="true">magnetic fields</a> to the supergranulation-cell boundaries, creating a network of magnetic-field elements.</p><span class="marker MOD2 mod-inline"></span> <span class="marker PREMOD3 mod-inline"></span><p class="topic-paragraph">The photospheric magnetic fields extend up into the atmosphere, where the supergranular pattern dominates the conducting <a href="https://www.britannica.com/science/gas-state-of-matter" class="md-crosslink " data-show-preview="true">gas</a>. While the temperature above the average surface areas continues to drop, it does not fall as rapidly as at the network edges, and a picture of the Sun at a wavelength absorbed somewhat above the surface shows the network edges to be bright. This occurs throughout the <a href="https://www.britannica.com/science/ultraviolet-radiation" class="md-crosslink " data-show-preview="true">ultraviolet</a>.</p><span class="marker MOD3 mod-inline"></span> <span class="marker PREMOD4 mod-inline"></span><div class="assemblies"><div class="w-100"><figure class="md-assembly m-0 mb-md-0 card card-borderless print-false" data-assembly-id="143513" data-asm-type="image"><div class="md-assembly-wrapper card-media" data-type="image"><a href="https://cdn.britannica.com/02/96902-050-95C9DAC6/wavelengths-spectrum-lines-Fraunhofer-elements-light.jpg" class="gtm-assembly-link position-relative d-flex align-items-center justify-content-center media-overlay-link card-media" data-href="/media/1/573494/143513"><picture><source media="(min-width: 680px)" srcset="https://cdn.britannica.com/02/96902-050-95C9DAC6/wavelengths-spectrum-lines-Fraunhofer-elements-light.jpg"><img src="https://cdn.britannica.com/02/96902-050-95C9DAC6/wavelengths-spectrum-lines-Fraunhofer-elements-light.jpg?w=300" alt="visible solar spectrum" data-width="1600" data-height="540" loading="eager"></picture><button class="magnifying-glass btn btn-circle position-absolute shadow btn-white top-10 right-10" aria-label="Zoom in"><em class="material-icons link-blue" data-icon="zoom_in"></em></button></a></div><figcaption class="card-body"><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><a class="gtm-assembly-link md-assembly-title font-weight-bold d-inline font-sans-serif mr-5 media-overlay-link" href="https://cdn.britannica.com/02/96902-050-95C9DAC6/wavelengths-spectrum-lines-Fraunhofer-elements-light.jpg" data-href="/media/1/573494/143513">visible solar spectrum</a><span>The visible solar spectrum, with prominent Fraunhofer lines representing wavelengths at which light is absorbed by elements.</span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div></figcaption></figure></div></div><p class="topic-paragraph"><a href="https://www.britannica.com/biography/Joseph-von-Fraunhofer" class="md-crosslink " data-show-preview="true">Fraunhofer</a> was the first to observe the solar spectrum, finding emission in all colours with many dark <a href="https://www.britannica.com/science/Fraunhofer-lines" class="md-crosslink " data-show-preview="true">lines</a> at certain wavelengths. He assigned letters to these lines, by which some are still known, such as the D-lines of <a href="https://www.britannica.com/science/sodium" class="md-crosslink " data-show-preview="true">sodium</a>, the G-band, and the K-lines of ionized <a href="https://www.britannica.com/science/calcium" class="md-crosslink " data-show-preview="true">calcium</a>. But it was the German physicist <a href="https://www.britannica.com/biography/Gustav-Robert-Kirchhoff" class="md-crosslink " data-show-preview="true">Gustav R. Kirchhoff</a> who explained the meaning of the lines, explaining that the dark lines formed in cooler upper layers, absorbing the light <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="emerging" href="https://www.britannica.com/dictionary/emerging" data-type="EB">emerging</a> from below. By comparing these lines with laboratory data, we can identify the elements responsible and their state of <a href="https://www.britannica.com/science/ionization" class="md-crosslink autoxref " data-show-preview="true">ionization</a> and excitation.</p><span class="marker MOD4 mod-inline"></span> <span class="marker PREMOD5 mod-inline"></span><p class="topic-paragraph">The spectral lines seen are those expected to be common at 6,000 K, where the <a href="https://www.britannica.com/science/thermal-energy" class="md-crosslink autoxref " data-show-preview="true">thermal energy</a> of each particle is about 0.5 volt. The most abundant elements, hydrogen and <a href="https://www.britannica.com/science/helium-chemical-element" class="md-crosslink autoxref " data-show-preview="true">helium</a>, are difficult to excite, while atoms such as <a href="https://www.britannica.com/science/iron-chemical-element" class="md-crosslink " data-show-preview="true">iron</a>, <a href="https://www.britannica.com/science/sodium" class="md-crosslink " data-show-preview="true">sodium</a>, and <a href="https://www.britannica.com/science/calcium" class="md-crosslink " data-show-preview="true">calcium</a> have many lines easily excited at this temperature. When Cecilia Payne, a British-born graduate student studying at Harvard College Observatory in <a href="https://www.britannica.com/place/Cambridge-Massachusetts" class="md-crosslink " data-show-preview="true">Cambridge</a>, Massachusetts, U.S., recognized the great abundance of hydrogen and helium in 1925, she was persuaded by her elders to mark the result as spurious; only later was the truth recognized. The strongest lines in the visible spectrum are the H- and K- (Fraunhofer’s letters) lines of ionized calcium. This happens because calcium is easily ionized, and these lines represent transitions in which energy is absorbed by ions in the ground, or lowest energy, state. In the relatively low density of the photosphere and higher up, where atoms are only <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="illuminated" href="https://www.merriam-webster.com/dictionary/illuminated" data-type="MW">illuminated</a> from below, the electrons tend to fall to the ground state, since excitation is low. The sodium D-lines are weaker than Ca K because most of the sodium is ionized and does not absorb radiation.</p><a class="link-module shadow-sm d-block qa-read-more-module" href="/science/climate-meteorology/Solar-radiation-and-temperature#ref149479" data-link-module-iframe-link=""> <img loading="lazy" src="https://cdn.britannica.com/23/204523-138-A67F2633/more-weather-climate.jpg" alt="What's the difference between weather and climate?" class="rounded-sm mr-15" width="70" /> <div class="line-clamp clamp-5"> <div class="module-title bg-navy-dark">More From Britannica</div> <div class="font-weight-semi-bold mt-5">climate: Distribution of radiant energy from the Sun</div> </div> </a><span class="marker MOD5 mod-inline"></span> <span class="marker PREMOD6 mod-inline"></span><p class="topic-paragraph">The intensity of the lines is determined by both the abundance of the particular element and its state of ionization, as well as by the excitation of the <a href="https://www.britannica.com/science/nuclear-energy" class="md-crosslink autoxref " data-show-preview="true">atomic energy</a> level involved in the line. By working backward one can obtain the abundance of most of the elements in the Sun. This set of abundances occurs with great regularity throughout the universe; it is found in such <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="diverse" href="https://www.merriam-webster.com/dictionary/diverse" data-type="MW">diverse</a> objects as <a href="https://www.britannica.com/science/quasar" class="md-crosslink " data-show-preview="true">quasars</a>, <a href="https://www.britannica.com/science/meteorite" class="md-crosslink " data-show-preview="true">meteorites</a>, and new stars. The Sun is roughly 90 percent hydrogen by number of atoms and 9.9 percent helium. The remaining atoms consist of heavier elements, especially <a href="https://www.britannica.com/science/carbon-chemical-element" class="md-crosslink " data-show-preview="true">carbon</a>, <a href="https://www.britannica.com/science/nitrogen" class="md-crosslink " data-show-preview="true">nitrogen</a>, <a href="https://www.britannica.com/science/oxygen" class="md-crosslink " data-show-preview="true">oxygen</a>, <a href="https://www.britannica.com/science/magnesium" class="md-crosslink " data-show-preview="true">magnesium</a>, <a href="https://www.britannica.com/science/silicon" class="md-crosslink " data-show-preview="true">silicon</a>, and iron, making up only 0.1 percent by number.</p><span class="marker MOD6 mod-inline"></span> </section> </section><span class="marker end-of-content"></span><span class="marker after-article"></span></div> <div id="chatbot-simplify-root"></div> <div id="chatbot-root"></div> </div> </div> </div> <div class="ai-dialog-placeholder"></div> </div> </div> <aside class="col-md-da-320"></aside> </div> </div> </div> </div> </article> </div> </div> </div> </div> </main> <div id="md-footer"></div> <noscript><iframe src="//www.googletagmanager.com/ns.html?id=GTM-5W6NC8" height="0" width="0" style="display:none;visibility:hidden"></iframe></noscript>  <script> window.ap3c = window.ap3c || {}; var ap3c = window.ap3c; ap3c.cmd = ap3c.cmd || []; ap3c.cmd.push(function() { ap3c.init('ZO4siT4cLwnykPnzZWJtd3Byb2Q', 'https://engage.email.britannica.com/'); ap3c.track({v: 0}); }); ap3c.activity = function(act) { ap3c.act = (ap3c.act || []); ap3c.act.push(act); }; var s, t; s = document.createElement('script'); s.type = 'text/javascript'; s.src = "https://engage.email.britannica.com/app.js"; t = document.getElementsByTagName('script')[0]; t.parentNode.insertBefore(s, t); </script> <script class="marketing-page-info" type="application/json"> {"pageType":"Topic","templateName":"DESKTOP","pageNumber":4,"pagesTotal":11,"pageId":573494,"pageLength":898,"initialLoad":true,"lastPageOfScroll":false} </script> <script class="marketing-content-info" type="application/json"> [] </script> <script src="https://cdn.britannica.com/mendel-resources/3-133/js/libs/jquery-3.5.0.min.js?v=3.133.36"></script> <script type="text/javascript" data-type="Init Mendel Code Splitting"> (function() { $.ajax({ dataType: 'script', cache: true, url: 'https://cdn.britannica.com/mendel-resources/3-133/dist/topic-page.js?v=3.133.36' }); })(); </script> <script class="analytics-metadata" type="application/json"> {"leg":"D","adLeg":"C","userType":"ANONYMOUS","pageType":"Topic","pageSubtype":null,"articleTemplateType":"PAGINATED","gisted":false,"pageNumber":4,"hasSummarizeButton":false,"hasAskButton":true} </script> <script type="text/javascript"> EBStat={accountId:-1,hostnameOverride:'webstats.eb.com',domain:'www.britannica.com', json:''}; </script> <script type="text/javascript"> ( function() { $.ajax( { dataType: 'script', cache: true, url: '//www.britannica.com/webstats/mendelstats.js?v=1' } ) .done( function() { try {writeStat(null,EBStat);} catch(err){} } ); })(); </script> <div id="bc-fixed-dialogue"></div> </body> </html>

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