--- title: "The Sun as a Star: Photosphere, Corona and the Activity Cycle" description: "The nearby Sun can be imaged in fine detail. Granulation traces convection, while sunspots and eruptions trace magnetic activity. Differential rotation helps reshape the field during a variable, rough" canonical: https://lightmysky.com/learn/science/the-sun-as-a-star-photosphere-corona-and-the-activity-cycle-mt_AMIkiU7wRz source: https://lightmysky.com/learn/science/the-sun-as-a-star-photosphere-corona-and-the-activity-cycle-mt_AMIkiU7wRz.md retrieved: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # The Sun as a Star: Photosphere, Corona and the Activity Cycle The nearby Sun can be imaged in fine detail. Granulation traces convection, while sunspots and eruptions trace magnetic activity. Differential rotation helps reshape the field during a variable, roughly eleven-year activity cycle. The sparse corona reaches temperatures around a million kelvin; explaining its heating requires tracing how energy reaches the gas and becomes heat. Subject: Science · Area: Astronomy & Astrophysics · Ages 21 to 22 Page: https://lightmysky.com/learn/science/the-sun-as-a-star-photosphere-corona-and-the-activity-cycle-mt_AMIkiU7wRz ## Ready when they can - Places photosphere, chromosphere and corona by temperature and density and says how each is observed - Ties sunspots, flares and the eleven-year cycle to a magnetic field that differential rotation twists - States why the corona being hotter than the photosphere is a problem and what would count as a solution ## Lesson: The Sun wears a hot crown The Sun's effective temperature is about 5800 K. Its photospheric light has a visible continuum crossed by absorption lines, as in other stars. The photosphere is gas, not a solid surface. Granulation traces convection: hotter gas rises in bright granules and cooler gas sinks between them. Study observatory images. Looking at the Sun can damage your eyes. Never use an unfiltered telescope or ordinary sunglasses to view it. Outward, the photosphere is followed by the chromosphere, then the transition region and corona. Density generally falls outward. Temperature first drops above the photosphere, then rises: parts of the chromosphere reach about 10000 K; the corona is often near 1000000 K. Narrowband visible and ultraviolet images are used to study the chromosphere. Coronagraphs block the bright disc to reveal faint coronal light; X-ray and extreme-ultraviolet images show hot coronal gas. *(drawing: Selected regions in outward order, not a scale drawing. Temperatures vary within regions; the temperature minimum and transition region are omitted.)* The equator rotates faster than higher latitudes: differential rotation. Moving, conducting gas drives a dynamo, converting motion into magnetic energy. Rotation and convection reshape the field. Emerging loops can make spot pairs with opposite polarities. Strong fields inhibit convective heat transport, making spots cooler and darker than nearby gas. Spots are not holes, shadows or explosions. Reconnection changes field connections and can release stored magnetic energy in flares. Sunspot counts rise and fall in an activity cycle averaging about eleven years. Cycle length and strength vary. The typical polarity pattern of spot pairs reverses between successive cycles; the magnetic polarity cycle lasts about twenty-two years. Motions of conducting gas sustain a dynamo, converting motion into magnetic energy. Differential rotation and convection both matter; their role does not fully explain the cycle's timing. Magnetic reconnection changes how parts of a magnetic field connect and can release stored magnetic energy. A flare heats gas and accelerates particles, producing a burst of radiation. A coronal mass ejection (CME) expels plasma and magnetic field into space. Flares and CMEs may occur together or separately. The solar wind is a continuing plasma outflow, distinct from these eruptions. Light from a flare takes about eight minutes to reach Earth. An Earth-directed CME may take days; its speed and path affect the arrival estimate. Arrival time alone does not predict storm severity: the cloud's magnetic field affects its interaction with Earth's magnetic environment. Disturbances can produce auroras and risks to satellites, communications and power grids. The million-kelvin corona needs ongoing heating. Passive heat flow from the cooler photosphere cannot maintain it. Waves can carry energy and lose it as heat. Reconnection can release stored magnetic energy; small impulsive heating events are called nanoflares. A proposed process must deposit enough heat to balance radiation and other losses and match the observed heating pattern. The relative contributions of different processes remain under study. **Recap.** The photosphere supplies visible light. Magnetic activity varies over a roughly eleven-year cycle. The hotter corona needs energy delivery and heating beyond passive heat flow. *Lesson, questions and flashcards written for this dataset. OpenStax Astronomy was consulted for astronomy facts; its prose is not reproduced.* ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Energy Transport and Nuclear Burning in Stars](https://lightmysky.com/learn/science/energy-transport-and-nuclear-burning-in-stars-mt_2rc4UAMyl-) - [Stellar Spectra, Spectral Classes and Surface Composition](https://lightmysky.com/learn/science/stellar-spectra-spectral-classes-and-surface-composition-mt_4FBh5G7Gdc)