Astronomy 2e · The Sun: A Garden-Variety Star
The Solar Cycle
On this page 9 sections
In 30 seconds
Look at the Sun through a safely filtered telescope and you will usually see a few dark blemishes: sunspots. Count them over the years, and a pattern emerges — the number rises and falls in a roughly 11-year cycle (actually varying from about 9 to 14 years). At solar maximum, dozens of spots cover the disk; at solar minimum, months can pass with none.
Sunspots are not blemishes on a smooth surface; they are anchors of intense magnetic fields poking through the photosphere. The magnetic field is the whole story: spots come in pairs of opposite polarity, the field flips its global orientation every cycle (full magnetic period 22 years), and spots march from mid-latitudes toward the equator as each cycle ages — the Butterfly diagram Plot of spots migrating from mid-latitudes to the equator Full entry →. The cycle also varies across centuries: during the Maunder minimum ~1645–1715 period with almost no sunspots Full entry → (roughly 1645–1715) sunspots nearly vanished.
This topic explains what sunspots are, how their magnetism is measured, and how the Babcock model Differential rotation + convection = magnetic dynamo Full entry → ties differential rotation and convection into a magnetic dynamo.
Why this matters
The solar cycle is the Sun's heartbeat — its strength controls how much solar activity reaches Earth, so space-weather forecasters (Topic 4) track it to predict flares, CMEs, and their effects on satellites, GPS, and power grids. The cycle also lets astronomers test dynamo theory — how rotating, convecting plasma generates magnetic fields — on the one star we can watch in detail. The Maunder minimum's rough coincidence with the "Little Ice Age" keeps the cycle a live climate-research question (correlation is real; causation is debated). For exams, the 11-year period, butterfly diagram, polarity flip, and Zeeman effect Splitting of spectral lines in a magnetic field Full entry → are classic items.
The college version
Core Concepts
Sunspots: anatomy of a magnetic plug
A Sunspot A dark, cooler, strongly magnetic patch on the photosphere Full entry → looks dark because it is cooler than its surroundings: roughly 3,800 K (commonly cited) against the photosphere's ~5,800 K — still hot enough to glow, but far less bright, so it looks black by contrast. Each spot has:
- Umbra: the dark center; penumbra: the lighter, striated fringe.
- Spots usually appear in pairs of opposite polarity, like the two ends of a bar magnet through the surface.
The fields are enormous: 1,000–4,000 gauss (commonly cited), vs. Earth's ~half gauss.
Counting spots: the 11-year cycle and the butterfly diagram
The standard measure is the sunspot number — a weighted count of individual spots and spot groups. Plotted over time it traces a cycle: a rapid rise to solar maximum, a slower decline to solar minimum, then repeat, averaging ~11 years (range ~9–14).
In 1904, E. W. Maunder showed that plotting spots by latitude and time forms a butterfly diagram: spots appear at mid-latitudes (~30–35°) early in a cycle and migrate toward the equator as it ages.
The 22-year magnetic (Hale) cycle
Sunspots are magnetic, so polarity matters: in a given cycle, leading spots in the north have one polarity and leading spots in the south the opposite; at the next cycle the polarities flip. Because the field needs two 11-year cycles to return to its original configuration, the full magnetic cycle is ~22 years (the Hale cycle The ~22-year full magnetic cycle (two 11-yr cycles) Full entry →), and the Sun's dipole field reverses near each maximum.
Measuring magnetism: the Zeeman effect
How do we know the fields are thousands of gauss? In 1896 Pieter Zeeman showed that a magnetic field splits spectral lines into multiple components — the Zeeman effect. Sunspot spectra show exactly this splitting, and its size measures the field strength. Zeeman measurements are the direct evidence behind everything else in this topic.
The Babcock model: a magnetic dynamo
The leading explanation (H. W. Babcock, 1961) chains the observations together:
- Differential rotation (equator ~25 days, poles ~35 days) drags the poloidal (north–south) field lines around the equator faster than the poles, wrapping them into a toroidal (east–west) field — like winding a rubber band.
- Over several rotations the wrapped field becomes tangled and buoyant; loops ("flux tubes") rise to the surface.
- Where a loop breaks through the photosphere, it makes a sunspot pair — one foot of the loop, one polarity; the other foot, the opposite. Pair orientation encodes the wrapped field's direction.
- As activity peaks, loops reconnect, polarities reverse, and the cycle starts over — convection supplies the energy and turbulence that sustain the dynamo.
The model explains the 11-year rhythm, butterfly migration, and 22-year polarity flip, though the real dynamo is messier — helioseismology (Chapter 16) points to a tachocline at the base of the convective zone where much of the field is generated.
Cycles within cycles: the Maunder minimum and variability
The cycle is not a metronome. Between about 1645 and 1715 — the Maunder minimum — sunspots almost completely disappeared. The coincidence with the coldest part of the "Little Ice Age" is documented, but whether reduced solar output caused the cooling is still debated. Other grand minima (e.g., the Dalton minimum ~1790–1830) and the weak Cycle 24 (peak ~2014) show the dynamo varies on many timescales.
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| Sunspots are holes or missing gas | Cooler, darker regions | Still glowing gas at ~3,800 K — dark only by contrast |
| The cycle is exactly 11 years | An average of ~11 years | Real cycles range from ~9 to 14 years |
| Sunspots cause the cycle | The magnetic dynamo causes both | Spots are a symptom, not the cause |
| 11-year cycle = full magnetic cycle | The 22-year Hale cycle | Polarity flips every 11 years, so the field repeats only every 22 |
| Sunspot number = number of spots | Weighted Wolf number | Groups are weighted to smooth the count |
| The Maunder minimum caused the Little Ice Age | They coincide | Correlation is documented; causation and magnitude are debated |
| Sunspots are permanently fixed features | They drift and evolve | Spots last days to weeks; their latitudes migrate over the cycle |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Sunspots are like the Sun's freckles — dark patches where its magnetic field pokes through. They come and go: lots of freckles for a few years, then almost none, over and over about every 11 years — the Sun's own weather seasons, driven by a giant invisible magnet inside it.
Worked example
Suppose you are handed daily sunspot drawings for a year and asked where the Sun is in its cycle:
- Count. Compute the sunspot number for each month. It is high and still climbing.
- Plot latitudes. Spot groups cluster around 15–25° and drift equatorward.
- Check polarity. In the north, leading spots have positive polarity; in the south, negative — the new cycle's orientation.
- Conclude. Rising numbers + equatorward drift + fresh polarity = the Sun is climbing toward the next maximum — a forecast of more flares and CMEs, and a heads-up for satellite operators and grid managers (Topic 4).
Rubber-band analogy. Wrap a rubber band around a ball while turning only its equator: it twists tighter until kinks pop out. Differential rotation does the same to the Sun's magnetic field — the kinks that pop through the surface are sunspots.
Key takeaways
- Sunspot cycle: ~11 years average (varies ~9–14 yr); full magnetic (Hale) cycle ~22 years.
- Sunspots are cooler (~3,800 K vs 5,800 K) and strongly magnetic (1,000–4,000 G; measured by the Zeeman effect).
- Butterfly diagram: spots start at mid-latitudes (~30–35°) and migrate toward the equator as the cycle progresses.
- Sunspot polarity flips every cycle — leading/trailing spots swap orientation; global dipole reverses near maximum.
- Babcock model: differential rotation wraps the field → buoyant loops rise → sunspot pairs → reconnection resets polarity.
- Maunder minimum (1645–1715): few sunspots; coincides with the Little Ice Age (causation debated).
- Sunspot number = weighted count (Wolf number), not raw spot count.
- Space weather intensity tracks the cycle: more spots → more flares and CMEs (Topics 3–4).
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
What is the average period of the sunspot cycle, and how much can it vary?
Show answer
About 11 years on average, ranging roughly 9–14 years; the full magnetic cycle is ~22 years.
Why do sunspots look dark if they are still glowing gas at thousands of kelvin?
Show answer
They are cooler than the surrounding photosphere (~3,800 K vs ~5,800 K) and therefore emit much less light per unit area — black by contrast, not truly black.
What does the butterfly diagram show, and who first plotted it?
Show answer
It shows sunspot latitudes over time: spots appear at mid-latitudes (~30–35°) early in a cycle and migrate toward the equator as the cycle ages; first plotted by E. W. Maunder in 1904.
What is the Hale cycle, and why is it 22 years rather than 11?
Show answer
The Hale cycle is the ~22-year magnetic cycle: sunspot pair polarities flip every 11-year cycle, so the global field returns to its original configuration only after two cycles.
How does the Zeeman effect let astronomers measure sunspot magnetic fields?
Show answer
A magnetic field splits spectral lines into multiple components; the size of the splitting in sunspot spectra measures the field strength (1,000–4,000 G).
Summarize the Babcock model in three steps.
Show answer
(1) Differential rotation wraps the poloidal field into a toroidal field; (2) twisted, buoyant loops rise to the surface and emerge as sunspot pairs; (3) near maximum, loops reconnect and the field's polarity reverses, starting the next cycle.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Sunspot
- A dark, cooler, strongly magnetic patch on the photosphere
- Umbra / penumbra
- Dark center / lighter striated fringe of a sunspot
- Sunspot number (Wolf number)
- Weighted count of spots and spot groups
- Solar maximum / minimum
- Peak / trough of the ~11-year activity cycle
- Butterfly diagram
- Plot of spots migrating from mid-latitudes to the equator
- Hale cycle
- The ~22-year full magnetic cycle (two 11-yr cycles)
- Zeeman effect
- Splitting of spectral lines in a magnetic field
- Flux tube
- A bundle of magnetic field lines; a sunspot pair is a loop's two feet
- Babcock model
- Differential rotation + convection = magnetic dynamo
- Maunder minimum
- ~1645–1715 period with almost no sunspots
Sources & references
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