Solar core

Key takeaways

Solar core belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Solar core to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Solar core from memory before moving on to harder problems.

Reference excerpt

The solar core is the hot, dense region at the center of the Sun where energy is generated by nuclear fusion. It is considered to extend from the Sun's center to about 0.2 of the solar radius (139,000 km; 86,000 mi). It is the hottest part of the Sun and of the Solar System. It has a density of 150,000 kg/m3 (150 g/cm3) at the center, and a temperature of 15,000,000 K (27,000,000 °F; 15,000,000 °C). The core is made of hot, dense plasma (ions and electrons), at a pressure estimated at 26.5 million gigapascals (3.84×1012 psi) at the center. Due to fusion, the composition of the solar plasma drops from about 70% hydrogen by mass at the outer core, to 34% hydrogen at the center. The core contains 34% of the Sun's mass, but only 3% of the Sun's volume, and it generates 99% of the fusion power of the Sun. There are two distinct reactions in which four hydrogen nuclei may eventually result in one helium nucleus: the proton–proton chain reaction – which is responsible for most of the Sun's released energy – and the CNO cycle.

Composition The composition of the Sun varies with depth. In the photosphere, it is about 73–74% hydrogen by mass, the rest being primarily helium, which is the same composition as the atmosphere of Jupiter, and the primordial composition of gases at the earliest star formation after the Big Bang. However, as depth into the Sun increases, fusion decreases the fraction of hydrogen. Traveling inward, hydrogen mass fraction starts to decrease rapidly after the core radius has been reached (it is still about 70% at a radius equal to 25% of the Sun's radius) and inside this, the hydrogen fraction drops rapidly as the core is traversed, until it reaches a low of about 33% hydrogen, at the Sun's center (radius zero). All but 2% of the remaining plasma mass (i.e. 65%) is helium.

Energy conversion Approximately 3.7×1038 protons (hydrogen nuclei), or roughly 600 million tonnes of hydrogen, are converted into helium nuclei every second, releasing energy at a rate of 3.86×1026 joules per second. The core produces almost all of the Sun's heat via fusion; the rest of the star is heated by the outward transfer of heat from the core. The energy produced by fusion in the core, except a small part carried out by neutrinos, must travel through many successive layers to the solar photosphere before it escapes into space as sunlight, or else as kinetic or thermal energy of massive particles. The energy conversion per unit time (power) of fusion in the core varies with distance from the solar center. At the center of the Sun, fusion power is estimated by models to be about 276.5 watts/m3. Despite its intense temperature, the peak power generating density of the core overall is similar to an active compost heap, and is lower than the power density produced by the metabolism of an adult human. The Sun is much hotter than a compost heap due to the Sun's enormous volume and limited thermal conductivity. The low power outputs occurring inside the fusion core of the Sun may also be surprising, considering the large power which might be predicted by a simple application of the Stefan–Boltzmann law for temperatures of 10–15 million kelvins. However, layers of the Sun are radiating to outer layers only slightly lower in temperature, and it is this difference in radiation powers between layers which determines net power generation and transfer in the solar core. At 19% of the solar radius, near the edge of the core, temperatures are about 10 million kelvins and fusion power density is 6.9 W/m3, which is about 2.5% of the maximum value at the solar center. The density here is about 40 g/cm3, or about 27% of that at the center. Some 91% of the solar energy is produced within this radius. Within 24% of the radius (the outer "core" by some definitions), 99% of the Sun's power is produced. Beyond 30% of the solar radius, where temperature is 7 million K and density has fallen to 10 g/cm3 the rate of fusion is almost nil. There are two distinct reactions in which four hydrogen nuclei may eventually result in one helium nucleus: "proton–proton chain reaction" and the "CNO cycle".

Proton–proton chain reaction

The first reaction in which 4 H nuclei may eventually result in one He nucleus, known as the proton–proton chain reaction, is:

{

1 H +

1 H →

2 D + e + + ν e then

2 D +

1 H →

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Solar core

Start with the simplest possible case. Write down what Solar core claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In astronomy, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Solar core before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Solar core ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Solar core

In research: Solar core appears in astronomy research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Solar core in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Solar core is common in secondary-school and first-year university syllabi. It links to neighbouring topics Structure of the Sun, so understanding it makes those chapters shorter.
In everyday life: Look for Solar core outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Solar core in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Solar core means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Solar core out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Solar core in simple terms?

The solar core is the hot, dense region at the center of the Sun where energy is generated by nuclear fusion. It is considered to extend from the Sun's center to about 0.2 of the solar radius (139,000 km; 86,000 mi).

Why does Solar core matter?

Because it connects several astronomy ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Solar core?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Solar core.