The Sun fuses 600 million tons of hydrogen every second. It has stayed the same size for 4.6 billion years.
Every second, the Sun fuses 600 million metric tons of hydrogen into helium. That releases enormous energy. Yet the Sun neither blows apart nor collapses. Gravity pulls it in, pressure pushes it out, and the two stay equal. This page shows how that balance works and how the energy reaches Earth.
Core Claims
- Balance. A star stays one size when the pull of gravity equals the push of pressure. This is hydrostatic equilibrium.
- Fusion. In the core, hydrogen nuclei join to make helium. This nuclear fusion releases the energy that keeps the pressure up.
- The journey. Energy takes about 100,000 years to leave the Sun. Light then crosses space to Earth in 8 minutes.
- Mass. A star with more mass is brighter and lives a shorter time.
Gravity against pressure
Retrieval Checklist
- Name the two pushes that balance inside a star.
- Say what happens to the Sun if its core cools a little.
- Say how long energy takes to leave the Sun, and how long light takes to reach Earth.
- Say how a star’s mass changes its brightness and its lifetime.
The Sun does not blow apart, and it does not collapse.
The Sun has about 333,000 times the mass of Earth, or 1.989 × 1030 kg. All of that mass pulls inward. Why does gravity not crush the Sun into a black hole?
The core also makes huge amounts of energy. Why does that energy not blow the Sun to pieces?
The answer is hydrostatic equilibrium: two pushes that stay equal.
1. Gravity pulls in
Every part of the Sun pulls on every other part. The pull squeezes the Sun toward its center. In the core, the squeeze raises the pressure to 250 billion atmospheres.
2. Pressure pushes out
The core is at 15,000,000 °C. Hydrogen nuclei fuse there and release energy. The hot gas pushes outward, and that push matches the pull of gravity.
The Sun is its own thermostat. If the core cools, gravity squeezes it and the squeeze heats it again. If the core overheats, it expands and cools. Either way, the Sun returns to the same size.
Thinking the Sun is a bomb that only luck holds together. Fusion pushes out all the time and gravity pulls in all the time. The two stay equal, so the Sun does not explode and does not collapse.
Fusion in the core powers the Sun.
In the Sun’s core, hydrogen nuclei join to make helium nuclei. This is nuclear fusion. Each joining releases energy.
The Sun fuses about 600 million metric tons of hydrogen every second. That energy keeps the core hot, and the hot core keeps the pressure high.
Thinking the Sun burns like a campfire. Burning is a chemical reaction that rearranges atoms. Fusion changes the nuclei themselves, and it releases far more energy for each atom.
EnrichmentThe proton-proton chain and mass defect
Extra depth. The rest of the lesson works without it.
In the Sun, four hydrogen nuclei become one helium-4 nucleus in three steps. This is the proton-proton chain. Each step releases energy.
Mass defect
Weigh the four hydrogen nuclei (protons) and the helium-4 nucleus they make. The helium nucleus is lighter.
- Four separate protons: 4 × 1.007276 u = 4.029104 u
- One helium-4 nucleus: 4.001506 u
- Missing mass, called the mass defect: 0.027598 u (0.71%)
The missing mass is not lost. It becomes energy, as Einstein’s equation says:
The speed of light squared is huge: c2 ≈ 9 × 1016 m2/s2. Turning 1.0 gram of mass into energy releases 90 trillion joules, the same as burning about 3,500 tons of coal. In the Sun, 4.3 million tons of mass become energy every second.
Energy takes 100,000 years to leave the Sun and 8 minutes to reach Earth.
Light made in the core cannot go straight out. The core is 150 times denser than liquid water. Light travels only 0.1 millimeters before it hits a particle and bounces off in a random direction.
1. The inner 70%: a slow zigzag
Light is absorbed and sent out again trillions of times. It zigzags outward for 100,000 to 170,000 years. Each time, it loses a little energy. Gamma rays from the core become visible, infrared and ultraviolet light.
2. The outer 30%, then space
Rising gas carries the energy up, like a pot of boiling oatmeal. At the surface (5,778 K), light escapes into space. It crosses 150 million kilometers to Earth in 8 minutes and 20 seconds.
Thinking the sunlight on your face left the core a few minutes ago. The trip across space takes 8 minutes. The trip out of the Sun takes about 100,000 years.
A star with more mass is brighter and lives a shorter time.
Astronomers measure the mass, brightness and lifetime of many stars. They find a pattern. The more mass a star has, the brighter it is and the shorter it lives. The Sun, at 10 billion years, sits in the middle.
More mass means more weight pressing on the core. The core must be hotter to hold that weight up. A hotter core fuses hydrogen much faster.
Lifetime depends on the fuel and on how fast the star burns it. A star with twice the Sun’s mass has twice the fuel. It is 16 times as bright, so it burns that fuel 16 times as fast.
Thinking a star with more fuel lives longer. A star with more mass burns its fuel so much faster that it lives a shorter time.
Push the Sun out of balance, then change its mass.
Cool or heat the core, release the star, and watch the two pushes. Then change the star’s mass and compare its brightness and lifetime.
Why does the Sun stay the same size for billions of years?
The explanation appears after you predict.
- Gravity: pulls the star inward (arrows from outside)
- Pressure: pushes the star outward (arrows from the star)
- The star, with its core at the center
- The star’s size over time (1.0 is the balanced size)
- Cool the core. Load 12 MK, press Start run, and press Record once the star settles. Look for: gravity is the bigger push at first. The star shrinks to about 0.85 of its size while the core heats to about 18 MK. Then it swells back to 1.00 and 15 MK.
- Heat the core. Load 18 MK, press Start run, and press Record once the star settles. Look for: pressure is the bigger push at first. The star swells to about 1.15 while the core cools to about 13 MK. Then it shrinks back to 1.00 and 15 MK.
- Change the mass. Load 2.0 Suns and press Record. Load 0.5 Suns and press Record. Look for: 2.0 Suns is 16 times as bright and lives 1.25 billion years. 0.5 Suns is 0.06 times as bright and lives 80 billion years.
Use your evidence
Fill the blanks from memory.
Type each blank from memory, then press Check. Press Show answers after your first check if you are stuck.
1. Inside a star, gravity pulls in and pushes out.
2. The two pushes are equal, so the Sun stays the same .
3. In the Sun’s core, hydrogen nuclei join to make helium. This is nuclear .
4. Energy zigzags out of the Sun for about 100,000 .
5. A star with more mass is and lives a shorter time.
A student says: “Fusion makes huge amounts of energy in the Sun’s core. A bomb does the same, and a bomb explodes. The Sun should explode.”
Explain why the Sun does not explode. Use the two pushes.
Say what happens to the core if fusion speeds up. Write your answer first, then compare it with the model answer.
A bomb has nothing holding it together. The Sun has its own gravity. Gravity pulls the gas inward, and the hot gas pushes outward. The two pushes stay equal.
If fusion speeds up, the core gets hotter and pushes harder than gravity pulls. The core expands. Expanding cools the gas and slows fusion. The push falls until it equals gravity again.
Write your answer first. Then grade yourself.
Give yourself 1 mark for each idea you wrote down.
Star Zeta has twice the mass of the Sun. Its core is hotter than the Sun’s core. Star Zeta is 16 times as bright as the Sun. The Sun lives about 10 billion years.
(a) Name the two pushes that balance inside Star Zeta. Say which way each one acts. [1 mark]
(b) Star Zeta’s core cools a little. Say what happens to the star’s size and why it returns to its old size. [1 mark]
(c) Star Zeta has twice the Sun’s fuel. Find how long it lives. Explain why a star with more mass lives a shorter time. [2 marks]
- Two pushes (1 mark): You say gravity pulls inward and pressure from the hot core pushes outward.
- Thermostat (1 mark): You say the star shrinks, the squeeze heats the core, and fusion speeds up again.
- Lifetime (1 mark): You find 10 billion years × 2 ÷ 16 = 1.25 billion years.
- Reason (1 mark): You say Zeta burns fuel 16 times as fast, so extra fuel gives no longer life.
Engineers want to copy the Sun’s fusion on Earth, in magnetic reactors such as ITER and in laser facilities such as the National Ignition Facility. Earth has no crushing gravity, so these reactors must heat deuterium-tritium fuel above 100 million °C, six times hotter than the solar core. If they succeed, fusion could make carbon-free electricity from seawater.
Adaptive Competency Practice: HS-ESS1-1
Test your understanding of the Sun one question at a time. The adaptive engine covers balance in a star, fusion in the core, how energy reaches Earth, and star lifetimes.