How a star is born and shines
Follow a cold gas cloud as gravity turns it into a star, see what fusion really is and why it is not burning, and learn the balance that keeps the Sun steady for billions of years.
From cold cloud to star
- A cold, dark cloud
Between the stars drift huge clouds of gas, mostly hydrogen, at around −260 °C. They are called nebulae. A big one holds enough gas for thousands of stars.
- Gravity gets a grip
As you saw last lesson, a slightly denser clump pulls in more gas and grows. Often a nudge helps, such as the blast from a nearby dying star. The clump starts to collapse.
- Squeezing makes heat
Gas heats up when you compress it; a bicycle pump gets warm for the same reason. As the clump falls inward, its centre gets hotter and denser. It begins to glow. This is a protostar, and it shines only from the heat of its own collapse.
- Ignition
After some millions of years the centre passes roughly 10 million °C. Hydrogen nuclei now hit each other hard enough to fuse. A new energy source switches on, the collapse stops, and a star is born.
Check yourself
Parisa reads that a protostar glows for a long time before any fusion starts. She asks where that heat comes from. What is the best answer?
- Fusion runs weakly from the very first moment of collapse
- Light from neighbouring stars warms the cloud
- Gravity squeezes the gas, and compressed gas heats up
- The gas rubs against empty space as it falls
Show the answer
Gravity squeezes the gas, and compressed gas heats up
Right. Falling inward turns gravitational energy into heat, just as pumping a tyre warms the pump. Fusion only begins once this squeezing has pushed the centre past about 10 million °C.
THE ENGINE
Fusion: mass turned into energy
In the Sun's core, at about 15 million °C, hydrogen nuclei slam together and, through a chain of steps, four of them become one helium nucleus. Here is the key fact: that helium nucleus weighs about 0.7% less than the four hydrogens that made it. The missing mass has not gone anywhere as matter. It has become energy, following Einstein's E = mc². Because c, the speed of light, is a huge number, a tiny bit of mass is an enormous amount of energy.
The Sun turns about 4 million tonnes of its mass into light every second. It is so massive that after 4.6 billion years of this it has lost well under a thousandth of itself.
fuse 1 kg of hydrogen into helium
helium produced = 993 g
mass that 'disappears' = 7 g (0.7%)
energy from those 7 g, compared with burning coal:about the same as burning 20,000 tonnes of coalBurning only rearranges atoms into new molecules. Fusion changes the nuclei themselves, and releases millions of times more energy per kilogram. This is why the Sun can last billions of years instead of thousands.
Check yourself
The Sun is essentially a very large fire: a ball of gas burning in space.
Show the answer
False
False. Fire is a chemical reaction that needs oxygen and only reshuffles atoms. The Sun runs on nuclear fusion: hydrogen nuclei merge into helium and 0.7% of their mass becomes energy. A Sun made of fire would have gone out within a few thousand years.
THE BALANCE
A tug of war that nobody wins
A star is a fight between two things. Gravity pulls all of its gas inward. Pressure from the hot gas in the core pushes outward. While they match, the star keeps a steady size. The clever part is that this balance repairs itself. If fusion slows, the core cools, pressure drops and gravity squeezes the core. Squeezing heats it, and fusion speeds back up.
This built-in thermostat is why the Sun's output has stayed steady enough for life on Earth, changing only slowly over billions of years.
A balloon with no skin
In a balloon, the stretched rubber squeezes in and the air inside pushes out, and the balloon holds its size where the two match. A star is similar, with gravity playing the rubber. Where the analogy breaks: a balloon can sit there for free. A star leaks energy into space as light every second, so it must keep making heat in its core to hold the pressure up. When the fuel runs out, the balance is lost. That is the next lesson.
Check yourself
Suppose fusion in a star's core briefly runs a little too fast. Using the balance you just learned, what happens next?
- The extra heat feeds on itself and the star blows up like a bomb
- The core heats and expands, which cools it and slows fusion back down
- Nothing changes, because the fusion rate is fixed
- Gravity immediately wins and the star collapses
Show the answer
The core heats and expands, which cools it and slows fusion back down
Yes. Extra heat means extra pressure, so the core swells slightly. An expanded core is cooler and less dense, so fusion eases off. The star settles back to where it was.
Mass decides how long a star lives
Small stars
A red dwarf with a tenth of the Sun's mass has weak gravity, so its core is only just hot enough for fusion. It is cool, dim and red, and it sips its fuel. Such stars can last for trillions of years, far longer than the universe's present age of 13.8 billion years. None has ever died of old age.
Massive stars
A star of 10 Suns has crushing gravity, so its core must be far hotter to hold it up. Fusion runs wildly faster and the star blazes blue-white. It is gone in about 20 million years. The Sun sits in between, with a total life of about 10 billion years. It is roughly halfway.
Check yourself
- Star A: 1 Sun of mass, shines as brightly as 1 Sun, lives about 10 billion years.
- Star B: 10 Suns of mass, shines as brightly as roughly 5,000 Suns, lives about 20 million years.
Dara expected star B to last 10 times longer, since it has 10 times the fuel. What did he miss?
- Big stars contain fuel that cannot be fused
- It has 10 times the fuel but spends it thousands of times faster
- Big stars are older, so they have less time left
- Brightness has nothing to do with fuel use
Show the answer
It has 10 times the fuel but spends it thousands of times faster
Right. Brightness is the rate of spending. Ten times the fuel, spent about 5,000 times faster, lasts roughly one five-hundredth as long: 10 billion years becomes about 20 million.
Check yourself
Match each object to its fate as a hydrogen-fusing star
Show the answer
- A clump with 5% of the Sun's mass → Never sustains hydrogen fusion
- A red dwarf, 10% of the Sun's mass → Shines for trillions of years
- The Sun → Shines for about 10 billion years
- A blue-white star of 10 Suns → Shines for about 20 million years
Lesson recap
- Stars form when gravity collapses a clump of cold gas; squeezing heats the centre until fusion starts at around 10 million °C.
- Fusion turns four hydrogen nuclei into one helium nucleus. About 0.7% of the mass becomes energy, millions of times more per kilogram than burning.
- A star is a balance between gravity pulling in and hot-gas pressure pushing out, and the balance corrects itself like a thermostat.
- More massive stars use their fuel far faster and die far sooner: trillions of years for red dwarfs, 10 billion for the Sun, tens of millions for giants.