author: Mara Krause, 11.08.2025
How was our sun born, how did it develop, and how will it end up?
The sun gives us life, grows plants, spends heat, and is indispensable for life on earth. But it is only one of many stars. Milky Way alone already has about 100-400 billion stars.
At this very moment thousand of stars form, explode and transform.
During the life of a star heavier elements such as carbon, oxygen, and even gold are created. Not only our sun is reason life on earth is possible, but also the lifecycles of other stars. We breathe oxygen, wear gold jewelry, and our bodies are largely made of carbon. Without stars, the universe would contain little more than hydrogen and helium.
Stellar Mass Categories
The end of a star’s life mainly depends on its mass. Stars with smaller mass take a different path than heavy ones. Let’s differentiate between three categories (all compared to the sun’s mass), though the limits are not that strict:
· Low: 0,5-8 sun masses
· Medium: 8-20 sun masses
· Massive: >20-25 sun masses
Important to mention, the mass of the sun is about 2 x 10^30 kg (!). This is about 333,000 times the mass of earth.
Overview
The Beginning: Star Formation
It all starts with a big cloud of nebula: remnants from the Big Bang and light-years in diameter.
The nebula gas (mainly hydrogen) collapses under its own gravity. (Dark matter might play a role here). More and more gas collapses and a protostar arises. Gravity gets stronger, the protostar gets bigger and denser. This raises its temperature and pressure which eventually starts nuclear fusion. And voila, a star is born.
Nuclear fusion sets energy free which causes immense outward pressure. The outward pressure is in balance with the inward pressure form gravity. This stabilises the star.
Once the star is stable and happily fuses hydrogen, the main sequence started.
Main Sequence: Nuclear Fusion
Our sun is currently a main sequence star, fusing hydrogen to helium.
The main sequence is the initial, energy-producing phase and lasts the longest. Now, how exactly do stars set energy free?
The secret is nuclear fusion. During the main sequence fusion has three stages:
1. Two hydrogen atoms fuse to a deuterium atom. Deuterium is the isotope of hydrogen with one proton and one neutron (p+p -> np+energy)
2. Hydrogen and deuterium fuse to a helium nucleus with two protons and one neutron (np+p -> ppn+energy)
3. Two helium-3 nuclei fuse to one helium-4 with two protons and neutrons plus two hydrogen (pnn + ppn -> pnpn + p + p + energy)
All steps release energy in form of photons, the light-particles. Photons are the reason we can see something on earth, have energy, and can live.
The first step, fusing hydrogen to helium, is called beta plus decay: a proton decays into a neutron, a positron and electron-neutrino.
The second and third step fuses nuclei with more than one proton. But which energy is released during this fusion? Where does it come from? According to energy conservation no energy can be produced.
Bounding energy is released. It’s the energy needed to split a nucleus against the strong nuclear force and the energy that is released when a nucleus forms.
For example, two deuterium nuclei have potential energy on their own. But extreme temperature and pressure inside the star gives them enough energy to overcome their potential energy. The two nuclei come so close together that they fuse. The emitted energy might reach you right now.
Okay, so stars like our sun fuse and fuse hydrogen to helium. But what happens when they run out of hydrogen? After all, they cannot have infinitely much.
Star Explosions: Red Giants and Supernova
Eventually, hydrogen fusion stops. Gravity is now stronger than the outward pressure and everything is pulled inward. The star is becoming a red giant.
Two things are happening:
1. The core of the star contracts and heats: helium flash
2. The outer layers of the star expand and drop in temperature: shell burining
The core becomes so hot and dense that helium begins to fuse. It is called helium flash:
· 3 x He -> C
Heavier elements like Carbon are created which generates a great amount of heat. This process corresponds to burning about 10 earth masses of helium per second .
The comforting news are that our sun will probably stay another 5 billion years in the main sequence.
While the core of the star contracts and heats, outer hydrogen shells begin fusion. This so-called shell burning expands the outer layer, causing the temperature to drop and the star to look reddish. Hence its name.
Stars with low mass become red giants while medium to massive stars become red supergiants. Red supergiants are even larger and brighter.
Supernova
The core of red supergiant stars continues fusing heavier and heavier elements up to iron with 26 protons in the nucleus. This is called supernova Nucleosynthesis and is the main source for elements like carbon, oxygen, magnesium, neon, silicon, and iron in the universe and on earth.
When the core builds up heavy iron, fusion stops because creating heavier elements requires more energy than emitted. The outward pressure can no longer hold against gravity of the heavy core.
It collapses. A massive explosion blasts outer layers into space: a supernova type 2.
This massive explosion is so powerful that it creates heavier elements such as gold or silver and pushes them into space.
New stars or planets arise from that ejected matter which means that our gold and silver jewelry originated in ancient supernovae.
„The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of star-stuff.” -Carl Sagan, Cosmos (1980)
End state
Red giant stars become white dwarfs (low mass stars) while supernova stars either turn to a neutron star (medium mass stars) or a black hole (massive stars).
White dwarfs (the Future of our Sun)
The core compresses to and extremely dense white dwarf and outer layers go into space after the red giant phase. They have roughly the mass of the sun but only the radius of the earth, which makes them very dense.
White dwarfs are not stabilised by nuclear fusion, but by electrons. Gravity is so strong that electrons are pressed together and provide enough outward pressure to balance out gravity.
According to the Schools‘Observatory, about 6% of all known stars in our part of the Milky Way are white dwarfs.
The Sun will probably turn to a white dwarf in 7 to 8 billion years. Already during the red giant phase, oceans of earth will be boiled. Mercury and Venus will probably be vaporised.
But will earth be pushed further out? Or will it burn? We don’t know, but either way, things don‘t look great for earth either way. But humanity will likely be long extinct by then.
Neutron star
Supernova stars with medium mass have extremely dense cores with heavy elements and collapse and push outer layers away. Gravity and density are so strong that protons and electrons turn to neutrons via the beta plus decay.
A neutron star is born. They are slightly more massive than our sun but have a radius of only 10-15km. Hence, they have immense pressure.
Black holes
Massive supernova stars also collapse, push outer layers away, and become black holes.
The star has so much matter that constantly more of it is falling in and, presumably, create a point of zero volume and infinite density. This point is called singularity. Our laws of physics break down at this point.
The escape velocity of a black hole exceeds the speed of light. No light can escape; hence it is a black hole.
Black holes have extreme gravity and is a popular object for all kind of science fiction. Its complexity exceeds our ability to understand it. Can it be a clue for more dimensions? Or a way to a different or parallel universe?
Conclusion
Stars have an exciting lifecycle. Explosions, immense temperatures, extreme conditions, and all that fuels us daily.
As Fred Hoyle once said: “The stars are not merely points of light in the sky, they are the crucibles in which the elements were formed.”
In short, this is how their life goes:
Gravity pulls gas and dust together until fusion starts. After a few billion years in the main sequence the star runs out of hydrogen. Depending on ist mass, it enters a red giant phase or red supergiant followed by a supernova. In the end it becomes a white dwarf, a neutron star, or a black hole.
Further
The most interesting question for our further life on earth is probably if we can also win energy through fusion. It would take the world’s population about 800,000 years to use the energy emitted by the sun in one second. However, it is extremely hard to create temperatures over 100 million degrees Celsius. Also, regulating pressure and magnetic forces is not that easy for us. But we keep researching. In fact, massive improvement was made in recent years.
We consist of elements formed in stars. But what about other creatures? Maybe some alien astronomer wears the same gold jewelry form the same supernova, just paused into a different direction.
We know the fascinating stellar objects that arise during star evolutions, but what else is in the universe? Are there even more interesting objects? Objects that challenge our cognitive abilities?
Sources
- Vaia. Stellar Evolution. https://www.vaia.com/en-us/explanations/physics/astrophysics/stellar-evolution
- Space.com. (2012). How Long Will the Sun Burn? https://www.space.com/14732-sun-burns-star-death.html
- Science Focus. (n.d.). Will the Sun Explode? https://www.sciencefocus.com/space/sun-explode-ulimate-fate
- Space.com. (2013). White Dwarf Stars: Facts, Definition & the Fate of the Sun. https://www.space.com/23756-white-dwarf-stars.html
- Encyclopaedia Britannica. Black Hole. https://www.britannica.com/science/black-hole
- Northwestern University. Life Cycle of a Large Star. https://faculty.wcas.northwestern.edu/infocom/The%20Website/large.html
- IAEA. (n.d.). What Is Nuclear Fusion? https://www.iaea.org/newscenter/news/what-is-nuclear-fusion
- YouTube. (n.d.). What Happens to the Sun After It Dies? [PBS Space Time]. https://www.youtube.com/watch?v=quaqEJU7ANc