Stars are giant balls of hot, glowing gas that create their own light and heat through nuclear reactions.

Key idea: Stars are born from collapsing gas clouds, powered by nuclear fusion of hydrogen into helium for most of their lives, and their final form depends on their initial mass.

A star's journey begins when a large cloud of gas and dust in space, called a nebula, starts to collapse under its own gravity. This cloud is mostly made of hydrogen and helium. The total amount of material in this cloud, or its mass, is the most important factor that decides how the star will live its life and what it will become in the end.

For most of its life, a star shines because it is performing a process called thermonuclear fusion in its center, or core. This is like a tiny nuclear reaction where hydrogen atoms combine to form helium, releasing a huge amount of energy. This energy then travels through the star and radiates out into space, making the star glow.

When a star runs out of hydrogen fuel for fusion, it reaches the end of its active life. Its core then becomes a leftover, called a stellar remnant. This remnant can be a white dwarf, a neutron star, or, if the star was very massive, a black hole. Almost all the elements in the universe heavier than lithium, including the ones that make up our bodies and our planet, were created inside stars or during their explosive deaths.

“Almost all the elements in the universe heavier than lithium, including the ones that make up our bodies and our planet, were created inside stars or during their explosive deaths.”

Key idea: Stars have been vital for navigation, timekeeping, and cultural practices throughout human history, and early observations laid the groundwork for modern astronomy.

People have looked at stars for thousands of years. Ancient civilizations used them to tell time, navigate, and create calendars. They noticed that some stars seemed to stay in the same place, which they called "fixed stars," while others moved, which they called "wandering stars" (these were actually planets).

Early astronomers grouped stars into constellations. This helped them track the movements of planets and understand the seasons. For example, the calendar we use today, the Gregorian calendar, is based on Earth's position relative to our Sun.

For a long time, many people thought stars never changed. However, Chinese astronomers were the first to record seeing new stars appear in the sky, which we now know were supernovae, or exploding stars. One of the brightest events ever recorded was a supernova in 1006. Medieval Islamic astronomers also made big contributions, naming many stars we still use today and building early observatories.

Key idea: Astronomers study a star's light and movement to determine its properties, and many stars exist in multi-star systems or larger groups like galaxies.

Scientists can learn a lot about a star by observing its light. They look at how bright it appears, the colors in its light (its spectrum), and how its position in the sky changes over time. These observations help them figure out things like a star's mass, age, what it is made of, how far away it is, and how it is moving.

Stars do not always exist alone. Many stars are part of orbital systems, meaning they orbit around each other or have planets orbiting them, just like our Sun has planets. When two stars are very close, their gravity can strongly affect each other's lives. Stars also often gather in much larger groups called star clusters and galaxies.

“Astronomers can determine stellar properties by carrying out observations of a star's apparent brightness, spectrum, and changes in its position in the sky over time.”

Key idea: Stars spend most of their lives on the main sequence, fusing hydrogen, with their lifespan determined by their mass and fuel consumption rate.

The life of a star is mostly spent on the "main sequence." During this time, it is steadily fusing hydrogen into helium in its core. Our Sun is currently a main sequence star. As a star ages on the main sequence, the amount of helium in its core slowly increases, causing it to become a little hotter and brighter. For example, the Sun has become about 40 percent brighter since it first reached the main sequence 4.6 billion years ago.

Stars also lose a tiny bit of their material into space through a "stellar wind." For most stars, like our Sun, this loss is very small. However, very massive stars can lose a lot of their mass this way, which affects how they evolve.

How long a star stays on the main sequence depends on how much fuel it has and how fast it burns it. Massive stars burn through their fuel very quickly and live for only a few million years. Smaller stars, like red dwarfs, burn their fuel very slowly and can live for trillions of years. This means that no red dwarf has ever reached the end of its life yet, because the universe is not old enough!

Key idea: Stars evolve off the main sequence, with average stars becoming red giants and shedding material, while massive stars undergo a series of fusion stages before exploding as supernovae.

After a star like our Sun uses up the hydrogen in its core, it begins to change. It starts fusing hydrogen in a shell around its core, and its outer layers expand and cool down a lot. This turns it into a red giant. As it expands, it can shed some of its outer material, which is enriched with heavier elements, back into space. This material can then be used to form new stars and planets.

Our Sun, in about 5 billion years, will become a red giant. It will expand so much that it will reach about the orbit of Earth, becoming 250 times bigger than it is now, and it will lose about 30 percent of its current mass.

For very massive stars, their lives end in a much more dramatic way. After they run out of helium fuel, they continue to fuse heavier and heavier elements in their core, like carbon, neon, oxygen, and silicon. This process creates layers inside the star, like an onion, with different elements fusing in each layer. The fusion process stops when the star tries to fuse iron, because fusing iron does not release energy. At this point, the core collapses very quickly, leading to a massive explosion called a supernova.

Key idea: The final stage of a star's life depends on its mass, leading to white dwarfs, neutron stars, or black holes, and enriching the universe with new elements.

What happens after a star's core collapses depends on its original mass. If the leftover core is less than about 1.4 times the mass of our Sun, it shrinks into a very dense, Earth sized object called a white dwarf. White dwarfs slowly cool down over billions of years, eventually becoming black dwarfs.

If the core of a massive star is larger than 1.4 times the Sun's mass after a supernova, it collapses even further. The intense pressure forces electrons and protons to combine, forming neutrons. This creates an incredibly dense object called a neutron star. These stars are so dense that a sugar cube sized piece would weigh billions of tons.

For the very largest stars, if the core remaining after the supernova is more than about 4 times the Sun's mass, it collapses completely to form a black hole. A black hole has such strong gravity that nothing, not even light, can escape from it. The heavy elements created in supernovae are then scattered into space, becoming the building blocks for new stars, planets, and even life.