Star

A star is a bright, glowing ball of hot gas held together by its own gravity. The Sun is the closest star to Earth and gives us light and warmth. Many other stars can be seen in the night sky, but because they are very far away, they look like tiny points of light. Some of the brightest stars have special names and are grouped into patterns called constellations.

Stars are born when clouds of gas and dust collapse under gravity. They shine because of a process called thermonuclear fusion, where hydrogen atoms combine to form helium and release energy. This energy travels out from the star and gives it its bright light.

Eventually, stars run out of fuel and change into different kinds of objects, such as white dwarfs, neutron stars, or black holes, depending on how big they were. Stars also help create new elements and spread them into space, which allows new stars, planets, and even life to form.

Etymology

The English word star comes from an ancient language root that also means 'star'. This root is linked to words meaning 'to burn', which makes sense since stars give off light. Many other languages have similar words for star, like stella in Latin, aster in Greek, and Stern in German. These shared words show how different cultures have long used similar terms to describe these bright objects in the sky.

Observation history

Stars have played an important role in many cultures around the world. They were used in religious practices, myths, and to help with navigation and tracking the seasons. Ancient astronomers noticed that some stars stayed in the same place while others, called planets, moved across the sky. They grouped stars into patterns called constellations to help track the movement of planets and the Sun.

Early civilizations made star charts and catalogues to record the positions of stars. The Greeks created detailed star lists, and later, astronomers in China and the Islamic world made important observations, including the first recorded supernovae. Over time, astronomers discovered that stars could change in brightness and that some stars moved through the sky. By studying stars, scientists learned about their distances, movements, and even their compositions.

Designations

Long ago, people noticed patterns in the stars they saw in the night sky. These patterns are called constellations, and different cultures told stories and myths about them. Many bright stars have special names, often coming from ancient languages like Arabic or Latin.

Scientists today use organized systems to name stars. One system uses Greek letters, like Alpha or Beta, to label stars within a constellation. Another system numbers stars based on their position in the sky. The International Astronomical Union is the group that decides the official names for stars.

Units of measurement

Stars are measured using special units based on the Sun, called solar units. These help scientists talk about a star’s mass, brightness, and size in a simple way. For example, the solar mass M_☉ is a way to describe how heavy a star is compared to the Sun.

Very large distances, like how far stars are from Earth, are measured using the astronomical unit. This unit is about the distance between the Earth and the Sun, which is around 150 million kilometers or 93 million miles. These units make it easier to share information about stars and their properties.

Formation and evolution

Main article: Stellar evolution

Stars begin their lives in clouds of gas and dust called molecular clouds. These clouds are mostly made of hydrogen, with some helium and a few heavier elements. Over time, parts of these clouds collapse under gravity, forming new stars.

Most stars spend the majority of their lives as main-sequence stars, where they fuse hydrogen into helium in their cores. Smaller stars like our Sun become red giants before shrinking down to white dwarfs. Larger stars go through more dramatic changes, sometimes ending in powerful explosions called supernovae, leaving behind neutron stars or black holes.

Distribution

Stars are grouped into galaxies along with gas and dust. A big galaxy like the Milky Way has hundreds of billions of stars. There are more than 2 trillion galaxies, and overall, there may be between 10²² and 10²⁴ stars — more than all the grains of sand on Earth.

Many stars are found in groups called multi-star systems, where two or more stars orbit each other. The most common is a binary star system, but there are also systems with three or more stars. Larger groups are called star clusters, which can have from a few stars to hundreds of thousands. The nearest star to Earth, besides the Sun, is Proxima Centauri, which is 4.2465 light-years away.

Characteristics

Almost everything about a star is determined by its initial mass, including such characteristics as luminosity, size, evolution, lifespan, and its eventual fate.

Most stars are between 1 billion and 10 billion years old. Some stars may even be close to 13.8 billion years old—the observed age of the universe. The more massive the star, the shorter its lifespan, primarily because massive stars have greater pressure on their cores, causing them to burn hydrogen more rapidly. The most massive stars last an average of a few million years, while stars of minimum mass (red dwarfs) burn their fuel very slowly and can last tens to hundreds of billions of years.

Stars are made mostly of hydrogen and helium, with a small fraction of heavier elements. These heavier elements may indicate whether the star has a planetary system. Stars range in size from tiny neutron stars to huge supergiants like Betelgeuse in the Orion constellation.

Radiation

Stars shine because of energy made by nuclear fusion in their centers. This energy travels into space as both electromagnetic radiation and particle radiation. The particle radiation is called the stellar wind, made of charged particles like protons and beta particles. Stars also send out nearly massless particles called neutrinos.

The light we see from stars comes from this energy. When tiny parts of atoms called atomic nuclei join together, they give off bright light called gamma rays. By the time this light reaches the outside of a star, it becomes the kind of light we can see, like the colors of the rainbow. The color of a star depends on its temperature — hotter stars look blue, and cooler ones look red. Stars also send out other kinds of energy we can't see, like radio waves, infrared, ultraviolet, X-rays, and gamma rays.

The brightness of a star is called its luminosity. It depends on the star’s size and temperature. Some stars, like Vega, shine differently at their poles than at their equators.

We describe how bright a star looks from Earth using something called apparent magnitude. The lower the number, the brighter the star looks. For example, a star with a magnitude of +1 looks about 2.5 times brighter than one with a magnitude of +2. The brightest stars can even have negative numbers. The star Sirius looks very bright to us because it is close, even though it is not the most powerful star.

Classification

Main article: Stellar classification

Stars are grouped into different types based on their temperature and the light they give off. The most common types are labeled from O, the hottest, to M, the coolest. Our Sun is a type G star, which is neither the hottest nor the coolest. Each type has ten sub-groups, making it easier to tell just how hot or cool a star is.

Besides temperature, stars can also be grouped by their size. Some stars are huge, called giants, while others, like our Sun, are called main-sequence stars. There are even tiny, very dense stars called white dwarfs, each with its own special way of giving off light.

Variable stars

Main article: Variable star

Variable stars are stars that change in brightness over time. These changes can happen for different reasons. Some stars, called pulsating variables, expand and contract regularly, causing their brightness to change. Examples include Cepheid and Cepheid-like stars, and long-period variables such as Mira.

Other stars can suddenly become brighter due to explosions or flares. These are called eruptive variables and include stars like Wolf-Rayet stars and flare stars. Some stars also change in brightness because of their orbits around other stars, like the eclipsing binary Algol, which dims when one star passes in front of the other.

Structure

Main article: Stellar structure

Stars are giant balls of hot gas that glow because of the energy they produce inside. The Sun is the star closest to Earth, and many other stars can be seen in the night sky. Inside a star, gravity pulls everything together, but the heat and pressure from nuclear reactions push back, keeping the star stable.

The center, or core, of a star is very hot and busy. Here, tiny particles called atoms fuse together, creating energy that makes the star shine. This energy travels outward through different layers of the star. Some stars have areas where hot material rises and cools, and then sinks back down, creating a swirling motion called convection. The outer layer we can see is called the photosphere, and it’s where things like dark spots, called sunspots, appear. Above this layer is a very hot but thin region called the corona, which we can sometimes see during a solar eclipse.

Nuclear fusion reaction pathways

Main article: Stellar nucleosynthesis

Stars produce energy through nuclear fusion, where small atomic nuclei combine to form larger ones. When nuclei fuse, the mass of the new nucleus is slightly less than the combined mass of the original nuclei. This "lost" mass is turned into energy, which keeps the star glowing brightly.

In our Sun and similar stars, hydrogen atoms fuse to form helium through a process called the proton–proton chain reaction. In bigger stars, a different process called the carbon-nitrogen-oxygen cycle does the same job. Later in a star's life, it can fuse helium into carbon and even create heavier elements, up to iron. Fusion stops at iron because creating heavier elements would actually absorb energy rather than release it.