Star

A star is a bright, glowing ball of hot gas held together by 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 come together. They shine because of a process called thermonuclear fusion, where hydrogen atoms combine to form helium and give off energy. This energy makes the star bright.

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 word star comes from an old language that also means 'star'. This word is connected to words meaning 'to burn', because stars shine with light. Many languages have similar words for star, like stella in Latin, aster in Greek, and Stern in German. These words show that many cultures have used similar names for these bright lights in the sky.

Observation history

Stars have been important in many cultures. People used them in stories, for navigation, and to know the seasons. Ancient astronomers saw that some stars stayed still while others, called planets, moved. They made groups of stars called constellations to help track the planets and the Sun.

Early people made maps of the stars. The Greeks made lists of stars, and later, astronomers in China and the Islamic world made important discoveries, including the first records of supernovae. Over time, scientists found that stars could change in brightness and move across the sky. By studying stars, they learned about how far away stars are, how they move, and what they are made of.

Designations

Long ago, people saw patterns in the stars and called them constellations. Different cultures told stories about these patterns. Many bright stars have special names from old languages like Arabic or Latin.

Today, scientists have systems to name stars. One system uses Greek letters, like Alpha or Beta, for stars in a constellation. Another system numbers stars by where they appear in the sky. The International Astronomical Union 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 more easily. 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 start in clouds of gas and dust called molecular clouds. These clouds are mostly hydrogen, with some helium and a few heavier elements. Over time, parts of these clouds come together because of gravity, forming new stars.

Most stars spend most of their time as main-sequence stars. During this time, they change hydrogen into helium in their centers. Smaller stars like our Sun become red giants before becoming small, dim stars called white dwarfs. Larger stars change in more dramatic ways. Sometimes they explode, leaving behind special objects called 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 many galaxies, and there may be a lot of 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. The nearest star to Earth, besides the Sun, is Proxima Centauri.

Characteristics

Almost everything about a star is decided by how much it weighs when it forms. This includes how bright it is, how big it is, how it changes over time, how long it lives, and what happens to it in the end.

Most stars are between 1 billion and 10 billion years old. Some stars may be almost as old as the universe itself, which is about 13.8 billion years old. Bigger stars live shorter lives because they burn their fuel faster. The biggest stars live only a few million years, while the smallest stars, called red dwarfs, can last for tens to hundreds of billions of years.

Stars are mostly made of hydrogen and helium, with a little bit of heavier elements. These heavier elements can tell us if the star has planets. Stars come in many different sizes, from very small neutron stars to very big 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 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 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. 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 in the middle—not the hottest and not the coolest. Each type has ten sub-groups, making it easier to tell 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 how bright they look over time. These changes happen for many reasons.

Some stars, called pulsating variables, get bigger and smaller in a regular pattern. This makes their brightness go up and down. Examples include Cepheid and Cepheid-like stars, and long-period variables such as Mira.

Other stars can suddenly get much brighter because of big 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 they orbit around another star. When one star moves in front of the other, the pair looks dimmer. An example is the eclipsing binary Algol.

Structure

Main article: Stellar structure

Stars are huge balls of hot gas that shine because of the energy they make 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 steady.

The center, or core, of a star is very hot and full of activity. Here, tiny particles called atoms join together, creating energy that makes the star glow. This energy moves outward through different layers of the star. Some stars have places where hot material moves up, cools, and then sinks back down, making 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, show up. Above this layer is a very hot but thin area called the corona, which we can sometimes see during a solar eclipse.

Nuclear fusion reaction pathways

Main article: Stellar nucleosynthesis

Stars make energy through nuclear fusion. This is when tiny parts of atoms called nuclei join together to make bigger nuclei. When this happens, the new nucleus has a little less mass than the original ones. The "lost" mass becomes energy. This is what makes the star shine brightly.

In our Sun and stars like it, hydrogen atoms join to make helium. This process is called the proton–proton chain reaction. In bigger stars, another process called the carbon-nitrogen-oxygen cycle does the same thing. As a star gets older, it can turn helium into carbon and even make heavier elements, up to iron. Fusion stops at iron because making elements heavier than iron would take energy instead of giving it out.