Star

A star is a luminous spheroid of plasma held together by self-gravity.

This means the star is a glowing, rounded object made of superheated gas. Gravity acts as a powerful inward force that keeps the plasma from flying apart. Our Sun is the closest star to Earth. While many other stars are visible at night, they look like tiny, fixed points of light. This appearance is due to their immense distances from our planet. The observable universe is estimated to contain a vast number of stars. However, only about 4,000 stars are visible to the naked eye. All of these visible stars are located within our own Milky Way galaxy.

The life of a star begins within a gaseous nebula. A nebula is a massive cloud of material consisting mostly of hydrogen and helium. Traces of heavier elements are also present in these clouds. A star forms when gravity causes this nebula to undergo a gravitational collapse. Once the star is established, it shines through thermonuclear fusion. This process occurs in the star's core, where hydrogen atoms fuse to create helium.

This nuclear reaction releases incredible amounts of energy. This energy travels through the star's interior and eventually radiates into outer space. A star's total mass is the most important factor in its life. The mass determines how the star evolves and what its eventual fate will be.

When a star reaches the end of its life, fusion finally ceases. The core then transforms into a stellar remnant. The type of remnant depends on the star's original mass. Smaller stars may become white dwarfs. More massive stars can become neutron stars. If a star is sufficiently massive, it may collapse into a black hole.

During these late stages, stars also participate in stellar nucleosynthesis. This is the process of creating new chemical elements. Stars or their remnants create almost all naturally occurring elements heavier than lithium. When stars die in supernova explosions, they return this enriched material to the interstellar medium. This recycled material then becomes the building blocks for new stars.

Humans have studied the stars for thousands of years. Ancient Egyptians created an accurately dated star chart in 1534 BC. The Babylonians compiled some of the earliest known star catalogues during the late 2nd millennium BC. In the 2nd century BC, the astronomer Hipparchus created a catalogue containing 1,020 stars. He is also credited with discovering the first recorded nova, which is a "new star."

During the medieval period, Islamic astronomers gave many stars their Arabic names. They also built the first large observatory research institutes. In the 17th century, Isaac Newton suggested that stars were distributed in every direction. This helped explain why their gravity did not pull our Solar System in one direction. Later, William Herschel discovered that some stars are physical companions in binary star systems.

Modern astronomers use several methods to determine stellar properties. They observe a star's apparent brightness and its motion through space. They also study the spectrum, which is the pattern of light colors emitted by a star.

By analyzing these spectra, scientists can find a star's metallicity, or its chemical composition. In 1838, Friedrich Bessel made the first direct measurement of a star's distance. He used a technique called parallax to measure the star 61 Cygni. In 1925, Cecilia Payne-Gaposchkin made a major discovery regarding stellar composition. She proposed that stars are made primarily of hydrogen and helium. This work helped advance the field of astrophysics significantly.

Stars rarely exist in total isolation. They often form orbital systems with other astronomical objects. Some stars exist in planetary systems, much like our own. Other stars form binary star systems where two stars orbit each other.

The gravitational interaction between two close stars can change how they evolve. On a larger scale, stars form part of gravitationally bound structures. These include star clusters and massive galaxies. Even in very distant galaxies, we can observe specific types of stars. For example, astronomers have used gravitational lensing to see a single star named Icarus 9 billion light-years away.

Understanding stars helps us understand the history of the universe. The study of stars connects to many different scientific fields. It links chemistry through the creation of elements to physics through the study of gravity and fusion.

By studying how stars change, we learn how galaxies grow and change over time. The patterns of stars, known as constellations and asterisms, have also helped humans throughout history. They were used for navigation, to mark the seasons, and to create calendars. From the ancient Babylonian period to modern space telescopes, our connection to the stars continues to grow.