Stars
Stars are massive, luminous celestial bodies primarily composed of hydrogen and helium, held together by their own gravity. They undergo a process known as nuclear fusion, where lighter atoms fuse to create heavier ones, producing vast amounts of energy. This energy generation enables stars like our sun to shine for billions of years, although their lifespans can vary significantly. The life cycle of a star often begins in nebulae, where gas clouds collapse under gravity to form protostars, which eventually evolve into main sequence stars. Depending on their mass, stars can transform into different types over time, such as red giants or white dwarfs, and may end their lives in dramatic explosions known as supernovas, potentially leaving behind neutron stars or black holes.
Historically, stars have had profound cultural significance, guiding navigation and influencing ancient religions and mythologies. The study of stars, or astronomy, is one of humanity's oldest sciences, evolving through innovations like the telescope and photography, which expanded our understanding of the universe. Advances in technology, including space telescopes, continue to enhance our knowledge, revealing billions of stars and their intricate properties. Despite these advancements, the sheer number of stars in the universe remains challenging to quantify, suggesting a vast and largely unexplored cosmic landscape.
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Stars
Stars are giant, glowing astronomical objects made of plasma—gas made of ions and electrons. Stars primarily consist of helium and hydrogen, and hold themselves together with gravity. The sun is the star closest to Earth. Many stars are visible in the night sky, but these stars appear much smaller than the sun due to their distance. Stars undergo a process called nuclear fusion throughout their existence, during which they fuse atoms together to produce energy. Some stars have life cycles of billions of years, while others only exist for a few million years. Stars vary in size, color, and luminosity, all of which determine what they become when they have reached the end of their life cycle.
Background
Throughout history, stars have played an important role in the cultures of ancient civilizations. Astronomy, or the study of the motions of the stars and planets, is one of the oldest sciences in human history. Humans calculated observations about stars to navigate their way around the world. The motion of stars also helped people note important days. Stars were also a major component of many ancient religions, and many cultures observed patterns in the stars shaped like humans, animals, and objects. The Greeks named many of these patterns, or constellations, after the heroic figures of Greek mythology. The invention of the telescope in the seventeenth century helped scientists and astronomers better understand the structure of the stars. Telescopic studies led to a better understanding of the motion of stars. Astronomers soon figured out that all stars, including the sun, followed the same laws of physics.
Knowledge of the stars advanced further in the nineteenth century with the advent of photography and spectroscopy. Photography allowed scientists to take pictures of faraway stars and study the quality of their surroundings. Spectrometers were capable of measuring the wavelengths of light that stars emit, giving insight into their molecular composition. These understandings eventually led to the establishment of the field of astrophysics—the study of the origin, development, and physical properties of stellar objects.
The first radio telescope was built in 1937, giving astronomers the ability to detect invisible energy waves and particles emitted from stars. This information further developed human understanding of a star's composition. Over the next few decades, ventures in space exploration led to the invention of space telescopes. The first interstellar optical telescope, the Hubble Space Telescope, was launched in 1990. Hubble provided scientists with detailed images of the visible universe; as it traveled, it observed many more stars. Astronomers named and cataloged each new star they discovered.
In 2013, the European Space Agency launched the Gaia space telescope in an effort to catalog the brightness and position of all the stars in the Milky Way galaxy. Within three years, it had cataloged more than one billion stars, and its 2022 data release included over 1.8 billion cataloged stars. As the telescope's focus only continued to expand, an announcement came in 2023 that Gaia's study of a large star cluster identified around half a million new, fainter stars. Scientists believe the Milky Way galaxy alone contains more than one hundred billion stars, however. Research suggests there are tens of trillions of galaxies in the universe, making the total number of stars in the universe nearly impossible to estimate.
Overview
Stars are primarily made of hydrogen, a gas that circles through space within dust clouds called nebulae. Gravity eventually causes a nebula to shrink until it begins folding into itself. As the clouds grow smaller and narrower, they spin faster, building pressure. This build in pressure increases the core temperature. Once the temperature reaches about 27 million degrees Fahrenheit (15 million degrees Celsius), nuclear fusion occurs, a process that involves lighter atoms such as hydrogen combining together to form heavier atoms. This process produces a great deal of energy as temperatures rise further. At this point, a very young star, or protostar, has formed. Several protostars can emerge from the spinning clouds of a nebula, as the dust is known to split up and separate.
As a protostar continues to produce more energy, it attracts surrounding matter and grows bigger. Once it has accumulated enough mass, a protostar becomes a main sequence star. It takes approximately fifty million years for a protostar to mature into a main sequence star. This type of star exists in a state of nuclear fusion for approximately ten billion years, emitting energy by converting hydrogen to helium. The production of energy creates enough pressure within the star to keep it from collapsing. This energy also gives the star its brightness. The sun in our solar system is a main sequence star.
The lifespan of main sequence stars depends on the intensity of their energy emission. Small stars emit less energy than bigger stars. Main sequence stars also range in brightness and color depending on how much energy they have. Scientists classify stars based on these factors. The smallest and least bright stars are called red dwarfs. Red dwarfs can have as little as 10 percent of the mass of the sun and emit just 0.01 percent of its energy. Red dwarfs make up the bulk of all stars in the universe. The biggest stars in the universe are classified as hypergiants. These stars can be one hundred times bigger than the sun and can get hotter than 50,000 degrees Fahrenheit (27,760 degrees Celsius). Hypergiants emit massive amounts of energy during their lifetime, and only exist for a few million years. Hypergiants once populated vast areas of the universe, but only a few still exist in today's universe.
When all the hydrogen atoms within a star's core have been fused, nuclear fusion stops and the core stops producing energy. The star's core then begins to collapse in on itself and rise in temperature. As the temperature rises, the star's outer layer expands, transforming the star into what is called a red giant. The red giant's core continues to destabilize as the star tries to cool down.
Depending on the size of the star's core, the star could transform into several states. Average-sized stars like the sun eventually shed their outer layers to become white dwarfs, extremely dense stars that cool and fade in brightness. White dwarfs sometimes accumulate so much mass that they explode into extremely bright stars called supernovas. If the star was big enough when it exploded, its supernova can turn into a neutron star, an even denser star composed of a compact mass of neutrally charged particles. The biggest of exploded star cores simply collapse completely to form a black hole, an astronomical object with infinite density that absorbs any matter it encounters. The remnants of old, dead stars eventually give life to new stars by combining with surrounding dust clouds, starting the star life cycle over again.
Bibliography
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