The night sky may appear still, but it is filled with stars, planets, galaxies, clouds of gas and dust, and many other objects moving and changing over enormous timescales. Stars form inside large molecular clouds when gravity begins pulling denser regions of material together. As the gas collapses, the center becomes increasingly compressed and hot until conditions are sufficient for nuclear fusion. Hydrogen nuclei begin combining to form helium, releasing energy and creating the outward pressure that allows a star to remain stable against gravity. The lifetime of a star depends largely on its mass. Smaller stars consume their fuel slowly and can remain active for extremely long periods, while massive stars burn through their fuel much more quickly because their cores are hotter and denser. The Sun is one example of a medium-sized star. It formed about 4.6 billion years ago from a collapsing cloud of gas and dust. Most of that material collected at the center and became the Sun, while the remaining material formed a rotating disk. Over time, particles within that disk collided and accumulated, eventually producing the planets, moons, asteroids, comets, and other objects of the Solar System. Beyond our own system, astronomers have discovered thousands of planets orbiting other stars. These exoplanets vary greatly in size, temperature, composition, and distance from their parent stars, showing that planetary systems can take many different forms. Some planets orbit extremely close to their stars, while others travel on much wider paths. Stars themselves also vary enormously. Some are cooler and smaller than the Sun, while others are many times larger, hotter, brighter, and more massive. Their colors provide clues about their temperatures, and their light contains even more information. By separating starlight into different wavelengths, astronomers can study patterns that reveal the chemical elements present in a star's atmosphere. This technique, known as spectroscopy, can also provide information about temperature, motion, rotation, and other physical properties. As stars age, their internal structure changes. Stars similar to the Sun eventually use much of the hydrogen available in their cores and expand into red giants. Later, they can release their outer layers into space and leave behind dense remnants known as white dwarfs. More massive stars follow a different path. They can produce heavier elements through successive stages of fusion until their cores can no longer support the star against gravity. The resulting collapse can trigger a supernova, an extremely energetic explosion that sends material outward into surrounding space. The remaining core may become a neutron star or, if enough mass is present, a black hole. Neutron stars are extraordinarily dense objects, while black holes have gravitational fields so strong that beyond a boundary known as the event horizon, light cannot escape. Astronomers often detect black holes indirectly by observing the motion and radiation of nearby matter. Gas falling toward a black hole can become extremely hot and form a bright accretion disk. Some black holes contain only several times the mass of the Sun, while supermassive black holes at the centers of galaxies can contain millions or billions of solar masses. The Milky Way contains one such object, Sagittarius A*, near its center. The Milky Way itself is a vast spiral galaxy containing hundreds of billions of stars along with gas, dust, stellar remnants, planets, and dark matter. The Solar System is located far from the galactic center within one of the galaxy's smaller spiral structures. Beyond the Milky Way are enormous numbers of other galaxies. Some are spiral-shaped, some are elliptical, and others have irregular forms. Galaxies can pass near one another, interact through gravity, distort each other's structures, and eventually merge. On even larger scales, galaxies form groups and clusters that are distributed across immense filaments and regions often described as the cosmic web. Astronomers study these structures using many different kinds of instruments. Optical telescopes collect visible light, while radio, infrared, ultraviolet, X-ray, and gamma-ray observatories reveal physical processes that cannot be seen with ordinary vision. Gravitational-wave detectors provide another way of studying the universe by measuring tiny distortions in spacetime produced by events such as the mergers of black holes and neutron stars. Distance is one of the major challenges in astronomy. Nearby stars can be measured using parallax, while more distant objects require other techniques based on brightness and other known properties. A light-year, one of the common units used for these distances, is the distance light travels in one year, roughly 9.46 trillion kilometers. The nearest stars beyond the Sun are several light-years away, the Milky Way is roughly one hundred thousand light-years across, and the Andromeda Galaxy is about 2.5 million light-years from Earth. Because light travels at a finite speed, every astronomical observation also records the past. The Sun is seen as it was about eight minutes earlier, while a star one thousand light-years away is seen through light that began its journey about one thousand years ago. Extremely distant galaxies are therefore observed as they existed during much earlier periods of cosmic history. Modern telescopes continue to extend how far and how precisely astronomers can observe. Large ground-based observatories use advanced optical systems to improve image quality, while space telescopes avoid the effects of Earth's atmosphere and can observe wavelengths that are difficult to detect from the ground. Large sky surveys collect enormous datasets containing information about billions of stars and galaxies. Researchers compare these observations with mathematical models and computer simulations to improve our understanding of how stars form, how galaxies develop, how black holes behave, how matter is distributed across space, and how the universe has changed over time. Many questions remain open, including the nature of dark matter, the cause of accelerated cosmic expansion, and the detailed history of the earliest stars and galaxies. Each new observation provides additional measurements that help refine the physical picture of the universe and the structures within it.