Many stars do not live alone. A binary star is a pair of stars bound by gravity, orbiting a common center of mass. Roughly half of Sun-like stars have at least one companion, and the share is higher for massive stars. Types by how we detect them Visual binaries: both stars can be seen separately … Read more
A neutron star is what remains when a massive star’s core collapses in a supernova. It packs more mass than the Sun into a sphere only about 20 kilometers (12 miles) across, roughly the size of a city. How extreme are they? The core is thought to be made mostly of neutrons, though what happens … Read more
Stars are usually born in groups. A star cluster is a family of stars that formed from the same cloud at about the same time, so they share a similar age and chemical makeup. This makes clusters ideal laboratories for studying how stars evolve. Open clusters Famous examples: the Pleiades (the Seven Sisters), visible to … Read more
Polaris, the North Star, is not the brightest star in the sky. It ranks only around 50th. Its importance comes from its position: it lies almost exactly above Earth’s North Pole, so it barely moves while the rest of the sky turns around it. Why it seems fixed Earth rotates around an axis. Extend that … Read more
An exoplanet is a planet orbiting a star other than the Sun. The first ones were confirmed in the 1990s, and the NASA Exoplanet Archive now lists more than 6,000. Most are never seen directly: they are detected through their effects on their star. The transit method When a planet passes in front of its … Read more
A supernova is the explosion of a star, briefly bright enough to outshine an entire galaxy. These explosions forge and scatter many of the elements in our bodies and planets, and leave behind neutron stars and black holes. Two main kinds Core-collapse supernovae (Types II, Ib and Ic) happen when a massive star, more than … Read more
Every star is born in a cloud of gas and dust and spends its life in a balance between gravity, which pulls inward, and the pressure of nuclear fusion pushing outward. Its mass at birth decides how long it lives and how it dies. Birth in molecular clouds Stars form in cold, dense clouds of … Read more
A nebula is a cloud of gas and dust in space. Some are nurseries where new stars form, others are the remains of dying or exploded stars. They are classified mainly by how they shine, or fail to. Emission nebulae Hot, young stars flood nearby hydrogen gas with ultraviolet light. The gas is ionized and … Read more
Astronomers have not yet found a true “Earth twin”, a rocky planet of Earth’s size orbiting a Sun-like star at the right distance and with signs of life. But they have found many planets that share some of Earth’s key features, and statistics suggest that such worlds are common. What “Earth-like” means Scientists usually look … Read more
A black hole is a region where gravity is so strong that nothing, not even light, can escape once it crosses the boundary called the event horizon. Black holes are not cosmic vacuum cleaners: from a distance, their gravity acts like that of any object with the same mass. Stellar black holes: from dying stars … Read more
