Gravitational waves are ripples in the fabric of space-time, produced when massive objects accelerate. Einstein predicted them in 1916 but thought they would be too weak to detect. A century later, he was proven right on the first point and wrong on the second. The first detection On September 14, 2015, the two LIGO detectors … Read more
According to Einstein’s general relativity, mass curves space, so light passing near a massive object follows a bent path. A galaxy or cluster can act like a giant, imperfect lens that magnifies and distorts the light of objects behind it. A famous confirmation During the total solar eclipse of 1919, expeditions led by Arthur Eddington … 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
How the universe will end depends mainly on dark energy, the mysterious component that is making the expansion of space speed up. Nobody knows for sure, but current observations point toward a long, cold fade. The discovery that changed everything In 1998, two teams studying distant Type Ia supernovae found that they were dimmer than … Read more
Dark matter is matter that does not emit, absorb or reflect light, yet its gravity shapes galaxies and the whole universe. Ordinary matter, everything made of atoms, is only about 5 percent of the universe’s total content. Dark matter accounts for roughly 27 percent, and dark energy the remaining 68 percent or so. The evidence … Read more
The Big Bang theory says the universe began about 13.8 billion years ago in an extremely hot, dense state and has been expanding and cooling ever since. It does not describe an explosion into empty space: space itself is expanding. Several independent lines of evidence support it. 1. The universe is expanding In the late … 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
A galaxy is a huge system of stars, gas, dust and dark matter held together by gravity. In the 1920s Edwin Hubble showed that many fuzzy “nebulae” were galaxies far beyond the Milky Way, and in 1926 he sorted them by shape. His “tuning fork” scheme, refined since, is still used. Spiral galaxies A flat, … Read more
