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
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
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
You cannot run a tape measure to a star. Astronomers instead use a chain of methods, the cosmic distance ladder, where each step calibrates the next, reaching from nearby planets to the edge of the observable universe. Units Step 1: radar in the solar system Bouncing radar off Venus and other planets and timing the … Read more
Stars look like identical points of light, but they differ enormously in temperature, size, brightness and lifespan. Astronomers sort them with two simple tools: a letter for temperature and a Roman numeral for size. Spectral classes: sorting by temperature The Harvard system, developed in the early 1900s by Annie Jump Cannon and colleagues, groups stars … Read more
