Galaxies did not exist at the very beginning. The early universe was a nearly smooth hot gas. Tiny differences in density, visible today in the cosmic microwave background, grew over hundreds of millions of years into the galaxies we see. Dark matter first According to the standard model of cosmology, dark matter clumped first, because … Read more
The Milky Way is our home galaxy: a barred spiral of hundreds of billions of stars, including the Sun. It is an ordinary galaxy among hundreds of billions in the observable universe, but it is the only one we can study from the inside. Size and structure Where are we? The Sun sits about 26,000 … Read more
A quasar is the extremely bright core of a distant galaxy, powered by a supermassive black hole swallowing gas. A single quasar can outshine all the stars of its host galaxy combined, which makes quasars visible across billions of light-years. How quasars were found In the 1950s, radio astronomers found sources that looked like stars … 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
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
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
A formation describes how ten outfield players are arranged, counted from defense to attack. It is a starting point rather than a fixed shape: teams look different when they have the ball and when they defend. 4-4-2 Two banks of four and two strikers. Simple, balanced and easy to organize defensively, with clear partnerships on … Read more
