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
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 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
