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 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
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
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
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
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
