No one has drilled into Jupiter or breathed the air of a planet around another star. Yet astronomers know a surprising amount about what planets and their atmospheres are made of, by combining several methods. Density: rock, ice or gas? If you know a planet’s mass (from the orbits of its moons, or how it … Read more
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
The difference comes down to one criterion: whether the object has cleared its orbital neighborhood. This definition was adopted by the International Astronomical Union (IAU) in 2006, and it is why Pluto is no longer counted among the planets. The IAU definition of a planet A planet in our solar system is a body that: … Read more
Spectroscopy means spreading light into its colors, a spectrum, and studying it in detail. It is one of the most powerful tools in astronomy: from a point of light, it can reveal what a star is made of, how hot it is, how fast it moves and much more. Spectral lines: chemical fingerprints Every element … Read more
Astronomy is probably the oldest science. People watched the sky to keep time, plan harvests, navigate and make sense of their place in the world long before writing existed. Ancient skywatchers Greek models Greek thinkers sought geometric explanations. Eratosthenes estimated Earth’s circumference around 240 BCE. Aristarchus proposed that Earth orbits the Sun, but the Earth-centered … Read more
Almost everything we know about the universe comes from light. A telescope’s main job is not to magnify but to collect as much light as possible and focus it into an image. Refractors and reflectors An eyepiece or a camera then magnifies or records the focused image. Aperture is what counts Seeing through the atmosphere … 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
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
