Parallax is the apparent change in direction of a remote object when the observer's viewpoint changes. The smaller the parallax, the more distant the object. The first successful measurements of stellar parallax came in 1838. Bessel, Henderson and Struve independently measured the parallaxes of the stars 61 Cygni, Alpha Centauri and Vega. A star with a parallax of one arcsecond is at a distance of one parsec, which is 3.26 light-years. A star with a parallax of 0.1 arcsecond is 10 parsecs away. As of November 2026, the record shows that ESA's Gaia spacecraft made more than three trillion observations of two billion stars and other objects between July 2014 and January 2025.
A star is a giant ball of gas, and the Sun is the closest star to Earth. Except for the Sun, stars are so far away that they look like pinpoints of light. A light-year is the distance that light travels in one year, and Proxima Centauri is the nearest star to the Sun. As of November 2026, it is about 4.24 light-years away. Our nearest stellar neighbor, the Alpha Centauri system, includes three stars. The farther away an object in space is, the farther in the past we see it, because light takes time to travel.
An exoplanet is a planet outside our solar system that usually orbits another star, and many stars belong to systems with two or more stars. Binary stars are two stars that orbit each other, held together by gravity. The first planet found around a Sun-like star was discovered in 1995: a hot, star-hugging gas giant believed to be about half the size of Jupiter. The wobble method measures a star's radial velocity. Starlight wavelengths are alternately squeezed and stretched as a star moves slightly closer, then slightly farther from us. When a planet passes in front of its star, the star's light dips by a small amount that telescopes can measure.
In transit spectroscopy, light from a star that travels through the atmosphere of an orbiting planet reaches our telescopes and tells about where it has been. As of November 2026, Proxima Centauri b, a planet of the star Proxima Centauri, is the closest known exoplanet to Earth, about 4 light-years away. Stars are gathered into huge groups called galaxies, and the Sun belongs to a galaxy called the Milky Way. As of November 2026, the Milky Way galaxy contains more than 100 billion stars. As of November 2026, NASA says scientists have confirmed 6,300+ exoplanets in three decades of discovery, out of billions believed to exist.
Words to know
- parallax: apparent shift
- parsec: a distance unit
- exoplanet: far-off planet
- spectroscopy: light study
