Delta Cephei helps measure cosmic distances

The constellation Cepheus the King is situated in the northern sky near Cassiopeia and Polaris, the north pole star. Within this constellation lies a star known as Delta Cephei, which played a crucial role in determining cosmic distances. Delta Cephei is a pulsating star that exhibits regular brightness changes, doubling in brightness and fading back to minimum every 5.366 days.

These types of stars, called Cepheid variables, are vital for establishing the distance scale of our galaxy and the universe. The star's name is honored in the entire class of supergiant stars called Cepheid variables. In 1912, astronomer Henrietta Swan Leavitt made groundbreaking observations of Cepheid variable stars, which led to the discovery by Edwin Hubble that the Andromeda "nebula" is a galaxy like our own Milky Way.

Cepheid stars have a directly correlated relationship between their periodic brightness changes and their intrinsic brightness, known as the Leavitt law or period-luminosity relationship. This phenomenon occurs due to the stars expanding (becoming brighter) and contracting (becoming fainter) in a regular manner. The light curve of Delta Cephei shows these brightness variations over time, with the star reaching its minimum brightness every 5.366 days.

Astronomers utilize the regularity of Cepheids' brightness changes to measure cosmic distances. By measuring the distance to a nearby Cepheid star using the parallax method and observing its pulsations, which indicate its intrinsic brightness, astronomers can then investigate more distant Cepheids. By comparing the apparent brightness and pulsation rate of these more distant Cepheids, astronomers can calculate their distance using simple calculations.

Therefore, Cepheid variable stars are often referred to as standard candles by astronomers due to their significant role in measuring distances across space.