Why are Cepheids called the “beacons” of the universe

(ORDO NEWS) — Cepheids are a class of pulsating variable stars, the prototype of which was the double star Delta Cephei, 887 light-years away from Earth.

Cepheids are often referred to as the “beacons” of the universe, as they are a natural tool for measuring distances in outer space.

How do astronomers use Cepheids to measure distances?

The simple answer is that the periods of change in the brightness of Cepheids are closely related to their luminosity.

This striking property of Cepheids was discovered by the American astronomer Henrietta Swan Leavitt (July 4, 1868 – December 12, 1921) more than a century ago.

Thus, astronomers can predict the absolute magnitude (that is, the average intrinsic luminosity) of any observed Cepheid by measuring the time it takes for its brightness to change systematically.

Why are Cepheids called the beacons of the universe 2
Light curve of Delta Cephei, the prototype of classical Cepheids

By observing the apparent inverse-square luminosity, and comparing it to absolute magnitude, astronomers can accurately calculate the distance to a given Cepheid.

Why does the period-luminosity relationship exist at all?

Most stars, including Cepheids, are in hydrostatic equilibrium, where there is a balance between gravitational contraction and expansion provided by the energy the star generates.

As stars evolve, the equilibrium can be disturbed, and in some cases this leads to fairly stable oscillations, as in the case of Cepheids.

For mechanical systems (including those controlled by gravity), the natural period of oscillation is largely determined by the average density, which is mass divided by volume.

Why are Cepheids called the beacons of the universe 1

Low density stars have longer periods, and variables like Cepheids also tend to have larger radii (and lower density). Larger radii result in more surface area, which at a fixed surface brightness means higher brightness.

Then Cepheids with a longer period will have a higher luminosity (the longer the period of change in the brightness of a Cepheid, the greater the power of its radiation). It follows that the periods predict luminosity.

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