How do we measure cosmic distances?


 
The simplest and the oldest way to measure distances to stars is the parallax method. This utilizes variation of the angle between a star and a benchmark direction as an observer changes its position in space, basically moving with the Earth around the Sun. When looking from the Earth, that angle can be measured not better than hundredth of an arcsecond because of atmospheric turbulence and scattering of star’s light. Fortunately space missions are able to measure angles down to tens microarcseconds which correspond to distances of tenth of the Milky Way size. To calculate distances to other galaxies, we use the observables about which we know how they should look at distances. Those include so called standard sirens, rulers and candles. The convenient and best known standard candles are Cepheid variables and supernovae. A Cepheid is a star of a very high yet pulsing luminosity. Its luminosity defines the period of pulsation of its apparent brightness. Using the period we can calculate the luminosity and then  distance to the Cepheid judging by its apparent brightness​.  As for supernova explosion it can be of different nature. Among them, we can identify those happening as a star is acquiring mass and finally reaches a critical one starting the carbon fusion. The luminosity of explosion is then defined by this mass so is the same for all the supernovae of this kind. Then, again using the apparent brightness of the explosion, we can calculate the distance to the supernova.

Ways of measuring distances to stars

Cepheids served for Hubble’s law discovery though only being useful up to tens megaparsecs. Supernovae allow us to look at a thousand megaparsecs so were used to confirm the acceleration of the universe expansion.

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