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Originally published in the [May 2021 issue of the CTA Newsletter](https://mailchi.mp/892f3f0c743d/cta-newsletter-december2020-english). *Written by: Paolo Goldoni*
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*Only by knowing the distance of the objects we observe can we begin to understand their physical nature. In 1923, Edwin Hubble demonstrated that some of the so-called “nebulae” he saw were, actually, galaxies located millions of light years away. He did so by observing a particular type of variable star in the “nebulae,” the Cepheids, discovered by Henrietta Leavitt a few years before, whose period of variation is linked to their luminosity. The period of the Cepheids in the nebulae that Hubble observed implied such a luminosity that they were undoubtedly extragalactic. In doing so, he also established a correlation between the distance and the redshift of the optical spectra of the galaxies he observed. Since then, the redshift is the quantity most used to measure the distance of extragalactic objects.
The redshift is an increase of the light’s wavelength (decrease of energy) that occurs when a light source moves away from the observer. It is typically measured in optical and near-infrared by the spectral lines of the source (Figure 1). In extragalactic astronomy, the redshift (also called cosmological redshift) is due to the expansion of the Universe that increases the distance between the galaxies and the Earth.
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*Figure 1. Absorption spectral lines in the optical spectrum of a supercluster of distant galaxies (upper panel) compared to a close-by object, the Sun (bottom panel). Arrows indicate the redshift, i.e. the increase of the wavelengths (lower energy). Credit: Georg Wiora*
### Blazars and their Distance in Very High-Energy Astronomy
## Blazars and their Distance in Very High-Energy Astronomy
As for all astronomical instruments, the distance of the observed object is very important to CTA, too. This is particularly true for blazars, the most numerous class of extragalactic sources in the very high-energy (VHE) domain (above tens of GeV). Blazars are a class of Active Galactic Nuclei (AGNs) – compact regions at the centre of galaxies with strong and variable emission across the electromagnetic spectrum. Their emission is caused by accretion on the central supermassive black hole. During the accretion process, a jet of relativistic particles is emitted from the vicinity of the black hole, whose radiation spans from radio to gamma rays. In blazars, the jet is pointed towards the observer.