The galaxy NGC 3370 is very similar to our Milky Way and is about 100 million light years away, in the direction of the constellation Leo. This photo obtained by the Hubble Space Telescope allows you to see many of its details. It has been possible to identify some individual pulsating stars, called Cepheids, that can be used to calculate the distance to NGC 3370. This spiral galaxy was chosen because in 1994 one of its stars exploded as a type Ia supernova. Knowing the distance to the galaxy, it has been possible to calibrate this type of supernova to determine distances to other similar supernovae occurring at much greater distances, thus revealing the size and expansion of the Universe.
By comparing nearby supernovae with the more distant ones, we can determine that the Universe is accelerating its expansion and that it contains a mysterious dark energy. But, to measure the size of the Universe and its expansion rate, we must calibrate the true brightness of these supernovae. Hence its importance, since its distance can be determined thanks to the existence of dimmer stars of known brightness in its vicinity and, with it, calibrate the measurements in the Universe.
The faint stars that are used as brightness standards are the variable stars known as Cepheids, whose brightness varies regularly with a period that is directly related to their intrinsic brilliance. This allows to know directly the distance of the galaxy NGC3370 and the SN1994ae supernova by observing the variation of one or several of these stars individually, something that can only be done with Hubble. The observations detect several Cepheids and indicate that they are the most distant that have been observed.
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