The distance problem

One of the principal problems in astronomy is the measurement of stellar distance. We have already seen that all objects are “squashed” on a spherical projection, known as celestial sphere, at the centre of which we have Earth. A lack of depth obviously brings to  misled calculations of luminosity and distances among the objects. The sun for instance is a medium sized object, yet because it is also the star which is closer to us, it seems larger and brighter than many other stars that, even if they are much brighter, they seem smaller and weaker because of the distance.
There are many ways to calculate the distance between the stars; one of these goes by the yearly parallax. The parallax is the apparent movement of an object compared to its background, when looked at from two different points. The farther an object is, the smaller and less noticeable its movement will be when increasing the distance between the two points of observation. Since the stars are so distant, in order to be able to notice the parallax, they are watched every six months, that is when Earth is at two opposite ends of its orbit around the sun. Thus the yearly parallax method. By measuring the angle of this movement and knowing the radius of earth’s orbit, we can calculate the distance between us and the object with a simple trigonometry formula: D = R earth : tg angle expressed in parsec. Parsec is the unit of measurement used by astronomers for distances in the Universe; the name stands for the abbreviation of  “second parallax” which is the distance from which one can see the radius of earth’s orbit under an angle of 1 arcsecond. 1 parsec is the equivalent of 3.26 light years. In the past years we have been able to calculate with remarkable accuracy the distance of most of the closest stars with the parallax method thanks to the Hypparcos satellite.
Nevertheless one is easily inclined to think that the validity of this method is limited to stars near us; in the case of very distant stars the parallax angle becomes so small that it cannot be measured so we have to turn to indirect measuring methods. For instance, one can take into consideration some variable stars, whose variability is linked to their intrinsic luminosity. By measuring their apparent luminosity, meaning the one that we measure from Earth, it is possible to calculate their distance.
Several objects belong to this class of candle samples, the best known are the Cepheid, luminous stars that can be seen also in other galaxies beyond the Milky Way. The measuring precision of these stars allowed the Astronomer Edwin Hubble to measure the distance of the closest galaxies and discover their recession, paving the way to modern cosmology and the Big Bang theory. Today we are able to evaluate, even if not with perfect accuracy, galactic distances of hundreds of millions, and even billions, of light years.

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