Scientists

Hi. It is a pleasure to help. If you mean the distance from the earth to the stars, astronomers use a technique called parallax which is a way to infer the distance to a star based on triangulation. Astronomers use a unit called the light-year, which is the distance travelled by the light in one year. Since light travels at a speed of 300,000 km/s, in one year light leaves behind a distance of 9.4605284 × 1015 meters. Some stars are only a few light- years away. Others are very far away, in the limits of the observable universe. If the astronomer wants to measure the distance to a very far away star, then parallax is no longer useful. Other techniques and properties are employed such as the Cepheids, which are a very particular class of stars in which the period of variation in their brightness is related to their distances. So by measuring the period, astronomers are able to infer the distance.

Measuring distances to stars and to distant galaxies is one of the central problems in Astronomy. It is VERY important. Several research efforts are devoted to measuring distances. In fact, in the 1920's E. Hubble first measured the distance to other galaxies using cepheids. The Hubble Sapce Telescope still measures distances to distant galaxies using a variety of observational techniques.

If you mean the measurement of the radius of a star, astronomers do that by knowing the details of the cameras used to photograph the star and the known distance to it. Sometimes studying the properties of the light emitted by stars it is possible to infer its radius based on the theory of stellar evolution.

Prof. João Rodrigo Leão

Rio Grande Federal University - BRAZIL

This exclamation is most famously attributed to the ancient Greek scholar Archimedes. He reportedly proclaimed "Eureka!" when he stepped into a bath and noticed that the water level rose-he suddenly understood that the volume of water displaced must be equal to the volume of the part of his body he had submerged. (This relation is not what is known as Archimedes' principle-that deals with the upthrust experienced by a body immersed in a fluid[2][3]) He then realized that the volume of irregular objects could be measured with precision, a previously intractable problem. He is said to have been so eager to share his discovery that he leapt out of his bathtub and ran through the streets of Syracuse naked.

Archimedes' insight led to the solution of a problem posed by Hiero of Syracuse, on how to assess the purity of an irregular golden votive crown; he had given his goldsmith the pure gold to be used, and correctly suspected he had been cheated, by the goldsmith removing gold and adding the same weight of silver. Equipment for weighing objects already existed, and now that Archimedes could also measure volume, their ratio would give the object's density, an important indicator of purity.

This story first appeared in written form in Vitruvius's books of architecture, two centuries after it supposedly took place.[4] Some scholars have doubted the accuracy of this tale, saying among other things that the method would have required precise measurements that would have been difficult to make at the time.[5] For the problem posed to Archimedes, though, there is a simple method which requires no precision equipment: balance the crown against pure gold in air, and then submerge the scale with crown and gold in water to see if they still balance.[6]