# Imagine you lost at night in the desert. A bright spot suddenly appeared in the distance. Is it a flashlight to 100 meters or 10 kilometers powerful projector? At night, no sound, it is impossible to determine the distance from a point light. The problem is the same for both celestial bodies. A faint star near the Earth but can exceed a star shine bright but distant.

We must therefore distinguish between two concepts: the apparent brightness measuring the brightness of a star measured from the Earth and the absolute luminosity which measures the actual amount of light emitted by the star. The apparent brightness depends on the distance of the star and does not provide direct information on the nature of it. The absolute luminosity depends only on the object itself and can therefore inform us about the nature of the body and is considered it to look for to determine.
The absolute brightness of stars
This is where the measurements of distance stars . Physicists have long known that the intensity of radiation follows a well defined: it decreases as the inverse square of the distance traveled by light. With this law, it is easy to establish the relationship between absolute luminosity, distance and apparent brightness of a star. In addition, if two parameters can be measured, the third can be calculated easily. So, if we can determine the distance to a star, simply measure its apparent brightness and apply a mathematical relationship to reach its absolute luminosity.
Measures of this type began as soon as the data on distances were available. They put in evidence an enormous range in absolute luminosities possible. Some stars do not emit one ten-thousandth the brightness of the Sun . Others emit a million times more energy than our star. Range of luminosities proved enormous, with a factor of ten billion between the minimum and maximum absolute luminosities.
The temperature of stars
It is possible to easily determine the temperature of a star with the spectral analysis . Just find the wavelength at which the light intensity of the star is up and enforce the law that connects the wavelength temperature.Note that the measured temperature is that which prevails at the surface of the star. The temperature inside is not directly measurable, it is possible to estimate that using theoretical models.
Spectroscopic observations have shown that the coldest stars are red and have a temperature of about 3000 degrees. The hottest stars are blue and reaches 50,000 degrees. The ratio between maximum and minimum temperatures is therefore only slightly greater than 10.
Spectral types
The status of various gases on the surface of a star is strongly dependent on the temperature therein. And the spectra of two stars of different temperatures have characteristics that distinguish them easily. This property has led astronomers of the nineteenth century to classify stars into different categories, depending on the aspect of their spectrum .
These groups, called spectral types are designated by the following letters: O, B, A, F, G, K and M types O and B correspond to surface temperatures above 10,000 degrees and their spectra are dominated by lines for helium. Type A, a little less than 10,000 degrees, has the hydrogen lines. Types F, G and K, with temperatures between 3500 and 7500 degrees, exhibit calcium lines. Finally, M-type stars, less than 3500 degrees, provide a spectrum dominated by webs, that is to say very wide lines due to some molecules, in particular titanium oxide.
An example of a spectrum for each primary spectral type, since the blue stars to the stars of type O red type M. Credit: Wikimedia Commons
The next step in understanding the nature of the star is then to analyze all this new information, in particular to establish a possible relationship between absolute luminosity and surface temperature, the raison d'être of the Hertzsprung-Russell .

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