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The brightest stars were called stars of first magnitude as far back as the ancient times. Ancient Greek astronomer Hipparchus compiled the catalog of stars visible by the naked eye in the II century BC. He suggested that all visible stars should be broken down into six classes. Hipparchus called the brightest of them first magnitude stars. The weakest stars were called sixth magnitude stars. Astronomers of later days decided to follow Hipparchus. The weaker is the star, the higher is its magnitude.

Around 5 000 stars may be observed in the sky by the naked eye (up to the sixth magnitude). Billions of stars are visible with the aid of telescopes.

British astronomer Norman Pogson proposed the modern scale of magnitudes in the middle of XIX century. Visible brightness of stars varies by the magnitude of approximately 2.5 times as compared to the differences proposed by Hipparchus. Difference of 5 magnitudes corresponds to a change in star brightness by 100 times. Thus, the difference of one magnitude corresponds to 2.5-time difference of brightness.

The fact that star magnitudes are different does not provide any concrete information about the stars. A very bright star may have a high luminosity but be situated too far to have any decisive pronouncement regarding its brightness. In order to cope with that obstacle and to determine the true brightness of stars, the notion of absolute magnitude was introduced.

Absolute magnitude M is a visible magnitude that any given star would have if it were situated at a standard distance of 10 pc, or 32.6 light-years.

The relationship between the absolute magnitude M, visible magnitude m and distances to stars R in parsecs is shown below:

M = m + 5 - 5 lg R.

This model illustrates the scale of magnitudes. The slide of the central display box presents a variety of celestial bodies, cross-referencing them against their visible magnitudes. You may also look at absolute magnitudes of celestial bodies by selecting "Absolute" switch.

Switch to a different slide by using the scale at the bottom part of the model by dragging the scroll bar to a different mark.

You may also choose to view the slides in a continuous mode. Press "Run" button. The slides will change every 3 seconds in a pre-determined sequence, starting with the current one. Pressing "Stop" button will suspend the animation. Press "Reset" to return the model to its initial state.

© OpenTeach Software, 2007