The brightest star would have a magnitude of 1 or less and a very faint star would have a magnitude of 6. The system of classifying stars based on their brightness was developed by a Turkish astronomer Hipparchus of Rhodes around BC.
He divided the stars into six groups, with the brightest star being first magnitude and the faintest as sixth magnitude. Although, measuring brightness of stars is an ancient idea, the technology has become more sophisticated now with astronomers using more precise tools to obtain more accurate readings. Astronomers now use apparent and absolute magnitude scale to define the brightness of stars. It refers to the fact that to determine the true brightness of a light source, we need to know how far away it is.
Astronomers take 10 parsecs as the standard distance and refer to the intrinsic brightness of the star as its absolute visual magnitude, the apparent magnitude of the star as it would appear if it were 10 parsecs, or Absolute magnitude is related to the intrinsic luminosity of the star. In simple terms, it is defined as the apparent magnitude at a distance of 10 parsecs from the star.
Apparent magnitude is a measure of how bright the star appears when viewed from Earth. Apparent brightness is one way of expressing how bright a celestial object appears as viewed from Earth from a dark-viewing site. Magnitude and apparent magnitude mean the same thing; namely how bright a celestial object appears to us on Earth ranked on the historic logarithmic magnitude system.
The apparent magnitude depends on three things: how big it is, how far away it is from Earth, and how much light it emanates per diameter of the star. Apparent magnitude is related to the observed energy flux from the star. Apparent magnitude, on the other hand, is a measure of how bright the star appears when viewed from Earth.
The apparent magnitude of a celestial object is a measure of its brightness as seen from the Earth. Absolute magnitude is related to the intrinsic luminosity of the star, whereas apparent magnitude is related to the observed energy flux from the star.
An intrinsically faint, nearby star can appear to be just as bright to us on Earth as an intrinsically luminous, distant star. There is a mathematical relationship that relates these three quantities—apparent brightness, luminosity, and distance for all light sources, including stars.
Why do light sources appear fainter as a function of distance? The reason is that as light travels towards you, it is spreading out and covering a larger area.
This idea is illustrated in this figure:. Again, think of the luminosity—the energy emitted per second by the star—as an intrinsic property of the star. As that energy gets emitted, you can picture it passing through spherical shells centered on the star. In the above image, the entire spherical shell isn't illustrated, just a small section.
Each shell should receive the same total amount of energy per second from the star, but since each successive sphere is larger, the light hitting an individual section of a more distant sphere will be diluted compared to the amount of light hitting an individual section of a nearby sphere.
The amount of dilution is related to the surface area of the spheres, which is given by:. The apparent magnitude is denoted by the symbol, m v. The scale used in apparent magnitude is reverse logarithmic. Similar to an absolute magnitude, the numeric value of apparent magnitude decreases as its luminous intensity increases. For instance, the star Sirius would be visible to naked eyes in a clear sky. Absolute magnitude can also be measured from apparent magnitude. The formula which relates absolute magnitude M v and apparent magnitude m v is as follows.
The absolute and apparent magnitude is widely used by astronomers to validate the magnitude of the luminous intensity. To know the true value of the body, the absolute magnitude can be used. If one needs to know the luminosity of the star when observed from a point, an apparent magnitude can be used.
Skip to content There is a countless number of stars present in this universe. It gives the magnitude of the brightness of the celestial body when observed at a fixed distance. Another measure of brightness is luminosity, which is the power of a star — the amount of energy light that a star emits from its surface. It is usually expressed in watts and measured in terms of the luminosity of the sun. For example, the sun's luminosity is trillion trillion watts.
One of the closest stars to Earth, Alpha Centauri A , is about 1. To figure out luminosity from absolute magnitude, one must calculate that a difference of five on the absolute magnitude scale is equivalent to a factor of on the luminosity scale — for instance, a star with an absolute magnitude of 1 is times as luminous as a star with an absolute magnitude of 6.
While the absolute magnitude scale is astronomers' best effort to compare the brightness of stars, there are a couple of main limitations that have to do with the instruments that are used to measure it. First, astronomers must define which wavelength of light they are using to make the measurement.
Stars can emit radiation in forms ranging from high-energy X-rays to low-energy infrared radiation. Depending on the type of star, they could be bright in some of these wavelengths and dimmer in others. To address this, scientists must specify which wavelength they are using to make the absolute magnitude measurements. Another key limitation is the sensitivity of the instrument used to make the measurement.
In general, as computers have advanced and telescope mirror technology has improved over the years, measurements that are made in recent years have more weight among scientists than those that are made long ago. Paradoxically, the brightest stars are among the least studied by astronomers, but there is at least one recent effort to catalog their luminosity.
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