There is a countless number of stars present in this universe. When we look at a clear sky during the night, the entire space would be lit by the stars which appear as tiny bright colour spots. Some appear bright and some appear dark. The reason for the varying magnitude of brightness could be many. Either the distance from the earth or its energy level to radiate the electromagnetic wave (light) could be the factor. The factor for determining the magnitude was developed at a very early age. The scale for standardizing the magnitude of the star was first conceptualized by Hipparchus (Turkish astronomer), thousands of years before.
Later, two standards were developed from the earlier scale referred by Hipparchus to determine the magnitude of the luminosity of the celestial bodies. The two standards which are in use are the absolute magnitude and apparent magnitude. Absolute magnitude helps us to know the luminosity of any celestial body from a fixed distance of ten parsecs (one parsec equals 3.25 light-years).
Absolute vs Apparent Magnitude
The main difference between absolute and apparent magnitude is that absolute magnitude does not take into account the size of the celestial body and the point from where it is viewed. It is the apparent magnitude that ascertains the degree of luminosity of any celestial object from the point of reference. Absolute magnitude measures the intensity of the star for a fixed distance only.
Absolute magnitude is a measure of intrinsic luminance of the celestial body (star). Apparent magnitude gives us a clear picture of the intensity of any celestial body when viewed from earth. This apparent magnitude has got evolved from the earlier version of the magnitude scale developed by Hipparchus.
To get a clear idea of the intensity of any celestial body from the point of reference, apparent magnitude gives us a clear picture of the above-mentioned criteria.
Comparison Table Between Absolute and Apparent Magnitude
| Parameter of Comparison | Absolute Magnitude | Apparent Magnitude |
| Meaning | It gives the magnitude of the brightness of the celestial body when observed at a fixed distance. | It denotes the brightness of the celestial body with respect to the point of observation (The Earth). |
| Scale used | Inverse logarithmic scale. | Reverse logarithmic scale. |
| Symbol | It is denoted by the symbol Mv | It is denoted by the symbol mv |
| Factors on which it depends | The energy being radiated from the star or the celestial body. | Size of the celestial body, the amount of energy being radiated from the body and its distance from the earth. |
| Highlights | It does not take into account energy loss due to absorption of light by cosmic dust when measuring the brightness. | It considers all the factors like energy absorbed by the interstellar matter when measuring the luminosity. |
What is Absolute Magnitude?
Absolute magnitude refers to the degree of the luminosity of a celestial body when observed at a fixed distance of 10 parsecs (this is equivalent to thirty times the distance travelled by light in a year). Absolute magnitude uses an inverse logarithmic scale to refer to the intensity of the light emitted by the celestial bodies. This tells us that as the luminosity of the object increases, the value of absolute magnitude decreases. It is denoted by the symbol Mv.
The absolute magnitude can be calculated from apparent magnitude as follows.
Mv = m – 2.5log([d/10]2)
where
- d refers to the distance in parsecs
- m refers to the apparent magnitude measured
The instrument which is used to measure absolute magnitude is called bolometer. It is a device which is used to measure the incident electromagnetic radiation.
Absolute magnitude gives the intrinsic brightness of the celestial object. This system does not consider the energy absorbed by the matter in space while it is measured. Absolute magnitude gives us a picture of the true luminosity of the celestial body.
What is Apparent Magnitude?
Apparent magnitude refers to the luminosity of any celestial body observed from a point of observation, i.e. the earth. It takes into account of all practical factors which would obstruct or absorb light on its path of travel. Thus, it gives us a clear picture of the actual brightness of a star when viewed from the earth.
The apparent magnitude is denoted by the symbol, mv. 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. Celestial bodies whose apparent magnitude’s range from -1 to 6.5 can be seen in the naked eye. 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 (Mv) and apparent magnitude (mv) is as follows
Mv – mv = 5 – 5log10(d)
where d is the distance in parsecs.
Main Differences Between Absolute and Apparent Magnitude
- Absolute magnitude is a way of measuring the intrinsic brightness of the celestial body. Apparent magnitude denotes the magnitude of the brightness relative to the distance measured from the point of observation.
- Absolute magnitude can also be stated as apparent magnitude for measuring the luminosity at a distance of ten parsecs from the celestial body. In the case of apparent magnitude, the point of reference is Earth.
- Absolute magnitude does not consider the factor which may obstruct the path of light emitted from the object. It denotes the true luminosity of the celestial body. But apparent magnitude is measured by considering all the factors which would affect the intensity of the light measured from the body.
- Absolute magnitude is denoted by Mv while the apparent magnitude is denoted by mv.
- Photometric systems help in measuring apparent magnitude whereas bolometer is the instrument which helps in measuring absolute magnitude.
Conclusion
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.
References
- https://www.researchgate.net/publication/324106257_Apparent_and_Absolute_Magnitudes_of_Stars_A_Simple_Formula
- https://royalsocietypublishing.org/doi/abs/10.1098/rspa.1966.0203