Absolute vs Apparent Magnitude: Difference and Comparison

There is a countless number of stars present in this universe. When we look at a clear sky during the night, the entire space is 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. 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 star’s magnitude was first conceptualized by Hipparchus (Turkish astronomer) thousands of years before.

Later, two standards were developed from the earlier scale referred to 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).

Key Takeaways

1. Absolute magnitude is the actual brightness of a celestial object, while the apparent magnitude is how bright it appears from Earth.
2. Absolute magnitude is measured on a standardized scale, while the apparent magnitude is affected by distance and other factors.
3. Absolute magnitude is used to compare the brightness of celestial objects, while the apparent magnitude is used to locate and identify them.

Absolute vs Apparent Magnitude

The difference between absolute and apparent magnitude is that absolute magnitude does not consider the size of the celestial body and the point from which it is viewed. The apparent magnitude 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 measures the 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 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 criteria mentioned above.

Comparison Table

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 concerning 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 is 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	When measuring brightness, it does not consider energy loss due to light absorption by cosmic dust.	When measuring luminosity, it considers all the factors, like energy absorbed by interstellar matter.

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 the value of absolute magnitude decreases as the object’s luminosity increases. It is denoted by the symbol M_v.

The absolute magnitude can be calculated from the apparent magnitude as follows.

M_v = m – 2.5log([d/10]²)

where

1. d refers to the distance in parsecs
2. m refers to the apparent magnitude measured

The instrument which is used to measure absolute magnitude is called a 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 the point of observation, i.e. the Earth. It considers 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 symbol mv denotes the apparent magnitude.} 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.

The naked eye can see celestial bodies whose apparent magnitudes range from -1 to 6.5. For instance, the star Sirius would be visible to the naked eye 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

M_v – m_v = 5 – 5log₁₀(d)

where d is the distance in parsecs.

Main Differences Between Absolute and Apparent Magnitude

1. 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.
2. 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.
3. 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.
4. _{Mv denotes absolute magnitude,} while the apparent magnitude is denoted by m_v.
5. Photometric systems help measure the apparent magnitude, whereas a bolometer is an instrument that helps measure absolute magnitude.

References

1. https://www.researchgate.net/publication/324106257_Apparent_and_Absolute_Magnitudes_of_Stars_A_Simple_Formula
2. https://royalsocietypublishing.org/doi/abs/10.1098/rspa.1966.0203