Difference Between Absolute and Apparent Magnitude (With Table)

If you have looked up at the stars, you would know that not every star shines with the same brightness.

The brightness of stars is measured on a scale called the magnitude scale. The technology keeps getting more sophisticated.

Astronomers use Absolute and Apparent magnitude scales to measure a sky’s brightness.

Absolute Magnitude vs Apparent Magnitude

The main difference between absolute and apparent magnitude is that absolute magnitude measures the star’s luminosity when measured from a distance of 10 parsecs (32.58 light years).

On the other hand, apparent magnitude is a measure of how bright a star appears when it is viewed from the Earth.

Absolute magnitude refers to the fact that we need to know how far away a star is to determine the true brightness of a light source.
The standard distance astronomers use is 10 parsecs, or 32.58 light years.

Apparent magnitude is simply a measure of how bright a star appears when viewed from Earth. It is related to the observed energy flux from the star.

Comparison Table between Absolute Magnitude and Apparent Magnitude

What is Absolute Magnitude?

Absolute magnitude is a measurement of the brightness of a celestial object from a standard distance. The standard distance astronomers use is fixed as 10 parsecs, or 32.58 light years away.

Absolute magnitude is represented using the symbol “M_v“.

Absolute magnitude is calculated using the formula –
M_v = m – 2.5log[(d/10)² ]

In the above formula,
M_v represents the Absolute Magnitude
m represents how bright the star appears to be
d represents the distance of the star measured in parsecs

Absolute magnitude is a star’s apparent magnitude if measured from 10 parsecs away.

The smaller the value of the absolute magnitude of a celestial body, the more luminous it would be.

The brightest stars would have an absolute magnitude of 1 or less, whereas stars barely visible to the naked eye have an absolute magnitude value of 6 or more.

Magnitude scales ignore anything that would interfere with the observations. The numbers represent what the brightness of a celestial body would look like without any atmospheric effect.

For example, light pollution is not considered when calculating absolute magnitude.

Magnitude scales also work on a logarithmic scale, as evident by the formulas used for their calculation.

A star with an absolute magnitude of 1 is not twice as bright as a star with an absolute magnitude of 2.

Instead, each number is 2.512 times brighter than the one next.

A star with an absolute magnitude of 1 is 2.512 times as bright as a star with an absolute magnitude of 2.
A star with an absolute magnitude of 1 is 6.31 times (2.512 x 2.512) brighter than a star with an absolute magnitude of 3.

What is Apparent Magnitude?

Apparent Magnitude is a measure of the brightness of a star or any other celestial body when observed from Earth.

Apparent magnitude does not measure the absolute brightness of a celestial body.

The apparent magnitude of a body depends not only on its intrinsic brightness but also its distance from the Earth and any obstruction of the celestial body’s light caused by interstellar dust along the line of sight of the observer.

Apparent magnitude is calculated using the formula –

m_v = m₂-m₁ = -2.50log(B₂/B₁).

In the above formula,

m_v represents the apparent magnitude
B₂/B₁ represents the brightness ratio

Today, astronomers use a more advanced version of the Hipparchus’ apparent magnitude scale to measure the apparent magnitude of a star by using photographic and electronic methods.

The apparent magnitude is related to the observed energy flux from the star.

Main Differences Between Absolute Magnitude and Apparent Magnitude

1. Absolute magnitude measures the intrinsic luminosity of a celestial body from a standard distance, that is, 10 parsecs away. At the same time, apparent magnitude can be used to measure the brightness of a celestial body from any distance away.
2. Where absolute magnitude is always measured from a distance of 10 parsecs (32.58 lightyears), an apparent magnitude can theoretically be measured from any distance. However, it is mostly measured from a dark-viewing site on Earth.
3. Absolute magnitude is represented with a capital M, with v as the subscript (M_v). The apparent magnitude is represented with a small M, with v as the subscript (m_v).
4. Absolute magnitude is related to a star’s intrinsic luminosity, whereas apparent magnitude is related to the observed energy flux from the star.
5. Formula for Absolute Magnitude – M_v = m – 2.5log[(d/10)²]
   Formula for Apparent Magnitude – m_v = m₂-m₁ = -2.50log(B₂/B₁)

Conclusion

Astronomers use absolute and apparent magnitudes to measure a star’s brightness.

While speaking about the brightness of a star, we must be careful to distinguish between the actual luminosity of a star and its apparent brightness.

Apparent magnitude is how bright a star would appear through a telescope or the naked eye. At the same time, absolute magnitude is not so easy to measure.

References

1. https://adsabs.harvard.edu/pdf/2006JAHH….9..173H
2. https://www.sciencedirect.com/science/article/pii/S002073737580037X