Universe Guide

Absolute, Apparent and Visual Magnitudes

In short, Magnitude is the measure of how bright an object in space is. The brightest object in space is measured at -26.74 and it comes as no surprise that that object is the Sun. The dimmest objects that we can see unaided, that is without using a pair of binoculars or a telescope has a magnitude of 6.5. To see something at that magnitude without assistance, you would need to go somewhere that is very dark, somewhere away from street lights.

The star magnitude system was first devised by the ancient Greek astronomer Hipparchus. His original system had star brightness starting from 1(Brightest) to 6 (Dimmest). The scale stayed. Stars and objects that are brighter than the stars he recorded at the time have therefore negative numbers. The scale has been revised over time hence why some stars such as Sirius has a negative number. Caltech

Apparent and Visual Magnitudes

Apparent Magnitude is the magnitude of an object as it appears in the sky on Earth. Apparent Magnitude is also referred to as Visual Magnitude. The magnitude of a star excluding the Sun has no bearing on how big the star is or how near the star is.

The star UY Scuti has apparent magnitude of 11.2 which means it is not something that can be seen from Earth with the naked eye. UY Scuti is bigger than Sirius which has an apparent magnitude of -1.44 and is the brightest star in the night sky. Sirius is visible in the night sky whereas UY Scuti isn't.

Below is a small selection of what objects are possible at a selected amount of magnitudes. For a complete list, you should visit Wiki. The values in brackets are the values that Astronomy Notes give for examples and are more precise excluding Vega.

Anything that is dimmer than 32 magnitude is something we are not able to see at the moment even with the most powerful telescope. When the James Webb Space Telescope eventually goes into orbit, we could well see something of a lower magnitude, whether it be a galaxy that is further than the current record holder GN+z11 is open to debate. GN+z11 is not something that can be seen unaided.

Apparent MagnitudeAbsolute MagnitudeExample
-27 (-26.7)The Sun
-13 (-12.6)Full Moon
-7SN 1006 supernova
-6International Space Station
-5 (-4.4)Venus
-4Faintest Daytime Object
-3Jupiter / Mars
-1 (-1.4)1.45Sirius
0 (0.3)0.58 / N.A. Vega / Saturn
5Vesta / Uranus
6 (6.5)Naked Eye Maximum
107x50 Binoculars Max
1115.46Proxima Centauri
134.5 - 6 in Telescope Max, 3C 273 quasar at 12.8
14Pluto - 8-10in Telescope Max
278m Telescope Max
32hubble space telescope Max

Formula for Absolute Magnitude Calculation

Absolute Magnitude is a calculated value of how bright the star would be at a distance of 10 parsecs (32.6 Light Years). Using this value, you can get an idea of just how bright the object really is and compare like for like. The value is calculated by the following formula.

Mv = mv -5 (log d -1)

Lets take Regulus as an example, the distance (d) of Regulus as measured in parsecs is 41.13 using the 2007 figures. mv is the apparent visual magnitude which in 2007, it was 1.36 which means our absolute magnitude is -0.57.

The formula can make the star on some occasions seem dimmer than it is seen from Earth. Take Promixa Centauri, the reason for the star to seem dimmer is because from Earth, the star is less than 10 parsecs from Earth, it is only 1.3 parsecs from Earth so the absolute magnitude is how it is seen further away than from the Earth.

The below is a selection of stars, their apparent magnitude, the absolute and how far away they are in Light Years. The reason for choosing V762 Cassiopeiae is because it is the furthest star you can see with the naked eye in good conditions.

Apparent MagnitudeAbsolute MagnitudeDistance (Light yr)Star
11.0115.464.243Proxima Centauri
-0.014.344.32Rigil Kentaurus
5.450.86280.93Polaris Australis
5.87-7.422764.1V762 Cassiopeiae

What is the difference between Apparent and Absolute Magnitude?

Essentially, apparent magnitude is the magnitude of the object that we see from Earth. Absolute magnitude is the magnitude of an object that we see if we are 32.6 light years away from it.

Lets take two stars with the same apparent magnitude (6.5) but are at different distances, Star A (9 Monocerotis) and Star B (TY Fornacis). They have the same brightness in the sky but they are different distances to the Earth (937.25 LY and 328.13) respectively. Taking a short-cut, we can see that 9 Monocerotis has a lower absolute magnitude and is therefore the brighter of the two when both seen at the same distance.

The Absolute Magnitude is a means of comparing like for like. When you take into account the distance they are, you can then work out the real solar magnitude and which is the brighter. When you apply the calculation, you find that 9 Monocerotis has a brighter absolute magnitude and therefore is the real brighter of the two stars. The lower the value of the Absolute Magnitude, the brighter the object is.

Absolute Magnitude and Luminosity

Luminosity is the amount of energy that a star pumps out every second. The lower the absolute magnitude of a star is, the more luminous the star is. Take for instance Sirius, the absolute magnitude of the star is 1.45 and its Luminosity is 25.82. Compare it to Tarazed which has an absolute magnitude of -3.03 and a Luminosity of 2948.48.

What is the Brightest / Luminous Star?

Brightest (Apparent Magnitude

The brightest star is the Sun but you probably want that excluded from the answer. The brightest star in the night sky using the apparent magnitude system is Sirius with an apparent magnitude of -1.44. The Sun for comparison is as mentioned above -27.

Luminous (Absolute Magnitude)

The brightest in terms of absolute magnitude is R136a1 star in the nearby Large Magellanic Cloud. The cloud can be located in the constellation of Dorado. The star is a large Wolf-Rayet Star which are stars that have used up or nearly used all their hydrogen and are blowing out huge amount of gases.

Absolute and Apparent Magnitude of an Asteroid

Magnitudes don't just apply to stars, they can apply to other objects such as planets. When talking about Asteroids, the Apparent Magnitude is how we see it on Earth. The Absolute Magnitude is how bright the object is from 1 Astronomical Unit (A.U.), that is the distance from the Sun to Earth. Absolute Magnitude of asteroids are denoted with a H. Ref: N.A.S.A.

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