Absolute and Apparent Magnitude: Measures of Brightness

By Peter Forrester | January 17, 2019

I’ve sometimes referred to the brightness of different stars and other objects in the sky. Let me tell you how astronomers measure brightness.

The first term you need to know is “apparent magnitude,” which is just a fancy way of saying how bright an object appears to be, usually when seen from the surface of the Earth.

The second term is “absolute magnitude”, which means if you put the stars at the same distance from Earth, what would the brightness be then?

Now let me explain about the numbering scale used to express these brightness. This is where it gets to seeming crazy. You see, the brighter an object is, the lower the number is. For instance, the brightest object, the Sun, has an apparent magnitude of -26.74. The next brightest star, Sirius, comes in at “only” -1.46.

The peculiar scale goes back to the Greek writer Hipparchus in the 1st century BC. He labelled the brightest 20 stars as first degree stars, and the dimmest as 6th degree. This was a rather simple way of describing brightness, before there were telescopes or instruments for measuring brightness precisely. Stars that are too dim to see with the naked eye are 7th degree or lower.

Eventually, after measurement of magnitude started, a decimal form started to be written. This system was formalized in 1856 by an English astronomer named Norman Robert Pogson (1829-1891).  Under his system, a first degree star is 100 times as bright as a sixth degree star, and so each degree represents a ratio of about 2.5, sometimes called “Pogson’s ratio”. For you math nerds, the exact amount is the 5th root of 100.

Pogson’s system assigned the North Star, Polaris, as being of degree 2.0; however this was later changed because Polaris’ magnitude varies slightly over time. The star Vega is now defined as 0.00 magnitude. There are four stars brighter than Vega, which necessarily means they have negative numbers in their magnitude.

Here is a list of the 10 brightest objects in the sky (the planets and Moon are listed at their brightest but they vary over time). It should be noted that there are different possible ways of measuring apparent magnitude of stars, and you may see these in a slightly different order. See below Wikipedia, List of brightest stars for more information on these variations.

  1. The Sun: -26.74
  2. The Moon: -12.74
  3. Venus: -4.89
  4. Jupiter: -2.94
  5. Mars: -2.91
  6. Mercury: -2.45
  7. Sirius: -1.46 (star in Canis Major)
  8. Canopus: -0.74 (star in Carina*)
  9. Saturn: -0.49
  10. Rigil Kentaurus: -0.27 (star in Centaurus*)

*Note: Carina and Centaurus can only be seen from the Southern Hemisphere.

Now to Absolute Magnitude. This is defined as the brightness an object would have if seen from a standard distance (10 parsecs, or 32.6 light-years), adjusting for interstellar dust. They can also measure it in different light bands, but I won’t bore you with an explanation of that. See Wikipedia, Absolute magnitude for more on this. Warning: there’s a lot of complicated math on this page. Also, note that a parsec is a measure of distance, not time as implied in the first Star Wars movie.

Measurement of absolute magnitude is made with an instrument called a bolometer, and varies based on what type of light wavelength you’re looking at.

Some stars are so bright that they would appear brighter than the planets and cast shadows if they were only 10 parsecs away. For example, Rigel is -7.0, Deneb is -7.2, and Betelgeuse in Orion has an absolute magnitude of -5.6. By comparison, Sirius is 1.4, much brighter than the Sun’s absolute magnitude 4.83.

Apparent magnitude for objects in the solar system is based on supposing that the object were a standard distance of 1 Astronomical Unit (about 93 million miles, or the distance between the Sun and Earth) from both the Sun and the observer.

So now you know the difference between apparent and absolute magnitude. Now if someone asks how to measure the brightness of stars, you’ll know the answer!


References:

Wikipedia. (2019, January 14). Absolute magnitude. Retrieved from https://en.wikipedia.org/wiki/Absolute_magnitude.

Wikipedia. (2019, January 11). Apparent magnitude. Retrieved from https://en.wikipedia.org/wiki/Apparent_magnitude.

Wikipedia. (2018, 24 October). First magnitude star. Retrieved from https://en.wikipedia.org/wiki/First_magnitude_star.

Wikipedia. (2019, January 1). List of brightest stars. Retrieved from https://en.wikipedia.org/wiki/List_of_brightest_stars.

Wikipedia. (2018, 23 July). N. R. Pogson. Retrieved from https://en.wikipedia.org/wiki/N._R._Pogson.

Author: Peter Forrester

I have been interested in astronomy and stargazing for many years, and now delight to offer some of my learning to others through my weekly blog posts.

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