When you look up at the stars, some appear to be brighter than others. Several factors can affect the relative brightness of a star when viewing it from Earth. This includes both the size and temperature of the star. The Apparent Magnitude, which is represented as “m,” is used to indicate a star’s brightness. The measurement can be used to learn a ton of information about a star and plays a crucial role in photometry.
Hipparchus
Apparent Magnitude was first established long before the first telescopes were invented. The concept was created around 150 B.C. by Hipparchus, a Greek astronomer. Back then, the measurement was very simple. Hipparchus simply assigned stars a number based on how visible it was in the night sky. The brightest stars were given a 1 while the dimmest were assigned a 6. The Apparent Magnitude scale was only used for stars, not planets or the moon.
The brightest stars that he saw received a rating of one. Though Hipparchus observed planets, comets and other objects, he did not include them on his scale. Though astronomers use a similar scale today, they now have access to technology that helps them locate and assign numbers of each star.
The simple measurement system was used until 1856. Then, the English astronomer Norman Robert Pogson decided to add some formality to the rating system. He established a new scale. It still used the 1 to 6 measurement system. However, his new Apparent Magnitude system said that stars with a measurement of 1 were 100 times brighter than stars with a measurement of 6. Essentially, the differences between magnitudes were the fifth root of 100. This became known as Pogson’s ratio.
Apparent Magnitude Overhaul
Thanks to the advent of high-powered telescopes and satellites, the Apparent Magnitude scale saw another overhaul. The scale scientists use today still follow the same concepts as the original system used by Hipparchus. Pogson’s ratio also comes into play.
The main difference is that the new Apparent Magnitude scale is much wider. The faintest objects that can be seen by the Hubble Space telescope are given a magnitude of 30. Rather than stopping at 1 to determine brightness, the new scale dips into negative values. For example, the full moon has an Apparent Magnitude of -12.6m. The sun has a magnitude of -26.7m.
As you can guess, the new scale takes on a more scientific approach. It also takes into account larger celestial bodies that don’t necessarily give off light themselves. It’s used to measure planets, distant solar systems, and more. The high-tech equipment used by scientists and astronomers provides more in-depth measurements using Hipparchus’s scale as a basis.
Negative Values
One of the biggest changes between the apparent magnitude used today and the figures that Hipparchus used is that some objects can have a negative value. Astronomers assign the lowest numbers to the objects that have the highest brightness. Unlike Hipparchus, astronomers now use apparent magnitude to assign numbers to objects too. The sun has an apparent magnitude of -26.7. During a full moon, astronomers give it an apparent magnitude of -12.6. Both Venus and Mars have a low apparent magnitude when the planets are at their brightest. This ranges from -3 for Mars to -4.4 for Venus.
What is the Apparent Magnitude on Earth?
Astronomers assign different values to the Earth based on the surrounding environment. The limit to the apparent magnitude that the eye can see is +6. This is the absolute maximum though and can vary based on where you stand and if there is any surrounding light pollution. If you stood outside on the street or in your yard with lights on the street and from your house shining, you would experience an apparent magnitude of +3. You can reach the higher limit if you stand in a spot with less nearby lights.
Visible to the Naked Eye
You might wonder what you can see before you peer through a telescope. Humans can see any object with an apparent magnitude of up to +6.5. This lets you see more than 9,100 stars with the naked eye. Objects with an apparent magnitude of above +7 are not visible with the naked eye and include more than 300,000 stars. That does not mean that you can never see those stars though. As long as you have a telescope, you can use it to scan through space and locate Polaris and other bright stars.
Apparent Magnitude Formulas
Astronomers use different formulas to determine the apparent magnitude of an object in space. They look at the spectral band, which determines how sensitive the object is to a certain type of radiation. The formula also uses the flux density of an object, which is a measurement that refers to how quickly it can move through space. Astronomers can use formulas to determine the brightness ratio between two objects too. One famous formula showed that the apparent magnitude of the sun was more than -26, which made it twice as bright as the moon on an average day.
Absolute Magnitude
Another term you may hear is absolute magnitude, which describes the intrinsic brightness of an object. This refers to how bright an object appears to observers who are far away. Apparent magnitude gives you an idea of how bright an object is in space and how that brightness compares to other objects. With absolute magnitude, you can get an idea of how bright an object appears to you and others on Earth. If you want to get a better look at the stars and see how these two terms compare to each other, you can simply observe objects through the naked eye and then the lens of a telescope.