What Affects the Apparent Magnitude of a Star or Planet?

by Carson
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When you go stargazing, you see some bright stars, some barely visible ones, and others only seen when assisted by tools. This is because the apparent brightness, or apparent magnitude, of a star or planet, is affected by many factors. Let’s find out why this happens in this article.

The Apparent Magnitude of a Star

What makes a star brighter or dimmer from Earth’s perspective? There are two main factors: luminosity and distance. The closer the star is to Earth, the brighter it looks. Luminosity is influenced by the star’s size and temperature. If a star is larger and/or hotter than another, it is the more luminous of the two if all other factors are equal. In other words, if those three parameters (size, temperature, distance) are present, we can calculate how bright a star will look from Earth.

Let’s say two stars have the same luminosity. If one is closer to our planet, it will look brighter than the other star from our view. That’s why extremely bright stars might be invisible to the naked eye because Earth is just too far away. If two stars are equally close to Earth, the star that emits more light will be brighter than the other.

No matter what, we have to use the inverse square law. This applies to gravitation and other physical phenomena, but let’s talk about luminosity to catch up with the article’s topic. If the distance is doubled, the star will look only 0.25 times as bright. On the contrary, if the distance is halved, the object will be four times as bright as viewed from the original distance. This algorithm is helpful in calculating the actual luminosity of the star, as we can just do some calculations and compare the brightness of two stars conveniently.

The Apparent Magnitude of a Planet

This part is more complicated because more factors are involved: Its bond albedo, its distance to Earth, the apparent brightness of the star (from the planet’s perspective), and the relative position of the planet from Earth.

The bond albedo of an object is basically how much light is reflected divided by how much light hits its surface. For instance, if the bond albedo of an object is 0.7, it reflects 70% of all light that arrives at it. An object’s bond albedo is affected by its surface and atmospheric composition. For example, if an object is covered with white materials, its bond albedo will be near 1 (100%). The larger an object’s bond albedo, the more light it reflects.

Of course, its bond albedo will only come into effect if the star that illuminates it has sufficient apparent brightness from the planet’s perspective. That means rogue planets are very dim and cannot be observed with simple tools from far away, no matter how large the object’s bond albedo is.

The relative position of the planet from the Earth is an essential factor, too. For instance, if a planet is between the Sun and Earth, all light reflecting off of it will not be observable by Earth. However, when more of the reflected light reaches our planet, we will perceive the planet as getting brighter. This cycle repeats as both objects go around our star or as an exoplanet orbits another star.

How Is Apparent Magnitude Calculated?

After discussing the parameters that can change it, it’s time to talk about how apparent magnitude is calculated. Remember that the smaller the magnitude, the brighter the star. And this is a logarithmic scale, meaning that a 1st-magnitude star is not 2 “magnitudes” brighter than a 3rd-magnitude star.

An object with a magnitude 5 lower than the other is 100 times as bright as the other object. For instance, a 2nd-magnitude star is 100 times brighter than a 7th-magnitude star, while a 6th-magnitude star is 100 times dimmer than a 1st-magnitude star.

Because

\[⁵\sqrt{100}\ ≈ 2.512\]

An object with a certain magnitude is about 2.512 times brighter than one with a magnitude of one greater than the magnitude of the original object. Confused? Let’s understand this by providing examples. For instance, a 1st-magnitude star is about 2.512 times brighter than a 2nd-magnitude star, about 6.31 times brighter than a 3rd-magnitude star, approximately 15.849 times brighter than a 4th-magnitude star, about 39.811 times brighter than a 5th-magnitude star, and 100 times brighter than a 6th-magnitude star.

Absolute Magnitude

Don’t forget absolute magnitude! It’s an object’s apparent brightness viewed from a universal distance of 10 parsecs (about 32.616 light-years) away. Other than that, the logarithmic scale works the same way. The only parameter is the star’s luminosity, which can be figured out from the star’s size and temperature, as mentioned before.

Conclusion

Remember that many parameters influence the apparent brightness of an object. This includes the luminosity of the object or the associated star, and most importantly, the distance from the object to Earth. If you want to learn more, visit the websites in the references below. Moreover, if we missed crucial parameters and/or points, please leave that in the comments below.

References and Credits

  1. (n.d.). Absolute Magnitude. Retrieved July 16, 2021, from https://astronomy.swin.edu.au/cosmos/a/Absolute+Magnitude
  2.  (n.d.). What is absolute magnitude? Retrieved July 16, 2021, from https://lco.global/spacebook/distance/what-absolute-magnitude/
  3.  (n.d.). Albedo. Retrieved July 16, 2021, from https://astronomy.swin.edu.au/cosmos/a/Albedo
  4. Barbara Ryden. (2020, January 14). How Bright is a Star? Retrieved July 16, 2021, from http://www.astronomy.ohio-state.edu/~ryden/ast162_2/notes7.html
  5.  (n.d.). The Magnitude Scale. Retrieved July 16, 2021, from http://burro.case.edu/Academics/Astr221/Light/magscale.html
  6. Joe Rao. (2021, July 14). What makes a planet look bright? It’s complicated. Retrieved July 16, 2021, from https://www.space.com/planet-apparent-brightness-factors-for-skywatching

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