Is the brightest star in the night sky really the brightest star out there?

We see many stars of varying brightness in the night sky, but our view of each star’s brightness does not necessarily reflect the reality in space!

In a clear night sky you can see many stars of different brightness. Of course, these stars appear to us with a certain luminosity. Several bright stars piqued the curiosity of ancient people so much that they received their own stories and lore. There are also thousands of faint stars that many of us might consider “inconsequential.”

But are the bright stars we see from Earth bright because they give off a large amount of energy and radiation? Or does our point of view from Earth just make it appear that way?

First we need to know about the brightest stars in the night sky. First on the list is Sirius, found in the Canis Major constellation. After Sirius, next on the list are the stars Canopus, Rigil Kentaurus (also known as Alpha Centauri, the star system closest to Earth after the Sun), and Arcturus. It is interesting to note that this list of magnitudes is from Earth’s point of view. In any other star system, this list can vary significantly.

Apparent sizes

Astronomers compiled this list of stars according to their brightness using a system of stellar sizes.

The Greek astronomer and mathematician Hipparchus was the first to introduce this system. In it he grouped the brightest stars as “first magnitude” stars and the dimmer stars as “sixth magnitude” stars. All other stars are given second, third, fourth, and fifth magnitude values, depending on their relative brightness. The sizing system is in reverse order; The brighter the star, the smaller its magnitude.

This is a scale that provides a comparison between the stars and planets and some of the telescope’s limitations.

Over time, this size scale has been refined and is now officially referred to as the “apparent size” of stars. It is “apparent” as it refers to the brightness of the stars as seen from Earth. It currently makes out stars much dimmer than what the human eye can see. It also contains stars of zero magnitude and even negative magnitude! Sirius, the brightest star in the night sky, has an apparent magnitude of -1.46, while our Sun has that of -26.72.

Mathematically, we calculate apparent magnitude using a logarithmic equation given by:

mB – mA = 2.512 * protocol10 ( IA / IB )

Where m refers to the apparent sizes, and I refers to the intensity (or brightness) of two stars, denoted as A and B. We can see that this is a relative scale. Here, if star A is 100 times brighter than star B, star A has an apparent magnitude five orders of magnitude lower (lower because the brightness is in reverse order of their brightness) than star B.

Now, apparent magnitude is not a measure of the actual magnitude of stars. As seen from Earth, the brightness of celestial objects depends on two things: the intensity of the radiation emitted by that object, which is a measure of the amount of energy (in the form of light) it gives off, and the distance. Distance is an important factor here. A star that is significantly more distant would still be visible to us if it emits sufficient light energy.

This is a collection of images of the 25 brightest stars in the night sky, along with their apparent magnitudes. (Image credit: lerrosbershka/Shutterstock)

There is also the possibility that the light coming from distant stars is scattered or obscured by interstellar gas and dust particles. This would result in some reduction in the star’s brightness when viewed from Earth. However, this effect is not very significant and has been largely ignored in this article.

absolute sizes

To account for the total amount of light emitted by stars, another system called absolute magnitude was introduced. This system describes the sizes of stars according to their brightness if they were all viewed from the same distance.

Formally, the absolute magnitude of the star would be the apparent magnitude we would observe if an object were placed 10 parsecs away from us. So absolute magnitude is a much better measure of the star’s inherent brightness, as it only accounts for its radiance without considering its distance.

We can get the absolute size of any celestial object using the relationship given by:

M = m – ( 5 * protocol10(d) ) + 5

Where M is the absolute size of the object, m is the apparent magnitude, and i.e is the distance between the earth and the star.

The arrow in this image points to one of the most distant stars ever observed, Earendel, discovered by the Hubble Space Telescope in 2022. Despite being a million times brighter than the Sun, it’s barely visible even through the Hubble Space Telescope because it’s so far away. (Image credit: Spectrum)

Another measure of the intrinsic brightness of stars is the absolute bolometric magnitude. This amount makes up all the radiation emitted by a star of all wavelengths, not just the wavelengths of visible light. It also takes into account the electromagnetic radiation that we would observe if the object were 10 parsecs from Earth (equivalent to absolute size).

Since both apparent and absolute magnitudes only consider the wavelengths of visible light, absolute bolometric magnitude can be considered a better measure of the star’s intrinsic magnitude. However, for this article we will not consider the absolute bolometric magnitude.

brightness comparisons

Now we will use both apparent and absolute magnitudes to understand the magnitude of the difference between the star’s actual magnitude and the magnitude we observe here on Earth. Consider the first two brightest stars, Sirius and Canopus. Sirius’ apparent and absolute magnitudes are -1.46 and 1.43, respectively, while Canopus’s are -0.72 and -5.6, respectively. This means that while Sirius appears brighter than Canopus from Earth, Canopus would outshine Sirius a bit if they were placed the same distance from an observer.

This is an image of the brightest star in the night sky, Sirius, taken with the help of a telescope. (Image credit: AstroStar/Shutterstock)

Of course, what we observe does not give us the whole picture. Another example of this is the Star Rigel. Although only the seventh brightest star in the night sky with an apparent magnitude of 0.12, it has an absolute magnitude of -7.0, making it one of the brightest stars.

The main factor in this contrast is that these stars are at different distances from Earth. Sirius is one of the stars closest to our solar system and is only about 8.6 light-years away from us, while Canopus is much further away, 309 light-years. Rigel, on the other hand, is about 860 light-years from Earth but still shines brighter than most.

An artist’s rendering of the star R136a1 and the near-infrared image of its parent cluster, taken with ESO’s Very Large Telescope. R136a1 is considered one of the brightest stars ever observed. (Image credit: Flickr)

So which star has the highest intrinsic brightness value? This question is quite difficult to answer. There are so many stars out there whose absolute size has yet to be measured. Some of the possible candidates in this list are R136a1, pistol star and BAT99-98. However, a good rule of thumb would be that the very distant stars that are visible to us have high characteristic magnitudes.

Conclusion

Bottom line, Sirius isn’t really the “brightest” star. It appears so because it is relatively closer to us than most other stars out there. There are many stars out there that are brighter than Sirius, but unfortunately they cannot take the spotlight as they are positioned very far away.

Determining the absolute magnitude and the total amount of energy emitted by stars helps us develop numerical models of star formation. It also allows us to understand and study the evolution of stars and all the processes that take place within them. Observing and understanding high brightness stars plays a crucial role in our knowledge of the star formation process, which in turn helps us learn more about gas evolution in galaxies, supernovae, etc.

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