Which is the brightest star in the night sky?

Produced by: Science Popularization China

Author: Qi Yifan [China University of Geosciences (Wuhan)]

Producer: China Science Expo

"Can the brightest star in the night sky hear

The loneliness and sigh in the heart of the person looking up

Many people may be familiar with the song "The Brightest Star in the Night Sky", but have you ever thought about which is the "brightest star in the night sky"?

In fact, strictly speaking, our star the Sun and our satellite the Moon are also stars; the brightness of several planets in the solar system (such as Venus, Mars, and Jupiter) varies greatly from the Earth because their distances from the Earth vary greatly; in addition, the stars that emit light themselves are large and small, and they are far and near from the Earth, so relatively speaking, the closer they are to us, the larger they are, and the higher their energy, the brighter they will appear.

So, for the sake of rigor, we need to ask the question more specifically: Which star appears to be the brightest when looking at the night sky from Earth?

The champion who stands out from the crowd

First, let me announce the answer. The champion is Sirius!

There are many bright stars in the winter night sky. It is easy to find three stars side by side in the southern sky. This is the "belt" of Orion, also known as the "three stars of fortune, wealth and longevity" in Chinese folklore. Looking roughly to the lower left along the direction of the three stars, there is an extremely bright star with blue-white starlight, which is Sirius.

Sirius is not only the brightest star in the winter night sky, it is also the brightest star in the entire sky.

It should be noted here that people have defined two standards to evaluate whether a star is bright: one is the "apparent magnitude" which takes into account both the luminous ability of the star and the distance from the observer; the other is the "absolute magnitude" which is measured assuming that all stars are placed at a distance of 10 parsecs from the earth ("parsec" is a unit of distance, about 32.6 light-years). For these two standards, the smaller the value, the greater the brightness.

The absolute magnitude of Sirius is not ranked very high, but because it is the sixth closest star to the Earth, only 8.6 light-years away, its apparent magnitude can reach -1.45, which is more than twice that of the second-ranked Canopus (αCar, -0.65). It can be said to be the true crown of bright stars.

Figure 1 How to find Sirius in the night sky

(Image source: Homemade based on Stellarium)

Su Dongpo's Puns

Speaking of Sirius, many people may think of a line from Su Dongpo's "Jiangchengzi·Mizhou Hunting":

I can draw a bow like a full moon, look northwest, and shoot the wolf in the sky. "

This sentence is written with boldness and can be regarded as a model of the bold and unrestrained style. But this sentence actually contains a pun. When we understand this pun and read it again, we may have a different experience.

If you have found the position of Sirius, you will find that Sirius is usually in the southern sky. It rises from the southeast horizon in autumn and winter, reaches the zenith in the south sky, and sets in the southwest. Similarly, it is impossible for Su Dongpo to see Sirius in the northwest in Mizhou (now Zhucheng, Shandong). However, Sirius was also called "Dog Star" in ancient times, which means invasion from outside. So, is this sentence a scientific mistake made by Su Dongpo forcibly using allusions without considering the actual situation?

Actually not, because we have to read it together with the first half of the sentence: In addition to the name of the celestial body used in the sentence "shooting the wolf in the sky", "being able to draw a carved bow like a full moon" is also an implicit celestial phenomenon. This "bow" does not simply refer to a weapon for fighting, but also refers to a star official in the Jing constellation among the twenty-eight constellations in the sky - "Hu Shi". Hu Shi is composed of nine stars, and as you can see from the name, it is the shape of a fully drawn bow and arrow, which is actually the true meaning of "being able to draw a carved bow like a full moon".

Figure 2 The star official "Arculus" of Jingxiu and Sirius

(Image source: Homemade based on Stellarium)

We can see that Sagitta 1, 2, and 3 form an arrow, Sagitta 4, 5, 6, 2, 7, and 8 form a bow, and Sagitta 4, 3, 9, and 8 form a fully drawn bowstring. The approximate direction of the arrow is the position of Sirius. The coordinate system of the celestial map is defined as follows: when you look up at the zenith, the geographic east, west, south, and north are the east, west, and north of the celestial sphere. Therefore, on the celestial map, north is up, south is down, east is left, and west is right. The direction of the arrow is exactly northwest.

Figure 3 The position of the sagitta and Sirius on the celestial sphere

(Image source: Homemade based on Stellarium)

Looking back at this sentence, we can understand it from two levels: first, from a literary perspective, Sirius refers to the invading enemy troops on the northwest border, and the full moon carved bow really refers to the weapon in the hand, looking forward to going to the northwest border to fight; second, from an astronomical perspective, the arc and arrow in the sky have been aiming at Sirius, and it has been like this for thousands of years, just like the guards on the Great Wall have been vigilant against the invading enemy.

Sirius, more than one

When you find Sirius in the night sky and wander in the sky with the famous quote, I will tell you a secret: in fact, the Sirius you see is not just a star.

Sirius is actually a binary star system, consisting of a main sequence star with a spectral type (astronomers can infer the surface temperature of a star based on its spectrum, and classify stars based on this temperature, with temperatures ranked from high to low as O, B, A, F, G, K, and M, and each category is further subdivided by numbers) of A0 or A1 - Sirius A (the main sequence stage is the most stable stage in the life of a star, and the time a star stays in this stage accounts for more than 90% of its entire life span, and the sun is in this stage) and a white dwarf with a spectral type of DA2 - Sirius B (white dwarfs are usually very small in size, are the product of supernova explosions, and are the final destination of some stellar evolution).

In 1844, German astronomer Friedrich Wilhelm Bessel wrote in a letter that, based on observations of Sirius's inherent motion, Sirius should have an undiscovered companion star. In January 1862, astronomer Alvan Graham Clark observed Sirius B for the first time based on calculations using the largest telescope in the United States at the time (about 470 mm in diameter). Since Sirius was called the Dog Star in ancient times, Sirius B was also called the Pup Star after it was discovered.

Figure 4 The relative position changes of Sirius A and B

(Image source: Wikipedia)

The orbit of Sirius B relative to Sirius A is an ellipse. Seen from Earth, their maximum distance from each other is no more than 11 arc seconds (arc seconds is a unit for describing angles, which is 1/3600 of a degree. In astronomical observations, arc seconds are used to describe parameters such as the visual size and visual distance of celestial bodies. 11 arc seconds is equivalent to a person observing a one-yuan coin about 20 km away), and their closest distance is 3 arc seconds. Therefore, if you want to observe this binary star system, the best chance is to observe them when they are farthest apart.

The good news is that in recent years, they are the farthest apart (if observed from the Earth), so everyone should take out the astronomical telescopes at home to observe this "Puppy Star"!

Figure 5 Sirius A and Sirius B

(Photo credit: Damian Peach & Qi Yifan)

We can see that Sirius A looks big and bright, while Sirius B is very small and dim, almost drowning in the light of Sirius A. This is because Sirius B is already an old white dwarf, it is small and non-luminous, and its diameter is even smaller than that of the earth, which is why we ignore it when observing Sirius.

But don't underestimate it, its density is very high: its current mass is about the same as that of the sun, and it evolved from a star with five solar masses. It was once a B5V main sequence star, which is larger than the current Sirius A! Now Sirius A is a star on the main sequence, with a mass of about 2 suns and a diameter of 1.7 times that of the sun, and it is continuously undergoing nuclear fusion, emitting light and heat.

Figure 6 Size comparison of Sirius A, Sirius B, the Sun, and the Earth

(Image source: self-made based on the Wikipedia "Sirius" entry)

Now look up at Sirius again. From "Look Northwest, Shoot Sirius" to "Puppy Star", do you have a more comprehensive understanding of this blue and white "brightest star in the night sky"?

Acknowledgements:

We would like to thank the following people for their comments on this article: Lin Kai, Liang Yu [China University of Geosciences (Wuhan)]; Peng Xiaogang (Shijiazhuang No.2 Middle School Experimental School); Yang Han (Beijing Motie Culture Group Co., Ltd.).

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