Explore | The brightest star in the night sky and the brightest star in the universe

In the writings of ancient poets, the stars were endowed with rich symbolic meanings. For example, "The night is like water flooding the steps of heaven, and the stars Altair and Vega are reflected in the eyes", "The carved bow is like a full moon, shooting towards Sirius". These two lines of poetry mention three bright stars: Altair (Hegu II), Vega and Sirius. We are familiar with the legend of Altair and Vega, and their story is told every year on the Chinese Valentine's Day. Sirius has been regarded as an evil star symbolizing war and aggression since ancient times. Today, let us explore the mystery and charm of these two stars and feel the charm of the stars described by the ancients.

Summer Triangle

▏How to find Vega and Sirius?

In the autumn night sky, the stars are twinkling. Jupiter shines brightly in the east. As the brightest star, its brightness level is as high as -2.93. In the southeast, Saturn attracts people's attention with its apparent magnitude of 0.44. There is also an orange lone star in the southwest, which is Arcturus, with a brightness level of -0.04, as if it is blinking constantly.

Looking up at the center of the sky, three bright stars form a huge triangle, which is the well-known Summer Triangle. Among them, Altair and Vega mentioned in Du Mu's poem are among them. Altair, also known as Hegu 2, is the most prominent with its magnitude of 0.75; Vega, which is across the Milky Way, has a magnitude of 0. The two stars together weave a romantic picture of the summer night. Deneb, located in the Milky Way, is a member of the Summer Triangle, but it is the darkest one among them, with a magnitude of 1.25.

Winter Triangle

As time goes by, when the season steps into the threshold of winter, the Summer Triangle will quietly hide in the western horizon after nightfall. Instead, the beautiful Winter Triangle is replaced. It is composed of Betelgeuse in Orion (magnitude 0.45), Procyon in Canis Minor (magnitude 0.4) and Sirius in Canis Major (known as the first star in the night sky). The apparent magnitude of Sirius reaches an astonishing -1.47. The ancients said: "Look northwest and shoot Sirius", and this star carries countless meanings and yearnings.

Vega and Sirius are both treasures in the starry sky. One stands in the Summer Triangle with its unique brilliance, evoking people's romantic reverie; the other emits a cold light in the Winter Triangle, causing people to think about its mystery. Their peculiarity lies in that they are not only symbols of light, but also carriers of culture and legends. Behind each star are countless stories and people's yearning for the unknown.

▏Which is bigger, Vega or Sirius?

All stars have the ability to emit their own light, and the brilliance of massive stars is more dazzling. Although Sirius is brighter than Vega, this does not mean that its mass or volume is larger. In fact, after observation by astronomers, Vega is slightly better in both mass and volume. Specifically, Vega's mass is about 2.14 times that of the sun, and its radius is the product of 2.26 times and 2.78 times that of the sun. In comparison, although Sirius' mass is 2.02 times that of the sun, its radius is slightly smaller, only 1.71 times that of the sun. Surprisingly, the 0.12 solar masses that Vega has over Sirius are equivalent to the mass of Proxima Centauri. This subtle difference allows us to have a deeper understanding of the diversity of stars.

▏The shapes of Vega and Sirius

Vega's striking feature is its unique shape. According to astronomers, its equatorial radius is 23% larger than its polar radius, forming an ellipsoidal appearance. This shape is caused by Vega's extremely fast rotation speed. It can complete a rotation every 12.5 hours. Its rotation speed at the equator is as high as 274 kilometers per second, as if it is "flattened" by its own rotation force. In comparison, Sirius is more gentle, with a rotation speed of only 16 kilometers per second, so its shape still maintains a relatively standard spherical shape.

▏Which is brighter, Vega or Sirius?

In the night sky of stars, Vega in summer nights attracts attention with its dazzling brilliance, while Sirius in winter also shines brightly. So, which of these two stars is brighter? We can answer this question through a more precise measurement method - apparent magnitude. The apparent magnitude is an indicator to measure the brightness of stars that we can see with our naked eyes. The smaller the value, the brighter the star is visually.

Vega's apparent magnitude is 0, while Sirius's is as low as -1.47, which undoubtedly makes Sirius appear brighter visually. According to astronomers' research, for every increase in magnitude, the brightness of a star will differ by about 2.512 times. Based on this rule, we can infer that Sirius' brightness is about 4 times higher than Vega's.

Although both Vega and Sirius are bright stars in the night sky, through the calculation of apparent magnitude, we can clearly conclude that Sirius is visually brighter. However, in addition to the apparent magnitude, we also need to consider the actual luminosity of the star. Vega is 37 times brighter than the Sun, while Sirius is slightly less luminous, but still 25.4 times brighter than the Sun. This also means that in terms of real luminosity, Vega is actually brighter.

Why do we feel that Vega is dimmer than Sirius when we observe from Earth? This is mainly because Vega is farther away from Earth, at a distance of 25 light years. Sirius is relatively close, at only 8.6 light years. Due to the difference in distance, the brightness we observe will also be different. This also emphasizes once again that the apparent magnitude cannot fully reflect the true luminous ability of a star.

In order to more accurately measure the true luminous ability of stars, astronomers proposed the concept of absolute magnitude. Absolute magnitude refers to the brightness of a star measured at a distance of 32.6 light years from the Earth. In this way, it is like putting all stars on the same starting line, and their true luminous ability is clear at a glance.

In terms of absolute magnitude, Vega is brighter than Sirius. If we assume that the positions of Vega and Sirius are swapped, so that Vega is farther away from us and Sirius is closer, how will their brightness change? By converting the apparent magnitude and absolute magnitude, we can know that if Vega's position is closer, its brightness will increase greatly; while Sirius's brightness will decrease accordingly. Specifically, if Vega's apparent magnitude is converted to -2.3, its brightness will be close to that of Jupiter; while Sirius's apparent magnitude will become 0.85, and its brightness will be comparable to that of Altair.

Although Sirius may appear brighter visually, Vega is actually brighter in terms of luminosity. The difference depends not only on the brightness of the stars themselves, but also on their distances from Earth.

▏The Age of Vega and Sirius

Although these two stars are still young, they are both short-lived. Vega is only about one-tenth the age of the Sun, about 450 million years old. In comparison, Sirius' life span is between 200 million and 300 million years. Nevertheless, they and the Sun have entered a mature stage. The reason is their mass. Vega's mass is 2.14 times that of the Sun, and Sirius's is 2.02 times that of the Sun. The life span of a star is often inversely proportional to its mass. Experts estimate that the remaining life span of both stars will not exceed about a billion years. In a blink of an eye, hundreds of millions of years will pass, and the two stars will swell into red giants, and then reach the end of their lives and eventually turn into white dwarfs. Compared with the long life of the Sun, their short life span is particularly regrettable.

Tarantula Nebula

▏The brightest star in the known universe

Located in the Large Magellanic Cloud, 165,000 light-years from Earth, there is a huge Tarantula Nebula that spans more than a thousand light-years. This nebula is full of stars, and one of them, R136a1, is particularly eye-catching.

In July 2010, astronomers discovered a stunning star here. Its brightness reached an astonishing level, 6.16 million times that of the sun, making it the brightest celestial body in the universe so far. This star, R136a1, is undoubtedly a shining pearl in the universe.

R136a1 is not only incredibly bright, but also has an incredible mass. It is estimated to be 230 to 345 times the mass of the Sun, making it one of the most massive stars known. In addition, its surface temperature is as high as 52,000°C, almost ten times that of the Sun.

In the vast universe, R136a1 is also quite large. Its radius is about 35.4 times that of the sun, equivalent to about 24,638,400 kilometers. Amazingly, this star can accommodate 44,000 suns. Although R136a1 is not large compared to other fat red giants, in the universe, mass is the symbol of the value of celestial bodies.

R136a1 is also a Wolf-Rayet star. The nuclear fusion reaction inside it is extremely rapid, which makes its gravity and the pressure of the star's outward radiation form a delicate balance. This balance leads to the generation of stellar winds of up to several thousand kilometers per second on the surface of the star, causing it to lose about 50 solar masses in just one million years. This also means that the mass of R136a1 was once even larger. Compared with the gentle sun, R136a1 is undoubtedly a terrifying monster in the universe.

R136a1 vs. the Sun

▏Can R136a1 illuminate the entire solar system?

If R136a1 is placed at the core of the solar system, can it illuminate the entire solar system? First, let's discuss the sun's ability to illuminate. As the brightest celestial body in the solar system, how amazing is its brightness?

Scientists tell us that the total radiation power of the sun is as high as 3.86×10^26 watts. In comparison, the power of the electric lights we use in daily life is usually between 10 and 100 watts. The greater the power of the electric light, the brighter it is. From this, we can imagine how amazing the brightness of the sun is. Even on Earth, we are 150 million kilometers away and still dare not look directly at the strong light of the sun. Here on Earth, the apparent brightness of the sun reaches an astonishing minus 26.74.

However, as we head toward the edge of the solar system, we will find that the sun's light gradually becomes dimmer. Scientists estimate that the radius of the solar system is about 1 light year. So, when we reach the edge of the solar system, how much will the sun's brightness decrease? By using the conversion formula between apparent magnitude and absolute magnitude, we find that its brightness is comparable to that of Venus in the sky at its dimmest. This shows that, although the sun is so bright, it cannot illuminate the entire solar system alone.

So, what about the star R136a1? It is 6.16 million times brighter than the sun. What does that mean? Scientists have determined that the absolute magnitude of R136a1 is minus 12.24. This means that if R136a1 were 32.6 light years away from Earth, its apparent magnitude would be about the same as that of the full moon. Imagine traveling by moonlight on a full moon night, and you will deeply appreciate the powerful luminescence ability of R136a1.

If we put R136a1 at the position of the sun, its apparent magnitude from the earth will become extremely bright, reaching minus 43.71. This brightness is equivalent to the scene when we observe the sun from 60,000 kilometers outside the surface of the sun. However, to achieve the same brightness as the sun, we must observe it at a distance of about 0.4 light years from R136a1 - which is equivalent to a distance of 378.4 billion kilometers. This is enough to prove how strong the brightness of R136a1 is.

So how bright would R136a1 be at one light-year from the edge of the solar system? After calculation, we know that its apparent magnitude is minus 19.8. This brightness is similar to or brighter than the sun we see from Neptune, or similar to the sky in the evening when the sun has just set. But it should be noted that if the brightest star in the universe was located at the center of the solar system, the earth would be scorched by its intense heat in an instant.

Therefore, we can conclude that R136a1, the brightest star known in the universe, is fully capable of illuminating the entire solar system. However, this does not mean that we should place it at the center of the solar system. Despite its brilliance, being too close to a star also means great risks and unknown effects.

(Picture from the Internet)

Author | Kiwi graduated from Lincoln University in New Zealand. He has a strong interest in popular science knowledge and has published popular science articles in many popular science journals. He pays attention to facts and actively explores cutting-edge technology.