Looking up at the night sky, the stars are bright and dim. How long does it take for the starlight to travel to our retinas?

How long it takes for starlight to reach our retina depends on which star it is. The human eye can see about 6,000 stars, some near and some far away. If the moon is included, it is the closest and brightest star. Theoretically, the sun is also a star, because most of the stars we see are stars like the sun, but the sun is too close, so it appears particularly bright, so people do not regard it as a star.

Only a few of the stars in the night sky are planets. Although planets are very small and do not emit light themselves, they reflect sunlight. Because they are very close to us and are not at the same order of magnitude as stars, they appear very bright. In addition to the moon, the brightest stars in the sky at night are: Venus, Jupiter, Mars, Mercury, Sirius, and Saturn. Except for Sirius, the rest are planets.

There are only a few planets that we can see with our naked eyes. Their brightness magnitudes are: Venus -4.6, Jupiter -2.9, Mars -2.9, Mercury -1.9, Sirius -1.46, Saturn -0.4. Here we can talk about the brightness of stars divided into numerical levels. The smaller the value, the brighter it is, and vice versa. There are also negative numbers. The more negative, the brighter it is. The dimmest magnitude that the human eye can see is 6th magnitude.

Everything humans observe in this world depends on light. Therefore, the stars we see are all due to the light of these stars being transmitted to our retina. To know how long it takes for the light of these stars to reach our retina, we need to know the speed of light. The current accurate value of the speed of light in a vacuum is c=299792458m/s. Generally, when too much precision is not required, the value is 300,000 kilometers per second.

On the interstellar scale, because the distances between stars are too great, ordinary distance units are inconvenient, so the unit of light year is used. Here, we need to understand the concept of light year.

A light year is a unit of distance, which is the distance that light travels in one year. In astronomy, a Julian year is 365.25 days, 1 hour 3600 seconds, and 24 hours a day. A Julian year is 31557600 seconds. Based on the precise value of the speed of light c=299792458m/s, a light year is about 9.46 trillion kilometers.

Since light has speed, the world we see is in the past tense. The human eye relies on the speed of light to observe the world, so any image we see has a delay, which is the time it takes for the light emitted or reflected by the target object to reach our retina.

In this way, the stars we see are not what they look like at the moment, but what they looked like a few seconds ago based on how much time the light has traveled. For example, the sun is 150 million kilometers away from us, and it takes 500 seconds for light to travel, so the sun we see is what it looked like 500 seconds ago, and it will always look like it did 500 seconds ago; the moon is about 384,000 kilometers away from us on average, and it takes more than 1 second for light to travel, so the moon we see is always the moon that looked like more than 1 second ago.

In fact, the distance of a star from us is the same as the number of years its light has traveled, and what we see is what the star looked like many years ago. For some planets that are very close to us, it doesn't take long for their light to reach us. For example, Venus is about 42 million kilometers away from us at its closest point, and it only takes 140 seconds for the sunlight it reflects to reach us, and what we see is what it looked like 140 seconds ago. Similarly, Mars is about 60 million kilometers away from us at its closest point, and it takes 200 seconds for the sunlight it reflects to travel; Jupiter is about 630 million kilometers away from us at its closest point, and it takes 2,100 seconds for the sunlight it reflects to travel; and Saturn is about 1.2 billion kilometers away from us at its closest point, and it takes 4,000 seconds for the sunlight it reflects to travel.

The distance between stars and us is measured in light years, so the time it takes for light to travel is much longer. The farthest star that the human eye can see is Eta Carinae, which is about 7,500 light years away from us. The reason why we can see such a distant star is that Eta Carinae is very large and bright, with a mass of about 120 to 200 times that of the sun and a brightness of about 5 million times that of the sun, so it can be seen by us. Its apparent magnitude is +4.3, but it is a variable star in the late stage of evolution. Historically, its apparent magnitude has reached -0.8, which is brighter than Saturn. It takes 7,500 years for the light from Eta Carinae to reach our retina, so we see Eta Carinae as it was 7,500 years ago.

Most of the stars that humans can see with the naked eye are tens to hundreds of light years away from us. For example, the Big Dipper's first star, Shu, is 124 light years away, the second star, Tianxuan, is about 79 light years away, the third star, Ji, is about 84 light years away, the fourth star, Tianquan, is about 81 light years away, the fifth star, Yuheng, is about 81 light years away, the sixth star, Kaiyang, is about 78 light years away, and the seventh star, Yaoguang, is about 101 light years away.

There are also several more famous stars: Sirius is the brightest star visible to the naked eye, 8.6 light-years away from us; Alpha Centauri A is the closest star to us visible to the naked eye, about 4.3 light-years away; Altair is about 16 light-years away; Vega is about 25 light-years away; Polaris is about 400 light-years away, etc.

These stars are so many light years away from us that the light takes so many years to travel, and what we see are how they looked like so many years ago.

That’s it, welcome to discuss, thanks for reading.

The copyright of Space-Time Communication is original. Infringement and plagiarism are unethical behavior. Please understand and cooperate.