There are about 6000 stars visible to the naked eye. These stars are far and near. If you count the moon, it is the nearest and brightest star. Theoretically, the sun is also a star, because most of the stars we see are the same as the sun, but the sun is too close, so it looks particularly bright, so people don't regard it as a star.
Only a few stars in the night sky are planets. Although the planet is small and does not emit light, it reflects sunlight, and because it is close to us, it is not an order of magnitude with the star, so it looks bright. Besides the moon at night, the brightest stars in the sky are Venus, Jupiter, Mars, Mercury, Sirius and Saturn. Here, except Sirius, everything else is a planet.
We can only see these planets with the naked eye. Their brightness apparent magnitude is Venus -4.6, Jupiter -2.9, Mars -2.9, Mercury-1.9, Sirius-1.46, Saturn -0.4, etc. Here, magnitude brightness is divided into numerical grades. The smaller the value, the brighter it is. Conversely, the darker, the negative number. The more negative, the brighter. The darkest apparent magnitude that humans can see with the naked eye is 6.
Everything that human beings observe in this world depends on light.
So all the stars we see are because the light of this star has reached our retina. In order to know how long it takes for light from these stars to reach our retina, we need to know the speed of light propagation. At present, the accurate value of vacuum light speed is c=299792458m/s, and it is generally 300,000 kilometers per second under inaccurate conditions.
On the interstellar scale, because the distance between them is too far, ordinary distance units are very troublesome, so there are light-year units. Here, we need to understand the concept of light years.
Light years are units of distance, that is, the speed of light is one year. Astronomically, a year is 326.25 days, 1 hour is 3600 seconds, and a day is 24 hours. A Julian year is 3 1.557600 seconds. According to the exact value of the speed of light c = 299,792,458 m/s, the scale of a light year is about 9.46 trillion kilometers.
Since light has speed, the world we see is the past tense.
The human eye observes the world by the speed of light, so any image it sees has a delay. Delay time is the time required for the light emitted or reflected by the target object to reach our retina.
In this way, we can see the stars not in real time, but according to the time when they are away from the light. For example, the sun is 500 seconds away from us1.500 million kilometers, so the sun we see is what it was 500 seconds ago, and it will always be what it was 500 seconds ago; In terms of the distance to the moon, our average distance is about 384,000 kilometers, and the light propagation needs more than 1 second, so the moon we see is always the moon before 1 second.
In fact, how many light years has this star been away from us and how many years has it traveled? What we see is what this star looked like many years ago. It doesn't take long for the light from several very close planets to reach us. For example, when Venus is about 42 million kilometers away from us recently, it only takes 140 seconds for its reflected sunlight to reach us. What we saw was 140 seconds ago. By analogy, when Mars is closest to us, it is about 60 million kilometers, and the reflected sunlight takes 200 seconds. Jupiter is about 630 million kilometers away from us recently, and it takes 2 100 seconds to reflect sunlight. Saturn is about1200 million kilometers away from us recently, and it takes 4000 seconds to reflect sunlight.
The distance between the star and us is measured in light years, so it takes longer for light to travel.
The farthest star that the human eye can see is Haishan 2, which is about 7500 light years away from us. The reason why we can see stars at such a distance is that Haishan II is very big and bright, with a mass of about 120~200 times 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 later stage of evolution. Historically, its apparent magnitude has reached -0.8, which is brighter than Saturn. It takes 7500 years for the light of Haishan II to reach our retina, so we see Haishan II as it did 7500 years ago.
The vast majority of stars seen by human naked eyes are between tens of light years and hundreds of light years away from us. For example, the distances between Beidou-Shu Tian, Beidou -2 Tian Xuan and Beidou -7 are 124 light years, 79 light years, 84 light years, 8 1 light years, 8 1 light years, 78 light years and 10655 light years respectively.
There are also several famous stars: Sirius is the brightest star seen by the naked eye, 8.6 light years away from us; Nanmen2a star is the nearest star visible to the naked eye, about 4.3 light years away. Altair is about 16 light years; Vega is about 25 light years; Polaris is about 400 light years away from us.
How many light years are these stars from us, how many years have light traveled, and how many years ago are we seeing.
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