The star is 20 light years away. What is a light year and what is it equal to? What is this Alpha Centauri anyway?
Proxima Centauri.
Here's a classic catch-up question. Ask your friends, " Which one is closest to us?" and then watch them list nearest stars. Maybe Sirius? Alpha is there something there? Betelgeuse? The answer is obvious - this is; a massive ball of plasma located approximately 150 million kilometers from Earth. Let's clarify the question. Which star is closest to the Sun?
Nearest star
You've probably heard that the third brightest star in the sky is only 4.37 light years away. But Alpha Centauri not a single star, but a system of three stars. First, a double star (binary star) with a common center of gravity and an orbital period of 80 years. Alpha Centauri A is only slightly more massive and brighter than the Sun, and Alpha Centauri B is slightly less massive than the Sun. There is also a third component in this system, a dim red dwarf. Proxima Centauri.
Proxima Centauri- That's what it is the closest star to our Sun, located only 4.24 light years away.
Proxima Centauri.
Multiple star system Alpha Centauri located in the constellation Centaurus, which is visible only in the southern hemisphere. Unfortunately, even if you see this system, you will not be able to see Proxima Centauri. This star is so dim that you'll need a fairly powerful telescope to see it.
Let's find out the scale of how far Proxima Centauri from U.S. Think about . moves at a speed of almost 60,000 km/h, the fastest in. He covered this path in 2015 in 9 years. Traveling at such speed to get to Proxima Centauri, New Horizons will require 78,000 light years.
Proxima Centauri is the closest star over 32,000 light years, and it will hold this record for another 33,000 years. It will make its closest approach to the Sun in about 26,700 years, when the distance from this star to Earth will be only 3.11 light years. In 33,000 years, the nearest star will be Ross 248.
What about the northern hemisphere?
For those of us in the northern hemisphere, the closest visible star is Barnard's Star, another red dwarf in the constellation Ophiuchus. Unfortunately, like Proxima Centauri, Barnard's Star is too dim to be seen with the naked eye.
Barnard's Star.
Nearest star, which you can see with the naked eye in the northern hemisphere is Sirius (Alpha Canis Majoris). Sirius is twice the size and mass of the Sun, and is the brightest star in the sky. Located 8.6 light-years away in the constellation Canis Major, it is the most famous star that haunts Orion in the winter night sky.
How did astronomers measure the distance to stars?
They use a method called . Let's do a little experiment. Keep one arm extended and place your finger so that some distant object is nearby. Now open and close each eye one by one. Notice how your finger seems to jump back and forth as you look with different eyes. This is the parallax method.
Parallax.
To measure the distance to stars, you can measure the angle to the star with respect to when the Earth is on one side of the orbit, say in the summer, then 6 months later when the Earth moves to the opposite side of the orbit, and then measure the angle to the star compared to which some distant object. If the star is close to us, this angle can be measured and the distance calculated.
You can actually measure the distance this way to nearest stars, but this method only works up to 100,000 light years.
20 nearest stars
Here is a list of the 20 closest star systems and their distance in light years. Some of them have multiple stars, but they are part of the same system.
| Star | Distance, St. years |
| Alpha Centauri | 4,2 |
| Barnard's Star | 5,9 |
| Wolf 359 (Wolf 359; CN Leo) | 7,8 |
| Lalande 21185 (Lalande 21185) | 8,3 |
| Sirius | 8,6 |
| Luyten 726-8 | 8,7 |
| Ross 154 | 9,7 |
| Ross 248 | 10,3 |
| Epsilon Eridani | 10,5 |
| Lacaille 9352 | 10,7 |
| Ross 128 | 10,9 |
| EZ Aquarii (EZ Aquarii) | 11,3 |
| Procyon | 11,4 |
| 61 Cygni | 11,4 |
| Struve 2398 (Struve 2398) | 11,5 |
| Groombridge 34 | 11,6 |
| Epsilon Indian | 11,8 |
| DX Cancri | 11,8 |
| Tau Ceti | 11,9 |
| GJ 106 | 11,9 |
According to NASA, there are 45 stars within a radius of 17 light years from the Sun. There are more than 200 billion stars. Some are so faint that they are almost undetectable. Perhaps, with new technologies, scientists will find stars even closer to us.
Title of the article you read "Nearest star to the Sun".
Cosmic distances are difficult to measure in ordinary meters and kilometers, so astronomers use other physical units in their work. One of them is called a light year.
Many fantasy fans are very familiar with this concept, as it often appears in films and books. But not everyone knows what a light year is, and some even think that it is similar to the usual annual calculation of time.
What is a light year?
In reality, a light year is not a unit of time, as one might assume, but a unit of length used in astronomy. It refers to the distance traveled by light in one year.
It is usually used in astronomy textbooks or popular science fiction to determine lengths within the solar system. For more accurate mathematical calculations or measuring distances in the Universe, another unit is taken as a basis - .
The appearance of the light year in astronomy was associated with the development of stellar sciences and the need to use parameters comparable to the scale of space. The concept was introduced several years after the first successful measurement of the distance from the Sun to the star 61 Cygni in 1838. 
Initially, a light year was the distance traveled by light in one tropical year, that is, in a period of time equal to the full cycle of seasons. However, since 1984, the Julian year (365.25 days) began to be used as a basis, as a result of which the measurements became more accurate.
How is the speed of light determined?
To calculate a light year, researchers had to first determine the speed of light. Astronomers once believed that the propagation of rays in space was instantaneous, but in the 17th century this conclusion began to be questioned.
The first attempts to make calculations were made by Galileo Gallilei, who decided to calculate the time it takes light to travel 8 km. His research was unsuccessful. James Bradley managed to calculate the approximate value in 1728, who determined the speed at 301 thousand km/s.
What is the speed of light?
Despite the fact that Bradley made fairly accurate calculations, they were able to determine the exact speed only in the 20th century, using modern laser technologies. Advanced equipment made it possible to make calculations corrected for the refractive index of rays, resulting in this value being 299,792.458 kilometers per second. 
Astronomers operate with these figures to this day. Subsequently, simple calculations helped to accurately determine the time that the rays needed to fly around the orbit of the globe without the influence of gravitational fields on them.
Although the speed of light is not comparable to earthly distances, its use in calculations is explained by the fact that people are accustomed to thinking in “earthly” categories.
What is a light year equal to?
If we take into account that a light second is equal to 299,792,458 meters, it is easy to calculate that light travels 17,987,547,480 meters in a minute. As a rule, astrophysicists use this data to measure distances inside planetary systems.
To study celestial bodies on the scale of the Universe, it is much more convenient to take as a basis a light year, which is equal to 9.460 trillion kilometers or 0.306 parsecs. Observing cosmic bodies is the only case when a person can see the past with his own eyes.
It takes many years for light emitted by a distant star to reach Earth. For this reason, when observing cosmic objects, you see them not as they are at the moment, but as they were at the moment of light emission.
Examples of distances in light years
Thanks to the ability to calculate the speed of movement of rays, astronomers were able to calculate the distance in light years to many celestial bodies. Thus, the distance from our planet to the Moon is 1.3 light seconds, to Proxima Centauri - 4.2 light years, to the Andromeda nebula - 2.5 million light years. 
The distance between the Sun and the center of our galaxy takes rays approximately 26 thousand light years, and between the Sun and the planet Pluto - 5 light hours.
On February 22, 2017, NASA reported that 7 exoplanets were found around the single star TRAPPIST-1. Three of them are in the range of distances from the star in which the planet can have liquid water, and water is a key condition for life. It is also reported that this star system is located at a distance of 40 light years from Earth.
This message caused a lot of noise in the media; some even thought that humanity was one step away from building new settlements near a new star, but this is not so. But 40 light years is a lot, it’s a LOT, it’s too many kilometers, that is, it’s a monstrously colossal distance!
From a physics course, the third escape velocity is known - this is the speed that a body must have at the surface of the Earth in order to go beyond the solar system. The value of this speed is 16.65 km/sec. Conventional orbital spacecraft take off at a speed of 7.9 km/sec and orbit the Earth. In principle, a speed of 16-20 km/sec is quite accessible to modern earthly technologies, but no more!
Humanity has not yet learned to accelerate spaceships faster than 20 km/sec.
Let's calculate how many years it will take a starship flying at a speed of 20 km/sec to travel 40 light years and reach the star TRAPPIST-1.
One light year is the distance that a beam of light travels in a vacuum, and the speed of light is approximately 300 thousand km/sec.
A human-made spaceship flies at a speed of 20 km/sec, that is, 15,000 times slower than the speed of light. Such a ship will cover 40 light years in a time equal to 40*15000=600000 years!
An Earth ship (at the current level of technology) will reach the star TRAPPIST-1 in about 600 thousand years! Homo sapiens has existed on Earth (according to scientists) for only 35-40 thousand years, but here it is as much as 600 thousand years!
In the near future, technology will not allow humans to reach the star TRAPPIST-1. Even promising engines (ion, photon, cosmic sails, etc.), which do not exist in earthly reality, are estimated to be able to accelerate the ship to a speed of 10,000 km/sec, which means that the flight time to the TRAPPIST-1 system will be reduced to 120 years . This is already a more or less acceptable time for flight using suspended animation or for several generations of immigrants, but today all these engines are fantastic.
Even the nearest stars are still too far from people, too far, not to mention the stars of our Galaxy or other galaxies.
The diameter of our Milky Way galaxy is approximately 100 thousand light years, that is, the journey from end to end for a modern Earth ship will be 1.5 billion years! Science suggests that our Earth is 4.5 billion years old, and multicellular life is approximately 2 billion years old. The distance to the closest galaxy to us - the Andromeda Nebula - 2.5 million light years from Earth - what monstrous distances!
As you can see, of all the living people, no one will ever set foot on the earth of a planet near another star.
Surely, having heard in some science fiction action movie an expression a la “twenty to Tatooine light years", many asked legitimate questions. I'll mention some of them:
Isn't a year a time?
Then what is it light year?
How many kilometers is it?
How long will it take to overcome light year spaceship with Earth?
I decided to devote today’s article to explaining the meaning of this unit of measurement, comparing it with our usual kilometers and demonstrating the scale that it operates Universe.
Virtual racer.
Let's imagine a person, in violation of all the rules, rushing along a highway at a speed of 250 km/h. In two hours it will cover 500 km, and in four – as much as 1000. Unless, of course, it crashes in the process...
It would seem that this is speed! But in order to circumnavigate the entire globe (≈ 40,000 km), our racer will need 40 times more time. And this is already 4 x 40 = 160 hours. Or almost a whole week of continuous driving!
In the end, however, we will not say that he covered 40,000,000 meters. Because laziness has always forced us to invent and use shorter alternative units of measurement.
Limit.
From a school physics course, everyone should know that the fastest rider in Universe- light. In one second, its beam covers a distance of approximately 300,000 km, and thus it will circle the globe in 0.134 seconds. That's 4,298,507 times faster than our virtual racer!
From Earth before Moon the light reaches on average 1.25 s, up to Sun its beam will reach in a little more than 8 minutes.
Colossal, isn't it? But the existence of speeds greater than the speed of light has not yet been proven. Therefore, the scientific world decided that it would be logical to measure cosmic scales in units that a radio wave (which light, in particular, is) travels over certain time intervals.
Distances.
Thus, light year- nothing more than the distance that a ray of light travels in one year. On interstellar scales, using distance units smaller than this does not make much sense. And yet they are there. Here are their approximate values:
1 light second ≈ 300,000 km;
1 light minute ≈ 18,000,000 km;
1 light hour ≈ 1,080,000,000 km;
1 light day ≈ 26,000,000,000 km;
1 light week ≈ 181,000,000,000 km;
1 light month ≈ 790,000,000,000 km.
Now, so that you understand where the numbers come from, let’s calculate what one is equal to light year.
There are 365 days in a year, 24 hours in a day, 60 minutes in an hour, and 60 seconds in a minute. Thus, a year consists of 365 x 24 x 60 x 60 = 31,536,000 seconds. In one second, light travels 300,000 km. Therefore, in a year its beam will cover a distance of 31,536,000 x 300,000 = 9,460,800,000,000 km.
This number reads like this: NINE TRILLION, FOUR HUNDRED AND SIXTY BILLION AND EIGHT HUNDRED MILLION kilometers.
Of course, the exact meaning light years slightly different from what we calculated. But when describing distances to stars in popular science articles, the highest accuracy is, in principle, not needed, and a hundred or two million kilometers will not play a special role here.
Now let's continue our thought experiments...
Scale.
Let's assume that modern spaceship leaves solar system with the third escape velocity (≈ 16.7 km/s). First light year he will overcome it in 18,000 years!
4,36 light years to the closest star system to us ( Alpha Centauri, see the image at the beginning) it will overcome in about 78 thousand years!
Our Milky Way galaxy, having a diameter of approximately 100,000 light years, it will cross in 1 billion 780 million years.
Astronomers have discovered the first potentially habitable planet outside the solar system.
The reason for this conclusion is provided by the work of American “exoplanet hunters” (exoplanets are those that revolve around other stars, and not around the Sun).
It is published by the Astrophysical Journal. The publication can be found on the website arXiv.org.
The red dwarf Gliese-581, which, when viewed from Earth, is located in the constellation Libra at a distance of 20.5 light years (one light year = the distance that light travels in a year at a speed of 300 thousand km/sec.), has long attracted attract the attention of “exoplanet hunters”.
It is known that among the exoplanets discovered so far, most are very massive and similar to Jupiter - they are easier to find.
In April last year, a planet was found in the Gliese-581 system, which at that time became the lightest known solar planet outside the Solar System, orbiting stars similar in parameters to the Sun.
Planet Gliese-581e (the fourth in that system) turned out to be only 1.9 times more massive than Earth.
This planet orbits its star in just 3 (Earth) days and 4 hours.
Now scientists are reporting the discovery of two more planets in this star system. Of greatest interest is the sixth planet discovered - Gliese-581g.
It is what astronomers call the first suitable for life.
Using their own data and archival data from the Keck Telescope, which is based in the Hawaiian Islands, the researchers measured the parameters of this planet and came to the conclusion that there may be an atmosphere and the existence of liquid water.
Thus, scientists have established that this planet has a radius from 1.2 to 1.5 Earth radii, a mass from 3.1 to 4.3 Earth masses and a period of revolution around its star of 36.6 Earth days. The semimajor axis of this planet's elliptical orbit is about 0.146 astronomical units (1 astronomical unit is the average distance between the Earth and the Sun, which is approximately 146.9 million km).
The acceleration of free fall on the surface of this planet exceeds a similar parameter for the Earth by 1.1-1.7 times.
As for the temperature regime on the surface of Gliese-581g, it, according to scientists, ranges from -31 to -12 degrees Celsius.
And although for the average person this range cannot be called anything other than frosty, on Earth life exists in a much wider range from -70 in Antarctica to 113 degrees Celsius in geothermal springs where microorganisms live.
Since the planet is quite close to its star, there is a high probability that Gliese-581g, due to tidal forces, is always turned to one side towards its star, just as the Moon always “looks” at the Earth with only one of its hemispheres.
The fact that in less than 20 years, astronomers have gone from discovering the first planet around other stars to potentially habitable planets, indicates, according to the authors of the sensational work, that there are many more such planets than previously thought.
And even our Milky Way galaxy may be rife with potentially habitable planets.
To discover this planet, it took more than 200 measurements with an accuracy of, for example, a speed of 1.6 m/sec.
Since our galaxy is home to hundreds of billions of stars, scientists conclude that tens of billions of them have potentially habitable planets.
