On February 22, 2017, NASA announced that 7 exoplanets have been found around the single star TRAPPIST-1. Three of them are in the range of distances from the star where 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 made a lot of noise in the media, it even seemed to some 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, this is a monstrously colossal distance!

From the course of physics, the third cosmic speed 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/s. Ordinary orbiting spacecraft start at a speed of 7.9 km / s, and revolve around the Earth. In principle, a speed of 16-20 km/s is quite affordable for modern earthly technologies, but no more!

Mankind has not yet learned how to accelerate spaceships faster than 20 km/sec.

Let's calculate how many years it will take for a starship flying at a speed of 20 km/s to overcome 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 300,000 km/sec.

A human-made spacecraft flies at a speed of 20 km/sec, that is, 15,000 times slower than the speed of light. Such a ship will overcome 40 light years in a time equal to 40*15000=600000 years!

An earth ship (with the current level of technology) will fly to the star TRAPPIST-1 in about 600 thousand years! Homo sapiens exists on Earth (according to scientists) only 35-40 thousand years, and here as much as 600 thousand years!

In the near future, technology will not allow a person to reach the star TRAPPIST-1. Even promising engines (ion, photon, space sails, etc.), which are not in earthly reality, can be estimated to accelerate the ship to a speed of 10,000 km / s, 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 flying with the help of suspended animation or for several generations of migrants, 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 path from end to end for a modern earthly ship will be 1.5 billion years! Science suggests that our Earth is 4.5 billion years old, and multicellular life is about 2 billion years old. The distance to the nearest galaxy to us - the Andromeda Nebula - is 2.5 million light years from Earth - what monstrous distances!

As you can see, of all people living today, no one will ever set foot on the earth of a planet near another star.

At some point in our lives, each of us has asked this question: how long does it take to fly to the stars? Is it possible to make such a flight in one human life, can such flights become the norm of everyday life? There are many answers to this complex question, depending on who asks. Some are simple, others are more difficult. To find a comprehensive answer, there are too many things to consider.

Unfortunately, no real estimates exist to help find such an answer, and this is frustrating for futurologists and interstellar travel enthusiasts. Like it or not, space is very big (and complex) and our technology is still limited. But if we ever decide to leave the "native nest", we will have several ways to get to the nearest star system in our galaxy.

The closest star to our Earth is the Sun, quite an “average” star according to the Hertzsprung-Russell “main sequence” scheme. This means that the star is very stable and provides enough sunlight for life to develop on our planet. We know that there are other planets orbiting stars near our solar system, and many of these stars are similar to our own.

In the future, if humanity wants to leave the solar system, we will have a huge selection of stars that we could go to, and many of them may well have favorable conditions for life. But where are we going and how long will it take us to get there? Don't forget that this is all just speculation and there are no guidelines for interstellar travel at this time. Well, as Gagarin said, let's go!

Reach for the star
As already noted, the closest star to our solar system is Proxima Centauri, and therefore it makes a lot of sense to start planning an interstellar mission from it. As part of the Alpha Centauri triple star system, Proxima lies 4.24 light-years (1.3 parsecs) from Earth. Alpha Centauri is, in fact, the brightest star of the three in the system, part of a tight binary system 4.37 light-years from Earth - while Proxima Centauri (the dimmest of the three) is an isolated red dwarf 0.13 light-years away from a dual system.

And while conversations about interstellar travel evoke thoughts of all sorts of "faster-than-light" (FSL) travel, ranging from warp speeds and wormholes to subspace drives, such theories are either highly fictional (like the Alcubierre drive) or exist only in science fiction. . Any mission to deep space will stretch over generations of people.

So, starting with one of the slowest forms of space travel, how long does it take to get to Proxima Centauri?

Modern methods

The question of estimating the duration of travel in space is much simpler if existing technologies and bodies in our solar system are involved in it. For example, using the technology used by the New Horizons mission, 16 hydrazine monopropellant thrusters can reach the Moon in just 8 hours and 35 minutes.

There is also the SMART-1 mission of the European Space Agency, which moved to the Moon using ion propulsion. With this revolutionary technology, a variant of which was also used by the Dawn space probe to reach Vesta, it took the SMART-1 mission a year, a month and two weeks to get to the moon.

From fast rocket spacecraft to economical ion propulsion, we have a couple of options for getting around in local space - plus you can use Jupiter or Saturn as a huge gravitational slingshot. However, if we plan to go a little further, we will have to increase the power of technology and explore new opportunities.

When we talk about possible methods, we are talking about those that involve existing technologies, or those that do not yet exist but are technically feasible. Some of them, as you will see, are time-tested and confirmed, while others remain in question. In short, they represent a possible, but very time-consuming and financially expensive scenario for traveling even to the nearest star.

Ionic movement

Now the slowest and most economical form of propulsion is the ion propulsion. A few decades ago, ionic motion was considered the subject of science fiction. But in recent years, ion thruster support technologies have moved from theory to practice, and quite successfully. The SMART-1 mission of the European Space Agency is an example of a successful mission to the Moon in 13 months of spiral motion from the Earth.

SMART-1 used solar-powered ion thrusters, in which electrical energy was collected by solar panels and used to power the Hall effect motors. It took only 82 kilograms of xenon fuel to get SMART-1 to the Moon. 1 kilogram of xenon fuel provides a delta-V of 45 m/s. This is an extremely efficient form of movement, but far from the fastest.

One of the first missions to use ion thruster technology was the Deep Space 1 mission to Comet Borrelli in 1998. The DS1 also used a xenon ion engine and used 81.5 kg of fuel. In 20 months of thrust, the DS1 reached speeds of 56,000 km/h at the time of the comet's flyby.

Ion thrusters are more economical than rocket technologies because their thrust per unit mass of propellant (specific impulse) is much higher. But ion thrusters take a long time to accelerate a spacecraft to substantial speeds, and top speeds depend on fuel support and power generation.

Therefore, if ion propulsion is used in a mission to Proxima Centauri, the engines must have a powerful source of energy (nuclear energy) and large fuel reserves (albeit less than conventional rockets). But if you start from the assumption that 81.5 kg of xenon fuel translates into 56,000 km / h (and there will be no other forms of movement), you can make calculations.

At a maximum speed of 56,000 km/h, Deep Space 1 would take 81,000 years to cover the 4.24 light-years between Earth and Proxima Centauri. In time, this is about 2700 generations of people. It's safe to say that an interplanetary ion drive would be too slow for a manned interstellar mission.

But if the ion thrusters are larger and more powerful (i.e., the ion outflow rate is much faster), if there is enough rocket fuel to last the entire 4.24 light years, the travel time will be significantly reduced. But there will still be much more than a human lifespan.

Gravity maneuver

The fastest way to space travel is by using the gravity assist. This method involves the spacecraft using the relative motion (i.e. orbit) and gravity of the planet to change path and speed. Gravity maneuvers are an extremely useful spaceflight technique, especially when using the Earth or another massive planet (like a gas giant) for acceleration.

The Mariner 10 spacecraft was the first to use this method, using the gravitational pull of Venus to accelerate towards Mercury in February 1974. In the 1980s, the Voyager 1 probe used Saturn and Jupiter for gravitational maneuvers and acceleration to 60,000 km / h, followed by an exit into interstellar space.

The Helios 2 mission, which began in 1976 and was supposed to explore the interplanetary medium between 0.3 AU. e. and 1 a. e. from the Sun, holds the record for the highest speed developed with the help of a gravitational maneuver. At that time, Helios 1 (launched in 1974) and Helios 2 held the record for the closest approach to the Sun. Helios 2 was launched by a conventional rocket and put into a highly elongated orbit.

Due to the large eccentricity (0.54) of the 190-day solar orbit, Helios 2 managed to achieve a maximum speed of over 240,000 km/h at perihelion. This orbital speed was developed due to only the gravitational attraction of the Sun. Technically, Helios 2's perihelion speed was not the result of a gravitational maneuver, but a maximum orbital speed, but the craft still holds the record for the fastest man-made object.

If Voyager 1 were moving towards the red dwarf Proxima Centauri at a constant speed of 60,000 km/h, it would take 76,000 years (or more than 2,500 generations) to cover this distance. But if the probe were to reach the record speed of Helios 2 - a constant speed of 240,000 km / h - it would take 19,000 years (or more than 600 generations) to travel 4,243 light years. Substantially better, though not close to practical.

EM Drive Electromagnetic Motor

Another proposed method of interstellar travel is the resonant cavity RF drive, also known as the EM Drive. Proposed back in 2001 by Roger Scheuer, the British scientist who created Satellite Propulsion Research Ltd (SPR) to carry out the project, the engine is based on the idea that electromagnetic microwave cavities can directly convert electrical energy into thrust.

While traditional electromagnetic thrusters are designed to propel a certain mass (like ionized particles), this particular propulsion system is independent of mass response and does not emit directed radiation. In general, this engine was met with a fair amount of skepticism, largely because it violates the law of conservation of momentum, according to which the momentum of the system remains constant and cannot be created or destroyed, but only changed by force.

However, recent experiments with this technology have obviously led to positive results. In July 2014, at the 50th AIAA/ASME/SAE/ASEE Joint Propulsion Conference in Cleveland, Ohio, NASA advanced jet scientists announced they had successfully tested a new electromagnetic propulsion design.

In April 2015, scientists from NASA Eagleworks (part of the Johnson Space Center) said they had successfully tested this engine in a vacuum, which could indicate a possible application in space. In July of the same year, a team of scientists from the Department of Space Systems at the Dresden University of Technology developed their own version of the engine and observed tangible thrust.

In 2010, Professor Zhuang Yang from Northwestern Polytechnic University in Xi'an, China, began publishing a series of articles about her research into EM Drive technology. In 2012, she reported a high power input (2.5 kW) and a recorded thrust of 720 mn. It also conducted extensive testing in 2014, including internal temperature measurements with built-in thermocouples, which showed that the system worked.

NASA's prototype (which was given a power estimate of 0.4 N/kilowatt) calculated that an electromagnetically propelled spacecraft could make a trip to Pluto in less than 18 months. This is six times less than the New Horizons probe, which was moving at a speed of 58,000 km / h, required.

Sounds impressive. But even in this case, the ship on electromagnetic engines will fly to Proxima Centauri for 13,000 years. Close, but still not enough. In addition, until all the e is dotted in this technology, it is too early to talk about its use.

Nuclear thermal and nuclear electrical propulsion

Another possibility to carry out interstellar flight is to use a spacecraft equipped with nuclear engines. NASA has been exploring such options for decades. A nuclear thermal propulsion rocket could use uranium or deuterium reactors to heat the hydrogen in the reactor, converting it into ionized gas (hydrogen plasma), which would then be directed into the rocket nozzle, generating thrust.

A nuclear electric-powered missile includes the same reactor, which converts heat and energy into electricity, which then powers an electric motor. In both cases, the rocket will rely on nuclear fusion or fission for thrust, rather than the chemical propellants that all modern space agencies run on.

Compared to chemical engines, nuclear engines have undeniable advantages. First, it has a virtually unlimited energy density compared to propellant. In addition, a nuclear engine will also produce powerful thrust compared to the amount of fuel used. This will reduce the amount of fuel required, and at the same time the weight and cost of a particular device.

Although thermal nuclear engines have not yet gone into space, their prototypes have been created and tested, and even more have been proposed.

And yet, despite the advantages in fuel economy and specific impulse, the best proposed nuclear thermal engine concept has a maximum specific impulse of 5000 seconds (50 kN s/kg). Using nuclear engines powered by nuclear fission or fusion, NASA scientists could get a spacecraft to Mars in just 90 days if the Red Planet were 55,000,000 kilometers from Earth.

But if we're talking about the journey to Proxima Centauri, it would take centuries for a nuclear rocket to accelerate to a substantial fraction of the speed of light. Then it will take several decades of travel, and after them many more centuries of deceleration on the way to the goal. We are still 1000 years away from our destination. What is good for interplanetary missions is not so good for interstellar missions.

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 sci-fi fans are familiar with this concept, as it appears frequently in films and books. But not everyone knows what a light year is equal to, and some even think that it is similar to the usual annual calculation of time.

What is a light year?

In fact, the light year is not a unit of time, as one might assume, but a unit of length used in astronomy. It is understood as the distance traveled by light in one year.

It is commonly used in astronomy textbooks or popular science fiction to determine lengths within the solar system. For more accurate mathematical calculations or measurements of 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 a few years after the first successful measurement of the distance from the Sun to the star 61 Cygni in 1838.


Originally, a light year was called the distance traveled by light in one tropical year, that is, for a period of time equal to the full cycle of the seasons. However, since 1984, the Julian year (365.25 days) has been taken as the basis, as a result of which the measurements have become more accurate.

How is the speed of light determined?

To calculate the light year, the researchers had to first determine the speed of light. Once upon a time, astronomers believed that the propagation of rays in space occurs instantly, but in the 17th century, such a conclusion began to be questioned.

The first attempts to make calculations were made by Galileo Gallilei, who decided to calculate the time during which light travels 8 km. His research was unsuccessful. James Bradley managed to calculate the approximate value in 1728, who determined the value of the speed at 301 thousand km / s.

What is the speed of light?

Despite the fact that Bradley made fairly accurate calculations, it was only in the 20th century that they could determine the exact speed using modern laser technology. Perfect equipment made it possible to make calculations adjusted for the refractive index of the rays, as a result of which this value was 299,792.458 kilometers per second.


Astronomers operate with these figures to this day. In the future, simple calculations helped to accurately determine the time that the rays need to fly around the orbit of the globe without being affected by gravitational fields.

Although the speed of light is not comparable to earthly distances, its use in calculations is explained by the fact that people are used to thinking in "earthly" categories.

What is a light year?

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 these data to measure distances within 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. Observation of cosmic bodies is the only case when a person can see the past with his own eyes.

It takes many years for the light emitted by some distant star to reach Earth. For this reason, when observing space 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 the rays, astronomers were able to calculate the distance in light years to many celestial bodies. So, 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 is about 26 thousand light years, and between the Sun and the planet Pluto - 5 light hours.

And how many potentially explosive stars are located at an unsafe distance?

A supernova is an incredible explosion of a star - and almost beyond the limits of human imagination. If our Sun were to explode as a supernova, the resulting shock wave would probably not destroy the entire Earth, but the side of the Earth facing the Sun would disappear. Scientists believe that the temperature of the planet as a whole would increase by about 15 times. Moreover, the Earth will not remain in orbit.

A sudden decrease in the mass of the Sun could free the planet and send it wandering into space. It is clear that the distance to the Sun - 8 light minutes - is not safe. Fortunately, our Sun is not a star destined to explode in a supernova. But other stars, outside our solar system, might. What is the closest safe distance? Scientific literature shows 50 to 100 light years as the closest safe distance between Earth and a supernova.

Optical image of the remnant of supernova 1987A, taken by the Hubble Space Telescope

What happens if a supernova explodes near the Earth? Let's consider the explosion of a star other than our Sun, but still at an unsafe distance. Let's say a supernova is 30 light years away. Dr. Mark Reid, senior astronomer at the Harvard-Smithsonian Center for Astrophysics, says:

“... if there was a supernova that was about 30 light years away, it would lead to strong impacts on the Earth, possibly mass extinctions. X-rays and more energetic gamma rays from a supernova can destroy the ozone layer that protects us from the sun's ultraviolet rays. It could also ionize nitrogen and oxygen in the atmosphere, leading to the formation of large amounts of nitrous oxide-like smog in the atmosphere."

Moreover, if a supernova were to explode 30 light years away, phytoplankton and reef communities would be particularly affected. Such an event severely depletes the base of the ocean food chain.

Let's assume that the explosion was a little more distant. The explosion of a nearby star could leave Earth, its surface, and ocean life relatively untouched. But any relatively close explosion would still "douse" us with gamma rays and other high-energy particles. This radiation can cause mutations in earthly life. In addition, the radiation of the nearest supernova could change our climate.

It is known that a supernova has not exploded at such a close distance in the known history of mankind. The most recent supernova visible to the eye was supernova 1987A, in 1987. It was about 168,000 light years away. Prior to this, the last flash visible to the eye was recorded by Johannes Kepler in 1604. Approximately 20,000 light years away, it shone brighter than any star in the night sky. This explosion was visible even in daylight! As far as we know, this did not cause noticeable consequences.

How many potential supernovae are closer to us than 50 to 100 light years away? The answer depends on the type of supernova. A Type II supernova is an aging massive star that is collapsing. There are no stars massive enough to do this within 50 light-years of Earth.

But there are also Type I supernovae - caused by the collapse of a small, pale white dwarf star. These stars are dim and hard to spot, so we can't be sure how many there are around. Probably several hundred of these stars are within 50 light years.

Relative sizes of IK Pegasi A (left), B (bottom, center) and the Sun (right).

The star IK Pegasi B is the closest candidate for the role of a supernova prototype. It is part of a binary star system located about 150 light years from our Sun and solar system.

The main star in the system, IK Pegasi A, is an ordinary main sequence star not unlike our Sun. A potential Type I supernova is another star, IK Pegasi B, a massive white dwarf that is extremely small and dense. When star A begins to evolve into a red giant, it is expected to grow to a radius where it will collide with a white dwarf or start pulling matter from A's expanded gas envelope. When star B becomes massive enough, it can explode as a supernova.

What about Betelgeuse? Another star often mentioned in the history of supernovae is Betelgeuse, one of the brightest stars in our sky, part of the famous constellation Orion. Betelgeuse is a supergiant star. It is inherently very bright.

However, this brilliance comes at a price. Betelgeuse is one of the most famous stars in the sky because it will explode someday. The enormous energy of Betelgeuse requires that the fuel be used up quickly (relatively speaking), and in fact Betelgeuse is already nearing the end of its life. Someday soon (astronomically speaking) it will run out of fuel and then experience a spectacular Type II supernova explosion. When this happens, Betelgeuse will become brighter for weeks or months, perhaps as bright as the full moon and visible in broad daylight.

When will it happen? Probably not in our lives, but no one knows for sure. It could be tomorrow or a million years in the future. When this happens, everyone on Earth will witness an impressive event in the night sky, but earthly life will not be affected. This is because Betelgeuse is 430 light years away.

How often do supernovae break out in our galaxy? No one knows. Scientists have suggested that high-energy supernova radiation has already caused mutations in terrestrial species, maybe even humans.

According to one estimate, there may be one dangerous supernova event in the vicinity of the Earth every 15 million years. Other scientists say that, on average, a supernova explosion occurs within 10 parsecs (33 light years) of Earth every 240 million years. So you see what we really don't know. But you can compare those numbers to a few million years—the time humans are thought to have existed on the planet—and four and a half billion years for the very age of the Earth.

And, if you do, you will see that a supernova is sure to explode near the Earth - but probably not in the foreseeable future of mankind.

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The light year is known to many from fantastic. Despite the fact that its name is similar to the time period of the year, the year does not measure time at all, but distance. This unit is designed to measure huge.

A light year is a non-systemic unit of length. This is the distance that light travels in a vacuum in one year (365.25 days or 31,557,600 seconds).

The comparison of a light year with a calendar year began to be used after 1984. Before that, a light year is the distance traveled by light in one tropical year.

The length of the tropical year does not have an exact value, since its calculations are related to the angular velocity of the Sun, and there are variations for it. For a light year, an average value was taken.

The difference in calculation between a tropical light year and a Julian light year is 0.02 percent. And since this unit is not used for high-precision measurements, there is no practical difference between them.

The light year as a length is used in popular science literature. In astronomy, there is another off-system unit for measuring large distances - the parsec. The calculation of the parsec is based on the average radius of the earth's orbit. 1 parsec is equal to 3.2616 light years.

Calculations and distances

The calculation of a light year is directly related to the speed of light. For calculations in physics, it is usually taken to be 300,000,000 m/s. The exact value of the speed of light is 299,792,458 m/s. That is, 299,792,458 meters is just one light second!

The distance to the moon is approximately 384,400,000 meters, which means that the light beam will reach the surface of the moon in approximately 1.28 seconds.

The distance from the Sun to the Earth is 149,600,000,000. Therefore, a sunbeam hits the Earth in a little less than 7 minutes.

So, there are 31,557,600 seconds in a year. Multiplying this number by a distance equal to one light second, we get that one light year is equal to 9,460,730,472,580,800 meters.

1 million light years, respectively, will be equal to 9,460,730,472,580,800,000,000 meters.

According to approximate calculations of astronomers, the diameter of our galaxy is about 100,000 light years. That is, within our Galaxy there cannot be distances measured in millions of light years. Such numbers are applicable for measuring distances between galaxies.

The Andromeda galaxy closest to Earth is 2.5 million light-years away.

To date, the largest cosmic distance from Earth that can be measured is the distance to the edge of the observable universe. It is about 45 billion light years.

Tip 2: How long is a light year in the cosmic dimension

The term "light year" is found in many scientific articles, popular TV shows, textbooks, and even in the news from the world of science. However, some people believe that a light year is a certain unit of time, although in fact, distance can also be measured in years.

How many kilometers in a year

In order to understand the meaning of the concept of "light year", you first need to remember the school physics course, especially the section that concerns the speed of light. So, the speed of light in vacuum, where it is not affected by various factors, such as gravitational and magnetic fields, suspended particles, refraction of a transparent medium, and so on, is 299,792.5 kilometers per second. It must be understood that in this case, light means those perceived by human vision.

Less well-known distance units are the light-month, week, day, hour, minute, and second.

A sufficiently long light was considered an infinite quantity, and the first person to calculate the approximate speed of light rays in a vacuum was the astronomer Olaf Roemer in the middle of the 17th century. Of course, his data were very approximate, but the very fact of determining the final value of the speed is important. In 1970, the speed of light was determined to within one meter per second. More accurate results have not been achieved so far, as there were problems with the error of the meter standard.

Light year and other distances

Since the distances in are huge, measuring them in customary units would be irrational and inconvenient. Based on these considerations, a special light year was introduced, that is, the distance that light travels in the so-called Julian year (equal to 365.25 days). Considering that each day contains 86,400 seconds, it can be calculated that in a year a ray of light covers a distance of several more than 9.4 kilometers. This value seems huge, however, for example, the distance to the nearest star to the Earth, Proxima Centauri, is 4.2 years, and the diameter of the Milky Way galaxy exceeds 100,000 light years, that is, those visual observations that can be made now display a picture that existed about hundreds of thousands of years ago.

A beam of light covers the distance from the Earth to the Moon in about a second, but sunlight reaches our planet for more than eight minutes.

In professional astrophysics, the concept of a light year is rarely used. Scientists mainly operate with units such as the parsec and the astronomical unit. A parsec is the distance to an imaginary point from which the radius of the Earth's orbit is seen at an angle of one arc second (1/3600 of a degree). The average radius of the orbit, that is, the distance from the Earth to the Sun, is called the astronomical unit. A parsec is about 3 light years or 30.8 trillion kilometers. An astronomical unit is approximately equal to 149.6 million kilometers.

Tip 3: Is there a distance unit greater than a light year?

Meters, kilometers, miles and other units of measurement have been successfully used and continue to be used on Earth. But space exploration raised the question of introducing new measures of length, because even within the solar system you can get confused in zeros, measuring the distance in kilometers.

To measure the distance within the solar system, an astronomical unit was created - a measure of distance, which is equal to the average distance between the Sun and the Earth. However, even for the solar system, this unit does not seem quite suitable, which can be shown by a good example. If we imagine that the center of a small table corresponds to the Sun, and the astronomical unit is taken as 1 cm, then to designate the Oort cloud - the "outer boundary" of the solar system - you will have to move away from the table by 0.5 km.

If the astronomical unit was not large enough even for the solar system, all the more needed other units to measure the distances between stars and galaxies.

Light year

The unit of measurement of distance on the scale of the universe had to be based on some absolute value. This is the speed of light. Its most accurate measurement was made in 1975 - the speed of light is 299,792,458 m/s or 1,079,252,848.8 km/h.
The unit of measurement was taken as the distance that light, moving at such a speed, travels during an earthly non-leap year - 365 Earth days. This unit was called the light year.

At present, light years are more often indicated in non-fiction books and science fiction novels than in scientific works. Astronomers often use a larger unit, the parsec.

Parsec and its derivatives

The name "parsec" means "parallax of an arc second". An arc second is a unit of measurement of an angle: a circle is divided into 360 degrees, a degree is divided into 60 minutes, a minute is divided into 60 seconds. Parallax is the change in the observed position of an object depending on the position of the observer. According to the annual parallax of the stars, the distance to them is calculated. If we imagine a right-angled triangle, one of the legs in which is the semi-axis of the earth's orbit, and the hypotenuse is the distance between the Sun and another star, then the size of the angle in it is the annual parallax of this star.

At a certain distance, the annual parallax will be equal to 1 arc second, and this distance was taken as a unit of measurement called the parsec. The international designation of this unit is pc, the Russian one is pc.

A parsec is equal to 30.8568 trillion km or 3.2616 light years. However, for cosmic scales, this was not enough. Astronomers use derived units: equal to 1000 pc, - 1 million pc, and - 1 billion pc.