A parsec is a unit of measurement equal to 3.26 light-years, calculated using the parallax of 1 arcsecond. Parallax is used to measure the distance to stars, with 1 arcsecond of parallax indicating a distance of 1 parsec. Parsecs are more convenient than light years for astronomical distances, with a kiloparsec equal to 1000 parsecs and a megaparsec equal to 1 million parsecs. For stars over 100 parsecs away, other methods such as spectroscopic parallax are used to calculate distance.
A parsec is an astronomical unit of measurement equivalent to 3.26 light-years of distance, or the distance that photons will travel in a vacuum over a period of 3.26 years. Light travels at an approximate speed of 186,000 miles per second (300,000 kilometers per second), so this distance is just over 19 trillion miles (about 31 trillion kilometers).
By comparison, the Sun’s average distance from Earth is only 93 million miles (150,000,000 km). This distance is referred to as 1 astronomical unit (AU). A person would have to make 103,000 round trips to the Sun to cover the distance indicated by a single parsec. Earth’s solar system, defined for example by the orbit of Pluto, is only 1/800 of a light-year across. It would have to be 2.608 times larger to equal 1 parsec in diameter.
This distance is calculated using the parallax of 1 arcsecond, which leads to the shortest term, parsec. To understand what this means, it will be helpful to define the terms parallax and arcsecond.
In a spherical plane or simple circle bisected uniformly by 180 lines forming 360 equal sections, the distance between two adjacent lines is equal to 1° of arc. All arcs added together equal 360° or the entire circle. If each degree of arc is further bisected into 60 other equal sections, each such section equals 1 arc minute, so 60 arc minutes equals 1 arc degree. Each minute of arc can be divided into another 60 equal sections, representing seconds of arc. An arcsecond is therefore an angular measure that equals 1/60 of an arcminute, or 1/3600 of a single degree of arc.
Parallax refers to the apparent movement of a fixed object along an angular trajectory due to a change in the observer’s position. For example, if a person uses one eye to look at a computer monitor and then switches eyes, the monitor will appear to “jump” horizontally against the background. Scientists use parallax to measure distance to stars.
To achieve the parallax effect, an object is photographed against a background of stars from a fixed position on Earth. Six months later, when the Earth has completed half its orbit at a relative distance of 186 million miles (2 AU) from the first location, a second photograph is taken. By measuring how far the object ‘jumped’, scientists can calculate the arcseconds of parallax to reveal the distance. (Incidentally, a third photograph is taken within a full year of the original location to calculate and subtract any effects from natural seasonal shift.) If a star generated 1 arc of parallax per year, scientists would know that the distance to that star is 1 parsec, although no stars lie neatly at this distance.
The further away the object is, the less parallax it has, while the closer the object is, the more parallax. This means that the distance is inversely proportional to the parallax: an object with a parallax of 0.5 arcseconds would be twice the distance of an object with 1 arcsecond of parallax. Conversely, if a star were close enough to have 2 arcseconds of parallax, it would be twice as close as an object with 1 arcsecond of parallax.
In reality, there are no stars this close to Earth, apart from the Sun. Parallax is therefore measured in fractional increments corresponding to larger distances. Scientists also use milliarcseconds (mas) or 1/1000th of an arcsecond to indicate parallax in whole numbers. For example, the Sirius system was approximately 2.6 parsecs, (0.37921 arcsecond), or 379.21 mas away.
Parsecs are more convenient for indicating astronomical distances than light years. A thousand of these is known as a kiloparsec, or kpc, while a megaparsec is equal to 1 million, abbreviated as Mpc. A trip from Earth to the center of the Milky Way would be a long journey at just over 8.5 kpc.
While the kpc and Mpc units are useful, to actually measure very distant stars over 100 parsecs or over 400 light years, parallax is no longer viable. If so, scientists use other methods that involve calculating brightness, sometimes referred to as spectroscopic parallax.
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