The second difference (Parsec, abbreviated as pc) isastronomyUpperLength unit。The second difference is the oldest and the most standard measurementfixed starDistance method.It is based onTrigonometric parallaxOn the basis of.The average of orbits in the orbit of the earthradius(OneAstronomical unit, AU) is the internal angle of the triangle corresponding to the bottom edgeparallax。When the size of the angle is 1s, the length of one side (the distance from the earth to the star) of the triangle (because the length difference between the two sides corresponding to the 1s angle can be completely ignored, the triangle can be imagined as an acute angle triangle or an isosceles triangle) is called the 1s difference.
Imagine the triangle formed by the star to be measured and the radius line segment (i.e. the length of one astronomical unit) of the earth's orbit, and measure the side length from the star to the sun and to the earth, and the angle between the two sides.When the included angle is one quarter second, the triangle is so narrow and long that the two sides can be regarded as equal, then the side length is called the one second gap.At this time, the triangle can be regarded as both isosceles triangle and right triangle (because the remaining two angles are very close to right angles).More specifically, the distance between a star with an annual parallax equal to 1 and the Earth is 206265AU, which is defined as a one second difference of 1pc.[1]
Seconds: The unit of distance used in astronomy.It is mainly used to measure the distance of extrasolar objects.The 1-second gap is defined as the distance from the celestial body to the earth (sun) when the semi annual parallax of the celestial body is 2 ", that is, the distance when the corresponding angle of view of the earth's orbital radius is 1".The second gap is the reciprocal of the viewing angle. When the viewing angle of an object is 0.1 ", its distance is 10 seconds. When the viewing angle of an object is 0.01", its distance is 100 seconds, and so on.
One second difference is equal to 3.2615637771418798291 light years, or 206264.806245480309553 astronomical units, or 30.856775814671915808 trillion kilometers.When measuring distant galaxies, the second difference unit is too small, which is commonly usedKilosecond gap(kpc) andMillisecond gapIs the unit.
Use the length unit of the second gap
Length unit
abbreviation
express
remarks
Second gap
pc
three point two six oneLight year(approximate value)
Can be used to measure proximityfixed starDistance between.
Kilosecond gap
kpc
1000 second gap
Can be used for measurementGalaxyorGalaxy clusterThe distance between internal celestial bodies.
Millisecond gap
Mpc
1000000 second gap
It is used to measure the distance between neighboring galaxies or galaxy clusters.
Taking the distance between the earth and the sun as the bottom edge, the distance r between the star with angle of view π=1 '' and the earth is 206264AU (=3.08568 × 10sixteenM=3.2616 light years), this distance is defined as a one second gap (1pc).Astronomers usually use the second difference rather than the astronomical unit to describe the distance of celestial bodies.
This is not only because it is easier to calculate when the number of second difference is small, but also for historical reasons.The greater the parallax of a celestial body, the closer its distance will be.On the contrary, the smaller the parallax, the farther away from us.The nearest star (except the sun)Adjacent starThe second difference of is about 1.29 pc (4.22 light years).One second difference=3.2615637771418798291 light year=206264806245480309553 astronomical units=3085677581467191580.8 billion kilometers=30856775814671915.808 meters.
verification:
In advanced mathematics, when θ angle (unit: radian) → 0, sin θ → θ
1 ''=2 π/(360 × 3600) (radians)
1AU/1pc = sin 1'' ≈2π/(360×3600)
1pc=1AU/[2 π/(360 × 3600)]=(360 × 3600/2 π) * AU ≈ 206265AU, while 1Au=1.496 × 10eightkm，
Therefore, 1Pc/1 light year=3.08572 × 10thirteen/9.4608×10twelve=3.2616, that is, 1pc=3.2616 light years
If the parallax is 0.1, 0.01 and 0.001 respectively:
0.1 arcsec: second difference=1Au/0.1 arcsec (arc)=10Au/1 arcsec (arc)=2062650Au=10Pc=32.616 (light year)
0.01 arcsec: second difference=1Au/0.01 arcsec (arc)=20626500Au=100PC=326.16 (light year)
0.001 angular second: second difference=1Au/0.001 angular second (arc)=206265000Au=1000PC=3261.6 (light year)
Use and measurement
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Parallax method is a basic calibration step for measuring distance in astrophysics.However, the precision of ground-based telescope for measuring parallax angle is limited to about 0.01 angular second, so it is impossible to range objects with a distance of more than 100 pc.[2]This is because the Earth's atmosphere limits the clarity of star images.[3]However, space-based telescopes are not limited by this effect, and can accurately measure the distance of objects beyond the ground observation limit.Between 1989 and 1993, the Hipparcos satellite launched by the European Space Agency (ESA) measured the parallax of about 100000 stars with an astronomical measurement accuracy of about 0.97 milliseconds, and obtained an accurate measurement of the distance between stars of 1000 pc.[4-5]
ESA's Gaia satellite was launched on December 19, 2013, aiming to measure the distance of 1 billion stars in 20 microseconds, with an observation error of 10%, and is about 8000 pc away from the Milky Way center of Sagittarius constellation.[6]
Distance of second difference
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Less than 1 second gap
The distance expressed as a fraction of the second difference usually involves objects within a single stellar system.For example:
One astronomical unit (AU), the distance from the sun to the earth is slightly less than 5 × 10-6Second gap.
The farthest space probe, Voyager 1, was 0.00066 parsecs away from Earth in August 2016.
It is estimated that the diameter of the Oort cloud is about 0.6 seconds apart.
Second gap and kilosecond gap
The distance in seconds (pc) includes the distance between close stars, such as in the same spiral arm orGlobular clusterDistance in.The distance of 1000 pc (3262 light years) is usually represented by kiloparsec (kpc).Astronomers usually use the millisecond gap to express the distance between parts of a galaxy or within a galaxy cluster.So, for example:
A second difference is about 3.26 light-years.
In addition to the nearest star [Centaur] neighbor star known to the sun, the known star closest to the Earth is about 1.30pc (4.24 light years).
The absolute magnitude is the brightness of the star measured on the assumption that the star is placed 10 seconds away from the Earth (32.6 light years).
The center of the Milky Way is more than 8 thousand seconds away from the Earth (26000 light years), and the Milky Way is about 34 thousand seconds away (110000 light years).
Andromeda galaxy (M31) is about 780 kpc (2.5 million light years) away from Earth.
Millions of seconds and billions of seconds
The one million second gap is usually expressed by the megasecond gap (Mpc).Astronomers usually use it to express the distance between adjacent galaxies and galaxy clusters.
The one billion second gap (Gpc) is one of the maximum length units commonly used, about 3.26 billion light years, or 1/14 of the observable universe, and the distance to the horizon of the observable universe (determined by the cosmic background radiation).Astronomers usually use Gpc to represent the size of large-scale structures, such as the distance between galaxy clusters and the distance to quasars.
For example:
Andromeda galaxy is about 0.78 Mpc (2.5 million light years) away from Earth.
The nearest large galaxy cluster, the Virgo Cluster, is about 16.5Mpc (54 million light years) away from Earth.[7]
For galaxy RXJ1242-11, a supermassive black hole core similar to the Milky Way was observed, about 200 Mpc (650 million light years) away from the Earth.
The galaxy filamentHercules Corona Borealis Great Wall is the largest known structure in the universe, about 3 Gpc (10 billion light years).
The radius of the particle event horizon (the boundary of the observable universe) is about 14.0 Gpc (46 billion light years).[8]
practical application
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In our daily life, we have this experience: when we move our own line of sight, objects that are different from our own distance will shift. The closer we move, the greater the distance will be. Therefore, we can also calculate the distance of objects by using this potential difference.Astronomers use this potential difference to measure the distance of celestial bodies. Although the stars are far away from us, and the Earth's orbit is insignificant relative to this distance, distant stars still have potential differences that we cannot feel with the naked eye. The potential difference is so small that we need to use seconds on the circular arc as the unitEarth orbitIn terms of the relative distance between the two sides, if the potential difference of a celestial body moving on the celestial sphere is one second, astronomers will name the distance between the celestial body and us as "one second difference", which is about 3.26 light-years.
