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Collinder 70 and the belt stars of the constellation of the Orion

Description of object:

Collinder 70 Little physical details are known about Collinda 70. With an apparent diameter of nearly 3 degrees it is one of the largest open star clusters in our galaxy. It belongs to the Orion-OB1 association, the brightest star is Alnilam, the middle belt star of Orion. Alnitak, the left belt star is also assigned to Collinder 70. It is classified as type III 3 r n according to Trumpler.

With an estimated age of only 5 million years it is one of the youngest star clusters in our Milky Way. Its distance to the solar system is about 1000 light years. About 130 stars are assigned to Collinder 70.

Per Collinder was a Swedish astronomer at the Lund Observatory who researched the structure of open star clusters and their spatial distribution in the Milky Way. In 1931 he compiled a catalogue of open star clusters, listing 471 such objects.

The belt stars of Orion

Alnitak is the easternmost (left) belt star of Orion. In the immediate vicinity are the nebula areas IC 434, NGC 2024 and Barnard 33, the famous Horsehead Nebula, which are parts of the large Orion nebula complex.

Alnitak is a triple star system, the main component is a luminous blue supergiant with 20 times the diameter of our sun, the luminosity is by a factor of 10 000 higher than the sun, it is assigned to spectral class O9. Its distance is given as 800 light years.

Alnitak has two physical companion stars, Alnitak Ab. The star was only discovered in the year 2000, it is a main sequence star of spectral class O with about 23 times the mass of the Sun. Alnitak B is located at a distance of 2.3 arcseconds from the main star and can therefore be visually separated even in smaller telescopes. It is a star of spectral type B0 III, its mass is estimated at about 13 solar masses.

Alnilam the middle of the 3 belt stars and like Alnitak a luminous blue supergiant of spectral class O9, it is one of the most luminous stars known. It has about 26 times the diameter of our sun. It is located in the - visually seen - center of the large open star cluster Collinder 70, but physically it is far behind the cluster.

Strangely enough, its distance from the solar system is only uncertain. Classical methods for measuring the distance are around 1350 light years, the parallax measured by the astrometry satellite Hipparcos in 1995 gives 2000 +/- 500 light years. Alnilam is surrounded by a very faint reflection nebula.

It was discovered on 1 February 1786 by William Herschel and listed in the New General Catalogue under NGC 1990.

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Mintaka is the westernmost (right) belt star of Orion. It is a slightly variable blue supergiant of spectral class B0. It belongs to the 10% of the most luminous stars in the galaxy. Mintaka is a multiple star system whose two closest components form a mutually occulting double star of the Algol type, orbiting each other with a period of 5.7 days. Probably the star HD 36 485 (Delta Orionis C) also belongs to the Mintaka system, which itself is a spectroscopic double star. Mintaka has a distance of about 1000 light years to the Solar System and is - together with Alnitak - a member of the open star cluster Collinder 70.

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