THE LARGE RADIAL VELOCITY OF NGC 7619

In 1929, Milton L. Humason from the Mount Wilson Observatory, affiliated with the Carnegie Institution of Washington, communicated a significant discovery regarding the radial velocity of a celestial object known as NGC 7619. This object is situated near the celestial coordinates of Right Ascension 23h 16m and Declination +7° 50′.

NGC 7619 is a part of a cluster that includes small spiral, globular, and elongated nebulae. Among the brightest nebulae in this cluster are NGC 7619 and 7626, with the former being a globular nebula.

Two years earlier, astronomer Edwin Hubble had proposed observing a list of fainter and more distant extragalactic nebulae, particularly those found in groups, to investigate potential large displacements in their absorption lines towards longer wavelengths. This investigation was intended to support de Sitter's theory of curved space-time.

Hubble's suggestion led to the acquisition of two spectrograms of NGC 7619 using the Cassegrain spectrograph VI attached to the 100-inch telescope at Mount Wilson Observatory. The dispersion of the spectrograph was 183 Angstroms per millimeter at 4500, and the exposure times were 33 and 45 hours for the two spectrograms.

Miss MacCormack, from the computing division at Mount Wilson Observatory, and Humason themselves measured the radial velocity of NGC 7619 from these spectrograms. The weighted mean radial velocity was calculated to be +3779 km/second. This velocity is notably twice as large as any previously observed, with the previous record being +1800 km/second for NGC 584, which was measured by Vesto Slipher.

The two individual velocities derived from the spectrograms were +3828 km/second and +3754 km/second, with the latter being considered more reliable due to its longer exposure time.

Interestingly, Hubble had previously compiled approximate distances for 24 extragalactic nebulae and found a consistent pattern of increasing radial velocity with distance. The high velocity of NGC 7619 fits into this pattern, suggesting a possible influence of distance on the observed line shift. This effect could be due to the apparent slowing down of light vibrations with distance or a real tendency of material bodies to scatter in space, as proposed by de Sitter's theory.

NGC 7619's spectral type is estimated to be F8, with a probable error of the measured velocity not exceeding 100 km/second. However, the poor quality of the absorption lines resulted in a large probable error. The spectrum of NGC 7619 resembled that of Milky Way clouds in Sagittarius and Cygnus, as well as binary stars of the W Ursae Majoris type, where rapid rotation affects the widening and depth of the lines.

It is believed that the wide and shallow absorption lines observed in NGC 7619's spectrum are due to a dispersion in velocity and a blending of the spectral types of numerous stars within the nebula. The lack of depth in the absorption lines seems to be more pronounced in smaller and fainter nebulae, and NGC 7619's absorption is notably weak. The researchers hope that velocities of other intriguing objects will soon be available for further study.