NGC 638 is a barred spiral galaxy located in the constellation of Pisces. Its velocity speed to the cosmic microwave background is 2,864± 21 km/s, which corresponds to a Hubble's law of 42.2 ± 3.0 Mpc (~138 million light-year). NGC 638 was discovered by American astronomer Lewis A. Swift in 1886.
NGC 638 has a broad Hydrogen line. NGC 638 is a galaxy whose nucleus shines in the ultraviolet domain. It is listed in the Markarian catalogue under the code Mrk 1003 (MK 1003).
To date, two non-redshift measurements give a distance of 55.330 0 ± 1.273 Mpc (~180 million light-year), which is outside the Hubble distance values. Note that the NASA/IPAC database calculates the diameter of a galaxy using the average value of independent measurements, when available, and that consequently the diameter of NGC 638 could be about 11.1 kpc (~36,200 light-year) if the Hubble's law were used to calculate it.
NGC 645 Group
NGC 638 is part of the NGC 645 group which includes at least five galaxies. The other three galaxies in this group are NGC 632, UGC 1137 and UGC 1172.