NGC 6164-6165 (Cederblad 135a,b)

Distance: 4200 Light Years

text by Robert Gendler

NGC 6164-5 is a bipolar nebula surrounding the peculiar O 7f star HD 148937. Although the nebula was believed initially to represent a planetary nebula, it is now understood to represent a shell-like ejected nebula, formed by the winds of its young central star. Wind blown nebulae are usually considered the outcome of Wolf-Rayet type stars (WR), evolved O-type stars that have left the main sequence, molding their surroundings by way of fierce stellar winds. In the case of the S-shaped bipolar nebula NGC 6164-5, the central star is not a WR star, but a young O-type supergiant of 40 solar masses. Relatively few examples of such objects are known to exist, making NGC 6164-5 particularly unique. The true age of the star is not known although estimates of its age vary from a pre-main sequence age of 350,000 years to a more evolved age of one to two million years. HD 148937 is a peculiar type of Of star with exotic emission lines (carbon emission lines (CIII) equal in strength to Nitrogen (NIII)) and other peculiar spectra indicative of significant mass loss. Only five such stars are known to exist, three in our galaxy and two in Small Magellanic Cloud.


The nebula complex is located within a large cavity expanded by the Ara OB1 stellar association. The axes of the bipolar nebula measure 8.5 x 6.2 light years. The material in the nebula was ejected from the poles of a precessing HD 148937 in several bursts beginning about 4000 years ago. The bursts are superimposed on a lower intensity continuous mass flow from the star. The central star is now bathing the surrounding condensations of dense gas with its fierce stellar winds that blow at 8 million kilometers per hour. The winds are molding the surrounding gas into a symmetric bipolar structure through photoevaporation and ionization. The composition of the outer knots of NGC 6164-5, show enhancements in helium and nitrogen, and depletion in oxygen similar to the Eta Carinae nebula, and the Wolf-Rayet nebulae NGC 6888 and NGC 2359. NGC 6164-5 may be an example of a young ejected nebula formed from a massive young star which may ultimately evolve into a more classical WR shell-type object.