M76 represents a stunning example of a double lobed or bipolar type planetary nebula. It is now believed that most planetary nebulae, even spherically symmetric appearing ones have a bipolar structure. In the current model of PN formation (see M57) the progenitor star loses a considerable amount of mass in its final stage on the asymptotic giant branch, culminating in a strong superwind just prior to planetary nebula formation, where the star loses as much as one ten thousandth of a solar mass per year at wind speeds up to ten kilometers per second. The elliptical and bipolar shapes are produced because the superwind mass loss occurs primarily into an equatorial plane. The existence of a close binary companion may have an influence on whether the mass loss occurs in an equatorial distribution or not. Ultimately the core of the dying star is finally exposed and its hot, fast wind (1000 kilometers per second) collides with the equatorial shaped envelope of gas and preferentially expands it into a bipolar shape.
M76 is about 4.5 light years across and its hot central stellar remnant is about 140,000 degrees Kelvin. Visibly the central star (magnitude 16.6) appears to be a binary system but HST studies reveal that the southern component of the apparent binary is an unrelated stellar system located some 20,000 light years in the background. M76 is a fairly evolved planetary nebula and is likely well over 10,000 years old. The basic structure of the nebula is a complex napkin ring shaped core with two attached inner lobes and two fainter outer lobes. The core shows concentrated h-alpha emission from inner and outer rims with the predominant OIII emission (teal color) concentrated between the two rims. The two lobes have inner and outer components. M76 contains considerable small scale structure in the form of clumps which is a feature seen in many PN's. The inner lobes have a substantial outward velocity consistent with a pressure driven bubble phenomenon. The fainter outer lobes are similar to the faint outer halo of M57 which show very low velocity and likely represent ionized remnants of previously ejected winds during the dying stars red giant phase.