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# 2 Planetary nebulae

As a low-mass or intermediate-mass star ascends the AGB, mass loss from the star becomes more and more significant. A strong stellar wind may develop in which a substantial fraction of the mass is carried away from the star. This usually corresponds to most of the hydrogen envelope! One consequence of extreme mass loss can be the formation of a planetary nebula, described below. Another effect is that even intermediate-mass stars, which had main-sequence masses in the range 3–8 M☉, end up with a mass < 1.4 M☉. The significance of this final mass will become clear when white dwarfs are discussed later.

The mass lost from stars at the end of their AGB phase is not immediately visible in optical light, because dust grains in the cool ejecta obscure the star (although it may be seen in the infrared). The underlying helium-rich star – with a carbon and oxygen core – gets smaller and hotter, and a fast, radiation-driven stellar wind develops. Once the central star’s surface temperature reaches ~ 10 000 K it ionises the ejected envelope, which is then seen as a planetary nebula, expanding at ~ 30–60 km s-1. (Note: the term planetary nebula is a misnomer originating from when these objects were first observed as fuzzy planet-like objects through 18th century telescopes.)

Figure 2(a) Hubble Space Telescope (HST) image of the bipolar planetary nebula NGC 6302. (b) A composite of data from the Chandra X-ray Observatory and HST showing the multipolar planetary nebula NGC 6543. (c) A composite view of the AGB star L2 Puppis in visible light (blue) and radio (orange) revealing a giant planet shaping the outflow from the star. (d) ALMA radio observations reveal a spiral structure in the material around the AGB star R Sculptoris.

The majority of planetary nebulae are asymmetric, with a small fraction being largely spherical or elliptical. Most are bipolar (with lobes either side of a central star) or multipolar (with at least two axes of symmetry), as shown in Figures 2a and 2b. The origin of these asymmetries has been the subject of considerable debate over recent decades. However, recent observations with the ALMA radio telescope and the SPHERE coronagraphic adaptive optics system on the Very Large Telescope (VLT) show that many of the asymmetries in planetary nebulae derive from structures that first formed while the star was on the AGB. They appear to be the result of orbiting companions, either a binary companion star or a massive planet (see Figure 2c). These can either transfer angular momentum to the ejecta and form a constraining torus, leading to a bipolar nebula, or create an orbital motion of the AGB star around the system’s centre of mass, giving rise to a spiral pattern in the ejecta (see Figure 2d).

The physical diameter of a planetary nebula can be derived from its angular extent and distance. Because the expansion velocity of the nebula gas can be measured by its Doppler shift, the duration of the expansion – and hence of the post-AGB phase – can be calculated. By extrapolating the observed expansion of the nebula backwards, it can be shown that the whole evolution of a planetary nebula lasts only about 20,000 years.

The star at the centre of the planetary nebula continues to get smaller and hotter. Its hydrogen-burning shell is extinguished when it gets too close to the surface and hence too cool, and the same happens eventually to the helium-burning shell. What remains is a hot carbon-and-oxygen core, surrounded by thin shells of helium and hydrogen, with a surface temperature of ~ 100,000 K. All nuclear burning has been extinguished by this stage, and there is no prospect of thermonuclear reactions being re-ignited. Observations show that, although the nebula continues to expand, the central star now begins a long slide to lower luminosity and temperature at almost-constant radius. The star has become a cooling white dwarf.

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