2.1 From point-like stars to diverse nebulae

Stars are fascinating and complicated objects, but for the most part this complexity does not reveal itself in individual images of single stars. Most stars are unresolved to our telescopes (i.e. their light is contained within a region smaller than the size of details we can distinguish).

To understand the structure of stars, astronomers have had to construct models based on information from spectroscopy, studies of binary star systems, and tools such as asteroseismology. However, there are a few exceptions.

We have exquisite structural information about the surface of the Sun (Figure 7a), from space missions such as the Solar and Heliospheric Observatory (SOHO) and Solar Orbiter. This information has enabled astronomers to understand the interplay of energy transport and magnetic fields in our nearest star. We also have a small number of resolved images of nearby giant stars. An example is Gruis (Figure 7b), which shows similar convective structure on its surface to that of the Sun.

Figure 7 Stars as extended astronomical objects: (a) the Sun, taken by the SOHO mission, and (b) the nearby giant star Gruis.

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The left-hand image (a) shows a picture of the Sun, taken through an ultraviolet filter that makes particular features clearer. The surface appears highly mottled due to convection cells. Several bright spots are visible from active regions. On the edge of the Sun’s disc, several pillars can be seen, as well as a large arch on the Sun’s top-right.

The right-hand image (b) shows a picture of the distant red supergiant star Gruis. The image is blurry, but still shows a circular disc. Dark lines can be seen on this image, indicating what are thought to be large convective cells. A bright spot is seen in the lower-right of the disc.

Figure 7 Stars as extended astronomical objects: (a) the Sun, taken by the SOHO mission, and (b) the nearby giant star MathJax failure: MathML - Unexpected text node: '

For unresolved objects such as stars, the main properties that can be measured from images alone are location, brightness, and colour. But astronomers have known for several centuries that there are objects in the sky that are not point-like – they show extended and varied structure in images. (Extended astronomical objects are defined as those that have an angular size larger than the PSF of an image.) Identification of such fuzzy, ‘cloud-like’ astronomical objects dates back as far as early tenth-century Islamic astronomy.

Early astronomers classified these extended objects as nebulae: nebula being the Latin word for a cloud or fog. The term became used to describe any object that had a fuzzy appearance, rather than the sharper form of a star.

In modern astronomy, the term nebula is now used in a more precise way. However, until the early twentieth century the physical nature of many of the ‘fuzzy’ extended objects in the sky was not well understood, and so the class of nebulae provided a useful distinction between unresolved (point-like) and resolved (extended) structures.

Figure 8 shows some hand-drawn sketches of nebulae dating to the early nineteenth century. It is apparent that they do not all show the same structure: there are various different types of fuzzy, extended – and in some cases a little peculiar – objects visible in the night sky.

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(Source: The Earl of Rosse (1805) ‘Observations on the nebulae’, Philosophical Transactions of the Royal Society of London, 140(499), pp. 499–514, www.jstor.org/stable/108449)

Figure 8 Images of nebulae, as viewed by the most powerful telescope of the nineteenth century, the Leviathan of Parsonstown, and hand-sketched by William Parsons, 3rd Earl of Rosse.

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Four pencil drawings of astronomical objects are shown. The drawings are negative, so that darker shading indicates a brighter source. The nature of each object is not indicated.

The top-left image shows an elongated, asymmetric object, with rings that become more prominent in the middle.

The top-right image shows a cone-shaped object. A section at the point of the cone appears separated by a waist, and a dark spot is visible at its apex.

The bottom-left image shows an elongated structure that becomes progressively darker in the middle, before becoming lighter again.

The bottom-right image shows a circular structure with bands or rings, with rays emanating from its edges. Two large holes appear in its centre in which points are drawn.

Figure 8 Images of nebulae, as viewed by the most powerful telescope of the nineteenth century, the Leviathan of Parsonstown, and ...