Galaxies come in a wide range of forms. Among these are some especially beautiful examples with prominent ring structures. These galactic exceptions have varied causes.

THE MORPHOLOGY OF RING GALAXIES

Like the irregular and interacting galaxies, ring galaxies are galactic exceptions whose history has played a deciding role. They are the result of collisions of two galaxies which led to something new – a ring galaxy. In addition to a bright nucleus, this type of galaxy has a ring, which is usually symmetrically oriented about the centre. The size of the ring greatly exceeds the size of the central component. This ring is very prominent in photos since it contains bright starforming regions with young, massive stars. One usually recognizes a blue ring which contrasts in colour with the yellowish central component. The colours of these components remind one of a spiral galaxy with its bulge and the spiral arms, thus a ring is comparable to a closed spiral arm. This morphological peculiarity was explicitly taken into account by de Vaucouleurs in his classification scheme: “(R)” is placed at the beginning of the description when a ring is seen. When the morphology of the ring is observed in more detail, differences from galaxy to galaxy become apparent. One thus distinguishes two cases: a) the ring formed via a genuine collision of two partner galaxies, b) the ring structure is the result of wound-up tidal tails of two mutually orbiting partner galaxies.

In the latter case, perspective also plays an important role, since tidal tails result in a sufficiently bright ring only when seen one on top of the other. Ring galaxies formed by a collision are often referred to in the literature as “true colliding galaxies”. But in this case as well, a small angle between the plane of the ring and the line of sight can mean that one doesn't recognize the ring, since it is covered by dust regions or parts of the spiral arm. It is assumed that only about 30 per cent of all ring galaxies can be reliably identified.

THE EXTRAGALACTIC DISTANCE SCALE

The drama of the heavens unfolds on a two-dimensional stage. Observations readily supply the two coordinates, right ascension and declination, that specify an object's position on the celestial sphere. However, the object's distance is not so easily obtained. Astronomers today use many methods to estimate the distances of remote galaxies. In this section we will introduce a variety of techniques; some depend on the properties of individual stars, some use other objects (globular clusters, planetary nebulae, supernovae, HII regions), and some rely on the statistical properties of whole galaxies and galaxy clusters. The third dimension of distance is doubly significant because, as we saw at the end of Section 26.2, when astronomers peer deeper and deeper into space, they are looking back in time at increasingly ancient light. It is vital to understand not only a galaxy's position in space, but its depth in time as well.

Unveiling the Third Dimension

The techniques used to determine distances within the solar neighborhood were described in Section 3.1. In 1761, the method of trigonometric parallax was used to measure the distance to Venus, thereby calibrating the size of Kepler's Solar System. When Friedrich Wilhelm Bessel measured the subtle annual shift in the position of the star 61 Cygni in 1862, he combined the parallax method with his knowledge of the true size of Earth's orbit to discover that 61 Cygni is 650,000 times farther away than the Sun. Today, the surveyor's method of trigonometric parallax can reach out to a kiloparsec or so.We have also seen how the moving cluster method (Section 24.3) made it possible to determine the distance of the Hyades star cluster. From there, the technique of main-sequence fitting (Sections 13.3 and 24.3) can be used to find the distances to open clusters out to about 7 kpc by comparing their main sequences on an H–R diagram with that of the Hyades cluster. The repeated application of a variety of methods using this pattern of calibration and measurement constitutes the steps of the extragalactic distance scale, also referred to as the cosmological distance ladder.

EVOLUTION ONTHE MAIN SEQUENCE

In Section 10.6 we learned that the existence of the main sequence is due to the nuclear reactions that convert hydrogen into helium in the cores of stars. The evolutionary process of protostellar collapse to the zero-age main sequence was discussed in Chapter 12. In this chapter we will follow the lives of stars as they age, beginning on the main sequence. This evolutionary process is an inevitable consequence of the relentless force of gravity and the change in chemical composition due to nuclear reactions.

Stellar Evolution Timescales

To maintain their luminosities, stars must tap sources of energy contained within, either nuclear or gravitational. Pre-main-sequence evolution is characterized by two basic timescales: the free-fall timescale (Eq. 12.26) and the thermal Kelvin–Helmholtz timescale (Eq. 10.24). Main-sequence and post-main-sequence evolution are also governed by a third timescale, the timescale of nuclear reactions (Eq. 10.25). As we saw in Example 10.3.2, the nuclear timescale is on the order of 1010 years for the Sun, much longer than the Kelvin–Helmholtz timescale of roughly 107 years, estimated in Example 10.3.1. It is the difference in timescales for the various phases of evolution of individual stars that explains why approximately 80% to 90% of all stars in the solar neighborhood are observed to be main-sequence stars (see Section 8.2); we are more likely to find stars on the main sequence simply because that stage of evolution requires the most time; later stages of evolution proceed more rapidly. However, as a star switches from one nuclear source to the next, gravitational energy can play a major role and the Kelvin–Helmholtz timescale will again become important.

Width of the Main Sequence

Careful study of the main sequence of an observational H–R diagram such as Fig. 8.13 or the observational mass–luminosity relation (Fig. 7.7) reveals that these curves are not simply thin lines but have finite widths. The widths of the main sequence and the mass–luminosity relation are due to a number of factors, including observational errors, differing chemical compositions of the individual stars in the study, and varying stages of evolution on the main sequence.