Most galaxies are not isolated in space, but rather form groups. Galaxy clusters are the largest structures visible in wide-angle images of the universe. These galactic groups are challenging subjects for astrophotographers.

THE CLASSIFICATION AND MORPHOLOGY OF GALAXY GROUPS AND GALAXY CLUSTERS

Independent of the focal length, when studying images of galaxies one notes that galaxies are not solitary objects. This does not necessarily imply spatial proximity, since even if the angular distance of two suspected neighbours is small, their true separation can be much larger. Since the third dimension, the distance of the individual galaxies from the Milky Way, is not obvious from the photograph, there is room for speculation. It is thus a logical assumption that galaxies which appear about the same size and display a similar amount of detail are located at similar distances and are thus neighbours in space. However, sizes are just a rough estimate since the appearances and thus relative sizes of galaxies in a group can be greatly changed by interactions. Redshift - derived galaxy distances in astronomical databases, by contrast, allow firmer statements to be made concerning which galaxies have a high probability of being part of a pair or group. Collections of up to 50 galaxies are deemed to be a group if the diameter is in the range three to five million light years. Due to the common gravitational potential, the members of the group differ only slightly in redshift. The so-called virial theorem implies that the mass of a bound group is always less than 2–3 × 1013 solar masses. The Milky Way is itself part of such a group – the Local Group. It consists of about 35 galaxies, although the number is increasing as further members have been discovered in the last few years. These new members are usually dwarf galaxies in the surroundings of large group members. Apart from the Milky Way, M 31 and M 33 are the largest and brightest members of the Local Group. Looking beyond the Local Group, nearby galaxies in our Galactic neighbourhood are also arranged in groups.

Since the first edition of An Introduction to Modern Astrophysics and its abbreviated companion text, An Introduction to Modern Stellar Astrophysics, first appeared in 1996, there has been an incredible explosion in our knowledge of the heavens. It was just two months before the printing of the first editions that Michel Mayor and Didier Queloz announced the discovery of an extrasolar planet around 51 Pegasi, the first planet found orbiting a main-sequence star. In the next eleven years, the number of known extrasolar planets has grown to over 193. Not only do these discoveries shed new light on how stars and planetary systems form, but they also inform us about formation and planetary evolution in our own Solar System.

In addition, within the past decade important discoveries have been made of objects, within our Solar System but beyond Pluto, that are similar in size to that diminutive planet. In fact, one of the newly discovered Kuiper belt objects, currently referred to as 2003 UB313 (until the International Astronomical Union makes an official determination), appears to be larger than Pluto, challenging our definition of what a planet is and how many planets our Solar System is home to.

Explorations by robotic spacecraft and landers throughout our Solar System have also yielded a tremendous amount of new information about our celestial neighborhood. The armada of orbiters, along with the remarkable rovers, Spirit and Opportunity, have confirmed that liquid water has existed on the surface of Mars in the past. We have also had robotic emissaries visit Jupiter and Saturn, touch down on the surfaces of Titan and asteroids, crash into cometary nuclei, and even return cometary dust to Earth.

Missions such as Swift have enabled us to close in on the solutions to the mysterious gamma-ray bursts that were such an enigma at the time An Introduction to Modern Astrophysics first appeared.We now know that one class of gamma-ray bursts is associated with core-collapse supernovae and that the other class is probably associated with the merger of two neutron stars, or a neutron star and a black hole, in a binary system.