Recent progress on the astrophysics of globular clusters is discussed. Highlights are (a) developments in color-magnitude survey work, (b) globular cluster structures and the “fundamental plane,” and (c) the relation between globular clusters and the halo field stars in the same host galaxy.

Color-Magnitude studies: The beginning and end of an era

Above almost all other types of astronomical systems, globular clusters offer the chance to take a broad historical perspective. Much of the history of astrophysics in the twentieth century—stellar structure, stellar evolution, the distance scale, galactic structure and evolution, stellar populations, variable stars, high-energy sources—was driven by the need to understand the complex array of phenomena taking place inside these dense stellar systems.

No review of this kind should fail to mention the continuing efforts to understand the stellar content of these elegant systems through color-magnitude studies (CMDs), which are penetrating to ever-greater detail and depth. The very first such studies (see, for example, Shapley & Davis 1920) barely showed the red-giant stars and brightest horizontal-branch stars for the nearest clusters. This past year, a watershed in this kind of basic color-magnitude survey work was reached with the publication of the monumental survey project of Piotto et al. (2002), who used the WPFC2 camera on board HST to obtain (B, V) CMDs for 74 globular clusters, very nearly half of the entire Milky Way globular cluster population. The objects range from nearby, high-latitude clusters with beautifully precise, classic CMD sequences, down to sparse objects deeply embedded in the heavy field contamination and differential reddening of the Galactic bulge.

I present the preliminary results of a program to measure the star formation history in the halo of the Andromeda galaxy. Using the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope, we obtained the deepest optical images of the sky to date, in a field on the southeast minor axis of Andromeda, 51′ (11 kpc) from the nucleus. The resulting color-magnitude diagram (CMD) contains approximately 300,000 stars and extends more than 1.5 mag below the main sequence turnoff, with 50% completeness at V = 30.7 mag. We interpret this CMD using comparisons to ACS observations of five Galactic globular clusters through the same filters, and through χ2-fitting to a finely-spaced grid of calibrated stellar population models. We find evidence for a major (∼30%) intermediate-age (6–8 Gyr) metal-rich ([Fe/H]> −0.5) population in the Andromeda halo, along with a significant old metal-poor population akin to that in the Milky Way halo. The large spread in ages suggests that the Andromeda halo formed as a result of a more violent merging history than that in our own Milky Way.

Introduction

One of the primary quests of observational astronomy is understanding the formation history of galaxies. An impediment to this research is the relative paucity of galaxies in the Local Group, which contains no giant ellipticals, and only two giant spirals-our own Milky Way and Andromeda. Fortunately, Andromeda (M31, NGC 224) is well situated for studying the formation of giant spiral halos, due to its proximity (770 kpc; Freedman & Madore 1990), small foreground reddening (EB-V = 0.08 mag; Schlegel, Finkbeiner, & Davis 1990), and low inclination (i ≍ 12.50°; de Vaucouleurs 1958).

The galaxies of the Local Group that are currently forming stars can serve as our laboratories for understanding star formation and the evolution of massive stars. In this talk I will summarize what I think we've learned about these topics over the past few decades of research, and briefly mention what I think needs to happen next.

Introduction

My talk today will be restricted to giving a brief introduction to the study of massive stars in the Local Group; I'll begin by discussing why I think the subject is important, and giving you a few of the complications and caveats. I'll spend most of my time then talking about what I think we've learned, first about star formation (stories of star formation, the initial mass function, and the upper mass cut-off), and second about the evolution of massive stars (including Luminous Blue Variables, Wolf-Rayet stars, and red supergiants). Finally I'll conclude with a brief discussion of what I think we need to do next. This talk is based in large part on an Annual Reviews of Astronomy & Astrophysics paper that I have coming out in October (Massey 2003), and the reader is referred there for a more in-depth analysis. I have used this opportunity to update some of the figures and thoughts from that, so hopefully the two will be somewhat complementary.

It is suggested that M31 was created by the early merger and subsequent violent relaxation of two or more massive metal-rich ancestral galaxies within the core of the Andromeda subgroup of the Local Group. On the other hand, the evolution of the main body of the Galaxy appears to have been dominated by the collapse of a single ancestral object that subsequently evolved by capturing a halo of small metal-poor companions. It remains a mystery why the globular cluster systems surrounding galaxies like M33 and the LMC exhibit such striking differences in evolutionary history. It is argued that the first generation of globular clusters might have been formed nearly simultaneously in all environments by the strong pressure increase that accompanied cosmic reionization. Subsequent generations of globulars may have formed during starbursts that were triggered by collisions and mergers of gas-rich galaxies.

The fact that the [G]alactic system is a member of a group is a very fortunate accident. Hubble (1936, p. 125)

Introduction

According to Greek mythology, the goddess of wisdom, Pallas Athena, clad in full armor, emerged from Zeus's head after Hephaestus split it open. In much the same way the Local Group sprang forth suddenly, and almost complete, in Chapter VI of The Realm of the Nebulae (Hubble 1936, pp. 124–151). Hubble describes the Local Group as “a typical small group of nebulae which is isolated in the general field.” He assigned (in order of decreasing luminosity) M31, the Galaxy, M33, the Large Magellanic Cloud, the Small Magellanic Cloud, M32, NGC 205, NGC 6822, NGC 185, IC 1613 and NGC 147 to the Local Group, and regarded IC 10 as a possible member.

Introduction

A census of baryons in the local universe indicates that the majority of this normal matter is undetected, or “missing.” At high redshifts (z ∼ 3), big-bang nucleosynthesis models and QSO absorption line observations indicate a baryon mass fraction of Ωb ∼ 0.04 (e.g., Fukugita, Hogan & Peebles 1998). The stars and gas detected in local galaxies account for only 20% of this (Ωb ∼ 0.008). The absence of a local Lyα forest indicates that these baryons are likely in a hot (T > 105 K), diffuse medium which has heretofore remained undetectable (e.g., Fukugita, Hogan & Peebles 1998; Cen & Ostriker 1999a; Davé et al. 2001).

Introduction

Starbursts are not simply scaled-up versions of the disks of normal spiral and irregular galaxies. The composite HST WFPC2 image of the classic starburst galaxy M82 in Figure 1 illustrates some of the differences. Star formation is localized in a well-defined central zone, where it is concentrated in clumps, beyond which there is virtually no star-forming activity (O'Connell & Mangano 1978). The well-known superwind extends above and below the plane out to kiloparsecs beyond the main starburst zone (Shopbell & Bland-Hawthorn 1998 and references therein). In M82 we can observe the combined effects of stellar feedback and a weak interaction with M81 in sufficient detail to test our models of galactic star formation. This is critical for understanding how the cycling of baryonic matter through stars relates to the overall structure of a galaxy, including its dark matter halo; e.g., through its influence in varying the luminosity part of the Tully–Fisher relationship (van Driel, van den Broek & Baan 1995).

Introduction: The inner structure of AGNs

There is now general consensus that the long-standing paradigm for active galactic nuclei (AGNs) is basically correct, i.e., that AGNs are fundamentally powered by gravitational accretion onto supermassive collapsed objects. Details of the inner structure of AGNs, however, remain sketchy, although both emission lines and absorption lines reveal the presence of large-scale gas flows on scales of hundreds to thousands of gravitational radii. The accretion disk produces a time-variable high-energy continuum that ionizes and heats this nuclear gas, and the broad emission-line fluxes respond to the changes in the illuminating flux from the continuum source. The geometry and kinematics of the broad-line region (BLR), and fundamentally, its role in the accretion process, are not understood. Immediate prospects for understanding this key element of AGN structure do not seem especially promising with the realization that the angular size of the nuclear regions projects to only microarcsecond scales even in the case of the nearest AGNs.

Introduction

Abundance studies of the oldest stars provide critical clues to—and constraints upon—the characteristics of the earliest stellar populations in our Galaxy. Such constraints include those upon: light element production and BBN; the early star-formation and nucleosynthesis history of the Galaxy; the characteristics of heavy-element nucleosynthesis mechanisms; and the ages of early stellar populations from nuclear chronometers. Discussions of many of these issues are to be found in a number of review papers (Wheeler et al. 1989; McWilliam 1997; Truran et al. 2002; Gratton, Sneden, & Caretta 2004).

While much of the available data has been obtained with ground-based telescopes, there is much to learn with HST. Studies in the wavelength region accessible with HST can, in fact, address issues ranging from the origin of the light elements Li, Be, and B to the production mechanisms responsible for the synthesis of the heaviest elements through thorium and uranium. In the following two sections, we will review specifically first boron abundance studies at low Z and then abundances of the heavy elements Ge, Zr, Os, Pt, Au, and Pb, at low Z.

Boron abundances in halo stars

Knowledge of lithium, beryllium, and boron abundances in stars play a major role in our understanding of Big Bang nucleosynthesis, cosmic-ray physics, and stellar interiors.

In the standard model for the origin and evolution of the light elements, only 7Li is produced in significant amounts from Big Bang (primordial) nucleosynthesis.

Introduction

When both the photometric transits and the radial velocity variations due to an extrasolar planet are observed, we are granted access to key quantities of the object that Doppler monitoring alone cannot provide. In particular, precise measurements of the planetary mass and radius allow us to calculate the average density and infer a composition.

The Space Telescope Science Institute Symposium on Planets to Cosmology: Essential Science in the Final Years of the Hubble Space Telescope took place during 3–6 May 2004.

These proceedings represent only a part of the invited talks that were presented at the symposium. We thank the contributing authors for preparing their manuscripts.

With some uncertainty concerning Hubble's next Servicing Mission still hanging, identifying the most crucial science to be performed by this superb telescope has become of paramount importance. With this goal in mind, the symposium examined a wide range of topics at the forefront of astronomy and astrophysics. The result is a magnificent collection of results, with a special emphasis on future research.

We thank Sharon Toolan of ST ScI for her help in preparing this volume for publication.

Introduction

HST and quasar host galaxy studies have grown up together over the past 30 years. Indeed, “the imaging of low-redshift quasars at high angular resolution (∼0″.1) is one of the principal scientific goals for which the Hubble Space Telescope was designed” (Bahcall, Kirhakos, & Schneider 1994). The nice demonstration by Kristian (1973) that nearby quasars are, in fact, surrounded by “fuzz” in deep 200-inch photographs provided timely input for the design of HST and its instruments, the specifications for which were outlined by the Large Space Telescope Science Working Group in 1974 (HST website). While HST has changed the way we look at quasar hosts, the ultimate goal of our studies has not changed over the decades. Then, as now, we strive to understand the roles played by quasars in galaxy evolution.

Introduction

Infrared (IR) luminous galaxies represent highly active stages in galaxy evolution that are not generally inferred in optically selected galaxy surveys (e.g., Rieke & Low 1972; Soifer, Neugebauer, & Houck 1987).

Introduction

The last decade has witnessed amazing progress in our empirical and theoretical understanding of galaxy formation and evolution.