Overview

When we look at photographs of the Milky Way (see Fig. 16.1), we note large regions where no light is seen. We think that these are due to dust blocking the light between us and the stars. We can see the same effect on a smaller scale (Fig. 14.1). Note that there is a high density of stars near the edges of the image. As one moves close to the center, the density of stars declines sharply. Near the center, no stars can be seen. This apparent hole in the distribution of stars is really caused by a small dust cloud, called a globule. The more dust there is in the globule, the fewer background stars we can see through the globule. We can use images like this to trace out the interstellar dust. We find that it is not uniformly distributed. Rather, it is mostly confined to concentrations or interstellar clouds.

We detect the presence of the gas by observing absorption or emission lines from the gas. By tracing these lines, we find that the gas also has an irregular distribution. Often the gas appears along the same lines of sight as the dust clouds. From this apparent coincidence we form the idea that the gas and dust are generally well mixed, with the gas having about 99% of the mass in a given cloud. In this chapter, we will see how the masses of different types of clouds are determined.

The past decades have seen dramatic improvements in our observing capabilities. There have been improvements in our ability to detect visible radiation, and there have also been exciting extensions to other parts of the spectrum. These improved observing capabilities have had a major impact on astronomy and astrophysics. In this chapter we will first discuss the basic concepts behind optical observations. We will then discuss observations in other parts of the spectrum.

What a telescope does

An optical telescope provides two important capabilities:

1. (1) It provides us with light-gathering power. This means that we can see fainter objects with a telescope than we can see with our naked eye.

2. (2) It provides us with angular resolution. This means that we can see greater detail with a telescope than without.

For ground-based optical telescopes, light-gathering power is usually the most important feature.

Light gathering

We can think of light from a star as a steady stream of photons striking the ground with a certain number of photons per unit area per second. If we look straight at a star, we will see only the photons that directly strike our eyes. If we can somehow collect photons over an area much larger than our eye, and concentrate them on the eye, then the eye will receive more photons per second than the unaided eye. A telescope provides us with a large collecting area to intercept as much of the beam of incoming photons as possible, and then has the optics to focus those photons on the eye, or a camera, or onto some detector.

Overview

Throughout this book we have discussed the components of our galaxy: stars, clusters of stars, interstellar gas and dust. We now look at how these components are arranged in the galaxy. The study of the large scale structure of our galaxy is difficult from our particular viewing point. We are in the plane of the galaxy, so all we see is a band of light (Fig. 16.1). The interstellar dust prevents us from seeing very far into the galaxy. We see a distorted view.

The first evidence on our true position in the galaxy came from the work of Harlow Shapley, who studied the distribution of globular clusters (Fig. 16.2). He found the distances to the clusters from observations of Cepheids and RR Lyrae stars. Shapley found that the globular clusters form a spherical distribution. The center of this distribution is some 10 kpc from the Sun. Presumably, the center of the globular cluster distribution is the center of the galaxy. This means that we are about 10 kpc from the galactic center.

In Chapter 13, when we studied HR diagrams for clusters, we introduced the concept of stellar populations I and II. The distribution of these populations in the galaxy can help us understand how the galaxy has evolved. Population I material is loosely thought of as being the young material in the galaxy.

Now that we know the basic properties of stars, we look at how the laws of physics determine those properties, and then how stars change with time – how they evolve. Stars go through a recurring full life cycle. They are born, they live through middle age, and they die. In their death, they distribute material into interstellar space to be incorporated into the next generation of stars.

In describing the life cycle, we can start anywhere in the process. In Chapter 9, we discuss the most stable part of their life cycle, life on the main sequence – stellar middle age. In Chapters 10, 11 and 12 we will look at the deaths of different types of stars. After discussing the interstellar medium in Chapter 14, we will look at star formation in Chapter 15.

Brightness of starlight

When we look at the sky, we note that some stars appear brighter than others. At this point we are not concerned with what causes these brightness differences. (They may result from stars actually having different power outputs, or from stars being at different distances.) All we know at first glance is that stars appear to have different brightnesses.

We would like to have some way of quantifying the observed brightnesses of stars. When we speak loosely of brightness, we are really talking about the energy flux, f, which is the energy per unit area per unit time received from the star. This can be measured with current instruments (as we will discuss in Chapter 4). However, the study of stellar brightness started long before such instruments, or even telescopes, were available. Ancient astronomers made naked eye estimates of brightness. Hipparchus, the Greek astronomer, and later Ptolemy, a Greek living in Alexandria, Egypt, around 150 BC, divided stars into six classes of brightness. These classes were called magnitudes. This was an ordinal arrangement, with first-magnitude stars being the brightest and sixth-magnitude stars being the faintest.

When quantitative measurements were made, it was found that each jump of one magnitude corresponded to a fixed flux ratio, not a flux difference. Because of this, the magnitude scale is essentially a logarithmic one. This is not too surprising, since the eye is approximately logarithmic in its response to light.

The contributions of the Hubble Space Telescope to our understanding of starburst galaxies are reviewed. Over the past decade, HST's imagers and spectrographs have returned highquality data from the far-ultraviolet to the near-infrared at unprecedented spatial resolution. A representative set of HST key observations is used to address several relevant issues: Where are starbursts found? What is their stellar content? How do they evolve with time? How do the stars and the interstellar medium interact? The review concludes with a list of science highlights and a forecast for the second decade.

Overview

Almost exactly 10 years ago ST ScI hosted its annual symposium entitled Massive Stars in Starbursts (Leitherer et al. 1991). Those were the weeks immediately prior to HST's launch, and the conference organizers felt it appropriate to have a meeting on the subject of starbursts because HST had the potential for significant contributions. Starbursts are compact (10°—103 pc), young (∼ 106—108 yr) sites of star formation, often with high dust obscuration. These properties make starbursts ideal targets for HST, given its superior spatial resolution, ultraviolet (UV) sensitivity, and (later-on) infrared (IR) capabilities.

As we all know, the high hopes were not immediately fulfilled, and it was not until after the First Servicing Mission that HST lived up to the expectations.

One of the important topics of current astrophysical research is the role that supermassive black holes play in shaping the morphology of their host galaxies. There is increasing evidence for the presence of massive black holes at the centers of all galaxies and many efforts are directed at understanding the processes that lead to their formation, the duty cycle for the active phase and the question of the fueling mechanism. Related issues are the epoch of formation of the supermassive black holes, their time evolution and growth and the role they play in the early ionization of the Universe. Considerable observational and theoretical work has been carried out in this field over the last few years and I will review some of the recent key areas of progress.

Introduction

It is now widely accepted that quasars (QSOs) and Active Galactic Nuclei (AGN) are powered by accretion onto massive black holes. This has led to extensive theoretical and observational studies to elucidate the properties of the black holes, the characteristics of the accretion mechanisms and the mechanisms responsible for the production and transportation of the energy from the central regions to the extended radio lobes.

However, over the last few years there has been an increasing realization that Massive Dark Objects (MDOs) may actually reside at the centers of all galaxies (Ho 1998, Magorrian et al. 1998, Richstone et al. 1998, Gebhardt et al. 2000a, Gebhardt et al. 2000b, Merrit & Ferrarese 2001, van der Marel 1999).

Supernova 1987A has been a prime target for the Hubble Space Telescope since its launch, and it will remain so throughout the lifetime of HST. Here I review the observations of SN1987A, paying particular attention to the rapidly developing impact of the blast wave with the circumstellar matter as observed by HST and the Chandra Observatory.

Introduction

If there was ever a match made in heaven, it is the combination of SN1987A and the Hubble Space Telescope. Although the HST was not available to witness the first three years after outburst, it has been the primary instrument to observe SN1987A since then.

SN1987A in the Large Magellanic Cloud is the brightest supernova to be observed since SN1604 (Kepler), the first to be observed in every band of the electromagnetic spectrum, and the first to be detected through its initial burst of neutrinos. Although the bolometric luminosity of SN1987A today is ≈ 10-6 of its value at maximum light (Lmax ≈ 2.5 × 108 L⊙), it will remain bright enough to be observed for many decades in the radio, infrared, optical, UV, and X-ray bands.

SN1987A is classified as a Type II supernova (SNeII) by virtue of the strong hydrogen lines in its spectrum. It was atypical of SNeII in that its light curve did not reach maximum until three months after outburst and its maximum luminosity was about 1/10 the mean maximum luminosity of SNeII.

The Space Telescope Science Institute Symposium on “A Decade of HST Science” took place during 11–14 April 2000.

There is no doubt that the Hubble Space Telescope (HST) in its first decade of operation has had a profound impact on astronomical research. But HST did much more than that. It literally brought a glimpse of the wonders of the universe into millions of homes worldwide, thereby inspiring an unprecedented public curiosity and interest in science.

HST has seen farther and sharper than any optical/UV/IR telescope before it. Unlike astronomical experiments that were dedicated to a single, very specific goal, HST's achievements are generally not of the type of singular discoveries. More often, HST has taken what were existing hints and suspicions from ground-based observations and has turned them into certainty.

In other cases, the level of detail that HST has provided forced theorists to re-think previous broad-brush models, and to construct new ones that would be consistent with the superior emerging data. In a few instances, the availability of HST's razor-sharp vision at critical events provided unique insights into individual phenomena.

These proceedings represent a part of the invited talks that were presented at the symposium, in order of presentation. We thank the contributing authors for preparing their papers.

It is rare in astronomy to have a purely physics-based technique for studying the distant universe. Rooted in General Relativity, the image distortion and time delay of light from distant objects caused by foreground gravitational lenses offers such a window on the universe. Using only combinations of measured redshifts, angles, and arrival times of source intensity fluctuations, lensing observations can probe the mass distribution of the lens, the rate of expansion of the universe (the Hubble constant), the acceleration of expansion (dark energy), and the total amount of matter in the universe. The HST has made and will continue to make unique contributions to this new window on the universe.

Introduction

The universe is not as it seems: distant galaxies and quasars are in the wrong places. Their apparent positions on the sky have moved relative to where they would normally appear, and the culprit is mass-energy. Specifically, a massive object (a star, a galaxy, a cluster of galaxies) will warp space-time around it, causing light rays to bend as they pass by. If a mass concentration lies between us and a distant source, that source will appear in an altered location. The effect is called gravitational lensing, and it also systematically distorts the images of resolved sources like galaxies.

The high spectral resolution and high signal to noise capabilities of the Goddard High Resolution Spectrograph (GHRS) have permitted very accurate measurements of the gas phase abundances and physical conditions in interstellar clouds found in the Galactic disk and low halo and of the matter in several Galactic high velocity clouds. The interstellar gas phase abundances provide important clues about the composition of dust grain mantles and cores, and about the origins of intermediate and high velocity gas in the Galactic disk and halo. The processes that circulate gas from the disk into the low halo do not destroy dust grain cores. The gas in Complex C in the direction of Mrk 290 has a metallicity of 0.089 ± 0.024 solar, which implies the accretion of low metallicity gas by the Milky Way at a rate per unit area sufficient to solve the long standing Galactic G-dwarf problem. GHRS studies of interstellar Si IV, C IV, and N V absorption toward stars and AGNs have yielded measures of the 3 to 5 kpc extension of hot gas into the halo of the Milky Way. The GHRS results coupled with new measurements from the Far-Ultraviolet Spectroscopic Explorer (FUSE) satellite of O VI absorption by hot halo gas permit a study of the physical conditions in the hot Galactic Corona originally envisioned by Lyman Spitzer in his classic 1956 paper “On a Possible Interstellar Galactic Corona.”

HST observed Mars during all 5 oppositions between 1990 and 1999, providing unique new observations of the planet's atmosphere and surface during seasons which are typically poorly-observed telescopically and in wavelength regions or at spatial scales that are not at all observed by spacecraft. HST observations also filled a crucial gap in synoptic observations of Mars prior to 1998, during a time when no spacecraft were observing the planet. HST data have provided important new insights and understanding of the Martian atmosphere, surface, and satellites, and they continue to fulfill important spacecraft mission support functions, including atmospheric aerosol characterization, dust storm monitoring, and instrument cross-calibration.

Introduction

Mars has been the subject of intense telescopic observations for centuries (see, for example, reviews by Martin et al. 1992 and Sheehan 1988). Interest in the red planet stems partly from its prominent appearance in the night sky as a bright extended object roughly every 26 months, and also from historic telescopic observations and more recent spacecraft encounters that have revealed many similarities between Mars and the Earth in terms of surface and atmospheric characteristics and climatic histories. While cold and arid today and probably inhospitable to most forms of life, evidence exists indicating that Mars once may have had a much more clement climate, during a postulated “warm and wet” epoch early in solar system history (e.g. Pollack et al. 1987; Carr, 1998).

The Galactic Center has been the subject of a variety of HST observing programs, mainly since the installation of NICMOS. The observational strengths of NICMOS lie with its sensitivity and very stable point spread function which enables a variety of studies including sensitive searches for variable sources and accurate colors across the 1 to 2.5 µm region. The emission line filters in NICMOS enable studies of the interstellar medium and a search for [SiVI] emission as a ‘smoking gun’ for gas clouds near a black hole powered accretion disk.

Introduction

The center of the Milky Way is of course the closest galaxy nucleus and is a natural area to choose to study in detail. The discovery of a peculiar radio source, SgrA*, and the subsequent demonstration that it is a black hole has only heightened interest in the center. Figure 1 shows a contour plot at 1.04 µm compared to a NICMOS image at 1.45 µm which clearly shows why the Galactic Center requires use of infrared instrument like NICMOS with Av ∼ 30 while AK ∼ 3.3.

The Galactic Center has been studied with HST from the first observing cycle using WFPC proposed in an era where the nature of many of the stars was not understood, and the existence of a cluster in very close proximity to the black hole, SgrA*, was unknown.