Showing papers in "Annual Review of Astronomy and Astrophysics in 2001"
TL;DR: The cosmic infrared background records much of the radiant energy released by processes of structure formation that have occurred since the decoupling of matter and radiation following the Big Bang as discussed by the authors, with additional constraints coming from studies of the attenuation of TeV γ-rays.
Abstract: ▪ Abstract The cosmic infrared background records much of the radiant energy released by processes of structure formation that have occurred since the decoupling of matter and radiation following the Big Bang. In the past few years, data from the Cosmic Background Explorer (COBE) mission provided the first measurements of this background, with additional constraints coming from studies of the attenuation of TeV γ-rays. At the same time, there has been rapid progress in resolving a significant fraction of this background with the deep galaxy counts at infrared wavelengths from the Infrared Space Observatory (ISO) instruments and at submillimeter wavelengths from the Submillimeter Common User Bolometer Array (SCUBA) instrument. This article reviews the measurements of the infrared background and sources contributing to it and discusses the implications for past and present cosmic processes.
850 citations
TL;DR: The rotation curves of spiral galaxies are the major tool for determining the distribution of mass in spiral galaxies and provide fundamental information for understanding the dynamics, ev... as discussed by the authors,...
Abstract: ▪ Abstract Rotation curves of spiral galaxies are the major tool for determining the distribution of mass in spiral galaxies. They provide fundamental information for understanding the dynamics, ev...
786 citations
TL;DR: In this paper, the mass-ejection history of the newly born driving sources and their mass-accretion history is reconstructed using the Herbig-Haro (HH) objects.
Abstract: ▪ Abstract Outflow activity is associated with all stages of early stellar evolution, from deeply embedded protostellar objects to visible young stars. Herbig-Haro (HH) objects are the optical manifestations of this powerful mass loss. Analysis of HH flows, and in particular of the subset of highly collimated HH jets, provides indirect but important insights into the nature of the accretion and mass-loss processes that govern the formation of stars. The recent recognition that HH flows may attain parsec-scale dimensions opens up the possibility of partially reconstructing the mass-ejection history of the newly born driving sources and, therefore, their mass-accretion history. Furthermore, HH flows are astrophysical laboratories for the analysis of shock structures, of hydrodynamics in collimated flows, and of their interaction with the surrounding environment. HH flows may be an important source of turbulence in molecular clouds. Recent technological developments have enabled detailed observations of outf...
621 citations
TL;DR: The inner few parsecs at the Galactic Center have come under intense scrutiny in recent years, in part due to the exciting broad-band observations of this region, but also because of the...
Abstract: ▪ Abstract The inner few parsecs at the Galactic Center have come under intense scrutiny in recent years, in part due to the exciting broad-band observations of this region, but also because of the...
450 citations
TL;DR: In this article, the formation of the first stars and quasars marks the transformation of the universe from its smooth initial state to its clumpy current state, and the study of high-redshift sources is likely to attract major attention in observational and theoretical cosmology over the next decade.
Abstract: ▪ Abstract The formation of the first stars and quasars marks the transformation of the universe from its smooth initial state to its clumpy current state. In popular cosmological models, the first sources of light began to form at a redshift z = 30 and reionized most of the hydrogen in the universe by z = 7. Current observations are at the threshold of probing the hydrogen reionization epoch. The study of high-redshift sources is likely to attract major attention in observational and theoretical cosmology over the next decade.
351 citations
TL;DR: The search for evidence of extraterrestrial intelligence is placed in the broader astronomical context of the search for extrasolar planets and biomarkers of primitive life elsewhere in the world as mentioned in this paper, where the authors place the search in a more general astronomical context.
Abstract: ▪ Abstract The search for evidence of extraterrestrial intelligence is placed in the broader astronomical context of the search for extrasolar planets and biomarkers of primitive life elsewhere in ...
317 citations
TL;DR: The Orion Nebula (M 42) is one of the best studied objects in the sky as mentioned in this paper, which is a thin blister of ionized gas on the front of a giant molecular cloud and the extremely dense associated cluster.
Abstract: ▪ Abstract The Orion Nebula (M 42) is one of the best studied objects in the sky. The advent of multi-wavelength investigations and quantitative high resolution imaging has produced a rapid improvement in our knowledge of what is widely considered the prototype H II region and young galactic cluster. Perhaps uniquely among this class of object, we have a good three dimensional picture of the nebula, which is a thin blister of ionized gas on the front of a giant molecular cloud, and the extremely dense associated cluster. The same processes that produce the nebula also render visible the circumstellar material surrounding many of the pre–main sequence low mass stars, while other circumstellar clouds are seen in silhouette against the nebula. The process of photoevaporation of ionized gas not only determines the structure of the nebula that we see, but is also destroying the circumstellar clouds, presenting a fundamental conundrum about why these clouds still exist.
306 citations
TL;DR: In this paper, it was shown that distant type Ia supernovae (SNe Ia) appear fainter than their local counterparts, and this implies a significant change in how we see the distant universe and what we understand of these stellar explosions.
Abstract: ▪ Abstract Distant type Ia supernovae (SNe Ia) appear fainter than their local counterparts. Independent of what explanation will eventually be found to be correct, this implies a significant change in how we see the distant universe and what we understand of these stellar explosions. The observational characteristics of nearby SNe Ia show some differences from event to event. Despite their considerable range in observed peak luminosity, they can be normalized by their light-curve shape. Through this normalization, SNe Ia can be used as exquisite distance indicators. The Hubble diagram of nearby SNe Ia, demonstrating the linear cosmic expansion at small scales, is the simplest observational proof for the standard character of these objects. Compared with Friedmann models of the universe, the distant SNe are too faint even for a freely coasting, “empty” universe, barring other influences that could dim the events. This result is independent of the absolute calibration of the peak luminosity, which is neede...
251 citations
TL;DR: In this article, the properties of some dusty disks orbiting, first, pre-main-sequence stars, then main-sequence and post-main sequence stars, and ending with white dwarfs are described.
Abstract: ▪ Abstract Dusty circumstellar disks in orbit around main-sequence stars were discovered in 1983 by the infrared astronomical satellite. It was the first time material that was not another star had been seen in orbit around a main-sequence star other than our Sun. Since that time, analyses of data from the infrared astronomical satellite, the infrared space observatory, and ground-based telescopes have enabled astronomers to paint a picture of dusty disks around numerous main-sequence and post–main-sequence stars. This review describes, primarily in an evolutionary framework, the properties of some dusty disks orbiting, first, pre–main-sequence stars, then main-sequence and post–main-sequence stars, and ending with white dwarfs.
211 citations
[...]
TL;DR: In this paper, the authors focus on new observational capabilities (Yohkoh, SoHO, TRACE) observations, modeling, approaches, and insights into physical processes of the solar corona.
Abstract: We focus on new observational capabilities (Yohkoh, SoHO, TRACE) observations, modeling, approaches, and insights into physical processes of the solar corona. The most impressive new results and problems discussed in this article can be appreciated from the movies and available on the Annual Reviews web site.
119 citations
TL;DR: In this article, it was shown that the time for an orbit to make a close encounter with a perturbing planet, Tc, is a function of the Lyapunov time, T ly.
Abstract: We have accumulated thousands of orbits of test particles in the Solar System from the asteroid belt to beyond the orbit of Neptune. We find that the time for an orbit to make a close encounter with a perturbing planet, Tc, is a function of the Lyapunov time, T ly. The relation is log(T c /T o) = a + b log(T ly /T o) where T o is a fiducial period which we have taken as the period of the principal perturber or the period of the asteroid. There are exceptions to this rule interior to the 2/3 resonance with Jupiter. There, at least in the restricted problem, for sufficiently small Jupiter mass, orbits may have a positive Lyapunov exponent and still be blocked from having a close approach to Jupiter by a “zero velocity curve”. Of more serious concern is whether the relation holds for purely secular resonances, and if it does, how to choose T o. This is the case of interest for the planets in the solar system.
TL;DR: Since the first radio astronomy observations in the 1930s, the angular resolution of radio telescopes has improved from tens of degrees to better than one thousandth of a second of arc as mentioned in this paper.
Abstract: ▪ Abstract Since the first radio astronomy observations in the 1930s, the angular resolution of radio telescopes has improved from tens of degrees to better than one thousandth of a second of arc. This advancement has been the result of technological innovations such as stable, sensitive, short-wavelength radio receivers, digital correlators, atomic clocks, and high-speed tape recorders, as well as the development of sophisticated image processing algorithms implemented on inexpensive, fast, digital computers.
TL;DR: In this paper, the authors take a critical look at the cosmological scenario at the turn of the century by examining the available cosmology models in the light of the present observational evidence.
Abstract: ▪ Abstract This review takes a critical look at the cosmological scenario at the turn of the century by examining the available cosmological models in the light of the present observational evidence. The center stage is held by the big bang models, which are collectively referred to here as standard cosmology (SC) and its extensions. SC itself is characterized by a seven parameter set of models based on Einstein's general theory of relativity. The seven parameters are H0, ΩB, ΩDM, ΩΛ, ΩR (describing the background universe, and A, n (specifying the amplitude and power law index of initial fluctuation spectrum). The extended SC includes extrapolations of the SC to earlier epochs when the mean energies of the particles were greater than about 100 GeV. The strength of the SC is seen to lie in its successful prediction of the expansion of the universe, the abundance of light nuclei, and the spectrum and anisotropies of the cosmic microwave background (CMBR). The SC has led to a whole class of theories of stru...