共查询到20条相似文献，搜索用时 23 毫秒
1.
F Hill PB Stark RT Stebbins ER Anderson HM Antia TM Brown TL Duvall DA Haber JW Harvey DH Hathaway R Howe RP Hubbard HP Jones JR Kennedy SG Korzennik AG Kosovichev JW Leibacher KG Libbrecht JA Pintar EJ Rhodes J Schou MJ Thompson S Tomczyk CG Toner R Toussaint WE Williams 《Science (New York, N.Y.)》1996,272(5266):1292-1296
The Global Oscillation Network Group (GONG) project estimates the frequencies, amplitudes, and linewidths of more than 250,000 acoustic resonances of the sun from data sets lasting 36 days. The frequency resolution of a single data set is 0.321 microhertz. For frequencies averaged over the azimuthal order m, the median formal error is 0.044 microhertz, and the associated median fractional error is 1.6 x 10(-5). For a 3-year data set, the fractional error is expected to be 3 x 10(-6). The GONG m-averaged frequency measurements differ from other helioseismic data sets by 0.03 to 0.08 microhertz. The differences arise from a combination of systematic errors, random errors, and possible changes in solar structure. 相似文献
2.
Wallerstein G 《Science (New York, N.Y.)》1988,240(4860):1743
Analysis of the chemical and isotopic composition of stellar surfaces reveals the types of nuclear reactions that have occurred in the stellar interiors as well as the timing and depths from which material once deep in the star has reached the surface. Mass loss from the stellar surface and, in some cases, mass transfer from a companion enhance the opportunity to observe material that is the product of internal nuclear reactions. Detailed studies show substantial deficiencies in current models with the timing and depth of convective and other forms of mixing. 相似文献
3.
The roughly 25 percent increase in luminosity over the life of the sun shared by many different solar models is shown to be a very general result, independent of the uncertainties suggested by the solar neutrino experiment. Superficially, this leads to a conflict with the climatic history of the earth, and if basic concepts of stellar evolution are not fundamentally in error, compensating effects must have occurred, as first pointed out by Sagan and Mullen. One possible interpretation supported by recent detailed models of the earth's atmosphere is that the greenhouse effect was substantially more important than at present even as recently as 1 billion to 2 billion years ago. 相似文献
4.
Drake JJ Laming JM Widing KG Schmitt JH Haisch B Bowyer S 《Science (New York, N.Y.)》1995,267(5203):1470-1473
The chemical composition of the solar corona is not the same as that of the underlying photosphere. In the corona, elements with a first ionization potential (FIP) of /=10 electron volts (for example, oxygen, neon, and sulfur) by factors of 3 to 10 with respect to the photosphere. The origin of this FIP effect is unknown. The launch of the Extreme Ultraviolet Explorer Satellite (EUVE) opened up the spectroscopic capability required to determine elemental abundances in the coronae of other stars. Spectroscopic observations of the corona of the nearby F5 IV star Procyon obtained with EUVE have yielded estimates of the relative abundances of high- and low-FIP species. The results provide evidence that Procyon, unlike the sun, does not exhibit the FIP effect. Whether the sun or Procyon is more typical of the general late-type stellar population is of fundamental interest to the physics of stellar outer atmospheres and has a bearing on the origin of cosmic rays. 相似文献
5.
Helioseismology is probing the interior structure and dynamics of the sun with ever-increasing precision, providing a well-calibrated laboratory in which physical processes can be studied under conditions that are unattainable on Earth. Nearly 10 million resonant modes of oscillation are observable in the solar atmosphere, and their frequencies need to be known with great accuracy in order to gauge the sun's interior. The advent of nearly continuous imaged observations from the complementary ground-based Global Oscillation Network Group (GONG) observatories and the space-based Solar and Heliospheric Observatory instruments augurs a new era of discovery. The flow of early results from GONG resolves some issues and raises a number of theoretical questions whose answers are required for understanding how a seemingly ordinary star actually operates. 相似文献
6.
High-resolution microwave observations are providing new insights into the nature of active regions and eruptions on the sun and nearby stars. The strength, evolution, and structure of magnetic fields in coronal loops can be determined by multiple-wavelength observations with the Very Large Array. Flare models can be tested with Very Large Array snapshot maps, which have angular resolutions of better than 1 second of arc in time periods as short as 10 seconds. Magnetic changes that precede solar eruptions on time scales of tens of minutes involve primarily emerging coronal loops and the interactions of two or more loops. Magnetic reconnection at the interface of two closed loops may accelerate electrons and trigger the release of microwave energy in the coronal parts of the magnetic loops. Nearby main-sequence stars of late spectral type emit slowly varying microwave radiation and stellar microwave bursts that show striking similarities to those of the sun. 相似文献
7.
Chaplin WJ Kjeldsen H Christensen-Dalsgaard J Basu S Miglio A Appourchaux T Bedding TR Elsworth Y García RA Gilliland RL Girardi L Houdek G Karoff C Kawaler SD Metcalfe TS Molenda-Żakowicz J Monteiro MJ Thompson MJ Verner GA Ballot J Bonanno A Brandão IM Broomhall AM Bruntt H Campante TL Corsaro E Creevey OL Doğan G Esch L Gai N Gaulme P Hale SJ Handberg R Hekker S Huber D Jiménez A Mathur S Mazumdar A Mosser B New R Pinsonneault MH Pricopi D Quirion PO Régulo C Salabert D Serenelli AM 《Science (New York, N.Y.)》2011,332(6026):213-216
In addition to its search for extrasolar planets, the NASA Kepler mission provides exquisite data on stellar oscillations. We report the detections of oscillations in 500 solar-type stars in the Kepler field of view, an ensemble that is large enough to allow statistical studies of intrinsic stellar properties (such as mass, radius, and age) and to test theories of stellar evolution. We find that the distribution of observed masses of these stars shows intriguing differences to predictions from models of synthetic stellar populations in the Galaxy. 相似文献
8.
Oscillations of the sun make it possible to probe the inside of a star. The frequencies of the oscillations have already provided measures of the sound speed and the rate of rotation throughout much of the solar interior. These quantities are important for understanding the dynamics of the magnetic cycle and have a bearing on testing general relativity by planetary precession. The oscillation frequencies yield a helium abundance that is consistent with cosmology, but they reinforce the severity of the neutrino problem. They should soon provide an important standard by which to calibrate the theory of stellar evolution. 相似文献
9.
Manchester RN 《Science (New York, N.Y.)》2004,304(5670):542-546
Pulsars are remarkable clocklike celestial sources that are believed to be rotating neutron stars formed in supernova explosions. They are valuable tools for investigations into topics such as neutron star interiors, globular cluster dynamics, the structure of the interstellar medium, and gravitational physics. Searches at radio and x-ray wavelengths over the past 5 years have resulted in a large increase in the number of known pulsars and the discovery of new populations of pulsars, posing challenges to theories of binary and stellar evolution. Recent images at radio, optical, and x-ray wavelengths have revealed structures resulting from the interaction of pulsar winds with the surrounding interstellar medium, giving new insights into the physics of pulsars. 相似文献
10.
Dwarf galaxies pose substantial challenges for cosmological models. In particular, current models predict a dark-matter density that is divergent at the center, which is in sharp contrast with observations that indicate a core of roughly constant density. Energy feedback, from supernova explosions and stellar winds, has been proposed as a major factor shaping the evolution of dwarf galaxies. We present detailed cosmological simulations with sufficient resolution both to model the relevant physical processes and to directly assess the impact of stellar feedback on observable properties of dwarf galaxies. We show that feedback drives large-scale, bulk motions of the interstellar gas, resulting in substantial gravitational potential fluctuations and a consequent reduction in the central matter density, bringing the theoretical predictions in agreement with observations. 相似文献
11.
Boss AP 《Science (New York, N.Y.)》1995,267(5196):360-362
The sensitivities of astrometric and radial velocity searches for extrasolar planets are strongly dependent on planetary masses and orbits. Because most nearby stars are less massive than the sun, the first detection is likely to be of a Jupiter-mass planet orbiting a low-mass star, with a possible theoretical expectation being that Jupiter-like planets will be found much closer [inside the Earth-sun separation of 1 astronomical unit (AU)] to these low-luminosity stars than Jupiter is to the sun (5.2 AU). However, radiative hydrodynamic models of protoplanetary disks around low-mass stars (of 0.1 to 1 solar mass) show that Jupiter-like planets should form at distances (approximately 4 to 5 AU) that are only weakly dependent on the stellar mass. 相似文献
12.
The determination of chemical abundances of the interstellar medium is related to three different areas of astronomy: pregalactic conditions, stellar evolution, and chemical evolution of galaxies. The observed abundances permit the testing of theories associated with these areas. Knowledge of the abundances of helium, carbon, nitrogen, and neon relative to the abundance of hydrogen in the interstellar medium puts limits on current models of cosmology, stellar evolution, and galactic evolution. 相似文献
13.
Radio waves from the sun were detected 50 years ago, but the microwave detection of other single solar-type stars has remained a challenge. Here, the discovery of four solar-type radio stars is reported. These "solar twin" G stars are radio sources up to 3000 times stronger than the quiet sun. The microwaves most likely originate from a large number of relativistic electrons, possibly produced along with coronal heating, a process that is not understood. Two of the stars are younger than the sun and rotate more rapidly; the dynamo process in the stellar interior is therefore presumably more vigorous, resulting in enhanced coronal activity. One of the detections, however, is an old, metal-deficient G dwarf. 相似文献
14.
Images of an active region on the far side of the sun were derived by applying seismic holography to recent helioseismic observations from space. Active regions are the centers of energetic phenomena such as solar flares and coronal mass ejections, whose resulting electromagnetic and particle radiation interfere with telecommunications and power transmissions on Earth and can pose significant hazards to astronauts and spacecraft. Synoptic seismic imaging of far-side solar activity will now allow anticipation of the appearance of large active regions more than a week ahead of their arrival on the east solar limb. 相似文献
15.
An array of problems in astronomy, cosmology, and particle physics is dependent on our understanding of the evolution and structure of stars. Stellar structure and evolution, in turn, depend on how the nuclear energy generated in the stellar center is transported to the surface. Energy transport by photons is a primary transfer mechanism. Recent improvements in the calculation of the radiative properties of stellar matter have helped resolve several long-standing discrepancies between observations and the predictions of theoretical models. 相似文献
16.
Because of instrumental sensitivity limits and stellar distances, the types of x-ray flares observable on stars have been intrinsically much more energetic than those on the sun. Such enormous events are a useful extrapolation of the solar phenomenon if the underlying assumption is correct that they form a continuous sequence involving similar physical processes as on the sun. The Advanced Satellite for Cosmology and Astrophysics (ASCA), with its greater sensitivity and high-energy response, is now able to test this hypothesis. Direct comparison with solar flares measured by the x-ray-monitoring Geostationary Operational Environmental Satellites (GOES) is possible. The detection of flares on Proxima Centauri that correspond to GOES M-class events on the sun are reported. 相似文献
17.
Hot massive stars represent only a small fraction of the stellar population of the galaxy, but their enormous luminosities make them visible over large distances. Therefore, they are ideal standard candles, used to determine distances of near galaxies. Their mass loss due to supersonic winds driven by radiation pressure contributes significantly to the interstellar medium and thus to the chemical evolution of galaxies. All hot stars are soft x-ray sources; in contrast to the sun with its highly variable x-ray flux, long time scale x-ray variability is not common among hot stars. An analysis is presented here of an unusual increase in x-ray flux observed with the roentgen observatory satellite during a period of 2 days for the hot supergiant zeta Orionis, the only episode of x-ray variability that has been found in a hot star. These observations provide the most direct evidence so far for the scenario of shock-heated gas in the winds of hot stars. 相似文献
18.
Braun DC 《Science (New York, N.Y.)》2012,336(6079):296; author reply 296
Ilonidis et al. (Reports, 19 August 2011, p. 993) report acoustic travel-time decreases associated with emerging sunspot regions before their appearance on the solar surface. An independent analysis using helioseismic holography does not confirm these travel-time anomalies for the four regions illustrated by Ilonidis et al. This negative finding is consistent with expectations based on current emerging flux models. 相似文献
19.
Huchra JP 《Science (New York, N.Y.)》1992,256(5055):321-325
The Hubble constant is the constant of proportionality between recession velocity and distance in the expanding universe. It is a fundamental property of cosmology that sets both the scale and the expansion age of the universe. It is determined by measurement of galaxy The Hubble constant is the constant of proportionality between recession velocity and development of new techniques for the measurements of galaxy distances, both calibration uncertainties and debates over systematic errors remain. Current determinations still range over nearly a factor of 2; the higher values favored by most local measurements are not consistent with many theories of the origin of large-scale structure and stellar evolution. 相似文献
20.
Ackermann M Ajello M Allafort A Baldini L Ballet J Barbiellini G Bastieri D Belfiore A Bellazzini R Berenji B Blandford RD Bloom ED Bonamente E Borgland AW Bottacini E Brigida M Bruel P Buehler R Buson S Caliandro GA Cameron RA Caraveo PA Casandjian JM Cecchi C Chekhtman A Cheung CC Chiang J Ciprini S Claus R Cohen-Tanugi J de Angelis A de Palma F Dermer CD do Couto E Silva E Drell PS Dumora D Favuzzi C Fegan SJ Focke WB Fortin P Fukazawa Y Fusco P Gargano F Germani S Giglietto N Giordano F 《Science (New York, N.Y.)》2011,334(6059):1103-1107
The origin of Galactic cosmic rays is a century-long puzzle. Indirect evidence points to their acceleration by supernova shockwaves, but we know little of their escape from the shock and their evolution through the turbulent medium surrounding massive stars. Gamma rays can probe their spreading through the ambient gas and radiation fields. The Fermi Large Area Telescope (LAT) has observed the star-forming region of Cygnus X. The 1- to 100-gigaelectronvolt images reveal a 50-parsec-wide cocoon of freshly accelerated cosmic rays that flood the cavities carved by the stellar winds and ionization fronts from young stellar clusters. It provides an example to study the youth of cosmic rays in a superbubble environment before they merge into the older Galactic population. 相似文献