Reviews in Modern Astronomy: Variabilities in Stars and Galaxies

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In numerous branches of natural philosophy, the ways in which major, transformative advances are achieved are often cloaked in mystery, or arrived at through a fortunate concatenation of circumstances. This theme is pursued here with the aid of some examples from my own work on catalysis the speeding up of the attainment of chemical equilibria , as well as from the work of others. The emergence of the maser forerunner of the laser , the development of positron emission tomography, and the creation of blood-glucose sensors for use by those suffering from type 2 diabetes are among the innovations adumbrated here.

In addition to describing the unpredictable nature of much scientific discovery, I also describe areas in which new chemical technology will be especially beneficial to society. Our current civilization belongs to the organic materials age. Organic materials science pervades nearly all aspects of our daily life. This essay sketches the evolution of materials science up to the present day.

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Plastics as textiles and structural materials dominate human civilization. The element carbon is at the core of this development because of its diverse interconnections with itself and other elements of the periodic table. While silicon will not be supplanted from its role in electronics, carbon will provide the most versatile electronics applications, through inexpensive, flexible electronic devices. As its name suggests, the field draws upon chemical insights and tools to understand or engineer living things.

This essay focuses on the scientific, societal, and pedagogical potential of an emerging frontier for chemical biologists: namely, the study of Homo sapiens. My goal is to highlight the opportunities and challenges presented to chemistry by human biology at a time when it costs less to sequence an individual's genome than it does to buy a car. Computational chemistry is the computer modeling of chemistry using mathematical equations that come from physics.

The field was made possible by advances in computer algorithms and computer power and continues to flourish in step with developments in those areas. Computational chemistry can be thought of as both a time-lapse video that slows down processes by a quadrillion-fold and an ultramicroscope that provides a billion-fold magnification.

The chemical elements are created in nuclear fusion processes in the hot and dense cores of stars. The energy generated through nucleosynthesis allows stars to shine for billions of years. When these stars explode as massive supernovae, the newly made elements are expelled, chemically enriching the surrounding regions.

Pdf Reviews In Modern Astronomy Variabilities In Stars And Galaxies

Subsequent generations of stars are formed from gas that is slightly more element-enriched than that from which previous stars formed. This chemical evolution can be traced back to its beginning soon after the Big Bang by studying the oldest and most metal-poor stars still observable in the Milky Way today. Through chemical analysis, they provide the only available tool for gaining information about the nature of the short-lived first stars and their supernova explosions more than thirteen billion years ago.

These events set in motion the transformation of the pristine universe into a rich cosmos of chemically diverse planets, stars, and galaxies. Cosmologists and philosophers had long suspected that our sun was a star, and that just like the sun, other stars were also orbited by planets. These and similar ideas led to Giordano Bruno being burned at the stake by the Roman Inquisition in It was not until , however, that the first exoplanet — a planet outside the solar system — was discovered.

While the rate of subsequent discoveries was slow, most of these were important milestones in the research on extrasolar planets, such as finding planets around a pulsar a compact remnant of a collapsed star and finding Jupiter-mass planets circling their stars on extremely short period orbits in less than a few Earth-days.

But the first decade of our millennium witnessed an explosion in the number of discovered exoplanets. Astronomers can map the sky in many ways: observing in different regions of the electromagnetic spectrum, obtaining spectra of stars and galaxies to determine their physical properties and distances, and repeatedly observing to measure the variability, explosions, and motions of celestial objects. In this review I describe recent surveys of the sky astronomers have carried out, focusing on those in the visible part of the spectrum.

I describe in detail the Sloan Digital Sky Survey, an ongoing imaging and spectroscopic survey of over one quarter of the celestial sphere. I also discuss some of the major surveys planned for the next decade, using telescopes both on the ground and in space. Quasars emit more energy than any other object in the universe, yet are not much bigger than our solar system. Quasars are powered by giant black holes of up to ten billion times the mass of the sun.

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Their enormous luminosities are the result of frictional forces acting upon matter as it spirals toward the black hole, heating the gas until it glows. We also believe that black holes of one million to ten billion solar masses — dead quasars — are present at the centers of most galaxies, including our own. The mass of the central black hole appears to be closely related to other properties of its host galaxy, such as the total mass in stars, but the origin of this relation and the role that black holes play in the formation of galaxies are still mysteries.

Deneb , in the constellation of Cygnus is the prototype of this class. Their periods are around one day and their amplitudes typically of the order of 0. These non-radially pulsating stars have short periods of hundreds to thousands of seconds with tiny fluctuations of 0. Known types of pulsating white dwarf or pre-white dwarf include the DAV , or ZZ Ceti , stars, with hydrogen-dominated atmospheres and the spectral type DA; [13] DBV , or V Her , stars, with helium-dominated atmospheres and the spectral type DB; [14] and GW Vir stars, with atmospheres dominated by helium, carbon, and oxygen.

The Sun oscillates with very low amplitude in a large number of modes having periods around 5 minutes. The study of these oscillations is known as helioseismology. Oscillations in the Sun are driven stochastically by convection in its outer layers. The term solar-like oscillations is used to describe oscillations in other stars that are excited in the same way and the study of these oscillations is one of the main areas of active research in the field of asteroseismology.

Eruptive variable stars show irregular or semi-regular brightness variations caused by material being lost from the star, or in some cases being accreted to it. Despite the name these are not explosive events, those are the cataclysmic variables. Protostars are young objects that have not yet completed the process of contraction from a gas nebula to a veritable star.

Most protostars exhibit irregular brightness variations. Orion variables are young, hot pre—main-sequence stars usually embedded in nebulosity. They have irregular periods with amplitudes of several magnitudes. A well-known subtype of Orion variables are the T Tauri variables. Variability of T Tauri stars is due to spots on the stellar surface and gas-dust clumps, orbiting in the circumstellar disks.

These stars reside in reflection nebulae and show gradual increases in their luminosity in the order of 6 magnitudes followed by a lengthy phase of constant brightness. They then dim by 2 magnitudes six times dimmer or so over a period of many years. V Cygni for example dimmed by 2.

FU Orionis variables are of spectral type A through G and are possibly an evolutionary phase in the life of T Tauri stars. Large stars lose their matter relatively easily. For this reason variability due to eruptions and mass loss is fairly common among giants and supergiants. Also known as the S Doradus variables, the most luminous stars known belong to this class. They have permanent high mass loss, but at intervals of years internal pulsations cause the star to exceed its Eddington limit and the mass loss increases hugely.

Visual brightness increases although the overall luminosity is largely unchanged. Giant eruptions observed in a few LBVs do increase the luminosity, so much so that they have been tagged supernova impostors , and may be a different type of event. These massive evolved stars are unstable due to their high luminosity and position above the instability strip, and they exhibit slow but sometimes large photometric and spectroscopic changes due to high mass loss and occasional larger eruptions, combined with secular variation on an observable timescale.

The best known example is Rho Cassiopeiae. While classed as eruptive variables, these stars do not undergo periodic increases in brightness. Instead they spend most of their time at maximum brightness, but at irregular intervals they suddenly fade by 1—9 magnitudes 2. Most are classified as yellow supergiants by luminosity, although they are actually post-AGB stars, but there are both red and blue giant R CrB stars. DY Persei variables are a subclass of R CrB variables that have a periodic variability in addition to their eruptions.

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Wolf—Rayet stars are massive hot stars that sometimes show variability, probably due to several different causes including binary interactions and rotating gas clumps around the star. They exhibit broad emission line spectra with helium , nitrogen , carbon and oxygen lines. Variations in some stars appear to be stochastic while others show multiple periods.

In main-sequence stars major eruptive variability is exceptional. It is common only among the flare stars , also known as the UV Ceti variables, very faint main-sequence stars which undergo regular flares. They increase in brightness by up to two magnitudes six times brighter in just a few seconds, and then fade back to normal brightness in half an hour or less. Several nearby red dwarfs are flare stars, including Proxima Centauri and Wolf These are close binary systems with highly active chromospheres, including huge sunspots and flares, believed to be enhanced by the close companion.

Variability scales ranges from days, close to the orbital period and sometimes also with eclipses, to years as sunspot activity varies.

Pdf Reviews In Modern Astronomy: Variabilities In Stars And Galaxies

Supernovae are the most dramatic type of cataclysmic variable, being some of the most energetic events in the universe. A supernova can briefly emit as much energy as an entire galaxy , brightening by more than 20 magnitudes over one hundred million times brighter. This collapse "bounces" and causes the star to explode and emit this enormous energy quantity. The outer layers of these stars are blown away at speeds of many thousands of kilometers an hour. The expelled matter may form nebulae called supernova remnants. A well-known example of such a nebula is the Crab Nebula , left over from a supernova that was observed in China and North America in The core of the star or the white dwarf may either become a neutron star generally a pulsar or disintegrate completely in the explosion. Supernovae can result from the death of an extremely massive star, many times heavier than the Sun. At the end of the life of this massive star, a non-fusible iron core is formed from fusion ashes. This iron core is pushed towards the Chandrasekhar limit till it surpasses it and therefore collapses.

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A supernova may also result from mass transfer onto a white dwarf from a star companion in a double star system. The Chandrasekhar limit is surpassed from the infalling matter. The absolute luminosity of this latter type is related to properties of its light curve, so that these supernovae can be used to establish the distance to other galaxies.

Novae are also the result of dramatic explosions, but unlike supernovae do not result in the destruction of the progenitor star. Also unlike supernovae, novae ignite from the sudden onset of thermonuclear fusion, which under certain high pressure conditions degenerate matter accelerates explosively. They form in close binary systems , one component being a white dwarf accreting matter from the other ordinary star component, and may recur over periods of decades to centuries or millennia. Novae are categorised as fast , slow or very slow , depending on the behaviour of their light curve.

Several naked eye novae have been recorded, Nova Cygni being the brightest in the recent history, reaching 2nd magnitude. Dwarf novae are double stars involving a white dwarf in which matter transfer between the component gives rise to regular outbursts. There are three types of dwarf nova:. DQ Herculis systems are interacting binaries in which a low-mass star transfers mass to a highly magnetic white dwarf. The white dwarf spin period is significantly shorter than the binary orbital period and can sometimes be detected as a photometric periodicity.

An accretion disk usually forms around the white dwarf, but its innermost regions are magnetically truncated by the white dwarf. Once captured by the white dwarf's magnetic field, the material from the inner disk travels along the magnetic field lines until it accretes. In extreme cases, the white dwarf's magnetism prevents the formation of an accretion disk.

Reviews in Modern Astronomy: Variabilities in Stars and Galaxies Reviews in Modern Astronomy: Variabilities in Stars and Galaxies
Reviews in Modern Astronomy: Variabilities in Stars and Galaxies Reviews in Modern Astronomy: Variabilities in Stars and Galaxies
Reviews in Modern Astronomy: Variabilities in Stars and Galaxies Reviews in Modern Astronomy: Variabilities in Stars and Galaxies
Reviews in Modern Astronomy: Variabilities in Stars and Galaxies Reviews in Modern Astronomy: Variabilities in Stars and Galaxies
Reviews in Modern Astronomy: Variabilities in Stars and Galaxies Reviews in Modern Astronomy: Variabilities in Stars and Galaxies
Reviews in Modern Astronomy: Variabilities in Stars and Galaxies Reviews in Modern Astronomy: Variabilities in Stars and Galaxies

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