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Topic Review
The Andromeda Galaxy
Star formation histories of galaxies are critically important for understanding the process of galaxy formation and the structure and contents of galaxies. Star formation can and has been studied in local galaxies for which the stellar populations are resolved and in more distant galaxies for which stars are unresolved, which are instead modeled as populations. Structural components of a galaxy can be resolved at much larger distances. The structural components include those long recognized, such as bulge, disk and halo. More recently recognized structures include separation of disks into thin and thick disk components and stellar streams, as well as recognition of significant numbers of dwarf companion galaxies. Stellar streams are the most recently recognized components of galaxies, mainly using observations of the Milky Way and the Andromeda Galaxy (M31).
  • 614
  • 14 Aug 2023
Topic Review
SWCNTs in Magnetic Recording
Applications in magnet-recording devices rely on the combination and emergence of unique properties of single-walled carbon nanotubes (SWCNTs) and encapsulated materials. The entry “SWCNTs in magnetic recording” focuses on applications of filled SWCNTs in magnetic recording.
  • 613
  • 11 Nov 2021
Topic Review
Onium Compound
In chemistry, an onium ion is a cation formally obtained by the protonation of mononuclear parent hydride of a pnictogen (group 15 of the periodic table), chalcogen (group 16), or halogen (group 17). The oldest-known onium ion, and the namesake for the class, is ammonium, NH+4, the protonated derivative of ammonia, NH3. The name onium is also used for cations that would result from the substitution of hydrogen atoms in those ions by other groups, such as organic radicals, or halogens; such as tetraphenylphosphonium, (C6H5)4P+. The substituent groups may be divalent or trivalent, yielding ions such as iminium and nitrilium. A simple onium ion has a charge of +1. A larger ion that has two onium ion subgroups is called a double onium ion, and has a charge of +2. A triple onium ion has a charge of +3, and so on. Compounds of an onium cation and some other negative ion are known as onium compounds or onium salts. Onium ions and onium compounds are inversely analogous to -ate ions and ate complexes:
  • 612
  • 16 Nov 2022
Topic Review
Music Without Sound
Music without sound can refer to music that falls outside the range of human hearing (typically 20 Hz–20 kHz) or to compositions, such as visual music, that are analogous to or suggestive of conventional music but in media other than sound, such as color, shape, position, motion and literature (see Discursive music below). It is commonly taken for granted that music is wont to be performed or recorded, but some sound works simply won't fit on a disc or on stage, being either extremely discreet (like Robin Minard's Silent Music) or incomplete (Varèse's Unfinished music). Additionally, silence can be regarded as the via negativa of music and has induced long lasting fascination to music composers of all kinds. A composer deals with the absence of sound as much as they deal with sounds. Therefore, this article includes several examples of apophasis in music (like Algorithmic music or Gesture Music).
  • 612
  • 27 Oct 2022
Topic Review
Holographic Optical Tweezers for Biomedical Applications
Holographic optical tweezers (HOT) is a programmable technique used for manipulation of microsized samples. In combination with computer-generation holography (CGH), a spatial light modulator reshapes the light distribution within the focal area of the optical tweezers. HOT can be used to realize real-time multiple-point manipulation in fluid, and this is useful in biological research.
  • 610
  • 27 Oct 2022
Topic Review
Bismuth Indium
The elements bismuth and indium have relatively low melting points when compared to other metals, and their alloy Bismuth Indium is classified as a fusible alloy. It has a melting point lower than the eutectic point of the tin lead alloy. The most common application of the alloy is as a low temperature solder, which can also contain, besides Bismuth and Indium, lead, cadmium and tin.
  • 610
  • 10 Nov 2022
Topic Review
Epoch
In astronomy, an epoch or reference epoch is a moment in time used as a reference point for some time-varying astronomical quantity. It is useful for the celestial coordinates or orbital elements of a celestial body, as they are subject to perturbations and vary with time. These time-varying astronomical quantities might include, for example, the mean longitude or mean anomaly of a body, the node of its orbit relative to a reference plane, the direction of the apogee or aphelion of its orbit, or the size of the major axis of its orbit. The main use of astronomical quantities specified in this way is to calculate other relevant parameters of motion, in order to predict future positions and velocities. The applied tools of the disciplines of celestial mechanics or its subfield orbital mechanics (for predicting orbital paths and positions for bodies in motion under the gravitational effects of other bodies) can be used to generate an ephemeris, a table of values giving the positions and velocities of astronomical objects in the sky at a given time or times. Astronomical quantities can be specified in any of several ways, for example, as a polynomial function of the time-interval, with an epoch as a temporal point of origin (this is a common current way of using an epoch). Alternatively, the time-varying astronomical quantity can be expressed as a constant, equal to the measure that it had at the epoch, leaving its variation over time to be specified in some other way—for example, by a table, as was common during the 17th and 18th centuries. The word epoch was often used in a different way in older astronomical literature, e.g. during the 18th century, in connection with astronomical tables. At that time, it was customary to denote as "epochs", not the standard date and time of origin for time-varying astronomical quantities, but rather the values at that date and time of those time-varying quantities themselves. In accordance with that alternative historical usage, an expression such as 'correcting the epochs' would refer to the adjustment, usually by a small amount, of the values of the tabulated astronomical quantities applicable to a fixed standard date and time of reference (and not, as might be expected from current usage, to a change from one date and time of reference to a different date and time).
  • 610
  • 22 Nov 2022
Topic Review
Colloidal Quantum Dots-Based Upconversion Devices
Colloidal quantum dots (CQD) have narrow emission linewidth and adjustable bandgap, so that CQD based infrared detectors can realize a widely tunable infrared spectral range. In addition, the luminescence spectrum of CQDs is extremely narrow, the color saturation and purity are high, and the optical stability is excellent, which can be obtained by solution procession. Therefore, CQDs-based LEDs (QLEDs) have excellent performances of a wide color gamut, long life, and low cost. For CQD baesd upconverters, except for the top electrode, the entire device can be prepared by solution method, which greatly simplifies the preparation of the device and make the upconverters are available for use in the fields of flexible devices.
  • 610
  • 31 Mar 2022
Topic Review
ATOM Program System
The ATOM computer system is designed to study the structure of atoms and the physical processes occurring with their participation. 
  • 610
  • 07 Jun 2022
Topic Review
International Space Station Maintenance
Since construction started, the International Space Station (ISS) programme has had to deal with several maintenance issues, unexpected problems and failures. These incidents have affected the assembly timeline, led to periods of reduced capabilities of the station and in some cases could have forced the crew to abandon the space station for safety reasons, had these problems not been resolved.
  • 608
  • 02 Nov 2022
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