An Etymological Dictionary of Astronomy and Astrophysics
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An small → irregular galaxy undergoing → violent star formation activity. These objects appear blue by reason of containing clusters of hot, → massive stars which ionize the surrounding interstellar gas. They are chemically unevolved since their → metallicity is only 1/3 to 1/30 of the solar value. Same as → H II galaxy.

→ blue; → compact; → dwarf; → galaxy.

1) Closely and firmly joined or packed together; Occupying little space compared with others of its type.
2) To reduce in size or volume.

M.E., from L. compactus "concentrated," p.p. of compingere "to fasten together," from → com- "with, together" + pangere "to fix, fasten," cf. Gk. pegnunai "to fasten, coagulate;" PIE *pag-/*pak- "to fasten."

1) Hampak, from ham-, → com-, + pak, from pakidé [Mo'in, Dehxodâ] "thick, dense, compact," in Hamadâni pukida "much, full, abundant," Kordi pêk "together, joint," pêk hatin "to be made up of," pêk hênan "to collect, constitute," from PIE *pag-/*pak- as above?
2) Hampakidan, from hampak + -idan infinitive suffix; hampak kardan, from hampak + kardan "to do, make, perform," → -ize.

A binary star system which is composed of a collapsed object (→ degenerate dwarf, → neutron star, or → black hole) in orbit with a low-mass (≤ 0.5 Msol) secondary star, wherein the collapsed star → accretes matter from its → companion. These two objects form a binary system of overall dimensions 10⁶ km with an orbital period of only hours or less. See also: → X-ray binary.

→ compact; → binary; → star; → system.

An → X-ray source detected close to the center of young → supernova remnant (SNR)s that has no apparent emission in other wave-bands and no binary companions. Although these sources have been known and studied for several decades without much understanding of their nature, exciting results over the past few years have brought them into the forefront of → neutron star studies. They have soft, exclusively thermal spectra in the few hundred eV range and X-ray luminosities around 10³³ - 10³⁴ erg s^-1. About ten CCOs are presently known, including the central sources of CasA, Puppis A and Kes 79 supernova remnants. Several, J1852+0040 in Kes79, J0822.0-4300 in Puppis A and 1E 1207.4-5209 in PKS 1209-51/52, have detected pulsations in the hundreds of milliseconds range. J1852+0040 has a detected → period derivative, indicating that it is spinning down like a → rotation-powered pulsar (RPP). The measured period and either measurements or constraints on period derivative indicate that these sources have very low → magnetic fields in the range 10¹⁰ - 10¹¹ G assuming magnetic dipole braking. Since their SNRs are all young, ~ 10³ - 10⁴ yr, they were probably born with unusually low magnetic fields, which makes them "anti-magnetars" (A. K. Harding, 2013, Front. Phys. 8, 679 and references therein).

→ compact; → central; → object.

A galaxy belonging to a comparatively rare class of galaxies possessing very small radii and high central → surface brightnesses. The prototype is the → Local Group  → dwarf galaxy M32. At the low mass end of the → early-type galaxy population, the well-known → mass-size relation splits into diffuse and compact branches. The compact branch is composed of compact elliptical galaxies (cEs) and may even extend to the regime of → ultracompact dwarfs. Compact ellipticals have → effective radii (R_e) generally less than 0.6 kpc, while their diffuse counterparts, the → dwarf elliptical galaxies (dEs) or → dwarf spheroidals (dSphs), have R_e ~ 0.6-3 kpc at similar mass. One formation scenario for cEs proposes that they are low-mass classical → elliptical galaxies, in accordance with the fact that they follow the same trend on the fundamental plane as the giant ellipticals. This implies formation through hierarchical mergers, as in "normal" ellipticals. Most cEs are notably more → metal-rich than dEs and are outliers from the → mass-metallicity relation of massive early type galaxies and low-mass galaxies in the Local Group. An alternative formation scenario addresses the problem of high metallicity by proposing that cEs are the remnants of larger, more massive galaxies. In this scenario, their disks are stripped by strong tidal interactions (→ tidal stripping) with an even more massive host galaxy, leaving only the compact, metal-rich bulges (Du et al., 2018, arxiv/1811.06778 and references therein).

→ compact; → elliptical; → galaxy.

A galaxy with no disk or nebulous background and a high surface brightness that appears only barely larger than a star-like point on a sky survey photograph.

→ compact; → galaxy.

A Galactic H II region with an electron density ≥ 10³ cm^-3 and of a linear dimension ≤ 1 pc.

→ compact; → H II region.

A population of relatively small (typically < 2°) → high-velocity clouds, which are spatially and kinematically isolated from the gas distribution in their environment. They are thought to be located in the → intergalactic medium of the → Local Group.

→ compact; → high-velocity cloud.

A galaxy with a stellar mass of M ≥ 10¹¹Msun and an → effective radius of R_e ≤ 1.5 kpc. Many studies have shown that massive galaxies with low → star formation rates were remarkably compact at a → redshift of z≥ 2. At fixed stellar mass of Mstars ≅ 10¹¹Msun, quiescent galaxies are a factor of ~ 4 smaller at z = 2 than at z = 0. As the stellar mass of the galaxies also evolves, the inferred size growth of individual galaxies is even larger. It is unlikely that all massive galaxies in the present-day Universe had a compact progenitor. However, the vast majority of CMGs that are observed at z = 2 ended up in the center of a much larger galaxy today. Their size growth after z = 2 is probably dominated by minor → mergers. Such mergers are expected because other mechanisms cannot easily produce the observed scaling between size growth and mass growth (P. G. van Dokkum1 et al., 2015, ApJ 813, 23).

→ compact; → massive; → galaxy.

An astronomical object that is substantially denser or more compact than most objects of its class. More specifically, a → neutron star or a → stellar black hole.

→ compact; → object.

A star whose spectrum shows striking similarities to → B[e] stars and is evolving into a → planetary nebula (→ preplanetary nebula).

→ compact; → planetary; → nebula; → B[e] star.

An object emitting intense energy in radio wavelength from a small, unresolved central region.

→ compact; → radio source.

A topological space for which every collection of open sets that covers the space has a finite subset that also covers the space.

→ compact; → space.

1) Math.: A process applied to topological spaces having many dimensions to make them compact spaces.
2) Physics: In string theory, the process of "curling up" the extra dimensions (six in the superstring theory). According to these theories, the Universe actually has 10 spatial dimensions but that 7 of them have become "compactified" down to subatomic size and thus are unobservable.

Compactification, n. from → compactify.

Verbal form of → compactification.

Compactify, from → compact + -ify "cause to become," M.E. -fien, from O.Fr. -fier, from L. -ficare, root of facere "to make, do;" PIE base *dhe- "to put, to do" (cf. Skt. dadhati "puts, places;" Av. dadaiti "he puts," O.Pers. ada "he made," Gk. tithenai "to put, set, place."

1) The act of compacting or the state of being compacted.
2) Geology: The physical process whereby the volume of a fixed mass of sediment reduces either by the weight of overlying deposits or by the pressure of earth movements.

→ compact; → -tion.

1) General: The quality of being compact.
2) For a → compact object, a rough measure of the importance of gravity, defined by a dimension-less quantity which is the ratio between its gravitational radius (→ Schwarzschild radius) and its physical radius. It is expressed as φ = 2 GM / c² R, where M and R are the mass and radius of the object respectively, and G and c are the → gravitational constant and speed of light respectively. For a → black hole  φ is of order unity (by definition), for a → white dwarf it is of order 10^-4, and for a → neutron star  φ is around 0.5.

→ compact + → -ness.

A mapping of an infinite → space-time onto a finite one that may make the far away parts of the former accessible to study. The technique invented by Penrose defines an equivalence class of → metrics, g_ab being equivalent to ĝ_ab = Ω²g_ab, where Ω is a positive scalar function of the space-time that modifies the distance scale making the asymptotics of the physical metric accessible to study.

→ conformal; → compactification.

A list of 100 compact groups of galaxies that were identified by a systematic search of the → Palomar Observatory Sky Survey red prints. Each group contains four or more galaxies, has an estimated mean surface brightness brighter than 26.0 magnitude per arcsec² and satisfies an isolation criterion.

Hickson, Paul, 1982, ApJ 255, 382; → compact; → group.

Extremely compact with respect to a comparison value. → ultracompact H II region, → ultracompact dwarf galaxy.

→ ultra-; → compact.