Astronomy & Celestial Mechanics
the sky, computed
Concept

X-ray emission — where it appears

Light at energies of hundreds to tens of thousands of electronvolts, emitted by gas at millions of degrees or by particles accelerated to high energies. In astronomy it marks shocks, hot halo gas, accretion close to compact objects and stellar coronae.

Named by 2 essays across 2 fields — each of them below, with the objects they name alongside it.

Gas kept inside 3.6 to 6.8 kiloparsecs, depending only on the speed of the fall. The pressure holding a disc galaxy's gas down, against radius, with the ram pressure of the cluster gas it is falling through drawn as three levels. The restoring pressure is two pi times the gravitational constant times the product of the stellar and gaseous surface densities, both exponential with scale lengths of 3 and 5 kiloparsecs, so it falls steeply outward; the ram pressure is the intracluster density times the square of the infall speed and does not depend on radius at all. Where the level crosses the curve, the gas goes. A galaxy entering at 700 kilometres a second keeps everything inside 6.8 kiloparsecs; at 1600 it keeps only 3.6. Nothing in this touches the stars, which are not a fluid and feel no pressure at all, so the galaxy emerges with its stellar disc and its rotation curve intact and its star formation stopped from the outside in. That is the mechanism behind the most conspicuous fact about clusters: their spirals are red, gas-poor and still spiral-shaped, which no process acting on the stars could produce.

Red, gas-poor, and still spiral-shaped

A galaxy falling into a cluster meets a headwind of hot gas at two thousand kilometres a second. Where that ram pressure exceeds the disc's own grip on its gas, the gas goes; the stars, which are not a fluid and feel no pressure, do not. The result is a spiral with no fuel, and it is the commonest kind of galaxy in a cluster.

galaxies · Clusters Thin winds cannot cool their own shocks. The ratio of the cooling time of gas behind a shock to the time the wind takes to carry it away, against the wind's mass-loss rate, for shocks of 300, 500, 800 km/s at three stellar radii from a star of 10 solar radii. The ratio goes as the fourth power of the shock speed and inversely as the density, and so as one over the mass-loss rate. Above the dashed line the shocked gas leaves the star before it can radiate its heat and stays at millions of kelvin. The crossing falls at 2·10⁻⁹, 10⁻⁸, 9·10⁻⁸ solar masses a year. Winds thinner than that stay hot behind their shocks: the measured weak winds, 10⁻¹⁰ to 10⁻⁸, are hot behind any shock faster than about 500 km/s and the thinnest of them behind any shock at all, while the dense winds of the most luminous stars, above 10⁻⁷, cool their shocks except the fastest. The weak-wind problem begins about where the shocks stop cooling.

A wind that may be too hot to see

Below a certain luminosity the winds of hot stars are measured at a tenth to a hundredth of what the theory of line-driven winds predicts. Either the theory fails for thin winds, or the winds are there and hot — heated by their own shocks and unable to cool, invisible to the ultraviolet lines that measure them. X-ray line shapes, which weigh the whole wind regardless of its ions, have begun to say which.

starlight · Stellar winds

Named alongside it

The objects these essays reach for when they reach for this one.

Galaxy clusterGalaxy populationsInterstellar mediumIntracluster mediumIonisationLine driven windMass loss rateO type starP cygni profileQuenchingRam pressureShock heating

All concepts