ESO Astronomical Glossary - I
Infrared (IR) radiation
Infrared light is a form of electromagnetic radiation with lower energy than visible light but more energetic than radio waves. Further subdivisions in this region of the spectrum are made according to the subregion's proximity to the visible region: near-infrared or NIR (wavelength 1-10 microns), mid-infrared or MIR (wavelength 10-50 microns) and far-infrared or FIR (wavelength 50 to 300 microns).
Integral Field Spectroscopy
Integral Field Spectroscopy (IFS) is a modern method of spectroscopy and a powerful discovery tool. In traditional spectroscopy a spectrum is obtained of a single source in the telescope's field of view. With IFS, optical techniques are used to produce a spectrum of the entire field, which gives much more information on the region being observed in one single observation. This is achieved by dividing the field of view into several regions, each of which is is sent to the spectrograph and the detector. The Visible Multi-Object Spectrograph (VIMOS) on ESO's VLT uses integral field spectroscopy.
Interferometer
An interferometer is a system of two or more widely separated telescopes that achieves the image quality of a much larger telescope of the size of the distance between the telescopes. ESO operates the largest visible light interferometer in the world with the Very Large Telescope Interferometer (VLTI). In full operation, VLTI will be able to separate objects just two thousandths of an arcsecond apart.
Interferometry
In interferometry, the data from more than one telescope are used simultaneously to improve the resolution (see also Interferometer).
Irregular galaxy
Irregular galaxies are galaxies that cannot be classified as either spiral or elliptical in shape. They often look chaotic with no discernible structure, and are thought to make up around 25% of all galaxies.
Interstellar medium
The interstellar medium is the dust and gas, mostly hydrogen, that is located between stars in a galaxy. The interstellar medium is of very low density: at its densest, it is emptier than the best vacuum we can produce on Earth.
Ionisation
Ionisation is the process by which an atom loses electrons. Electrons can be lost by an atom when high-energy radiation transfers enough ennergy to an electron for it to escape the attractive force of the atom's nucleus. Ionisation is very common in astronomical environments where gas is heated by the radiation from nearby hot stars, such as nebulae. Special notation is used to indicate that a gas is ionised: hydrogen in its 'neutral' state, with its one electron present, is denoted 'HI'; when the electron is lost, the ionised hydrogen is called 'HII'. Similarly, neutral oxygen is written as 'OI'; when one electron is lost, 'OII', and when a second also escapes, the gas is called 'OIII'.
Isotope
Forms of a chemical element in which the atoms all have the same number of protons but different numbers of neutrons are called isotopes.