The Interstellar Medium as Observed by COBE

Abstract

The Cosmic Background Explorer (COBE) was developed by NASA Goddard Space Flight Center to measure the diffuse infrared and microwave radiation from the early universe. It also measured emission from nearby sources such as the stars, dust, molecules, atoms, ions, and electrons in the Milky Way, and dust and comets in the Solar System. It was launched 18 November 1989 on a Delta rocket, carrying one microwave instrument and two cryogenically cooled infrared instruments. The Differential Microwave Radiometers (DMR) measured the fluctuations in the Cosmic Microwave Background Radiation (CMBR) originating in the Big Bang, with a total amplitude of 11 parts per million on a 10° scale. It also measured the synchrotron and free-free emission of the Galaxy. The Far Infrared Absolute Spectrophotometer (FIRAS) mapped the sky at wavelengths from 0.01 to 1 cm, and compared the CMBR to a precise blackbody. The spectrum of the CMBR differs from a blackbody by less than 0.03%. The FIRAS observed interstellar dust, showing that up to three different temperatures of dust are needed to explain the spectra. It also measured the emission of CO, [C I], [C II], and [N II], tracing several distinct phases of the gas. The Diffuse Infrared Background Experiment (DIRBE) spanned the wavelength range from 1.2 to 240 μm and mapped the sky at a wide range of solar elongation angles to distinguish foreground sources from a possible extragalactic Cosmic Infrared Background Radiation (CIBR). It measured foreground emission from stars, interstellar and interplanetary dust, the polarization of starlight from the Galactic Center, and the reddening of the Galactic plane by the dust absorption.