Much of the interstellar medium (ISM) is filled with a mix of optically-opaque dust and molecular gas. The diffuse dust attenuates optical and ultraviolet (UV) light from background stars. At certain locations instabilities in the ISM allow it to gravitationally collapse — increasing the density drastically, in some cases enough for a star (or group of stars) to form in the middle. Once a star has formed the increase in power output can raise the temperature of its immediate surroundings. Young stars can produce outflows, and push the parent cloud material outward. Very massive, short-lived O and B stars produce enough UV radiation to photodissociate molecular gas around them. Over time these young stellar objects either destroy the clouds from which they were born, or simply drift away from them, eventually becoming typical “naked” main-sequence stars. All of these processes make it difficult to learn about the “pre-conditions” required for star-formation to occur.

At optical wavelengths the very first stages of this process are completely invisible. At the densities of star-forming cores all of the optical-UV light is completely absorbed by the dust, heating it to ~10–30 K. At these temperatures the the dust radiates most of its power as a near-blackbody at long wavelengths (> 100 μm), where the dust is completely transparent. Stars only become optically visible once the material in their vicinity reaches sufficiently low densities (and have therefore already evolved for some time). Radio observations can detect free-free emission from slightly younger (and massive) objects whose first UV light ionizes their immediate environment. This light is also emitted at long wavelengths where the dust is optically thin, but of course requires the presence of a young massive star capable of producing ionizing radiation. The only way to see the very earliest phase of star-formation that precedes this is to observe the cold-dust re-processed light directly, which is emitted at wavelengths ~1000–100 μm, the submillimeter band.

Over the last 15 years submillimeter imaging arrays operating from the ground, such as SCUBA on the JCMT and MAMBO on the IRAM 30-m, have enabled the first detailed studies of these pre-stellar collapsed (or collapsing) cores. Surveys now covering several tens of square degrees across a number of known star-forming regions in the Galactic plane have uncovered large numbers of compact peaks in the submm emission, many of which are new objects, and which may form (or are forming) new stars. However, this first generation of surveys is limited by the inability to measure total luminosities and temperatures (and hence masses) of cores accurately, since they typically only sample a single point on their SEDs. While plausible temperatures can be assumed in order to extrapolate from these single-wavelength measurements to produce order-of-magnitude results, multi-band imaging near the peak of the thermal SED (~100–500 μm) is required to accurately constrain their SEDs.

4 deg² BLAST survey of the Galactic plane in the constellation Vulpecula. Several example features have been identified in this figure: Submm-luminous Ultra-compact H II regions which BLAST colours reveal to have dust temperatures ~30 K; cooler (T ~10–20 K), less-luminous objects which we believe are pre-stellar candidates; and large-scale filamentary structures. For further details on this map see the Vulpecula paper.

BLAST is presently the most powerful submm mapping telescope in the world: it is unique in its ability to detect and characterize cold dust emission from a range of pre- and proto-stellar sources, constraining the temperatures of objects with T < 25 K using its three-band photometry near the peak of the spectrum at 250, 350 and 500 μm. Furthermore, by operating above most of the atmosphere its sensitivity (and therefore mapping speed) is approximately an order-of-magnitude faster than any other existing submm facilities in terms of detecting compact cores. In terms of measuring diffuse submm structures in the ISM, BLAST provides an even more drastic improvement over existing facilities since it is designed for fast-scanning (modulating observed signals faster than the detectors drift), and sky emission is negligible. Together these features of the experiment make it possible to control large-scale noise at the map-making stage out to angular scales much larger than previously possible from the ground (from ~ 10s of arcmin to degree scales depending on the observing strategy).

BLAST successfully surveyed ~20 deg² of the Galactic plane visible from the northern hemisphere during its 2005 flight, and over 200 deg² of the southern plane during its 2006 flight (including a particularly deep map covering 50 deg² in the Vela molecular ridge). First results from the 2005 flight have now been published, and may be accessed from the results page, and the first 2006 flight results will hopefully be published in the next few months. The combined BLAST data set will be unrivalled for area and spectral coverage until the regular operation of SCUBA-2 and Spire begin in the next ~1–2 years.