This intriguing compilation of images published last week shows a variety of young stars each surrounded by a disc of cool matter. What fascinates me most are the clear patterns of concentric circles within many of the discs – scientists believe that these are most likely paths swept clear by unseen young planets as they accumulate matter and grow, and for this reason these features are known as protoplanetary discs. How cool is that, being able to take snapshots of developing solar systems beyond our own!
The images in this compilation were obtained by the ALMA observatory (the Atacama Large Millimeter/submillimetre Array) located high in the desert in the Chilean Andes Mountains. This observatory and others like it enable us to view objects that would be impossible to see with an optical telescope. Why? Because objects such as these young stars are located deep within huge clouds of dust –small particles of matter, typically the remnants of supernovae, i.e. exploding stars, found almost everywhere in space.
Obscured by dust
Light (or more correctly speaking, electromagnetic radiation) at visible wavelengths cannot pass through these clouds of dust because the particles are just the right size to absorb or scatter it. As a result, dust clouds appear dark at visible wavelengths, even if there are bright objects within or beyond.
Cool Radiation
So how are ALMA and similar telescopes able to peer through the dust? The obscured objects shine at a variety of wavelengths, not only visible, and the clue to ALMA’s approach lies in its name – it observes electromagnetic radiation emitted at wavelengths that are not blocked by dust, in the millimetric and submillimetric range. These wavelengths are longer and less energetic than visible light, falling between infrared and radio waves, and enable us to see cooler objects.
However, although these wavelengths are not blocked by dust, they do get blocked to a great extent by water vapour in our atmosphere. For this reason, earth-based telescopes observing in this range such as ALMA and the James Clark Maxwell Telescope on Mauna Kea in Hawaii are typically located at high altitude, very low humidity locations.
As you can see here, millimeter/submillimeter telescopes appear more similar to radio dishes than the traditional mirror-based telescope used for visible wavelengths. ALMA itself is a fascinating facility - rather than one instrument, it is made up of an array of 66 separate antennae, whose combined signals enable a much more detailed image, a system known as an ‘interferometer’.
The antennae can be moved around into various formations depending on the needs of the observations. This is not a minor operation: each antenna is between 7 and 12 metres in diameter, weighs over 100 tonnes and has to be moved by one of two giant specialised transporters….named, rather cutely, Otto and Lore!
https://almascience.eso.org/about-alma/alma-basics
https://www.almaobservatory.org/en/about-alma-at-first-glance/how-alma-works/how-does-alma-see/
http://www.sci-news.com/astronomy/alma-protoplanetary-disks-06717.html
1. ALMA (ESO/NAOJ/NRAO), S. Andrews et al.; N. Lira https://www.almaobservatory.org/en/images/almas-high-resolution-images-of-nearby-protoplanetary-disks/
2. Y. Beletsky (LCO)/ESO https://www.almaobservatory.org/en/images/antenna-relocation-19/
3. Carlos Padilla – ALMA (ESO/NAOJ/NRAO) https://www.almaobservatory.org/en/images/antenna-relocation-19/