Space telescopes' new views of the Horsehead
Sen—One of the most striking objects in the night sky is the Horsehead Nebula in Orion. Though far less conspicuous than its near neighbour the Great Nebula, or M42, it is distinctive because of its resemblance to a knight on the chess board.
A large amateur backyard telescope will show the Horsehead apparently silhouetted against brighter nebulosity if you have a clear dark sky. Now two space telescopes have produced stunning views that are unlike anything you will ever see for yourself.
Most objects through an amateur telescope look pretty two-dimensional. Hubble’s infrared image, taken to celebrate its 23rd year orbiting the Earth, has a remarkable 3D-feel as it reveals plumes of gas a structure that is usually hidden by dust.
The nebula, otherwise known as Barnard 33 and close to Alnitak, the easternmost of three stars that make up Orion the hunter’s belt, was formed from a collapsing cloud of material that is lit up by a hot star nearby. It lies about 1,300 light-years away.
Most of the gas around the Horsehead has thinned out over millions of years but the usually dark pillar is composed of thick clumps of material that have managed to hold themselves together. It is thought the structure has another five million years or so before it is also eroded away and disintegrates.
Apart from the need for a telescope of Hubble’s quality, another reason we cannot see the Horsehead like this for ourselves is because our eyes are not sensitive to infrared light. This special view was captured with Hubble’s high-resolution Wide Field Camera 3, fitted in 2009, which can see in near-infrared light.
The Horsehead is dwarfed in a view from another space telescope, Herschel, which is kept super-chilled to view deeper in the infrared. That has taken a wide-angled look at a star-forming region of nebulosity known as the Orion B molecular cloud. You can just glimpse the Horsehead as a little extension on the far right of the image.
The Orion B molecular cloud from Herschel, showing the Horsehead as a tiny appendage on the right. Credit: ESA
The region is a vast stellar incubator where dust heated by newborn stars shines brightly in far-infrared wavelengths. The bright yellow, white and pink areas in the picture are the densest regions containing many protostars and newborn stars. Darker regions indicate colder parts of the cloud where star formation is less active.
The cloud shows a very sharp edge to the right, including the Horsehead formation, where the material within Orion B is being compressed by powerful stellar winds from clusters of massive stars located outside the field of view. The image is a composite from observations made with Herschel's PACS and SPIRE instruments.
It seems no time at all since this writer saw Herschel in 2007 as it was being prepared for its mission in a clean room at Frederickshafen, Germany. The telescope is named after the famous astronomer William Herschel. As well as finding Uranus in 1781, it was he who discovered the infrared when he observed that the temperature on a thermometer rose just beyond the red end of a visible rainbow spectrum.
The Herschel telescope was launched in May 2009 from Kourou, French Guiana, but is already nearing the end of its working life. That is because the coolant it carries to allow it to observe at -271C is almost exhausted. The day it runs out, this amazing telescope will have lost its special observing powers for ever.
European Space Agency video showing the Horsehead and the region surrounding it at different wavelengths. Images from Herschel and Hubble are combined with those from the VLT and VISTA telescopes run by the European Southern Observatory (ESO) in Chile and the Digitized Sky Survey 2 (DSS2). Credit: ESA