Physics Photo of the Week

Physics Photo of the Week

February 21, 2025

The Horsehead Nebula (IC 434) - Photo by Don Collins - College View Observatory

The Horsehead Nebula in Orion is a famous icon of deep sky astronomy for it's unusual well-fitting image of a horse.  It is also unusual in that it is a dark nebula -  a giant cloud of dust suspended in the cosmos - rather than a luminous nebula.  The size of the horse's head is about 7 light years across.  The head appears as an extension of a larger cloud of dust and molecules in the constellation Orion.  

This color photo was obtained with the CCD camera on the College View Observatory telescope over a span of one week in late January, 2025.  The process involves taking about an hours' worth of pictures in one night for several nights during the week.  Each night would concentrate on a single filter eventually obtaining a stack of white light monochrome photos and a stack of each of the red, green, and blue filtered images.  Stacking each color to minimizes noise.  Finally,  we combine the monochrome, red, green, and blue stacked images to obtain a reasonable rendition of the actual colors.

The right side of the image consists of a faint red glow due to ionized hydrogen atoms.  The atoms are illuminated by ultraviolet light from a star that is off the right side of the photo (not seen).  The electrons of the hydrogen atoms are ionized by the high energy of the ultraviolet photons.  When the electrons recombine with the hydrogen nuclei the atoms emit the characteristic red radiation so characteristic of hydrogen at a very precise wavelength of 656 nano-meters called "H-alpha".  The fringes of the dark clouds also show the characteristic H-alpha red color indicating the presence of Hydrogen within the cloud.

There is also a lot of dust within the cloud that obliterates any stars that may lie within.  The few stars that we see "within" the cloud may be foreground stars outside of the cloud.  Giant clouds in space are often the sites of intense star formation.  Clumps of the cloud develop enough self-gravity to attract more of the cloud to concentrate and build up larger clumps.  As the clumps become more massive, they attract more cloud particles to enlarge the cloud further.  This also self-attracts the cloud to shrink and increase its density.  When is this process going to stop?  Before all the dust in the cloud contracts into one massive clump, the density becomes great enough, and the gravitational pressure due to the self-gravity of the cloud becomes great enough, that the temperatures rise, the atoms of hydrogen become close enough together that they fuse into helium.  A STAR IS BORN!  This is happening at several places all through the cloud.  Other clouds, all through our Milky Way Galaxy, and within the Orion constellation are also forming stars.  These regions are called "stellar nurseries".  

Astronomers designed the James Webb Space Telescope (JWST) with only infrared cameras, so that it could see through the clouds and observe objects further away in space.  The great distance to the furthest known cosmological objects exhibit such a large red shift that the distant light - emitted as visible light from the distant galaxies - has shifted to infrared wavelengths which our eyes do not see.  

Astronomers also designed JWST  to image in the infrared so its cameras could see through the dust clouds and examine the star-forming activities within dust clouds.  The JWST has recently imaged many dark clouds within our galaxy and successfully imaged many star clusters within the opaque dust clouds, making those stars visible for the first time.  (C. Harper, "The Birth of Stars...", American Scientist, March-April, 2025).  The NASA has also used the JWST to see into the clouds of the Horsehead and has published excellent images of the "Horsehead" to show details of new stars being formed. (Link here).

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Physics Photo of the Week is published periodically during the academic year on Fridays by Donald F. Collins, professor emeritus of Warren Wilson College. These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to dcollins@warren-wilson.edu.

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