Physics Photo of the Week

 Physics Photo of the Week

February 7, 2025

The Flame Nebula  (or the "Burning Bush")

Astrophoto by Donald Collins - the College View Observatory

 

This is "The Flame Nebula" - an interstellar dark cloud surrounding by a glowing cloud.  I like to compare to the Burning Bush that is described in the Bible.  The burning bush in the Bible appeared to Moses - a bush on fire, but not burning up.  A perpetual flame.  This cosmic burning bush likewise is not burning up, but appears to last forever.

Ancient stories aside, the cosmic flame is not the burning up of stars.  On the contrary, the "burning bush" trunk and branches are cosmic clouds of dust particles as well as hydrogen gas.  Stars are actually being formed deep inside the dark clouds.  The glowing cloud in the astrophoto above is about 6 light years across calculated from the field of view of the camera-telescope system (18 arc minutes) and the approximate distance of the nebula from Earth (~1300 light years).  Inside the dark cloud we see hardly any stars because the dust cloud is very opaque.  The dust is probably silicates - accumulated from the ashes of very ancient supernova explosions of giant stars millions of years ago.  The few stars visible in the midst of the dark clouds are most likely foreground stars - stars that are closer to the Earth than the nebula.  The orange glow of the flames is caused by very young stars within the cloud.  The cloud is so immense that its self gravity attracts the cloud particles and hydrogen gas atoms.  The self gravity becomes so immense in parts that stars are compressed to such a high density that the hydrogen atoms' nuclei begin to fuse into new stars, similar to the ignition of a hydrogen bomb.  The new stars that are forming are deep within the clouds that block our view.  Some of the diffuse ultraviolet light (UV) from the new massive internal stars reaches the outer edge of the hydrogen cloud.  The UV light is so energetic that it ionizes the hydrogen in the cloud - the single electrons of the hydrogen atoms are "torn" away from the proton in the center of each hydrogen atom.  The color is emitted when the electron falls back into its orbit within a new hydrogen atom, which emits the red light from the energy that is released.  This light has a very special wavelength: 656 nano meters or 0.656 microns which is red light.

The Flame Nebula (cataloged as NGC 2024) is called an emission nebula because the light is caused by the emission of discrete spectral lines of atoms in the nebula.  Most of the light emitted by stars not spectral lines, but thermal emission - the light coming from a hot body such as a hot filament, burning embers in a campfire, or the glow within a hot pottery kiln.

Another nebula close by to the Flame Nebula is the Great Nebula in Orion.  The Orion Nebula consists of a similar stellar nursery where stars are forming from condensing dust clouds, except that the Great Nebula we can see the new stars as extremely hot blue stars (See PPOW for March 27, 2024).

The lower right hand corner of the photograph is a blue quarter-circle artifact and some blue streaks emanating from off the right side of the photo.  This artifact is caused by scattered light from the bright star Alnitak.  Alnitak is the eastern-most stars of the three stars of Orion's belt.  The light is scattered from edges of internal baffles within the telescope and the edges of the telescope tube opening.  The scattered light is blue because Alnitak star is a very hot star (about 20,000 K compared to the white Sun at about 6,000 K).  K means Kelvin degrees.

The photo above shows the constellation Orion along with the position of Alnitak belt star and the Flame Nebula's position very close to Alnitak.
 

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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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