Spectral Colors – A Fossil Record of Stellar Death

Studying stellar colors is more than learning about the emission of light which falls in the optical portion of the electromagnetic spectrum. It means beautiful pictures! The images shown below were all taken with the Hubble Space Telescope, showing some of the incredible emission features found in nearby galactic objects.

Throughout most of its lifetime, a star generates the energy to shine by burning hydrogen and converting it into helium in its core. Once it has exhausted the available nuclear fuel, it begins to die. The length of its life depends upon its mass: the heavier the star, the faster it will burn through its hydrogen, and the sooner it will die. The most massive stars die very violently, in huge explosions called supernovae. These bright blasts tear the stars apart, and their remains are blown out into space. We can explore the elemental abundances of these remnants by looking at their colors.

Image of the Cygnus Loop Nebula shows a black background with individual faint red-white stars dotting the sky. Prominent in the image is the light from the nebula, vivid green, yellow, orange, and red veils of gas which intersect and show clear substructures (bright curved lines which arc across the image, and tangled webs of different colors lying on top of each other).
The bright colors of the Cygnus Loop Nebula show clear emission from a hot gas, and expanding shells of materials emitted from a supernova explosion which occurred some 15,000 years ago. [NASA/HST]

Most stars, those like our Sun or up to eight times heavier, lead less exciting lives (and die less exciting deaths). After the nuclear fuel supply is exhausted, the outer layers of the star begin to collapse inwards. As they are compressed they ignite, in turn causing the outermost shells of gas to explode outwards in all directions. As the dregs of the hydrogen resources dwindle away, more and more material is ejected in shells, leaving only a small remnant of the hot stellar core. As this heats up, it radiates more and more strongly, and the surrounding planetary nebula of gas will begin to glow brightly.

Image of the Cat's Eye Nebula shows a black background. Centered in the the image is a bright violet-white star, surrounded by intersecting ovals and arcs of red gas, dotted with green on the edges. The effect is somewhat like the petals of a gigantic flower, with lines of substructure crossing each individual petal.
The Cat's Eye Nebula shines with red (hydrogen) and green (oxygen) features tracing concentric gas shells, jets of high-speed gas, and shock-induced knots of gas. [NASA/HST]

The remainder of the stellar core is called a white dwarf. With a mass just a bit less than the Sun stuffed into a volume comparable to that of planet Earth, it is far more dense than its progenitor star. The white dwarf has no means to generate new energy, so over time it radiates away its remaining heat. It slowly cools and fades, and over several billions of years decays into a dark lump of coal. The surrounding nebula of gas will continue to expand into the surrounding space, at a rate of almost 100,000 kilometers per hour. As the white dwarf cools and the nebular gas shifts further and further away from it, it becomes harder and harder for the white dwarf to heat the gas. In a mere 25,000 years, the nebula will fade away. But until then, appreciate the complexity and beauty of these stellar graveyards, as revealed by the Hubble Space Telescope!

The first in a mosaic of 12 nebula images shows a light blue-white central region surrounded by orange and reddish oval shells. The overall appearance of the Helix Nebula is of a giant eye looking out towards us inquisitively. Nebula #2 has a greenish-white central region surrounded by orange and white gas. The overall shape is quite boxlike, in contrast to the oval shapes shown for Nebula #1. Nebula #3 (the Ring) has a green-blue central region surrounded by successive smooth ovals of yellow, orange, and red gas. The colors are quite similar to those seen in the hot springs of Yellowstone Park, such as the beautiful Opal Pool.
Nebula #4 (the Egg) is a glaring blue-white core surrounded by pulsing white and blue circles of gas (as if a single pebble had been dropped into a calm pool of blue water, producing ripples on the surface). Nebula #5 has a golden-red central region surrounded by a thick golden ring of gas, with surrounding green, yellow, and red arcs. Nebula #6 (the Cat's Eye) is shown with a new set of camera filters; displaying complicated shells of blue, white, and orange light surrounding a bright white star.
Nebula #7 (the Crab) is a jumbled mixture of red and white bright arcs and gauzy plumes. Nebula #8 has a bright white central star surrounded by pink and green plumes of gas. In this case the gas is mainly confined along a tube centered around the star, extending out in two directions and broadening in radius with distance from the star. The effect is somewhat like the two long wings of a raptor, spread out on either side of the star Nebula #9 is shown through a red light filter, and appears to have a patchy distribution of gas being thrown off from the central star in all directions.
Nebula #10 (another view of the Egg, through different camera filters) still shows a rippled pattern of circles surrounding the central star, this time illuminated in violet, green, and white colors. Nebula #11 shows a mosaic of purple and orange circles intersecting around a bright yellow central star. Nebula #12 looks far more disordered than the others, with messy, unsymmetrical splotches of green-blue and yellow-orange-white gas grouped irregularly.
Nebulae come in all sorts of shapes, sizes, and colors. [NASA/HST]

The nebula will eventually disperse into the surrounding interstellar medium, enriching it with key elements such as carbon, nitrogen and oxygen that are produced only deep in stellar interiors. This material will be recycled to form a later generation of stars, and even planets. The atoms of many of the elements that comprise our own bodies were created in the depth of the stars above. You and I are all made of star stuff!