Interstellar clouds

Ninety-nine percent of the material in the Universe is hydrogen and helium. Some of that is locked up in stars but the vast portion floats in the spaces that lay between them. Because the distances between stellar bodies is huge, the density of this material is exceedingly thin but some of it is shepherded by gravity and exploding stars into enormous nebulous clouds, often stretching thousands of light-years, to eventually spawn new stars under the right conditions.

The remaining percentage of material in space is comprised of cosmic dust. This dust is exceptionally small- much smaller that our Earth-bound namesake that is typically fine bits of fabric, dirt, or dead skin cells. These tiny, irregularly shaped particles range from less than 1/100th of a micron to 10 microns in size- a micron is only one millionth of a meter. For comparison, a particle of smoke is about 1/10th of a micron large.

Cosmic dust is comprised of carbon compounds and additional materials that include silicon, oxygen, nitrogen, nickel and, most likely, other heavy elements. Because of its carbon content, this cosmic fluff has been compared to diamond dust- it stretches the imagination, but the universe is, in fact, filled with this stuff!

Cosmic dust originates from red giant stars and titanic supernova explosions , although there are other sources. It's created when material from a dying star is expelled into surrounding space as grains of heavy elements that coalesce into a cloud of debris.

Over time, these particles drift from the scene of their creation, mix with the hydrogen and helium suspended throughout space and are herded into enormous stellar cauldrons- vast nebula clouds that incubate new stars and the planets which will one day circle them. These seemingly insignificant free floating grains are responsible for much of the world we see around us. They are also responsible for the inhabitants of our planet (you and me), too!

The Trifid Nebula

About 9,000 thousand light-years from Earth towards the southern constellation of Sagittarius, the Trifid Nebula (NGC 6514) offers a relevant portrait showing the early stages of a star’s life from conception to birth. Violent winds and extremely high temperatures of newly kindled stars agitate and transform the Trifid's clouds into a seething cauldron of unimaginable proportions.

Positioned along our line of site so that it appears to be near the much closer Lagoon Nebula- another stellar cauldron- the Trifid hangs in front of the central region of our galaxy that lies farther behind. As such, thousands of farther stars comprising part of the Milky Way's galactic plane are also seen in this picture.

Also known as NGC6514 or M20, based on Charles Messier's catalog of glowing night sky objects, the Trifid is a relatively new star nursery- astronomers estimate that it is only 300,000 years old. To gain a better perception of scale, consider that it takes light about 50 years, traveling at 186,000 miles a second, to travel from one side of the Trifid Nebula to the other.

Interstellar dust has several optical properties that are also of interest when it comes to viewing a deep space picture. For example, the Trifid Nebula offers an opportunity to see the three types of nebula that occur throughout the Universe: reflection, emission and absorption.

Reflection nebula

The large blue-hued region that stretches across the upper portion of this picture of the Trifid Nebula is called a reflection nebula. The color comes from cosmic dust as it scatters light from bright new stars that were formed nearby. The largest of these stars shines most brightly in the hot, blue portion of the visible spectrum.

Interestingly, the blue tinted dust seen in this picture was produced in the same manner that makes our skies on Earth the same color! For example, when the rainbow of colors that makes up the light spectrum are mixed together, as in Sun or star light, they seem almost white.

When light from the sun strikes the earth's atmosphere, some of it is reflected back to space but some of it bounces around before a glancing eyeball detects it. This bouncing is called scattering and its ability to color our sky and explain the blue color of dust in a distant nebula was first recognized by John William Strutt, 3rd Baron Rayleigh, a British physicist who discovered the element argon and the phenomenon known as Raleigh scattering back in the late 19th century.

Light is made up of weightless particles, called photons, that are arranged into waves. The distance between the tops of the wave crests for each color is different with blue light having a shorter wavelength than red. When light strikes an object that is larger than its wavelength, the color is reflected.

The molecules of oxygen and nitrogen in our atmosphere are larger than a wavelength of blue light but smaller than a wavelength of red. Therefore, the gas in our atmosphere reflects blue light while the other colors in sunlight, including red, simply passes through. A similar situation creates the blue hues in deep space nebula clouds. Instead of atmospheric gasses, however, the blue scattering comes from tiny particles of interstellar dust.

Emission nebula

An enormous, ruby-hued bubble of hydrogen gas molecules, several light years in diameter, represents the second nebula type in the Trifid- an emission nebula. Seen below the reflection nebula, hydrogen molecules at the Trifid’s core is heated by hundreds of brilliant young stars causing it to emit red light just as hot neon gas glows red-orange in illuminated signs.

Absorption nebula

The dark veins of gases and dust that trisect the Trifid (its name means 'divided into three lobes') represent the third kind of nebula in this cosmic cloud. They are known as an absorption nebulae because of their light-obscuring effects. Within these dark lanes, the remains of previous star births continue to collapse under gravity’s unrelenting attraction. The rising density, pressure and temperature inside these dark blobs will eventually trigger nuclear fusion and form additional new stars.

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