Trailing Normal: Nebulae

Portion of the Carina Nebula [1]

In my opinion, if not the most beautiful objects in the cosmos, definitely in the top three.

Mesmerizingly beautiful. And I will confess that this entry is as much about me wanting to learn more about Nebulae as it is about me wanting to assemble a beautiful collection of pictures on my blog.

I know very little of astronomy, and as such I was only able to skim the surface on this subject. Because in truth, when I find myself having to look up the meaning of every other word, I know I’m in over my head.

A Little Bit About Nebulae

Nebulae are interstellar clouds of dust, hydrogen, helium and other ionized gases. [1]

They can reach hundreds of *light years in diameter. [1] (*1 light year = just under 10 trillion kilometers.) [2]

A nebular cloud the size of the Earth would weigh only a few kilograms. [1]

Nebulae are often star-forming regions, where gas, dust, and other materials aggregate to form larger masses, which attract further matter, and eventually become massive enough to form stars. The remaining materials are then believed to form planets, and other planetary system objects. [1]

Many nebulae or stars form from the gravitational collapse of gas in the *interstellar medium. As the material collapses under its own weight, massive stars may form in the center, and their ultraviolet radiation ionizes the surrounding gas, making it visible at optical wavelengths. [1]

*Interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. [3]

The red giant Mira [12]

A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution.

These will slowly lose their outer layers during pulsations in their atmospheres.

When a star has lost enough material, its temperature increases and the ultraviolet radiation it emits can ionize the surrounding nebula that it has thrown off. The nebula is almost 97% hydrogen and 3% helium, plus trace amounts of other elements.

In only a few billion years, our own sun will turn into a red giant star, expand and engulf the inner planets, possibly even Earth. [4]

Nebulae can be categorized under four major groups:

DIFFUSE, PLANETARY, SUPERNOVA REMNANT and DARK. [1]

Diffuse nebulae

Most nebulae fall under this category, which, as the name suggests, means that they are extended and contain no well-defined boundaries. [1][14]

In visible light these may be divided into emission and reflection nebulae:

The Omega Nebula, an example of an emission nebula [1]

Emission nebulae emit spectral line* radiation from ionized gas (mostly ionized hydrogen.) [1][15]

*Spectral lines are dark or bright lines in an otherwise continuous spectrum. They can be used as an "atomic fingerprint," because gases emit light at very specific frequencies when exposed to electromagnetic waves, which are displayed in the form of spectral lines. [5]

The Witch Head reflection nebula [11]

Reflection nebulae themselves do not emit significant amounts of visible light, but are near stars and reflect light from them. [1][15]

Planetary nebulae

Are ring-shaped nebulae consisting of an expanding glowing shell of ionized gas around an aging star. [6]

The term 'planetary nebula' is a misnomer that originated in the 1780's with astronomer William Herschel because when viewed through his telescope, these objects appeared to him to resemble the rounded shapes of planets. [6][17][18]

A mechanism for formation of most planetary nebulae is thought to be the following: at the end of the star's life, during the red giant phase, the outer layers of the star are expelled by strong stellar winds.

Eventually, after most of the red giant's atmosphere is dissipated, the exposed hot, luminous core emits ultraviolet radiation to ionize the ejected outer layers of the star. [6][13]

Absorbed ultraviolet light energizes the shell of nebulous gas around the central star, appearing as a bright colored planetary nebula at several discrete visible wavelengths. [6][7][8]

"In about 5 billion years, when the sun shucks off its outer layers, it will create a beautiful shell of diffuse gas known as a planetary nebula. About 10,000 of these short-lived, glowing objects are estimated to exist in the Milky Way."[9]

The Cat's Eye Nebula, an example of a planetary nebula [1]

Supernova remnants

A supernova occurs when a high-mass star reaches the end of its life. It is a stellar explosion that briefly outshines an entire galaxy, radiating as much energy as the Sun is expected to emit over its entire life span, before fading from view over several weeks or months. [10][16]

When the nuclear fusion reactions in the core of the star stop, the star collapses. The gas falling inward either rebounds or gets so strongly heated that it expands outwards from the core, thus causing the star to explode. [1][15] The expanding shell of gas forms a supernova remnant, a special diffuse nebula. [1][15]

The Crab Nebula, an example of a supernova remnant [1]

Dark nebula

Nebulae which are not illuminated by stars and do not exhibit visible radiation, but may be detected as opaque clouds blocking light from luminous objects behind them. [1][15]

Although these nebulae have different visibility at optical wavelengths, they are all bright sources of infrared emission, chiefly from dust within the nebulae. [1][15]

The Horsehead Nebula, an example of a dark nebula [1]

And because no blog on Nebulae would be complete without this:

The "Pillars of Creation" from the Eagle Nebula (Diffuse Emission).
Evidence from the Spitzer Telescope suggests that the pillars may
already have been destroyed by a supernova explosion, but the light
showing us the destruction will not reach the Earth for another millennium. [13]

Updates / Related

A Rose by Any Name: Nebula Blossoms in Sweet Space Photo, Video / space.com / September 22, 2015

Charles Darwin

Thursday, September 25, 2014

Nebulae

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