What is a Nebulae?

[harry]

Hello All

http://astro.nineplanets.org/twn/types.html#planetary

Originally, the word "nebula" referred to almost any extended astronomical object (other than planets and comets). The etymological root of "nebula" means "cloud". As is usual in astronomy, the old terminology survives in modern usage in sometimes confusing ways. We sometimes use the word "nebula" to refer to galaxies, various types of star clusters and various kinds of interstellar dust/gas clouds. More strictly speaking, the word "nebula" should be reserved for gas and dust clouds and not for groups of stars.

What do you think?

[randall cameron]

The "popular" usage goes back to before guys like Messier. To the naked eye, except for a few objects like the Pleiades (which as a young star cluster is surrounded by a faint nebula), it was not possible to resolve the stars. They all looked like fuzzy clouds.

This use persisted even in astronomy books into the 50s and 60s, but I do not think anyone calls a galaxy or star cluster a nebula anymore. It disappeared from scholarly literature long before it dropped out of popular usage.

[The Meal]

I learned the visible-eye night sky with HA Rey's wonderful books. I called it "The Andromeda Nebula" for many months before I picked up on the new information.

Interestingly enough, I was also curious about trying to make further investigations with "field glasses" before I finally settled on a nice pair of 10x50 binos.

~Neal

[harry]

Hello


Types of nebulae


Emission Nebulae
Emission nebulae are clouds of high temperature gas. The atoms in the cloud are energized by ultraviolet light from a nearby star and emit radiation as they fall back into lower energy states (in much the same way as a neon light). These nebulae are usually red because the predominant emission line of hydrogen happens to be red (other colors are produced by other atoms, but hydrogen is by far the most abundant). Emission nebulae are usually the sites of recent and ongoing star formation. (M 42 shown)

http://antwrp.gsfc.nasa.gov/apod/ap051119.html

Reflection Nebulae
Reflection nebulae are clouds of dust which are simply reflecting the light of a nearby star or stars. Reflection nebulae are also usually sites of star formation. They are usually blue because the scattering is more efficient for blue light. Reflection nebulae and emission nebulae are often seen together and are sometimes both referred to as diffuse nebulae. (NGC 7023 shown)
http://www.aao.gov.au/images/general/reflection.html
http://antwrp.gsfc.nasa.gov/apod/ap051124.html

Dark Nebulae
Dark nebulae are clouds of dust which are simply blocking the light from whatever is behind. They are physically very similar to reflection nebulae; they look different only because of the geometry of the light source, the cloud and the Earth. Dark nebulae are also often seen in conjunction with reflection and emission nebulae. A typical diffuse nebula is a few hundred light-years across. (NGC 2264 shown; see also the Horsehead Nebula)


http://www.aao.gov.au/images/general/dark_frames.html

Planetary Nebulae
Planetary nebulae are shells of gas thrown out by some stars near the end of their lives. Our Sun will probably produce a planetary nebula in about 5 billion years. They have nothing at all to do with planets; the terminology was invented because they often look a little like planets in small telescopes. A typical planetary nebula is less than one light-year across. (M 57 shown)


http://www.aao.gov.au/images/general/planetary.html


Supernova Remnants
Supernovae occur when a massive star ends its life in an amazing blaze of glory. For a few days a supernova emits as much energy as a whole galaxy. When it's all over, a large fraction of the star is blown into space as a supernova remnant. A typical supernova remnant is at most few light-years across. (M 1 shown)
http://www.aao.gov.au/images/general/supernova.html

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[orin stepanek]

Nebula means cloud; or mist. Most nebula look similar to clouds. Any interstellar object that is in a gaseous state seems to get the term nebula. Star forming, and planetary, and emission nebula are my favorites.
Orin

[dcmcp]

Except maybe this one and this one which were named clouds.
On a clear night in the right place they are spectacular - even to the naked eye.

[harry]

Hello

It really depends who define the Nebula

Once a Nebula has been looked at it can be classified even under galaxy or cluster of stars or cluster of galaxies.

The ones you talk about dcmcp can also be included as a Nebula.
The Small Cloud of Magellan
http://antwrp.gsfc.nasa.gov/apod/ap050617.html
The Large Cloud of Magellan
http://antwrp.gsfc.nasa.gov/apod/ap060510.html

It all depends on the definition.