Why is Thors Helmet White?

[Tenacious Del]

Hi Guys (and girls if there are any)

Why is this Nebula white -

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

I know that most nebula are either red or blue, but sometimes I see greens, yellows, whites. What makes those colours? :hmm:

Ta

Del

[Astro Dave]

Hi Delio

Gooooood Question. Should be more of 'em.

Try this info: With regards to nebula: (plural nebulae although this looks likes it too is being dispensed with, along with most of our other accepted words) If they are large and massive enough they are frequently places of star formation,, thus generating big associations or clusters of stars.

Some of the young stars are often very massive and so hot that their high energy radiation can excite the gas of the nebula (mostly hydrogen) to shine; such nebula is called emission nebula. If the stars are not hot enough, their light is reflected by the dust and can be seen as white or bluish reflection nebula.

As most diffuse emission nebulae also contain dust, they typically have a reflecting nebula component also.

The word "nebula" is derived from the Latin word for "clouds". Indeed, a nebula is a cosmic cloud of gas and dust floating in space. Nebulae are the basic building blocks of the universe.

They contain the elements from which stars and solar systems are built. They are also among the most beautiful objects in the universe, glowing with rich colours and swirls of light. Stars inside these clouds of gas cause them to glow with beautiful reds, blues, and greens. These colours are the result of different elements within the nebula.

Most nebulae are composed of about 90% hydrogen, 10% helium, and 0.1% heavy elements such as carbon, nitrogen, magnesium, potassium, calcium, iron. These clouds of matter are also quite large. In fact, they are among the largest objects in the galaxy.

Many of them are dozens or even hundreds of light-years across. Nebulae have been divided into five major categories.

These are emission nebulae, reflection nebulae, dark nebulae, planetary nebulae, and supernova remnants. Emission and reflection nebulae tend to be fuzzy in appearance and lack any noticeable shape or structure. They are also known as diffuse nebulae.

[gyro]

Hi Dellio.
Not withstanding Dave's comprehensive reply, or wishing to detract from using the Forum, I find the index and search [in the quoted site] useful for quicky/condensed explanations for most queries on the picture content, drawing on their archive of related subjects.

The only problem I have with the site [my home page], is getting off it !!
After an hour or so I am still link hopping, stuffing my 'favourites' to the gills.