I want to know what is different between nebula and a death star?
Nebula is the latin word for cloud. It can refer either to star forming regions ( example: Orion nebula), or to the region surrounding a neutron star (Crab nebula), or the end stage of a star like our sun (Ring nebula). Historically, that word was used to denote anything that appeared "fuzzy" (like a cloud) in the night sky, including the objects that we call today as galaxies.
Technically, there is nothing known as a death star. The object that comes closest to such a description is probably a supernova, and it refers to "explosion" that occurs when a massive white dwarf (at least 1.5 times the mass of the sun) yields to its own gravity. In the end you're left with a neutron star (or pulsar) with material from the star pushed out to form a supernova remnant (again, the example is the crab nebula, aka M1).
Just google these terms; you're bound to get a lot of links to educational websites.
Here something to start with http://astro.nineplanets.org/twn/types.html
You can usually tell by the shape if you're looking at the remnants of a dead star or a just a cloud that hasn't collapsed into one. Ie, the ring nebula is a pretty symmetrical ring, and it would be tough to get something like that from an amorphous cloud collapsing into a star.
Just adding to what russ said: You need to use a good (not necessarily big) telescope/binoc... (I think) I've seen the Andromeda galaxy and I have seen Omega Centauri, but I couldn't find any difference between the two! :rofl:
When you approach a nebula, you say,
"That's no moon, it's a cloud of gas and dust."
"Naw, It's too small to be a cloud of gas and dust. Turn the ship around."
The difference between a nebula and a death star is that Darth Vader never used a nebula as a weapon. :)
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