Selected Questions - Answers From All Experts Astronomy Forum (Origin of Planetary Nebula)

Question:  I am interested in knowing about the origin of the planetary nebula, such as the Ring nebula in the constellation Lyra. Basically, how did the term originate and also what is the physical natures of these nebulas?  - Amateur astronomer, Dallas TX


Answer:  The Ring Nebula you referenced is shown in the accompany image from Mt. Palomar Observatory.  But, of course even in a good telescope you will not see it like that but rather more like the image shown below, taken with my Schmidt -Cassegrain Celestron scope in August,  1980..


The reason for the radical difference  is that no color enhancement has been applied to the second image.  Apart from some extra magnification it "is what it is".   This is pretty well how it appears in the telescope eyepiece, except much dimmer and fuzzier!

As to the origin of the name "planetary" this can be traced to the early observers being misled by the condensed, roundish appearance by surmising the objects were planetary like hence planetary nebula.   But when observed in a small telescope this is easily dispelled. After all what planet like object has a hole in its center?  It was only after the invention of the spectroscope that the real nature of the object was established.

It was found in particular that the atoms of the ring  like gas cloud (such as for the object shown) emitted a great deal of radiant energy.  This was compatible with temperatures in the thousands of degrees Celsius.  Again, this would confirm the objects were in no way "planet like".  The detection of the direction of expansion of the gaseous material also substantiated this.

What exactly is a planetary nebula and how does it come about? We are still not 100 % certain of the physical details but there is mounting evidence these objects are associated with the later stages of a star's life, especially moderate to relatively low mass stars like the Sun. Basically, it is fond that the central star typically belongs to the white dwarf class. A star which has already had its heyday as far as nuclear fuel consumption in its core is concerned.

The best way to think of a white dwarf is as the partially collapse core of such a star, e.g. like the Sun. Most plausibly, the white dwarf stage is triggered after a star like Sun becomes a Red Giant.  This would coincide with the time  when all the nuclear fuel (H, He)  is exhausted.  In the explosive instability process itself, two things happen:   1) The ejected gaseous surface- the ring if you will -   becomes the expanding shell of gas around the remnant core, and 2) the remnant itself collapses since with the outer, radiative layers blown away the star can no longer support itself against gravity.  

Note here that the time between when the star ceases to fuse Helium and the time to ejection of its outer gas layers - to become a planetary nebula- is inconceivably long - on the order of 8 billion years. 

 The outer expanding gas shell is what we are actually calling the planetary nebula. The central  remnant core meanwhile can attain temperatures up to 100,000 K.   As the last of the fuel is exhausted the hot core  becomes a white dwarf.  In his excellent book, 'The Collapsing Universe', the author Isaac Asimov describes in detail the collapse process peculiar to the white dwarf phase and might be worth a read.  From his parameters he estimate that just one teaspoonful of such an object cold tilt the scaled at 15 tons.

Ultimately - as Asimov and many others have pointed out- the white dwarf's further contraction (from which it extracts its energy) is prevented by degenerate gas pressure. The white dwarf can now only radiate what limited stored energy it has left.   How long would it take to radiate all the remaining stored energy away and then become a black dwarf?  We can't say for sure, but some,   like Asimov, believe it could be up to several trillion years, maybe more.

Parsing the origin of the white dwarf definitely gives one a cosmic perspective.

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