APOD Robot wrote:
The planetary nebula phase represents a final stage in the evolution of low mass stars like the sun as they transform themselves from red giants to hot white dwarf stars and in the process shrug off their outer layers.
Okay, so... "low mass stars like the Sun" are destined to end up as white dwarfs, after having briefly gone through the planetary nebula stage. Maybe we should start defining the term "low mass star", then. What kind of stars are they? Are they all like the Sun?
Well... It turns out that all stars lower in mass than 8 solar masses qualify as low mass stars!
Physics Questions and Answers
Why is the cut off mass for massive stars 8 solar masses? Why can't it be 10-11 solar masses or so?
Answer: The division is conventionally made at the boundary between where stars end their lives as white dwarf stars and where more massive stars will end their lives in core collapse supernovae.
The reason for the 8 solar mass division (it is uncertain by about 1 solar mass and also depends to a certain extent on rotation and the initial metallicity of the star, so is not a sharp threshold) is that this is where the carbon/oxygen core (during He shell burning)
becomes hot enough to ignite further fusion
. Core burning continues through to iron-peak elements, then there is a core mass collapse, a violent supernova and large quantities of processed material (O, Mg, Ne, Si, r-process elements) are ejected at high speeds. A neutron star or black hole remnant is formed.
In lower mass stars, the core becomes degenerate, supported by electron degeneracy pressure, and core nucleosynthesis halts. The star ends its life by expelling the majority of its envelope (mostly H and He, with some enrichment with C, N and s-process elements) at low speeds through stellar winds. The degenerate core becomes a white dwarf.
Right. So, at about 8 solar masses, the star's core gets hot enough to run through all possible core fusion processes until the star explodes as a supernova. Below 8 solar masses, the core doesn't get hot enough to explode as a supernova, and the stellar remnant becomes a white dwarf.
So let's have a look at one of those low-mass stars, shall we?
Bellatrix in Orion. Credit: Derrick Lim/NASA.
Yes! Bellatrix qualifies as a low mass star. It's a little guy or gal. Because its mass is believed to be 7.7 solar masses, and that is less than 8 solar masses, isn't it?
So how big is Bellatrix? Well, its radius is almost 6 times that of the Sun, and its V (visible) luminosity is some 1,000 times solar, and its bolometric (total) energy output is some 9,000 times that of the Sun.
All right. So what percentage of all stars in the Milky Way are believed to be low mass stars, i.e., less than 8 solar masses? Let's look at a Wikipedia table of the properties of (main sequence) stars:
The table is found here
(just scroll down a little bit).
As you can see, the "8 solar mass dividing line" falls within spectral class B, which comprises stars of 2.1—16 solar masses. The luminosity of these stars vary between 25 solar luminosities and 30,000 solar luminosities. So main sequence B-type stars vary enormously in mass and energy output.
0.13% of all main sequence stars belong to spectral class B. But we can be certain that most of them are going to be "low mass B-type stars" of spectral classes B9.5V—B8V. The "8 solar mass dividing line" is likely found at spectral class B2V (or B2.5V). What fraction of all stars are going to belong to spectral classes B2.5V—B0V? If you ask me, no more than ~0.01%.
Now let's add the O-type stars, the most massive ones. Only ~0.00003% of all main sequence stars belong to spectral class O!
Okay, I'll stop doing math in a minute, because that is certainly not my strong point. Nevertheless, I think we can conclude that if massive stars are those that contain 8 solar masses or more, and low mass stars are all the others, then 99.9% of all stars are low mass stars!
It's like the world's population, you know?
Some are rich and shiny and stand out against the sky. Others throng much closer to the ground and don't stand out at all. But there are many more of the latter.
Yes, but when it comes to the myriads and myriads of low mass stars, they eventually get to shine, for a few thousand years (a cosmic blink of an eye) when they clothe themselves in their ejected outer atmospheres and ionize them and make them shine in bright colors, like the most glorious (if quickly dispersing) burial shrouds.
, move over! (Even though your stunning funerary mask will last longer than almost any planetary nebula!)
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