State the steps in the life of the following stars: Average Mass Star, Massive Star, Super Massive Star?

Question

State the steps in the life of the following stars: Average Mass Star, Massive Star, Super Massive Star

Answer 1

The main evolutionary stages in the lives of stars are as follows.

First, all stars go through the immediately following early stages:

1. Gravitational collapse from the interstellar medium.

2. Quasi-static pre-main-sequence contraction, lasting from ~ 10^4 years (very massive stars) to ~ 3 x 10^7 years (average mass stars like the Sun), to ~ 10^9 or on up to 10^10 years (for the smallest mass stars destined to "burn" hydrogen, of mass ~ 0.08 solar masses, or the so-called "brown dwarfs" of even smaller mass that will never "burn" hydrogen but just cool to oblivion), respectively.

3. Main Sequence (MS) stage. Hydrogen "burning" (nuclear reactions converting H --> He in central-most regions). This stage lasts from ~ 10^6 years (super massive stars) through ~ 10^8 years (massive stars) to ~ 10^10 years for stars slightly less massive than the Sun. (For the Sun, it's ~ 8 x 10^9 years.)

After that, there are greater differences in the subsequent evolution. It's most natural and best to state the evolutionary stages in reversed mass order from what you asked, because the NUCLEAR evolution proceeds further, the more massive the star. In what follows A, M, or S will stand for Average Mass, Massive, or Super Massive Star respectively.

4A. (I'll use the Sun as an example.) The central, exhausted-hydrogen (now in fact largely helium) core starts becoming isothermal, while contracting more and more to also become degenerate. (The latter is a condition in which pressure support is provided by "degenerate electrons." That pressure ultimately derives from the electrons being forced to satisfy the Pauli exclusion principle.)

Meanwhile, the star slowly expands, becoming brighter and cooler (redder). It ascends the characteristic low mass Red Giant Branch (RGB) with a deep external convective envelope and a growing, degenerate He core. It ultimately expands to more than 100 or 200 solar radii, and reaches a luminosity of ~ 2000 solar luminosities.

5A. After modest mass loss near the highest end of the RGB, helium "ignites" unstably in the star's centre, releasing enormous (though externally unseen) amounts of nuclear energy. This energy, absorbed by the rest of the core, "lifts" it out of the deep gravitational well it was digging for itself. The star (specifically the Sun) will settle down on the very red (cool) stubby end of the Horizontal Branch (HB), jammed against the original RGB and observationally indistinguishable from it. (In contrast, stars with very much lower "metal contents" will move over to the hot (blue) end of the HB.) Stars then stay on this HB for ~ 6 x 10^7 - 10^8 years, with two major nuclear energy sources during this time. They burn H --> He in their cores, but He --> C and O in a relatively thin shell surrounding the He core.

6A. After central He exhustion, it's back to and along the "Asymptotic" RGB and up the former RGB again, losing yet more mass at fairly modest but accelerating rates.

7A. An instability finally removes the last parts of the now radiative envelope, and the star becomes a so-called Planetary Nebula. This short-lived phase (20,000 to 30,000 years) is nevertheless quite spectacularly beautiful, with glowing shells of ejected matter seen surrounding dense, hot, extremely blue cores. Although the time spent by any one star in this phase is a very small fraction of its total lifetime, there are SO MANY stars in our own Galaxy that we know of order a hundred or more of these quite stunning looking objects.

8A. The shell has finally dispersed, and the previous PN now cools to become a White Dwarf, which itself proceeds "down its cooling track" of essentially constant radius (only about the Earth's size) in the "HR diagram." This phase can ultimately take longer than the currently known age of the Universe to complete. In fact, very long ages have been found for some stellar systems from where their "coling tracks" simply peter out --- there hasn't been enough time for their white dwarfs to cool any further.

You know, I simply don't have the time to deal so completey with the other two mass ranges. Let me simply state the main differences from a nuclear and ultimate end-point point of view:

4M. A Massive star develops a more massive He core on the MS, and moves to the RGB relatively quickly, igniting central He on the way. This quick motion acros the HR diagram makes an observable "gap" (the lowest part of the "Hertzsprung Gap") in the that diagram.

Nuclear evolution proceeds further, with He --> C, O followed by Ne, Na, Mg, ... Si, ... and finally, in the very centre, Fe, ... alpha particle break-up, ... p, n, e, ... and then nothing but neutrons.

Surrounding this neutronized core are effectively "onion shell" layers consisting of different nuclear materials. These shells correspond to places where nuclear processing has gone on only up to some point along the route outlined above. The deeper one is in the star, the further nuclear evolution will have proceeded.

(In this intermediate "massive star" range, the more massive the star, the further the evolutionary travel at the very centre along this otherwise inevitable nuclear track. There is also a fair amount of tracking quickly back and forth at fairly high luminosities in the HR diagram, with each "ignition" or exhaustion of successive central fuels.)

It appears that stars up to ~ 8 solar masses may shed enough outer material during this time, to still end up as white dwarfs, whose masses themselves cannot exceed about 1.4 solar masses. For masses beyond about 8 solar masses initially, one gets instead supernovae and neutron star (i.e. pulsar) remnants.

4S. For Super Massive stars, the evolution proceeds for much of their (overall shorter) lifetimes like that of the most massive of the stars considered in 4M. The main difference in the end is that the cores grow too massive to even find a stable end state as neutron stars. Instead, these SM stars leave (presumably rapidly spinning) Black Holes behind them when the rest of the star explodes and disperses into space.

So: that's how stellar evolution proceeds in the three broad mass ranges. It's really quite fascinating. Stars are the largest physical systems for which we can fairly say that we now know almost all of the significant stages in their evolution. As you may well know, the attention of most astronomers and astrophysicists has now turned to the evolution of galaxies and of structure formation in the whole Universe. Interesting new observations and ideas are pouring forth almost daily, to make our knowledge of that approach the relative completeness of our knowledge and understanding of stars' lives.

Live long and prosper.

Answer 2

Generally, smaller stars live longer. Larger stars could very well be at end of life as stars will expand. It has to do with surface to volume ratio. Assuming the star is a sphere, the surface area is X and the volume is Y. Comparing a smaller star to a larger star, the larger star will have a high volume to surface area ratio. This is important because stars are large fusion reactors. The bigger the star, the more the gravity, the faster the reaction. For larger stars, the volume of fuel is consumed faster, all things being equal.

Answer 3

usually they're the same star. an average mass becomes a massive star, and then becomes super massive. after this two things can happen. the super massive can either gradually cool down into nothing or it can explode (supernova) and possibly create a black hole.