Difference between a red giant and red supergiant?

[21joanna12]

These two types of stars clearly must have different properties because they form distinct groups on a Hertzsprung-Russell diagram. I have also read that red supergiants can form elements up to around carbon by nuclear fusion, whereas red supergiants can form up to iron, however there doesn't seem to be a clear cut answer as to what main sequence star mass produces a ed giant and which produces a red supergiant, or what exactly happens after the red giant and red supergiant phase. Some sources seem to use only one term when describing the star cycle, and other sources use both but not consistently... I am very much confused! I would appreciate if someone could clear up the differences between these two star types. Thank you :)

 

[Simon Bridge]

You are correct - there is no clear-cut answer; just rules of thumb. The classes are guidelines - don't sweat it.
Note: the giant and supergiant classifications involve a big variation in size within each classification.
Possible endings are: black hole, neutron star, white dwarf (which have well defined mass limits).

 

[Ken G]

So all distinctions are somewhat arbitrary, since there are always "borderline cases" that have some properties of both, but stars do offer us some rather important differences worth keeping track of that relate to your question. The most important distinction is the one between "high-mass stars" and "low-mass stars," the Sun being in the latter camp (but not too far from those borderline cases). At what mass the line is drawn is not precisely known, and depends a bit on what aspect of stars is being used to make the high-mass / low-mass dichotomy, but that dichotomy is always of crucial conceptual importance. In regard to your question, supergiants are late evolutionary stages of high-mass stars, and red giants are late evolutionary stages of low-mass stars.

The reason this distinction is so important is not that there is such a big size difference between the classes (the variation in size within the classes is even larger), it is because there are so many other important distinctions between the physics going on in the core of high-mass stars versus low-mass stars, and all these distinctions tend to refer to generally the same dichotomy (though not exactly, that's the "borderline" problem). These other distinctions include:
the core of a high-mass star:
has H fusion by CNO cycle, is convective, goes relativistic and vulnerable to contraction before it becomes quantum mechanical. can fuse all elements up to iron, ultimately collapses and can cause a core-collapse supernova
the core of a low-mass star:
has H fusion by p-p chain, is not convective, becomes quantum mechanical and reaches a ground state before it goes relativistic, cannot fuse all elements and often won't go past carbon/oxygen, ultimately becomes a white dwarf.

The problem is that the transition between these types can be for masses on the main sequence anywhere between 2 and 8 solar masses, depending on which particular aspect we are talking about, and some uncertainties and details like rotation or magnetic effects. That's why some authors like to identify a third class called "intermediate-mass stars", to account for that whole transition domain.

 

[CygnusX-1]

Two books you should read (any good library will have both):

Stars by James Kaler
The Lives of Stars by Ken Croswell.

With a few exceptions, red supergiants go supernova, and red giants don't. Red supergiants leave behind neutron stars or black holes; red giants leave behind white dwarfs.

The dividing line between the two is a birth mass of about 8 solar masses.

Kaler discusses the difference on pages 176-177 of Stars: "The name [supergiant] implies that these stars are just extreme examples of the giants; they are not...the red supergiants are profoundly different."

Contrary to popular belief, during their lives both types of stars manufacture elements much heavier than iron via the s-process. For example, most tin and lead form in red giants. Croswell discusses the s-process on page 55 of The Lives of Stars.

 

[Chronos]

Agreed Cygnus, the basic difference between a red giant and a red super giant is mass. Obese stars grow up to be super giants, pleasingly plump and moderate mass stars become mere giants.