Are the four stars mentioned in the title stars that are going to go supernova, or are they stars that are in mid-life, like our Sun, but just happen to be so much larger than our Sun?

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    $\begingroup$ All three appear to be red-hypergiants which should end their (short) lives as type II super-novae. They are already off of the main sequence. $\endgroup$ – Conrad Turner Jul 8 '15 at 9:43

NML Cygni and VY Canis Majoris

Zhang et al. (2012) considered several values for $T_{\text{eff}}$ (effective temperature) and $L$ (luminosity) and found that NML Cygni matches up with the evolutionary track of a star of mass $\sim25M_\odot$, near the HR-diagram location of a similar star, VY Canis Majoris (denoted as VY CMa). Using these data points, the model estimates that NML Cygni is approximately 8 million years old and in the post-main sequence phase of its life.

However, NML Cygni is thought to be related to the Cygnus OB2 association, which has an age of 2-3 million years. This is strange because all the stars in the association should be nearly the same age (see Knodlseder (2008)). That age may be questionable, however, as evidence for older stars nearby has been presented (see Wright et al. (2010)).

The exact location of VY Canis Majoris on the evolutionary tracks is not certain. Massey et al. (2006) created models that placed it on a lower-mass track to help it avoid the "forbidden zone" beyond the Hayashi limit.

VV Cephei

Based on standard parameters, stellar wind measurements and Geneva grids, Bennett (2010) suggested that the red supergiant component of VV Cephei could be near the end of its life, assuming a mass of $\sim20$-$25M_\odot$.

UY Scuti

Wikipedia claims

UY Scuti should fuse lithium, carbon, oxygen, neon, and silicon in its core within the next million years. After this, its core will begin to produce iron, disrupting the balance of gravity and radiation in its core and resulting in a core collapse supernova.

As far as I know, that claim is made only through rough estimates of evolutionary tracks, not precise calculations. We do know that UY Scuti is certainly in the later period of its hydrogen-burning phase, if it is indeed at that point in its life. Arroyo-Torres et al. (2013) believe observations place it near evolutionary tracks of stars originating with masses of $\sim25$-$40M_{\odot}$. Therefore, it may have already lost a substantial fraction of its original mass - not surprising for a red hypergiant.

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