Since there is no single definition of yellow giant stars, the best way to describe them is to say that they have either a F or G spectral classification, and a mass of between 0.5 and 11 suns. Yellow giant stars also come in a supergiant variety, which have an added luminosity classification of Ia or Ib, and while these stars are very big, they are generally smaller than red supergiants.
Typical examples of yellow giant stars include the bright stars Canopus in Carina, and Polaris in Ursa Minor. Many, if not the majority of yellow giant stars are also variables that for the most part fall into the Cepheid class of variables, with the star d Cephei being a classic example. Below are some more interesting facts about yellow giant stars.
Yellow giant stars are best identified through their spectra
Although yellow supergiant stars generally have F and G spectral classifications, in some cases stars with late A or early K classifications are counted as yellow giants as well. These classifications have characteristic strong hydrogen lines that are prominent in A-class stars, but weaken progressively, or are completely absent in K-class stars. Similarly, Calcium H and K spectral lines are strongest in G-class stars, but weaker in F-class stars, and are barely present in late A-class stars. Below are some examples of yellow supergiant stars that have been classified by using their spectra. The value in brackets is the stellar classification, followed by the stars’ name:
• (F0 Ib): a Leporis
• (F2 Ib): 89 Herculis
• (F5 Ib): a Persei
• (F8 Ia): d Canis Majoris
• (G0 Ib): µ Persei
• (G2 Ib): a Aquarii
• (G5 Ib): 9 Pegasi
• (G8 Ib): e Geminorum
Yellow giant stars all about the same temperature
While the possible masses of yellow giant stars can and do vary greatly, all have temperatures that fall into a very narrow temperature range; from 4,000K to 7,000K, regardless of the star’s mass, which can range from less than one solar mass in the case of the star W Virginis, to more than 20 solar masses in the case of the star V810 Centauri.
Like their masses, yellow giant stars also vary widely in brightness, with the least luminous shining with about 1,000 solar luminosities, to the most luminous examples that shine with 100,000 or more solar luminosities.
Yellow giant stars live in the instability strip on the H-R diagram
The image above shows a simplified Hertzsprung-Russel diagram that plots the luminosity of stars against their temperatures. This diagram shows where on the graph various stellar populations rank, but note the area that is labelled as the “Instability Strip”, in which most, if not all yellow giant stars live.
This is because the temperatures and luminosities of yellow stars make them unstable. In fact, most yellow giant stars are Cepheid variables, named from the star Delta Cephei, the prototype for this class of stars. The most notable aspect of Cepheid-class stars is the fact that their periods of variability are directly linked to their luminosities. Thus, yellow giant stars that are also Cepheids can be used to calculate interstellar distances.
Yellow giant stars are rare
Compared to the high numbers of main sequence and red supergiant stars, yellow giant stars are exceedingly rare. This is evident from the fact that a recent study identified only 16 yellow giant stars in a region of M31 (Andromeda galaxy) that contained more than 25,000 O-type stars. The findings of this study also suggest that at least some of the 16 yellow giant stars found could have evolved from O-type stars.
Yellow giant Cepheids come in two flavours
While the variability of Cepheid stars is closely linked to their luminosity, there are two distinctly different types of Cepheid stars that have different luminosity – period relationships. One type, known as “Classical Cepheids”, are young and generally massive Population 1 stars, while Type II Cepheids are ancient, low-mass Population II stars that also include the W Virginis variables, BL Herculis variables, and RV Tauri variables. The factor that differentiates the two classes is the higher luminosities of Classical Cepheids, as compared to Type II Cepheids that have the same periods of variability.
Yellow supergiant stars can also be R Coronae Borealis variables
Yellow supergiant stars are sometimes classified as R Coronae Borealis variables, in which variability in luminosity is produced by an altogether different mechanism. In these cases, these stars are obscured by dust that condenses from discs or clouds around the star at regular intervals. Depending on the amount of dust that condenses out of the surrounding area, such a star can dim by several orders of magnitude.
Yellow giant stars were all main-sequence stars once
As a class, yellow giant and yellow supergiant stars were all main sequence stars that have exhausted the hydrogen in and around their cores. As such, these stars have all evolved off the main sequence, but they do not migrate across the H-R diagram at the same time, or at the same rate.
Typically, stars that have between 8 and 12 solar masses spend only a few million years on the main sequence as O-type stars before they cool and expand greatly into a yellow giant phase as their hydrogen fuel becomes depleted. This state typically lasts only for a few thousand years, before they cool further to become red supergiants. The short time spent as yellow giants, coupled with the fact that 10+ solar-mass stars account for less than 1% of all stars explains the rarity of yellow giant and supergiant stars.
Some yellow giant stars are actually evolving red giant stars
For the most part though, this usually applies only to those stars that fall into the bottom end of the red giant mass-range. In some cases, a yellow giant that has briefly evolved from a red giant will perform an extended blue loop, and cross the instability strip to pulsate like regular Cepheid variables with variability periods of about ten days.
Yellow supergiant stars are unlikely to produce supernovas
Current models of stellar evolution state that yellow supergiant do not exist in that state long enough to explode in supernova events, before they cool down to become red supergiant. However, it seems that it is possible for the cores of some post-red supergiant (now) yellow supergiants, to collapse to produce a core-collapse supernova. In fact, some supernova events have been linked to apparent yellow supergiant progenitors that were not sufficiently luminous to have been post-red supergiant stars.
Some yellow supergiant stars can become blue supergiant stars
Yellow supergiant stars that are especially unstable and luminous are classified as yellow hyper giant stars. These stars are typically extremely massive post-red giant stars that have blown off considerable amounts of their bulk in very energetic solar winds. This mass loss is the starting point for a complex evolutionary process in which the star evolved into either a blue supergiant star, or a very hot and luminous Wolf-Rayet star.