Yellow giant stars are evolved stars that have expanded and brightened after leaving the main sequence. Their yellow or yellow-white classification reflects their surface temperature, while their increased size makes them considerably more luminous than stars such as the Sun. But yellow giants are far less common than their red giant counterparts. They should also not be confused with yellow supergiants and hypergiants, which are far more massive and luminous stars despite having similar surface colors.
Yellow Giants Are Evolved Stars
Yellow giants begin their lives on the main sequence, where stars spend most of their existence converting hydrogen into helium in their cores. Once that core hydrogen becomes depleted, the star’s internal structure changes and its outer layers expand and cool as it evolves away from the main sequence. The term giant refers to the star’s luminosity class rather than simply its size. Ordinary yellow giants belong to luminosity class III, distinguishing them from the much more luminous yellow bright giants and supergiants. Their exact properties vary depending on their mass and stage of evolution.
Their Yellow Color Comes From Their Temperature
A star’s color is closely related to its surface temperature. Yellow giants are generally F- or G-type stars, with surface temperatures typically around 5,000 to 7,500 K, depending on their spectral type. This places them between hotter blue-white stars and cooler orange and red stars. The Sun has a surface temperature of about 5,800 K and occupies the same broad color range, but it is a main-sequence star rather than a giant. This is a useful reminder that color alone does not tell us a star’s size or evolutionary stage.
They Are Larger and Brighter Than the Sun
After leaving the main sequence, a yellow giant expands to several times the Sun’s radius. Its larger surface area allows it to radiate much more energy overall, even though its surface may be a similar temperature to the Sun’s. The two dominant stars in the Capella system provide good examples. Both are evolved luminosity-class III giants, each roughly 10 times the Sun’s radius and around 70–80 times as luminous. Despite appearing as a single bright star to the naked eye, each radiates dozens of times more energy than the Sun while having a broadly similar surface temperature.
Yellow Giants Are a Short-Lived Stage of Stellar Evolution
Yellow giants are relatively uncommon because stars generally spend much less time in this part of the H-R diagram than they do on the main sequence or as red giants. As their internal structure changes, they can move through the yellow-giant region comparatively quickly on their way toward another stage of evolution.
This does not mean the star itself is short-lived, though. A star may already be millions or even billions of years old before entering the yellow-giant stage, and it will continue evolving long after leaving it. The time spent as a yellow giant may therefore represent only a small fraction of its total lifetime.
Some Stars Pass Through the Yellow Giant Stage More Than Once
Stellar evolution does not always follow a simple path from hotter to cooler temperatures. Some stars undergo a blue loop, becoming hotter for a time before cooling again and potentially passing through the yellow-temperature region more than once.
The wider yellow-temperature region of the H-R diagram also overlaps the instability strip, where some evolved stars become unstable and pulsate. The best-known examples are Cepheid variables, whose regular changes in brightness reveal their intrinsic luminosity and allow astronomers to measure distances far beyond our solar neighborhood.
Yellow Giants Are Not the Same as Yellow Supergiants
Yellow giants and yellow supergiants can have similar surface temperatures and colors, but they are very different stars. Ordinary yellow giants belong to luminosity class III, while yellow supergiants belong to class I and are generally much more massive, larger and more luminous.
Polaris, for example, is a yellow supergiant rather than an ordinary yellow giant. More extreme still are yellow hypergiants, exceptionally rare and unstable massive stars such as HR 5171 A. Although all of these stars can occupy a similar temperature range, their masses, luminosities and evolutionary paths can be dramatically different.
What Eventually Happens to a Yellow Giant?
A yellow giant does not have one predetermined next stage. The term describes an evolved star by its temperature and luminosity class rather than a single point on one evolutionary pathway. Some yellow giants are moving toward cooler temperatures and will become red giants, while others may cross the same region in the opposite direction during a later phase of evolution.
Ordinary yellow giants are not massive yellow supergiants destined for core-collapse supernovae. After passing through further stages of stellar evolution, lower- and intermediate-mass stars that appear as yellow giants will ultimately shed their outer layers and leave behind white dwarfs.
A Brief but Important Stage
Yellow giants offer a glimpse of stars in transition. Their color, increased size and luminosity reveal how dramatically a star can change after leaving the main sequence, while their position on the H-R diagram provides a snapshot of stellar evolution in progress. Studying them also helps astronomers piece together the much larger story of stellar evolution, connecting the long main-sequence lifetime of a star with the giant stages and stellar remnants that eventually follow.