Yellow Dwarf Stars (Spectral Type G)

Updated : 29/04/2025

Stars with a Spectral Type of G are either Yellow or White dwarf star. They are typically called Yellow stars, although some can be a pale yellow bordering on white. Our Star, the Sun, is classed as a Yellow Dwarf Star even though it's technically not one. We see it in the sky as a yellow star, but it is a white star outside the atmosphere. What colour is the Sun?

As mentioned earlier, our star is a yellow dwarf star, the star by which other stars are measured. For example, when talking about Luminosity, the Sun has a Luminosity of 1, and the other star's Luminosity is a multiple of the Sun's. Other yellow dwarfs have been known to have planets orbiting them.

Whilst we may call them Yellow Dwarfs, the star classification can also include white stars. Our star, the Sun, is white; it's just that the atmosphere makes our star appear yellow.

Our star should be green due to the spectrum wavelength. We don't see green because other colours in the spectrum drown out the green. There are no green stars, although there is debate as to whether Zubeneschamali in the constellation of Libra is a green star or not. Astronomy Trek

Unlike red dwarf stars, Yellow Dwarf stars are not efficient with their fuel. A red dwarf star could live for trillions of years, whereas a yellow dwarf Star will typically live for only about ten billion years. Our star is relatively middle-aged. There are another billion years left on it before the Earth becomes inhospitable, and then another four billion before it ends as a Planetary Nebula. Hopefully, humanity will have found a way to get off this planet and go to the stars to continue.

G-Type Stars Facts

• About 10% of the stars in the Milky Way are Yellow Dwarf stars. The most common type of star is a Red Dwarf Star.
• The Sun is bigger than 70% of stars in the galaxy but is tiny compared to super and hypergiants such as UY Scuti which is a red star.
• Stars like the Sun are called yellow dwarf stars to distinguish them from stars that are giant yellow stars, such as Polaris, the Pole Star or HR 5171, the largest known yellow hypergiant. Both of these stars are smaller than red hypergiants.
• A Yellow Dwarf typically has a surface temperature of about 6,000 Celsius.

Yellow Super Giants

Although more commonly associated with Dwarf Stars, there are a few Yellow Super Giants. The largest Yellow Super Giant that is currently known is HR 5171. It looks like a strange bowling pin. Eventually, the small one will be consumed by the larger one. The radius of HR5171 is estimated at around 1,060 - 1,160 Solar Radius Arxiv. An artist's impression of the star is below. The image is taken from E.S.O. video on Youtube.

The Sun, a Yellow G Star

The Sun is at the centre of our solar system, and all the planets orbit around it. 70% of its mass is hydrogen, 28% is Helium, and all the other materials make up just 2%. The Sun consumes and uses a mass of 4 million tonnes a second. This might seem like a lot, but it's nothing to worry about. The Sun still has plenty of hydrogen to go through and will be here long after we've gone.

The core of the Sun is about 400,000 kilometres in diameter. To put that in context, the Earth's diameter is 12,742 km, according to Google. The Sun's diameter is 1,392,000 kilometres, so the core is just under a third of the star.

The Sun emits more than 30 billion times the energy that all the nuclear power plants on the planet produce. We capture that energy using solar panels to provide clean energy. Cronodon

The Sun weighs in at a massive 1,989,100,000,000,000,000,000,000,000,000 kilograms and is more than 332,900 times heavier. Planet Facts

Exoplanets

We've known since life existed that life could exist on a planet that orbits a Yellow Dwarf Star. Our planet is one. Other Yellow Dwarf stars have been found to have planets in orbit.

The first exoplanet to be discovered was found orbiting a pulsar now called Lich. The first exoplanet to be found around a hospitable star was a yellow star called 51 Pegasi, now known as Helvetios. It turned out that the star was boiling away its planet, but it was enough to show that stars other than our own Sun could have orbiting planets.

Evolution

A Yellow Dwarf star will be created in the same way as other stars from a dust cloud. An event in nearby space would have caused the cloud to collapse, first creating a Protostar, a T-Tauri star, then eventually a Yellow Dwarf star. Not all stars become T-Tauri stars. Only those less than three solar masses were T-Tauri stars. T-Tauri stars are named after the prototype star, which is T-Tauri.

When the star becomes a main sequence star, it will fuse hydrogen into Helium. Once it has finished fusing into Helium, it leaves the main sequence and moves on to the next stage in its evolutionary path. It will eventually end up as a white dwarf star.

A Yellow Dwarf star will grow in size because gravity will no longer be able to contain the outward pressure. Don't worry; we have at least a billion years before that stage. When the stage begins, the star will consume the inner planet. The star will turn to a cooler red. Planets that were once able to support life will no longer be in the Goldilocks Zone, as the Sun will have moved out further from its current location. If we were still alive then, we'd have had to move to the outer planets or another star system.

In a paper published on 30th August 2021 on the Nature website, scientists revealed that between 20-35% of Yellow stars engulf their exoplanets. We can feel lucky that the Sun is in the other 70%.

Yellow Dwarf Stars Examples

• Sadalsuud (Aquarius)
• Sadalmelik (Aquarius)
• RW Cassiopeiae (Cassiopeia)
• RW Cephei (Cepheus)
• Rigil Kentaurus (Centaurus)
• RZ Velorum (Vela)
• SAO 209008 (Scorpius)
• Omega Serpentis (Serpens)
• Nervia (Auriga)
• Moriah (Delphinus)
• Lambda Pegasi (Pegasus)
• Kappa Aurigae (Auriga)
• GW Orionis (Orion)
• FZ Leonis (Leo)
• 63 Eridani (Eridanus)