10 Interesting Facts about Yellow Dwarf Stars

10 Interesting Facts about Yellow Dwarf Stars
Image Credit: Sun by NASA's STEREO space-based observatories

Yellow dwarf stars are not true dwarf stars, at least not in the sense that red or white dwarf stars are dwarfs. Often used imprecisely, if not erroneously, the term “yellow dwarf” refers to stars of the “G” spectral class on the main sequence, with such stars usually having a mass of about 0.8 to 1.2 times that of the Sun, and surface temperatures of between 5,300K, and 6,000K.

Below are 10 more interesting facts about yellow dwarf stars you may not have known. Bear in mind that since our Sun is a prime example of a G-type star, some of the items on this list will refer to our nearest star.

The term “yellow dwarf” is a misnomer

Unlike the terms “red dwarf stars” or “white dwarf stars” that describe a class of star, the term yellow dwarf refers to a spectral class, in this case, G-type main sequence stars having a G2V classification. In reality, though, the Sun is a white star, and only appears to be yellow because of the way Earth’s atmosphere scatters some of the Sun’s light. Nevertheless, the color of yellow dwarf stars range from white to yellow depending upon their age – the Sun being relatively young at 4.6 billion years is just under halfway through its life cycle and so is white.

Yellow dwarfs outshine most other stars

The most useful application of the term “yellow dwarf” is to distinguish G-type Sun-like stars from yellow giant stars. In fact, Sun-like stars outshine more than 90% of the stars in our Milky Way galaxy, which consists primarily of dimmer orange, red, and white dwarf stars that are themselves often the remains or remnants of G-type yellow dwarf stars.

G-type yellow dwarfs are mega-power generators

Being a main-sequence star, the Sun is converting hydrogen into helium at the rate of about 600 tons per second, which means that in practice, the Sun is converting about 4 million tons of matter into energy every second. To put this into perspective, we can think of the Sun as a generator that creates 30 billion times more energy per second than all the power generators on Earth combined. Other G-type yellow dwarfs that produce similar amounts of energy include the stars 51 Pegasi  Tau Ceti, and Alpha Centauri A.

The Sun is really “lazy”

While the Sun and other G-type stars produce prodigious quantities of energy, they only do so because they are as big as they are. The fact is that on small scales, the Sun and similar stars produce only about 276 or so Watts of energy per square meter, which is typically about the energy levels produced by reptiles or compost piles.

G-type stars live for only about 10 billion years

G-type stars like the Sun will convert hydrogen into helium only for about 10 billion years or so, after which they will evolve into red giants, such as Aldebaran in Taurus is now. In this state, the Sun will engulf the planets Mercury, Venus, and quite possibly Earth as well. At the end of its red giant phase, the Sun will then blow off its outer layers to become a planetary nebula, while the core will contract into an Earth-sized remnant that will likely outlive the Milky Way.

By rights, Sun-like G-type stars should be green

The wavelength of light emitted by objects depends on the temperature of that object. So, since the light emitted by the Sun and some similar stars peaks in the blue-green part of the visible spectrum because of its temperature, we should see it as green, right?

Well, no, since the Sun also emits a large amount of red and yellow light (among others), which means that the green portion of the spectrum is drowned out. What we do see is all the light emitted by the Sun mixed together, which produces white, which is stronger than the blue-green portion of the Sun’s emitted light. While the above is an oversimplification of a complex issue, this is the basis for the reason why we do not see green stars. Images of the Sun that are colored green are either the result of enhancement or views of the Sun through filters that only admit green light.

The Sun is a near-perfect sphere

The Sun and other slowly rotating G-type stars are nearly perfectly spherical, since their rotational velocity is not high enough to deform them. In fact, the difference between the polar and equatorial diameters of the Sun is only 10 km (6.2 miles), which given the 695,700 km radius of the Sun, means that the Sun is one of the most spherical structures ever observed in the expanse of space.

G-type stars have extremely active cores

Generally speaking, G-type stars produce around 99% of the energy they create in their cores. In the case of the Sun, for example, the core comprises only 24% of its radius, and by 30% of its radius almost no nuclear fusion takes place at all.

G-type stars are self-correcting

G-type stars are in almost perfect hydrostatic equilibrium, but not quite. As the core heats up, it expands slightly, which has the effect of cooling the core down. This reduces the rate at which fusion reactions take place, which reduces the pressure. As the upper layers exert increased pressure against the core, the core heats up again, increasing the fusion rate, which increases the pressure against the overlaying layers, thereby correcting the initial expansion.

Light can take up to 1 million years to escape from a G-type star

While G-type stars like the Sun create enormous numbers of photons, those that we see as optical light can take up to 1 million years to escape through the “surface” of the star, as is the case with the Sun. The reason for this is that photons interact with billions of particles on their outward travels, and each collision deflects the light photon into a different direction. In essence, a light photon only escapes from the Sun by sheer chance, but once it does, it can travel unimpeded to reach Earth in about eight minutes and twenty seconds.

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