# Stars/Yellows

Based on their Planckian spectra, yellow stars have peak wavelengths between 570 and 590 nm. This is an actual temperature range from 5100 to 5300 K and their spectral types range from K0 to K2.
Additional higher temperature stars appear yellow to hominin eyes because our peak sensitivity is actually in the green. These are yellow-green stars that are also G spectral type stars.

## Objects

"An abundance analysis of the yellow symbiotic system AG Draconis reveals it to be a metal-poor K-giant ([Fe/H]=-1.3) which is enriched in the heavy s-process elements. ... the other yellow symbiotic stars are probably low-metallicity objects as well."
"A comparison of the heavy-element abundance distribution in [AG Draconis] with theoretical nucleosynthesis calculations shows that the s-process is defined by a relatively large neutron exposure (τ=1.3 mb$^{-1}$), while an analysis of the rubidium abundance suggests that the s-process occurred at a neutron density of about 2 [x] 10$^8$ cm$^{-3}$."
The "K giant in AG Dra [has a] T$_{eff}$ ~ 4100 - 4400 K. ... [With a best fit to spectroscopic data of T$_{eff}$ = 4300 K.]"

## Yellow degenerates

EG 5 is a yellow degenerate. EG 5 is another designation for Van Maanen's star.
Van Maanen's star (van Maanen 2) is a white dwarf that is the third closest to the Sun, after Sirius B and Procyon B, in that order, and the closest known solitary white dwarf.
The optical negative at right was taken earlier than the current coordinates for Van Maanen's star, which are at the center of the negative.
Van Maanen's star has a radius of 9,000 ± 1,400 km. It's effective surface temperature is 6,220 ± 240 K.
Degenerate stars are white dwarfs of spectral luminosity class VII.
Some yellow degenerate stars are of white dwarf spectral type DC (which show no detectable lines) mostly below T$_{eff}$ < 10,000 K.
At left is an Hertzsprung-Russell diagram which shows that luminosity class VII has color class G stars within.
At lower right is a close to true color visual image of GJ 3223, a yellow degenerate white dwarf. It is similar to other luminosity class VII yellow degenerates LHS 3369 and LHS 3399. Each is color class G, often written "g" when referring to white dwarfs.

## Yellow subdwarfs

Yellow subdwarfs are in luminosity class VI. "[Y]ellow high-velocity subdwarfs are easily confused with white dwarfs in a proper-motion selection."
HD 64090 is a color class G0 subdwarf.

## Yellow main sequence stars

The closest G2V yellow main sequence star is the Sun described above.

### 70 Ophiuchi A

Temperature = 5,300 K.
70 Ophiuchi is a variable star with a magnitude range for the two stars combined of 4.00 to 4.03. The type of variability is uncertain and it is not clear which of the two components causes the variations. It has been suspected of being either a BY Draconis variable or an RS Canum Venaticorum variable, and a period of 1.92396 days has been measured.
The primary star is a yellow-orange main sequence dwarf of spectral type K0, while the secondary is an orange dwarf of spectral type K4. The two stars orbit each other at an average distance of 23.2 AUs. But since the orbit is highly elliptical (at e=0.499), the separation between the two varies from 11.4 to 34.8 AUs, with one orbit taking 83.38 years to complete.

### 107 Piscium

Temperature = 5242 ± 3.2.
107 Piscium (abbreviated 107 Psc) is a K-type main sequence star in the constellation of Pisces, about 24.4 light years away from the Earth.
The star is somewhat older than the Sun—approximately 6 billion years old.
It has 83% of the mass and 80% of the radius of the Sun, but shines with only 46% of the Sun's luminosity.
The effective temperature of the star is 5,242 K. It is rotating slowly with a period of 35.0 days. The abundance of elements other than hydrogen and helium—the star's metallicity—is slightly lower than that of the Sun.

### HD 86226

At right is a visual image in close to true color of the main sequence single star HD 86226. It has a parallax of 22.20 mas, but is not an X-ray source. A substellar companion HD 86226b has been detected.

## Yellow subgiants

A subgiant star is slightly brighter than a normal main-sequence (dwarf) star of the same spectral class, but not as bright as true giant stars. Although certain subgiants appear to be simply unusually bright metal-rich hydrogen-fusing stars (in the same way subdwarfs are unusually dim metal-poor hydrogen-fusing stars), they are generally believed to be stars that are ceasing or have already ceased fusing hydrogen in their cores.
Many subgiants are rich in metals, and commonly host orbiting planets.
At right is a visual image in close to true color of V972 Scorpii, which is a variable star of the delta Sct type. It has spectral type G2IV and is a star in a cluster. The system includes components CCDM J16234-2622 A and CCDM J16234-2622 B. Component A is a dwarf star in a double star system with component B. Component A is apparently V972 Scuti.

## Yellow giants

Alpha Microscopii is a spectral type G7III yellow giant star in a double system.
This star has a visual companion, CCDM J20500-3347B, of apparent visual magnitude 10.0 approximately 20.4 arcseconds away at a position angle of 166°. It has no physical connection to the star described above.

## Barium stars

Barium stars are spectral class G to K giants, whose spectra indicate an overabundance of s-process elements by the presence of singly ionized barium, Ba II, at λ 455.4 nm. Barium stars also show enhanced spectral features of carbon, the bands of the molecules CH, CN and C$_2$.
Observational studies of their radial velocity suggested that all barium stars are binary stars Observations in the ultraviolet using [the] International Ultraviolet Explorer detected white dwarfs in some barium star systems.
Barium stars are believed to be the result of mass transfer in a binary star system. The mass transfer occurred when the presently-observed giant star was on the main sequence. Its companion, the donor star, was a carbon star on the asymptotic giant branch (AGB), and had produced carbon and s-process elements in its interior. These nuclear fusion products were mixed by convection to its surface. Some of that matter "polluted" the surface layers of the main sequence star as the donor star lost mass at the end of its AGB evolution, and it subsequently evolved to become a white dwarf. We are observing these systems an indeterminate amount of time after the mass transfer event, when the donor star has long been a white dwarf, and the "polluted" recipient star has evolved to become a red giant.
Barium stars exhibit carbon and s-process elements at their surfaces suggesting surface fusion possible during mass transfer or without it.
The mass transfer hypothesis predicts that there should be main sequence stars with barium star spectral peculiarities. At least one such star, HR 107, is known.
Prototypical barium stars include zeta Capricorni, HR 774, and HR 4474.

## Yellow supergiants

A yellow supergiant (YSG) is a supergiant star of spectral type F or G. These stars usually have masses between 15 and 20 solar masses. These stars, like any other supergiant, are older and swing between blue and red phases depending on the chemical elements they consume in their cores. Until now it had been thought that few supergiants spend a long time in the transitional yellow phase. These systems may be the progenitors of rare supernovae linked to yellow supergiants. Only a few such supernovae have been detected - most supergiants go supernova when at the blue (or hot) phase or red (or cool) phase.
Calculations of "the changes in the surface chemical composition of intermediate-mass stars in the first phase of convection dredge-up ... has been used to determine the changes in the surface chemical composition of stars with masses 2.5, 5, 10, 20 M$_ʘ$ due to nuclear reactions of the pp chains, the triple CNO cycle, and the NeNa and MgAl cycles."
For surface fusion or just above surface fusion a convection dredge-up may not be necessary.
"Boyarchuk and Lyubimkov [2] proposed that the excess sodium observed in yellow supergiants is synthesized in reactions of the NeNa cycle in the interior of stars on the main sequence (MS) and then is carried to the surface during the red-giant stage."

### DY Persei variable

DY Persei variables are a subclass of R Coronae Borealis variables. They are carbon-rich asymptotic giant branch stars that exhibit pulsational variability of AGB stars and irregular variability of RCB stars.
The star DY Persei is the prototype of this tiny class of variable stars.
DY Persei pulsates like a red variable, and fades from sight like an R Coronae Borealis variable.

### R Coronae Borealis variable

"[An] R Coronae Borealis variable (abbreviated RCB) is an eruptive variable star that varies in luminosity in two modes, one low amplitude pulsation (a few tenths of a magnitude), and one irregular unpredictably sudden fading by 1 to 9 magnitudes."
"The prototype star [is] R Coronae Borealis ... [O]nly about 100 RCB variables have been identified, making this class a very rare kind of star."
"The fading is caused by condensation of carbon to soot, making the star fade in visible light while measurements in infrared light exhibit no real luminosity decrease. R Coronae Borealis variables are typically supergiant stars in the spectral classes F and G (by convention called "yellow"), with typical C$_2$ and CN molecular bands, characteristic of yellow supergiants. RCB star atmospheres do however lack hydrogen by an abundance of 1 part per 1,000 down to 1 part per 1,000,000 relative to helium and other chemical elements, while the universal abundance of hydrogen is about 3 to 1 relative to helium."
"There is a considerable variation in spectrum between various RCB specimens. Most of the stars with known spectrum are either F to G class ("yellow") supergiants, or a comparatively cooler C-R type carbon star supergiant. Three of the stars are however of the "blue" B type, for example VZ Sagittarii, and one is a "red" giant star, V482 Cygni of type M5III. Four stars are unusually and inexplicably poor in iron absorption lines in the spectrum. The constant features are prominent Carbon lines, strong atmospheric Hydrogen deficiencies, and obviously the intermittent fadings."

## Yellow hypergiants

"ρ Cas, HR 8752 and IRC+10420, three well-studied yellow hypergiants, are situated at or close to the red border of the [yellow evolutionary] void."
"Generally speaking, a yellow hypergiant is a massive star with an extended atmosphere, which can be classified as spectral class from late A to K, with a mass of as much as 20-50 solar masses. Yellow hypergiants, such as Rho Cassiopeiae in the constellation Cassiopeia, have been observed to experience periodic eruptions, resulting in periodic or continuous dimming of the star, respectively. Yellow hypergiants appear to be extremely rare in the universe. Due to their extremely rapid rate of consumption of nuclear fuel, yellow hypergiants generally only remain on the main sequence for a few million years before destroying themselves in a massive supernova or hypernova. Yellow hypergiants are post-red supergiants, rapidly evolving toward the blue supergiant phase."
"According to the current physical models of stars, a yellow hypergiant should possess a convective core surrounded by a radiative zone, as opposed to a sun-sized star, which consists of a radiative core surrounded by a convective zone (Seeds, 2005). Due to the extremely high pressures which exist at the core of a yellow hypergiant, portions of the core or perhaps the entire core may be composed of degenerate matter."
These stars have "powerful magnetic fields".

## Yellow evolutionary void

"G is host to the "Yellow Evolutionary Void". Supergiant stars often swing between O or B (blue) and K or M (red). While they do this, they do not stay for long in the G classification as this is an extremely unstable place for a supergiant to be."
"[T]he yellow evolutionary void ... is an area in the Hertzspung-Russell diagram where atmospheres of blueward evolving super- and hypergiants are moderately unstable ... For [such stars] (in hydrostatic equilibrium)
* a negative density gradient occurs,
* the sum of all accelerations, including wind, turbulence and pulsations, is zero or negative,
* the sonic point of the stellar wind is reached in or below photospheric levels, and
* Γ$_1$ ≤ 4/3 indicating some level of dynamic instability in part of the atmosphere."

## Yellow galaxies

The image at right "shows several blue, loop-shaped objects that are multiple images of the same galaxy, duplicated by the gravitational lens effect of the cluster of yellow galaxies near the middle of the photograph. The lens is produced by the cluster's gravitational field that bends light to magnify and distort the image of a more distant object."