Stars/Os

O stars are the hottest stars.

The peaks of their Planckian spectra start at about 38,000 K and increase, and 79 nm and decrease in the ultraviolet.

Alnitak is the blue star in lower right corner of the image on the right. It is an O9.7Ib.

Luminosities
Stars including Os can be put into classes based on luminosity, size or apparent mass. These may be 0 - hypergiants, I - supergiants, II - bright giants, III - giants, IV - subgiants, V - main sequence, VI - dwarfs and VII - subdwarfs. But, these are seldom rigidly followed. For example, class Ia may be supergiants, while class Ib may be bright giants. Another example, class V may be main sequence dwarfs, class VI may be subdwarfs, while class VII may be white dwarfs.

For simplicity and emphasis on size and special characteristics, star classes may be by diameter, when known.

Temperatures
Each star class is subdivided into temperature ranges using numbers: hottest - 0 to coolest - 9.9. For example, O00 or O0 0 is an O hypergiant star having the hottest known photospheric temperature.

A "new subclass, O1, has been created".

"Clegg and Walsh (1989) determined Teff = 120 000 K for the central star NGC 7293 from the nebular lines. This is in reasonable agreement with the temperature determined from Hδ (Teff = 110 000 K)."

An O2 VIII [BD+28 4211, sdO2VIII:He5, on the right] can have an effective temperature of 82,000 K and an O9 V can have an effective temperature of 38,000 K.

"For BD+28° 4211 [...] the He I 5876 Å line in a high-resolution spectrum taken at the McDonald Observatory [has a] best fit [of] Teff = 82000 K in good agreement with Hε [on the right at the bottom]."

The "effective temperatures [are] derived from [spectral] lines".

If "the [helium] lines are significantly wider than the average [class VII], we use luminosity class “VIII”."

Subdwarf O stars
US 708 is a high-velocity star of spectral type sdOHe, where sd stands for subdwarf, O stands for an O-type star, and He stands for helium star.

"Hypervelocity stars (HVSs) travel with velocities so high that they exceed the escape velocity of the Galaxy."

"A multinational team of astronomers led by Dr Stephan Geier from the European Southern Observatory in Garching, Germany, has determined that a hypervelocity star known as US 708 is traveling at about 1,200 km per second."

"By measuring the velocity, trajectory and rotation of the star, known as US 708, researchers at the European Southern Observatory determined that it started life as one half of a close binary pair — two stars that closely orbited one other."

"Scientists using the W. M. Keck Observatory and Pan-STARRS1 telescopes on Hawaii have discovered a star that breaks the galactic speed record, traveling with a velocity of about 2.7 million mph (1,200 km/s). This velocity is so high, the star will escape the gravity of our galaxy. In contrast to the other known unbound stars, the team showed that this compact star was ejected from an extremely tight binary by a thermonuclear supernova explosion."

"A helium star is an O star or B star in which the absorption lines of helium are abnormally strong and those of hydrogen are absent or weak. Extreme helium stars (also called hydrogen-deficient stars) show no trace of hydrogen, while intermediate helium-rich stars have hydrogen lines that are visible but weaker than in normal stars. The loss or depletion of the star's hydrogen envelope, leaving essentially an exposed helium core, may have happened because of a powerful stellar wind, as in Wolf-Rayet stars, or because of mass transfer to a close binary companion."

Dwarf O stars
Ton S 61 (2MASS J22361663-3142130, PHL 334) is a spectral type OVI.

Main sequence O stars
Spectral-class "or temperature classification criteria in [the O3-B0 main sequence] are He II λ4541/He I λ4471 and He II λ4200/He I(+II) λ4026; at the later types He II λ4541/He I λ4387 and He II λ4200/He I λ4144 are useful checks, since the former ratios become very small, but with care since the latter ratios are also sensitive to luminosity."

"The definition of type 03 in the 60 Å mm-1 photographic classification was that He I is not seen, but λ4471 appears to have been detected in the present observation of HDE 303308, so that the distinction between the 03 V and 04 V spectra is not clear in [the] digital data."

"Type 07 is defined by He II λ4541 = He I λ4471."

By "definition, the O-type luminosity class V spectra all have strong He II λ4686 absorption; the notation ((f)) signifies that in addition weak Ν III λλ4634-4640-4642 emission is present."

Subgiant O stars
HD 93250 is a class O4 IV(fc).

Giant O stars
The "distinction between giant and supergiant spectra at class O3 may not correspond to a significant physical luminosity difference."

Supergiant O stars
For 03-08f supergiant spectra the "notation f* signifies Ν IV λ4058 emission stronger than Ν III λ4640, which is a characteristic of 03 spectra, while f+ denotes Si IV λλ4089,4116 in emission as well as the Of features."

Class O2
According to SIMBAD, HD 93129 in the image on the right is a double or multiple star, where the two stars are of spectral types: O2If*+O3.5V.

Further, HD 93129A is spectral type O2If*, a spectroscopic binary, an X-ray source: CXOGNC J104357.47-593251.3 and 2E 1042.0-5917, and a Wolf-Rayet star MR 27.

Class O3
A "luminosity sequence at spectral class 03 [for] the prototype O3 supergiant (Walborn 1971&); [has] the strong, narrow Ν IV λ4058 emission feature and Ν V λλ4604,4620 absorption lines [as] the outstanding spectral characteristics of the type."

According to SIMBAD, HD 93129B is spectral type O3.5V((f)), a star in a cluster, and X-ray source CXOGNC J104357.65-593253.7.

Class O6
"On the main sequence [for spectral classes 06-06.5, 07, and 08] one finds strong He II λ4686 absorption often accompanied by weak Ν III λλ4634-4640-4642 emission, a combination denoted ((f)); in the intermediate luminosity classes the λ4686 absorption weakens and may become neutralized while the Ν III emission strength increases—the (f) category; and finally, the Of supergiants have both of these selective He II and Ν III features strongly in emission."

AB7 (a binary of WN4 + O6I(f)) in Tucana in the image on the right is the brightest white star at the centre of the cavity within the nebula, not the brighter reddish star. False colour image: red is HI; green is OIII; blue is HeIII

Class O9
09-09.7 supergiant spectra [include] "several of types ON and OC. The CNO anomalies are very well-defined [...], particularly by Ν III λ4097 and the ratio Ν III λ4640/C III λ4650."

"The primary defining criterion for the [...] interpolated type 09.7 is He II λ4541 ≈ Si III λ4552 (Walborn 1971c)."

There is "a correlative increase with luminosity in the strengths of the Si IV absorption lines flanking Ηδ, [...] the Si IV absorption strength is a primary luminosity criterion at types O9-B0".

Gamma-ray stars
Gamma-ray stars have surface temperatures starting at 300,000,000 K (300 MK) corresponding to a peak wavelength of 0.010 nm (10 pm) for the beginning of super soft gamma-ray sources.

X-ray stars
X-ray stars have surface temperatures starting at 300,000 K corresponding to a peak wavelength of 10 nm for the beginning of super soft X-ray sources.

Ultraviolet stars
Stellar class O stars have surface temperatures high enough that most of their luminescence is in the ultraviolet.