42

Models for the future behaviour of the Sun do vary, mainly as a result of uncertainty of mass loss during the red giant (H shell burning) and asymptotic red giant (H+He shell burning) phases. A highly cited paper by Schroeder & Smith 2008 claims that the Sun will reach its maximum size of about $256 R_{\odot}$ (1.18 au) at the very tip of the red giant ...


17

Arcturus is a RGB star, probably fairly similar how the sun will look when it becomes a red giant. Arcturus is slightly more massive than the sun ($m_{\rm Arc}=1.08 m_{\odot}$), but the main difference is the lower metallicity of $[Fe/H]\approx-0.5$. This low metallicity reduces the opacity in the stellar radiative zone (which fills a significant portion of ...


17

Hydrogen bombs aren't like little stars. The process of fusion in stars is slow, releasing very little energy per cubic metre. As a result of this, and their large scale, stars are close to thermodynamic equilibrium throughout most of their interior and have a stable photosphere that can be characterised by a spectral type. Further, stars contain a ...


11

It's all to do with the relationships between mass, spectral-type and luminosity and the initial mass function of stars. I think your explanation of points 1 and 2 are completely correct. O and B stars are rarely born and short-lived; so even though they have enormous luminosities relatively few make it into a list of stars ordered by apparent brightness. A ...


8

There isn't a one-to-one relationship between spectral type and absolute magnitude. Instead, there is a mean relationship with a fair bit of scatter around it. The reason is that the luminosity of a star of a given effective temperature depends on its composition/metallicity and how far along in its main sequence lifetime it is. Basically, late B-type main ...


8

Your question may ulitmately be about the physiology of the eye, which is off-topic here. The spectrum of the Sun seen low on the horizon is quite different to the spectrum of an M-type red dwarf. The reason that a red dwarf is red, is not just that it is cool, but that there are great chunks of the spectrum that are absorbed by molecules in the photosphere ...


7

This is a well-studied problem. The effect of rotation on the structure of a low-ish mass star (like the Sun) is summarised by Eggenberger (2013). Such stars are never observed to rotate so fast that the rotation plays any significant role in their hydrostatic equilibrium, however rotation does play a role by causing additional mixing in the star. This is ...


6

Yes, you can see one tonight. Arctaurus is a red giant star with a mass of about 1.1 times the solar mass, so rather similar to the sun. It currently has a spectral type of K0 III. It is ascending the red giant branch, so it's luminosity and spectrum are not stable in the longer term. The sun will pass through this phase, and following a hydrogen flash ...


6

This is what Wikipedia says about it: When the MKK classification scheme was first described in 1943, the only subtypes of class O used were O5 to O9.5. The MKK scheme was extended to O9.7 in 1971 and O4 in 1978, and new classification schemes that add types O2, O3, and O3.5 have subsequently been introduced. It references the paper A New Spectral ...


6

A star with magnitude 0 would be 85 times brighter than the sun (since Magnitude=-2.5 log(Luminiosity)) Referring to the H-R diagram on Wikipedia shows that there is quite a range of spectral types possible with this luminosity: from B type main sequence stars, and A type sub-giants, such as 4 Sco There are also G and K and M type Giant stars with this ...


4

It isn't. You've just got dodgy table from wikipedia. A more modern (and well-used) version is here. It lists G1V 1.07 5880 G2V 1.02 5770 G3V 1.00 5720 This is an average relationship. The closest and most consistent relationship will be between spectral type and effective temperature and indeed the Sun is normally attributed a spectral class of G2V and $...


4

The following table gives the mass, radius, temperature, and luminosity of an average star of several selected spectral types: $$\begin{array} {d|c|c|} \text{Spectral Type} & \text{Mass} (\odot) & \text{Radius} (\odot) & \text{Temperature (K)} & \text{Luminosity} {(\odot)} \\ \hline \text{M8V} & \text{0.082} & \text{0.111} & \...


3

If present, an a (or b or ab) do not refer to spectral peculiarities but are part of the luminosity class definition explained further up on the page. Occasionally, letters a and b are applied to luminosity classes other than supergiants; for example, a giant star slightly less luminous than typical may be given a luminosity class of IIIb, while a ...


3

The spectral classes (O, B, A, F, G, K, M) and their 10 subtypes (0 to 9) were initially meant only as differentiators of spectral type. Annie Jump Cannon was the creator of this system. Through her work for/with Edward Pickering, she ended up classifying nearly a third of a million stars over a few decades. She (and many others) did not realize that this ...


3

There are no exact boundaries in temperature, luminosity, surface gravity etc. for spectral classes because the classification system works in a different way - it is fundamentally an empirical system, with classification based only on the appearance of features in the spectra. The Yerkes or Morgan-Keenan (MK) system is based only on a set of standard stars ...


3

I'd say there are fewer M stars on list because they are least bright. they may outnumber other star types but only nearest ones would make list of brightest stars. Type O stars are rare hence the small number. This info is available from a Wikipedia page on 'stellar classifications' and refer mostly to the mail sequence stars on the Hertzsprung–Russell ...


3

The spectrum of a star is almost certainly a unique fingerprint. Even though stars are born in clusters, formed from the reasonably chemically homogeneous environment of a giant molecular cloud, there are likely to be small differences in their local environment. Furthermore their formation environments, and the stars themselves at later times, can be ...


2

A very low mass M-dwarf (what you have there is something like an M5-M6 dwarf) will remain a highly magnetically active star for several billion years. As such, the spectrum of light from such an object has a much higher proportion, by several orders of magnitude, of UV and X-ray emission than the Sun. Therefore, if your planet is close enough to its star ...


2

Even the optical spectrum alone has lots of goodies. In addition to chemical abundance, size, and other outer properties from emission and absorption lines you can get the rotational rate from a narrow line's doppler profile, and very careful spectroscopy information on vibrational modes of the star through spectral asteroseismology. Also most stars are ...


2

Both $B-V$ and $U-B$ are a reflection of how hot the surface of a star is. To first order (inhomogeneous surface temperatures do exist) this has a single value for a given stellar photosphere and so there ought to be a direct relationship between $B-V$ and $U-B$. Here it is, shown below as a graph appropriate for main sequence stars with a composition ...


2

TiO is used as a sunscreen, it has high absorption and emittance rates and a high radiation potential. SiO2 is used for fiber optic cables and probably deflects and transmits photons a lot more, and it is very chemically inert, that's why the desert is made of quartz, it's all that is left after the rest has been weathered, it's orbitals are very stable and ...


2

Stars born together in clusters have more-or-less the same age. As a rule of thumb, any spread in age, measured in millions of years, is smaller than the extent of the cluster in parsecs. For most stellar clusters, smaller than a few pc, the only chance of measuring age differences occurs in the first 10 million years of life. There is no evidence for age ...


1

The main effect would be the radiation environment. A planet in the habitable zone of an M-dwarf would likely be subject to far more ultra-violet and X-ray observation for longer than a planet orbiting a G-dwarf of similar overall age. The reason for this lies in the physics of stellar dynamos that power the magnetism of cool stars. Fast rotating cool stars ...


1

The absolute magnitude quantifies the luminosity of an object at a standard distance of $10\ \mathrm{pc}$ from Earth. For example, in the case you mentioned, Vega becomes dimmer than at its actual distance (about $7\ \mathrm{pc}$). To answer your question, I don't think there is an actual star with exactly 0 absolute magnitude. If there is then, following ...


1

I don't believe that O0 is a real classification(see this chart), but if it were following the temperature steps it would probably be around 80,000-90,000 degrees Kelvin. The hottest star we know of is WR 102, which is 210,000 degrees Kelvin, and that is much, much hotter than my predicted O0 temperature. So the short answer is yes, O0 temperature stars ...


1

Yes, the spectral type changes with age. The spectral type is a function of temperature, gravity and chemical composition at the photosphere. All of these can change during a star's life. A star spends most of its life on the main sequence and changes in temperature and gravity are relatively slow. But thereafter there are comparatively rapid changes. For ...


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