Celestia Forums

Hi folks:

As an Astronomy teacher, I'm embarrassed to admit that I've been having a bit of trouble understanding/clarifying the nomenclature for spectral classes. I have been aware of the original "OBAFGKM" classes for years, and have always understood they represent the colors of stars based on the temperature at which there surfaces glow, with M being only red-hot (3000 - 4000 K and O being blue-hot (30000 K).

Then, I began encountering more and more classes. First class "N", then class "L" (representing cooler and cooler stars, dwarfs and brown dwarfs). Then, more classes appeared, based on what a star is made of. Class "C" for example, is used by Celestia to apparently represent not temperature, but stars rich in "carbon".

If I am to teach these properly, could someone please help me understand the answers to four questions:

1. Although it is not clear to me, can I presume that a "C" star is the same thing as an "N" or "L" star? That is, does a class "C" star glow only dully red and have a spectral color class of N or L?

2. Is class "L the lowest color class, based on temperature or is there a dark brown glowing star/"brown" dwarf?

3. Does Celestia display any "L" class stars? If so, what would one of them be named?

4. According to references I have consulted, neutron stars and pulsars have surface temperatures of several million degrees K. At that temperature, the star is so hot it is far beyond blue-hot. It is ultraviolet or X-ray hot. If true, then what visual color is a young neutron star? Does it glow deep violet or is it invisible visually? If so, then does that mean you could look at a young neutron star close up and see .... nothing??? If true, then why don't we have a spectral class for violet-colored stars or hotter ... something above the "O" class? Obviously, these stars all glow hotter than blue?

Your wise and sage counsel will all be very welcomed.

Thanks in advance.

Frank

fsgregs wrote:If I am to teach these properly, could someone please help me understand the answers to four questions:

1. Although it is not clear to me, can I presume that a "C" star is the same thing as an "N" or "L" star? That is, does a class "C" star glow only dully red and have a spectral color class of N or L?

No... The full sequence goes WOBAFGKMRNSLT.

W is a Wolf-Rayet Star, much hotter than O, usually a very high mass star with a solar wind so powerful that it's literally blowing itself away.

OBAFGKM you know...

R,N, and S are all types of evolved red giant/supergiant, IIRC. I think they might have all been combined into the C "Carbon Star" category - I know they have odd metals in their spectral lines though.

L and T are types of Brown Dwarf. L is warmer (generally 1000ish K to about 2500K?) and T are colder (below 1000 K). T dwarfs usually have methane clouds in their atmospheres, I think L often have silicate clouds.

2. Is class "L the lowest color class, based on temperature or is there a dark brown glowing star/"brown" dwarf?

See above - T is the 'coldest'.

3. Does Celestia display any "L" class stars? If so, what would one of them be named?

I dunno, I'll leave this for others to answer. I think Gliese 229B might be L?

4. According to references I have consulted, neutron stars and pulsars have surface temperatures of several million degrees K. At that temperature, the star is so hot it is far beyond blue-hot. It is ultraviolet or X-ray hot. If true, then what visual color is a young neutron star? Does it glow deep violet or is it invisible visually? If so, then does that mean you could look at a young neutron star close up and see .... nothing??? If true, then why don't we have a spectral class for violet-colored stars or hotter ... something above the "O" class? Obviously, these stars all glow hotter than blue?

I'd guess that they'd be intense blue/violet/white to look at - there'd still probably be a lot of emission in their blackbody spectrum that is in the lower wavelengths. I'm not sure on that though.

Evil Dr Ganymede wrote:No... The full sequence goes WOBAFGKMRNSLT.

W is a Wolf-Rayet Star, much hotter than O, usually a very high mass star with a solar wind so powerful that it's literally blowing itself away.

OBAFGKM you know...

W is the hottest type, but they are stars that have lost their outer atmosphere. Type O3 stars are the hottest (and most massive) main sequence stars known.

Evil Dr Ganymede wrote:R,N, and S are all types of evolved red giant/supergiant, IIRC. I think they might have all been combined into the C "Carbon Star" category - I know they have odd metals in their spectral lines though.

R and N have been combined into class C, S is different type of carbon stars. C and S types are parallel with the spectral types G, K, and M. In normal stars, oxygen is more abundant than carbon, but C stars have more carbon. Type S stars have equal amount of both.

There are also few known carbon dwarf stars.

Evil Dr Ganymede wrote:L and T are types of Brown Dwarf. L is warmer (generally 1000ish K to about 2500K?) and T are colder (below 1000 K). T dwarfs usually have methane clouds in their atmospheres, I think L often have silicate clouds.

L dwarfs are lithium dwarfs, T dwarfs are methane dwarfs. In normal stars, lithium is quickly consumed by nuclear reactions, and methane can exist only in relatively low temperatures.

Some of this may be a bit technical, but the links below should give you a bit more info on what's going on at the M/L/T end of the spectrum:

http://www-int.stsci.edu/~inr/ldwarf1.html
http://www.iap.fr/SitesHeberges/com45ua ... node3.html

Remember, these are spectral types - the classification is not just down to surface temperature, it's down to what elements are in their spectral absorption/emission lines. This seems to get a lot more important at this end of the HR-diagram. L Dwarfs don't just contain lithium - they can contain a wide range of other elements.

Question: for one of my Systems I have a Planet that is roughly nine Jupiter masses. Would this be conceder the lower limit of the class T Brown dwarf? I have heard that the distinction between Brown dwarf and a Gas Super giant is anywhere between 8-14 J Masses.

OK. If I get this correctly, Type L stars are small dwarf stars that have either not enough mass to begin fusing, or just barely enough to do so. As such, they would be hotter brown dwarfs, but still dull red in color.

Type T stars are cooler than that, and are small brown dwarfs that clearly have insufficient mass to even come close to being a normal star. They are very dull red, almost brown.

In neither case would a type L or T spectral class contain any stars that have already undergone a full life cycle and are on the way out (e.g. - cooling white dwarfs or neutron stars). That is because the rate of cooling of these stars is way too long for any of them to have reached a stage of cooling to a dull red color. All the white dwarfs and neutron stars ever born are still too hot to be a Type L or T in color.

Is that all correct?


As to my last question of the original post about stars at the other end of the spectrum, I still don't have a clear answer. The universe should be filled with VERY hot white dwarfs, neutron stars and pulsars that are over 1 million degrees K at their surfaces. If they are that hot, their color should be either deep violet or even invisible. Since the universe has potentially trillions of them, why isn't there a spectral class above "O", which ends at a surface temperature of only 30,000 degrees K? (given that class "W" is not supposed to represent colors)

Frank

The lower limit for a brown dwarf (ie something that started fusing Deuterium at one point but isn't necessarily anymore) is about 13 Jupiter masses.

L dwarfs are the more massive/younger brown dwarfs. They're massive enough to have started fusing deuterium in their cores (about 35-70 Jupiter masses), and may still be doing it - hence, they're probably glowing red. As they get older (after about 1 billion years after formation, tops), the core fusion will stop and they'll just cool off, becoming T Dwarfs.

The problem is that you can have T dwarfs that are at the low end of the mass spectrum - they just weren't massive enough to start any fusion in the first place, but still put out much more energy than Jupiter would through continuing gravitational contraction.

T Dwarfs would actually be more like a magenta colour. Have a look at this page:
http://spider.ipac.caltech.edu/staff/da ... rison.html

L and T stars are not old stars that have already been through their full life cycle.

Evil Dr Ganymede wrote:The lower limit for a brown dwarf (ie something that started fusing Deuterium at one point but isn't necessarily anymore) is about 13 Jupiter masses.

L dwarfs are the more massive/younger brown dwarfs. They're massive enough to have started fusing deuterium in their cores (about 35-70 Jupiter masses), and may still be doing it - hence, they're probably glowing red. As they get older (after about 1 billion years after formation, tops), the core fusion will stop and they'll just cool off, becoming T Dwarfs.

To make things more complicated, young brown dwarfs can have M-type spectrum. And massive planets can have a L-type spectrum, when they are very young. (See 2M1207)

Evil Dr Ganymede wrote:L and T stars are not old stars that have already been through their full life cycle.

Indeed, as brown dwarfs age, they just get cooler and cooler and move out from the T type. No cooler than T-type brown dwarfs have yet been discovered.

Red dwarfs shine for hundreds of billions to trillions of years, so no dead red dwarfs exist yet.

Frank,

Only Celestia 1.4.0 has L and T type stars in the base distribuion. The only type L star is Alula Australis Bb, a member of the Alula Australis quadruple system.

There are several type T stars:
Eps Ind Ba
Eps Ind Bb
2MASS 0415-0935
Luyten 668-21 B
33 Lib D
2MASS 0937+2931

The first two are neat--two brown dwarfs orbiting each other, and the pair in orbit around a main sequence star.

--Chris

The Alula Australis (main star HD 98231) is beautiful. But how do I select its barycenter ?

The name "Alula Australis" without a suffix refers to the barycenter itself, not to any particular member.

If you look in the file Celestia/data/nearstars.stc you'll find it listed with several synonyms:
Barycenter "Alula Australis:XI UMa:53 UMa:Gliese 423"

You can use Celestia name selection feature to select it:

type [return] (the carriage-return or Enter key)
type 53 UMa (or any other unique synonym)
type [return]

its name will appear in the upper left corner of Celestia's window along with the description
Star system barycenter

I'm guessing that neutron stars can even appear green because of their compositions. But I'm probably wrong.

Not sure what this has to do with this thread from 2005, but no, stars can't be green, or purple for that matter. Here's a page about it.

Cant we just ban Joey P for necromancing?

Gurren Lagann, I hope you are joking.

Forum Guidelines wrote:| Among other things, this means that you should not write on this Forum
| about the personal failings of people who post to the Forum.
| The Celestia Web Forum is for discussing Celestia.
| The Forum is not for arguing with or about people
| who do not meet with your approval.

Joey P. did use a sockpuppet once so this may apply:

Forum Guidelines wrote:No sockpuppets allowed, anyone suspected to use one or be one will be banned permanently after 1 warning.

But that was over a year ago and the rule is from many years ago when these people were disrupting the forum. I think we can just forgive Joey P. and move on.

I'm not joking, SevenSpheres. Some forums (the KSP forums for example) dont allow necromancing (aka revival of really old threads), so i assumed that this forum doesnt allow necromancing as well.

Why necromancing?

Gurren Lagann wrote:I'm not joking, SevenSpheres. Some forums (the KSP forums for example) dont allow necromancing (aka revival of really old threads), so i assumed that this forum doesnt allow necromancing as well.

Well, this forum doesn't ban people for necroposting. I agree Joey shouldn't do it, but I was trying to be helpful and answer his question. You replied "Can't we ban him?" which was completely unnecessary.

Gurren Lagann wrote:Some forums (the KSP forums for example) dont allow necromancing (aka revival of really old threads), so i assumed that this forum doesnt allow necromancing as well.

Indeed, it's common practise, but personally I dont find it reasonable. Topics that shoudnt be cotinued, should be explicitely closed.

Oh, now I know. Got it.