Spectral Classes

[TimberWolf]

Hello,

I've been a lifelong astronomy enthusiast and I confused over some matter of spectral classes. There are the main sequence stars, such as O, B, A, F, G, K and M. But, some stars are giants within classes that are normally not giants. Am I seeing this wrong, or is there something that I'm missing?

Cordially,

TimberWolf

[Brady Yoon]

Hello,

I've been a lifelong astronomy enthusiast and I confused over some matter of spectral classes. There are the main sequence stars, such as O, B, A, F, G, K and M. But, some stars are giants within classes that are normally not giants. Am I seeing this wrong, or is there something that I'm missing?

Cordially,

TimberWolf

Hey! Welcome to the board.

To answer your question, I'll provide an example. Red stars. There are brilliant red giants and dim red dwarfs, but not much in between. This is because red giants are not on the main sequence. They are not steadily fusing hydrogen into helium. Nuclear fusion powers these red giants, but the effects are different. This is why luminosity classes are used. Stars of similar colors or temperatures (they are the same thing), may not be at the same evolutionary stage, so they may have different luminosities. If you have any more questions, ask away.

[TimberWolf]

Hello,

Interesting. Sorry, I should have phrased the question better. I already know about the difference between main sequence and non-main sequence stars. For example: Antares is a red supergiant fusing helium instead of hydrogen, whereas Barnard's Star is a red dwarf still fusing hydrogen.

So, would Antares then still be classed as a M-class, even though it's definitely not main sequence?

Cordially,

TimberWolf

[Crimson]

Yes, Antares is spectral type M.

The spectral type--O, B, A, F, G, K, M, and now L and T--refers to the color or temperature of the star.

The luminosity class--I, II, III, IV, V, VI--refers to the luminosity:

I = supergiant
II = bright giant
III = giant
IV = subgiant
V = main-sequence star (sometimes called dwarf)
VI = subdwarf

Antares is M1 Ib, meaning it's a red (M-type) supergiant. (The b after the I means it's a faint supergiant--faint compared to a Ia supergiant, that is.) The Sun is G2 V, meaning it's a yellow (G-type) main-sequence star.

[TimberWolf]

Hello,

Cool. Non-main sequence stars still belong within the letter classes. But what is L and T?

Cordially,

TimberWolf

[PhantomWolf]

Not an answer to your questions, just a friendly welcome to the Wolf Pack from a fellow Lupine. Watch out for that WolfKC though, he's a real loon. (in a good way of course)

[TriangleMan]

But what is L and T?

L & T are stars further along the sequence and are cooler stars than class M. I think Class T stars have a temperature of just a few thousand degrees so they are very faint.

[Crimson]

Yes, L and T stars are even cooler than M stars. They are probably all brown dwarfs--stars born with so little mass (under 0.08 Suns) that they never sustain hydrogen-1 fusion, the reaction that powers all main-sequence stars.

[Ut]

Not an answer to your questions, just a friendly welcome to the Wolf Pack from a fellow Lupine. Watch out for that WolfKC though, he's a real loon. (in a good way of course)

Oh dear, they're forming a stable. Soon, there'll be dozens of them wandering across the grassy plains and shady woods of the board.

[ToSeek]

Not an answer to your questions, just a friendly welcome to the Wolf Pack from a fellow Lupine. Watch out for that WolfKC though, he's a real loon. (in a good way of course)

Oh dear, they're forming a stable. Soon, there'll be dozens of them wandering across the grassy plains and shady woods of the board.

And hunting the halt and the lame. Better look healthy!

[Doodler]

So L and T stars are new classes for Brown Dwarf stars?

[sol_g2v]

The dividing line between brown dwarfs and true stars is around L4-L5.

[tjm220]

The dividing line between brown dwarfs and true stars is around L4-L5.

What does the number represent after the spectral class letter? What is the difference between L4 and L5?

[tjm220]

Never mind, found out about it. Methinks I should use that search feature before making posts like that one.

[TimberWolf]

Hello,

What was the keywords in that search? I'd like to know the answer to that one too.

Cordially,

TimberWolf

[Argos]

What about the Harvard Spectral sequence, with classes R, N and S? I´ve heard of classes DB, DA, DF, DG for white dwarfs, WR, WN, WC for Wolf-Rayet stars. Is there a standard classification?

[tjm220]

Hello,

What was the keywords in that search? I'd like to know the answer to that one too.

Cordially,

TimberWolf

I entered spectral class number as the parameters in the 'Search for Keywords' slot and rediscovered this thread.

[Kullat Nunu]

The dividing line between brown dwarfs and true stars is around L4-L5.

I don't think that the dividing line is that clear. Even some M9 stars are classified as brown dwarfs (for example LP 944-020). As the brown dwarfes get older, they cool down since they have no internal fusion. As they cool their spectral class must also change.

[Captain Z]

What about the Harvard Spectral sequence, with classes R, N and S?

Originally Class N stars were listed as stars of equal (and a bit lower) temperature to that of an M class star, but with strong Carbon lines. Class R were closer to that of G and K class stars, also with more carbon. Later on they decided to lump class R and N stars together into a common group, now known as the C class star. Both C and S class stars are in the general range of M and lower K class stars (between 2000°-4000°K)

Basic Temprature breakdown
W class - 50000°K or higher (ex: Suhail al Muhlif)
O class - 30000°-50000°K (ex: Naos)
B class - 11000°-25000°K (ex: Rigel)
A class - 7500°-11000°K (ex: Sirius)
F class - 5900°-7500°K (ex: Procyon)
G class - 5200°-5900°K (ex: Sol)
K class - 3900°-5200°K (ex: Aldebaran)
M class - 2500°-3900°K (ex: Betelgeuse)
L class - 1300°-2500°K (ex: Gilese 229B)
T class - below 1300°K

Also to add to the list of luminocity classes, there is also a class 0 for hypergiants now. Best example I can think of is Rho Cassiopeia (G2Ia-0), which has a diameter of 4.3AU, roughly 450x the diameter of our Sun.

[The Bad Astronomer]

R, N, and S are not used much anymore. They are subclasses of other stars.

L is just an extension of the Main Sequence to lower temperatures. Before their inclusion, M just dwindled off into nothing. Now there are definitions for the separation (basically, an L dwarf has spectral features showing carbon in the form of CO).

T dwarfs are also called methane dwarfs. They are so cool (roughly 1500 Kelvins and lower) that methane forms in their atmosphere, and that's where all the carbon is found in their spectrum (separating them from the L dwarfs).

L and T were added to the sequence in 1998. Astronomers voted on the names at a meeting at La Palma in the Canary Islands; here is alist of astronomers who attended. Perusing that list might be amusing. 8)

[TimberWolf]

Hello,

At the risk of sounding inane, I was wondering about spectral class numbers. Roman numerals denote luminosity. But what does the numeral immediately after the letter class of the star denote? Example: Antares is a M1 Ib class red giant.

If I'm not mistaken, the numeral after the M denotes the size of the star within the class and the Roman numeral denotes it's luminosity. From what I gather, many stars in the observable Universe are not main sequence stars.

Cordially,

TimberWolf

[Brady Yoon]

Hello,

At the risk of sounding inane, I was wondering about spectral class numbers. Roman numerals denote luminosity. But what does the numeral immediately after the letter class of the star denote? Example: Antares is a M1 Ib class red giant.

If I'm not mistaken, the numeral after the M denotes the size of the star within the class and the Roman numeral denotes it's luminosity. From what I gather, many stars in the observable Universe are not main sequence stars.

Cordially,

TimberWolf

Close, but no cigar. The numeral after the letter is used to show the spectral class/temperature in more detail. A smaller number means a hotter star in the same letter designation. For example, a G0 star is hotter than a G2 star (Sol). The Roman Numeral does denote the lumiosity as you said. Actually, 90% of stars are main sequence stars, which are mostly red dwarfs.

[sol_g2v]

The dividing line between brown dwarfs and true stars is around L4-L5.

I don't think that the dividing line is that clear. Even some M9 stars are classified as brown dwarfs (for example LP 944-020). As the brown dwarfes get older, they cool down since they have no internal fusion. As they cool their spectral class must also change.

Right, well this is true in the sense that spectral class is a reflection of what an object looks like, not what it actually is. For true stars the spectral class will be pretty much the same throughout the main-sequence lifetime, and there is a direct relationship between mass and temperature. Since the hydrogen-burning limit is around 1800 K surface temp., that corresponds to around the middle of the L class. A brown dwarf of type M9 is a very young object that has the same spectrum as an M9 red dwarf star but less mass.

[tjm220]

...L and T were added to the sequence in 1998. Astronomers voted on the names at a meeting at La Palma in the Canary Islands; here is alist of astronomers who attended. Perusing that list might be amusing. 8)

Did you enjoy the conference?

[Ricimer]

Timber:

Brady's got it. The # is merely a finer gradation of the spectral class sequence.

A star right on the spectral divide is a G0 (for example) and as it heads closer next one, it shifts down through G9. The G10, is actually a K0.

While it may happen, its rare to see the spectral class be specified any more specific than that, since it's a pretty fuzzy designation overall.