Becky Coretti

Stellar Spectrums and Classifications

Carbon Stars


You Know You’re a Deep Sky Observer When….

Stellar Spectrums and Classifications

The Spectral Sequence
Class Spectrum Color Temperature (K)
O Ionized and neutral helium; weakened hydrogen bluish 31,500 – 49,000
B Neutral helium, stronger hydrogen blue-white 10,000 – 31,500
A Strong hydrogen, ionized metals white 7,500 -10,000
F Weaker hydrogen, ionized metals yellowish-white 6,000 – 7,500
G Still weaker hydrogen, ionized and neutralized metals yellowish 5,300 – 6,000
K Weak hydrogen, neutral metals orange 3,800 – 5,300
M Little or no hydrogen, neutral metals, molecules reddish 2,100 – 3,800
L No hydrogen, metallic hybrids, alkalai metals red-infrared 1,200 – 2,100
T Methane bands infrared Under 1,200
Yerkes Luminosity Scheme
Class Description
Ia Most luminous supergiants
Ib Less luminous supergiants
II Luminous supergiants
III Normal giants
IV Subgiants
V Main sequence stars (dwarf)
Some Spectral Peculiarity
Lowercase letters are sometimes added to the end of a spectral type to indicate peculiarities
Codes Code Meaning
comp Composite spectrum; two spectral types are blended, indicating the star is an unresolved binary
e Emission lines are present (usually hydrogen)
m Abnormally strong metals (elements other than hydrogen and helium) for a star of a given spectral type, usually applies to “A” stars
n Broad (nebulous) absorption lines due to rapid rotation
nn Very broad nebulous lines due to very rapid rotation
neb Nebula’s spectrum is mixed with the star’s spectrum
p Unspecified peculiarity; except in type “A” where it indicates abnormally strong metal lines (related to Am stars)
s Very narrow (sharp) lines
sh Shell star ( “B” – “F” main sequence star with emission lines from a shell of gas)
var Varying spectral type
wl Weak line (suggesting and ancient “metal”-poor star)

Carbon Stars
(the “coolest” stars out there…)

My purpose in this page is to provide some insight into the stellar life cycle that results in a carbon star. I would like to provide a limited scientific understanding in order to fully appreciate the privilege we have in observing these objects - as well as to aid in searching out targets to observe.

A carbon star is a class of stars with a high carbon to hydrogen ratio, and a relatively low temperature (2000 – 3000k). They are a rare and beautiful class of red giant stars unusually rich in carbon. As a star ages, and it depletes its’ store of hydrogen, it can begin to burn helium into carbon and become a giant star. These carbon compounds absorb most of the stars’ blue light, thus appearing red.

All carbon stars are variable stars, with semi-regular or irregular periods. This is an indication of the stars’ internal instability.

The final evolutionary stage of most stars before they die is the Asymptotic Giant Branch. The star brightens and cools similar to a red giant, but with greater luminosity. These stars expel much of their carbon-rich envelopes as a wind. These envelopes will eventually mix with the surrounding environment; so that much of the carbon throughout the Milky Way comes from mass-losing carbon stars.

Carbon stars are currently known as “C” stars. They used to be split into two series: N (equal to “M” class) and R (equal to “K” class). “N” stars are older than “R” stars.

Each star classification has a number assigned to it, and it points toward the lower temperature. “0” is on the hotter end, “9” is at the cooler end. For example, a star with a rating of “G0” would be around 6000k while a “B9” would be around 10000k.

Carbon stars generally have two numbers assigned to them. The first number applies to temperature and the second number indicates the strength of its’ carbon bands on a scale of 1 - 5. The higher this number, the more reddish the appearance.

Most lists of carbon stars will classify a carbon star in the Morgan-Keenan C System, which lists BOTH temperature and carbon abundance. Some lists still utilize the older Harvard classification. Other lists will incorporate both the Harvard classification and the Morgan-Keenan C system.

Morgan-Keenan C System
MK Type Giant Teff
C0 G4-G6 4500
C1 G7-G8 4300
C2 G9-K0 4100
C3 K1-K2 3900
C4 K3-K4 3650
C5 K5-M0 3450
C6 M1-M2 --
C7 M3-M4 --
For example, T Lyrae is: C 6,5 (carbon star about 2780k, highest carbon band strength) or an “R6” (old classification). So, T Lyrae is a very red star.

Carbon stars, like all astronomical objects, are also classified by its’ magnitude. The lower the magnitude; the brighter the object (star in this case). Each increment in magnitude translates to 2.512 times brighter. For example, a 10th magnitude star is 2.512 times brighter than an 11th magnitude star. Or 5.024 times brighter than a 12th magnitude star. Our above example of T Lyrae has an mV (visual magnitude) of 8.5 (this is averaged). Its’ range is 7.5 – 9.3

Since carbon stars are variables, they have a magnitude range. This is reported as the “period”. During its’ “bright” stage the star is producing less carbon, thus more blue light passes through. This results in a brighter, “less red” star. Knowing where a carbon star is in its cycle can greatly aid in locating it.

Putting this all together as it would look on a list:
Star MV Spec Mag Range Period
T Lyrae 8.5 C 6,5 (R6) 7.5 – 9.3 Irreg

We can see that T Lyrae is a very bright (mV of 8.5) and deep red (5 on the carbon band scale). This would make an excellent target, and in fact, I have observed this star from my driveway in Ft Lauderdale. It is my favorite carbon star to date.

Lets' look at another example: R Leporis
Star MV Spec Mag Range Period
R Lep 7.7 C7, 4e*(N4e) 5.5 – 11.7 ~ 430 days
* “e” denotes that there are emission lines present; usually hydrogen

How R Leporis will look in the eyepiece of a 6" telescope. Roll over image to identify. Click to enlarge.

(R Leporis, shot January 2001 through 6” AP refractor)

Carbon Star List
Star Location Spec. Mag. Range Period Notes
T Lyr 18h 32m;
+36 59 56.0
C6,5 7.84 – 9.6 Irreg. Bright, orange
R Lep 04h 59m 36.30;
-14 48 23.0
C7,6e* 5.5 – 11.7 427 days Somewhat dim, deep red
UU Aur 06h 36m 32.80;
+38 26 44.0
C5,4 7.83 – 10.0 234 days Very bright orange
W Ori 05h 05m 23.70;
+01 10 39.0
C5.4 8.2 – 12.4 212 days Orange, almost naked-eye
AQ And 00h 27m 31.70;
+35 35 15.0
C5,4 9.9 – 11.8 346 days Dim yellow-orange
BL Ori 06h 25m 28.20;
+14 43 19.0
C6,3 7.9 – 9.7 Irreg. Bright yellow-orange
Y Tau 05h 45m 39.40;
+20 41 42.0
C6.5,4e* 6.5 – 9.2 241.5 days Bright orange
VY UMa 10h 45m 04.00;
+67 24 41.0
C6,3 5.87 – 7.0 Irreg. Dim orange, near maximum
Y CVn 12h 45m 07.80;
+45 26 25.0
C5,4 7.4 – 10.0 157 days Bright, yellow
W Cas 00h54m 53.80;
+58 33 49.0
C7, 1e* 7.8 – 12.5 406 days Near minimum, deep red-orange
UV Cam 04h 05m 53.80;
+61 47 40.0
C5, 3 7.5 – 8.1 294 days Near maximum, bright yellow
S Cam 05h 41m 02.50;
+68 47 55.0
C7, 3e* 7.7 – 11.6 327 days Near maximum, bright yellow-orange
SZ Lep 05h 35m 47.70;
-25 44 19.0
C7, 3 7.4 – 7.93 Unknown Bright yellow
NQ Gem 07h 31min 54.50;
+24 30 13.0
C6,2 7.4 – 7.99 70 days Near maximum, bright orange to yellow-orange
RT Ori 05h 33m 13.4;
+07 08 58
C6, 4 8.0 – 8.9 327 days Dim, yellow-orange
W CMa 07h 08m 03.4;
-11 55 26
C6, 3 6.4 – 8.0 Irreg. Bright yellow-orange
RY Mon 07h 06m 56.8;
-07 33 07
C5, 5 Unknown Bright orange
V614 Mon 07h 01m 01.7;
-03 15 06
R6 Unknown Bright yellow-orange
R CMi 07h 08m 42.2;
+10 01 25
C7, 1e* 7.4 – 11.6 338 days Small, bright yellow
X Cnc 08h 55m 22.9;
+17 13 51
C5, 4 5.6 – 7.5 195 days Bright yellow, stands out in “empty field”

* “e” Denotes that emission lines are present; usually hydrogen

  • Magnitude and Period are provided by AAVSO’s International Variable Star Index

  • AAVSO is an invaluable source to aid in carbon star hunting. You can print star charts, and check the latest observation on each star. The latest observation on each star will demonstrate the latest visual magnitude measurement (with date) and will give you an idea of what to expect to see. Remember that the lower the magnitude; the redder the appearance. If the star is at its maximum; you can probably expect to see a bright yellow to yellow-orange star.


Yerkes Classification
The Yerkes classification involves using the spectrum of the stars in the galaxy
and the shape, real and apparent, and the degree of its central concentration.
Spectral Type Explanation
a Prominent “A” stars
a-f Prominent “A – F” stars
f Prominent “F” stars
f-g Prominent “F – G” stars
g Prominent “G” stars
g-k Prominent “G – K” stars
k Prominent “K” stars

Galactic Shape Explanation
B Barred spiral
D Rotational symmetry without pronounced spiral or elliptical structure
E Elliptical
Ep Elliptical with dust absorption
I Irregular
L Low surface brightness
N Small, bright nucleus
S Spiral

Inclination Explanation
1 Galaxy is “face-on”
7  Galaxy is “edge-on”