Spectral Classes of Stars

To understand the nature of stars on an individual basis and then in a comparative nature, one begins with the spectral classification system devised by Pickering's team at Harvard University Observatory in the early 1900's. This subject was covered in some detail earlier in the course, but we will go over the HR Diagram again here, and look at the different spectral classes more.

A modified HR Diagram is shown to your left and serves to introduce you to the spectral classes of stars. I have circles the major star groupings as well. On the bottom of the diagram are the spectral classes OBAFGKM. Please be aware that a new class of stars has recently been added to this chart ... C & S... and these stars occupy a region to the right of the M stars.

These star classes are based on surface temperatures, and follow Wein's Law that relates color to temperature. The law is shown below:

Wavelength = 3,000,000/T

In the formula, temperature is given in units of Kelvin, and the wavelength is given in units of nanometers. The hotter the star, the shorter the wavelength. As you move through the Electromagnetic Spectrum, looking primarily at the visible portion for this discussion, short wavelengths are blue or violet, while the longer wavelengths are yellow, orange, or red.

When Annie Cannon and Edward Pickering finalized the Draper Catalogue of stars, they grouped them according to surface temperature and also according to absorption spectrum features. Every star is a ball of glowing gas, and when light shines through a glowing gas, the burning elements under the gas yield dark lines that contain a unique signature of the element type. This is how astronomers learned about the chemical composition of stars, by studying their absorption lines.

The absorption lines of some stars are shown to your left. I am not concerned whether you learn which dark lines correspond to which elements, for that is material best saved for collegiate Astronomy or beyond. What I do want you to see is how the absorption spectra change as you move from the O class down in temperature to the M class. Some lines are more prominent than others in the various spectral classes and this allows us to do something more with the information contained in a star's spectrum. Now we can take the basic OBAFGKM(C,S) classes and subdivide them into subclasses (0 - 9). This is a simple refinement, but is the basis for the numbers you see next to the spectral class. The hotter the star, the more prominent the signature of Helium and Hydrogen. The cooler the star, the more readily other elements and even chemical compounds can be seen. This is not to say necessarily that other elements or chemicals are not burning in O or B class stars, but that the hydrogen burning is so hot as to mask the burning of other elements.

Hertzsprung and Russell were the first to try and find a relationship between a star's spectral class and their luminosity. The HR Diagram was the result of their work and is the basis for the rest of your study of stars. All stars that are in the process of burning Hydrogen into Helium are classified as Main Sequence Stars. About 90% of the stars in space are Main Sequence Stars. The rest of the stars are either in the process of dying or already are dead. To help you see how we classify stars from the HR Diagram and the Spectral Absorption Lines, please note the diagrams below. The first shows the basic spectral classes of stars and some notes about that class.

W

W is a very rare type of superhot star, with surface temperatures up to 50,000 K. There is only one example in the sky that is visible to the naked eye, in the Suhail al Muhlif system in the constellation Vela.

O

O-type stars are also relatively uncommon, but far more numerous than those of type W. These are bright blue stars which also have very high surface temperatures, in the range 25,000 K to 50,000 K. Examples are Alnitak (O9.5), Naos (O5), Hatysa (O9) and Meissa (O8).

B

The B type is the first of the really populous classes. Stars of this type are blue in color and burn hotly, with surface temperatures lying between 11,000 K and 25,000 K. Prominent examples of blue B-type stars are Rigel (B8), Achernar (B3), Agena (B1) and Spica (also B1).

A

A-type stars are those whose surface temperatures lie in the approximate range 7,500 K to 11,000 K. They are white in color, and some of the brightest and most famous stars in the sky belong to this classification, including Sirius (A0), Vega (A0), Altair (A7) and Deneb (A2).

F

F-type stars lie between the A-type white stars and G-type 'true' yellow stars, and have a distinctly yellowish light. Their surfaces have a temperature between 6,000 K and 7,500 K. Sometimes called Calcium Stars, examples of this type include Canopus (F0), Procyon (F5), Algenib in Perseus (F5) and Wezen (F8).

G

The cooler a star, the more complex its chemistry tends to be. G-type stars, with temperatures ranging between 5,000 K and 6,000 K, have spectra that betray the existence of 'metals' (in this context, 'metal' refers to any element heavier than helium). Examples of yellow G-type stars are Alpha Centauri (G2), Capella (G5), Kraz (G5) and Mufrid (G0). The Earth's Sun is a G2 star, and also belongs to this type.

K

K-type stars are occasionally referred to as Arcturian Stars, after the brightest of their number. Their surface temperatures are between 3,500 K and 5,000 K, low enough for simple molecules to form. K-type stars are orange in color, and among the brightest in the sky are Arcturus (K2), Aldebaran (K5), Pollux (K0) and Atria (K2).

M

The coolest of the common star types, red stars are classified as M-type. They have very cool surface temperatures below 3,500 K, allowing more complex molecules to form. Among the brightest red stars in the sky are Betelgeuse (M2), Antares (M1), Gacrux (M4) and Mirach (M0). The Sun's nearest neighbor in space, Proxima Centauri, is also a red star, classified as M5.

These same spectral classes show the classic absorption lines of various atomic nuclei and even molecules. The chart below will demonstrate this:

Class

Characteristic Spectra

Color

Surface Temperature (K)

Examples

O

He II; He I

blue

28,000-50,000

chi Per, epsilon Ori

B

He I; H

blue-white

9900-28,000

Rigel, Spica

A

H

white

7400-9900

Vega, Sirius

F

metals; H

yellow-white

6000-7400

Procyon

G

Ca II; metals

yellow

4900-6000

Sun, alpha Centauri

K

Ca II; Ca I; other molecules

orange

3500-4900

Arcturus

M

TiO; other molecules; Ca I

orange-red

2000-3500

Betelgeuse

R (carbon stars)

CN; C2

orange-red

3500-5400

 

S (mild carbon stars)

ZrO; other molecules

orange-red

2000-3500

R Cyg

N (carbon stars)

C2

red

1900-3500

R Lep

Two other rare classes are the C- and S-type stars. These are cool stars that overlap the K- and M-type classifications in terms of temperature, but are placed in separate categories due to unusual chemistry within the star. Very few of these stars are visible without optical aid, though the C-type U Hydrae, and the S-type Chi Cygni are unusual exceptions.

A star's full spectral classification often also includes a 'luminosity class', a Roman numeral from I to VII indicating the star's luminosity, which correlates with its mass. The luminosity class is simply appended to the spectral class. So, for example, the Sun's full spectral classification, including its luminosity class, is G2V. The seven luminosity classes are listed below.

I

Supergiants: extremely massive and luminous stars, usually nearing the end of their lifespan. They are subclassified as Ia or Ib, with Ia representing the most luminous stars of all. Examples include Rigel (B8Ia), Betelgeuse (M2Ib) and Antares (M1Ib).

II

Bright Giants: a relatively uncommon group of giant stars that are particularly luminous, and can be a thousand times more so than the Sun, or more. Examples include Adara (B2II), Sargas (F1II) and Kraz (G5II).

III

Normal Giants: the giant stars in this category are typically a hundred times more luminous than Earth's Sun, and considerably more massive. Examples of this populous group include Arcturus (K2III), Agena (B1III) and Aldebaran (K5III).

IV

Subgiants: though still far more massive and luminous than the Sun, subgiants fall short of the true giants. Examples include Acrux (B0.5IV), Shaula (B1.5IV) and Miaplacidus (A2IV).

V

Dwarfs: a very numerous class of main sequence stars, whose mass and luminosity is generally comparable with that of the Sun. Examples include Sirius (A0V), Alpha Centauri (G2V) and Vega (A0V).

VI & VII

These classes designate subdwarfs and white dwarfs, respectively. They are not now in common use, but are included here for completeness.

It is strange to think of our Sun as a Dwarf star. Afterall, with a diameter of 1,392,000 km and a volume of over 1.1 million Earths, the Sun is hardly a dwarf. However, the Sun just seems big to us because it is close. When you move outside the realm of what you know and are comfortable with and explore the depths of space, there are things out there that simply boggle.

In addition to the luminosity class, spectral classifications sometimes also carry commentary additions, usually lower case letters added before or after the main type. For example, the full spectral classification for Achernar is B3Vp, with 'p' indicating that it has a peculiar spectrum, while Castor in Gemini is classified as A2Vm, with 'm' demonstrating that the spectrum contains strong metal lines, and so on. These prefixes and suffixes lie rather outside the range of this page.

From this reading, you should now be aware that stars are grouped by spectral pattern. Time now to look back at our Sun, if you have not already done so, or move ahead to Star Lifecycles. Or you can take a break and return to Introduction to the Stars, or to the Syllabus.


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