Spectral Classes

The rapid spread of spectroscopy in the late Nineteenth century resulted in a large number of stellar spectra. Astronomers faced the major challenge of trying to make sense of them. This was analogous to the problem faced earlier in the biological sciences and tackled by Linnaeus through his classification system of living organisms. The system now used was adopted in 1910 following detailed and extensive work by Annie Cannon and her team at Harvard Observatory. It has been refined since then but in essence is still the same.

To see how this spectral classification scheme works study the sequence of spectra shown below. It shows spectra for different stars as photographic plots. In reality photographic spectra would not show colour as the plates were monochrome but the colour has been added here to highlight the different wavelengths.

Continuum

O5V

B1V

A1V

F3V

G2V

K2V

M0V

If you study the spectra above you will notice some trends. The 0-class spectrum has relatively weak lines but lines for ionised He+ are present. The B, A and F stars have a similar pattern of lines that are strongest in the A star. These are the H Balmer series for neutral hydrogen. F and G stars have lines corresponding to ionised Ca+. The K and M stars have many more lines visible but the Balmer series is very weak. These lines correspond to Fe, other neutral metals and molecules. TiO lines are visible in the spectrum of M stars.

Another way of comparing stellar spectra is by studying their intensity plots. The sequence below is for main sequence stars from about the middle of each spectral class. It shows the spectrum for a small region of the visible waveband from 390 - 450 nm.

Why do different stars have different lines? This question is the key to helping us classify stars. If we compare an O-class star with and M-class star they have very different lines. The O-class star has weak lines except those for ionised He+ and it also has a continuum that is strong in the UV region.

The key factor at work here is temperature. By temperature we really mean the effective temperature of the star (sometimes called the surface temperature). This is the temperature of a black body having the same size and luminosity as the star and is determined by Stefan's Law. The variations in spectral lines for different stars are due primarily to the difference in temperature of the outer layers of gas in the star.

In very hot stars, helium can be ionised so we can expect to see spectral lines due to absorption by helium ions. In most stars the temperature is too cool for helium to ionise so no such lines can form in the spectrum. Even though spectral lines due to helium are not found in cool stars it does not mean that helium is missing from the star. In fact helium is the second most abundant element in the Universe and in stars. The absence of helium lines simply means that the conditions are not right for helium lines to form or be abundant in that star.

Some stars are cool enough that molecules can exist in outer layers without being ripped apart. As the number of possible electron transitions is much greater in molecules than single atoms there are many possible spectral lines that can form hence cool stars typically have many lines.

The standard spectral class classification scheme is thus based on temperature. Most stars fit into one of the following types or spectral classes:

O, B, A, F, G, K, M

These classes go from hot to cool with O the hottest and M, cool. recent discoveries have led to tentative new classifications for even cooler L-class stars. For the moment, however, we will focus on the seven original classes. The letters assigned to each class seem confusing and out of order. This is an historical artefact as classes were assigned to spectra before the underlying physical relationship was known. Rather than reassign letters to different spectra, some classes were merged and the whole sequence arranged in order of decreasing temperature.

How can you remember the sequence?

Many people use a memory device or mnemonic to help them. Here is a common example but feel free to make up your own.

Oh Be AFine Girl (or Guy), Kiss Me!

The basic system of a letter to denote spectral class is further refined by adding a number from 0 to 9 following it. Each spectral class is thus broken down into ten subdivisions so that, for example, an F2 star is hotter than an F7 star.

The basic characteristics of each spectral class are summarised in the following table. The four columns on the right of the table provide comparison of a star's mass, radius and luminosity (power output) with respect to the Sun and the main sequence lifespan for a star of that spectral class. These factors are discussed in more detail in later sections of the site.

Spectral Class Summary

Spectral Class

Effective Temperature (K)

Colour

H Balmer Features

Other Features

M/MSun

R/RSun

L/LSun

Main Sequence Lifespan

O

28,000 - 50,000

Blue

weak

ionised He+ lines, strong UV continuum

20 - 60

9 - 15

90,000 - 800,000

1 - 10 Myr

B

10,000 - 28,000

Blue-white

medium

neutral He lines

3 - 18

3.0 - 8.4

95 - 52,000

11 - 400 Myr

A

7,500 - 10,000

White

strong

strong H lines, ionised metal lines

2.0 - 3.0

1.7 - 2.7

8 -55

400 Myr - 3 Gyr

F

6,000 - 7,500

White-yellow

medium

weak ionised Ca+

1.1 - 1.6

1.2 - 1.6

2.0 - 6.5

3 - 7 Gyr

G

4,900 - 6,000

Yellow

weak

ionised Ca+, metal lines

0.85 - 1.1

0.85 - 1.1

0.66 - 1.5

7 - 15 Gyr

K

3,500 - 4,900

Orange

very weak

Ca+, Fe, strong molecules, CH, CN

0.65 - 0.85

0.65 - 0.85

0.10 - 0.42

17 Gyr

M

2,000 - 3,500

Red

very weak

molecular lines, eg TiO, neutral metals

0.08 - 0.05

0.17 - 0.63

0.001 - 0.08

56 Gyr

L?

<2,000

Tentative new (2000) classification for very low mass stars.

<0.08

May or may not be fusing H in cores?

Some stars exhibit spectral anomalies resulting in them being given special classifications:

R-class stars have the same temperature as K-class stars but have high abundances of carbon and carbon molecules.

N-class stars are carbon-rich stars with the same temperature as M-class stars.

S-class stars are similar temperature to M stars but have bands of zirconium oxide and lanthanum oxide.

WN and WC are two types of Wolf-Rayet stars, the same temperature as O-class stars but showing strong broad emission lines of carbon and nitrogen respectively.

Luminosity Classes

One problem facing early attempts at classifying stellar spectra was the fact that two spectra could have the same lines present, indicating that the stars had the same effective temperature, but the lines in one star's spectrum were broader than in the other. When the star's were plotted on an HR diagram it also became apparent that two stars could have the same effective temperature (hence also colour and spectral class) but vary enormously in luminosity and thus absolute magnitude. To account for this a second classification scheme of Luminosity Class was added to the original concept of Spectral Class. A simplified version of the MK system of luminosity classes is shown in the table below.