The interpretation of the spectral class
Analyses of the spectra show that all the stars of the Main Sequence, those fusing hydrogen, have similar chemical compositions, all about 90% hydrogen, 10% helium, and only a fraction of other elements The differences in stellar spectra, at least for main sequence stars, are caused almost entirely by differences in ionisation (when electrons are removed from the atoms, thus producing no absorption lines) and thus changes with temperature from cool class M stars up to hot class O stars.
The image left is a modern version of the HR-diagram.
Note that the vertical scale shows both luminosity (energy output) as compared to the Sun, as well as absolute magnitude, which is luminosity expressed in the magnitude scale. (Read more about this in our EBook "Stellar Distance").
The horizontal scale at the top shows both spectral class and temperature, and at the bottom the Blue-Violet colour index.
Find a larger size of the image here.
Main properties spectral classes
The mass, radius, and luminosity listed in the table below for each class are appropriate only for stars on the main sequence. The spectral classes O through M are subdivided by Arabic numerals (0–9). For example, A0 denotes the hottest stars in the A class and A9 denotes the coolest ones. The Sun is classified as G2. (Table credit: Wikipedia).
|
Spectral Class |
Surface |
Mass |
Radius |
Luminosity |
Hydrogen |
Fraction of all |
|
O |
≥ 30,000 K |
≥ 16 |
≥ 6.6 |
≥ 30,000 |
Weak |
~0.00003% |
|
B |
10,000–30,000 K |
2.1–16 |
1.8–6.6 |
25–30,000 |
Medium |
0.13% |
|
A |
7,500–10,000 K |
1.4–2.1 |
1.4–1.8 |
5–25 |
Strong |
0.6% |
|
F |
6,000–7,500 K |
1.04–1.4 |
1.15–1.4 |
1.5–5 |
Medium |
3% |
|
G |
5,200–6,000 K |
0.8–1.04 |
0.96–1.15 |
0.6–1.5 |
Weak |
7.6% |
|
K |
3,700–5,200 K |
0.45–0.8 |
0.7–0.96 |
0.08–0.6 |
Very weak |
12.1% |
|
M |
2,400–3,700 K |
0.08–0.45 |
≤ 0.7 |
≤ 0.08 |
Very weak |
76.45% |
Giant and Super Giant stars are so large that the densities in their outer regions are low, which subtly changes the appearance of the stellar spectrum. Each spectral class in fact has its own set of criteria. Therefore we can tell if a star is a giant, super giant, or of any other category, only from studying its spectrum.
It is now clear that stars can have different combinations of luminosity and temperature. To identify the combined property (essentially the location in the HR-diagram) a luminosity classification has been developed, the Morgan-Keenan luminosity classification. Roman numerals are used to indicate their MK class, "I" for super giants, "II" for bright giants, "III" for giants, "IV" for "sub giants" (stars that are developing into giants), and "V" for the main sequence.
Examples:
- Vega is an A0 V star,
- Polaris is F7 I or II, and
- Aldebaran is K5 III
- The Sun is a G2 V star.
White Dwarfs are just called White Dwarfs, or class D. All these classes show up distinctly on the HR-Diagram. This diagram has now been improved very much with the help of additional information about typical stars, such as distance and luminosity, and this diagram now plays a very important role in modern astronomy.
It has become an essential tool for interpreting the "finger prints" of stars and reveals a great deal of information, no matter how far away the sources are.