Astronomical Spectral Analysis
Introduction
Oh, Be A Fine Girl, Kiss Me, Less Talk.
We generally categorize the stars of our universe into several spectral classes, which are as follows: O, B, A, F, G, K, M, L, T, Y. The last three (L, T, and Y) are less commonly used, but it's worth reviewing them as well.
The above mnemonic helps us remember the order of stellar spectral classification. Oh, Be A Fine Girl, Kiss Me, Less Talk. The first letters of each word represent the spectral categories in order. If we don't like this mnemonic, we can think of a funnier one:
Only Bad Astronomers Forget Generally Known Mnemomics
This works too, though I find it harder to say. Enough with the mnemonics!
These spectral classifications help answer a wide variety of questions. Once we've determined a star's classification, we learn many other things about it: we find out the star's chemical composition, temperature, density, mass, distance, and luminosity. Moreover, astronomical spectroscopy also helps determine the Doppler shift of stars.
In general, spectral classifications follow the Morgan-Keenan, or MK system. This simply means that we scale stars from O to M.
In the MK system, O is the hottest, M is the coolest. Each letter is further divided into nine (9) subclasses, where 0 is the hottest and 9 is the coolest.
Taking some examples:
- Our Sun is classified as a G2 star in this system, with a surface temperature of approximately 5778 Kelvin.
- The Aldebaran star, a red giant with only about 3910 Kelvin, is classified as a K5 star.
- Alcyone, in the Pleiades cluster, is classified as a B7 star, with a surface temperature of 12,700 Kelvin.
Spectral Types
In the following table, we can see the classifications from O-type stars to M-type stars.
| Class | Effective Temperature (K) | Vega-relative Spectrum | Spectrum (D65) | Main Sequence Mass (in solar masses) | Main Sequence Radius (in solar radii) | Main Sequence Luminosity (bolometric) | Hydrogen Lines | Occurrence of Main Sequence Stars |
|---|---|---|---|---|---|---|---|---|
| O | ≥ 33,000 | blue | blue | ≥ 16 M☉ | ≥ 6.6 R☉ | ≥ 30,000 L☉ | Weak | 0.00003% |
| B | 10,000–33,000 | bluish-white | deep bluish-white | 2.1–16 M☉ | 1.8–6.6 R☉ | 25–30,000 L☉ | Medium | 0.12% |
| A | 7,300–10,000 | white | bluish-white | 1.4–2.1 M☉ | 1.4–1.8 R☉ | 5–25 L☉ | Strong | 0.61% |
| F | 6,000–7,300 | yellowish-white | white | 1.04–1.4 M☉ | 1.15–1.4 R☉ | 1.5–5 L☉ | Medium | 3.0% |
| G | 5,300–6,000 | yellow | yellowish-white | 0.8–1.04 M☉ | 0.96–1.15 R☉ | 0.6–1.5 L☉ | Weak | 7.6% |
| K | 3,900–5,300 | light orange | pale yellow-orange | 0.45–0.8 M☉ | 0.7–0.96 R☉ | 0.08–0.6 L☉ | Very weak | 12% |
| M | 2,300–3,900 | light orange-red | orange-red | 0.08–0.45 M☉ | ≤ 0.7 R☉ | ≤ 0.08 L☉ | Very weak | 76% |
Simplified, with less technical information:
- O – greater than 30,000 Kelvin – color: blue
- B – 10,000 to 30,000 Kelvin – color: bluish-white
- A – 7,500 to 10,000 Kelvin – color: white
- F – 6,000 to 7,500 Kelvin – color: whitish-yellow
- G – 5,200 to 6,000 Kelvin – color: yellow
- K – 3,700 to 5,200 Kelvin – color: orange
- M – 2,400 to 3,700 Kelvin – color: red
Luminosity Classification
The luminosity classification system helps distinguish between different types of giant stars and main sequence stars. The various subclasses of stars with different luminosities are denoted by Roman numerals.
The previously mentioned Sun, Aldebaran, and Alcyone stars fall under the G2V, K5III, B7III classifications, respectively. Knowing these Roman numerals, we learn a lot:
- 0 or Ia+: hypergiant stars or extremely luminous supergiants.
- Ia – luminous supergiants
- Iab – intermediate luminous supergiants
- Ib – less luminous supergiants
- II – bright giants
- III – normal giants
- IV – subgiants
- V – main sequence stars, or dwarf stars
- sd – subdwarf
- D – white dwarf
Hertzsprung-Russell (HR) Diagram
It is worth presenting one by one what each classification means exactly, and beyond that, I will also introduce the Hertzsprung-Russell (HR) diagram.
In the HR diagram:
- the Y-axis shows the luminosity of different stars relative to the Sun
- the X-axis shows the surface temperature of stars in Kelvin.
As we move from left to right, the stars measured on the scale become increasingly "cooler", more common, and redder. On the left side of the scale, the bluish-colored stars shine at 30,000 Kelvin. These stars are the least common in the universe, yet the brightest.
As we might think, the previously discussed spectral classification plays a very important role here: moving from left to right, we can go from the O classification all the way to the Y classification. It is worth studying this diagram, as it is essential to know in astronomy.
Different Classifications
O-type Stars
O-type stars have the highest surface temperatures: 25,000–40,000 K, their radiation maximum falls into the ultraviolet range, and when observed in the visible spectrum, they appear blue. The O spectral type (exceptionally) has 5 subclasses; the numbering goes from O5 to O9, where O5 denotes the hottest stars within the class. Dominant lines: neutral helium (strengthening from O5 to O9), singly ionized helium, doubly ionized nitrogen, doubly ionized oxygen, doubly ionized carbon, and triply ionized silicon. About 0.00003125% of main sequence stars belong to this class – examples include ζ Puppis, λ Orionis, and the Trapezium stars in the Orion Nebula. The spectrum of an O5 V spectral type star is shown in the figure below.
B-type Stars
Their surface temperatures range between 12,300–25,000 K, and their color is bluish-white. Dominant are the hydrogen Balmer series lines, which strengthen from the B0 to B9 subclass. Neutral helium lines appear; these are strongest in the B2 type. Ionized metal lines occur: Mg II and Si II. The lines of ionized oxygen and carbon strengthen at the B3 type. Approximately 0.13% of main sequence stars fall into this spectral class, such as Rigel, Bellatrix, Spica, the belt stars of the Orion constellation, and the brighter stars of the Pleiades cluster.
A-type Stars
The surface temperatures of A-type stars are between 7,900–10,000 K, and their color is white. Their spectrum shows strong hydrogen Balmer lines (strongest at the A0 type) and ionized metal lines: Fe II, Mg II, Si II (strongest at the A5 type). Helium and calcium lines are weaker. About 0.63% of main sequence stars fall into this class, such as Vega, Deneb, and Sirius.
F-type Stars
Their surface temperatures are between 6,000 and 7,500 K. In their spectra, alongside weak (weakening from F0 to F9) hydrogen lines, we can see the singly ionized Fe and Cr metal lines (strengthening from F0 to F9). About 3.1% of main sequence stars belong to this class – examples include Canopus and Procyon.
G-type Stars
Their surface temperatures are 5,000–6,000 K. We know the most about this spectral class because our Sun belongs here. The neutral hydrogen lines weaken, and the neutral metal lines become the strongest. The singly ionized calcium H and K lines are also visible. The G-band is extremely strong (actually a group of CH and Fe lines). About 8% of main sequence stars belong here; besides the Sun, examples include τ Ceti and Capella. The spectrum of a G5 III spectral type star is shown in the figure below.
K-type Stars
Their surface temperatures are between 3,000 and 5,000 K, and their color is orange. Some of them are giants or red giants, but 13% of main sequence stars also belong here. Stars in this class include Arcturus, ε Eridani, α Centauri B, and Aldebaran. The spectrum of a K4 III spectral type star is shown in the figure below.
M-type Stars
This group corresponds to Class III in the Secchi system. Their surface temperatures are between 2,200–3,500 K. Their spectra show absorption molecular bands (CN, CH, CO, TiO, VO, MgH, and H₂) and neutral metal lines. More than 78% of main sequence stars are classified in this class – such stars include Betelgeuse (giant star), Barnard's Star (dwarf), Gliese 581 (red dwarf), LEHPM 2-59 (subdwarf), and GSC 08047-00232 B, a late M-type brown dwarf.
