Overview of Stellar Spectra | Finding Spectra | Plotting and Measuring Spectra | Classifying Spectra | Links
Findings | Tables | Figures | Conclusion
O stars are the hottest of all the stars, ranging of temperatures of 28,000 50,000 Kelvin. Due to these numbers being so great, O stars are difficult to analyze and have less data taken on them than other spectral types. Although Figure 1 shows a great trend, Table 1 shows that there were only 1 or 2 spectra used for each subclass, which limits the reliability of the trend. Even with the low amount of data points, since there are no outliers in the spectra measured, it can be concluded with a decent level of certainty that in general, O stars will follow the trend in the ratio 4471: 4541. With this quantitative data being known, when the ratio 4471: 4541 is measured from SDSS data, an accurate O star classification can be made.
The measured ratios from Morgan’s standards generally do show trends that can be summarized quantitatively. Each data point showing the trend of the ratio 4026: 3935 is the average of two or three different measurements per subclass, which increases reliability. Also, the trend is relatively constant which further supports the relation. However, the values for this ratio vary greatly even in the same subclasses. For example, the three B0 stars measured had ratio values of 25.4, 10.46, and 5.71. These values are extremely different and thus cannot be trusted to a great extent. The other ratio showing trends is 3935: 3920-3925. For subclasses B0 to B3, the ratio values go up and then down before steadily increasing from B3 to B9. When the two ratios for B stars are combined, as shown in Figure 2, it is safe to say an accurate classification of a star can be made. Each subclass having two different ratio trends to help in classification vastly increases the ability to classify B stars.
The measurement for A stars, and only A stars, was done by the matching of a Gaussian curve to the absorption line and calculating the equivalent width from there. For overlapping absorption lines, two or three Gaussian curves were used to insure precision. This method was slightly more accurate than the other method used and should make these ratio measurements more reliable. The trends found for A stars were also fairly good. Combining the ratio 4385: 4481 and the strength of 4103, all A star subclasses have a quantitative representation that follow a distinct trend, as shown in Figures 3 and 4.
Trends for classifying both spectral type and luminosity class were found for F stars. There were several MKK standard spectra used in the calculation of the trend for the ratio 4030-4034: 4128-4132 which supports reliability. The issue faced with these numbers is that the difference is rather small; on average there is only a 0.09 difference between F0 and F2 stars. If a measurement taken has even a small percent error, the classification of the star might be difficult to make. The ratio 4077: 4045 has a good trend of decreasing as luminosity class goes from I to V. This trend applies to all F star subclasses but is most evident in F2 stars, as shown in Figure 6. The difference in the value for this ratio used in dividing luminosity classes is fairly substantial and thus can be used effectively in the determination of luminosity class.
G stars are perhaps the easiest of the spectral types to classify. There is a great amount of data available for these stars since they are close to the middle of the stellar temperature range. The values for the two ratios shown in Figure 7, 4045: 4103 and 4226: 4342, although are small numbers, are extremely accurate. The standard deviations are very small and thus the data is reliable. There should be no problem in measuring these ratios in a random G star and being able to properly differentiate between a G0 and a G2 star. The other determining ratio for G stars is 4144: 4103, which spans the subclasses G3 to G8. Although this trend is not as reliable as the ones for G0 and G2 stars, it still can be trusted; the values for 4144: 4103 steadily increase from G3 to G8 stars.
The MKK system relies only on a very small portion of the electromagnetic spectrum for analysis. The range focused on does not work as well with cool stars as it does with warmer stars. For this reason, there is limited data and analysis on K stars. The one ratio used in spectral type classification was 4226: 4325 which increased from K2 to K5. The trend is not too strong as it utilizes two spectra for K5 stars and only one spectrum for K2 and K3 stars. Nevertheless, the trend is still present and can be used in classification.
M stars might be the hardest stars to analyze. Since they are even colder than K stars, the range of data observed by Morgan is not conducive to classifying them. Furthermore, the MKK book informs to look at the strength of the band 4900- 5200 for classification. This band is not a single absorption line and thus is very difficult to measure. Broad generalizations about the band for each spectrum need to be implemented in order to measure the strength somewhat accurately. The method used to measure this strength relied heavily on visualizing where the Gaussian curve would fit the band. This method may not be the best for quantitative data, but certainly shows strong trends. Figure 10 shows the trend in the increase of the strength of the band as subclass goes from M0 to M4.5. The ratio 4045: 4077 was acknowledged by the MKK book to differentiate luminosity classes in M stars. There was a found trend in M2 stars, but there was not sufficient data to make a reliable claim regarding luminosity class. However, there was enough data to declare a differentiation of subclasses using the ratio 4045: 4077. Figure 11 confirms there is a steady decrease in the value of this ratio as subclass goes from M0.5 to M4.5.