Spectral Classes have a somewhat convoluted history, but the modern system—called the Harvard Classification [1]—divides stars into seven classes based on surface temperature and/or color: O, B, A, F, G, K, and M (often remembered by students through the mnemonic "Ok, Be A Fine Girl/Guy, Kiss Me".
The most common representation of these classes and their relationship is the HertzprungRussel Diagram (HR Diagram), which plots color/temperature against luminosity (in Solar units). Above is an HR Diagram, unique in that it plots temperature from lowest on the left to highest on the right, which is reversed from the usual plotting. This particular diagram is also useful, because it shows the ranges of stellar radii in Solar units as well, giving a sense of size as related to temperature, and it indicates masses in Solar units along the Main Sequence.
The table below shows the surface temperature ranges associated with a given Spectral Class (temperatures are, of course, given in Kelvins):
The table below shows the surface temperature ranges associated with a given Spectral Class (temperatures are, of course, given in Kelvins):
Stellar surface temperatures as assigned to Spectral Classes are not a linear progression across the entire range, nor do they follow an exponential or geometric progression. The graph below illustrates the progression of surface temperatures from the coolest Mclass stars to the hottest Oclass stars.
Spectral Class and Spectral Type
A star's Spectral Class is the letter from the above table that specifies the temperature range in which the star's surface temperature falls.
A star's Spectral Type, on the other hand, is a numerical value indicating where within the given range a star's surface temperature falls.
For instance, the Sun is designated as a G2 star: Spectral Class G, Spectral Type 2.
Its Spectral Class of G means that it has a surface temperature is in the range between 5200K and 6000K.
Within each Spectral Class are ten Spectral Types, ranging from zero to nine. The zero type is at the high end of the temperature range, and the nine type is at the low end of the temperature range.
The Spectral Type of the Sun having a value of 2 means that it falls into the third subdivision of surface temperatures below the highest for Spectral Class G; specifically, the Sun's surface temperature is ~5822.22 K (more on this next).
A star's Spectral Type, on the other hand, is a numerical value indicating where within the given range a star's surface temperature falls.
For instance, the Sun is designated as a G2 star: Spectral Class G, Spectral Type 2.
Its Spectral Class of G means that it has a surface temperature is in the range between 5200K and 6000K.
Within each Spectral Class are ten Spectral Types, ranging from zero to nine. The zero type is at the high end of the temperature range, and the nine type is at the low end of the temperature range.
The Spectral Type of the Sun having a value of 2 means that it falls into the third subdivision of surface temperatures below the highest for Spectral Class G; specifically, the Sun's surface temperature is ~5822.22 K (more on this next).
Calculating Spectral Class from Known Surface Temperature
Below are a set of equations which will return a star's Spectral Type based on its known surface temperature:
If the star's effective (surface) temperature is known, the above equations can be used to determine the numerical portion of its spectral designation. Use the first table, above, to locate the appropriate Spectral Class by letter, then use the equation in the second table associated with that letter to determine the value of the Spectral Type.
For example, the Sun's surface temperature is listed in most sources as about 5800K. (The actual figure—by calculation—is closer to 5822.22 K ... more on this below, keep reading).
If we didn't already know that it's a Gclass star, we could consult the first table and find the range into which the temperature 5800K falls and see that it is between 5200K and 6000K, telling us that the Sun is a Gclass star.
If we then run the equation for Gclass stars, using the 5800K value for the Sun's surface temperature, we get a Spectral Type of 2.25:
For example, the Sun's surface temperature is listed in most sources as about 5800K. (The actual figure—by calculation—is closer to 5822.22 K ... more on this below, keep reading).
If we didn't already know that it's a Gclass star, we could consult the first table and find the range into which the temperature 5800K falls and see that it is between 5200K and 6000K, telling us that the Sun is a Gclass star.
If we then run the equation for Gclass stars, using the 5800K value for the Sun's surface temperature, we get a Spectral Type of 2.25:
... which doesn't quite match the specified Spectral Type of 2 given for the Sun in most sources [2]. We'll solve this problem in the next section.
Calculating Surface Temperature from Known Spectral Class and Type
If the star's Spectral Class and Spectral Type are already known, the surface temperature can be determined using one of the equations in the following table:
This allows us to (finally) address the incorrect Spectral Type calculation for the Sun from above.
If we take the Spectral Type for the Sun to be 2, as specified in most sources, and run this value through the equation for Spectral Class G in the above table:
If we take the Spectral Type for the Sun to be 2, as specified in most sources, and run this value through the equation for Spectral Class G in the above table:
... we find an accurate value for the surface temperature of the Sun: 5822.22K.
As another example, the star Zeta Tucanae is listed [3] as having a Spectral Class/Type of F9.5. Using the appropriate equation:
As another example, the star Zeta Tucanae is listed [3] as having a Spectral Class/Type of F9.5. Using the appropriate equation:
...we calculate a surface temperature that is quite close to the "official" value of 5956K.
1. en.wikipedia.org/wiki/Stellar_classification
2. en.wikipedia.org/wiki/Sun
3. en.wikipedia.org/wiki/Solar_analog
2. en.wikipedia.org/wiki/Sun
3. en.wikipedia.org/wiki/Solar_analog
