Spectral Class
Stars can be classified by their surface temperatures as determined from Wien's Displacement Law, but this poses practical difficulties for distant stars. Spectral characteristics offer a way to classify stars which gives information about temperature in a different way - particular absorption lines can be observed only for a certain range of temperatures because only in that range are the involved atomic energy levels populated. The standard classes are: Temperature O 30,000 - 60,000 K Blue starsB 10,000 - 30,000 K Blue-white starsA 7,500 - 10,000 K White starsF 6,000 - 7,500 K Yellow-white starsG 5,000 - 6,000 K Yellow stars (like the Sun)K 3,500 - 5,000K Yellow-orange starsM < 3,500 K Red stars The commonly used mnemonic for the sequence of these classifications is "Oh Be A Fine Girl, Kiss Me".
If you study the spectra above you will notice some trends.
The O-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.
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.
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 A Fine 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.
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 A Fine 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.
1. How many letters are used in describing the Spectral Class?
2. What are stars classified by?
3. What is the acronym used
4. The O-Spectrum has relatively...?
5.What is the name of the
chart that shows the lines of stars?