6 Stellar Spectra and Stellar Classification

V. B. Bhatia

1. Learning Outcomes

After studying this module, you should be able to

understand how spectral lines are formed
explain the formation of continuous, emission and absorption spectra
describe the solar spectrum and Fraunhofer lines
enumerate the various absorption lines found in stellar spectra
appreciate the diversity in stellar spectra
describe the Harvard System of stellar classification
name the classes of stars: O, B, A, F, G, K and M
state the characteristic spectral signatures for each spectral class
give reasons for the extension of classification to include Wolf-Rayet stars
state special features of Wolf-Rayet stars

2. Introduction

In the last few modules we have dealt with the stellar characteristics like apparent magnitudes, absolute magnitudes and their relationship with luminosities of stars. Since stars are not monochromatic radiators, UBV system of photometry became a necessity. This brought in the concept of colour index of stars and its relation to their surface temperatures. We found how useful the colour-colour diagram is for distinguishing between the various groups of celestial objects. It became clear that the true luminosity of a star can be found only by universal detectors of radiation like bolometers. However, these detectors are rather inefficient and are not very useful. So, concept of bolometric correction was introduced. Expression for bolometric correction was derived assuming that stars radiate like black bodies. Since this assumption is not correct, parametric formulae for bolometric correction have been introduced. Having dealt with these basic concepts, we are now ready to embark on important concepts like the stellar classification. We will find that the existence of certain absorption lines and their intensities play a pivotal role in defining the spectral class of a star. It is important to bear in mind that once the spectral class of a star has been fixed, it gives us significant information about many characteristics of the star.

3. Recap of Atomic Radiation

An important method of studying the stars is through the electromagnetic radiation that we receive from them. An analysis of spectra of stars can provide information not only about the layer from which the radiation is emanating, but also about internal constitution of stars, their atmospheric structure, and their geometric properties (such as, whether they belong to spectroscopic binary groups). Most of the theories of stellar structure are checked by comparing the predicted spectra against the observed spectra. It is because of the stellar spectra as a tool for the study of stars that spectroscopy is sometimes said to be the mother of astrophysics. A basic spectroscope, let us remember, consists of a prism (or a diffraction grating) which splits the light into its various colours (Fig. 11.1)

Fig. 11.1. A basic spectroscope to get the stellar spectra.

Before we discuss stellar spectra, let us briefly revise our ideas of atomic radiation. For the basic idea about the emission and absorption of radiation, it will suffice at this stage to consider the simple Bohr model of the atom, in which electrons orbit the nucleus in various fixed orbits. When an electron jumps from a higher energy level to a lower energy level, a photon is emitted with energy equal to the difference in energies of the two levels involved in the transition (Fig. 11.2). If the electron can absorb a photon of energy exactly equal to the difference in the two energy levels, then the electron jumps from a lower energy level to a higher energy level.

3.1. Types of Spectra

Consider now a large assembly of atoms of a hot gas such as hydrogen. Electrons are lifted to various states, from where they jump to lower energy states either in one go, or by way of two or more transitions. In the process photons of various energies are emitted. Photons emitted due to

Fig. 11.2. Emission and absorption of radiation by an atom. Notice that the frequency of an absorbed photon involving any two states is exactly equal to photon emitted involving a given transition cooperate to produce a bright emission line. Hydrogen spectrum has many such characteristic lines corresponding to the various transitions in the hydrogen atoms (Fig. 11.3). The emission spectra of other elements are characteristic of transitions in their atoms.

Fig. 11.3. Emission spectrum of hydrogen gas. Emission spectra of other elements are also similar. (Source: Wikipedia)

Suppose now that we have a hot sphere of solid, or gas at high pressure. The individual lines are broadened due to collisions so much that the spectrum appears continuous. We may understand the continuous spectrum as due to lines overlapping each other so that they cannot be recognized as separate. An incandescent light, for example, produces a continuous spectrum.

Fig. 11.4. A cool gas in front of a hot source of continuous radiation produces a characteristic absorption spectrum.

Fig. 11.5. Continuous spectrum, and emission and absorption spectra of hydrogen gas. (Source: Wikipedia)

Now imagine that a cool gas is located between a hot object and the specrtroscope. The cool gas will subtract out of the continuous spectrum of the hot object photons corresponding to transitions in its atoms. So, in the spectroscope we will have dark lines on a continous background, an absorption spectrum. The lines are characteristic of the cool gas (Fig. 11.4). Fig. 11.5 shows all three types of spectra.

4. Stellar Spectra

Stellar spectra are generally a continuous spectra marked with dark lines, sometimes only a few and sometimes in thousands. In rare cases there may also be emission lines alongside the absorption lines. The lines may be narrow or broad, diffuse or sharp, faint or bright. Out of this diversity we shall search for commonness.

4.1. Solar Spectrum

We are quite familiar with the structure of the Sun, which we can study it in great details because of its proximity. The solar spectrum consists of a continuous background crossed by a large number of dark lines. The existence of dark lines was explained correctly by Fraunhofer. These lines are, therefore, called Fraunhofer lines (Fig. 11.6). The continuous spectrum is produced in the photosphere (called the sphere of light for this reason) of the Sun. Photosphere is the visible surface of the Sun. Above the photosphere are the two layers of the solar atmosphere, the chromosphere and the corona. The thickness of the photosphere is ~ 500 km. The temperature at the base of the photosphere is ~ 6000 K. The temperature decreases outwards, reaching a minimum of ~ 4500 K at the top. The cooler layers are the source of dark absorption lines reflecting their chemical composition.

Fig. 11.6. Solar spectrum shows dark lines on a continuous background. Dark lines are called the Fraunhofer lines. (Source: https://commons.wikimedia.org/w/index.php?curid=7003857 )

4.2. Stellar Spectra

Stellar spectra is similar to the solar spectrum in that there are dark lines and bands overlying the continuous background. The important lines seen in the stellar spectra are those of HI, HeI, HeII, OI, OII, OIII, NII, NIII, CII, CIII, CIV, SiI and SiII (according to the astronomical nomenclature, I denotes neutral atom; II denotes singly ionized atom; III denotes doubly ionized atoms; and so on). Among metals, the more significant lines are those contributed by CaI, CaII, MgI, MgII, FeI, TiI, CrI, NiI and ScI. The prominent dark bands found in stellar spectra have been traced to molecules and radicals such as TiO, ZrO, LaO, C2, CH, CN, MgH, SiH, AlH, ScO, VO, CrO, C3 and SiC2. In addition to dark lines and bands, spectra of some stars also show emission features.

Stellar spectra display great diversity in the number and intensity of lines and bands, and in the intensity of background continuum. Fig. 11.7 shows a sequence of spectra. At the top, we see a spectrum with just a few lines. At the bottom, we see a spectrum with thousands of absorption lines. In this diagram, we can also notice the change in the brightness of the background continuum.

Fig. 11.7. Sequence of stellar spectra, from a very simple to a very complex. (Source: NOAO/AURA/NSF)

Despite the great diversity in stellar spaectra, common features in them can be noticed. Based on features which are common in some spectra and those features which distinguish one type of spectra from the other type, astronomers have devised a classification scheme for stars. Classification has led to a knowledge of conditions in the atmospheres of stars, including temeprature, chemical composition and strength of gravity. No wonder, the, that the assignment of spectral class to a star is so important.

At first, when the spectra of a large number of stars were collected, it was thought that the differences in spectra reflected the varying chemical compositions of the stellar atmospheres. It was argued that the gradual change in the spectral features reflected the gradual buildup of chemical elements from hydrogen. In 1920s, Saha, an Indian astrophysicist, was able to show that this conclusion was not correct and that the stellar spectral sequence from a very simple (with only a few lines) to a very complex spectrum (containing thousands of lines and bands) actually represents the condition of decreasing stellar surface temperature.

The system of classification of stars is known as Harvard System because it was developed at the Harvard Observatory, mainly due to the work of Annie Cannon, following the work of Henry

Draper. The Harvard system divides the stars on the basis of their spectra into seven major classes, namely, O, B, A, F, G, K, M. The sequence of jumbled letters may appear strange. Initially, the letters were in alphabetic order from A to Q, 22 classes, running from the simplest to the most complex spectra. This scheme was too unwieldy. Cannon was able to rearrange and merge the classes to only 7 major classes. It is remarkable that this scheme has survied till today. However, with continuous improvement in our understanding of stellar composition and stellar evolution, some adjustments became necessary. An interesting and popular pnemonic to remember the sequence is: O Be A Fine Girl/Guy, Kiss Me. Each major class is further subdivided into 10 subclasses, denoted by numbers from 0 to 9. Thus, the class O runs from O0 to O9, followed by B0. B9 is followed by A0, and so on. Ocasionally, even a subclass may be further subdivided. Stars belonging to the first three major classes are sometimes referred to as early type, while those belonging to the last four major classes are called the late type stars. Almost 99% of all stars belong to classes B, A, F, G, K and M. Fig. 11.8 shows the sample spectra for the various stellar classes.

Fig. 11.8. Spectra of various spectral classes of stars. Important lines have been identified. One can see how certain lines change with the spectral class. (Source:

https://peo ple.highline.edu/iglozman/cla sses/astronotes/me dia/specl ine_ class.jpg )

Table 11.1 summarizes the characteristics of the various spectral classes.

Table 11.1: Stellar Spectral Types and their Characteristics

Spectral Class	Approx Surface Temperature	Hydrogen Balmer Lines	Helium Lines	Other Characteristic Lines	Naked-eye Example	Colour
O	> 25000 K	Weak	HeII	Lines of Highly Ionized Atoms (e. g., SiIV, NIII)	Meissa (λ -Orionis, Class O8)	Blue
B	25000 – 11000 K	Medium	HeII absent, HeI strong	SiIII, OII	Achernar (α-Eridani, Class B6)	Blue/White
A	11000 – 7500 K	Strong	HeI absent	Weak CaII, strong MgII and SiII	Sirius (α- Canis Majoris, Class A1)	White
F	7500 – 6000 K	Medium	HeI absent	CaII strong, FeI, TiI Lines strong	Canopus (α-Carinae, Class A9)	Yellow/White
G	6000 – 5000 K	Weak	HeI absent	CaII very strong, neutral metals FeI strong	Sun (Class G2), α- Centauri A (Class G2)	Yellow
K	5000 – 3500 K	Very weak	HeI absent	Neutral metal lines strong, molecular bands	Arcturus(α- Boötis, Class K0)	Orange
M	< 3500 K	Very weak	HeI absent	CaI very strong, TiO bands stronger than in K	Betegeuse (α-Orionis, Class M1 – M2)	Orange/Red

(Bold text indicates the signature of the class. For example, strong HeI lines is the signature of Class B; very strong ionized calcium lines is the signature of class G (The Sun belongs to class G2); appearance of strong molecular lines and bands is the signature of class M.)

These days spectra is obtained as tracings of CCD (charge-coupled device) output at the focus of telescopes. Fig. 11.9 shows the tracings of stellar spectra of various classes.

Fig. 11.9. CCD tracings of spectra of stars. CCD gives directly the intensity of lines and the background. (Source: http://www.astronomy.ohio- state.edu/~pogge/TeachRes/Ast162/SpTypes/OBAFGKM_scans.jpg )

Summary

Stellar spectra is an important tool for studying stars.
A hot dense object, or a hot gas at high pressure, emits a continuous spectrum.
Spectrum of a thin hot gas is an emission spectrum, bright lines on a dark background. The lines are characteristic of the gas.
Spectrum of a cool thin gas placed between a hot dense object and the spectroscope is an absorption spectrum, dark lines crossing the bright background. The absorption lines have exactly the same frequency as the emission lines.
Stellar spectra are dark lines crossing a continuum background.
Stellar spectra can be very simple, containing only a few lines.
Stellar spectra can be very complex, containing thousands of lines and molecular bands.
Even within this diversity common characteristics can be found, which form the basis of classification.
Classification scheme was developed at the Harvard Observatory mainly through the efforts of Annie Cannon. The scheme is called the Harvard Scheme.
Assigning a spectral class to a star is very important because it gives all the information about the surface temperature and other characteristics of the star.
There are seven major classes, O, B, A, F, G, K, M. Each major class is subdivided into ten subclasses running from 0 to 9.
The colour of the classes changes from blue (O type) to red (M type).
Stars belonging to the first three classes in the sequence are called the early type stars.
Stars belonging to the last four major types are termed as the late type stars.

The following websites give more information on the topics covered in this module:

http://www.star.ucl.ac.uk/~pac/spectral_classification.html
https://en.wikipedia.org/wiki/Stellar_classification
http://hyperphysics.phy-astr.gsu.edu/hbase/starlog/staspe.html
http://www.atlasoftheuniverse.com/startype.html
https://www.cfa.harvard.edu/~pberlind/atlas/htmls/note.html
http://www.skyandtelescope.com/astronomy-equipment/the-spectral-types-of-stars/
http://www.atnf.csiro.au/outreach/education/senior/astrophysics/spectral_class.html
http://cas.sdss.org/dr7/en/proj/basic/spectraltypes/history.asp
https://people.highline.edu/iglozman/classes/astronotes/media/specline_class.jpg
http://www.astronomy.ohio-state.edu/~pogge/TeachRes/Ast162/SpTypes/OBAFGKM_scans.jpg
https://commons.wikimedia.org/w/index.php?curid=7003857