Spectral Classification of Stars

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1 Department of Physics and Geology Spectral Classification of Stars Astronomy 1402 Part 1: Background Spectral Classification of Stars 1.1 Spectral Types: O, B, A, F, G, K, M On a dark, clear night far from city lights, the unaided human eye can see on the order of five thousand stars. Some stars are bright, others are barely visible, and still others fall somewhere in between. A telescope reveals hundreds of thousands of stars that are too dim for the unaided eye to see. Most stars appear white to the unaided eye, whose cells for detecting color require more light. But the telescope reveals that stars come in a wide palette of colors. A few stars are bright enough to make out their color, such as the red star Betelgeuse in the constellation Orion or the yellow color of our own Sun. If you photograph the stars then their colors are easily seen. Astronomers normally group stars according to their surface temperature and each group known as Spectral Type or Spectral Classes is given an identifying letter; like (O,B,A,F,G,K,M). The peculiar ordering of the letters is a matter of historical accident, and the identifying letters themselves have no particular significance. Within each type, stars are further categorized by subdividing each lettered spectral class into 10 subdivisions, denoted by the numbers from 0 to 9. So for example, an M2 star is hotter than an M7 star. In the case of our own Sun it is classified as a G2 star, Betelgeuse is an M2 star, and so on. Table 1.1 lists the main properties of each spectral type for the stars with their surface temperature and color.

2 Spectral Class Surface Temperature (K) Color Examples O > 30,000 Violet - Blue Naos (O5) 42,400 K Mintaka (O9) 35,000 K B 10,000 30,000 Blue Spica A (B1) 22,400 K Rigel (B8) 11,000 K A 7,500 10,000 White Vega (A0) 10,000 K, Sirius A (A1) 9,900K Zaniah (A2) 8,800 K F 6,000 7,500 Yellow-White Canopus (F0) 7,500 K, Procyon A (F5) 6,600 K Wezen (F8) 6,200 K G 5,000 6,000 Yellow Capella [Binary System] (G0/G8) 6,000 K / 5,200 K Sun (G2) 5,800 K, Nihal (G5) 5,300 K K 3,500 5,000 Orange Arcturus (K2) 4,300 K Albederan (K5) 4,100 K M < 3,500 Red Antares (M1) 3,500 K Betelgeuse (M2) 3,400 K Barnard s Star (M4) 3,100 K Proxima Centauri (M5) 3,000 K Mira (M7) 2,900 K Table 1.1 p. 2

3 O Type: These are relatively rare. They have a very high surface Temperature, in the range of 30,000 K and above, and are violet-blue in color. B Type: This type of stars is the first of the really populous classes. These stars are blue in color and burn hotly, with surface temperatures lying between 10,000 K 30,000 K. A Type: These stars have surface temperatures in the range of 7,500 K 10,000 K and are white in color. Some of the brightest and most famous stars in the sky belong to this classification. F Type: This type of star has a yellow-white color and surface temperatures between 6,000 K 7,500 K. G Type: These stars, with temperatures ranging between 5,000 K 6,000 K, have spectra that betray the existence of metals or heavy elements (any element heavier than Helium) and are yellow in color. K Type: These stars are occasionally referred to as Arcturian Stars, after the brightest of their type. Their surface temperatures are between 3,500 K 5,000 K, which is a temperature low enough for simple molecules to form and are orange in color. M Type: The coolest of the common star types, these stars have very cool surface temperatures, below 3,500 K, which allows more complex molecules to form. These stars are red in color. p. 3

4 1.2 Luminosity Classes: I, II, III, IV, V Over the years, astronomers have developed a system for classifying stars according to the widths of their spectral lines. Because the line width depends on pressure in the stellar photosphere, and because this pressure in turn is well correlated with luminosity, this stellar property has come to be known as Luminosity Class. The information about the stars luminosity is given by a Roman numeral from I to V. This luminosity class is simply appended to the spectral class, for example the star Betelgeuse is a M2 I star, which means it has a M2 spectral class and a I luminosity class. The standard luminosity classes are listed in Table 1.2. Luminosity Class I II III IV V Type of Star Supergiants Bright Giants Giants Subgiants Main Sequence Table 1.2 I: Supergiants - These stars are extremely massive and luminous stars, usually nearing the end of their lifespan. II: Bright Giants - This is a relatively rare group of giant stars that are very luminous. As an example some of them are a thousand times brighter than our own Sun. III: Giants - Usually these stars are typically a hundred times more luminous than our own Sun, but considerably more massive. IV: Subgiants - Definitely these stars are far more massive and luminous than our own Sun but fall short of the true giants. V: Main Sequence - Is a very numerous class of main sequence stars, whose mass and luminosity is generally comparable with that of our own Sun. p. 4

5 SAFETY REMINDER Follow the directions for using the equipment. Part 2: Lab Activity Spectral Classification of Stars The purpose of this laboratory activity is to introduce you to stellar spectral classification. You will have an opportunity to study some of the features of the spectra and to classify some stars. Record your results in Part 3 - Lab Report. p. 5

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