Spectral Classification of Stars

Transcription

1 Department of Physics and Geology Astronomy 1402 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. Part 1: Background 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 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 Name Name Name Name Class Date Part 3: Lab Report Spectral Classification of Stars Spectral Classification Using the data of Table 1.1, fill up the following table: Star Name Spectral Type Surface Temperature (K) Color Luminosity (Lsun) Rigel B8 70,000 Betelgeuse M2 38,000 Vega A0 60 Sirius A A1 26 Sun G2 1 Arcturus K2 170 Alpha Centauri A G2 1.6 Proxima Centauri M Barnard s Star M Procyon A F5 7.4 Capella G0/G8 70/77 Table 3.1 p. 5

6 Put all the stars listed in Table 3.1 into Figure 3.1 the best you can. Remember to label each star that you plot on Figure 3.1. Figure 3.1 p. 6

7 Luminosity Classification Using Table 1.1 and Table 1.2 find the following information about these stars: Star Name Spectral Type Surface Temperature (K) Color Type of Star (Luminosity Class Name) Rigel B8 I Betelgeuse M2 I Antares M1 I Sun G2 V Procyon F5 IV Nihal G5 II Arcturus K2 III Spica A B1 V Aldeberan K5 III Acrux B1 IV Zaniah A2 IV Sirius A A1 V Mintaka O9 V Naos O5 I Wezen F8 I Canopus F0 II Deneb A2 I Algol B8 V Mira M7 III Muphrid G0 IV Table 3.2 p. 7

8 Put all the stars listed in Table 3.2 into Figure 3.2 the best you can. Remember to label each star that you plot on Figure 3.2. Figure 3.2 p. 8

9 Wein s Formula 1. Using the Wien s formula calculate λmax for the following cases [note that the visible spectra goes from 400 nm (violet) to 700 nm (red)]: λmax = ( / T ) x 10 7 (nm) Naos (O5): T= λmax = Rigel (B8): T= λmax = Vega (A0): T= λmax = Canopus (F0): T= λmax = Sun (G2): T= λmax = Arcturus (K2): T= λmax = Antares (M1): T= λmax = p. 9

10 General Spectral Classification Questions Note: the completed Figure 3.2 may be useful in answering these questions. 1. Which star is more luminous: Aldebaran or Naos? Why? 2. Which star is cooler: Rigel or Mira? Why? 3. Which star is less luminous: Canopus or Sirius A? Why? 4. Which star is hotter: the Sun or Betelgeuse? Why? 5. Which star is more luminous: the Sun or Betelgeuse? Why? 6. Which star is hotter: K3I or K8V? Why? 7. What is the luminosity class of Algol? 8. What color is Mintaka? p. 10