A stronomy Chapter III Light

1. Astronomy Chapter III Light

2. Can light say something to us? Look at the pictures and explain which gives more energy.

3. Chapter 3 Light We know a lot about the universe, and we also want to learn much more. The universe is so vast(big) that we must travel across immense (extremely large or great) distances to the stars to collect information. The most sophisticated(developed) probes of today can just make their way to the planets of our solar system, but not any further. The distance from our sun to the nearest star -alpha centauri- is about 700.000 times farther than the farthest planet, Pluto. It is impossible to even dream about going to - alphacentauri- with today's technology. Thus, we must think of another way of obtaining the information we need. We can make the information come to us by means of an information carrier. This vital(essential) information carrier is light.

4. Astronomers collect light using different types of telescopes and make conclusions about celestial objects by analysing this light. So the more light that is gathered, the more information we can retrieve(find and bring back). We can obtain the following properties of stars by analysing light gathered from them: distance temperature chemical composition diameter mass nuclear reactions luminosity (brightness) lifetimes (age and remaining lifetime) properties and effects of magnetic field and much more information about the stars. In this chapter we will learn how light reveals(make known) this information about the stars.

5. 3.1 The Electromagnetic Spectrum What we call light is actually the electromagneticradiation. We need to learn about electromagnetic radiation in order to understand why light is the source of such a mass of information about the stars. Light is a form of energy which displays both particle and wave properties. Let's look at the wave properties of light first. Each wave has a wavelength, which is the length of one complete wave, i.e. the distance between one wave crest and the following wave crest; it is denoted by l . The wavelength is inversely proportional to the energy,E, that the light carries i.e. the larger the wavelength, the lower the energy of the wave. where E is the energy in joules h is the Planck constant with the value: 6,62 x 10-34 joule second. lis the wavelength in meters c is the speed of light with the value: 3 x 108 m/s

6. Electromagnetic radiation is categorised according to its wavelength on a chart which is called the electromagneticspectrum.

7. The visible portion of the electromagnetic spectrum is between about 4000 A° and 7000 A° where one Angstrom (A°) equals 10-10 meters. However the radiation may take any wavelength value without maximum or minimum limits. In comparison with the entire spectrum, the visible portion takes very little space on the spectrum. An important concept in electromagnetic waves is frequency. Frequency is the number of waves produced in one second. The unit of frequency is Hertz (Hz), which is 1/second (s-1); and denoted with f. The frequency is inversely proportional to the wavelength. So the energy of a wave is proportional to its frequency.

8. 3.1.a. Continuous Spectrum - Discrete Spectrum A spectrum is a chart upon which the electromagnetic waves are arranged according to their wavelengths. This means that each position on the screen corresponds to a specific wavelength. Light of a certain wavelength can only represent a single line on the screen. If light at all wavelengths is spread out upon a screen, the entire screen will be bright. This kind of spectrum is called a continuousspectrum. If only some lines are bright and the rest of the chart dark in a spectrum, the spectrum is called a discrete (separate ) spectrum.

9. 3.1.b. Emission Spectrum - Absorption Spectrum An emission(sent out)spectrum is a spectrum which has bright lines on a dark background. Bright lines correspond to the emitted wavelengths. If there are dark lines on a continuous, bright background; the spectrum is called an absorption (take in)spectrum. Dark lines correspond to the absorbed wavelength.

10. In other words, an absorption spectrum is a spectrum which has dark lines on a bright background.

11. QUIZ: Tell the differences between continuous, emission and absorption spectra.

12. 3.1.c. Fraunhofer Lines Newton obtained the spectrum of sunlight by passing sunlight through a prism. Fraunhofer observed dark lines across the sun's spectrum, each of which corresponded to a wavelength. The dark lines means that the Sun emits (sends out) less light at these wavelengths than at other wavelengths. The lines on the spectrum of a star provide information about the star; likewise the spectrum as a whole may include some other information.

13. Fraunhofer Lines To obtain all the possible information from a spectrum, astronomers must consider the lines and the whole spectrum separately.

14. 3.2.a. Spectral Lines Spectral lines also carry lots of information about stars. German scientist Bunsen discovered a very important property of matter. Flames of each element emit light only at specific wavelengths. He then noticed that the Fraunhofer lines coincide exactly with the spectral lines of specific elements.

15. Flames of each element emit light only at specific wavelengths which means specific colors. Strontium Barium Lithium Sodium Cupper Calcium

16. Kirchhoff then used this information to show that elements on Earth are also present in the Sun since the wavelengths of emission spectral lines of these elements are exactly the same as the wavelengths of the Fraunhofer lines.

17. In order to understand this concept we need to study the behaviour of the energy levels of atoms. An electron is excited to an upper energy level from a lower one. After a short time, it spontaneously returns to the lower energy level and emits a photon. This emitted photon has an energy exactly equal to the energy difference between the two levels. As the dark lines of the sun's spectrum correspond to specific wavelengths, and as each wavelength corresponds to a specific energy value.

18. Bunsen clarified the fact that these energies are equal to the energies of the excited electrons of specific elements. Thus, it can be concluded that these elements exist in the chemical composition of the Sun. This property is very useful because the chemical compositions of stars can be determined by matching the energy values to the excited levels of specific elements. Kirchhoff’s Laws of Spectral Lines We have noted that the spectrum of the Sun is a continuous spectrum but there are dark lines on this spectrum. We call these lines Fraunhoferlines. How does the Sun produce a continuous spectrum? How are the dark lines produced? Can we come across other types of lines in the spectra of the billions of other stars?

19. The answers were found by Kirchhoff. He developed rules in order to define how the spectra are created. • A gas should be hot and dense in order to produce a continuousspectrum. No lines are observed, light is emitted at all wavelengths. • A hot but rarefied (low density) gas produces an emissionspectrum. Bright lines are produced on a dark background. • A cool gas absorbs light at specific wavelengths and produces an absorptionspectrum when it is placed in front of a continuous spectrum source. Dark lines are produced on a bright background. We can conclude that, in the Sun, there is a cool outer layer of gas around a hot interior core.

20. 3.2. Derivation(Obtaining) of Stellar Properties All objects at temperatures above absolute zero (–273 ºC or 0 ºK) emit radiation called thermal radiation. By analyzing this radiation on a screen, one can obtain a continuous spectrum. Thermal radiation is emitted over a broad range of wavelengths with maximum (peak) intensity at some particular wavelength (Figure 3.9). Stars have continuous spectra because they radiate thermal energy due to their very high temperatures.

21. Scientists discovered relationships between the properties of spectra and the properties of stars. They described these relationshipswith a number of laws. Wien's law: One of these laws was Wien's law which stated that: The absolute temperature isinversely proportional to the wavelength of maximum emission. Stars appear to have different colours because they have difference temperatures. Some stars appear blue, whereas others appear yellow and others are red. It doesn't mean that the star only emits light at only this wavelength, it just means that its maximum emission appears at that wavelength and less at other wavelengths. Blind: unable to see

22. The observed colour indicates the maximum emission value (the peak point of the spectrum of the star). By comparing these colours we are comparing the temperatures of the stars. Since the wavelength of blue light is shorter than that of red, we can conclude that blue stars are hotter than red stars. Yellow stars are cooler than blue stars and hotter than red stars (Figure 3.10). • Blue stars are hotter, thus the wavelength of maximum intensity is lower than that of yellow-white stars. • The wavelength of maximum emission reveals (shows) that the temperature of yellow-white stars is higher than red stars, but lower than blue stars. • The wavelength of maximum emission increases with decrease in temperature.

23. Stefan-Boltzmann law: Another law concerning the spectra of stars is the Stefan-Boltzmann law, whichdescribes the relationship between the total energy emitted per second (luminosity)and the temperature of an object. The Stefan-Boltzmann law states that: The totalenergy emitted per centimetre square is proportional to the fourth power of the temperature. For example if a star is 3 times hotter than another (be sure that the temperatures are measured in Kelvin) the first one will emit 34 = 81 times more energyper centimetre square. This is energy per centimetre square, not luminosity. So we have to consider thetotal surface area of the star which is proportional to R2 (Surface area of a sphereA = 4pR2) for luminosity(brightness). Question: Compare the luminosities of two stars which have equal temperatures if the radius of the first star is r1=2r and the radius of the second star is r2=r. Solution:First sketch a diagram to imagine the stars. Then calculate the surface area of the stars by A = 4pR2. A1 A2 r2 =r r1=2r

24. The third law, is the inverse square law which states that, the observed brightnessof an object is inversely proportional to the distance of the object from the pointof observation. For example an object appears 4 times fainter when we observe itat twice the distant away from the first point of observation.

25. Combining the Stefan-Boltzmann law and the inverse square law; the energyreceived from a star depends upon • its temperature, • radius and • distance away from the point of observation. That is why the Sun appears much brighter than all otherstars in the universe even though many of them are millions of times brighterthanthe Sun.

26. 3.2.b. Radial Velocity The radial velocity of a star with respect to the Earth is another property which can be determined by analysing the spectral lines. Radial velocity is the rotational velocity towards or away from the Earth. The wavelength of a receding (go or move back ) source is observed to be longer than its real wavelength and the wavelength of an approaching(come near or nearer to ) source is observed to be shorter than it really is. This phenomenon of light is called the Doppler Effect. We cannot measure the exact value of the velocity but we can measure the radial velocity using the Doppler Effect. Now remember that each line on the spectrum corresponds to a specific wavelength.

27. If the wavelength is shorter it means there has been a shift of the spectral lines to the blue side of the spectrum; and If the wavelength is longer it means there has been a shift in wavelength to the red side of the spectrum. This is also valid for stars. The spectrum of a receding(leaving) star is observed to be 'red shifted' and the spectrum of an approaching-star is observed to be 'blue shifted'. As a result; if the spectrum of a star is observed to be red shifted it is concluded that the star is receding; and if the spectrum of a star is observed to be blue shifted, it is understood that the star is approaching the point of observation.

28. Doppler Effect Shorter wavelength Longer wavelength

29. The amount of change in the wavelength indicates the velocity. So we can determine not only the type of motion (receding or approaching) but also the magnitude of the velocity. The formula is given by: Where v is the radial velocity, Dl is the difference between the observed and actual wavelengths l is the expected wavelength (assuming that the object has no radial velocity) c is the speed of light. If the observed wavelength is smaller than the expected value (blueshifted) X will be negative; thus, the velocity will be negative which means a motion towards the observer.

30. 3.2.c. Brightness Hipparchus, an early astronomer, classified the stars according to their brightness. He gave them numbers from 1 to 6 which are called the magnitude of the stars. The brightest stars were of magnitude 1, whereas the faintest are of magnitude 6. His classification was made according to the brightness's observed by eye. However, some stars caused confusion since they were between two consecutive magnitudes. Hipparchus could see up to only a magnitude of 6. However, astronomers today can take photographs of stars whose magnitudes are 32. That is, 32 – 6 = 26 magnitudes lower. This means that today's telescopes can detect stars over 2.5 x 1010 times fainter than those observed with the naked eye.

31. 3.2.d. Spectral Type After the the re-definition of the magnitudes, the stars were classified according to the maximum emitted wavelength in their spectra. Wien's law shows that maximum emission wavelength is directly related to the surface temperature. The hottest group belongs to O-type stars. The other groups are O, B, A, F, G, K, M in order from the hottest to the coolest. O-type stars have a surface temperature of about 50.000 K whereas the surface temperatures of M type stars are about 2000 to 3000 K. This classification is detailed further and each group has 10 sub-types A0, A1, A2 etc. The Sun is a G2 star with a surface temperature of 6000 K.

32. 3.2.e. Hertzsprung - Russell (H-R) Diagram Two astronomers Ejnar Hertzsprung and Henry Norris Russell simultaneously found the relationship between the luminosity and spectral type. They plotted the graph of luminosity versus spectral class and observed that, most of the stars lie on a line from upper left to lower right. The stars on the main line are called main sequence stars. Some stars are present in the upper right corner of the H-R diagram. They have high luminosity but low surface temperature compared with main sequence stars. Stars of the same surface temperatures will have the same luminosity if their dimensions are equal. We can conclude that the reason for the high luminosity is the large surface area. These stars are called Red giants and Super giants. They have larger surface areas compared to main sequence stars. Visual absolute magnitude Spectral Class

33. 3.2.f. Distance All objects are seen on the same spherical surface. This surface is called the celestial sphere. The depth of an object cannot be determined. However there are some methods available to calculate the distances of the stars. The distant stars are our background and the closer stars our objects to be measured. Astronomers measure the angle between two observation points, half of this angle is the parallax angle. Let A and B be the actual positions of two stars. The stars are observed to be at position A` and B`.

35. Questions 1. What is the importance of light in astronomy? 2. Which properties of stars can be ascertained by analysing the spectrum? 3. What is wavelength? What is the relationship between the wavelength and the energy of light? 4. What are the minimum and the maximum wavelengths of the electromagnetic spectrum? 5. Write down the electromagnetic waves from the smallest wavelength to the largest wavelength. 6. What are the regions of the electromagnetic spectrum? 7. What is the use of the relationship between Bunsen's discovery and Fraunhofer lines for astronomers? 8. Which laws help astronomers analyse whole spectra and derive stellar properties? 9. What information do we obtain from the spectrum of the Sun according to Kirchoff's rules of spectral lines? 10. Why can't the Doppler shift tell us the true velocity of a star? 11. Which is brighter: a star of magnitude 2 or a star of magnitude 5? 12. What is the spectral type of the Sun? 13. Where in a H-R diagram is the Sun located?