بسم الله الرحمن الرحيم محاضرة الاربعاء والخميس 23-24-2011 Chapter 5 Light The Cosmic Messenger

1. بسم الله الرحمن الرحيم محاضرة الاربعاء والخميس 23-24-2011 Chapter 5 LightThe Cosmic Messenger

2. The Two Laws of Thermal Radiation • Law 1 (Stefan-Boltzmann law): Each square meter of a hotter object’s surface emits more light at all wavelengths. • قانون ستيفان بوتلزمان • ينص قانون ستيفان بولتزمان على أن الطاقة الكاملة المنبعثة من الجسم الأسود (واط) في الثانية لكل وحدة مساحة تتناسب مع القوة الرابعة لدرجة حرارة الجسم كلفن. • E(T) = σ. T4 • حيث σ ثابت ستيفان-بولتزمان • σ =5.67.10-8 W/m2.K4

3. Law 2 (Wien’s law ) Hotter objects emit photons with a higher average energy, which means a shorter average wavelength. That is why the peaks of the spectra are at shorter wavelengths for hotter objects. For example, the peak for the 15,000 K star is in ultraviolet light, the peak for the 5800 K Sun is in visible light, and the peak for the 3000 K star is in the infrared. قانون فين للانزياح يقول أن طول الموجة العظمي لإشعاع الجسم الاسود تتناسب تناسبا عكسيا مع درجة حرارته : b=2.897786.10-3(K.m): انظر الشكل التالي

6. How does light tell us the speedof a distant object? Spectral lines shift to shorter wavelengths when an object is moving toward us, and to longer wavelengths when an object is moving away from us. Doppler Effect then:Object moves away from us, longer wavelength (redshifted) Object moves towards us, shorter wavelength (blueshifted)

12. How do telescopes help us learnabout the universe? • The Two Key Properties of a Telescope • Light-collecting area • Its angular resolution.

13. Light-collecting area The light-collecting area tells us how much total light the telescope can collect at one time. Telescopes are generally round, so we usually characterize a telescope’s “size” as the diameter of its light-collecting area. For example, a “10-meter telescope” has a light-collecting area that is 10 meters in diameter. Such a telescope has a light-collecting area more than a million times that of the human eye. Because area is proportional to the square of a telescope’s diameter, a relatively small increase in diameter can mean a big increase in light-collecting area. A 10-meter telescope has five times the diameter of a 2-meter telescope, so its light collecting area is 25 times as great.

14. Its angular resolution Angular resolution is the smallest angle over which we can tell that two dots—or two stars—are distinct. For example, the human eye has an angular resolution of about arcminute, meaning that two stars can appear distinct only if they have at least this much angular separation in the sky. If the stars are separated by less than arcminute, our eyes will not be able to distinguish them individually and they will appear as a single star.

15. للمزيد عن دراسة المناظير يرجي الاطلاع على الفصل السادس من كتاب مبادئ علم الفلك الحديثةصفحة 156للمؤلف أ.د. عبد العزيز بري أحمد

17. How is technology revolutionizing astronomy? Adaptive optics allows ground-based telescopes to overcome atmospheric distortion.

18. This chapter’s main purpose was to show how we learn about theuniverse by observing the light of distant objects. “Big picture” ideasthat will help you keep your understanding in perspective include thefollowing:• Light and matter interact in ways that allow matter to leave“fingerprints” on light. We can learn a great deal about the objectswe observe by carefully analyzing their light. Most of what weknow about the universe comes from information that we receivefrom light.• The visible light that our eyes can see is only a small portion of thecomplete electromagnetic spectrum. Different portions of the spectrummay contain different pieces of the story of a distant object, soit is important to study all forms of light.• There is far more to light than meets the eye. By dispersing the lightof a distant object into a spectrum, we can determine the object’scomposition, surface temperature, motion toward or away from us,and more.• Technology drives astronomical discovery. Every time we builda bigger telescope, develop a more sensitive detector, or open upa new wavelength region to study, we learn more about theuniverse.