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Answer to q. 1
In a visual binary system, the two stars are separately visible in the telescope and astronomers can watch the stars orbit each other over periods of years or decades. From that, astronomers can find the orbital period and, if the distance of the system from Earth can be found, the size of the orbits. That is enough to find the masses of the stars.If the stars in a binary system are close together, a telescopic view, limited by diffraction and atmospheric seeing, shows a single point of light. Only by looking at a spectrum, which is formed by light from both stars and contains spectral lines from both, can astronomers tell that there are two stars present and not one. Such a system is a spectroscopic binary system.Although spectroscopic binaries are very common, they are not as useful as visual binaries. Astronomers can find the orbital period easily, but they can't find the true size of the orbits because there is no way to find the angle at which the orbits are tipped. That means they can't find the true masses of a spectroscopic binary. All they can find is a lower limit to the masses. If the plane of the orbits is nearly edge-on to Earth, then the stars can cross in front of each other as seen from Earth. When one star moves in front of the other, it blocks some of the light, and the star is eclipsed. Such a system is called an eclipsing binary system.
Answer to q. 2
Hot stars emit a great deal of ultraviolet radiation that you can't see, and cool stars emit plenty of infrared radiation. To add in the energy you can't see, astronomers make a mathematical correction that depends on the temperature of the star. With that correction, astronomers can find the total electromagnetic energy output of a star, which they refer to as its luminosity (L).Astronomers know the luminosity of the Sun because they can send satellites above Earth's atmosphere and measure the amount of energy arriving from the Sun, adding up radiation of every wavelength, including the types blocked by the atmosphere. Of course, they also know the distance from Earth to the Sun very accurately, which is necessary to calculate luminosity. The luminosity of the Sun is about 4 x 1026watts (joules per second).
Answer to q. 3
In an eclipsing binary system, if the plane of the orbits is nearly edge-on to Earth, then the stars can cross in front of each other as seen from Earth. When one star moves in front of the other, it blocks some of the light, and the star is eclipsed. Such a system is called an eclipsing binary system. Seen from Earth, the two stars are not visible separately. The system looks like a single point of light. But, when one star moves in front of the other star, part of the light is blocked, and the total brightness of the point of light decreases. A light curve is a graph of brightness versus time commonly used in analyzing variable stars and eclipsing binaries.The light curves of eclipsing binary systems contain plenty of information about the stars, but the curves can be difficult to analyze. A light curve shows how long it takes for the stars to cross in front of each other, and multiplying these time intervals by the orbital speeds gives the diameters of the stars. There are complications due to the inclination and eccentricity of orbits, but often these effects can be taken into account, so observations of an eclipsing binary system can directly tell you not only the masses of its stars but also their diameters.
Answer to q. 4
A survey on the family of stars could reveal the following characteristics:a. Taking a survey is difficult because you must be sure to get an honest sample. If you don't survey enough stars, or if you miss some types of stars, your results can be biased.b. M dwarfs and white dwarfs are so faint they are difficult to find even near Earth and may be undercounted in surveys.c. Luminous stars, although they are rare, are easily visible even at great distances. Typical nearby stars have lower luminosity than our Sun.
Answer to q. 5
The set of star types, called the spectral sequence, is important because it is a temperature sequence. The O stars are the hottest, and the temperature continues to decrease down to the M stars, the coolest. For further precision, astronomers divide each spectral class into 10 subclasses. For example, spectral class A consists of the subclasses A0, A1, A2, . . . A8, A9. Next come F0, F1, F2, and so on. These finer divisions define a star's temperature to a precision of about 5 percent.The study of spectral types is more than a century old, but astronomers continue to discover and define new types. The L dwarfs, found in 1998, are cooler and fainter than M stars. They are understood to be objects smaller than stars but larger than planets and are called brown dwarfs. The spectra of M stars contain bands produced by metal oxides such as titanium oxide (TiO), but L dwarf spectra contain bands produced by molecules such as iron hydride (FeH). The T dwarfs are an even cooler and fainter type of brown dwarf than L dwarfs. Their spectra show absorption by methane (CH4 ) and water vapor.
Answer to q. 6
A
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