Astronomy

Seen at a glance with naked eye, all stars in the night sky have the same color. All looked to be like small white dot. Though, if we observe more accurately with the telescope, we can see that each star have its own color. It can be seen red, blue, or even yellow. Bluer star meant it have higher temperature. Blue stars typically have temperature around 30.000°C. Cooler stars with temperature of 6.000°C have red color. Our sun is yellow stars with intermediate temperature, about 6.000°C.

The information we get from a star will be more complete if used telescope is also provided with the special prism to elaborate the perceived starlight spectra. From its light spectra, we can obtain more complete picture concerning that star; for example, hitting the chemical composition, rotation, and its movement. Spectra of a star are very important for astronomer. They used it as one of the way to classify the star and also determine its movement.

Under the prism, star spectra have the light and dark lines which are ordinary to be referred as the absorption or emission lines. Where do the lines come from?

Atom of an element consists of the atomic nuclei encircled by electrons. Electrons have various kinds of level of energy, and can move from one level to another. Dark line, or absorption line, came from electron transition process, when atomic electron move to higher level of energy as it absorb energy. On the contrary, when it move to lower level, the electron will transmit energy and as a result, releasing the emission lines, the light one. Emission and absorption lines for every atom is very typical (always happened at certain wavelengths), so that, by analyzing the spectra, astronomer can easily determine which element that dominant in a star.

Using the Doppler Effect, astronomer can also determine the movement direction of a star. In short, if a star moved toward us, the spectra will shift up at blue wavelength (known as “blue shift”). Otherwise, if it moves away from us, its spectra are shift up to red wavelength (it call “red shift”). By comparing the spectra from a star with the standard spectra made in laboratory, astronomers can determine how far the spectra have shifted, as consequence by the Doppler Effect. An amount friction of spectral line is then used to determine the stars’ movement speed in our line of sight. Speed component in the line of sight direction is called radial speed. Radial speed itself actually not the true speed of star, hence it is relative speed from direction of our line of sight. The true speed of star (called space speed) can be obtained only if its proper motion, radial speed, and distance have been known (We will discuss it in upcoming posts).

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