A spectrum of an actual star is shown on the right. In addition to the continuous spectrum, a star's spectrum includes a number of dark lines absorption lines.
Absorption lines are produced by atoms whose electrons absorb light at a specific wavelength, causing the electrons to move from a lower energy level to a higher one. This process removes some of the continuum being produced by the star and results in dark features in the spectrum. In the actual stellar spectrum, shown above on the right, notice how the underlying shape the continuum is a thermal radiation curve with roughly the same peak as the spectrum on the left.
However, we can learn a lot more from the spectral lines than from the continuum. Two very important things we can learn from spectral lines is the chemical composition of objects in space and their motions. During the first half of the 19th century, scientists such as John Herschel, Fox Talbot, and William Swan studied the spectra of different chemical elements in flames.
Since then, the idea that each element produces a set of characteristic emission lines has become well-established. Each element has several prominent, and many lesser, emission lines in a characteristic pattern. Sodium, for example, has two prominent yellow lines the so-called D lines at The studies of the Sun's spectrum revealed absorption lines, rather than emission lines dark lines against the brighter continuum.
The precise origin of these 'Fraunhofer lines' as we call them today remained in doubt for many years, until Gustav Kirchhoff, in , announced that the same substance can either produce emission lines when a hot gas is emitting its own light or absorption lines when a light from a brighter, and usually hotter, source is shone through it. With that discovery, scientists had the means to determine the chemical composition of stars through spectroscopy. Stars aren't the only objects for which we can identify chemical elements.
Any spectrum from any object allows us to look for the signatures of elements. But humans experience the color of the sky from sunrise to noon as: red, orange, yellowish-white, white, blue. The difference between the scientific version of the sky's color and what humans experience are due to three facts: 1 our eyes can't see ultraviolet or infrared, 2 our eyes can't see violet very well, and 3 our eyes experience a nearly even mix of all colors as white.
In terms of how we experience the sky, the color spectrum seems to start at red and end at blue. The same goes for incandescent flames such as in a campfire or on a candle. Scientifically, flame colors go from coldest to hottest in the order of: infrared, red, orange, yellow, green, blue, violet, ultraviolet.
But humans experience the color of flames from coldest to hottest as red, orange, yellow-white, white, blue. From everyday experience, blue seems to be the upper end of the visible color spectrum, even though we can technically see violet. That is why an upwards Doppler shift is called a blue shift. A "violet shift" would therefore mean the same thing as a blue shift if the phrase were ever used: an upwards shift.
Use of the Doppler shift has allowed astronomers to make some interesting observations. On average, the light from all stars outside our local group of galaxies is red shifted.
Also, the farther away a star is, the more its light is red shifted. This fact indicates that our universe is expanding and all of the stars outside our local group of galaxies are moving away from us. Also, when a star rotates, one edge of the star is moving towards us relative to its center while the other edge is moving away.
As a result, light from one edge of a star is slightly red shifted while light from the other edge is slightly blue shifted.
Astronomers can use these two shifts in order to calculate how fast a star is rotating. The same approach can be used to calculate how fast a galaxy is rotating. Topics: Doppler shift , blue shift , color , expanding universe , light , movement , red shift , violet shift , wave , waves.
A blue shift does not mean that the object ends up blue.
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