Saturday, June 26, 2021


Stars emit light over the entire electromagnetic spectrum, from the X- ray to the radio. Dark absorption lines are produced by atoms at the cooler atmosphere (their electrons are in the ground state) they are excited, absorb radiation of  specific wavelength, and 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 the dark features in the spectrum. These lines are dimmer than the wavelength region around them.

Stars  are not perfect blackbodies. However, the spectrum of a star is close enough to the standard blackbody spectrum, thus we can use Wien's Law to calculate its surface temperature. Wien's Law tells us that that the objects of different temperatures emit spectra that peak at different wavelengths. Hotter objects emit most of their radiation at shorter wavelengths ; hence will appear to be bluer.

Let's calculate the  temperature of the Sun's surface. The peak wavelength  at a solar spectrum  is green 501.7 nm.

T=2898000 /501.7 

  =5776 K.

Each element in the periodic table can appear in gaseous form and will produce a series of bright lines unique to the element. For example, sodium has two prominent yellow lines, the so called D lines at 589.0 and 589.6 nm. Any sample that contains sodium (as table salt) will produce these lines. The same substance can either produce emission lines, bright lines, when a hot gas is emitting its own light or absorption lines, dark lines, when  light from a brighter and usually hotter source, is shone through it. The chemical composition  of a star can be determined through spectroscopy. When we measure the spectrum of a star, we determine the wavelength of each of its lines. If the star is not moving with respect to the Sun, then the wavelength  corresponding to each element will be the same as those we measure in a laboratory here on Earth. But if star is moving toward or away from us, we must consider the Doppler effect. If the star is moving away from us, its lines will appear in longer wavelengths (redshift). If the star is moving toward us, it's lines will be observed at shorter wavelengths (blueshift). The greater the shift, the faster the star is moving. Such motion, along the line of sight between the star and observer on Earth is called radial velocity and is measured in km per second. 

William Huggins, in 1868 made the first radial velocity determination of a star. He observed the Doppler shift of the hydrogen lines in the spectrum of Sirius and found that this star is moving toward the Solar system. The absorption spectrum is the exact  inverse of the emission spectrum of the same element. 

Light is the result of electrons moving between defined energy levels in an atom. When energy is absorbed by electrons of an atom, electrons move from lower energy levels to higher energy levels, these excited states are unstable and electrons have to radiate energy  to return to ground states. The emission spectrum is formed by  frequencies of this emitted light. Emission spectrum has different colored lines. To get an absorption spectrum just shine white light on a sample analyzed. White light is made up of all wavelengths of visible light put together. In the absorption spectrum will be gaps or dark lines. The dark lines correspond to the frequencies of light that have been absorbed by the gas. As the photons of light are absorbed by electrons, the electrons move into a higher energy level. This is the opposite of emission. The dark lines, absorption lines, correspond to the frequencies of the emission spectrum of the same element. When a gas is cool, it absorbs the same wavelengths of light as it would emit when it is hot. The energy of the photon corresponds to the difference in energy between two energy levels. Line emission spectrum is  unique to a particular element, it is  like a fingerprint and it can be used to identify the element present.

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