Basics of Astronomical Spectroscopy
‐‐ Most powerful weapon in the astrophysical arsenal
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EM Spectrum
EM Spectrum
Methodology
• To get the spectrum of a light source, one uses p
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either the phenomenon of refraction
means of a glass prism, or the phenomenon of diffraction by means of a grating.
by means of a grating
Let us take the image of the Sun as light source supplied by a telescope Different kinds of spectra can be obtained
by a telescope. Different kinds of spectra can be obtained according to the way by which we proceed: A. A spectrum from the complete image (incoherent source): one obtains a colored band with familiar colors but without any detail
B. Place a very fine slit in front of the image of the Sun (coherent source). If
image of the Sun (coherent source). If the focusing is done on this slit, the spectrum shows then a lot of more or less dark lines The spectrum can be
less dark lines. The spectrum can be then considered as the juxtaposition of a multitude of colored images of the entrance slit. t
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C. If the image of the Sun and the entrance slit of the spectroscope are in the same plane, correctly focusing, we can then perceive details in this fine slice of Sun. Spectrograph with Slit Layout
Spectrograph with Slit Layout
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Light from telescope supplying a real image of the object to be studied
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An entrance slit placed on the focal plane (Case C in the previous slide)
A collimator lens allowing to make parallel the beam coming from the entrance slit
A diffraction grating resulting in interference patterns
A camera mirror to form the real image of the diffracted beam (spectrum)
A detector placed at the camera focus plane for observation (spectroscopy) or A detector
placed at the camera focus plane for observation (spectroscopy) or
recording (spectrography)
Fraunhofer Lines
• The
The dark lines which streak the colored continuum of the spectrum are dark lines which streak the colored continuum of the spectrum are
provoked by gases situated between the photosphere and the observer. Every chemical element, by absorbing a very accurate quantity of energy, is going to leave its signature in the form of characteristic dark lines. Among these lines, some are due to the sun atmosphere and the others to the Earth atmosphere. p
• Fraunhofer, in 1814, indicated the main lines by letters. We still use some of these notations today; for example, we say “K line” about i i dC l i
ionized Calcium line or “D lines” about Sodium doublet.
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magnesium lines (triplet)
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sodium lines (doublet)
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Band of oxygen (telluric)
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How do we distinguish the earth and sun spectral lines? Continuous Spectrum
Continuous Spectrum
• Solids, liquids, and dense gases emit light of all g
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wavelengths, without any gaps. We call this a continuous spectrum. Emission Spectrum
Emission Spectrum
• Thin gases emit light of only a few g
or wavelengths. We call this an emission
bright line spectrum. NGC6543, also known as the Cat's Eye Nebula, is the type of nebula which our Sun will b l hi h
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produce as nuclear fusion slows and then dies in its core, and it expels its outer layers of gas
expels its outer layers of gas.
Spectroscopic pioneer William Huggins (1824‐1990) is the fi
first who pointed spectrographs at nebula: h
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“On the evening of the 29th of August, 1864, I directed the telescope for the first time to a planetary nebula in Draco (NGC 6543). The reader may now be able to
time to a planetary nebula in Draco (NGC 6543). The reader may now be able to picture to himself to some extent the feeling of excited suspense, mingled with a degree of awe, with which, after a few moments of hesitation, I put my eye to the spectroscope Was I not about to look into a secret place of creation? I
the spectroscope. Was I not about to look into a secret place of creation? I looked into the spectroscope. No spectrum such as I expected ! A single bright line only!“ Absorption Spectrum
Absorption Spectrum
• If there is a source of light behind it, a thin gas g
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will absorb light of the same wavelengths it emits. We call this an absorption or dark line spectrum. spectrum
Scientists who studied the solar spectrum were able to identify a number of elements in the Sun by matching solar absorption lines to absorption lines formed in the lab. Some of the solar lines didn't match any found in the lab. Among these was an absorption feature in the yellow portion of the spectrum During a solar eclipse spectra of the solar
spectrum. During a solar eclipse, spectra of the solar chromosphere showed this line in emission at 5876 A
Story of Helium
Story of Helium
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In 1868, English astronomer Joseph Norman Lockyer
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guessed that this line was d h hi li
due to an element which hadn't yet been discovered on Earth: he suggested the name "Helium", from the Greek word for the Sun. Most other astronomers th
thought he was wrong, and the yellow line simply due to some common element ht h
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in an unusual energy state. In1895, William Ramsey isolated small amounts of an inert gas after processing uranium compounds. This new gas had a line at exactly the same wavelength as i
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Lockyer had observed in the Sun ‐‐ and so it was given the name proposed for it almost three decades earlier. Joseph Norman Lockyer
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evidently was a bit of an egotist, judging by this little d l
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poem written by one of his contemporaries: And Lockyer, and Lockyer, grows cockier, and cockier, ki
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for he thinks he's the owner of the solar corona
Parahelium
(S=0, singlet state)
Orthohelium
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(S=1, triplet state)
• So, in the typical stellar spectrum, there must be a dense, hot source of light in the background (to create the continuum), with a tenuous cooler layer of gas above it (to create the dark absorption
tenuous, cooler layer of gas above it (to create the dark absorption lines). We call this outer layer the photosphere.
The End
The End