Astronomy

How is spectroscopy used to deduce what an object is made of?

How is spectroscopy used to deduce what an object is made of?


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Spectroscopy is an analysis of light (or other EM wavelengths) that is often used by scientists to examine what an object is made from or contains.

Apologies if this is a stupid question, but this sounds effectively like saying “this thing is absorbing this wavelength or colour, therefore it is this substance”.

I'm confused as to how this works in practice. As a familliar example: light bouncing off leaves in summer can be a completely different colour to leaves in winter. Water when frozen looks to be a different colour to when it is liquid. Copper is shiny brownish unless it gets a bit rusty, then it's bright green, etc etc.

Is it possible to explain how we can say with any amount of certainty that a given set of dips in EM wavelengths translates into identification of substances?


This is not exclusive to spectroscopy applied to astronomy but general.

Matter can interact with electromagnetic waves spanning a very wide range of frequency (energy). Also matter can emit electromagnetic radiation when in a kind of excited state.

Due to the internal mechanism of absorption/emission it happens that the spectral characteristics can be peculiar to the chemical nature and physical state of the matter under analysis.

This is basically what you are aware of. Your concern likely arises by over-focusing on colour, that per se in not the most powerful identification tool, or even by thinking that the particular state of the matter does not count, or changes are neglected.

Let me take your example of leaves. The fact that they colour undergoes changes does actually means that their composition changes. This is already an information rather than a problem.

Imagine that observing a planet one detect, say, carbon monoxide, it does not mean that its atmosphere won't change. Eventually different observations would prompt a planetologist to ask himself why, perhaps concluding that a kind of volcanic activity or degassing is ongoing , just to say.

A planet looking green then brown on a regular pattern would certainly suggest the presence of plants or anyway photosynthesis based on chlorophyll, providing that the green spectrum closely resembles that one we know, and does not come from narrow spectral features that would otherwise indicate, for instance, the presence of chlorine.

In other words, probing a sample, in astronomy as well as on a lab bench, leads to information about that sample in that moment. This is quite philosophical rather than a problem in spectroscopy or spectroscopy applied to astronomy. Moreover spectroscopy doesn't come alone and certainly requires considering the scenario.

But at its core is the existence of peculiar features, they can be single lines or more or less complicated spectra, and those are linked to the composition of the sample, elemental or molecular depending on the frequency window used.

I am not sure if this answer your question, but again this passage

"this thing is absorbing this wavelength or colour, therefore it is this substance”

is both correct and wrong. It must be taken with a grain of salt, or things must be analysed in depth.

What is certainly true is that there are specific lines typical of elements, or other spectral features typical of molecules. Explaining why it is so is way more complicated (for me) to be done in few lines here. But the reason resides on the discrete electronic structure of matter, as well as molecular shapes combined with limitations on which transitions can take places. Combining the two leads to a high specificity.

Unfortunately not the best example for astronomy, but consider that a given compound virtually has its own IR vibrational spectrum, different from that of any other compound!

You might want to read about spectroscopy in general, and have a look at Wikipedia's Astronomical spectroscopy; Chemical properties


Reducing errors in X-ray photoelectron spectroscopy

Grzegorz Greczynski, senior lecturer in the Department of Physics, Chemistry and Biology at Linköping University. Credit: Thor Balkhed

X-ray photoelectron spectroscopy (XPS) is often used to determine the chemical composition of materials. It was developed in the 1960s and is accepted as a standard method in materials science. Researchers at Linköping University, Sweden, however, have shown that the method is often used erroneously.

"It is, of course, an ideal in research that the methods used are critically examined, but it seems that a couple of generations of researchers have failed to take seriously early signals that the calibration method was deficient. This was the case also for a long time in our own research group. Now, however, we hope that XPS will be used with greater care," says Grzegorz Greczynski, senior lecturer in the Department of Physics, Chemistry and Biology at Linköping University.

Greczynski and his colleague in the department, Professor Lars Hultman, have shown that XPS can give misleading analysis results due to an erroneous assumption during calibration. The results have been published in Scientific Reports.

XPS is used to determine the chemical composition of materials. It is a standard method in materials science, and more than 12,000 scientific articles with results obtained by XPS are published each year. The technique was developed during the 1960s by Professor Kai Siegbahn at Uppsala University to become a useful and powerful method for chemical analysis, and the work led to him being awarded the Nobel Prize in physics in 1981.

"The pioneering work that led to the Nobel Prize is not in question here. When the technique was developed initially, the error was comparatively small, due to the low calibration precision of the spectrometers used at the time. However, as spectroscopy developed rapidly and spread to other scientific fields, the instruments have been improved to such a degree that the underlying error has grown into a significant obstacle to future development," says Hultman.

What the researchers have discovered is that the original method is being used wrongly, through an erroneous assumption used in the calibration process. When calibrating the experiment, the signal from elemental carbon accumulating on the sample surface is often used.

It turns out that the carbon-based compounds that are formed naturally by condensation onto most samples give rise to signals that depend on the surroundings and the substrate onto which they are attached, in other words, the sample itself. This means that a unique signal is not produced, and large errors arise when a more or less arbitrary value is used as reference to calibrate the measuring instrument.

Criticism against the method was raised as early as the 1970s and 1980s. After that, however, knowledge about the error fell into oblivion. Hultman suggests that several factors have interacted to allow the error to pass unnoticed for nearly 40 years. He believes that the dramatic increase in the number of journals that digital publishing has made possible is one such factor, while deficient reviewing procedures are another.

"Not only has a rapidly growing number of scientists failed to be critical, it seems that there is a form of carelessness among editors and reviewers for the scientific journals. This has led to the publishing of interpretations of data that are clearly in conflict with basic physics. You could call it a perfect storm. It's likely that the same type of problem with deficient critical assessment of methods is present in several scientific disciplines, and in the long term this risks damaging research credibility," says Hultman.

Greczynski hopes that their discovery can not only continue to improve the XPS technique, but also contribute to a more critical approach within the research community.

"Our experiments show that the most popular calibration method leads to nonsense results and the use of it should therefore be terminated. There exist alternatives that can give more reliable calibration, but these are more demanding of the user and some need refinement. However, it is just as important to encourage people to be critical of established methods in lab discussions, in the development department, and in seminars," Greczynski concludes.


What causes these lines?

Atoms consist of protons, neutrons, and electrons. Protons are positively charged, electrons are negative, and neutrons have no charge (electrically neutral). Danish physicist Niels Bohr devised a model of the atom which helps explain absorption and emission lines. In his model, protons and neutrons are in the nucleus, the electrons orbit the nucleus. In the Bohr model electrons are only allowed to orbit at certain distances from the nucleus, much in the same way as planets can only orbit the sun at certain distances. The further away from the nucleus, the more energy is needed. Each of these "distances" is called an energy level. Electrons can move between energy levels, but it does require an exchange of energy.

When we discuss the energy of a photon we can also talk about the wavelength since the two are related. The energy required is determined by the energy difference between the two levels and is different for every energy level and every element. Combining elements into molecules also changes the energy requirements.

The energy (E) of a photon (in Joules) is given by the formula:


Equation 29 - Energy of a Photon

Where h is the Planck constant (6.624 x 10 -34 joule-sec) and the frequency (f) is a function of wavelength (&lambda). Frequency is given by the formula below:


Equation 30 - Frequency of Light

Where c is the speed of light (3x10 8 ms -1 ) and &lambda it's wavelength in hertz.

For an electron to move to a higher energy level it must gain energy. One way is to absorb a photon having the right amount of energy. When the electron absorbs the photon the corresponding wavelength appears to be missing from the spectrum because it has been absorbed.

When electrons move to a lower energy level it releases the same amount of energy. This causes an emission line.

Energy levels are generally noted as n, the first energy level being n = 2 (n = 1 for the nucleus). A jump from n = 2 to n = 3 requires an absorption of energy, while moving from n = 3 to n = 2 releases it.

Going back to our hydrogen example, when it gains energy from a photon in the sun an electron makes the jump from n = 2 to n = 3 and an absorption line is formed. In this case light of 656.3nm (red). When we heat hydrogen in a burner we actually excite the electron with energy, then it releases it again. As the electron returns to n = 2 it emits the same amount of energy and we see an emission at 656.3nm.

Electrons can jump from n = 2 to n = 3, or to n = 4, 5 and so on. The amount of energy required is summarised in the table below for hydrogen. This is also known as the Balmer Series.

Transition of n3→4𔾶5𔾶6𔾶7𔾶8𔾶9𔾶&infin𔾶
NameH-&alphaH-&betaH-&gammaH-&deltaH-&epsilonH-&zetaH-&eta
Wavelength (nm)656.3486.1434.1410.2397.0388.9383.5364.6
ColorRedBlue-greenVioletVioletUltravioletUltravioletUltravioletUltraviolet

Each different element has it's own unique energy levels and when an elemental atom is combined in a molecule the energy levels again change. Because of this, we can use spectroscopy to identify almost any element or compound.

This post is part of the series Solar Physics. Use the links below to advance to the next tutorial in the couse, or go back and see the previous in the tutorial series.


How is spectroscopy used to deduce what an object is made of? - Astronomy

A laboratory spectroscope is an instrument that spreads out the spectrum of a light source, typically by means of a prism or a diffraction grating. The idea for such an instrument was developed by the famous glass-worker Joseph Fraunhofer in the second decade of the 19th century. With his spectrometer he was able to see detail in the solar spectrum that isn&rsquot visible in such naturally occurring phenomena as rainbows, detail that even Isaac Newton had missed during his experiments with prisms. As later spectroscopists found out, the detail was in the form of dark absorption lines, characteristic of the elements present in the outer layers of the Sun and characteristic of their state of ionisation. This work opened the window of stellar spectroscopy.

Stellar spectroscopy turns out to be an astonishingly powerful tool. It achieves what nineteenth century philosophers and indeed non-astronomers thought would be impossible, namely telling what stars are made of. Through spectroscopy one can deduce the composition of a star, its temperature, the presence on its surface of strong magnetic fields, even its speed of movement towards or away from us and, for very distant objects, their rough distance away. This list is not exhaustive. Spectroscopy is in a way the astronomer&rsquos microscope, revealing detail far beyond that accessible to our unaided senses. One unsung result of spectroscopy is that the stars for as far as we can see, and that is a very long way indeed, are made of the same elements and particles as are found on Earth. The strangeness of the heavens is the variety of ways these ingredients are organised, making Earth a rather special place.

Since motion of a star has a small effect on the positions of its spectral lines, binary stars show a well-defined periodic motion of their spectral detail as they orbit each other. Many binaries &lsquogive themselves away&rsquo by showing doubled spectroscopic lines even if we don&rsquot see two stars. This allows the period of their orbit around each other to be deduced, the eccentricity of their orbits and at least the ratio of their masses. Since the mid-1990s, the same general technique applied to the lines from a single star has been used to detect orbiting planets. It is the most reliable technique for identifying &lsquoextra-solar planets&rsquo.

There are broadly two spectroscopic techniques, both begun in the nineteenth century. Placing a large narrow-angle prism in front of the objective lens of a refracting telescope creates a spectrum for every object seen. Narrow angle prisms don&rsquot have much dispersion so the spectrum does not show fine detail. Moreover, if many stars appear close together in the image, their spectra will overlap. Nonetheless, this method provides far more detail than simple observations of the colour of a star and enables stars to be classified into a range of types. At first the significance of the classification wasn&rsquot apparent but in the twentieth century the classification was shown to be related to their temperature and to the evolution of stars over time.

The second method is to place the spectrometer at the eyepiece end of the telescope and display and record the spectra of individual stars or galaxies. This can be done with spectrometers that give a much wider spread to the spectrum than objective prisms and hence show more detail. A development of this technique at the end of the twentieth century was to position fibre-optic receptors at the known position of stars or galaxies in the final image plane. The fibre-optics transmit the signal to individual channels in a display that records each spectrum digitally. In the twenty-first century version, many hundreds of spectra are obtained simultaneously as the telescope follows the rotation of one patch of sky and the fibre-optics are then manoeuvred under computer control when a new image is selected. It is this technique in particular that allows astronomers to plot the large-scale structure of the universe, for each distant galaxy in a field of view reveals how far away it is through the wavelength of its spectral lines.


What is a spectroscope and what is it used for?

Spectroscopes are instruments that allow scientists to determine the chemical makeup of a visible source of light. The spectroscope separates the different colors of light so that scientists can discover the composition of an object.

Also Know, what is a spectrometer and how does it work? A spectrometer is a measuring device that collects light waves. When objects are hot enough, they emit visible light at a given point or points on the electromagnetic spectrum. Spectrometers split the incoming light wave into its component colors. Using this, they can determine what material created the light.

Subsequently, one may also ask, what does a spectroscope do?

A spectroscope helps us find out what stars are made of. It disperses, or separates, white light from a star into a very wide spectrum of colors &mdash much wider than a normal prism does. When spread very wide, black lines appear in the spectrum.

Where is the spectroscope used?

Spectrometers are used in many fields. For example, they are used in astronomy to analyze the radiation from astronomical objects and deduce chemical composition. The spectrometer uses a prism or a grating to spread the light from a distant object into a spectrum.


Uncovering the mystery of Quasars

The distant nature of quasars were discovered in the early 1960's, when spectral lines were noted to be substantially-shifted redder than they should normally be. This redshift can be attributed to the recession (speeding away) of quasars from us. In the standard Big Bang model of cosmology (the faster it's speeding away from you, the more distant it is), this rapid motion implies that quasars are the most distant objects known. Below is a typical spectrum of a quasar. The wavelength scale has been rescaled to the "appropriate" rest wavelengths for the spectral lines. The most noticeable feature is the broad emission line at 1216 Angstroms due to hydrogen atoms making the transition from the first excited state to the ground state. Although 1216 Angstroms lies deep in the ultraviolet, where the Earth's atmosphere is opaque, many quasars are receding from us so fast, this line is redshifted into the visible part of the spectrum (4000-7000 Angstroms).


2 thoughts on &ldquo The Importance of Spectroscopy &rdquo

However, when you have a number of elements overlapping in the same spectral observation, I think it is a bit difficult to identify the elements! Now that we have digitized this process for spectral analysis, it makes the job of the astronomer much easier!

I agree that the importance of spectroscopy in astronomy really can’t be overstated as there really is no substitute for determine the composition of celestial objects.

I will just add that to perform spectroscopy (that is make a spectra) scientists must spilt the light apart in some way like you prism picture illustrates. Modern instruments tend to use something called a diffraction grating to do this and how well how well the light is split is a very important limiting factor on a spectrograph’s performance.


How is spectroscopy used to deduce what an object is made of? - Astronomy

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Click on an underlined word for more information on that subject.
Spectroscopy
Twinkle, twinkle, little star,
How I wonder what you are?

Spectroscopy is a technique in which the visible light that comes from objects (like stars and nebulae) is examined to determine the object's composition, temperature, motion, and density.

When something is hot enough to glow (like a star), it gives you information about what it is made of, because different substances give off a different spectrum of light when they vaporize. Each substance produces a unique spectrum, almost like a fingerprint.

In addition, different cool gases will absorb different wavelengths of light (and generate a signature spectrum with dark lines at a characteristic places). Because of this, you can determine the composition of gases by observing light that has passed through them.

In fact, a substance will emit spectral lines (at a particular wavelength) when it is heated, and absorb light at the same wavelength when it is cool. When the substance emits light, a bright-line spectrum or an emission spectrum is generated (these look like a series of bright lines on a black background). When the substance absorbs light (at the same characteristic wavelength), the spectral pattern that is formed is called a dark-line spectrum or an absorption spectrum (these look like a series of dark lines on a rainbow).

For example, burning sodium (Na) will always produce two very close yellow lines (near the middle of the spectrum) on a black background, and it is the only element that will do exactly this. If you look at a light source and find these characteristic yellow lines, you know that there was sodium in the glowing object that produced this light. If you look at a light source and find dark lines in the same place on the spectrum, you know that the light you're seeing passed through sodium gas somewhere on its journey to you.

Examining the Sun:
Although we think of sunlight or starlight as white, it is really composed of a spectrum of colors - you can use a prism to break up sunlight into a rainbow (red, orange, yellow, green, blue, indigo and violet) - Isaac Newton was the first person to realize this. But when the spectrum is closely examined, the rainbow is interrupted by hundreds of tiny dark lines (called Fraunhofer lines). These lines show that some wavelengths are being absorbed by gases in the outer atmosphere of the Sun, and from this, we can determine which elements are in the Sun's atmosphere.

Why Does Each Element Have a Different Signature Spectrum?:
Each element has a different atomic structure, causing it to produce (or absorb) a different set of wavelengths. It's the actions of the electrons (tiny particles that surround the much heavier nucleus) jumping between different orbitals (the many places where the probability of finding an electron is the greatest) that produce the signature spectrum for an element.

When light (or other energy) is absorbed by the atom, an electron jumps from a low energy orbital to a higher energy orbital. When an electron returns to a less energetic orbital, light (or other electromagnetic radiation) is generated. There are actually many high energy orbitals that an electron can move to, so you can get emitted light in several different wavelengths. The bigger the difference in energy of the orbitals, the shorter the wave length of the light produced (or absorbed).

History of Spectroscopy:
Joseph von Fraunhofer (1787-1826), a German scientist and inventor, observed in the early 1800s that the continuous spectrum was marred by over 700 dark lines (now called Fraunhofer lines). No one knew what caused these lines until the work of G. R. Kirchhoff.

Spectroscopy was discovered in 1859 by Gustav Robert Kirchhoff and Robert Wilhelm Bunsen (of Bunsen burner fame). They made a prism-based device that separated the visible light emitted when substances were vaporized in the flame of Bunsen's specially-designed burner (it had a high-temperature, non-luminous flame). They determined that each gas had its own signature spectrum. Kirchhoff also realized that when emitted light passed through a cooler gas of the same substance, the bright spectral lines were replaced by dark ones - in the same position (1859). So a substance will emit spectral lines (at a particular wavelength) when it is heated, and absorb light at the same wavelength when it is cool.

Kirchhoff had explained the Sun's Fraunhofer lines - the dark lines in the solar spectrum (the light from the Sun) were the same as the emission lines observed by various heated chemical substances. Kirchhoff realized that the Sun was hot and gaseous.

The first person to use the technique of spectroscopy to examine celestial objects was William Huggins (in 1863). He determined that the Sun and the stars are composed mostly of the element hydrogen. He and his wife Margaret also examined the spectra of nebulae and comets.


Spectroscopy

For the purposes of this discussion we can regard light as a wave without consideration of its particle character. Since it is a wave it has a wavelength (λ) λ which is defined as the distance between two crests or troughs of the light wave as shown in figure #. (More generally speaking, it is defined as the distance between two points on the wave with the same phase.) If we know what the wavelength (λ) λ of light is, then we can determine a great deal of information about that light such as its color (or, more precisely, which region of the EM spectrum the light is in), energy, temperature, and so on. Each particular color of light has a specific wavelength. For example red light has a wavelength of roughly (λ=700nm)whereas violet light has a wavelength of roughly (λ=400nm) λ =400 nm . There are also forms of light whose wavelength corresponds to regions in the EM spectrum which are imperceptible to human vision (but, nonetheless, can still be “seen” by our detectors) such as infrared light, microwaves, etc. Given the wavelength then using Plank’s relationship (E=hc/λ) E = h c λ , we can determine the energy of the light. Furthermore from the Boltzmann relationship (E=kT) E = kT , we can also relate the energy of this light to the temperature of the matter which emitted it. On average the atoms composing matter of temperature (T) T will emit photons with an energy (E=kT) E = kT = h c λ where (T=k/λ) T = k λ . Thus given (λ) λ we can determine the temperature (T) T of the matter. The value (λ) λ of radiation emitted by the human body with temperature (T≈98 ext< degrees F>) T ≈98℉ corresponds to infrared radiation. If you consider hotter matter such as a piece of metal heated to (500 ext< degrees C>), it will emit wavelengths of light which correspond to the red region of the EM spectrum and the object will be glowing red. If you consider still hotter object such as the filament in a light bulb with a temperature of (3,000 ext< degrees C>) 3,000℃ , it will emit white light because (λ) λ will have shifted to the middle of the visible part of the EM spectrum.

When light is emitted by a distant stellar object (i.e. star or galaxy) with an initial wavelength (λe) λ e , by the time it reaches us its wavelength becomes “stretched” and increases by an amount (∆λ) ∆ λ . We say that the light was redshifted by an amount (∆λ) ∆ λ . In practice the radial velocity (V) V of recession of a distant stellar object a distance (D) D away from us can be calculated using Doppler’s equation if we know what the redshift (∆λ) ∆ λ is. During the 1920s the astronomer Edwin Hubble performed this calculation for many different stellar objects and plotted the velocity (V) V as a function of distance (D) on a graph. He then drew a line of regression through the data points (which was a straight line) and concluded that (V∝D) V ∝ D . Then by calculating the slope he was able to determine Hubble’s constant (Ho). (Hubble’s initial calculation of (Ho) H 0 was off by a factor of 10 but later on this error was corrected.) It is very important to emphasize that these stellar objects are not moving through space rather it is space itself that is “moving” and expanding. Therefore the redshift of these objects is due not to their motion relative to us but rather to the expansion of space (we will talk about this in detail later on). The redshift (∆λ) of such objects is measured using a device called a spectroscope which is an instrument that contains a glass (or some other refractive material) prism. When light passes through the prism it “spreads out” this makes it easier to distinguish between the different wavelengths of light. (For example when white light (which is composed of all visible wavelengths) passes through a prism a rainbow is produced and the different wavelengths are easier to distinguish.) This refracted light is then shone on a photographic plate (the detector) which records the “brightness” of each wavelength of light. The intensity as a function of wavelength, (I(λr)) I ( λ r ) , is obtained from the detector and, roughly speaking, is a measure of the “brightness” or “dimness” of each wavelength (λr).

According to the laws of atomic physics (which are derived from quantum mechanics), a particular atom can emit and absorb only certain wavelengths of light. Take for example sodium atoms which compose salt. Sodium atoms emit or absorb only two different wavelengths (which correspond to the “orange region” of the EM spectrum) of photons. If a light source shined light composed of all wavelengths in the EM spectrum through a bunch of sodium atoms all wavelengths of light would pass right through it except for two specific wavelengths (which are in the “yellow region” of the EM spectrum). If this emitted light, after passing through the sodium atoms, is shined through a spectroscope and onto a detector, two distinct bands (which lie in the yellow region) will appear on the detector as shown in Figure 3. Each different kind of atom leaves its own signature and produces different bands on the detector as shown in Figure 3. If light consisting of all wavelengths is shined through some object (that is composed of unknown kinds of atoms) and then through a spectroscope and onto a detector, we can determine what kinds of atoms it is made of by inspecting the dark lines (called spectral lines) on the detector. Throughout the early 20th century astronomers used this technique (which is called spectroscopy) to determine the composition of the Sun, other stars, our Milky Way galaxy, and other galaxies.

A star has an atmosphere made up of certain kinds of atoms each of which absorb only particular wavelengths. Stars generate their light through a process called nuclear fusion at its center. When this light passes through the star’s atmosphere most wavelengths of light will go straight through it but certain wavelengths of light will get absorbed. After the light passes through the star’s atmosphere some of it will passes through an astronomer’s telescope and spectroscope and onto the detector. All wavelengths of light will appear on the detector except for the ones that got absorbed—these will appear as black spectral lines. Suppose that we had two identical stars made of the same atoms: the first is stationary relative to us, but the second is moving away from us due to the expansion of space. The spectral lines will look exactly the same for both stars with one exception: the spectral lines corresponding to the star moving away from us will be slightly redshifted. This measured redshift (∆λ) ∆ λ is what astronomers use to calculate the recessional velocity (V) V of the star. The same exact argument applies to entire galaxies: most of a galaxy’s light is generated in the interiors of stars which then pass through the stars atmospheres (some wavelengths getting absorbed) and some of that light reaches the detectors of humanities most powerful telescopes. By examining the redshift in the spectral lines astronomers like Vesto Slipher and Edwin Hubble were able to determine what the galaxies recessional velocities were.

Finding massive exoplanets

Spectroscopy also plays an important role in the discovery of massive exoplanets which are several times as massive as the Earth. To understand how this works, it is very useful to start out by thinking about the Jupiter-Sun system. As Jupiter orbits around the Sun, it exerts gravitational forces causing the Sun to oscillate about its equilibrium position with an amplitude of hundreds of miles. If we were living on a distant exoplanet "watching" the Sun with out telescopes, what would we see? Well, if we were at the appropriate point in our orbit we would be able to see the Sun either moving towards us or away from us. This relative motion will cause the light emitted from the Sun to be either blueshifted or redshifted. From there, we could just use Doppler's equations to determine the Sun's relative velocity. After determining the relative velocity, we could then work out the mass of Jupiter, its orbital period, and lastly how far Jupiter is away from the Sun.

The entire aforementioned discussion we just had would also apply to an Eath-based observer watching distant stars with their telscopes. When astronomers see a star wobbling, this gives them a pretty good hunch that a massive exoplanet must be orbiting around it. Using the aforementioned techniques, astronomers ccould deduce the mass of the exoplanet, its orbital period, and its distance away from its parent star. Unfortunately, for smaller planets, this strategy of looking at the star's wobble doesn't work quite so well since small exoplanets (ones which are 0.1 to a few times the size of the Earth) exert very small tugs on their parent star making it difficult to notice any wobbles. To find smaller exoplanets, astronomers use a different strategy: namely, they try to spot a "twinkle" as the exoplanet passes by its neighboring star.

This article is licensed under a CC BY-NC-SA 4.0 license.

1. Singh, Simon. Big Bang: The Origin of the Universe. New York: Harper Perennial, 2004. Print.

2. Wikipedia contributors. "Spectroscopy." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 12 May. 2017. Web. 18 May. 2017.


Spectroscopy: Reading the Rainbow

What Is a Spectrum?

A spectrum is a rainbow! This rainbow is created when a beam of white light is broken into its component colors, such as with a prism. The colors formed are ordered according to their wavelength. When scientists look at this rainbow, they examine how intense the light is in each color. Is blue brighter than yellow, or is this specific red brighter than this other red?

When material interacts with light, properties of that material are stamped on the light. This stamp is like a specific fingerprint for each element and molecule. By examining the intensity of light in each color, scientist can work backward to infer the properties of the material that touched the light along the way.

What Is Spectroscopy?

An image tells us what something looks like a spectrum tells us what it is.

Spectroscopy is the study of the spectra produced when material interacts with or emits light. It is the key to revealing details that cannot be uncovered through a picture. A spectrograph &mdash sometimes called a spectroscope or spectrometer &mdash breaks the light from a single material into its component colors the way a prism splits white light into a rainbow. It records this spectrum, which allows scientists to analyze the light and discover properties of the material interacting with it. Spectroscopy is as crucial as imaging to understanding the universe.

Hubble and Spectroscopy

Hubble is famous for the images captured by its cameras, but it often also relies on its spectrographs. Spectrographs collect data that tell scientists how much light comes out at each wavelength. These data reveal important details about the makeup of atmospheres on exoplanets, the compositions of stars and nebulas, the motion of galaxies and more.

Ultraviolet spectroscopy is one of Hubble&rsquos most unique contributions to the astronomical community, and this capability will not be replaced or superseded by any mission in the near future. Ultraviolet spectroscopy tells us certain things about the universe, while visible and infrared spectroscopy tells us others. By combining Hubble&rsquos ultraviolet spectroscopy with the infrared spectroscopic capabilities of the James Webb Space Telescope, the two telescopes will achieve scientific results together that neither could achieve alone.

How Do You Read a Spectrum?

Light carries information about the material with which it interacts. Different materials interact differently with light, and we can use light to understand what something is made of. All matter is made of atoms. Electrons go around the nucleus of an atom at different allowed energies, like rungs on a ladder. Light with the exact energy needed to go between rungs can be absorbed, but not others. Electrons fall down to lower rungs, emitting light at the specific energy of the difference between the rungs. This allows different atoms to emit different colors of light. Sodium&rsquos spectrum does not look like nitrogen&rsquos spectrum &mdash nor like the spectrum of any other element.

All elements absorb and emit specific wavelengths of light that correspond to those energy levels. An absorption spectrum is a spectrum of light transmitted through a substance, showing dark lines or bands where light has been absorbed by atoms, causing a dip in the spectrum. An emission spectrum is made by electrons falling down the energy ladder. It&rsquos what you get when you look at hot gas, which is heated by something out of the line of sight. This heating moves the electrons up the ladder, then when they fall down the ladder some of the light they emit comes to you. This results in bright, colored spikes due to atoms releasing light at those wavelengths.

How Does a Spectrograph Work?

A spectrograph passes light coming into the telescope through a tiny hole or slit in a metal plate to isolate light from a single area or object. This light is bounced off a special grating, which splits the light into its different wavelengths (just like a prism makes rainbows). The split light lands on a detector, which records the spectrum that is formed.

What Has Hubble Found with Spectroscopy?

Hubble&rsquos spectrographs reveals important details of many aspects of our universe. Below are examples of the many spectroscopic findings from Hubble.


Watch the video: Spektroskopie - Sternenlicht zerlegen (May 2022).