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Referring to this answer to What's the rationale behind the false colours in solar observation photographs? which includes the table from Wikipedia's Fraunhofer lines:
In the Table of wavelengths there is an e designated Hg line of 546.07 nm which DOES NOT appear in the picture of the Fraunhofer spectrum, any explanation? Also there are about a dozen lines in the spectrum which are not tabulated and not identified at all.
The picture is a mocked-up fake and is not an actual picture of the solar spectrum. You can easily see this because the black "Fraunhofer lines" extend beyond the spectrum and H alpha should have an appreciable width.
The table is massively incomplete. It list only a tiny fraction (the strongest) absorption lines in the solar spectrum. There are thousands of others.
I guess whoever mocked up the spectrum didn't use that table.
A real solar spectrum is shown below. The Fraunhofer lines are not easy to see at all in a spectrum that has low enough resolution to include the whole thing in one linear image. Any lines due to mercury will be weak because mercury is not an abundant element in the solar photosphere.
A better view is offered by an echellogram taken at higher resolution. This begins to demonstrate just how many lines there are in the solar spectrum.
What are Fraunhofer lines?
The Fraunhofer lines are any of the dark absorption lines in the spectrum of stars (like the Sun), which is caused by selective absorption of a star’s radiation at specific wavelengths by various gas elements existing in the atmosphere.
First observed by an English physicist William Hyde Wollaston in 1802 but are named after a German physicist Joseph von Fraunhofer. Since 1814 Fraunhofer plotted more than 500 Fraunhofer lines and assigned the brightest by letters A to G, which is still in use today.
At the time of writing this, there are 25,000 Fraunhofer lines which are known to exist in our Solar System spectrum.
There are three kinds of spectra you should know about.
Continuous – All the colours, i.e. wavelengths, of the visible spectrum with nothing missing. What we see when white light is dispersed by a prism.
Image via: NASA
Emission – When atoms are excited they give off light. Different elements produce light with very definite wavelengths, i.e. very definite colours. Sodium lamps are yellow because sodium emits lots of yellow light. Neon lights are red because neon atoms emit lots of red and orange light. Mercury emits lots of blue and green light. Think of it as a fingerprint or a barcode. If we can measure the wavelength of the light given off by a substance, we can identify it. Here is the visible part of the emission spectrum for hydrogen.
Image via: NASA
Absorption – If white light, containing all the colours, passes through a gas then when it emerges some of the colours are missing. This is because the atoms in the gas absorb light with very definite wavelengths, actually exactly the same wavelengths that the same atoms would emit if they were excited. What we see is a continuous spectrum with black lines on it. Here is an absorption spectrum for hydrogen. Notice what colours are missing.
Image via: NASA
If we pass sunlight through a prism, we see the visible spectrum we expect but also many black lines. It is called the Fraunhofer spectrum after one of the first people to study it. Below is a German stamp featuring Fraunhofer’s original drawing.
This is, of course, an absorption spectrum. The black lines, over 600 of them, are due to the absorption of particular wavelengths of light by chemical elements in the outer layers of the Sun. This means that if we measure the wavelength of these black lines, we can identify what elements there actually are in the Sun. Their relative intensity also tells us how much of these elements are present and so we can build up a pretty good picture of what the Sun is made of.
One set of lines, including a strong yellow line, did not correspond to any known element. In 1870 Lockyer suggested that they corresponded to a new element which he named Helium after the Greek Sun god Helios. 25 years later Helium was discovered on Earth.
3 thoughts on &ldquo The Sun’s Absorption Spectrum &rdquo
I would be interested to compare the spectrum of our Sun with that of other stars. We talk a lot about what our Sun is made of, but are all stars the same? What differs between stars? Are the gases mentioned (hydrogen, oxygen, carbon, nitrogen, silicon, magnesium, iron, neon and sulfur) typical ingredients of stars?
Hey from your grader! Fun fact–Helium was actually first discovered in the Sun!
essay Grammar and punctuation aren’t important on a multipleoption. It maybe written as an argument or awareness or to raise your voice.” While you are at it, distinguish they’re, there and their.
5.5 Formation of Spectral Lines
We can use Bohr’s model of the atom to understand how spectral lines are formed. The concept of energy levels for the electron orbits in an atom leads naturally to an explanation of why atoms absorb or emit only specific energies or wavelengths of light.
The Hydrogen Spectrum
Let’s look at the hydrogen atom from the perspective of the Bohr model . Suppose a beam of white light (which consists of photons of all visible wavelengths) shines through a gas of atomic hydrogen. A photon of wavelength 656 nanometers has just the right energy to raise an electron in a hydrogen atom from the second to the third orbit. Thus, as all the photons of different energies (or wavelengths or colors) stream by the hydrogen atoms, photons with this particular wavelength can be absorbed by those atoms whose electrons are orbiting on the second level. When they are absorbed, the electrons on the second level will move to the third level, and a number of the photons of this wavelength and energy will be missing from the general stream of white light.
Other photons will have the right energies to raise electrons from the second to the fourth orbit, or from the first to the fifth orbit, and so on. Only photons with these exact energies can be absorbed. All of the other photons will stream past the atoms untouched. Thus, hydrogen atoms absorb light at only certain wavelengths and produce dark lines at those wavelengths in the spectrum we see.
Suppose we have a container of hydrogen gas through which a whole series of photons is passing, allowing many electrons to move up to higher levels. When we turn off the light source, these electrons “fall” back down from larger to smaller orbits and emit photons of light—but, again, only light of those energies or wavelengths that correspond to the energy difference between permissible orbits. The orbital changes of hydrogen electrons that give rise to some spectral lines are shown in Figure 5.19.
Similar pictures can be drawn for atoms other than hydrogen. However, because these other atoms ordinarily have more than one electron each, the orbits of their electrons are much more complicated, and the spectra are more complex as well. For our purposes, the key conclusion is this: each type of atom has its own unique pattern of electron orbits, and no two sets of orbits are exactly alike. This means that each type of atom shows its own unique set of spectral lines, produced by electrons moving between its unique set of orbits.
Astronomers and physicists have worked hard to learn the lines that go with each element by studying the way atoms absorb and emit light in laboratories here on Earth. Then they can use this knowledge to identify the elements in celestial bodies. In this way, we now know the chemical makeup of not just any star, but even galaxies of stars so distant that their light started on its way to us long before Earth had even formed.
Energy Levels and Excitation
Bohr’s model of the hydrogen atom was a great step forward in our understanding of the atom. However, we know today that atoms cannot be represented by quite so simple a picture. For example, the concept of sharply defined electron orbits is not really correct however, at the level of this introductory course, the notion that only certain discrete energies are allowable for an atom is very useful. The energy levels we have been discussing can be thought of as representing certain average distances of the electron’s possible orbits from the atomic nucleus.
Ordinarily, an atom is in the state of lowest possible energy, its ground state . In the Bohr model of the hydrogen atom, the ground state corresponds to the electron being in the innermost orbit. An atom can absorb energy, which raises it to a higher energy level (corresponding, in the simple Bohr picture, to an electron’s movement to a larger orbit)—this is referred to as excitation . The atom is then said to be in an excited state. Generally, an atom remains excited for only a very brief time. After a short interval, typically a hundred-millionth of a second or so, it drops back spontaneously to its ground state, with the simultaneous emission of light. The atom may return to its lowest state in one jump, or it may make the transition in steps of two or more jumps, stopping at intermediate levels on the way down. With each jump, it emits a photon of the wavelength that corresponds to the energy difference between the levels at the beginning and end of that jump.
An energy-level diagram for a hydrogen atom and several possible atomic transitions are shown in Figure 5.20. When we measure the energies involved as the atom jumps between levels, we find that the transitions to or from the ground state, called the Lyman series of lines, result in the emission or absorption of ultraviolet photons. But the transitions to or from the first excited state (labeled n = 2 in part (a) of Figure 5.20), called the Balmer series, produce emission or absorption in visible light. In fact, it was to explain this Balmer series that Bohr first suggested his model of the atom.
Atoms that have absorbed specific photons from a passing beam of white light and have thus become excited generally de-excite themselves and emit that light again in a very short time. You might wonder, then, why dark spectral lines are ever produced. In other words, why doesn’t this reemitted light quickly “fill in” the darker absorption lines?
Imagine a beam of white light coming toward you through some cooler gas. Some of the reemitted light is actually returned to the beam of white light you see, but this fills in the absorption lines only to a slight extent. The reason is that the atoms in the gas reemit light in all directions, and only a small fraction of the reemitted light is in the direction of the original beam (toward you). In a star, much of the reemitted light actually goes in directions leading back into the star, which does observers outside the star no good whatsoever.
Figure 5.21 summarizes the different kinds of spectra we have discussed. An incandescent lightbulb produces a continuous spectrum. When that continuous spectrum is viewed through a thinner cloud of gas, an absorption line spectrum can be seen superimposed on the continuous spectrum. If we look only at a cloud of excited gas atoms (with no continuous source seen behind it), we see that the excited atoms give off an emission line spectrum.
Atoms in a hot gas are moving at high speeds and continually colliding with one another and with any loose electrons. They can be excited (electrons moving to a higher level) and de-excited (electrons moving to a lower level) by these collisions as well as by absorbing and emitting light. The speed of atoms in a gas depends on the temperature. When the temperature is higher, so are the speed and energy of the collisions. The hotter the gas, therefore, the more likely that electrons will occupy the outermost orbits, which correspond to the highest energy levels. This means that the level where electrons start their upward jumps in a gas can serve as an indicator of how hot that gas is. In this way, the absorption lines in a spectrum give astronomers information about the temperature of the regions where the lines originate.
Link to Learning
Use this simulation to play with a hydrogen atom and see what happens when electrons move to higher levels and then give off photons as they go to a lower level.
We have described how certain discrete amounts of energy can be absorbed by an atom, raising it to an excited state and moving one of its electrons farther from its nucleus. If enough energy is absorbed, the electron can be completely removed from the atom—this is called ionization . The atom is then said to be ionized. The minimum amount of energy required to remove one electron from an atom in its ground state is called its ionization energy.
Still-greater amounts of energy must be absorbed by the now-ionized atom (called an ion ) to remove an additional electron deeper in the structure of the atom. Successively greater energies are needed to remove the third, fourth, fifth—and so on—electrons from the atom. If enough energy is available, an atom can become completely ionized, losing all of its electrons. A hydrogen atom, having only one electron to lose, can be ionized only once a helium atom can be ionized twice and an oxygen atom up to eight times. When we examine regions of the cosmos where there is a great deal of energetic radiation, such as the neighborhoods where hot young stars have recently formed, we see a lot of ionization going on.
An atom that has become positively ionized has lost a negative charge—the missing electron—and thus is left with a net positive charge. It therefore exerts a strong attraction on any free electron. Eventually, one or more electrons will be captured and the atom will become neutral (or ionized to one less degree) again. During the electron-capture process, the atom emits one or more photons. Which photons are emitted depends on whether the electron is captured at once to the lowest energy level of the atom or stops at one or more intermediate levels on its way to the lowest available level.
Just as the excitation of an atom can result from a collision with another atom, ion, or electron (collisions with electrons are usually most important), so can ionization. The rate at which such collisional ionizations occur depends on the speeds of the atoms and hence on the temperature of the gas—the hotter the gas, the more of its atoms will be ionized.
The rate at which ions and electrons recombine also depends on their relative speeds—that is, on the temperature. In addition, it depends on the density of the gas: the higher the density, the greater the chance for recapture, because the different kinds of particles are crowded more closely together. From a knowledge of the temperature and density of a gas, it is possible to calculate the fraction of atoms that have been ionized once, twice, and so on. In the Sun, for example, we find that most of the hydrogen and helium atoms in its atmosphere are neutral, whereas most of the calcium atoms, as well as many other heavier atoms, are ionized once.
The energy levels of an ionized atom are entirely different from those of the same atom when it is neutral. Each time an electron is removed from the atom, the energy levels of the ion, and thus the wavelengths of the spectral lines it can produce, change. This helps astronomers differentiate the ions of a given element. Ionized hydrogen, having no electron, can produce no absorption lines.
Every color of the Sun’s rainbow: Why are there so many missing?
What you see here is the entire gamut of our Sun’s visible light output. It clearly shows you how the Sun emits almost every color, but how the output of some colors, such as yellow and green, are brighter than others. Perhaps more interestingly, though, the black lines illustrate the portions of the visible light spectrum that are not emitted by the Sun — and to this day, we still don’t know why some portions of the visible solar spectrum are absent.
The image above (view the full-res version), which is called the absorption spectrum of the Sun, was observed by the Fourier Transform Spectrometer at the National Solar Observatory on Kitt Peak, near Tucson, Arizona. The data, which is in essence gathered by shining sunlight through a very accurate prism, was compiled into a Solar Flux Atlas. The Atlas recorded the entirety of Sun’s emitted light from 296nm to 1300nm, but for the absorption spectrum above, that range was narrowed to the visible light range — 400nm (purple) to 700nm (red). In the image above, each of the 50 rows represents 60 angstroms, or 6nm.
The black lines in the Sun’s spectrum are caused by gases on, or above, the Sun’s surface that absorb some of the emitted light. Every gas (such as helium, hydrogen, oxygen, and so on) has a very specific set of frequencies that it absorbs. If you shine some light through some gas, and then a prism, and record the absorption spectrum, you can say with certainty what that gas is — a valuable tool in chemistry called absorption spectroscopy. NASA’s Curiosity rover uses spectrometers (though not absorption spectrometers) to work out what gases and compounds are present on Mars.
Fraunhofer lines, on the Sun’s absorption spectrum. The letters correspond to various elements (such as helium, sodium) that cause the lines.
For the most part, we know exactly which gases cause each of the black lines — called Fraunhofer lines, after Joseph von Fraunhofer who discovered them in 1814 — in the Sun’s absorption spectrum. Some lines, however, remain mysteriously unidentified. It’s probably not the case that these lines are produced by weird and wonderful elements that don’t exist on Earth, but it’s a possibility.
Flamsteed Astronomy Society
For our first lecture of the 2012/13 season, the Flamsteed were delighted to welcome Dr Radmila Topalovic of the Royal Observatory Greenwich. Though Radmila had never lectured to us before, she was well known to many Flamsteed members, both through her work at the ROG and as tutor of the GCSE Astronomy course.
Radmila’s energetic and enthusiastic style kept us all thoroughly hooked throughout a wide-ranging lecture on the importance of spectroscopy in astronomy and how it has helped us in our understanding of the nature of the Universe.
Radmila began the lecture by saying that atomic spectra could be considered to be the ‘fingerprint’ of an element. It is possible to tell many different properties of objects by looking at this fingerprint.
We were then taken on a whistle-stop tour of the history of the Universe, with hydrogen, helium, lithium and beryllium created minutes after the Big Bang and stars producing all of the heavier elements in the periodic table, either through atomic fusion during their main sequence lifetime (for elements up to Iron) or via supernovae explosions of massive stars (for elements heavier than Iron).
The nature of light was then explained, with different types of light forming the entire electromagnetic spectrum, from gamma-rays at very short wavelengths to microwaves and radio waves at very long wavelengths.
So, what happens when light and matter interact? Radmila explained that electrons can move in several orbits, depending on the amount of energy absorbed by the atom. Photons can push electrons up to a higher energy, creating an absorption spectrum, which appears as ‘missing light’ or dark lines in the spectra. Emission spectrum are caused by atoms emitting photons of light, causing electrons to jump to a lower energy state.
By plotting the spectrum of an object, we can determine its temperature. A measurement of the ‘peak’ of an object’s spectrum gives an indication of the temperature of the object. Hotter stars have a peak in the blue end of the spectrum, whereas cooler stars have a peak in the red end.
We can also use spectra to determine the chemical make-up of plants on Earth. Radmila asked ‘why are plants green’? The answer is that plants contain the chemical chlorophyll, which has the property of reflecting green light and absorbing red, orange, blue and purple. Spectra are also used in medicine. For example, using spectroscopy to detect the chemical alanine is used as a diagnosis for brain tumours.
Spectra can also help us to understand how objects move. The Doppler effect shows that objects moving towards us have shorter wavelengths (i.e. the light is ‘blue-shifted’), whereas objects moving away have longer wavelengths (i.e. the light is ‘red-shifted’). We can determine that the Sun rotates by measuring the spectrum at each end of the Sun. This shows a red-shift on one side of the Sun and a blue-shift on the other, which confirms the Sun’s rotation.
Taking a spectrum of the Sun tells us that elements heavier than those that can be created in a star of the Sun’s size exist (for example, Iron). As the Sun cannot make these elements, they must have been made in another, bigger, star.
Radmila explained how spectra were used to determine that the Universe is expanding. Edwin Hubble, in the 1920s, was looking at light emitted by Hydrogen gas in galaxies. By measuring the spectra, he determined that all distant galaxies are moving away from us. In addition, the more distant the galaxy, the faster it was moving, showing that the Universe is expanding.
The age of the Universe has been determined at around 13.7 billion years by analysing the spectrum of the Cosmic Microwave Background radiation. Radmila also showed how the shape of the Milky Way can be determined by analysing the spin inversion of hydrogen gas in the spiral arms, thus allowing us to build a map of the Milky Way. The Doppler shift of the hydrogen gas shows us how fast the spiral arms are rotating. By analysing the rotation speed to that expected, given the amount of mass that we can see, shows that there would appear to be ‘missing mass’ in the Universe, which has been termed as ‘dark matter’.
Radmila then turned our attention to molecules. So far, 161 have been detected, such as Carbon Monoxide and Hydrogen Peroxide. A particularly interesting class of molecules are polycyclic aromatic hydrocarbons (PAHs), which are seen in nebulae. These molecules are important, as there is a theory that PAHs were the ‘scaffolding’ of the first DNA molecules.
Spectral evidence in asteroids and comets has shown that all of our water is extra-terrestrial in origin. Amino acids have been found in meteorites and it is also possible that bacteria could survive for long periods in space as we have found 250 million year-old dormant bacteria in the North Sea. The search for life goes on, with Curiosity on Mars finding evidence of an ancient stream of water only a few days ago.
Finally, Radmila ended by discussing the Kepler space observatory, which is designed to look for ‘Earth-like’ planets. So far, 838 exoplanets have been detected by various means. The exoplanet Kepler-22b is a planet larger than Earth, but within the ‘habitable’ zone of it’s Sun-line star. The next step must be to take a spectrum of the planet to see if water, methane and other building blocks of life can be detected.
This was an absolutely fascinating lecture that really highlighted the many varied uses of spectroscopy in astronomy and in areas closer to home. Our sincere thanks to Radmila for talking through such a wide-ranging topic in such a clear way. We hope she will come back to see us soon.
THE MODERN VIEW OF ATOMS
Supplementary Questions and Exercises
A spectral line represents (a)
the energy of an electron in an atom.
the light emitted as an atomic electron makes a transition from one atomic state to another.
the frequency of an orbiting electron.
the absorption of a photon by an electron.
Why does an atom emit light only with certain definite frequencies?
A beam of 4-eV electrons is incident on a quantity of hydrogen gas. Will the gas be excited to emit light? Explain.
The ground state of an atom is (a)
the state of largest mass.
the state of largest angular momentum.
the state with the largest orbit radius.
the lowest possible energy state.
The photon emitted when an electron makes a transition from the third to the first energy level of the hydrogen atom has a wavelength of (a)
A radius of 13.25 Å corresponds to the energy level n = ___ of the hydrogen atom. (a)
In what way did Bohr's atomic model contradict the laws of electrodynamics? How was this conflict resolved?
The principal quantum number n of an atomic electron determines (a)
the maximum value of the angular momentum that the electron can have.
the direction of spin of the electron.
the projection of the angular momentum in the direction of an external field.
the electron shell in which the electron is located.
What must be done to a 2S electron in a hydrogen atom to make it a 3P electron? What happens when a 3P electron becomes a 2S electron?
According to the Bohr model, what is the energy of the photon that is emitted when a hydrogen atom makes a transition from the L shell to the K shell?
has been replaced by quantum physics.
is valid when interpreting the behavior of atomic matter.
is valid when describing large-scale objects but not when describing atoms and molecules.
is based on the probabilistic interpretation of the behavior of atoms.
Discuss the meaning and importance of the Pauli exclusion principle. How is this principle useful in interpreting the periodic table?
An electron in the L shell of a hydrogen atom could have the quantum numbers (n, l, ml, ms) (a)
Which group of elements has the lowest ionization energies? The highest? Explain.
The chemical properties of an element are determined primarily by (a)
the attraction of the protons for the electrons.
the number of neutrons in the nucleus.
the electrons in the outermost shell.
The atomic electrons of boron (Z = 5) are arranged as follows: (a)
2 electrons in the first shell and 2 electrons in the second shell.
2 1S electrons and 3 2S electrons.
2 1S electrons and 3 2P electrons.
2 1S electrons and 3 electrons in the second shell.
What is the meaning of the acronym laser?
Describe the laser effect.
Missing line in solar spectrum - Astronomy
Now that you know what the lines mean and how to identify them, try classifying stars based on the "strength" of their hydrogen absorption lines, specifically the H&alpha line. The spectrum below is the same spectrum as from Question 5, again with a zoom-in of the spectrum's H-alpha line shown below it.
|Click on image to see a larger version|
Notice that the H&alpha absorption line dips down vertically, but it is not a straight drop the line has some width. The two triangles in the zoom-in mark two spots on the continuum spectrum. If you drew a line between the two triangles, you would see approximately how much light would have been emitted by the star if it had no hydrogen.
If you colored in the area between that line and the actual spectrum, you would see the amount of hydrogen "missing" from the spectrum due to hydrogen absorption. This amount is referred to as the strength of the absorption line. The strength of an absorption line depends not only on the depth of the valley, but also on its width.
Explore 2. Look at the spectra of these seven stars (the page will open in a new window). You can also see the seven stars as a FlashPaper File (requires free Flash Player) or as a PDF (requires free Adobe Reader).
Using the spectra, rank the seven stars according to the strength of their H&alpha absorption lines. If you have a hard time judging by eye, color in the area between the line connecting the triangles and the spectrum, then count the number of grid boxes to measure the line strength.
Rank the stars by writing each star number in the appropriate place in the second column of the table below.
Explore 3. Originally, astronomers classified those stars with the strongest hydrogen lines as 'A' stars, stars with the next strongest lines as 'B' stars, the next strongest 'C' and so on. Eventually, they realized that some letters were unnecessary, and dropped them from the classification system. The letter assigned to a star is called its spectral class .
The spectral classes that remain are: A,B,F,G,K,M and O. In the third column of the table from Explore 2, write the spectral class of each star in the table. There is only one star of each type in this data.
It is logical, according to Bode's Law (maybe should be called" theory'?) that there is or should be a planet there that we cannot find or see --IF it exists.
IT is easy to say well it is there but it is 2000 light years away in its perhilion and its apogee or aperhillion is also 2000 light years so that is why just cannot find it but the truth maybe is that Bode's planet numerology 'law' is wrong and if not then perhaps the hypothetical solution is that the missing planet is now a moon that was knocked into its present orbit thus as becoming a satellite of Jupiter, Neptune or Uranus or an asteroid. That could be why we cannot see the forest for the trees?
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Bode's "law" is an empirical observation that ends with Uranus. Since Neptune does not fit the pattern, there is no reason to expect it beyond Neptune.
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October 28, 2017 at 9:36 pm
I know you are all very logical and the Astronomer in Cali has a formula just for question sake. Which direction is he/you looking towards state constellation name area?
If by chance you're looking towards Pisces I would like to know? I this is the case then I may have an answer worth pursuing.
Thank you for your time please let me know as soon as possible, please.
Milky Way: Hydrogen halo lifts the veil of our galactic home
Sometimes it takes a lot of trees to see the forest. In the case of the latest discovery made by astronomers at the University of Arizona, exactly 732,225. Except that in this case, the "forest" is a veil of diffuse hydrogen gas enshrouding the Milky Way, and each "tree" is another galaxy observed with the 2.5-meter telescope of the Sloan Digital Sky Survey.
After combining this staggering number of spectra -- recorded patterns of wavelengths revealing clues about the nature of a cosmic target -- UA astronomers Huanian Zhang and Dennis Zaritsky report the first detections of diffuse hydrogen wafting about in a vast halo surrounding the Milky Way. Such a halo had been postulated based on what astronomers knew about other galaxies, but never directly observed.
Astronomers have long known that the most prominent features of a typical spiral galaxy such as our Milky Way -- a central bulge surrounded by a disk and spiral arms -- account only for the lesser part of its mass. The bulk of the missing mass is suspected to lie in so-called dark matter, a postulated but not yet directly observed form of matter believed to account for the majority of matter in the universe. Dark matter emits no electromagnetic radiation of any kind, nor does it interact with "normal" matter (which astronomers call baryonic matter), and is therefore invisible and undetectable through direct imaging.
The dark matter of a typical galaxy is thought to reside in a more or less spherical halo that extends 10 to 30 times farther out than the distance between the center of our galaxy and the sun, according to Zaritsky, a professor in the UA's Department of Astronomy and deputy director of the UA's Steward Observatory.
"We infer its existence through dynamical simulations of galaxies," Zaritsky explains. "And because the ratio of normal matter to dark matter is now very well known, for example from measuring the cosmic microwave background, we have a pretty good idea of how much baryonic matter should be in the halo. But when we add all the things we can see with our instruments, we get only about half of what we expect, so there has to be a lot of baryonic matter waiting to be detected."
By combining such a large number of spectra, Zaritsky and Zhang, a postdoctoral fellow in the Department of Astronomy/Steward Observatory, covered a large portion of space surrounding the Milky Way and found that diffuse hydrogen gas engulfs the entire galaxy, which would account for a large part of the galaxy's baryonic mass.
"It's like peering through a veil," Zaritsky said. "We see diffuse hydrogen in every direction we look."
He pointed out that this is not the first time gas has been detected in halos around galaxies, but in those instances, the hydrogen is in a different physical state.
"There are cloudlets of hydrogen in the galaxy halo, which we have known about for a long time, called high-velocity clouds," Zaritsky said. "Those have been detected through radio observations, and they're really clouds -- you see an edge, and they're moving. But the total mass of those is small, so they couldn't be the dominant form of hydrogen in the halo."
Since observing our own galaxy is a bit like trying to see what an unfamiliar house looks like while being confined to a room inside, astronomers rely on computer simulations and observations of other galaxies to get an idea of what the Milky Way might look like to an alien observer millions of light-years away.
For their study, scheduled for advance online publication on Nature Astronomy's website on Apr. 18, the researchers sifted through the public databases of the Sloan Digital Sky Survey and looked for spectra taken by other scientists of galaxies outside our Milky Way in a narrow spectral line called hydrogen alpha. Seeing this line in a spectrum tells of the presence of a particular state of hydrogen that is different from the vast majority of hydrogen found in the universe.
Unlike on Earth, where hydrogen occurs as a gas consisting of molecules of two hydrogen atoms bound together, hydrogen exists as single atoms in outer space, and those can be positively or negatively charged, or neutral. Neutral hydrogen constitutes a small minority compared to its ionized (positive) form, which constitutes more than 99.99 percent of the gas spanning the intergalactic gulfs of the universe.
Unless neutral hydrogen atoms are being energized by something, they are extremely difficult to detect and therefore remain invisible to most observational approaches, which is why their presence in the Milky Way's halo had eluded astronomers until now. Even in other galaxies, halos are difficult to pin down.
"You don't just see a pretty picture of a halo around a galaxy," Zaritsky said. "We infer the presence of galactic halos from numerical simulations of galaxies and from what we know about how they form and interact."
Zaritsky explained that based on those simulations, scientists would have predicted the presence of large amounts of hydrogen gas stretching far out from the center of the Milky Way, but remaining associated with the galaxy, and the data collected in this study confirm the presence of just that.
"The gas we detected is not doing anything very noticeable," he said. "It is not spinning so rapidly as to indicate that it's in the process of being flung out of the galaxy, and it does not appear to be falling inwards toward the galactic center, either."
One of the challenges in this study was to know whether the observed hydrogen was indeed in a halo outside the Milky Way, and not just part of the galactic disk itself, Zaritsky said.
"When you see things everywhere, they could be very close to us, or they could be very far away," he said. "You don't know."
The answer to this question, too, was in the "trees," the more than 700,000 spectral analyses scattered across the galaxy. If the hydrogen gas were confined to the disk of the galaxy, our solar system would be expected to "float" inside of it like a ship in a slowly churning maelstrom, orbiting the galactic center. And just like the ship drifting with the current, very little relative movement would be expected between our solar system and the ocean of hydrogen. If, on the other hand, it surrounded the spinning galaxy in a more or less stationary halo, the researchers expected that wherever they looked, they should find a predictable pattern of relative motion with respect to our solar system.
"Indeed, in one direction, we see the gas coming toward us, and the opposite direction, we see it moving away from us," Zaritsky said. "This tells us that the gas is not in the disk of our galaxy, but has to be out in the halo."
Next, the researchers want to look at even more spectra to better constrain the distribution around the sky and the motions of the gas in the halo. They also plan to search for other spectral lines, which may help better understand the physical state such as temperature and density of the gas.