# Effect of particulates on the visibility of stars?

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I am looking for a (mathematical) relationship - either empirical or theoretical - which quantifies how the visibility of celestrial objects decreases with increasing amount of particulates in the air. I would be already happy if somebody knows such a relationship for the atmospheric light attenuation effects of black carbon or soot. Those two are actually not the same as Alfred Wiedensohler from the Leibniz Institute for Tropospheric Research points out in a presentation on the topic:

Soot is carbon particles resulting from the incomplete combustion of hydrocarbons. Soot contains polycyclic aromatic hydrocarbons (PAHs) and heavy metals. Black carbon (BC) itself is neither a toxic nor a carcinogen.

What I expect (but I am happy to be proven wrong) is that I am actually after the three constants in the following expression:

$$f_{ m attenuation}(d) = frac{1}{k_c + k_l cdot d + k_q cdot d^2}$$

In this formula, $$d$$ is the distance between the light source and the observers, so in our case it would be the height of Earth's atmosphere (if we assume homogenous distribution of soot/ BC over the air column). $$k_c$$ is called the constant attenuation factor, $$k_l$$ is the linear attenuation factor, and $$k_q$$ stands for quadratic attenuation factor.

Such an expression would already be very helpful, but if there is science on the overall effect of particulates, that would be much cooler. I am dreaming of having a formula for visible magnitude of stars with naked eye in dependence of particulate concentration (for ideal, pitch-black nights without clouds).

### Background

My question is inspired by What effect do aircraft have on night-time visibility? which does not have an answer yet.

I am aware that in the physical oceanography and in marine biology, there are extensive studies on light attenuation due to BC or other dissolved substances in water, and most search results are on that.

### References and related questions

• C.Liousse, H.Cachier, S.G.Jennings: Optical and thermal measurements of black carbon aerosol content in different environments: Variation of the specific attenuation cross-section (1993)
• H. Horovath: Atmospheric light absorption - A review (1993)
• ESA on Desert dust plume over the Atlantic observed by Aeolus and Sentinel-5P

Aerosol physics is actually an old topic - yet still a very vibrant one.

One main factor on the extinction (which sums up scattering and absorption) other than the distance (which is about constant when looking at a a particular zenith distance) is the wavelength $$lambda$$ you look at compared to the typical aerosol particle radius $$r$$ as its ratio affects the scattering regime we look at ($$lambda gg r, lambda approx r, lambda ll r$$). There's multiple papers on the aerosol influence on observations, like this (Stubbs et al) for PanSTARRS or Patat et al (2018) on extinction over Paranal which also looks at the temporal variability.

Looking at the typical aerosol concentrations (e.g. here by DWD), it seems that the smaller particles are more abundant. Thus we are in the Rayleigh or Mie scattering regimes where the extinction is due to scattering and does not depend much on the actual particle properties other than their size. The actual absorption only becomes important for the larger particles which are the least abundant.

All in all this might only be half an answer, but too long for a comment and maybe gives you leads to follow-up on. If you get a hand on Friedlander's "smoke, dust and haze", it might be a good read, too (especially chapters 5 and 13).

## The Interstellar Medium (ISM)

When you observe the night sky, you see the stars as pinpoints of light against a black background. You have also probably been told that outer space is a “vacuum”—that is, that, other than stars and planets, it is very empty. It is true that space is so empty that it is a more perfect vacuum than we can create in the laboratory on Earth however, space is not completely empty. Here are some images that show some obvious examples of regions in space that contain some material between the stars:

In all of these images, you see different regions of space that are either filled with what appears to be glowing gas (similar to a neon light), or dark regions that are obscuring the stars behind them. All of these objects are called nebulae, a word which is Latin for "cloud." Taken as a whole, the gas and dust that fills the space between stars is called the interstellar medium.

If you look at the images at the links above in some depth, you'll see some or all of the following:

• Clouds that glow an almost uniform, bright red
• Dark clouds that appear to block all or most of the light of stars or bright clouds behind them
• Clouds that glow a blue color very similar to the blue of our sky.

At this point, let's address the physical processes responsible for the appearance of these various nebulae, and give them more descriptive names.

#### Emission Nebulae

As the name implies, emission nebulae emit emission spectra, not continuous or absorption spectra. If you refer to the lesson on the production of emission lines, what is required for a cloud of gas to emit an emission spectrum is for electrons in the gas to be in energy levels above level 1. As they move down to the ground state, they will emit photons of specific energies, producing emission lines at those specific energies. One point we glossed over in that lesson is: How do the electrons get into the higher energy levels in the first place? In the case of emission nebulae, there is a clue usually quite apparent in the image. For example, in the image below of massive stars in NGC 6357, you see a large region of glowing red emission nebula surrounding a number of massive stars (although there is no way to tell just from looking at the picture that they are massive stars).

The massive stars give off a lot of ultraviolet light, which the hydrogen gas absorbs. The electrons in the H atoms absorb so much energy that they are actually stripped completely from the H atoms, creating a sea of hydrogen nuclei and free electrons. This is a process called ionization. The free electrons can recombine with the hydrogen nuclei, and then, as they cascade down to the ground state, they emit photons. Because the clouds are primarily made of hydrogen, the spectrum of a typical emission nebula shows strong hydrogen emission lines. The strongest hydrogen line in the visible part of the spectrum is H-alpha at 656.3 nm, which is in the red part of the spectrum.

Astronomers refer to neutral hydrogen atoms as HI (pronounced "H-one") and they refer to ionized hydrogen nuclei as HII ("H-two"). For this reason, the glowing red emission nebulae found surrounding massive stars are often referred to as "HII Regions."

We will talk about these later, but there are objects known as planetary nebulae that also give off emission spectra. Here is an example emission spectrum from a planetary nebula taken by Prof. Robin Ciardullo of Penn State:

On the x-axis is wavelength in Angstroms (1 Angstrom is 1/10 of a nanometer, so 656.3 nm = 6563 Angstroms), and on the y-axis is the relative flux (or apparent brightness). You can see that there are several very prominent emission lines—the H-α line at 6563 Angstroms, the H-β line at 4861 Angstroms, a helium line, and several oxygen lines, for example. In this nebula, the oxygen line at 5007 Angstroms is stronger than H-α, so this nebula will appear more green than red. This is seen in the central regions shown in the photo below.

#### Dark Nebulae

In regions where there are a lot of stars (such as in the APOD of a Giant Molecular Cloud), or regions with bright emission nebulae (such as in the Hubblesite image of a Bok Globule), we can see dark nebulae that appear to be blocking the background light sources behind them. These dark nebulae are also interstellar clouds, but unlike the emission nebulae, they are very cold (10 K, as opposed to about 10,000 K for emission nebulae) and very dense. These clouds are called molecular clouds because the conditions in them are just right for the creation of molecular hydrogen. Be aware that two hydrogen atoms that have linked together form the molecule H2 (which is also pronounced "H-two", but should not be confused with HII). Another constituent of these dark clouds is interstellar dust. The dust particles are very tiny grains of different solids, and if you compare their chemical makeup to common materials on Earth, they are similar in many ways to soot and sand. As photons of light encounter a dark cloud of gas and dust, we observe two effects: first, the light is extinguished—that is, the background light sources appear dimmer than if there was no intervening cloud. This effect is referred to as interstellar extinction. The second effect is referred to as interstellar reddening. As the name suggests, the light from a background source will appear redder than if it had not passed through the cloud. As photons of light pass through the cloud, they get scattered from their original paths. Blue light scatters more than red light, so less blue light and more red light from the background object makes it through the cloud. If you look closely at the APOD image of Barnard 68, you will see that all the stars visible near the edge of the cloud look red or orange, while those just outside the boundaries of the cloud look blue or white, which is caused by this reddening effect. Again because of the wavelength dependence of scattering by dust, most of the infrared light will actually pass through the cloud. See the image below of Barnard 68 taken through infrared filters. You can see the reddening, but you can also see right through the cloud!

#### Reflection Nebulae

The bright blue regions seen in some of the images above are reflection nebulae, and they are also caused by the scattering of light by dust particles. The light from a star encounters dust particles and reflects. In most cases, the particles are the correct size to scatter blue light more efficiently than red light, so the reflection nebula appears to us in the reflected blue light from the nearby star. Since the light we see is reflected starlight, the spectrum of the reflection nebula is similar to the spectrum of the star's light.

#### More complex nebulae

Finally, let's consider some of the images of the regions that include overlapping nebulae of different types. First, look again at the very famous picture of a region of sky called the Eagle Nebula.

The dark pillars are part of the edge of a molecular cloud. Some nearby, very bright stars (off the edge of the picture) are illuminating these pillars of gas, and their intense radiation is causing the outer layers of the molecular cloud to evaporate (this process is called photoevaporation). What astronomers have found is that this process of photoevaporation is revealing that inside the densest knots in the pillars are newly formed, baby stars. If you study carefully some of the other images, for example the panorama of Carina, you can find similar structures to the ones seen in the Eagle Nebula.

In fact, we see that all of the youngest stars in the sky live in regions with a lot of interstellar matter. It seems clear that there must be a connection between the stars and the interstellar gas. By observing these regions in many wavelengths of light, astronomers have found that new stars are formed out of the gas and dust in these clouds. The process is not completely understood, but in the following section, we will discuss how stars form.

The Eagle Nebula image is one of the most famous Hubble images ever taken, and so, PBS has made a video on the making of the Eagle Nebula image.

## Quantum Astronomy: The Double Slit Experiment

This is a series of four articles each with a separate explanation of different quantum phenomena. Each of the four articles is a piece of a mosaic and so every one is needed to understand the final explanation of the quantum astronomy experiment we propose, possibly using the Allen Array Telescope and the narrow-band radio-wave detectors being build by the SETI Institute and the University of California, Berkeley.

With the success of recent movies such as "What the [email protected]# Do We Know?" and the ongoing -- and continuously surprising -- revelations of the unexpected nature of underlying reality that have been unfolding in quantum physics for three-quarters of a century now, it may not be particularly surprising that the quantum nature of the universe may actually now be making in-roads into what has previously been considered classical observational astronomy. Quantum physics has been applied for decades to cosmology, and the strange "singularity" physics of black holes. It is also applicable to macroscopic effects such as Einstein-Bose condensates (extremely cold conglomerations of material that behave in non-classical ways) as well as neutron stars and even white dwarfs (which are kept from collapse, not by nuclear fusion explosions but by the Pauli Exclusion Principle - a process whereby no two elementary particles can have the same quantum state and therefore, in a sense, not collapse into each other).

Well, congratulations if you have gotten through the first paragraph of this essay. I can't honestly tell you that things will get better, but I can say that to the intrepid reader things should get even more interesting. The famous quantum physicist Richard Feynmann once said essentially that anyone who thought he understood quantum physics did not understand it enough to understand that he did not actually understand it! In other words, no classical interpretation of quantum physics is the correct one. Parallel evolving universes (one being created every time a quantum-level choice is made), faster-than-light interconnectedness underlying everything, nothing existing until it is observed, these are a few of the interpretations of quantum reality that are consistent with the experiments and observations.

There are many ways we could go now in examining quantum results. If conscious observation is needed for the creation of an electron (this is one aspect of the Copenhagen Interpretation, the most popular version of quantum physics interpretations), then ideas about the origin of consciousness must be revised. If electrons in the brain create consciousness, but electrons require consciousness to exist, one is apparently caught in circular reasoning at best. But for this essay, we shall not discuss quantum biology. Another path we might go down would be the application of quantum physics to cosmology -- either the Inflationary origin of the universe, or the Hawking evaporation of black holes, as examples. But our essay is not about this vast field either. Today we will discuss the scaling of the simple double-slit laboratory experiment to cosmic distances, what can truly be called, "quantum astronomy."

The laboratory double-slit experiment contains a lot of the best aspects of the weirdness of quantum physics. It can involve various kinds of elementary particles, but for today's discussion we will be talking solely about light - the particle nature of which is called the "photon." A light shining through a small hole or slit (like in a pinhole camera) creates a spot of light on the screen (or film, or detector). However, light shown through two slits that are close together creates not two spots on the screen, but rather a series of alternating bright and dark lines with the brightest line in the exact middle of this interference pattern. This shows that light is a wave since such a pattern results from the interference of the waves coming from slit one (which we shall call "A") with the waves coming from slit two (which we shall call "B"). When peaks of waves from light source A meet peaks from light source B, they add and the bright lines are produced. Not far to the left and right of this brightness peak, however, peaks from A meet troughs from B (because the crests of the light waves are no longer aligned) and a dark line is produced. This alternates on either side until the visibility of the lines fades out. This pattern is simply called an "interference pattern" and Thomas Young used this experiment to demonstrate the wave nature of light in the early 19 th Century.

However, in the year 1900 physicist Max Planck showed that certain other effects in physics could only be explained by light being a particle. Many experiments followed to also show that light was indeed also a particle (a "photon") and Albert Einstein was awarded the Nobel Prize in physics in 1921 for his work showing that the particle nature of light could explain the "photoelectric effect." This was an experiment whereby low energy (red) light, when shining onto a photoelectric material, caused the material to emit low energy (slow moving) electrons, while high energy (blue) light caused the same material to emit high energy (fast moving) electrons. However, lots of red light only ever produced more low energy electrons, never any high-energy electrons. In other words, the energy could not be "saved up" but rather must be absorbed by the electrons in the photoelectric material individually. The conclusion was that light came in packets, little quantities, and behaved thus as a particle as well as a wave.

So light is both a particle and a wave. OK, kind of unexpected (like Jell-O) but perhaps not totally weird. But the double slit experiment had another trick up its sleeve. One could send one photon (or "quantum" of energy) through a single slit at a time, with a sufficiently long interval in between, and eventually a spot builds up that looks just like the one produced when a very intense (many photons) light was sent through the slit. But then a strange thing happened. When one sends a single photon at a time (waiting between each laser pulse, for example) toward the screen when both slits are open, rather than two spots eventually building up opposite the two slit openings, what eventually builds up is the interference pattern of alternating bright and dark lines! Hmm. how can this be, if only one photon was sent through the apparatus at a time?

The answer is that each individual photon must - in order to have produced an interference pattern -- have gone through both slits! This, the simplest of quantum weirdness experiments, has been the basis of many of the unintuitive interpretations of quantum physics. We can see, perhaps, how physicists might conclude, for example, that a particle of light is not a particle until it is measured at the screen. It turns out that the particle of light is rather a wave before it is measured. But it is not a wave in the ocean-wave sense. It is not a wave of matter but rather, it turns out that it is apparently a wave of probability. That is, the elementary particles making up the trees, people, and planets -- what we see around us -- are apparently just distributions of likelihood until they are measured (that is, measured or observed). So much for the Victorian view of solid matter!

The shock of matter being largely empty space may have been extreme enough -- if an atom were the size of a huge cathedral, then the electrons would be dust particles floating around at all distances inside the building, while the nucleus, or center of the atom, would be smaller than a sugar cube. But with quantum physics, even this tenuous result would be superseded by the atom itself not really being anything that exists until it is measured. One might rightly ask, then, what does it mean to measure something? And this brings us to the Uncertainly Principle first discovered by Werner Heisenberg. Dr. Heisenberg wrote, "Some physicist would prefer to come back to the idea of an objective real world whose smallest parts exist objectively in the same sense as stones or trees exist independently of whether we observe them. This however is impossible."

Perhaps that is enough to think about for now. So in the next essay we will examine, in some detail, the uncertainty principle as it relates to what is called "the measurement problem" in quantum physics. We shall find that the uncertainty principle will be the key to performing the double-slit experiment over astronomical distances, and demonstrating that quantum effects are not just microscopic phenomena, but can be extended across the cosmos.

• Physlink.com: a web reference for all types of Physics, including a section in which you can ask the experts. If you think you are an expert you can even try answering the questions yourself!
• Physics Virtual Bookshelf: Documents on various physics topics written by staff members at the University of Toronto.
• Hyperphysics: Clear and succinct explanations of general physics.
• Cern Public Site: CERN is the European Centre for Nuclear Research - the world's largest Particle Physics centre.

### How to ask a question?

If you have a question about another area of astronomy, find the topic you're interested in from the archive on the side bar or search using the below search form. If you still can't find what you are looking for, submit your question here.

## Heisenberg's Astrophysics Prediction Finally Confirmed After 80 Years

Light coming from the surface of a neutron star can be polarized by the strong magnetic field it . [+] passes through, thanks to the phenomenon of vacuum birefringence. Detectors here on Earth can measure the effective rotation of the polarized light. Image credit: ESO/L. Calçada.

Discovering that our Universe was quantum in nature brought with it a lot of unintuitive consequences. The better you measured a particle's position, the more fundamentally indeterminate its momentum was. The shorter an unstable particle lived, the less well-known its mass fundamentally was. Solid, material objects exhibit wave-like properties. And perhaps most puzzlingly of all, empty space -- space that's had all of its matter and radiation removed -- isn't empty, but is rather filled with virtual pairs of particles and antiparticles. 80 years ago, physicist Werner Heisenberg (who determined the two fundamental uncertainty relations), along with Hans Euler, predicted that because of these virtual particles, strong magnetic fields should affect how light propagates through a vacuum. Thanks to neutron star astronomy, that prediction has just been confirmed.

A neutron star, despite being mostly made of neutral particles, produces the strongest magnetic . [+] fields in the Universe. Image credit: NASA / Casey Reed - Penn State University.

We might take the name "neutron star" quite literally, and assume that it's made out of neutrons exclusively, but that's not quite right. The outer 10% of a neutron star consists mostly of protons and even electrons, which can stably exist without being crushed at the surface. Because neutron stars rotate extremely rapidly -- more than 10% the speed of light -- those charged particles are always in motion, meaning they produce electric currents and magnetic fields. The magnetic fields themselves should affect the particle/antiparticle pairs present in empty space differently, since they have opposite charges. And if you have light passing through that region of space, it should get polarized dependent on the strength of the field.

Direct laser pulse experiments seek to measure this vacuum birefringence under laboratory . [+] conditions, but have been unsuccessful so far. Image credit: Probing vacuum birefringence under a high-intensity laser field with gamma-ray polarimetry at the GeV scale, by Yoshihide Nakamiya, Kensuke Homma, Toseo Moritaka, and Keita Seto, via https://arxiv.org/abs/1512.00636.

This effect is known as vacuum birefringence, and occurs as the charged particles get yanked in opposite directions by the strong magnetic field lines. Because the effect scales as the square of the magnetic field strength, it makes sense to look at neutron stars for this effect. While Earth's magnetic field is about 100 microTesla, the strongest magnetic fields we produce on Earth are only about 100 Tesla: strong, but not strong enough. But with the extreme conditions of neutron stars, large regions of space contain magnetic fields in excess of 10^8 Tesla, making this an ideal place to look.

VLT image of the area around the very faint neutron star RX J1856.5-3754. The blue circle, added by . [+] E. Siegel, shows the location of the neutron star. Image credit: ESO.

Although not very much light is emitted from the surface of the neutron star, the light that is emitted must pass through the strong magnetic field on its way to our telescopes, detectors and eyes. Because the space exhibits this vacuum birefringence effect, the light passing through it must get polarized, and it should all exhibit a common direction of polarization. By measuring the light from the very faint neutron star RX J1856.5-3754 with the Very Large Telescope in Chile, a team led by Roberto Mignani was able to measure the polarization degree for the first time. The actual data show a large effect: a polarization degree of around 15%.

Measurement of the polarization around the neutron star RX J1856.5-3754. Image credit: Figure 3 from . [+] Evidence for vacuum birefringence from the first optical polarimetry measurement of the isolated neutron star RX J1856.5−3754, R.P. Mignani et al., MNRAS 465, 492 (2016).

If you do the calculation for what the effect of vacuum birefringence ought to be and subtract it out, as the authors do, you can clearly see that it accounts for nearly all of the polarization. The data and the predictions match practically perfectly.

Without the effects of vacuum polarization, practically no signal would be visible. The data and the . [+] theory match. Image credit: Figure 3 from Evidence for vacuum birefringence from the first optical polarimetry measurement of the isolated neutron star RX J1856.5−3754, R.P. Mignani et al., MNRAS 465, 492 (2016).

The reason this neutron star -- as opposed to others -- is so perfect for this measurement is that most neutron stars have their surface obscured by a dense, plasma-filled magnetosphere. If we tried to look at the pulsar in the Crab Nebula, for example, we'd have no chance of making this observation at all. The region around it is simply opaque to the types of light we'd like to measure.

Heisenberg and Euler made this prediction all the way back in 1936, and it's gone completely untested until now. Thanks to this pulsar, we have confirmation that light polarized in the same direction as the magnetic field has its propagation affected by quantum physics, in exact agreement with the predictions from quantum electrodynamics. A theoretical prediction from 80 years ago adds another feather in the cap of Heisenberg, who can now posthumously add "astrophysicist" to his resume. But RX J1856.5-3754 can, in the future, confirm vacuum birefringence even more strongly by looking in the X-rays.

The future Athena X-ray telescope from the European Space Agency. Image credit: MPE and the Athena . [+] team.

We don't have a space telescope capable of measuring X-ray polarization today, but the ESA's upcoming Athena mission will do exactly that. As opposed to the

15% polarization the visible light exhibits, X-rays ought to be

100% polarized. Athena is currently slated for launch in 2028, and combined with giant ground-based observatories like the Giant Magellan Telescope and the ELT, should deliver this confirmation for many such neutron stars. It's another victory for the unintuitive but fascinating quantum Universe.

Reference: Evidence for vacuum birefringence from the first optical polarimetry measurement of the isolated neutron star RX J1856.5−3754, R. P. Mignani, V. Testa, D. Gonzalez Caniulef, R. Taverna, R. Turolla, S. Zane and K. Wu, MNRAS 465, 492 (2016).

## Radio Telescopes Could Make Dark Matter Visible

With a big enough radio telescope, astronomers could create a map detailing the structure and distribution of invisible dark matter in the universe up to 10 times sharper than previous ones made using visible light telescopes.

Scientists think the stars and glowing gas visible to optical telescopes make up only about 10 percent of the matter in the universe. The rest is thought to be a mysterious form of non-luminous matter called "dark" matter.

As befits the name, dark matter does not absorb or emit light, and is therefore invisible to current instruments. While it can't be detected directly, dark matter's presence has been implied by the effect its gravity has on light.

Streaming light from distant stars and galaxies is bent slightly by the gravitational effects of large foregrounds objects-such as other stars, galaxies, or large dark matter clumps-that they travel past. The bending causes a detectable distortion of the images of the distant objects similar to the distortion observed in the reflections of rippled water in a lake. By measuring the strength of the distortion, scientists can calculate the strength of the gravity and mass of the foreground objects. Using this technique, called gravitational lensing, scientists have created density maps of large objects in the universe, including clumps of dark matter.

Previous maps created using optical telescopes suffer from blurriness, however, because many distant light sources are required to make a sharp image, and there are not enough visible galaxies in the sky for this. Only the very largest lumps of matter, corresponding to the very biggest galaxy clusters, can be pinpointed with any confidence on such maps.

Another problem is that many of the massive objects, such as primordial hydrogen clouds present before the first stars or galaxies ever formed, lie behind [image] the galaxies whose visible light scientists are measuring. As a result, the more ancient structures in the universe are invisible to many of today's instruments. To view these objects, scientists need many more distant light sources.

Radio waves could provide these sources, say Benton Metcalf and Simon White, researchers at the Max Plank Institute of Astrophysics in Germany.

Visible light from shining stars and galaxies is a relatively recent development in the history of the universe. Before the first star blazed into existence and the first galaxy shined, the universe was a dark place, pervaded by matter and energy but no light. More ancient than visible light are radio waves, some of which can be traced back to a time before galaxies.

There are also many more radio wave sources in the universe. Look in any direction in the sky, and there are up to 1,000 radio sources at different distances along that single line of sight.

About 400,000 years after the Big Bang, hydrogen and helium atoms permeated the universe. Over the course of a few hundred million years, gravity caused these gases to clump together to form dense clouds. Eventually, these clouds coalesced into the first stars and galaxies whose radiance put an end to the universe's "dark age."

Theory predicts that ultraviolet light from these early celestial bodies heated the hydrogen and helium and caused the hydrogen atoms to emit and absorb radio waves of a specific wavelength: 21 cm (8.3 inches). As the universe expanded, this wavelength expanded as well, and today should be about 2 to 20 meters (7 to 66 feet) long.

A new kind of radio telescope

In a study published in last November's issue of the Monthly Notice of the Royal Astronomical Society, Metcalf and White showed that a sufficiently large radio telescope that measures the gravitational lensing of these early universe radio waves could be used to create a density map of the universe rivaling, or even besting, today's sharpest optical telescopes [image]. The researchers predict that such a telescope could spot light from an object with a mass similar to our own Milky Way galaxy from when the universe was only 685 million years old, or about 5 percent of its predicted present age.

This kind of high-resolution imaging requires an extremely large radio telescope array, however. "To map individual galaxies, you need something like 100 km (62 miles)," Metcalf told SPACE.com. "You could have one that's smaller, but you wouldn't get as good resolution."

No radio telescope that big is currently planned, but a smaller project, called the Low Frequency Array, or LOFAR, being constructed in the Netherlands and Germany, could provide similar, albeit lower resolution, radio pictures of the sky. LOFAR's arrays will be arranged in clusters and scattered over an area about 350 km (217 miles) in diameter. However, its most sensitive core array-the part required to do the type of imaging the researchers propose-will only be about 7 km (4.34 miles) across, Metcalf said.

• Contributed by Andrew Fraknoi, David Morrison, & Wolff et al.
• Sourced from OpenStax

The nearest star is so far away that the fastest spacecraft humans have built would take almost 100,000 years to get there. Yet we very much want to know what material this neighbor star is composed of and how it differs from our own Sun. How can we learn about the chemical makeup of stars that we cannot hope to visit or sample?

In astronomy, most of the objects that we study are completely beyond our reach. The temperature of the Sun is so high that a spacecraft would be fried long before it reached it, and the stars are much too far away to visit in our lifetimes with the technology now available. Even light, which travels at a speed of 300,000 kilometers per second (km/s), takes more than 4 years to reach us from the nearest star. If we want to learn about the Sun and stars, we must rely on techniques that allow us to analyze them from a distance.

• 5.1: The Behavior of Light James Clerk Maxwell showed that whenever charged particles change their motion, as they do in every atom and molecule, they give off waves of energy. Light is one form of this electromagnetic radiation. The wavelength of light determines the color of visible radiation. Wavelength (&lambda) is related to frequency (f) and the speed of light (c) by the equation c = &lambdaf. Electromagnetic radiation sometimes behaves like waves, but at other times, it behaves as if it were a particle- called a photon.
• 5.2: The Electromagnetic Spectrum The electromagnetic spectrum consists of gamma rays, X-rays, ultraviolet radiation, visible light, infrared, and radio radiation. Many of these wavelengths cannot penetrate the layers of Earth&rsquos atmosphere and must be observed from space, whereas others&mdashsuch as visible light, FM radio and TV&mdashcan penetrate to Earth&rsquos surface. The emission of electromagnetic radiation is intimately connected to the temperature of the source.
• 5.3: Spectroscopy in Astronomy A spectrometer is a device that forms a spectrum, often utilizing the phenomenon of dispersion. The light from an astronomical source can consist of a continuous spectrum, an emission (bright line) spectrum, or an absorption (dark line) spectrum. Because each element leaves its spectral signature in the pattern of lines we observe, spectral analyses reveal the composition of the Sun and stars.
• 5.4: The Structure of the Atom Atoms consist of a nucleus containing one or more positively charged protons. All atoms except hydrogen can also contain one or more neutrons in the nucleus. Negatively charged electrons orbit the nucleus. The number of protons defines an element (hydrogen has one proton, helium has two, and so on) of the atom. Nuclei with the same number of protons but different numbers of neutrons are different isotopes of the same element.
• 5.5: Formation of Spectral Lines When electrons move from a higher energy level to a lower one, photons are emitted, and an emission line can be seen in the spectrum. Absorption lines are seen when electrons absorb photons and move to higher energy levels. Since each atom has its own characteristic set of energy levels, each is associated with a unique pattern of spectral lines. This allows astronomers to determine what elements are present in the stars and in the clouds of gas and dust among the stars.
• 5.6: The Doppler Effect If an atom is moving toward us when an electron changes orbits and produces a spectral line, we see that line shifted slightly toward the blue of its normal wavelength in a spectrum. If the atom is moving away, we see the line shifted toward the red. This shift is known as the Doppler effect and can be used to measure the radial velocities of distant objects.
• 5.E: Radiation and Spectra (Exercises)

Thumbnail: This photograph of the Sun was taken at several different wavelengths of ultraviolet, which our eyes cannot see, and then color coded so it reveals activity in our Sun&rsquos atmosphere that cannot be observed in visible light. This is why it is important to observe the Sun and other astronomical objects in wavelengths other than the visible band of the spectrum. This image was taken by a satellite from above Earth&rsquos atmosphere, which is necessary since Earth&rsquos atmosphere absorbs much of the ultraviolet light coming from space. (credit: modification of work by NASA).

## The Wave Properties of Light

To begin our study of light, we’re actually going to first discuss waves in general. For example, what happens when a pebble is thrown into a pond?

As shown in the image above, where the pebble enters, the water starts to oscillate up and down. The “pieces” of water right next to where the pebble entered “feel” the water next to them going up and down, and they start to move up and down, too. The disturbance in the water moves outward as more pieces of water start to move up and down. The water in each place only moved up and down, but a wave moved outward from where the pebble entered the water. No water moved outward—what moved outward is the disturbance in the pond's surface. The outward motion of the disturbance transports energy from one place (the location where the pebble entered the water) to another (all points outward from the pebble entry point). This example illustrates that a wave is really a mechanism by which energy gets transported from one location to another.

Electric fields and magnetic fields can be disturbed in a similar way to the surface of a pond. When a stationary charged particle begins to vibrate (or more generally, if it is accelerated), the electric field that surrounds the particle becomes disturbed. Changing electric fields create magnetic fields, so a moving charge creates a disturbance in both the electric field and magnetic field near the charged particle. The outward moving disturbance in the electromagnetic field is an electromagnetic wave. The phenomenon that we refer to as “light” is simply an electromagnetic wave.

Light (or any other wave) is characterized by its wavelength or its frequency. For any wave, the wavelength is the distance between two consecutive peaks. If you stand at one particular point and count how many peaks pass by you per second, this number is the frequency.

White light (for example, what comes out of a flashlight) is actually made up of many waves that each exhibit one of the different colors of light (red, orange, yellow, green, blue, and violet). The reason that different waves of light appear to be different colors of light is because the color of a light wave depends on its wavelength. For example, the wavelength of blue light is about 450 nanometers, while the wavelength of red light is about 700 nanometers. A light source that gives off white light is therefore emitting multiple waves of light with a wide range of wavelengths from 450 nanometers through 700 nanometers. All of these light waves move at the same speed (the speed of light), so you can determine their frequencies and see that red light has a lower frequency than blue light.

#### Try This!

There is an online, interactive tool created by the folks at HubbleSite called "Star-light, Star-bright" for younger students who want to investigate light. Go to that link and study the "Catch the Waves" and "Making Waves" content.

The wavelength of light can be extremely long (kilometers in length!) or smaller than the nucleus of an atom (one millionth of a nanometer!)—so, what do we call light that has a wavelength longer or shorter than the visible light that we are used to? Well, here is one example: light that has a wavelength just longer than red is called infrared light. The next example is light with a wavelength just shorter than violet light, which is called ultraviolet light. The entire range of possible types of light, from the longest wavelengths (radio waves) to the shortest wavelengths (gamma rays) is called the electromagnetic spectrum.

You may have learned in another course that light is peculiar in that it can be described (as we just did) as being a wave, but in some experiments it behaves, and can be described more accurately, as a particle. When we describe light as a particle, we'll refer to an individual "packet" of light as a photon. You can still refer to the wavelength and the frequency of that photon, even though you are considering it to be a particle rather than a wave. If you go back to the very first discussion at the beginning of this page, we talked about how waves transport energy. So, each photon of light does carry energy, and the amount of energy depends on the wavelength or frequency of that photon. The equation is:

In these equations, E is energy, h is Planck's constant, and c is the speed of light.

• The experiment that discovered infrared light
• The discovery of ultraviolet light (from The National Radio Astronomy Observatory) (from NASA)

Before we discuss the entire electromagnetic spectrum in detail, we will next discuss how astronomers represent the range of light emitted by a source in a diagram or image called a spectrum.

## Air Pollution & Visibility

Air pollution can create a haze that affects visibility, dulling national park views by softening the textures, fading colors, and obscuring distant features.

The National Park Service (NPS) keeps track of the visibility conditions in NPS areas and works with air regulatory agencies and partners to improve visibility.

In eastern parks and wilderness areas, the average distance a visitor can see has improved from 50 miles in 2000 to 70 miles in 2015 and very clear days, now regularly occur. In western parks and wilderness areas, the average distance a visitor can see has improved from 90 miles to 120 miles over the same period. Unfortunately, the clarity of park views is still affected by air pollution in virtually all national parks across the country.

### How is visibility impacted by pollution?

Air pollution can create a white or brown haze that affects how far we can see. It also affects how well we are able to see the colors, forms, and textures of natural and historic vistas.

Haze is caused when sunlight encounters tiny particles in the air. The particles scatter light into and out of the sight path and absorb some light before it reaches your eyes. The more particles in the air, the more scattering and absorption of light to reduce the clarity and colors of what you see. Some types of particles scatter more light, especially when it is humid. Haze is mostly caused by air pollution from human activity including industry, power generation, transportation, and agriculture. Natural haze from dust, wildfires, and more also occurs in many parks.

Tiny particles affect how well you can see by scattering light into and out of the sight path and absorbing some light before it reaches your eyes.

Air pollution does not significantly impact views on clear days. Also, sometimes weather and not haze is the main thing affecting how well you can see a view.

On hazy days, air pollution can be visible as a plume, layered haze, or uniform haze. A plume is a column-shaped layer of air pollution coming from a point source (such as a smoke stack). Layered haze is any confined layer of pollutants that creates a contrast between that layer and either the sky or landscape behind it. Plumes and layers can mix with the surrounding atmosphere, creating a uniform haze or overall decline in air clarity.

Plumes and layered haze are more common during cold winter months when the atmosphere moves less. Uniform haze occurs most often when warm air causes atmospheric pollutants to become well mixed.

Types of haze include plumes, layered haze, and uniform haze.

## New theory of gravity might explain dark matter

The rotation curve of typical spiral galaxy M33 (yellow and blue points with errorbars) and the predicted one from distribution of the visible matter (white line). The discrepancy between the two curves is accounted for by adding a dark matter halo surrounding the galaxy. Though dark matter is by far the most accepted explanation of the rotation problem, other proposals have been offered, including Modified Newtonian Dynamics (MOND) and this new theory of Emergent Gravity. Image credit: Wikimedia Commons. A new theory of gravity might explain the curious motions of stars in galaxies. Emergent gravity, as the new theory is called, predicts the exact same deviation of motions that is usually explained by inserting dark matter in the theory. Professor Erik Verlinde, renowned expert in string theory at the University of Amsterdam and the Delta Institute for Theoretical Physics, published a new research paper today in which he expands his groundbreaking views on the nature of gravity.

In 2010, Erik Verlinde surprised the world with a completely new theory of gravity. According to Verlinde, gravity is not a fundamental force of nature, but an emergent phenomenon. In the same way that temperature arises from the movement of microscopic particles, gravity emerges from the changes of fundamental bits of information, stored in the very structure of spacetime.

Newton’s Law from information
In his 2010 article, On the origin of gravity and the laws of Newton, Verlinde showed how Newton’s famous second law, which describes how apples fall from trees and satellites stay in orbit, can be derived from these underlying microscopic building blocks. Extending his previous work and work done by others, Verlinde now shows how to understand the curious behaviour of stars in galaxies without adding the puzzling dark matter.

Puzzling star velocities
The outer regions of galaxies, like our own Milky Way, rotate much faster around the centre than can be accounted for by the quantity of ordinary matter like stars, planets and interstellar gasses. Something else has to produce the required amount of gravitational force, and so dark matter entered the scene. Dark matter seems to dominate our universe: more than 80 percent of all matter must have a dark nature. Hitherto, the alleged dark matter particles have never been observed, despite many efforts to detect them.

Professor Erik Verlinde, University of Amsterdam, Delta Institute for Theoretical Physics. Image credit: University of Amsterdam. No need for dark matter
According to Erik Verlinde, there is no need to add a mysterious dark matter particle to the theory. In a new paper, which appeared today on the ArXiv preprint server, Verlinde shows how his theory of gravity accurately predicts the velocities by which the stars rotate around the center of the Milky Way, as well as the motion of stars inside other galaxies. “We have evidence that this new view of gravity actually agrees with the observations,” says Verlinde. “At large scales, it seems, gravity just doesn’t behave the way Einstein’s theory predicts.”

At first glance, Verlinde’s theory has features similar to modified theories of gravity like MOND (Modified Newtonian Dynamics, Mordehai Milgrom 1983). However, where MOND tunes the theory to match the observations, Verlinde’s theory starts from first principles. “A totally different starting point,” according to Verlinde.