# Can some stars not emit any energy in the visible spectrum?

Stars convert mass into energy. Even converting the slightest mass into energy is immense because the speed of light is so great and $E = mc^2$. This means that stars have to emit large amounts of energy. So can they emit so much energy in a form of electromagnetic radiation that is not visible light? Resulting in a star that cannot be detected using our senses(+ a telescope). If so what problems could this star pose?

Light that is not light

That's meaningless. All light is electromagnetic radiation. A finite part of the infinitely large range of the electromagnetic spectrum is visible light. So you should talk about EM radiation and to discuss the visible spectrum just say visible spectrum.

Stars emit lots of energy at frequencies that are outside the visible range.

Dark Matter

Dark matter does not mean that it's just not radiating in the visible part of the spectrum. It means it's not radiating in any part of the EM spectrum much. If dark matter exists (and that's not certain) it won't interact with most matter much at all.

If dark matter were in star-like objects, it sounds very unlikely that there would not be any EM radiation as a result of the interactions that would result in a star-like object. We'd detect it if it were there. So I don't think that's likely.

You mix up a lot of things in your assumptions.

Stars convert mass into energy.

$$E = mc^2$$

The way stars produce energy is nuclear fusion. One main process there is the tripple-alpha process, which releases a net energy of about $$E sim 7$$ MeV (that is not that much)

So can they emit so much energy in a form of light that is not light?

By light you are speaking of visible light. But radiation goes over all frequencies and wavelength, not just the one the human eye can see (which is roughly $$lambda = 400 - 700$$ nm). Here you want to check out the theory behind black body radiation, which describes - more or less - the intensity of the light at different wavelengths/frequencies at a given temperature.

For an overwiev, look at this figure. The sun's surface temperature ist about 6000 Kelvin, so you can infer, that it's radiation maximum is in the range, which the human eye would perceive as green.

So can they emit so much energy in a form of light that is not light? Resulting in a star that cannot be detected using our senses(+ a telescope).

Presumably by "light that is not light" you mean light outside of the visible spectrum. This is why the fields of gamma ray astronomy, x-ray astronomy, ultraviolet astronomy, infrared astronomy, microwave astronomy, and radio astronomy exist. Astronomy is not limited to the visible spectrum, and there's lots of interesting stuff to "see" from using a telescope that senses light outside of the visible spectrum.

The hottest of stars emit most of their radiation as x-rays and ultraviolet light, but they also emit a lot of visible light. These very hot stars are quite large, short-lived, and very rare. The coolest of stars emit most of their radiation in the infrared, but they also emit some light in the visible range. These very cool stars are quite small, very long-lived, and extremely numerous. Their cool temperatures makes them rather hard to see, even in the infrared.

If so what problems could this star pose? Could this be dark matter?

The very smallest of those cool stars, along with black holes, cool neutron stars, brown dwarfs, and rogue planets collectively form a set of candidates for dark matter called "massive astrophysical compact halo objects", or MACHOs for short. However, theoretical calculations along with observations says that MACHOs can account for at most about 20% of dark matter.

I will just point out two things that haven't really been discussed above.

Firstly, stars generally fall into two categories. 'Hot stars' are those that convert mass into energy by nuclear fusion, and 'cold stars' are those that have ended the nuclear burning phase, e.g. white dwarfs and neutron stars. These still have a finite temperature and therefore will emit radiation. As you can imagine, however, since they no longer produce energy, eventually they will become actually cold, hence not radiating (or radiating very little).

The second point to note is that radiations from stars far away are redshifted as they reach us, and they are likely to be attenuated by dust. Therefore, even if the radiation itself when emitted is visible light, by the time it gets to us it may no longer be. However, there are still many other different spectroscopic techniques that will allow us to 'see' it.

## Can some stars not emit any energy in the visible spectrum? - Astronomy

When the universe was younger, some galaxies produced a lot more radiation than galaxies do today. A typical galaxy shines with the energy from billions of stars and is tens of thousands of light years (or more) across. A peculiar group of galaxies are extremely luminous but very compact. The light from these active galaxies is produced by a strange process in the centers of the galaxies. Sometimes starburst galaxies are grouped with these peculiar galaxies, but here I will consider an active galaxy'' to be a galaxy with a very luminous nucleus. The great luminosity of starburst galaxies is not confined to their nucleus.

### Quasars

Which is the quasar and which is an ordinary star? (The quasar is the bright one on the left.)
Courtesy of Space Telescope Science Institute

Maarten Schmidt solved the mystery in 1963. In order to figure out the structure of the atoms producing the bizarre spectra, he set out to construct an energy level diagram from the pattern of the emission lines. He made some mistakes in his calculations because his calculations were not showing the regularity he could clearly see in the spectrum of a radio source called 3C 273 (the 273rd object in the third Cambridge catalog of radio sources). As a test of the regularity he compared the spectrum of 3C 273 with the spectrum of hydrogen. He was shocked because the pattern was the same but greatly redshifted! 3C 273 is moving at a speed of 47,400 kilometers/second (almost 16% the speed of light!). The Hubble-Lemaître Law says that this blue radio object is far outside the Galaxy. The other radio stars'' were also at great distances from us. They are called quasi-stellar radio sources or quasars for short. Later, some other blue star-like objects at large redshifts were discovered to have no radio emission, but they are still called quasars.

What is strange about the quasars is not their great distance, but, rather, their incredible luminosities. They are hundreds to thousands of times more luminous than ordinary galaxies. Yet, all of this energy is being produced in a small volume of space. Their luminosity varies on time scales of a few months to as short as a few days. Remember from the discussion of pulsars and black holes in the stellar evolution chapter that the light fluctuation time scale from any object gives you an estimate of the maximum possible size of the object. The maximum size = (speed of light) ×(light fluctuation time interval). The quasars that vary their light output over a few months are about the size of our solar system. This is tens of thousands of times smaller than a typical galaxy!

The shape of the continuous part of a quasar spectrum is also quite unusual. Stars are luminous in primarily the visible (optical) band of the electromagnetic spectrum. The hottest stars also emit a significant fraction of their light in the ultraviolet band and the coolest stars emit a significant fraction of their light in the infrared band. Regardless of the star, though, the spectrum of a star, and, hence, the spectrum of a normal galaxy, rises to a peak at some wavelength determined by the temperature (remember Wien's law?) and drops off at wavelengths shorter or longer than the peak wavelength. Such a spectrum is called a thermal spectrum because it depends on just the temperature.

Quasars have a decidedly non-thermal spectrum: they are luminous in the X-rays, ultraviolet, visible, infrared, and radio bands. They have about the same power at all of the wavelengths down to the microwave wavelengths (shortwave radio wavelengths). The spectrum looks like the synchrotron radiation from charged particles spiraling around magnetic field lines at nearly the speed of light (remember the emission from pulsars?).

Perhaps the quasars are not as far away as the Hubble-Lemaître Law says from their redshifts. If their large redshifts are due to some powerful explosive event that shot the quasars out at some tremendous speed, then you would not have to worry about the tremendous luminosities. That would be nice, but unfortunately (or fortunately, if you like a good mystery) that does not appear to be the case. Quasars are found in clusters of galaxies. The galaxies are much fainter than the quasars so only the largest telescopes can gather enough light to create a spectrum for those far away galaxies. Their spectra also have the same large redshift of the quasars in the cluster. Also, some quasars are close enough to us that some fuzz is seen around them. The color of the fuzz is like that of normal galaxies. The spectra of the fuzzy patches around the bright quasar shows that the light from the fuzz is from stars.

Equal opportunity quasar hosts: top left: core of normal spiral, bottom left: core of normal elliptical, top center: spiral galaxy hit face-on to make a quasar+starburst galaxy, bottom center: quasar merging with a bright galaxy and maybe another one, top right: tail of dust and gas show that the host galaxy collided with another one, bottom right: merging galaxies create a quasar in their combined nucleus. Select the image to view an enlarged version in another window.
Courtesy of Space Telescope Science Institute

In addition, the gravitational lensing of quasars by distant galaxies is only possible if the lensed quasars are farther away than the galaxy bending the quasar's light. Quasars are the exceptionally bright nuclei of galaxies!

## Kelt-9 b

The hottest exoplanet known so far is Kelt-9 b, which was discovered in 2016. Kelt-9 b orbits a star that is twice as hot as our Sun, at a distance ten times closer than Mercury orbits our star. It is a large gaseous exoplanet, with a radius 1.8 times that of Jupiter and temperatures reaching 5,000K. For comparison, this is hotter than 80% of all the stars in the universe and a similar temperature to our Sun.

In essence, hot Jupiters are a window into extreme physical and chemical processes. They offer an incredible opportunity to study physics in environmental conditions that are near impossible to reproduce on Earth. Studying them enhances our understanding of chemical and thermal processes, atmospheric dynamics and cloud formation. Understanding their origins can also help us improve planetary formation and evolution models.

We are still struggling to explain how planets form and how elements, such as water, were delivered to our own Solar System. To find out, we need to learn more about exoplanet compositions by observing their atmospheres.

## Chapter 3

Color and Temperature

This section explains the observations students made in the last section. Stars have different colors because they have different temperatures.

Have students play the hot plate animation to get an intuitive feel for how the visual color of an object changes with temperature. When the fire symbol beneath the hot plate disappears, the plate is removed from the heat and begins to cool. The color sequence reverses. Emphasize that this color sequence depends only on temperature: it does not just apply to hot plates, and the sequence will be the same for any object studied. Question 4 acts as a quick review of the electromagnetic spectrum: students should realize that ultraviolet light has shorter wavelengths than violet light.

Thermal radiation curves are often called blackbody curves. This term was chosen because the curve represents only the thermal radiation of "black bodies" - objects that do not reflect any light at any wavelength (remember that an object that does not reflect light will appear black). Stars are nearly perfect black bodies: their color comes almost completely from their own glow and not from other light that they reflect. The term "blackbody curve" is not used in this project because many students find the name confusing. "Thermal radiation curve" expresses the idea that any object that emits thermal radiation (that is, light emitted as a function of temperature) will show radiation whose intensity matches such a curve. The diagram shows thermal radiation curves for stars at three different temperatures.

Explore 4 lets students experiment with changing the temperature of an imaginary object and watching its thermal radiation curve change. Alternatively, they can drag the curve to change its peak wavelength, then see what temperature would be required to generate that curve. You should familiarize yourself with the Java applet before assigning this exercise to students. You may assign them to figure out the mathematical relationship between temperature and peak wavelength, or you may just have them develop an intuitive feel for how the curves change with temperature.

Temperature and Peak Wavelength

The equation l peak T = 2.897 x 10 -3 m K can be used to predict the peak wavelength (in Angstroms) from the temperature (in Kelvin) of the thermal radiation curve, or vice versa. Practice 2 and 3 give students practice in using this equation, called "Wien's Law" (pronounced VEEN) to solve problems.

If students ask, tell them that the Kelvin scale, named for British physicist Lord Kelvin, has the same degree size as the Celsius scale, which is larger than the Fahrenheit degree by a factor of 1.8. The Kelvin scale is set so that 0 K is equal to "absolute zero" - the temperature at which atoms stop moving. Kelvin temperatures are written without the degree symbol.

This section introduces students to spectra - graphs of the intensity (amount) of light coming from a star as the function of the light's wavelength. Be sure students understand what they are looking at. Actually, the thermal radiation curves in the last section are spectra of perfect thermal sources. However, on SkyServer, the term "spectrum" is reserved for an observation of a star. Point out to students how large the peaks and valleys of non-thermal radiation can be. Practice 4 and Explore 5 ask students to find stars' peak wavelength from their spectra. To find the peak wavelengths, students may look at the blue curve superimposed on the spectra. However, tell students that this curve is not a thermal radiation curve, but only an approximated polynomial curve that matches with the broad trend of the observed spectrum.

Explore 5 asks students to use the Get Plates tool to look at observed SDSS spectra. Students calculate the temperatures of stars from their peak wavelengths, find an average temperature for SDSS stars, and compare this temperature to the Sun's to see if the Sun is an average star. The Sun is about average for the stars in the SDSS database, but is hotter than average for the stars in our local part of the galaxy. The reason for this discrepancy is that a dim, distant star viewed from Earth will appear too faint to be detectable, even with the SDSS's powerful telescope.

Emphasize the point made in "A Word of Warning": the starlight that we see from Earth is all the light the star emits, from both thermal and non-thermal sources. Non-thermal sources are emission and absorption lines that arise from electrons changing energy levels in the star's atoms. Students should have a sense for what electron energy levels are, but do not need to understand the process of energy level jumping.

Remind students that they can't sort out the two effects, thermal and non-thermal, by looking at the star's color alone. Tell them that the spectrum lets them sort out the two effects. Question 7 asks students whether the spectrum shown is a thermal source. Either yes or no is an acceptable answer, as long as students give reasonable justification for their response.

If you do not intend to have the class do the Color-Color diagram page, stop after Question 7. The last section of this page, "The Other 3,995,000 Stars" is a hook for the Color-Color diagrams section, asking what, if anything, astronomers can learn from looking at color alone.

## Why are there no green stars?

Go outside on a dark, moonless night. Look up. Is it December or January? Check out Betelgeuse, glowing dully red at Orion’s shoulder, and Rigel, a laser blue at his knee. A month later, yellow Capella rides high in Auriga.

Is it July? Find Vega, a sapphire in Lyra, or Antares, the orange-red heart of Scorpius.

In fact, any time of the year you can find colors in the sky. Most stars look white, but the brightest ones show color. Red, orange, yellow, blue… almost all the colors of the rainbow. But hey, wait a sec. Where are the green stars? Shouldn’t we see them?

Nope. It’s a very common question, but in fact we don’t see any green stars at all. Here’s why.

Take a blowtorch (figuratively!) and heat up an iron bar. After a moment it will glow red, then orange, then bluish-white. Then it’ll melt. Better use a pot holder.

Why does it glow? Any matter above the temperature of absolute zero (about -273 Celsius) will emit light. The amount of light it gives off, and more importantly the wavelength of that light, depends on the temperature. The warmer the object, the shorter the wavelength.

Cold objects emit radio waves. Extremely hot objects emit ultraviolet light, or X-rays. At a very narrow of temperatures, hot objects will emit visible light (wavelengths from roughly 300 nanometers to about 700 nm).

Mind you – and this is critical in a minute – the objects don’t emit a single wavelength of light. Instead, they emit photons in a range of wavelengths. If you were to use some sort of detector that is sensitive to the wavelengths of light emitted by an object, and then plotted the number of them versus wavelength, you get a lopsided plot called a blackbody curve (the reason behind that name isn’t important here, but you can look it up if you care – just set your SafeSearch Filtering to “on”. Trust me here). It’s a bit like a bell curve, but it cuts off sharply at shorter wavelengths, and tails off at longer ones.

Here’s an example of several curves, corresponding to various temperatures of objects (taken from online lecture notes at UW:

The x-axis is wavelength (color, if you like) color, and the spectrum of visible colors is superposed for reference. You can see the characteristic shape of the blackbody curve. As the object gets hotter, the peak shifts to the left, to shorter wavelengths.

An object that is at 4500 Kelvins (about 4200 Celsius or 7600 F) peaks in the orange part of the spectrum. Warm it up to 6000 Kelvin (about the temperature of the Sun, 5700 C or 10,000 F) and it peaks in the blue-green. Heat it up more, and the peaks moves into the blue, or even toward shorter wavelengths. In fact, the hottest stars put out most of their light in the ultraviolet, at shorter wavelengths than we can see with our eyes.

Now wait a sec (again)… if the Sun peaks in the blue-green, why doesn’t it look blue-green?

Ah, this is the key question! It’s because it might peak in the blue-green, but it still emits light at other colors.

Look at the graph for an object as hot as the Sun. That curve peaks at blue-green, so it emits most of its photons there. But it still emits some that are bluer, and some that are redder. When we look at the Sun, we see all these colors blended together. Our eyes mix them up to produce one color: white. Yes, white. Some people say the Sun is yellow, but if it were really yellow to our eyes, then clouds would look yellow, and snow would too (all of it, not just some of it in your back yard where your dog hangs out).

OK, so the Sun doesn’t look green. But can we fiddle with the temperature to get a green star? Maybe one that’s slightly warmer or cooler than the Sun?

It turns out that no, you can’t. A warmer star will put out more blue, and a cooler one more red, but no matter what, our eyes just won’t see that as green.

The fault lies not in the stars (well, not entirely), but within ourselves.

Our eyes have light-sensitive cells in them called rods and cones. Rods are basically the brightness detectors, and are blind to color. Cones see color, and there are three kinds: ones sensitive to red, others to blue, and the third to green. When light hits them, each gets triggered by a different amount red light (say, from a strawberry) really gets the red cones juiced, but the blue and green cones are rather blasé about it.

Most objects don’t emit (or reflect) one color, so the cones are triggered by varying amounts. An orange, for example, gets the red cones going about twice as much as the green ones, but leaves the blue ones alone. When the brain receives the signal from the three cones, it says “This must be an object that is orange.” If the green cones are seeing just as much light as the red, with the blue ones not seeing anything, we interpret that as yellow. And so on. So the only way to see a star as being green is for it to be only emitting green light. But as you can see from the graph above, that’s pretty much impossible. Any star emitting mostly green will be putting out lots of red and blue as well, making the star look white. Changing the star’s temperature will make it look orange, or yellow, or red, or blue, but you just can’t get green. Our eyes simply won’t see it that way.

That’s why there are no green stars. The colors emitted by stars together with how our eyes see those colors pretty much guarantees it.

But that doesn’t bug me. If you’ve ever put your eye to a telescope and seen gleaming Vega or ruddy Antares or the deeply orange Arcturus, you won’t mind much either. Stars don’t come in all colors, but they come in enough colors, and they’re fantastically beautiful because of it.

Note: this is not the end of the story. There are green objects in space, and some stars do appear green… but that’s for another post, coming soon. Promise.

## Why are there no green stars?

Go outside on a dark, moonless night. Look up. Is it December or January? Check out Betelgeuse, glowing dully red at Orion’s shoulder, and Rigel, a laser blue at his knee. A month later, yellow Capella rides high in Auriga.

Is it July? Find Vega, a sapphire in Lyra, or Antares, the orange-red heart of Scorpius.

In fact, any time of the year you can find colors in the sky. Most stars look white, but the brightest ones show color. Red, orange, yellow, blue… almost all the colors of the rainbow. But hey, wait a sec. Where are the green stars? Shouldn’t we see them?

Nope. It’s a very common question, but in fact we don’t see any green stars at all. Here’s why.

Take a blowtorch (figuratively!) and heat up an iron bar. After a moment it will glow red, then orange, then bluish-white. Then it’ll melt. Better use a pot holder.

Why does it glow? Any matter above the temperature of absolute zero (about -273 Celsius) will emit light. The amount of light it gives off, and more importantly the wavelength of that light, depends on the temperature. The warmer the object, the shorter the wavelength.

Cold objects emit radio waves. Extremely hot objects emit ultraviolet light, or X-rays. At a very narrow of temperatures, hot objects will emit visible light (wavelengths from roughly 300 nanometers to about 700 nm).

Mind you – and this is critical in a minute – the objects don’t emit a single wavelength of light. Instead, they emit photons in a range of wavelengths. If you were to use some sort of detector that is sensitive to the wavelengths of light emitted by an object, and then plotted the number of them versus wavelength, you get a lopsided plot called a blackbody curve (the reason behind that name isn’t important here, but you can look it up if you care – just set your SafeSearch Filtering to “on”. Trust me here). It’s a bit like a bell curve, but it cuts off sharply at shorter wavelengths, and tails off at longer ones.

Here’s an example of several curves, corresponding to various temperatures of objects (taken from online lecture notes at UW:

The x-axis is wavelength (color, if you like) color, and the spectrum of visible colors is superposed for reference. You can see the characteristic shape of the blackbody curve. As the object gets hotter, the peak shifts to the left, to shorter wavelengths.

An object that is at 4500 Kelvins (about 4200 Celsius or 7600 F) peaks in the orange part of the spectrum. Warm it up to 6000 Kelvin (about the temperature of the Sun, 5700 C or 10,000 F) and it peaks in the blue-green. Heat it up more, and the peaks moves into the blue, or even toward shorter wavelengths. In fact, the hottest stars put out most of their light in the ultraviolet, at shorter wavelengths than we can see with our eyes.

Now wait a sec (again)… if the Sun peaks in the blue-green, why doesn’t it look blue-green?

Ah, this is the key question! It’s because it might peak in the blue-green, but it still emits light at other colors.

Look at the graph for an object as hot as the Sun. That curve peaks at blue-green, so it emits most of its photons there. But it still emits some that are bluer, and some that are redder. When we look at the Sun, we see all these colors blended together. Our eyes mix them up to produce one color: white. Yes, white. Some people say the Sun is yellow, but if it were really yellow to our eyes, then clouds would look yellow, and snow would too (all of it, not just some of it in your back yard where your dog hangs out).

OK, so the Sun doesn’t look green. But can we fiddle with the temperature to get a green star? Maybe one that’s slightly warmer or cooler than the Sun?

It turns out that no, you can’t. A warmer star will put out more blue, and a cooler one more red, but no matter what, our eyes just won’t see that as green.

The fault lies not in the stars (well, not entirely), but within ourselves.

Our eyes have light-sensitive cells in them called rods and cones. Rods are basically the brightness detectors, and are blind to color. Cones see color, and there are three kinds: ones sensitive to red, others to blue, and the third to green. When light hits them, each gets triggered by a different amount red light (say, from a strawberry) really gets the red cones juiced, but the blue and green cones are rather blasé about it.

Most objects don’t emit (or reflect) one color, so the cones are triggered by varying amounts. An orange, for example, gets the red cones going about twice as much as the green ones, but leaves the blue ones alone. When the brain receives the signal from the three cones, it says “This must be an object that is orange.” If the green cones are seeing just as much light as the red, with the blue ones not seeing anything, we interpret that as yellow. And so on. So the only way to see a star as being green is for it to be only emitting green light. But as you can see from the graph above, that’s pretty much impossible. Any star emitting mostly green will be putting out lots of red and blue as well, making the star look white. Changing the star’s temperature will make it look orange, or yellow, or red, or blue, but you just can’t get green. Our eyes simply won’t see it that way.

That’s why there are no green stars. The colors emitted by stars together with how our eyes see those colors pretty much guarantees it.

But that doesn’t bug me. If you’ve ever put your eye to a telescope and seen gleaming Vega or ruddy Antares or the deeply orange Arcturus, you won’t mind much either. Stars don’t come in all colors, but they come in enough colors, and they’re fantastically beautiful because of it.

Note: this is not the end of the story. There are green objects in space, and some stars do appear green… but that’s for another post, coming soon. Promise.

The structure of the atom explains the formation of spectral lines

This model was set forth by the Danish physicist Neils Bohr in 1912
• the state of lowest energy - the ground state - is the "normal" condition of the electron
• if an electron energy exceeds a maximum allowed energy in the atom, it will leave the atom, and the atom will be ionized
• the electron can exist only in certain well defined energy states, orbitals

Photons (the quantum of electromagnetic radiation) can be absorbed or emitted by an atom, boosting the electron to an excited state (on absorption) or bring the electron to a lower energy state (on emission).

## Discovery of chromophores that emit light in the ultraviolet region when excited with visible light

Fluorescence usually entails the conversion of light at shorter wavelengths to light at longer wavelengths. Scientists have now discovered a chromophore system that goes the other way around. When excited by visible light, the fluorescent dyes emit light in the ultraviolet region. According to the study published in the journal Angewandte Chemie, such light upconversion systems could boost the light-dependent reactions for which efficiency is important, such as solar-powered water splitting.

Fluorescent dyes absorb light at shorter wavelengths (high energy, e.g. blue light) and emit light at longer wavelengths (low energy, e.g. red light). Upconversion of light is much more difficult to achieve. Upconversion means that a fluorescent dye is excited with radiation in the visible range but emits in the ultraviolet. Such dyes could be used to run high-energy catalytic reactions such as solar-powered water splitting just using normal daylight as an energy source. Such dyes would expand the range of available excitation energy.

Nobuhiro Yanai and colleagues at Kyushu University, Japan, are exploring multi-chromophore systems for their ability to upconvert fluorescence light. Yanai explains how upconversion works: "Fluorescence upconversion occurs when two chromophore molecules, which have been excited in the triplet state by a sensitizer, collide. This collision annihilates the sensitized energy and lifts the chromophores to a higher energy level. From there, they emit the energy as radiation."

In practice, however, it is difficult to achieve effective upconverting chromophore designs -- existing systems need high-intensity radiation and still do not achieve more than ten percent efficiency. "The main reason for the low efficiency is that the sensitizer chromophore molecules also absorb much of the upconverted light, which is then lost," Yanai says.

In contrast, the donor-acceptor chromophore pair developed by Yanai and colleagues exhibits energy levels that are so finely adjusted that it achieved a record-high 20 percent upconversion efficiency. Almost no back-absorption and low nonradiative loss occurred. The novel chromophore pair consisted of an iridium-based donor, which was an established sensitizer, and a naphthalene-derived acceptor, which was a novel compound.

Low back-absorption and few radiative losses mean that the intensity of the exciting radiation can be low. The researchers reported that solar irradiance was sufficient to achieve high upconversion efficiency. Even indoor applications were possible using artificial light. The authors held an LED lamp above an ampoule filled with the chromophore solution and measured the intensity of the emitted UV light.

## Emission

Not all nebulae are galaxies some have completely different spectra than do the galaxies. Instead of looking like stars, they show only a few bright, narrow lines of pure red, blue, or green color from a continuous background. The specific way the emission occurs is complicated, but the net result is that photons from the exciting star cause this specific type of emission, which only occurs in the proximity of hot stars. Each of the pure colors can be specifically identified with a certain type of atom or ion. Nothing about the emission resembles a star. This type of nebula, called an emission nebula, can occur in our Galaxy or any other galaxy with hot stars.

As a specific example, consider hydrogen, the most abundant element in the Universe, which is widely spread throughout the Milky Way Galaxy. When a hot star illuminates nearby hydrogen atoms, starlight is absorbed by the hydrogen. The result is to produce a glowing gas with a spectrum of pure red, green, and blue emission lines in a pattern easily recognizable as hydrogen gas.

The Orion Nebula's Trapezium Cluster: Hubble Space Telescope image of the Orion Nebula. The Trapezium is the set of four optically visible stars at the core of the nebula.
Credit: NASA, C.R. O'Dell and S.K. Wong (Rice University)

Now, I said the process of emission is complicated. Unlike in stars, the hydrogen gas in emission nebulae does not shine because it is very hot. When a hydrogen atom in the nebula is in the path of a high-energy photon from the nearby star, the photon is absorbed by the atom, exciting an electron to a higher state. When the electron reaches this high-energy state, its natural tendency is to lose the energy as fast as possible by releasing new photons. However, an electron in an atom can only have specific energies that depend on the type of atom of which it is a part. So to release its energy, the atom has to emit unique wavelengths of light. Hydrogen, and every other element, shine in their own unique way by emitting a characteristic spectrum, like a unique shade of color identifying the element, and we are able to tell what elements make up an object in space by looking at the wavelengths of light that our detectors receive.

Spectrum of a Small Portion of the Orion Nebula Gas: The emission lines are part of the Balmer series of hydrogen, near 3680 Å. Note the series convergence to shorter wavelengths. The absorption spectrum of Merope, the reflection nebula, shows lines from the same hydrogen series. The nebular emission arises when stellar photons (from very hot stars) ionize the hydrogen gas. When the resulting electrons and protons recombine, the emission spectrum appears. It is obviously easy for an astronomer to distinguish a reflection nebula from and emission nebulae using their spectra, even though they may both look similar in some images. The spectrum was obtained with the Ultraviolet Visual Echelle Spectrograph on one of the four 8.5 meter telescopes called the Very Large Telescope (this particular one named Kueyen), in Chile. The spectrum took about 600 seconds to record. For more information about this spectrum, see Esteban, C, et al. 2004, MNRAS, 355, 229.
Credit: Courtesy of the Monthly Notices of the Royal Astronomical Society

## Can some stars not emit any energy in the visible spectrum? - Astronomy

There are special properties of light that we can take advantage of to understand even objects that are millions and billions of light years away. In this section we explore some of these properties and how we can use them to understand our Universe. In the previous section of this unit, you were told that superheated material created by the supernova explosion gives off X-rays and gamma-rays. X-rays and gamma-rays are really just light (electromagnetic radiation) that has very high energy.

### What is Electromagnetic (EM) Radiation?

Although it would seem that the human eye gives us a pretty accurate view of the world, we are literally blind to much of what surrounds us. A whole Universe of color exists, only a thin band of which our eyes are able to detect an example of this visible range of color is the familiar rainbow (an example of a “spectrum”). The optical spectrum ranges in color from reds and oranges up through blues and purples. Each of these colors actually corresponds to a different energy of light. The colors or energies of light that our eyes cannot see also have names that are familiar to us. We listen to radios, we eat food heated in microwaves, we have X-rays taken of our broken bones. Yet many times we do not realize that radio, X-ray, and microwave are really just different energies of light!

The entire range of energies of light, including both light we can see and light we cannot see, is called the electromagnetic spectrum. It includes, from highest energy to lowest: gamma-rays, X-rays, ultraviolet, optical, infrared, microwaves, and radio waves.

Because light is something that is given off, or radiated from an object, we can call it radiation. That’s why we often talk about X-ray radiation – it’s the same thing as saying X-ray light. When we refer to the whole spectrum of light, we can call it electromagnetic radiation.

Because we can see only visible light, we are put at a disadvantage because the Universe is actively emitting light at all different energies.

Light has different colors because it has different energies. This is true whether we are talking about red and blue visible light, or infrared (IR) and X-ray light. Of all the colors in the visible spectrum, red light is the least energetic and blue is the most. Beyond the red end of the visible part of the spectrum lie infrared and radio light, both of which have lower energy than visible light. Above the blue end of the visible spectrum lies the higher energies of ultraviolet light, X-rays, and finally, gamma-rays.

### What Units are Used to Characterize EM Radiation?

Light can be described not only in terms of its energy, but also its wavelength, or its frequency. There is a one-to-one correspondence between each of these representations. X-rays and gamma-rays are usually described in terms of energy, optical and infrared light in terms of wavelength, and radio in terms of frequency. This is a scientific convention that allows the use of the units that are the most convenient for describing whatever energy of light you are looking at. For example, it would be inconvenient to describe both low energy radio waves and high energy gamma-rays with the same units because the difference in their energies is so great. A radio wave can have an energy on the order of 4 × 10 -10 eV as compared to 4 × 10 9 eV for gamma-rays. That’s an energy difference of 10 19 , or ten million trillion eV!

Wavelength is the distance between two peaks of a wave, and it can be measured with a base unit of meters (m) (such as centimeters, or Ångstroms). Frequency is the number of cycles of a wave to pass some point in a second. The basic unit of frequency is cycles per second, or Hertz (Hz). Energy in astronomy is often measured in electron volts, or eV or its multiples (such as kilo electron volts, or 1,000 eV) .

Wavelength and frequency are related by the speed of light (c), a fundamental constant. Energy is also directly proportional to frequency (the constant of proportionality is Planck’s constant, h) and inversely proportional to wavelength. It was Max Planck who demonstrated that light sometimes behaves as a particle by showing that its energy (E), divided by its frequency (usually denoted using the Greek letter n) is a constant. Since we know that frequency is equal to the speed of light (c) divided by wavelength (the Greek letter l), we also know the relationship between energy and wavelength. The energy (or wavelength or frequency) of light can give important clues into how the light was produced, and it is this characterization of light emission that allows us to understand objects in the distant universe.

Since light can act like both a particle and a wave, we say that light has a particle-wave duality. We call particles of light photons. Low-energy photons (i.e. radio) tend to behave more like waves, while higher energy photons (i.e. X-rays) behave more like particles. This is an important difference because it affects the way we build instruments to measure light (telescopes!).

You are familiar with light in many forms, like sunlight, which you see every day. But how is this light created? Further, how can we use the properties of light to understand objects in the Universe?

### Observing Supernovae and Their Remnants at Different Energies

It pays to make multiple observations of astronomical objects bacause they emit light of different energies. Supernovae remnants can give off visible light, ultraviolet light, radio waves and X-rays. Each observation of a supernovae remnant can give us different information about it.

Let’s examine the Crab Nebula it is unique in that it contains one of only a few pulsars that are observable at so many different energies.

The Crab Nebula’s creation was witnessed in July of 1054 A.D. when Chinese astronomers and members of the Native American Anasazi tribe separately recorded the appearance of a new star. Although it was visible for only a few months, it was bright enough to be seen even during the day! In the 19th century, French comet hunter Charles Messier recorded a fuzzy ball of light near the constellation Taurus. This fuzzy ball turned out not to be a comet after all, but the remains of a massive star whose explosive death had been witnessed centuries before by the Chinese and the Anasazi.

Scientists now believe the Crab Nebula is the remains of a star which suffered a supernova explosion. The core of the star collapsed and formed a rapidly rotating, magnetic neutron star, releasing energy sufficient to blast the surface layers of the star into space with the strength of a 10 28 megaton bomb or a hundred million nuclear warheads. Nestled in the nebulous cloud of expelled gases, the rotating neutron star, or pulsar, continues to generate strobe-like pulses that can be observed at radio, optical, and X-ray energies. The Crab Nebula was one of the first sources of X-rays identified in the early 1960s when the first X-ray astronomy observations were made.

At radio wavelengths, the Crab Nebula, seen to the left, displays two distinctive physical features. The nebulous regions hide radio emission coming from unbound electrons spiraling around inside the nebula. The pulsar at the heart of the Crab Nebula generates pulses at radio frequencies roughly 60 times a second. In this image, the pulsar’s flashes are blurred together (since the image was “exposed” for much longer than 1/60 s) and it appears as the bright white spot near the middle of the nebula.

In the optical, both a web of filaments at the outer edges of the nebula and a bluish core become apparent. The blue core is from electrons within the nebula being deflected and accelerated by the magnetic field of the central neutron star. The red filaments surrounding the edges of the nebula are the remnants of the original outer layers of the star.

In the ultraviolet (or UV) the nebula is slightly larger than when seen in X-rays. Cooler electrons (responsible for the UV emission) extend out beyond the hot electrons near the central pulsar. This supports the theory that the central pulsar is responsible for energizing the electrons.

X-ray observations reveal a condensed core near the central pulsar, which is the bright dot visible slightly left and below center in the image to the right. The Crab Nebula appears smaller and more condensed in X-rays because the electrons which are primarily responsible for the X-ray emission exist only near the central pulsar. Scientists believe that the strong magnetic field near the surface of the neutron star “heats up” the electrons in it and that these “hot” electrons are responsible for the X-ray emission.

### For the Student

Using the text and any external references, define the following terms: radio waves, microwaves, infrared, visible, ultraviolet, X-rays, gamma rays, light energy, photon, electromagnetic spectrum, electromagnetic radiation, Hertz, wave peak, frequency, and wavelength.