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\n Travel the universe with Dr. Ethan Siegel as he answers the biggest questions of all. <\/p>\n
If you look at all the objects detectable in Earth\u2019s skies, including both naturally occurring bodies as well as artificial satellites, it should come as no surprise that the Sun appears as the brightest object of all. The Sun, after all, produces its own light, sustainably powered by nuclear fusion in its core. That core-generated energy helps keep the Sun from contracting under its own gravitation, but also propagates to the Sun\u2019s edge, the photosphere, where the Sun emits radiation over a wide range of wavelengths that correspond to a temperature of around 6000 K. Although the Moon is the second-brightest object in most wavelengths of light, it only appears so bright because of its very close proximity to Earth. From an intrinsic point of view, most of the Moon\u2019s light is merely reflected light from the Sun.<\/p>\n
Although this was first shown to be true in visible wavelengths of light, the 20th century revealed that the same physical process held true across a wide variety of other wavelengths. The Sun is the brightest object in Earth\u2019s skies, followed distantly by the Moon in second place, as seen from Earth in:<\/p>\n
X-rays,<\/p>\n
ultraviolet light,<\/p>\n
visible light,<\/p>\n
infrared light,<\/p>\n
and radio light.<\/p>\n
All of the other planets, stars, satellites, and galaxies trail far behind both the Sun and Moon in terms of brightness as viewed from Earth. However, when we began looking at the sky at the shortest, highest-energy wavelengths, we realized this picture was no longer universally true. The Sun emits practically no gamma-rays during most times, but the Moon can always be caught emitting copious quantities of them. It was a mystery upon first discovering this phenomenon, observationally, but science quickly puzzled out the reason why.<\/p>\n
<\/p>\n
As seen in X-rays against the cosmic background, the Moon\u2019s illuminated (bright) and non-illuminated portions (dark) are clearly visible in this early X-ray image taken by ROSAT. The X-rays emitted from the Moon, like almost all wavelengths of light (with the notable exception of gamma-rays), arise mostly from reflected emission from the Sun.\n<\/p>\n
Credit<\/a>: DARA, ESA, NASA, Max Planck Institut, J.H.M.M. Schmitt<\/p>\n Although the name \u201cHubble\u201d is synonymous with the original idea of NASA\u2019s Great Observatories, history\u2019s most famous space telescope was one of four missions designed and built to view the Universe from above the confines of Earth\u2019s atmosphere. While the atmosphere is either partly or fully transparent to certain wavelengths of light \u2014 with particularly good windows at visible and radio wavelengths \u2014 practically all of the microwave, far-infrared, X-ray, and gamma-ray wavelengths are completely obscured from Earth\u2019s surface, along with much of the near-and-mid-infrared and ultraviolet portions of the spectrum as well.<\/p>\n To overcome these limitations, NASA\u2019s original fleet of Great Observatories spanned many of those obscured wavelengths, with the four Great Observatories being:<\/p>\n Hubble, optimized for visible light observations that also extended a little bit of the way into the ultraviolet and the near-infrared,<\/p>\n Spitzer, optimized to reveal the near-infrared and mid-infrared Universe,<\/p>\n Chandra, specifically designed to view the Universe in X-ray light,<\/p>\n and Compton, tailored to observe the highest energies of all: gamma-rays.<\/p>\n The Sun, a notoriously bright object, would\u2019ve burnt out the optical systems for Hubble and Spitzer if they were ever pointed in the Sun\u2019s directions, so such observations were strictly forbidden. However, for the Compton gamma-ray observatory, the Sun was actually a safe target.<\/p>\n The Sun\u2019s light across the electromagnetic spectrum is due to nuclear fusion, which primarily converts hydrogen into helium. The nuclear reactions produce neutrinos and gamma-ray radiation at the source, but the Sun\u2019s thick outer layers reduce those gamma rays significantly by the time that they leave the Sun\u2019s photosphere. Only about one-trillionth of the Sun\u2019s energy is emitted in the form of X-rays.\n<\/p>\n Credit<\/a>: NASA\u2019s Goddard Space Flight Center\/Scott Wiessinger<\/p>\n The reason why is relatively simple: the Sun, bright though it is, emits the majority of its energy peaked in and around the visible portion of the spectrum. Even within the visible light range of wavelengths, the Sun emits more green-and-yellow light than either red or blue light, and the amount of energy continues to fall off as we move out of the visible range in both directions. The Sun emits less ultraviolet light than visible light, and even less X-ray light than ultraviolet light. Similarly, the Sun is less energetic at near-infrared wavelengths than visible light wavelengths, and continues to decline through the mid-infrared, far-infrared, microwave, and radio sets of wavelengths.<\/p>\n Because light is quantized into individual \u201cpackets\u201d of energy \u2014 in the form of photons \u2014 there are fewer and fewer photons to observe the higher the energy (and shorter the wavelength) we choose to examine. While the Sun is far and away the brightest light source from X-rays all the way down to radio wavelengths, it emits virtually no gamma-ray photons at all. Sure, there are energetic flaring conditions where the Sun does temporarily emit gamma-rays, but those are rare and inconsistent. Only the highest-energy sources of all<\/a>, such as:<\/p>\n the regions around black holes,<\/p>\n neutron stars and pulsars,<\/p>\n along with supernovae and their remnants,<\/p>\n are capable of consistently emitting significant quantities of high-energy gamma-rays.<\/p>\n This map shows a 1-year view of the entire gamma-ray sky from NASA\u2019s Fermi satellite. The growing-and-shrinking sources are active galaxies powered by supermassive black holes, but the transient \u201cblips\u201d that appear are the gamma-ray bursts that are so sought after, many of which are thought to also create black holes, albeit not the supermassive type. When the Moon enters the field-of-view of the telescope, it can temporarily become the brightest gamma-ray source in the entire sky, while the lack of gamma-rays from the galactic center constrains annihilating WIMP dark matter scenarios.\n<\/p>\n Credit<\/a>: NASA\u2019s Marshall Space Flight Center\/Daniel Kocevski<\/p>\n It came as no surprise, then, that the Compton gamma-ray observatory, specifically designed to look for photons with between 20,000,000 and 30,000,000,000 electron-volts of energy (compared to ~2-2.75 electron-volts for visible wavelengths of light), wasn\u2019t really seeing gamma-rays coming from the Sun. Although the Sun creates gamma-rays in its core \u2014 where nuclear fusion reactions occur \u2014 those gamma-rays typically bounce around for more than 100,000 years before making it to the Sun\u2019s outermost layers. By the time these photons are released, they\u2019ve collided with other particles within the Sun many times over, having thermalized and come down to much lower energies (and, correspondingly, with much larger numbers of photons) than the individual gamma-ray photons created by those nuclear reactions within the star\u2019s core.<\/p>\n But when it came to measuring gamma-rays, our Solar System had plenty for Compton to observe. Specifically, the Compton gamma-ray observatory had an instrument aboard it known as EGRET: the Energetic Gamma Ray Experiment Telescope. EGRET operated for 9 years: from 1991 until the Compton observatory was purposefully deorbited in 2000 after a gyroscope failure. As our first observatory capable of accurately measuring the highest-energy photons from across the Universe, Compton, with its EGRET instrument (along with three other instruments), opened a new window into the multiwavelength cosmos.<\/p>\n A diagram of the EGRET instrument, which was used for observing the highest-energy photons aboard the Compton Gamma-Ray Observatory. The EGRET instrument is the only one capable of measuring photons with energies between about 20 MeV up to around 30 GeV: higher energy photons than the Sun typically emits.\n<\/p>\n Credit<\/a>: NASA\/GSFC CGRO Science Support Center, EGRET Technical Information, Appendix G<\/p>\n Although the Sun can produce gamma-rays during certain flaring events, that isn\u2019t the case during most of the Sun\u2019s life. Under nearly all circumstances, the Sun is completely gamma-ray quiet, which Compton\u2019s EGRET instrument clearly confirmed.<\/p>\n However, during the early phases of the Compton mission, the Moon passed into the Compton gamma-ray observatory\u2019s field of view multiple times, where the EGRET instrument was capable of observing it. This wasn\u2019t expected to be of scientific interest, but nature had other plans. Lo and behold, over those same time periods where the Sun was emitting no gamma-rays at all, the Moon didn\u2019t just show up in Compton\u2019s data, but appeared incredibly bright to Compton\u2019s eyes. The Moon was doing something that the Sun wasn\u2019t \u2014 emitting gamma-rays \u2014 implying that, somehow, the Moon was doing a whole lot more than just reflecting sunlight.<\/p>\n This isn\u2019t the case for any of the other wavelengths of light emitted by the Moon. Almost all of the Moon\u2019s light, from X-rays to ultraviolet, visible light, infrared, microwave, and radio light, comes from reflecting the light that arrives from the Sun (or, at some narrow wavelengths of the radio, from Earth). Even though the Moon re-radiates its absorbed heat as infrared radiation, it\u2019s still the case that the majority of the Moon\u2019s infrared radiation comes from reflected sunlight. However, for gamma-rays, that explanation couldn\u2019t be the case at all.<\/p>\n Between 1991 and 1994, the Moon passed into the Compton Gamma-Ray Observatory\u2019s field-of-view multiple times, where the instrument was capable of observing it. Compton detected high-energy gamma-rays from the Moon with its EGRET instrument, and the energy spectrum of the lunar gamma radiation is consistent with a model of gamma ray production by cosmic ray interactions with the lunar surface. The Moon is brighter than the (non-flaring) Sun in these high energies.\n<\/p>\n Credit<\/a>: D. J. Thompson, D. L. Bertsch (NASA\/GSFC), D. J. Morris (UNH), R. Mukherjee (NASA\/GSFC\/USRA)<\/p>\n This phenomenon puzzles even most modern-day astrophysicists when they first learn about it. Why would the Moon, famous for not generating its own light but for reflecting the light of the brighter objects close to it \u2014 sunlight on the illuminated portion of the lunar surface, and Earth-light on the darkened portion (a phenomenon known as Earthshine) \u2014 make its own gamma-ray light?<\/p>\n The key is to put two distinct physical phenomena together: the presence of cosmic rays, or cosmic particles that come from all over the cosmos (including from the Sun), and the elemental composition of the Sun\u2019s photosphere as compared with the Moon\u2019s surface.<\/p>\n From all across the Universe, high-energy sources of particles are emitted. These particles, known as cosmic rays, are mostly composed of single protons, although heavier atomic nuclei, electrons, positrons (the antimatter counterpart of electrons), and neutrinos are all present as well. They arise from a wide variety of phenomena, including stars, stellar remnants, collisions of hot gas, colliding neutron stars and white dwarfs, tidal disruption events, and a variety of other steady and cataclysmic events. One of the biggest sources of cosmic particles in our corner of the cosmos, unsurprisingly, is our own Sun, which emits these energetic particles (mostly protons) in the form of the solar wind.<\/p>\n Cosmic rays, which are ultra-high energy particles originating from all over the Universe, including particles emanating from the Sun, strike atomic nuclei everywhere they exist. On Earth, they land in the upper atmosphere and produce showers of new particles, but on the Moon, they recoil off of the heavy atomic nuclei present on the airless Moon\u2019s surface.\n<\/p>\n Credit<\/a>: Asimmetrie\/INFN<\/p>\n From all directions in the sky, including outward from our own Sun, these cosmic particles bombard the planets, moons, and other objects present within our Solar System. When these fast-moving particles, largely protons, hit the Sun, they run into only relatively light, low-mass particles: other protons, helium nuclei, and electrons. Very few heavier elements are present in the Sun\u2019s photosphere, and hence cosmic rays are extraordinarily unlikely to hit them, being much more likely to hit a proton, electron, or a helium nucleus.<\/p>\n That\u2019s important, because the way to stimulate the emission of a gamma-ray is to have these incoming cosmic rays knock into a heavy atomic nucleus with the right energy to boost it into an excited state: a configuration of protons and neutrons that\u2019s in a high-energy, metastable state. Then, those metastable states de-excite, causing the emission of a gamma-ray.<\/p>\n However, here\u2019s a catch: protons and helium nuclei don\u2019t get boosted into metastable states from collisions with protons alone. This explains why the Sun can\u2019t emit gamma-rays through this mechanism. You need heavier atomic nuclei for that: elements farther up the periodic table like carbon, oxygen, calcium, silicon, iron, etc. The Sun has very little of these elements present in its photosphere, but the Moon\u2019s crust and regolith is almost exclusively composed of these heavier elements. When cosmic rays \u2014 including, in part, plain old protons from the Sun\u2019s solar wind \u2014 strike the Moon, the recoil from those heavy elements knocks them into an excited state. When they then de-excite in response, it leads to the emission of gamma-rays.<\/p>\n The only time the Sun produces gamma-rays is during flaring events, when accelerated, high-energy protons can collide with heavier nuclei, producing an excited-state nucleus that emits gamma-rays. During quiet conditions, these fast protons will only interact with hydrogen or helium nuclei, which do not produce these gamma-rays. On the Moon\u2019s surface, however, heavy nuclei abound, and the creation of excited-state nuclei that then emit gamma-rays is ubiquitous, irrespective of the Sun\u2019s activities.\n<\/p>\n Credit<\/a>: NASA\/GSFC\/Gordon D. Holman<\/p>\n The Sun does, however, affect the gamma-rays that are emitted from the Moon in one particularly intricate way: through the Sun\u2019s magnetic field. Just as the Sun has an 11-year solar cycle that involves periodically increasing and decreasing numbers of sunspots and solar flares, it also possesses a complex magnetic field. The same 11-year time cycle occurs for that as well, where the shape and orientation of the Sun\u2019s magnetic field, including how it extends throughout the Solar System, changes how the cosmic ray protons responsible for the Moon\u2019s gamma-rays actually impact the Moon itself. The Moon, whose magnetic field is negligibly small, does not regulate itself in this fashion at all.<\/p>\n Over time, the gamma-ray flux from the Moon fluctuates by about 20% due to these periodic changes in the Sun\u2019s magnetic field. You might wonder why the gamma-ray flux doesn\u2019t change by more over the course of a solar cycle, and that\u2019s a good question to ask. It turns out that it\u2019s mostly the cosmic rays from sources outside our Solar System, rather than from our Sun, that drive the production of gamma-rays on the Moon. As a result, the gamma-ray signal doesn\u2019t fluctuate with the month or the cycle of lunar phases. Instead, it\u2019s not only steady, save for the ~20% variation over an 11-year period, but the Moon always appears to be in a \u201cfull\u201d phase when looked at in any orientation in the gamma-ray sky, as cosmic rays, as far as we can tell, are produced isotropically (i.e., the same in all directions) in space.<\/p>\n Although the Sun doesn\u2019t typically generate either gamma-rays or cosmic-rays that account for what we see on the Moon, its complex magnetic field undergoes cyclical changes on an 11-year timescale. These changes can alter the gamma-ray flux from the Moon, over time, by up to about 20%.\n<\/p>\n![\"Sun](\"https:\/\/www.newsbeep.com\/us\/wp-content\/uploads\/2025\/09\/illustration_of_solar_eruption_and_gamma_rays.jpg\")
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