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This answer to Which kinds of astronomical observations most need to avoid the Moon being up? mentions
For completeness - radio, mid-infrared and mm-wave observations are unaffected (unless the Moon is in the way!)
The Moon is of course opaque to all wavelengths used in astronomy, from very low frequency radio waves to gamma rays. So if the moon eclipses a target radio signals will be blocked.
But are there any more subtle or non-obvious effects? For example if a target is very close to the Moon but not covered by it, would that affect an observation in any way?
Question: Is this routinely avoided out of an abundance of caution? On the other hand, has a lunar occultation of a radio source ever be leveraged in some way for a specific measurement?
"Bonus points" for any anecdotes of strange results or confounding measurements in Radio Astronomy that turned out to be the result of not taking the position of the Moon in to consideration.
Occulations of artificial probes has been used to investigate the ionosphere of the moon. See, for example http://adsabs.harvard.edu/full/2008MSAIS… 12… 53P
In this technique, radio signals from the probe are monitored as the probe passes behind the moon. There is refraction from the lunar ionosphere, which can be detected indirectly, using a doppler method. This gives information on the (very thin) lunar atmosphere and the electron density in the lunar ionosphere.
Yes, and lunar occultations have proved useful in several cases.
Hazard et al. 1963 used a lunar occultation to produce a high-resolution brightness profile of the now well-studied radio quasar 3C 273. Scheuer 1965 goes into a little bit of detail on general computations.
A slightly different tack was taken by Vedantham et al. 2015. They were attempting to create interferometric maps of the cosmic 21 cm line signal using LOFAR. Unfortunately, they needed a calibration source to be able to measure this "global signal". The Moon, occulting portions of the field of view, provided a calibration source as they observed that patch of sky.
4 mysterious objects spotted in deep space are unlike anything ever seen
There's something unusual lurking out in the depths of space: Astronomers have discovered four faint objects that at radio wavelengths are highly circular and brighter along their edges. And they're unlike any class of astronomical object ever seen before.
The objects, which look like distant ring-shaped islands, have been dubbed odd radio circles, or ORCs, for their shape and overall peculiarity. Astronomers don't yet know exactly how far away these ORCs are, but they could be linked to distant galaxies. All objects were found away from the Milky Way's galactic plane and are around 1 arcminute across (for comparison, the moon's diameter is 31 arcminutes).
In a new paper detailing the discovery, the astronomers offer several possible explanations, but none quite fits the bill for all four new ORCs. After ruling out objects like supernovas, star-forming galaxies, planetary nebulas and gravitational lensing &mdash a magnifying effect due to the bending of space-time by nearby massive objects &mdash among other things, the astronomers speculate that the objects could be shockwaves leftover from some extragalactic event or possibly activity from a radio galaxy.
"[The objects] may well point to a new phenomenon that we haven't really probed yet," said Kristine Spekkens, astronomer at the Royal Military College of Canada and Queen's University, who was not involved with the new study. "It may also be that these are an extension of a previously known class of objects that we haven't been able to explore."
Spekkens added that the objects could also be caused by different phenomena. All four ORCs are bright at radio wavelengths but invisible in visible, infrared and X-ray light. But two of the ORCs have galaxies at their center that can be seen at visible wavelengths, which suggests that these objects might have been formed by those galaxies . Two ORCs also appear to be very close together, meaning their origins could be linked.
Astronomers spotted three of the objects while mapping the night sky in radio frequencies, part of a pilot survey for a new project called the Evolutionary Map of the Universe (EMU). The EMU pilot used the Australian Square Kilometer Array Pathfinder, or ASKAP, from July to November in 2019. This radio telescope array uses 36 dish antennas, which work together to observe a wide-angle view of the night sky. They found the fourth ORC in archival data collected by the Giant MetreWave Radio Telescope in India. This helped the astronomers to confirm the objects as real, rather than some anomaly caused by issues with the ASKAP telescope or the way in which the data was analyzed.
With only four of these peculiar objects discovered so far, the astronomers can't yet tease out the true nature of these structures. But the EMU survey is just beginning, and astronomers expect it to reveal more unusual objects.
By combining an ability to see faint radio objects with a wide gaze, the survey is uniquely positioned to find new objects. EMU scientists have predicted the project will find about 70 million new radio objects &mdash&ndash expanding the current catalog of some 2.5 million.
"This is a really nice indication of the shape of things to come in radio astronomy in the next couple of years," Spekkens told Live Science. "History shows us that when we open up a new [avenue of looking at] space to explore … we always find new and exciting things."
The paper, which is available on the preprint site arXiv, has been submitted for publication to the journal Nature Astronomy, where it is still under review.
Is the moon really a 'been there done that' world?
In the past year, we've learned that the moon is a very different place than what we had thought. Should we be so quick to disregard a manned mission?
If there's only one thing we've learned from all the highly successful recent Moon missions – the Lunar Reconnaissance Orbiter, LCROSS, Chandrayaan-1 and Kaguya — it's that the Moon is perplexingly different from our perceptions of the past 40 years. The discovery of water and volatiles across the surface and in the permanently shadowed regions at the poles changes so many of the notions we've had about Earth's constant companion.
Basically, just within the past year we've realized the Moon is not a dry, barren, boring place, but a wetter, richer and more interesting destination than we ever imagined. And so, the proposal for NASA to effectively turn away from any human missions to the Moon, as well as Administrator Charlie Bolden's 'been there, done that' comments is quite perplexing – especially for the lunar scientists who have been making these discoveries.
"It's been quite a year for the Moon," said Clive Neal, a lunar geologist from Notre Dame, speaking last week at the NASA Lunar Science Institute's annual Lunar Forum at Ames Research Center. "And things got quite depressing around February 2010."
That's when President Obama proposed a new budget that effectively would end the Constellation program and a return to the Moon.
At the Forum, lunar scientists shared their most recent findings – as well as their attempts to model and comprehend all the data that is not yet understood. But they saved any discussion of NASA's future until the final presentation of the meeting.
"Hopefully at the end of this session you won't be running out of here ready to hang yourself or slit your wrists," quipped Neal, who led the final session.
The week began, however, with keynote speaker Andrew Chaikin – author of the Apollo 'bible,' "A Man on the Moon," and several other space-related books — saying, "We have to erase that horrendous 'been there done that' notion." Chaikin also shared a famous Peanuts cartoon showing Lucy pulling the football out from under from Charlie Brown. No caption was needed for everyone to understand to what Chaikin was referring.
"With all of these new discoveries, we should have ample reason to believe that humans will follow," said Chaikin. But right now, he added, the man in the Moon looks a little like Rodney Dangerfield. "The Moon wants – and deserves – respect."
"It appears NASA's focus might be shifting to Near Earth Objects," said Neal, "but the Moon is the nearest Near Earth Object. It's quicker, safer and cheaper to get humans there, and the important thing to recognized that there's a lot left to explore, and a lot to do on the Moon."
Only 5% of the Moon's surface has been explored by humans, and Neal showed scaled maps of the Apollo landing sites overlaid on maps of Africa, Europe and the US, revealing just how small a portion of the Moon has been explored directly by humans. The map below shows the Apollo 11 crew's movement on the Moon can fit within the size of a soccer (football) field.
Additionally, the latest data reveal that the Apollo sites were in no way representative of the entire Moon.
In light of the proposed plan to give up on the Moon, Neal said there probably is a lot of misperceptions by the American public, as well as in other countries that there's nothing to do or learn at the Moon. But he believes nothing could be further from the truth.
"What we've heard over the last couple of days are fantastic talks and seen wonderful posters in regard to the vibrancy of lunar exploration and science, and seen that exploration enables science and that science enables exploration. The Moon is a Rosetta Stone for solar system exploration and science. The recognition of a possible lunar magma ocean has resulted in terrestrial and Martian magma oceans being proposed. This could be the way terrestrial planets evolve and the Moon is begging us to go back and explore to figure that out."
There's also the studies of preserved impacts on the lunar surface which represents a look back in time where we can figure out how to do date planetary surfaces, test cataclysm hypotheses, and study how airless bodies undergo space weathering, which has a direct application to NEO research. Studying cold trap deposits has direct applicability to learning more about the planet Mercury, and lunar regolith contains information about the history of our Sun.
There are proposals for doing radio astronomy from the lunar farside, which will probe the dark ages of the Universe and look back to when the first stars turned on. "So the Moon is a gateway to the Universe," Neal said. "You can do so much more with the moon — its not just the moon, it’s the solar system and beyond."
In addition there are many unresolved scientific questions about the Moon. What are the locations and origins of shallow Moon quakes, and large lunar seismic events? How does the lunar regolith affect transmission of seismic energy? What is the nature of the lunar volatiles in the permanently shadowed regions at the lunar poles? What is the mechanism for the adsorption of water, hydroxyl and other minerals recently found on the Moon's surface? What is nature of lunar core?
When Constellation was proposed, returning to the Moon was said to be a testbed for going on to Mars. It would be a safe and more economical way to test out systems and technology needed for going to the Red Planet. So, what has changed?
Primarily the budget. There weren't enough funds in Constellation's coffers to go to the Moon and then Mars. It primarily became a Moon-only program, which many said, didn't bring us to the "real" destination that everyone really wants: Mars.
And money is still the real issue for not returning to the Moon in the new proposals of going to NEO's and then Mars. If money weren't an object, we'd do it all.
But the Moon offers a great local to test out human missions to Mars. "The Moon offers one-sixth of Earth's gravity," Neal said," and we do not know what happens to the human body over time in that gravity, and we can only extrapolate what happens there and on Mars' one-third gravity. We could test out life support, the growth of crops, the radiation environment and more. The 'feed forward' there is quite important where you can simulate a Mars mission on Moon. To develop and test your radiation shielding in the real environment on the Moon is more of a test than flying on the space station."
Both Neal and Chaikin said they could go on and on about the benefits of returning to the Moon, and they also book-ended the Lunar Forum by saying it is up to the lunar scientists and Moon enthusiasts to educate the public, other scientists and even NASA about the importance of the Moon.
"We have to do a better job of educating the public – even dealing with the conspiracy theorists," Neal said. "We need to get into schools and educate about what NASA has done, and what they are doing now. We all take responsibility for that."
"The Moon is not going to get the respect it deserves unless people are out there talking about it," said Chaikin.
Astrophysics is the branch of space science that involves the study of physical laws that explain the origin of stars, planets, and other objects in the universe. NASA describes astrophysics as a goal to observe and explore the universe and its evolution for the search of the existence of life on other planets. Astrophysics allows scientists to deduce theories for explaining the mechanism of radiation emitted by universe objects and extract important information in it. NASA focuses on the Physics of the cosmos, cosmic origins, exoplanet exploration, astrophysics explorer programs, and research in the field of astrophysics.
Current Scenario and Need for Innovations in Astrophysics
For Astrophysical research, NASA focuses on operational great observational tools that comprise Hubble Space Telescope, the Chandra X-ray Observatory, and the Spitzer Space Telescope. Other observational missions are Fermi Gamma-ray Space Telescope, Neil Gehrels Swift Observatory, NuSTAR, and TESS missions. Some complementary missions are in the process such as NICER and SOFIA. NASA also funded the development of astrophysics instruments for the observations and data analysis for their missions.
Most of the mission mentioned have achieved their initial goals, but they are prolonged to produce breathtaking results. All of these missions will work together for much of the human piled knowledge of the universe, and use this knowledge for humanity to touch new horizons. NASA also collaborates with international space platforms across the globe for astrophysics instrumentation development such as ESA’s XMM-Newton.
Despite the above currently operated missions, there is still a need for innovations in the field of Astrophysics. For example, the nature of dark matter can only be understood by a model of the microstructure of space. Astrophysicists assume that it involves a great phenomenon of physics called “Planck Length”. It is the greatest challenge to the present universe.
Future of Astrophysics
NASA proposed four future missions under Astrophysics space divisions to the American Astronomical Society (AAS). Once they get approved, NASA will start working on it. Each of these missions comprises
a space telescope primarily designed to study stars, galaxies, black holes, alien planets, and objects within Earth’s solar system. Only one of these four missions will be selected till mid-2030. Once the mission gets selected, the cost can rise above 1 billion dollars.
Large UV Optical Infrared Surveyor – LUVOIR
This candidate mission will be the larger and more sophisticated version of the Hubble Space Telescope to observe the universe in ultraviolet, infrared, and visible wavelengths of light. The space telescope will be designed in two different sizes such as larger and smaller depending on budget.LUVOIR- observatory concept LUVOIR 16-meter size telescope Credit NASA.
It will help astrophysicists to deal with various future astronomical research projects like the study of habitable exoplanets study of formation and evolution of stars and galaxies mapping of dark matter all around the universe and imaging objects in the solar system, like planets, comets, and asteroids.
Habitable Exoplanet Observatory – HabEx
As the name indicates, this space telescope will observe the potentially habitable exoplanets around sun-like stars.Representation of the Telescope Concept
If it is approved, it will become the first telescope to trace biosignatures like water and methane and image the Earth-like exoplanets where life will be possible. Habex would also be able to observe and map stars and galaxies, study the expansion of the universe, and investigate the dark matter by conducting ultraviolet and infrared observations.
Lynx X-Ray Observatory
The space telescope would be the upgraded form of Chandra X-Ray Observatory. Lynx would be powerful enough to reveal the invisible universe through very high energy X-ray radiations. The birth and death of stars, the perfect maps of exploding stars, the invisible supernovas, and black holes would only be seen because of Lynx.Space telescope would be the upgraded form of Chandra X-Ray Observatory A closer look of Lynx X-Ray Observatory concept
Origins Space Telescope
The next-generation version of the Herschel Space Observatory, the Origins Space Telescope would solve the mysteries of life in-universe. The questions like how habitable planets are formed will be easily get answered by this infrared surveyor telescope. It will be consisting of a cryocooler cooling system that would increase its sensitivity to about 1000 times. It will involve in keeping the track of the earliest stages of stars and other planets to check the ingredients of life.
Model of Origin Space Telescope
All of these future astrophysics’ missions have their importance. They will readily revolutionize the concept of life in the universe.
Laser Interferometer Gravitational-Wave Observatory – LIGO
The world’s largest gravitational wave observatory is not like a traditional telescope. It does not work by using light. Instead of light, it uses the ripples in space-time called gravitational waves. These waves are produced by big events that affect space-time such as mergers of pairs of neutron stars, black holes, and supernovae.
An aerial view of the Virgo interferometer near Pisa, Italy. Credit: The Virgo collaboration/CCO 1.0 An upgrade to the Advanced Laser Interferometer Gravitational-wave Observatory
It is an outstanding physics experiment on the scale and complexity of some of the world’s giant particle accelerators and nuclear physics laboratories. It consists of two enormous laser interferometers located 3000 kilometers apart, and two widely separated detectors in Washington State and Louisiana, USA.
It is aided by the Virgo detector in Italy and the GEO 600 instrument in Germany. Since LIGO is upgrading in series form, which is supported by 92 collaborating institutions, it is still expanding with a third detector site being established in India.
LIGO and Virgo operate together as a collaboration and will soon be joined by the KAGRA detector in Japan. The Collaborative operation allows examination of gravitational waves simultaneously produced by the same event to determine what is the origin and nature of signals. This collaboration addition of telescopes is introducing a whole new exciting field within multi-messenger astronomy.
By working in collaboration, these telescopes not only drive ahead in astronomical exploration but also the data obtained from the experiment will be used to improve our lives on earth in the future.
Gaia is a telescope designed to make the largest 3D map of our galaxy, the Milky Way. Its mission is to provide unprecedented positional and radial velocity measurements with the accuracies needed to produce a stereoscopic and kinematic census of about one billion stars in our Galaxy.
It will observe each star 70 times in planned 5 years. It is not only able to observe stars, but it is also able to detect a large no. of asteroids and comets, along with potential exoplanets and supernova explosions.
Astronomers spotted three of the objects while mapping the night sky in radio frequencies, part of a pilot survey for a new project called the Evolutionary Map of the Universe (EMU), using the Australian Square Kilometer Array Pathfinder, or ASKAP, from July to November in 2019.
This radio telescope array uses 36 dish antennas, which work together to observe a wide-angle view of the night sky.
They found the fourth ORC in archival data collected by the Giant MetreWave Radio Telescope in India. This helped the astronomers to confirm the objects as real, rather than some anomaly caused by issues with the ASKAP telescope or the way in which the data was analyzed.
With only four of these peculiar objects discovered so far, the astronomers can’t yet tease out the true nature of these structures. But the EMU survey is just beginning, and astronomers expect it to reveal more unusual objects.
This is a really nice indication of the shape of things to come in radio astronomy in the next couple of years. History shows us that when we open up a new [avenue of looking at] space to explore … we always find new and exciting things.
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A brief history of Radio Astronomy
To start with, a very short history of radio astronomy would be helpful. Radio astronomy was born in the early 1930s when Karl Jansky, working for Bell Laboratories, was trying to determine the origin of a source of noise that was showing up in receivers operating in the 20 MHz region of the radio spectrum.
Jansky built a steerable antenna and began searching for the source of the noise by taking directional measurements. To his surprise, he discovered that this noise was from extraterrestrial sources. Jansky, enthused by his discovery, published his work, however the majority of astronomers at the time were decidedly underwhelmed by this discovery and for the most part dismissed it as either irrelevant or simply curious. There were a few inventive individuals who saw the potential for this noise from space.
One of them, Grote Reber, an electronics engineer and avid radio armature, had reviewed Jansky's original discovery and speculated that the signals were of thermal origin (caused by very hot objects), and as such they should be easier to detect at higher frequencies. Since Jansky's original work was done at 20 MHz (about 15 metre wavelength) and a beam width of about 25 degrees, Reber wanted to narrow the effective beam width to obtain finer detail. Reber reasoned that he should build his first receiver and antenna to operate at 3000 MHz (10cm wavelength) an extraordinary frequency at that time. With his own resources and enthusiasm, Reber built the first parabolic reflector radio telescope. Since this was deemed a private 'extracurricular' activity, Reber received no sponsorship or support. Besides being the first of its kind, it was also a huge structure. Basically built by a single individual, it was 9.5 metres (31 feet or 3 stories) in diameter.
The term 'Radio Telescope' had not been coined at the time, however Reber gets the credit for building the first one. Although he did not prove his original hypothesis, his work went on to detail the first radio map of the galactic plane and large portions of the sky. Reber published his work "Cosmic Static" in the late 1930's.
It was the search for static or noise that led to the development of the radio telescope, and it is essentially noise from the universe that the radio telescope detects. Buried in this roiling confusion is information that is specific in nature to astronomical objects and phenomena. This noise bears witness to the physical characteristics of the universe. The information is presented as a mixture of signal properties such as frequency, phase, amplitude and in some cases repetitive patterns. Also present is information that can be mathematically assembled into 'radio pictures' of these cosmic objects. Some signals arrive from finely defined sources that can be, by and large, considered as point sources (quasars and pulsars for example).
Other sources cover vast areas and can be thought of as wide field objects. These are clouds of dust and gas, star 'nurseries', galaxies and a plethora of other interesting goodies. To obtain information from these sources, the radio telescope must receive not only specific information but also all the 'noise' from these objects and their surroundings then reject what isn't wanted and record the results.
Radio frequency signals of extraterrestrial origin are extremely weak. As an example, if all the signal energy ever received from all the radio telescopes ever built (viewing objects other than the sun) were combined, there would not be enough total energy to melt a single snowflake.
The radio telescope must first concentrate signals gathered over a wide area and focus them into a small area. This is the same principle on which the reflecting optical telescope operates. The term "radio optics" refers to this similarity. Since the term 'light' really means electromagnetic radiation, all the same basic equations, theories and principles are applicable to radio, infrared or visible light. The big difference is that optical telescopes operate at extremely high frequencies and microscopic wavelengths, while their cousins the radio telescopes work at lower frequencies and longer wavelengths.
Resolution, which can also be expressed as beam width, is a function of the wavelength of the signal and the diameter of the reflector. At optical frequencies (blue-green light 600,000 GHz or a wavelength of .0005 mm) a 1 meter diameter "perfect" mirror will have a beam width of about .00003 degrees. The same mirror operating at radio frequencies (30 GHz for example with a wavelength of 1 cm) will have a beam width of about 6 degrees. As can be seen, the beam width for the radio telescope is about 200,000 times wider, thus yielding lower resolution observations. At first the solution to this was to build bigger and bigger reflectors, giving narrower beam widths and higher resolutions.
By the late 1950's reflectors of 100 metres (300 feet) across were being built. At diameters larger than this, a steerable reflector becomes far too heavy and cumbersome to be effectively used. The big problem is that the surface warps and deforms due to gravity and thus the effectiveness of the reflector is compromised. The one advantage of large reflectors is that with their very large gathering surface area they offer significant signal strength the down side of this is that they are very expensive to operate, maintain, and build.
Even with the large areas, one still must remember that the beam width is still wide compared to optical instruments. A 100 metre diameter radio telescope, operating at 10 cm wavelength, still only has the individual resolving ability of an optical mirror of about 5 mm (less than 1/4 inch). Even with such seemingly myopic resolution, the sheer size of these instruments allows for detection of weak sources billions of light-years away. In a later article I will discuss interferometry, a technique by which multiple radio telescopes can be combined to give the effective resolution of a single telescope many miles across. This process changes the apparently fuzzy world of the radio telescope to one of crystal clarity. Modern radio telescope arrays such as the VLA in New Mexico and the Caltech OVRO millimetre array have resolving abilities far beyond even the Hubble telescope.
The temperature of the radio telescope, its reflector, and its receiver are all sources of noise with which the observer must contend. Since everything with a temperature above absolute zero gives off electromagnetic noise in one form or another, and the fact that what a radio telescope 'sees' is essentially electromagnetic noise, the radio telescope needs to be highly selective and reject as much superfluous noise as possible.
One method of counteracting noise is to cool the receiving electronics to a temperature just a few degrees above absolute zero. This eliminates thermally generated noise in the electronics. Once this noise has been removed, the amplified signal of interest is then selectively amplified again, converted to more manageable frequency bands, divided into a series of adjacent channels and finally processed to detect the relative power or energy of the source along with frequency and phase detection.
Because a radio telescope is so sensitive, other methods of reducing noise are used. One is to reduce reflected and thermal noise from the ground. This is why many radio telescopes have a Cassegrain configuration (a secondary mirror reflects the signals back through a hole in the centre of the main reflector). Since the receiving electronics input focus points to the sky, picking up thermal and reflected noise from the ground is avoided.
The final method is to reduce the contributed noise from terrestrial sources. This translated means move the telescope away from the high density cities to some remote location where the local denizens, i.e. rabbits, moss, and life forms found under rocks, do not pollute the radio spectrum. This also usually means placing the telescope in a valley surrounded by mountains so that the terrain blocks a great deal of unwanted radio noise. Add to this the help of the local authorities to declare the surrounding area of the telescope as a 'radio free' zone and you have a reasonably quiet observing site. Finally when all this is combined, the effective noise temperature of an entire radio telescope system can be reduced to only a few tens of degrees above absolute zero, (quite an improvement when considered that typical room temperature is about 300 Kelvin).
A signal arriving from a celestial source has now been gathered by a large reflector, concentrated into a small area and fed to a low noise electronic receiver that is isolated from strong external sources, quiet in its own operation and highly selective. The next part of the process is to store the information for subsequent processing. Since many of the radio source signals are so weak, it is often necessary for a telescope to stay fixed on a target for extended lengths of time to insure sufficient information has been gathered. The result of these long 'exposure times' (to borrow a phrase from photography), results in huge amounts of data. In the early days of radio astronomy, information was recorded on paper, which chart recorders spewed out by the mile, and consequently the astronomer had to inspect visually, by the mile. This was an arduous process and sometimes required months to extract the information.
In the 1960s magnetic tape was substituted for paper and computers were given the task of correlating the information. Today with inexpensive desktop computers, flash analogue to digital converters, and billion operation per second digital signal processing chips, much of the information obtained can be processed in real time. It is the results of the computations on the raw signal data that carries the ultimate useful information. With faster and faster real time processing, the storage of information has shifted from saving the raw incoming signals to saving the derivatives and ultimately to saving only the specific information. This not only reduces the total storage required (raw signals require magnitudes more storage) but allows for faster retrieval of pertinent information since the data has been prefiltered and formatted.
Last, but not least, is the interpretation of the data into a meaningful format. Despite our ability to interpret numbers and form abstract conclusions, we human beings are visually oriented. The information from a radio telescope can indeed be turned into a picture that is easy to understand. However, along with this visual presentation comes volumes of additional information that, when analysed, reveals the secret workings of much of the universe. This information is often intangible to our senses. Properties such as phase, coherence, polarisation and subtle frequency variations cannot be discerned from a simple picture. Additional signal processing and receiving techniques must be used to reveal these characteristics. Often, the presentation of these other qualities will be in a visual or pictorial format, but the colours and intensities will demonstrate properties not normally visible. These 'false colour' images present to the mind visualisations of concepts and properties heretofore unobservable.
The radio telescope, while not as basically easy to use as a simple optical instrument, actually reveals much more information to the observer. With its ability to cover a much wider portion of the electromagnetic spectrum, the radio telescope shows much more of the inner workings of the universe. The intrinsic composition of interstellar clouds, the birth of stars, and the properties of stars whose lives have passed, are all observable with the radio telescope where these mysteries are masked to the optical instruments. Now with the combination of highly accurate optical and radio imaging, the cosmos is beginning to become comprehensible.
Jim Fredsti is a Research Engineer at
Owens Valley Radio Observatory,
California Institute of Technology,
Big Pine, California, USA.
This article is the second in a series on Radio Astronomy, bookmark this page as the following articles will be uploaded shortly. To return to the first article: first radio astronomy article.
The most distant astronomical object ever seen… in 1962
One of the fun things about having written thousands (!!) of articles about astronomy over the past decade or two is going through old posts looking for relevant info. If I’m writing about a black hole, say, then it helps to link to older articles that have background info, saving me the trouble of writing it again.
Every now and again I’ll be writing about some particular object and then hit the archives to see what I’ve said about it before. It doesn’t happen often, but sometimes I’ll find… nothing. Meaning, I haven’t written about this particular object before. That’s fine I can’t write about everything, but what’s weird is when it’s about some famous or particularly iconic astronomical object. A few years ago, for example, I was looking for stuff I had written about Proxima Centauri, the closest star to the Sun, and discovered I had never written an article devoted to it! That was weird, and now happily fixed (many times over).
So. A little while back I was writing about a bright quasar found in the distant Universe, and decided to drop a line in about the very first one ever identified, called 3C273 — the story of how it was discovered is really fun, with lots of weird coincidences combined with both human failings and incredible cleverness. I looked for an old article about it to link to, but I was shocked to see I had never written about it in detail. But I could have sworn I wrote about it…
Then I remembered something funny: I did write an extensive piece about 3C273 for my book Death from the Skies!, in the chapter about how galaxies can be a danger to life inside them. But, due to space requirements, I had to leave most of the story out!
I almost never give tips on writing — it’s too individual a practice, with some advice that’s great and some being anathema to others — but here’s one that I give without hesitation: Never throw anything out.
To wit: An early draft of that chapter sitting on my disk still had all the 3C273 backstory in it. It was never published anywhere. But now I can remedy that! It took a lot of editing to make it a standalone article, but here you go: How we found out that not every galaxy is as clement as the Milky Way. Not by a long shot.
[Update (Dec. 24 2020): When researching this story, I read a bunch of papers, books, and articles, and talked to some friends about it as well. To the best of my knowledge at the time, what I wrote was correct. But not long after posting this article I got a note from an astronomer saying Milton Humason and his team had actually found objects with higher redshifts (and are therefore further away from us) years before this in 1956! I never found this paper in my research, obviously. While I'm delighted to learn something new, this does cast a different light on some of the aspects of the story, like Schmidt's confusion on the spectrum. Clearly I have some more digging to do. One of the important aspects of the story is that the object looks like a star and nothing like it had ever been seen before, and what that meant to astronomy. So that's still cool.]
In 1962, astronomers had an enigma on their hands. Radio astronomy was coming into its own, and huge dishes were scanning the heavens looking for cosmic objects that emitted radio waves. Cambridge University sponsored several such surveys, numbering them 1C through 5C. In the third catalog – called, surprise, 3C – was an object in the constellation of Virgo. It was the 273rd object listed, so it became known as 3C273. It was fairly bright in radio, and variable, too — its brightness fluctuated on a scale of days. But while the radio telescope used to make the surveys was sensitive, its eyesight was somewhat fuzzy, and an exact location for 3C273 was impossible to determine (this same situation was faced just a few years later by astronomers observing gamma-ray bursts). Without an exact location, it wasn’t possible to look for the object using optical telescopes and find out if it were a star, a galaxy, or some more exotic object. The sky is full of stars, and thousands of objects were within the uncertainty of 3C273’s location.
The star field around the extremely luminous quasar 3C 273. Now be honest: If it weren’t arrowed you’d never have noticed it, would you? Credit: SDSS / Aladin
But astronomer Cyril Hazard got an idea. Virgo is a constellation on the zodiac, which means that the Sun and planets appear to move through it… and so does the Moon. Hazard discovered that in 1962 the Moon would pass directly over the most likely position for 3C273. What Hazard realized is that he could point a radio telescope at the radio source, then wait for the Moon to cover it (what astronomers call an occultation). At that moment the radio emission would cease, and he could measure that exact time. Since the position of the Moon is very well known for any given time, that meant he could nail down the location of 3C273.
This was a brilliant idea, but ironically he missed the actual occultation because he took the wrong train! However, his team, well trained in the telescope’s use, was able to make the observation. It went well, and they found an optical object at 3C273’s position … but it was a bit of a shock. Sitting at that location was an unassuming blue star, about 1/600th as bright as the faintest star visible to the unaided eye. This was really weird— how could something so faint in visible light be so luminous in radio?
It is said that in astronomy, a picture is worth a thousand words, but a spectrum is worth a million. OK, I’m the only one who says that, but it’s still true: By taking the light of an object and splitting it up into thousands of individual colors, you can determine lots of physical characteristics of the object. Its temperature, velocity, chemical composition, magnetic field strength, whether it’s spinning or not and even how rapidly – all are revealed by a good spectrum.
Astronomer Maarten Schmidt knew this very well, and obtained a spectrum of 3C273 not long after the optical position was determined. What he found was, to be charitable, odd. It looked nothing at all like a star, a galaxy, or anything ever seen before. Schmidt puzzled over it, and then had a flash of insight many astronomers wait a lifetime to experience. He suddenly understood why the spectrum was so odd: It was hugely redshifted.
The spectrum of 3C 273 shows a huge redshift (at least by 1962 standards). For example, the H-alpha line is emitted at a wavelength of 6563 Angstroms, but is redshifted to 7600 by the time it gets here from the quasar. All the other lines are similarly shifted. Credit: Dietrich et al.
Just like sound waves can change pitch if the source is moving toward or away from you (via the Doppler shift), light can too. In this case, pitch = color. An object moving away from you has its wavelength stretched out, and we call that a redshift. If it’s heading toward you the wavelength is compressed: a blueshift * .
In the 1960s, most astronomical objects measured had relatively low shifts. Even something moving away from you at, say, a few hundred kilometers per second has a low redshift, changing its wavelength only a few percent.
Schmidt’s insight was that the spectrum of 3C273 was enormously redshifted, moved to longer wavelengths by a stunning 16%, the highest ever seen! He knew right away that this was a special object: Its great speed meant that it must be terribly far away.
Almost half a century before, astronomers discovered the Universe itself was expanding. This meant that galaxies that were farther away were moving away from us faster than ones closer in. This in turn meant they had higher redshifts. So, by measuring the redshift, the distance to an object could be found. Back then the numbers used weren’t as accurate as today’s, but the overall idea was correct.
Using this method, Schmidt realized that 3C273 was tremendously far away, farther away than anything ever before seen. Far from being an innocuously faint and nearby blue star, 3C273 must be the most luminous known object in the Universe. It was so far away that it had to be hellishly luminous to be seen at all!
A deep Hubble image of the quasar 3C 273 shows it as a blazing point source, almost like any other star. The linear feature to the upper left is a jet of material accelerated by the quasar’s black hole central engine. Credit: ESA/Hubble & NASA
3C273 was the first object of this kind to be identified, but several more were to follow (and in fact the very similar object 3C48 was actually found first, and even had an optical counterpart found, but it was too faint to analyze well enough to get a distance). These new celestial beasties were dubbed quasars, short for quasi-stellar radio sources (or sometimes QSOs for quasi-stellar objects). 3C273 is the nearest quasar, at a distance of a staggering 2 billion light years. It is truly a monster, emitting several trillion times the Sun’s energy, hundreds of times the total output of our whole Galaxy!
Eventually, of course, more were found. Optical telescopes, taking very deep exposures of quasars, found some have “fuzz” around them, a faint extended source of light. Eventually, as technology got better, astronomers figured out this fuzz was actually an entire galaxy, the light from which was dwarfed by the quasar phenomenon itself! More observations revealed more details — quasars and other types of so-called active galaxies had extremely bright and very small cores their light could change brightness on very short time scales, implying the source of the light was small they emitted light across the electromagnetic spectrum, from radio waves to gamma-rays. And whatever was powering them had a lot of energy at its disposal.
Only one object in the Universe can fit all those clues: a black hole.
And no ordinary black hole. It had to be a supermassive black hole. One with millions or even billions of times the Sun’s mass.
Now we know this in fact to be the case, and we also think almost every big galaxy in the Universe has one of these monsters in its heart. While the details are complex and fierce, all the different kinds of active galaxies we see, including quasars, are variations of the same type of object: A supermassive black hole greedily gobbling down material from its host galaxy.
Artist drawing of a blazar, a galaxy with a supermassive black hole spewing out energy. Credit: DESY, Science Communication Lab
The black hole at the center of 3C273 probably has a mass a million or more times the Sun, which ironically makes it a lightweight as such things go. It just happens to be actively feeding, making it extremely luminous. But far more luminous ones are known, some that positively dwarf 3C273.
We’ve come a long way since those first observations in the early 1960s. We now know of tens of thousands of active galaxies, and have learned vast amounts about them. They taught us about supermassive black holes, and that the birth and evolution of galaxies depend on them. We know the Milky Way has one, and that it profoundly affects the environment around it. We’ve used them to test relativity. And now we’ve even been able to take images of one!
Our Universe has grown considerably since that time, as has our understanding of it. But remember: This happened over a human lifetime. My lifetime I can remember when articles were written about the mysterious quasars, speculating on what they might be. And now I can look back on those and chuckle they were far, far weirder than anyone thought at the time.
Isn’t that always the way? The Universe is a pretty weird place, and it’s always throwing curveballs at us.
With the Moon slowly receding from Earth, there will come a time when it no longer has sufficient angular size to cover the Sun completely. When will the last total solar eclipse occur? When the Moon’s at its closest and biggest and it can no longer cover the Sun at&hellip
If the near-Earth asteroid 99942 Apophis ever strikes Earth, how big would the crater be? Apophis (formerly 2004 MN4) ranks at the top of the worrisome-asteroid list. On April 13, 2029 (yes, that will be a Friday the 13th), it will pass only 30,000 kilometers (18,600 miles) from Earth’s surface&hellip
The ORCs were detected in late 2019 after astronomer Anna Kapinska studied a Pilot Survey of the Evolutionary Map of the Universe (EMU), based on the Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope array.  Every detected ORC, about 1 arcminute in diameter, are some distance from the galactic plane, at high galactic latitudes. The possibility of a spherical shock wave, associated with fast radio bursts, gamma-ray bursts, or neutron star mergers, was considered, but, if related, would have to have taken place in the distant past due to the large angular size of the ORCs, according to the researchers.  Also according to the astronomers, "Circular features are well-known in radio astronomical images, and usually represent a spherical object such as a supernova remnant, a planetary nebula, a circumstellar shell, or a face-on disc such as a protoplanetary disc or a star-forming galaxy, . They may also arise from imaging artefact around bright sources caused by calibration errors or inadequate deconvolution. Here we report the discovery of a class of circular feature in radio images that do not seem to correspond to any of these known types of object or artefact, but rather appear to be a new class of astronomical object." 
Has radio astronomy ever been done on objects that appear very close to the Moon? Is this avoided? - Astronomy
[An earlier version of the article presented here appeared in the magazine Amateur Radio Action, volume 6, issue 8 (1983).]
GOLDEN JUBILEE OF RADIO ASTRONOMY
Nineteen eighty-three may well be regarded as the golden jubilee of radio astronomy. Fifty years ago man first realised that an astronomical source was transmitting radio waves. The man was Karl Jansky and the source was the milky way. Never again would the universe look the same. During the next 50 years ever more sensitive radio telescopes would wrest secrets from the cosmos forever hidden from their optical counterparts.
Karl Jansky was employed by the Bell Telephone Laboratories in Holmdel, New Jersey, and was given the task of investigating static and other radio interference to long distance communications. Commencing toward the end of 1931, he employed a sensitive receiver, together with a rotatable antenna in his studies on 20.5 MHz. By 1932 he had managed to discriminate between three different types of static or atmospheric. The first type originated from nearby thunderstorms. It was very intermittent, composed of "crashes" that often induced high voltages inthe receiving antenna. The second type was similar, although more continuous and weaker in amplitude. It was identified with distant thunderstorms whose signals were propagated to the receiver by reflections from the ionosphere. The third type was a steady hiss type static whose origin was initially quite unknown.
This 14.6 metre rotatable, directional antenna system was designed and used by Karl Jansky in 1931, to investigate interference problems on long distance communication circuits. Using this antenna, Jansky was able to make the first positive identification of radio waves of extra-terrestrial origin.
Careful observation and analysis of a year's worth of data led Jansky to confidently state the origin of this type of noise. In 1933 he announced to the world that the source of this noise was fixed in space at a position very similar to the centre of our own galaxy, in the astronomical constellation Sagittarius. This was the first time that anyone had proven that an astronomical source actually radiated radio energy. The science of radio astronomy was born.
Growth was at first very slow. Not until 1937 was an attempt made to systematically map the celestial sphere using radio waves. This attempt was made by a dedicated radio amateur Grote Reber in Wheaton, Illinois. Using his own money and time he constructed a 9.5 metre parabolic dish in his backyard (surely a tremendous engineering feat for an individual today, let alone over 4 decades ago). Together with a home-built 'ultra-short-wave' receiver for 160 MHz, he produced the first contour maps of the sky. It was not until some time after Reber published these maps that astronomers generally realised what Jansky had first discovered.
The second world deviated interest in radio astronomy, but from the mid-forties onward it grew in leaps and bounds. Even during the war, interest was not totally lacking, particularly when it was discovered that a branch of radio astronomy had military significance. In 1942, sever jamming of British Coastal radar operating around 60 MHz was experienced for several days. Detective work by Stanley Hey revealed that the interference was not produced by the enemy, but by the Sun. From that onward, the radio activities of our nearest star have continued to be of great interest to both military and civilian users of radio communication, navigation and radar systems. Man's first steps into space have served to further intensify this interest.
If radio astronomy was of some interest to the military, radio astronomers were considerably more interested in the radio equipment and techniques developed during the war for military use. Bernard Lovell, at Jodrell Bank in England, was quick to use military surplus gear to make radar observations of meteors. The first radar reflections from the moon were received at 111 MHz, in 1946. The world of microwaves was becoming available to radio-astronomical observations. Australians played a significant part in this early development, particularly in the field of solar radio astronomy. The key organisation was the Radiophysics Laboratory of the CSIRO. To this day they have continued to lead the world in many facets of this exciting science.
|In the 1940's, CSIRO scientist J G Bolton identified the radio source called Taurus A with the optical object called the Crab Nebula, the remnant debris left over from a supernova explosion in 1054 AD. This was the first identification of an extrasolar radio source with an optical counterpart. [Image: Hubble Space Telescope (STScI/NASA/ESA)]|
There have been many discoveries made in the field of radio astronomy during the fifty years since Jansky. Some have been point events of the 'Eureka' type, but many have resulted from years of painstaking data collection and sky surveys. The end results have come from the efforts of many people: from the physicists, engineers and technicians who designed and built the radio telescopes, to the observers and astronomers who made the final deductions. Although many of these discoveries are worthy of discussion, seven contributions in particular stand above the rest. They do so because they are contributions that have revolutionised our thinking about the universe, and they are contributions that is general could have been made only by radio astronomical observations.
In historical order, the first of these seven events was the detection by Harold Ewen at Harvard, of the discrete frequency signal emitted by vast numbers of hydrogen atoms that fill the void between the stars. This was in distinct contrast to the extra-terrestrial signals observed by Jansky and Reber. These were very wideband or continuum type emissions produced by the acceleration or deceleration of charged particles, mainly electrons. The bulk of such radiation occurs either due to thermal agitation (i.e. hot objects) or to circular-type motions in a magnetic field. The single-frequency radiation from hydrogen atoms occurs at 1421 MHz (a wavelength of 21 cm) when the electron in the atom rapidly flips the direction of its 'spin axis'. The presence of such radiation had been predicted on theoretical grounds, by a young Dutch astronomer Hendrick van de Hulst in 1944. The great significance of Ewen's detection of this emission (in 1951) was its use in mapping the structure of our home galaxy, the Milky Way. When we look at the southern sky on a dark cloudless night, it is obvious that the Milky Way is a planar or flattened structure like a saucer viewe edged on. However, we cannot see very far into the plane because the light is obscured by large amounts of intervening gas and dust. Radio waves are not so hampered. Furthermore, the Doppler effect allows us to sort out the various 'arms' of the galaxy. As each arm moves at a different velocity relative to us, so its hydrogen 21 cm emission is shifted slightly in frequency. In consequence, a map showing the galactic features within the plane can be constructed.
The second significant event in the field of radio astronomy was one that evolved quite gradually, and one that is still being refined. It was the discovery and explanation of the diverse radio activity emanating from the Sun. It revealed that the Sun is a much more active body than was previously expected. In a way it typifies the whole discovery process of modern astronomy. Each new revelation of the universe shows it to be more complex, violent and turbulent than was formerly imagined. The classification and physical investigation of solar radio bursts by a team led by J Paul Wild, an Australian radio-physicist, gave much impetus to the science of astrophysics and revealed the intricacies of the Sun's atmosphere. Plasma conditions unreproducible in Earthly laboratories were opened to study. Who would have believed that when the solar surface termperature was 6000 degrees Kelvin (degrees_Celcius = degrees_Kelvin - 273), the temperature of the outer atmosphere would turn out to be around one million degrees.
One of the authors speaks to a scientist at the CSIRO radioheliograph,
built to elucidate the nature and spatial extent of solar radio burst emissions
The third significant event was the accidental discovery by Bernard Burke and Kenneth Franklin of the Carnegie Institute in Washington, of radio noise emanating from the planet Jupiter. This discovery, made in 1955, was completely unexpected. The planets became alive, if not in a biological sense, then certainly in a physical one. This awareness has continued through the more intimate investigation of the solar system by spacecraft. The interaction between Jupiter's magnetic field and the moon Io to produce the chorus of radio waves first heard in 1955 at around 20 MHz is still being debated.
The fourth significant event in our saga was the discovery of a very peculiar class of astronomical phenomenon known as quasi-stellar objects (QSOs) or quasars in short.
The object in the middle of the field is the first identified quasar, 3C273.
It shows a jet of material emitted toward the lower right.
[Image: 4 metre Mayall Telescope, Kitt Peak National Observatory]
The initial discovery was made over the years 1961-63 of a radio source identified by the third radio sky survey conducted by Cambridge University. It was the 273rd object discovered in this survey, and was thus given the name 3C273. At this time radio telescopes did not have very great spatial resolving power and it was thus generally impractical to determine if the radio object could be located and studied with an optical telescope. However, Cyril Hazard, using the then newly commissioned CSIRO radio telescope at Parkes was able, with the help of a series of lunar occultations (whereby the moon obscured the source), to determine an accurate position for 3C273. Maarten Scmidt, a young Palomar Observatory astronomer was then able to examine 3C273 in great detail. Whereas it appeared on first inspection to look like a star, all other evidence said it wasn't. For a start, velocity determinations from the Doppler shift of its spectral lines seemed to indicate that it lay 2000 million light years away (our nearest well-defind galaxy, that in Andromeda, is only 1.7 million light years distant). At this distance the power output of 3C273 is 10 40 watts. Together with its small size this indicates an absolutely fantastic energy consumption. It is equivalent to consuming one whole sun every month! At the time no known physical mechanism to explain the operation of quasars had been satisfactorily proposed. Later it would be suggested that a massive black hole was the driving engine, and this is now the generally accepted explanation.
Nevertheless, the quest to discover quasars more distant than 3C273 continues to this day, with radio telescopes directing optical observations. The latest identification(1983), made by a team of Australian observers using the Parkes radio telescope and the Anglo-Australian telescope (optical) at Siding Spring (NSW) , is that of the quasar PKS 2000-330 estimated to lie at a distance of nearly 20,000 million light-years. At this distance, it is the most distant known object in the universe (1983) and is running away from us with a velocity of 92 percent of the velocity of light.
The fifth significant event is one that may yet turn out to be the most significant discovery that any astronomy has yet given to the human race. It was and continues to be the discovery of a wide range of diverse molecules scattered throughout interstellar space. These molecules were discovered in the same way that hydrogen atoms were discovered. That is, by the discrete but different spectral frequencies they emit when undergoing some change. Each molecule has its own unique spectral signature from which it can be identified. The first interstellar molecule discovered was the hydroxyl radical (OH) in 1963. Since then an incredible range of inorganic and organic molecules have been detected. An Australian team from Monash University was very active in this research. The molecules discovered include hydrogen sulphide, carbon monoxide, hydrogen cyanide, acetaldehyde and vast amounts of ethyl alcohol. The last item has caused eminent radio astronomer John Kraus to joke that "even if there is no life in space, there is at least one of the amenities of the good life in abundance." However, the range of molecules now seems ot include all the precursors necessary to construct proteins. The seeds of life in space may yet outweigh all the celestial vodka. The presence of either entity throughout our galaxy still remains a complete mystery.
The sixth major contribution of radio astronomy gained its discoverers the 1978 Nobel prize in physics. The discovery was made in the course of an unrelated investigation, much like that of Jansky. The coincidence however was much stronger, for Arno Penzias and Robert Wilson were working for Bell Telephone Laboratories in New Jersey in a part of the same laboratory where Jansky made his discovery. Penzias and Wilson discovered, after very careful experimentation, that the sky is filled with a background microwave radiation that indicates that the universe has an overall temperature of 3 o Kelvin (ie 3 degrees above absolute zero or -270 o C). It took some time to recognise the significance of this, but eventually it became very important in speculations about the origin of the universe. It provided decisive evidence in favour of the big bang theory. This theory states that some 14,000 million years ago (it was believed to be 20,000 million in 1983) the universe started as an extremely dense and compact primordial fireball at a temperature of 10,000 million degrees Kelvin. By our present age, the expanded (and expanding) universe should have cooled to 3 o K, precisely that measured in 1963.
The seventh and most recent contribution has ramifications that encompass almost everything from fundamental astrophysics through the feminist movement and into Hollywood. The discovery was again serendipitious, and was made by a young lady Jocelyn Bell, working at the University of Cambridge. It was, of course, the discovery of pulsars - those strange objects that emit radio energy in rapid bursts. For months after the initial discovery, unscrupulous tabloids were running stories about LGM's (little green men) in space. It turned out however, that the reality of pulsars was just as interesting, but in a purely physical sense. They form a vital link in the process of star development. When a large star is nearing the end of its life it may suddenly explode in a most spectacular way. The visual outburst may be seen on Earth as a supernova, or bright new star. Most of the mass of the star is blown off into the surrounding space, but a small dense core may remain. The core contracts under the influence of enormous gravitational forces until the parts of each atom are packed so closely together that everythng turns into neutrons. We then have a neutron star. If, as is most probable, the neutron star is rotating, it sends out a directional radio beam once per rotation much like a lighthouse. If the neutron star is dense enough it may continue to shrink, and eventually become, a black hole. At this stage, the escape velocity of the object is greater than the velocity of light and thus nothing, including light, can reach the rest of the universe.
In spite of its rather slow beginnings, radio astronomy has now progressed fast and far. Jansky could never have even dreamed, in 1933, that over the next 50 years fantastically sensitive radio ears would quite literally 'spring' from the Earth to probe the cosmos almost to its visible limits - and yet much remains to be done. Limited resolution, the ability to separate close objects, has until recently been a restricting factor in radio observations of the sky. However, the development of 'aperture-synthesis' techniques has recently enabled radio telescope to locate celestial sources with considerably more precision than any optical telescope now in existence. These techniques allow a number of small, widely spaced antennas to simulate the resolution of one much larger single antenna. The Australia Telescope, a CSIRO project to come on-line in 1988, will make extensive use of aperture-synthesis to keep Australia in the fore-front of radio astronomical research.