What Equipment Does One Need to Send a Radio Signal to a Distant Star?

Question: Hi. I’m doing some research for a piece of fiction and was wondering: if you had a radio transmitter/receiver aimed at a far star 1) what kind of equipment would one need and 2) how would the equipment’s behaviour vary from a conventional satellite receiver? I know it’s not quite an astronomy question and more a question of radio astronomy technology, but I do hope you might be able to help. Any information would be very much appreciated.  – Steve

Answer: As a prelude to the answer to this question, let me dispel a myth about the detectability of Earth’s radio transmissions.  Even though it is true that many of our radio and television broadcasts are “leaking” into space at the speed of light, those signals are likely too weak to be detectable by any of the inhabitants of planets orbiting even the most nearby stars.  This is due to the fact that the strength of a light signal, such as a radio broadcast signal, decreases as the inverse-square of the distance that it travels from its source.

To illustrate this inverse-square law let us assume that we transmit a radio signal with a power of one megawatt (1 MW) into space.  For reference, the Arecibo Observatory has a radar system that it uses to map the surfaces of the Moon and nearby planets that transmits a signal power of about 1 MW at a frequency of about 2.4 GHz.  Let us further assume that the signal is transmitted at a frequency that can penetrate the Earth’s atmosphere and travels unimpeded through space.  This 1 MW signal would be just barely detectable at the nearest star, Proxima Centauri, which is at a distance of 4.243 light years.  Furthermore, keep in mind that this distant civilization would have to be staring at Earth with its receiving system tuned to exactly the same frequency that we are transmitting.  Therefore, it takes a rather power transmitter to send a detectable radio signal to the nearest star.  This sort of equipment is far more sophisticated than your average home satellite signal receiving system.

Furthermore, as you can probably imagine, this scenario has been investigated by quite a few researchers.  There are many factors that affect the detectability of radio signals emitted or transmitted from the Earth, complicating the rather simplistic scenario discussed above.  For a nice summary of these investigations see the io9 article on the pros and cons of sending beacon signals into space.

Jeff Mangum

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Inconsistency Between the Age and Diameter of the Universe?

Question: I have read that the diameter of the universe is 96 billion light years.  How can that be if the universe is a mere 14 billion years old?  Am I to conclude that the 96 billion figure is some extrapolation based on rapid inflation?  – Carlton

Answer: One of my physicist colleagues, Frank Heile from Stanford University, has provided an excellent answer to this question.  First of all, the 13.8 billion light years is derived from the radius of a sphere of the Cosmic Microwave Background (CMB) radiation that is being observed by the Wilkinson Microwave Anisotropy Probe (WMAP) and Planck satellites.  These satellites have mapped the structures which are precursors to galaxies and clusters of galaxies.  Second, the 93 billion light year diameter estimate refers to the “observable” universe.

Now, if we waited another 46.5 – 13.8 = 32.7 billion years, we should actually be able to see the light emitted right now from those superclusters of galaxies which formed from the CMB structures.  The light is already on its way towards us but it will take a while to reach us since it will have to come from a sphere with a diameter of 93 billion light years.  This is the explanation for the difference; the “observable” universe is larger than we can see it today.

This is only, at best, a theoretical estimate of the diameter of the universe, though.  We now know that due to dark energy the expansion of the universe is accelerating.  This acceleration will not allow us to see those superclusters which are now 46.5 billion light years from us.  In fact, if we wait the requisite 32.7 billion years those superclusters will be receding from us at a rate that is greater than the speed of light.  We just will not be able to observe those galaxies and clusters of galaxies which have formed at the edge of the universe.

Jeff Mangum

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Academic Path to a Career in Astronomy

Question: Forgive me for asking a question that I’m sure you’ve seen many times over. I’m interested in radio telescopes and a career with them(particularly the prospect of working with the Square Kilometer Array). I’ve been contemplating an undergrad in physics with a possible double with chemistry (looking to go to a local university as I cannot afford going out of state for a university with an actual undergrad in astronomy or astrophysics) and then working towards an astrophysics graduate degree out of state. I’m thirty one and considered a sophomore. I’m finishing up my year in general chemistry and looking into calculus and physics starting the fall. Is shooting for an astrophysics degree a good way to go or should I go for an astronomy degree?

I’m trying to reach out into the field to get a better understanding now so I can make the right decisions in my academics. Thank you for taking the time to read this and if you do respond, I greatly appreciate that you took the time to do so. – Jonathan

Answer: At the graduate school level the astronomy and astrophysics degrees are effectively the same thing.  The difference is in name only.  You should choose a graduate school based on other factors like the research programs offered rather than whether the graduate degree is in astronomy or astrophysics.

Jeff Mangum

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Why Can’t Venus be Fully Lit by the Sun in the Ptolemaic System?

Question: The first known observations of the full planetary phases of Venus were by Galileo at the end of 1610 (though not published until 1613). Using a telescope, Galileo was able to observe Venus going through a full set of phases, something prohibited by the Ptolemaic system (which would never allow Venus to be fully lit from the perspective of the Earth, as this would require it to be on the far side of the sun, which is impossible if its orbit is, as the Ptolemaic system requires, between the Earth and the sun). This observation essentially ruled out the Ptolemaic system, and was compatible only with the Copernican system and the Tychonic system and other geoheliocentric models. [From the Wikipedia page on the Phases of the Venus].

Can you please explain this?  In the Ptolemaic system if Venus orbits the earth why would it not be fully lit when it is on the far side of its orbit around the earth just like the moon when it is not between the earth and the Sun.  – Joe

Answer: In the Ptolemaic system the order of the planets, the Sun, and the stars from the Earth was as follows:

  1. Moon
  2. Mercury
  3. Venus
  4. Sun
  5. Mars
  6. Jupiter
  7. Saturn
  8. Fixed Stars

A key fact in this discussion is the fact that in the Ptolemaic system Venus is always between us and the Sun.  Therefore, we would always see some of the night-side of Venus (the side which faces away from the Sun).  This arrangement would never allow Venus to be fully-lit.  Therefore, the fat that Galileo observed the full-phase of Venus required that Venus orbited the Sun between the Earth and the Sun.

Jeff Mangum


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A Career as an Astronomer

Question: I am 15 and I plan to become and astronomer. I’m a freshman in high school. What classes do you suggest I take my sophomore year? Also, do astronomers travel a lot? What do you suggest I do to learn hands-on about astronomy.  – Anthony

Answer: You should check out my Careers in Astronomy page on this blog for information on various aspects of a career choice as an astronomer.   Regarding the classes that one should take in high school to prepare for a career as an astronomer, the best tip that I could give to an aspiring high school student who is interested in astronomy is to learn as much math and science as possible.  Astronomers are basically physicists, and math is the language of physics.  A good background in physics and math is essential for a productive career in astronomy.  Regarding travel, yes astronomers do travel a fair bit, but I would not say that it is excessive.  Mostly we travel to attend meetings or to conduct observations at observatories.  Finally, to learn hands-on astronomy your best bet is to try to find a summer internship working at a physics or astronomy research institute.  There are a growing number of these summer programs for high school students and they are a great way to “get your feet wet” working as an astronomer for a summer.

Jeff Mangum

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Dream Job Questionnaire

Question: Hello. For school, I have to interview a person with my dream job and I would be very grateful if you could answer some of my interview questions.  – Leo

Q1: What events in your life inspired you to become an astronomer?

I have always had an interest in science.  I am not sure what influenced my decision to pursue a career in astronomy, but I have also always been interested in astronomy.  I did also have very good science teachers in elementary and high school, so I believe that they were very influential in my career path.
Q2: How does the typical work day as an astronomer look like?

The great thing about astronomy is that it is rarely typical.  I have quite a wide range of tasks that I can work on during any given day.  Most of them involve solving astrophysical problems or working on keeping the National Radio Astronomy Observatory’s facilities running well.  All of them are interesting, though.
Q3: What college majors and degrees did you take to become an astronomer?

I have three degrees in astronomy.  Bachelors, Masters, and Doctorate.
Q4: What is the history of this occupation?

I think that astronomy must be one of the oldest of the sciences, dating from ancient times when people tried to figure out what stars where and why they moved the way they did.
Q5: How many years of college did you take to become an astronomer?

It took me four years to finish my bachelors degree, then two years to finish my Masters degree, followed by another three years to finish my Doctorate.
Q6: What is the main purpose of being an astronomer in your point of view and what accomplishments do they achieve?

The main purpose of astronomers is to figure out how the universe works.  Since many people find the study of planets, stars, and galaxies fascinating, astronomy also inspires people to better understand the physical world around them.  This results in an increased number of people who seek to understand science, and perhaps pursue careers in the sciences.
Q7: What is the best part or benefits for working as this occupation?

I think that the best part of my job is that I get to work on a variety of interesting problems.
Q8: Who in your life gave you the motivation and helped you to become an astronomer?

I think that my high school teachers were the main motivators for what became a career path in astronomy.
Q9: What is your specific role and what kind of astronomer are you?

I am a scientist at the National Radio Astronomy Observatory (NRAO) where I work to develop, update, and maintain radio telescope facilities.  NRAO’s telescopes are used by astronomers all over the world.
Q10: What is your name?

Jeff Mangum


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What Radio Astronomy Can Be Done at S-Band?

Question: At work, we have a 14 foot dish with an S-Band receiver.  Are there any interesting radio astronomy frequencies around S-Band?  – Frank

Answer: The radio astronomy S-band runs from about 2655 to 3352.5 MHz.  Its use in radio astronomy is for continuum measurements from sources of synchrotron and free-free emission, such as supernova remnants and regions where stars are forming.  Note, though, that there are numerous communications services which operate at S-band, so you might find it hard to detect astronomical signals if your receiver is not tuned to the radio astronomy allocated frequency range.

Jeff Mangum

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What is the Large Scale Structure of the Universe?

Question: What does the structure of the universe look like at the largest scales?

  1. Galaxy cluster are distributed evenly throughout space with no large gaps
  2. There are many more galaxies and clusters in some directions (up and down the milky way’s disk) and very few galaxies in other direction
  3. Linear or wall like distributions of galaxies, clusters, and superclusters surrounding relatively empty regions- like soap bubbles
  4. Galaxies and cluster are very thiny spread out in the near distance, and are more closely packed at greater distances from the milky way

– Jared

Answer: I think that option 3 comes closest to the actual distribution of galaxies in the universe.  The large-scale structure of the Universe is made up of filaments and voids.  When we look closely at the filaments, we find that they can be broken down into superclusters, clusters, galaxy groups, and finally into galaxies.

Jeff Mangum

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What is the Age of Kapteyn’s Star?

Question: I’m interested in Kapteyn’s Star. July 2014 popular news reports about the discovery of Kapteyn b & c mentioned an age for the system of 11.5 GY. However, the EU exoplanet database lists an age of 8 GY. (For some reason, it also lists a few stars with ages older than 13.8 GY.) Can you provide a definitive age? Kotoneva’s quite thorough 2005 study of the star did not provide an age; the authors did use a 10 GY estimate for temperature purposes.  My guess is that the EU had an old list of stellar characteristics, and when someone finds a planet, they tack the new planet info onto the old star info. Did something happen to change the dating of Kapteyn?  Thanks for your help.  – Andrew

Answer: I am not sure what database the “EU exoplanet database” is, but I have reviewed a recent analysis by Anglada-Escude etal. (2014) which includes a nice description of the limitations associated with determining the age of Kapteyn’s Star.  Anglada-Escude etal. (2014) list the age as greater than 10.0 GY, but less than 13.8 GY (the upper limit set by the current value for the age of the universe).  I believe that this is the best current estimate for the age of Kapteyn’s Star.

Jeff Mangum

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Astronomer Career Interview

Question: Sorry for disturbing. I have to make an interview with an astronomer for my school project. You have already answered these questions 2 years ago but questions have to be asked by me. I asked the same questions again sorry for that and also sorry for grammar mistakes. Thank you.  – Selin

Q:What is your name ?
A: Jeff Mangum

Q: What is your job title?
A: Scientist

Q: What is your upcoming salary?
A: In general, astronomers are not paid extremely well, but well-enough to “get by”…

Q: How many years it took you to finish your university [education]?
A: I was an undergraduate in astronomy at the University of California at Berkeley for 4 years, which was followed by 5 years of graduate study at the University of Virginia.

Q: Do you like or dislike your job?
A: Like most astronomers, I do like my job.  What is there not to like about trying to figure out how the universe works every day!

Q: Was it hard to get your job?
A: Permanent jobs in astronomy are rather difficult to obtain.  It can sometimes take several years to “land” a permanent position.

Q: What university did you study in?
A: As I mentioned above, I was an undergraduate at the University of California at Berkeley and a graduate student at the University of Virginia.

Q: How many years you have been working on this job?
A: I have been on the Scientific Staff at the National Radio Astronomy Observatory for 20 years.

Q: Do you enjoy your job? Why?
A: Absolutely!  I have a fair amount of freedom to work on a wide-variety of interesting problems.

Q: How many hours do you spend working?
A: Most scientists spend quite long hours doing their work and research.  My typical week can consume up to 60 or more hours at work.

Q: When do you have free time?
A: Mostly on weekends and holidays, like most people.


Jeff Mangum

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