In this post I like to publish the way making the 21 cm radio signals of the socalled spiral arms of our galaxy, visible on the screen of my PC. I like to demonstrate the presense of numerous the spiral arms in our galaxy. It is another interest of mine.
To understand what I mean by making it visible, first some explanations about what is a galaxy and how it looks like. We call a galaxy also a milkyway.
In the pictures 1 and 2 we see an artist impression how our galaxy looks like from above. It consist of a core or bulge in the mid and several moving socalled spiral arms, starting from the core in the mid. In the spiral arms exist a lot of sun systems. A sun system consist of a sun in the mid with around it several planets. In one of the arms, called the Orion arm, our sun system is located. Our earth is one of the planets in our sun system.. More planets are Mars, Jupiter, Venus etc..
In our galaxy or milkyway, there exists a large spectrum of radiation with different wavelenghts or frequencies, called the electromagnetic spectrum. Like radio, atomic hydrogyn, infra red, optical or visible light, röntgen and gamma. Most of them are dangerous for our health. For instance gamma is very dangerous, but our atmosphere form a protecting shield for this.
We can divide the whole spectrum into 10 maps of wavelengths.
In the picture below 10 Multiwavelength maps, seen in a flattened view of our milkyway. In each map we see in the mid the core or bulge of our galaxy. These maps are seen from the side of the galaxy.
For making visible the spiral arms of our Milkyway, mentioned in the Header above. We use the Atomic Hydrogyn map in the picture above. The second map from above.
In this map we see the electromagnetic hydrogyn gaz emissions. We are able to receive this emission on a radio receiver.
That hydrogyn is neutral hydrogyn gaz, called H1. This gaz has the chracteriststic to send out a radiosignal at a frequency of 1420.402 Mhz. Or 21,1 cm wave length. Not a clean carrier but a signal with a broad spectrum of about 2 Mhz. With a maximum at 21,1 cm.
Again: In picture 1 and 2 an artist impression, how we see our galaxy from above and we can notice these spiral arms in visible light, from all the suns in it. But more kind of emissions, like IR, gamma x-ray. We see the Perseus arm, the Cygnus Orion arm. Also we see the location of our sun, which is located in one of the side branches of the Orion arm.
In picture 3 a visible impression of an other galaxy.
Between these sun ystems, the interstellar space, and there is a lot of neutral hydrogyn gaz (H1). The H1 atom consists of only one proton and one electron. When the tollong or spin of the electron is different for a moment from the tollong of the proton, radio emission occurs. But that happens only once in a million year. Actually it is forbidden law, but it still happens once in that million year. See picture below.
But because there are so many H1atoms, there exist enough emission to detect it. This with a maximum at 21,1 cm wavelengh. The radio emission is not a clean signal but it is noise emission with a bandwidth of 2 Mhz, but in that 2 Mhz a maximum noise signal at 21, 1 cm.
Foto 1, 2 and 3.
The artist impressions from the pictures above are pictures we never can see them from our position on earth. Because we only can see these arms one after the other in one flattened of dish. So our galaxy is a kind of dish with in the mid a thickening, the core. See picture below in visible light.
Or we have to build ourselves a space aircraft to go to upper side of our Galaxy and take a picture. But because of the state of the art, it takes thousands of lightyears to reach that point, and that is not possible. The distance is to large! The diameter of our galaxy is 100.000 light years. We have to travel more then 100.000 lightyears! To give an impression about distance: one light year is what the visible light, radio emission has the same speed, travels in one year with a speed of 300.000 km per second! That is why we speak of lightyears instead of km or miles. To decrease the amount of figures.
But how do we know, how our Galaxy looks like the same as the artists impressions? How can we see that there are more arms then what it looks like one, we can see from our position up to the sky?
When we look at night with a clear sky upwards, , we see a broad lightned stroke. This our milkyway. We see all the arms as one arm. See picture below.
We can demonstrate that there are other arms with the aid of a physical law, called the doppler frequency shift and applied in a software program.
But what is doppler frequency shift effect.
When an object moves towards us and moves away from us, the sound frequency is first increasing and later decreasing.
Example for instance: a train is passing you when you are waiting in front of a closed railway crossing. The train with a foghorn on, moves from the left to the right direction. You sure notize the frequency shift of the tone of the horn. Just try it out. When it approches you the tone frequency increases, when it just passes in front of you, you hear the original tone frequency of the horn, when it deletes you, the tone frequency decreases.
In our galaxy all these spiral arm moves away from the core. See picture 2, in clockwise direction. We ourselves are situated in the Orion arm. The Centaurus arm is moving much faster in relation to our sun, so earth in the Orion arm. So the Perseus moves slower.
So if we look to the outer Perseus arm, it runs slower. So we overtake the Perseus arm in speed. Also can be said, that the Perseus arm is approching us.
Compare this situation with the train. The train is approaching us, when comming from the left to our location in front of the railway passage. So the radio emission of a gaz cloud in the perseus arm is increasing in frequency! Just because oif the Doppler. In case of the Centaurus arm: we delete this arm. So its frequency is decreasing ( compare that train).
And these facts, we are using in a software program on our PC.
Our milky way is divided in 4 quadrants, number alpha, beta, gamma en delta.,
See picture below.
The Perseus arm, we spoke about, is in the alpha quadrant. The other arms in the gamma and delta quadrant are difficult to detect, because of the core.
When we run the program, we see at the right an FFT screen. The x-axis represents the frequency shift of the H1 radio emission. The y-axis represent the power or the strength of the emission. See pictures below.
The software I am using is SDR# with IF Average. This gives the screen above. At the right we see then the results of the H1 emissions from the different spiral arms. All these frequencies of these peaks are higher in frequency then the real frequency of H1 emission, 1420, 402 Mhz. (1420, 402 Mhz is our situation just in front of the railway passage, where the train is passing us).
This situation happens for instance when we pointing our antenna to the position of a longitude of about 150 degrees in quadrant alpha, pointing to the star system Perseus. See picture 2.
With the software program, we can demonstrate the presence of several spiral arms in our galaxy. In the FFT screen we see the peaks, each peak is a maximum emission of a gaz cloud in the spiral arm.
So by means of radio, we can demonstrate, that there are more spiral arms, the then only one we see visible. With a optical tescope we never can demonstrate that.
By making several FFT’s from different coordinates, we can construct a map of our milky way.
Some history:
The first systematic investigation of HI in the Galaxy was made in The Netherlands at Kootwijk by van der Hulst, Muller, and Oort (1954). The Dutch group used a reflector from a German radar installation used in WW2. Called the “Great Würzburg” type, 7.5 m in diameter.
They made the first map of our milkyway. See picture below. IAt the stairs of the Würzberg cabin Lex Muller.
About the galactictic coordinate system, seen in picture 2:
In picture 2 we were pointing our antenna to, for instance, the coordinates Perseus. Every sun or star system in space has its own coordinates. The coordinates is the galactic coordinates system in degrees of longitude and latitude. These coordinates are always realtime. Always the same degrees. To point your antenna to an object we have to convert the galactic coordinates into the azimutal system in degrees. Azimut and elevation settings of your antenna. And of course you have to follow that position, because our earth is rotating. To keep the direction of your antenna to the location.
A very helpful program can be downloaded from internet, called Stellarium. In Stellarium you can trace all the stars and star systems in space in and outside our galaxy. Also our planets in our sun system. When we choose the star system Perseus, the coordinates of both systems appear at the left above edge of the screen, realtime.
I use it always to point my antenna to an object. And keeping it.
A very interesting program.
But the signal strengh of these emissions are very , very weak. All below the offset of the noise level of our receiver. It is so weak, that we have to use in the software socalled FFT, Fast Fourrier Transform. a mathemethical calculation. The program is measurung the strengh of the emission in a band width of 2 Mhz. The does it over and over again. All these measurement are stored. It does it over and over again and averages the total of measurements out every time. Finally we get the picture of the peaks.
Also the strengh of the final peaks depend on how much gain your antenna has. I am using a parabole antenna with a diameter of 1,5 m. As a receiver a RTL SDR V3 dongle Also using a low noise amplifier (LNA), selectiv for 1420 Mhz. Especially the gain and noise figur are important. My LNA is build by G4DDK, an English radio amateur. It has a noise figur of 0,2 at a frequency of 1420 Mhz db and a gain of 37 db. Especially the noise figur is very good! It is one of the best LNA’s there exist at this moment.
In the picture below, my antenna, a parabole, is to be seen at a demonstration at an open day of Camras at the terrain of famous radio telescope of Dwingeloo.The LNA is mounrtinted just to the input connector of the feed. You have less losses then when using a cable between the LNA and the feed input. Every db of loss counts!
On the picture below of it, you can see the PC’s for the software. Note that the left PC shows a picture which is the same as picture 2. On the PC at the right the FFT screen. My homemade parabole antenna is pointing to Cassiopeia, a star system, existing of 6 stars.
But where that demonstration at that open day of the founding of CAMRAS? Wel, I am a volunteer, active in the founding of CAMRAS.
CAMRAS manages the former restaurated radio telescope of Dwingeloo. It has a parabole dish diameter of 25 meters. The gain of only the parabole without LNA is 55 db! They do observations in the radioastronomie field. Also they can use the telescope for EME (earth moon earth) purposes, and tracking satellites. Very interesting applications.
Picture below shows the advantage of more gain of a 25 meter diameter dish. I made an observation with my software and RTL SDR V3 dongle with it. A LNA is mounted already in the focusbox , located in the focal point in front of the dish of the telescope. The result is a much larger peak in the FFT screen.
It is an observation pointed to Cepheus, a star system. Watch the large peak due to the large gain of the telescope antenna of about 55 db plus gain of the LNA, at a frequency of 1420,402 Mhz.
A nice result.