The Science behind the SpaceAusScope
Radio astronomy is the study of objects, events, and phenomena in space using radio waves, which make up part of the electromagnetic (EM) spectrum.
Due to their long wavelengths, radio waves can enter Earth’s atmosphere and pass through clouds, which means radio astronomy can occur during the day and even when it is cloudy.
The telescope we built as part of this project is a horn antenna (although some teams built dish antenna), which were tuned into picking up small radio waves of 21cm wavelength. The source of these radio waves is from the most common element we find in the Universe, Hydrogen.
Massive Hydrogen clouds are found across our Galaxy’s spiral arms. This is the material that eventually forms stars, planets and everything else we see around us. We can’t see these cold interstellar clouds in optical telescopes, but we can with radio telescopes. To understand how we need to look at some atomic-scale science.
Hydrogen, as an atom, is fairly simple – it contains one Proton in its centre (which carries a positive (+) charge) and an orbiting electron (which carries a negative (-) charge). Let’s pretend that both the proton and the electron are spinning balls with their axis both pointing upwards
When this atom is most relaxed, or in a ground state, the two-axis are in opposite directions – so the proton points up and the electron points down. However, sometimes the electron gets an energy kick, which takes it to an excited state. When this happens, the electron has absorbed some energy. But it is no longer relaxed and really wants to get back to its relaxed state.
To do so, it has to release the energy it absorbed back out into the Universe. And so the electron gets rid of the energy and reverts back to having its axis pointing downwards relative to the proton, which points upwards.
Now that little package of energy it released is a radio wave and it has a wavelength of 21cm. This travels across our Galaxy and eventually is picked up by the telescope we are building.
By looking for 21cm wavelength radio waves coming from the Galaxy, our telescopes were able to tell where the huge clouds of Hydrogen are and if they are moving towards us or away from us. This paints a picture of the Galaxy’s spiral arms through our data.
We collecting our data from the end of March and through to June in 2020, and were able to capture an arm of the Milky Way and then the centre of our Galaxy, which usually is high in the sky by the middle of the year.
Our Community Questions – with Responses
During the course of our project, our community raised a few questions that we thought would be handy to document for folks who want to delve into some science, in addition to linking the science back to their telescope.
- What is the Milky Way and where are we (the Solar System) located in it?
- What are stars made from and where do we find this material in the Galaxy?
- What is the Doppler Shift and why do we measure it in the Galaxy?
- How do radio telescopes work and what is the electromagnetic spectrum?
Question 1: What is the Milky Way and where are we (the Solar System) located in it?
The Milky Way is a large barred-spiral galaxy, that the Earth and Solar System reside in. We can see the Milky Way Galaxy with our naked eyes (even from light-polluted cities) as the bright band of stars that arcs across the night sky. If you’re lucky enough to be in a dark sky location, you can even see the clouds of light that look ‘milky’ spread across this band.
Because we are inside the Milky Way Galaxy it is hard for us to see it from a holistic point of view. However, we can measure aspects of it (e.g. Hydrogen gas concentrated in certain locations to figure out its shape) – something our telescope builds will hopefully touch on.
The best way to imagine the Milky Way Galaxy is two saucepan plates sitting face on together – it is a disc with a central bulge.
When we look up and into the Milky Way band we are looking at the central regions – the brightest part, made of billions of stars. The reason why it appears jagged and broken is because there is a galactic arm in front of us with lots of dust in it, and that is blocking the light from our view.
There are a number of different shapes that galaxies come in, such as elliptical, lenticular, spiral and barred spiral. In a spiral galaxy, the majority of stars orbit the galaxy’s centre in the disc and generally move in the same direction.
In an elliptical or irregular galaxy however, the stars all orbit the centre in different directions and so the disc shape does not exist.
It’s thought that galaxy’s morph over time (galactic evolution) to go from spiral to elliptical through mergers and collisions. Our Milky Way galaxy is on a collision course with our giant neighbouring galaxy, The Andromeda, both colliding in about 4.5 billion years from now.
You can even see two small irregular-like galaxies with your naked eye in southern skies – The Large and Small Magellanic Clouds
We think there are about 100 billion stars in the Milky Way Galaxy, and some scientists even think that number might be as high as 200 billion.
The Milky Way itself is about 100,000 light years in diameter, which means if you could travel at the speed of light (spoiler: you can’t) it would still take 100,000 years to cross it. The main features of our Galaxy are the central nucleus – which contains the supermassive black hole known as Sgr A* (pronounced “Sagittarius A-Star”) – a gargantuan black hole with the mass of 4.3 million Suns. Surrounding this is the central core stars which form a bar shape, and then moving outwards the arms of the galaxy start to warp and snake their way around to form the spiral.
The Sun, and our Solar System resides in the outer suburbs of the galaxy, away from the intensity of the core region, about two thirds of the distance from the centre.
The Sun itself orbits the Milky Way once every 250 million years or so. Cool fact: The last time the Sun was in this region of the Milky Way (today’s date), was when Dinosaurs started to walk on the Earth!
The Milky Way has played a vital role in history for every ancient culture around the world – so humans have been looking up at it since civilisations began to form (and even prior in Neolithic ages and further historical ages).
Link to our DIY Radio Telescope build
The shape of the Milky Way and where we are located in it play an important role in our DIY Radio Telescope build project …. We are aiming to measure the Hydrogen Gas in the arms of the Milky Way around our Solar System – and hopefully be able to measure if the arms are moving towards us or away from us.
So knowing what the shape of the galaxy is and where we are located gives us the ability to track which parts of the Milky Way galaxy our telescopes will be observing in.
Question 2: What are stars made from and where do we find this material in the Galaxy?
To answer this, first we need to go back to the VERY beginning of the Universe and then talk about the life-cycle of stars.
So, very briefly (as cosmology is probably too deep for this), the Universe was created in an event called the Big Bang. During the course of this event – the first elements in the universe were created. These were Hydrogen (73%), Helium (25%) and small amounts of Lithium and Beryllium (2%).
Everything thereafter in the Universe, started off with these elements. So naturally, it is safe to assume that stars are made of these elements, especially Hydrogen. And if stars are made of Hydrogen, then galaxies should be made of hydrogen (stars are the building blocks of galaxies).
Side note: Astronomers are a little weird with what comes next in this discussion, so here’s a little handy hint about some language that will be used from here on. In the periodic table of elements, everything that is not Hydrogen or Helium is called a ‘metal’. We think of metals as iron structures, steel rods, copper coins and this is correct …. But for astronomers, even things like oxygen in the air we breathe, or calcium in our bones is considered a metal. Told you, we’re a weird bunch.
The very first stars to be born in our Universe were massive stars. They were made from mostly hydrogen (this was three quarters of the universe after the big bang, so there was plenty of it to go around) and they contained not many metals.
But stars are factories that generate new elements through their lives, and then spread those new elements around the galaxy and universe. How do we know this? Well, our very existence of many different elements means they had to come from somewhere. So let’s check out the lifecycle of stars.
How stars live (and die) is all dependent on how much mass they have when they are born. Bigger stars have higher mass, and this higher mass creates more energy for the nuclear fires in their cores, and so they are rather greedy – they consume their hydrogen fuel source quickly, live fast and die young in spectacular explosions known as supernovae.
Smaller mass stars are in it for the long run – they consume their hydrogen fuel slowly and last a very long time – some stars today (Red Dwarfs) will live longer than the existing age of the universe!
Stars produce energy by thermonuclear reactions – they fuse hydrogen atoms together to produce Helium. This reaction gives off enormous amounts of energy, so stars can shine for very long periods.
Then when all the hydrogen has run out, stars begin to fuse Helium, to produce carbon and oxygen. And so on the creation of new elements continues down the periodic table of elements until it reaches Iron.
Iron, absorbs heat from the reaction and this causes the star to stop the fusion process and end its life – usually when it is big enough, in a violent supernova explosion …. Or when the star is moderately sized, like our Sun, just by puffing its outer layers off gently.
The key here is that in both these scenarios, elements which were created during the fusing processes are now sprayed out into the galaxy and ready to form part of the next generation of stars.
Now that the new elements are spread across the galaxy, a new generation of stars can re-start the cycle and start forming with these new elements. These new stars (like our Sun) are considered metal-rich, but even so, they are mostly formed of Hydrogen gas.
The most important element in all of this process is Hydrogen. For stars to form, cold hydrogen must be present – and it usually bundles up in Giant Molecular Clouds – enormous masses of (mostly) hydrogen stretching fast distances across the galaxy.
Without this hydrogen (and it needs to be cold hydrogen), stars will not form. Most of the hydrogen near the Galaxy centre has been consumed by the billions of stars that already exist there. So we now find most of the remaining hydrogen, in the big spiral arms and outer regions of galaxies. It is in this region where new stars are being born.
And if we can find where the big clouds of hydrogen are, then we can find the star forming regions in our galaxy – which means we can find the arms of the spiral galaxy we live in.
Link to our DIY Radio Telescope build
Hydrogen is the key element we are looking for with our DIY Radio Telescopes – and in particular, the energy released from cold hydrogen gas that is floating in the spiral arms of the galaxy.
By being able to trace the regions in our sky with lots of hydrogen (and hopefully how fast these giant gas clouds are moving) we will be able to identify the spiral arms of the Milky Way.
Question 3: What is the Doppler Shift and why do we measure it in the Galaxy?
Nearly everyone you know has experienced the Doppler Shift here on Earth. Here’s when:
You’re standing or walking by the side of the road. Off in the distance, you see the flashing lights of a fire truck coming towards you – but it is too far to hear as yet. Once the truck comes within audible range and continues, the siren starts to sound like it is getting higher pitched and sharper.
The truck is loudest when it is right besides you.
But then when it passes you, the sound begins to sound stretched and like it is ‘off’.
This is the doppler shift (related to sound waves). As the source of the wave approaches the wave compresses in front of it. But as the source of the wave departs from you, the wave is stretched behind it.
Some time back, astronomers found that light itself was also a wave. Which meant that light from the stars would exhibit similar properties, like a wave (although at a much higher velocity – things get weird. Light moves at 300,000 km/s).
But astronomers, using an instrument known as a spectrometer (attached to telescopes) were able to split the components of visible light up and measure how fast things were approaching or receding from the Earth just by analysing the light of the object in space.
This revolution changed astrophysics and allowed us to measure many things – there is so much information inside light (i.e. the electromagnetic spectrum, which includes radio waves) that entire industries are now built around it.
In our galaxy, we can use the light across the electromagnetic spectrum (visible light, x-rays, radio waves, etc.) to measure if objects are moving towards us or away from us – and their velocity.
Using this method, astronomers have observed stars that move towards us and away from us in cycles – like they are going around in circles. In fact, those stars are orbiting and usually have another companion.
The really fun part is when we calculate the speed of the orbit of the first star, and have an understanding of its mass – because then we can indirectly determine how much mass the other object must way, even if we can’t see it!
There has been numerous candidates for black holes like this.
Link to our DIY Radio Telescope build
Much like star light, the 21cm wavelength emitted by cold Hydrogen (which can be measured at exactly 1,420 MHz) in the spiral arms of the Milky Way, will carry a doppler signature within it – it will either be moving towards us or away from us.
If it is moving towards us it will be compressed, so a shorter wavelength (roughly around 1,418 MHz to 1,419 MHz). If it is moving away from us, we should see a longer wavelength (roughly 1,421 MHz to 1,422 MHz). Of course this is dependant on the sensitivity of the telescopes we are building, including materials, shape etc. (hence we are running this experiment with all of you).
If we observe a shorter wavelength, it means the galaxy arm we are observing is rushing towards us. If it is a longer wavelength, it means the galaxy arm is moving away from us. And we’re caught in between two massive spiral arms of the galaxy!
Question 4: How do radio telescopes work and what is the electromagnetic spectrum?
Radio telescopes can come in all shapes and sizes – as we’ve seen from our project alone, there are Dish-like radio telescopes and Horn-like radio telescopes. Each is designed for a different specific need but they are all designed to pick up on waves in the lower end of the electromagnetic region of the spectrum – radio waves.
Whilst there is a variety in the telescope design, the core concepts behind the scenes are similar for nearly all cases. In a nutshell:
There is usually a collection tool, like a dish or a horn. This directs incoming radio waves from space into a focus – such as a waveguide. Here in the waveguide resides an antenna which picks up the signal and sends it to the amplifiers and receivers. They then send the signal to computers to process the data.
Each radio telescope design is in tune with a certain frequency range which allows it to study certain objects better than other telescope designs.
For example, the Parkes Telescope is a 64m-diameter parabolic dish which makes it perfect for studying pulsars. Where as the feed horn antenna that some of us are building, is an alternate design – which is good for studying the 21cm hydrogen gas within our galaxy.
Some radio telescopes look funny – in that their antenna sections are split off into sub-sections and scattered over large regions – thus making the distance between them, the bigger sized ‘dish’. The Murchison Wide-Field Array telescope in WA is made up these spider-like antennas arranged in a 4 x 4 configuration. This is just one of many stations like this – and when they are all switched on together, they act as one giant eye that looks deep into space at certain frequencies.
This telescope design is used to study the central galactic plane of the Milky Way at really low frequencies, with the ability to extract very fine detail about the expanding shells from past supernovas and even peer into the heart of the galaxy as well – the supermassive black hole Sgr A*.
So there are a variety of radio telescope types doing a variety of observation types but for most of them, the underlying concepts remain the same – collect data, amplify and record data, process data.
Link to our DIY Radio Telescope build
The electromagnetic (EM) spectrum is the range of wavelengths that EM radiation is transmitted in. It ranges from radio waves, which are big and long through to Gamma-Rays, which are tiny and short in wavelength. Our atmosphere blocks a lot of the spectrum out from space – and we have evolved around this – our eyes can see the visible portion of the EM spectrum. We can also experience some of the infrared section (this is heat: close your eyes and follow the heat until you find the Sun – you’ve just become a telescope!) and even UV light (which causes us to sunburn (and can be dangerous to the cells in our body)