Astronomical Concepts – Week 8 (Final)

For this final week of the course the focus was cosmology.


The main topics of learning we looked at included:

  • Galaxies
  • Quasars
  • Dark matter
  • Dark energy
  • The Big Bang Theory
  • Gravity
  • Expansion of the universe and inflation


There are three main types of galaxy: spiral, elliptical and irregular. Our Milky Way galaxy is a spiral type and contains billions of stars. It is about 100,000 light years in diameter and our solar system is located in the suburbs of the galaxy. At the centre of our galaxy, and also at the centre of most is a super massive black hole. Our nearest galaxy is called Andromeda and we are on a collision course with this galaxy and we will collide in about 4 billion years. Even though the universe is expanding, space is literally stretching like the surface of a balloon being blown up, our galaxies are locked in a gravitational embrace.

Our galaxy is within a local group of galaxies that also contains Andromeda. This local group was first recognized by Edwin Hubble. Even though our local group is a closely packed group of galaxies the distances between the galaxies is enormous. If we travelled at 17.3 km/s it would take us 40 billion years to get to the nearest galaxy (Andromeda). If we could travel at light speed it would only take us 2.3 million years, but this is not possible… yet!


Paul showed some stunning images during the evening and some are shown below.

The pinkish image is of the large magellanic cloud. It is nearly 200,000 light years from Earth and is a satellite galaxy of the Milky Way. It is a highly active star forming vast cloud of gas. Gas slowly collapses to form new stars which light up the gas around them.


This theory proposes a period of very fast expansion of the early universe. It offers solutions to some of the problems of the big bang theory. Inflation is said to have increased the size of the universe by a factor of 10^26 in only a fraction of a second. But, it also has problems! Here is why thanks to New Scientist.

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The cosmic microwave background was another featured topic tonight and here it is.IMG_2857

This image shows the universe in microwaves. It shows the temperature fluctuations of the early, early universe, about ~300,000 years after the big bang. The image is a record of a time when the early universe cooled to around 3,000 Celsius and protons and electrons were able to form atoms. As a result photons were able to escape and travel freely around the universe. The CMB was discovered in 1965 by Penzias and Wilson and they hared the Nobel prize in physics for this discovery in 1978. Today the CMB is very cold, just 2.725 degrees above absolute zero which means the radiation shines in the microwave part of the electromagnetic spectrum and is invisible to the naked eye. However, we know it is there, everywhere in the sky and if we could see it ourselves we would see the entire sky glow with a very uniform brightness in every direction. The temperature is uniform to better than 1 part in a thousand. This is the main reason to why scientists think it is the remnants from the big bang, because what other event could have been the cause. By studying the CMB further we can learn about the conditions of the early universe in great detail.

Dark matter and dark energy

These theories are still a mystery. We know a lot about our universe and one thing we know is that about 0.4% of the mass of the universe is made of stars, dark matter is about 27%, dark energy is about 68% and the remainder is gas, mainly hydrogen. Here, again thanks to New Scientist are dark matter and dark energy explained in more detail.


There, described beautifully, thank you New Scientist!

Limitations of the big bang theory


The fate of the universe! There are two theories: endless expansion and the big crunch. If the universe continues to expand forever then it will also continue to cool down until it is unable to to sustain life. On the other hand, if gravity wins and takes back control over expansion and there is sufficient mass to be able to do this then the universe will start to collapse back in on itself – the big crunch! Recent evidence suggests the universe is still expanding and at an increasing rate. This could be the dark energy mentioned earlier.

Paul left us where we started 8 weeks ago with the Hubble Deep Field image.


This is an image of a tiny patch of the night sky that was believed to be blank, empty space. The Hubble Telescope focused on this tiny patch of sky and took a long exposure image over 10 days, and this was the result. The image shows over 300 galaxies, everything in the image is a galaxy and some of the farthest and oldest ever seen. The image is very important to scientists and researchers to see how the universe has developed and changed over time. it is one of the most important images ever taken!

This was an amazing course packed full of super-interesting information about our universe, solar system, stars, planets and the theories that shape our lives. I recommend it to everyone! Follow the link to sign up for the next instalment.

Massive thanks go to Dr Paul Payne for your amazing lectures, graphics, stories, jokes, cups of tea and biscuits!

Astronomical Concepts – Week 7

This week – special relativity.


We started with the two main axioms of special relativity:

  1. The laws of physics are the same in all inertial frames of reference

  2. The speed of light is constant (299,792,458 m/s)

Tonight we had to imagine we were floating through space in a spaceship. Are we stationary or are we moving? Are the stars stationary or are they moving? Relative to us of course. With the window blinds down on our spacecraft we have no way of knowing if we are moving or not, there is no experiment we can do to find out.

However, an observer on another spacecraft has a different point of view. On our spacecraft if we bounce a ball we see it go down and then up in a straight line. If an observer on another spacecraft could see us bounce a ball as we drift past them in our spacecraft, and they are stationary they get a different point of view of the ball.


The definition of ‘now’

We accept that certain events that happen can be simultaneous. But how can we determine that two events that happen in different locations are simultaneous?

On Earth we carry clocks and they are synchronised, we accept that two events that happen happened at the same time. What we have to accept is that clocks, time, is influenced by motion. Einstein realised that space contracts and time dilates through motion. A moving clock runs more slowly as its velocity increases, until, at the speed of it stops running all together. So having clocks that run at different rates leads to strange effects – simultaneity is relative. Whether or not two events are simultaneous depend on your frame of reference.


So, one person’s definition of time is not the same as another’s. Also, the faster you travel the slower you age. At speeds close to c the effects are huge, at smaller speeds less so.

Further, the faster you move the more you contract. Close to c the amount of contraction is great, at slower speeds less so, tiny amounts. An observer at rest relative to the moving object would observe the moving object to be shorter in length of motion. As the object increases in speed and gets close to c the object would appear much shorter.


Special relativity is a work of pure genius by Albert Einstein. Our session tonight was an introduction, a mere taster of the theory and we were only able to scratch the surface. I loved the session, the concepts are so interesting, if a little hard to get your head around. Much more information about special relativity can be found here.

So, the only true constant is the speed of light. The faster you travel the more time slows down for you and the more you contract.

With the invention of atomic clocks we can now measure time to billionths of a second and can be accurate to within one second over 3.7 billion years. Einstein said that realising gravity and acceleration were the same thing was “the happiest thought of my life”.

Astronomical Concepts – Week 6

This weeks topic was the stars.

Their are billions of stars in our galaxy and there are billions of galaxies in the universe. There is an unbelievably huge amount of stars out there and the stars are separated by huge distances. How do we measure the distances to the stars?

The closest star to Earth is of course the sun at 1AU. The next nearest star is called Alpha Centauri and is about 4.24 light years away. This means the light from that star takes 4.24 years to reach us. It is a huge distance away from us, over 40 trillion kilometres! Our fastest spacecraft travelling at its top speed would take over 80,000 years to reach it.

Our closest star is actually a triple star system, three stars bound together by gravity. There is Alpha Centauri A and B and Proxima Centauri, shown below,


How do we measure the distance to the stars?

There are a few methods and the most popular is called stellar parallax. This works because we know the diameter of Earth’s orbit around the sun (300 million kilometres). By looking at a star one day and then 6 months later looking at it again an astronomer can see a difference in the viewing angle for the star. With a little trigonometry the different angles yield a distance . This method works for stars less than 400 light years from Earth.


For stars further away the measurement is made through the brightness and its colour spectrum. Once astronomers determine the colour spectrum they can then determine the the star’s actual brightness. By knowing the brightness and comparing it to the apparent brightness seen from Earth they can determine the distance to the star.

Our sun is an average star. Compared to other stars it is tiny, as these graphics illustrate.

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It is amazing how huge stars can be!

When studying the star the Hertzsprung Russell diagram is one of the most important. The diagram originated in 1911 by Ejnar Hertzsprung who plotted the absolute magnitude of stars against their colour. Hence their effective temperature. In 1913 Henry Norris Russell used spectral class against absolute magnitude. The result shows the relationship between the temperature and luminosity of the star. A version of the diagram is seen below,


Our sun can be seen in the main sequence about half way along the sequence.


A pulsar (or neutron star) is about 20km in diameter but has the mass of about 1.4 times that of the sun. These stars are so dense that on Earth one teaspoon would weigh a billion tons! They have intense gravity and also magnetic fields a million times stronger than the Earth. Pulsars are a possible end of a star. They result from massive stars about 4-8 times that of the sun. They finish burning their nuclear fuel and undergo a supernova explosion. Outer layers of the star are blown away and what is left in the centre is the remnant collapsed under gravity. It collapses and compresses so much that protons and electrons combine to form neutrons. They are made from almost pure nuclear matter, atomic nuclei packed side by side.

Pulsars were discovered in 1967 by Jocelyn Bell. They spin rapidly and have jets of fast moving particles almost at the speed of light streaming out above their magnetic poles. The jets produce very powerful beams of light. As the pulsar spins around the jets of light sweep around the star so from Earth we see the light turn on and off, like a lighthouse. A pulsar has been observed within the Crab Nebula, shown below,


The images are from the Einstein X-ray observatory.

Crab Nebula.


Black holes

Black holes are made from warped space and warped time – nothing else, no matter! The singularity is the point where the surface reaches a point and becomes infinitely warped and where tidal gravitational forces are infinitely strong. Matter gets stretched and squeezed out of existence. Nothing can escape a black hole, not even light.

Black holes can spin, it drags space around it into a vortex type whirling motion. We have not observed a black hole directly but we know they exist thanks to Einstein’s relativistic laws. Properties of black holes have been deduced from Einstein’s equations by many physicists, such as by Stephen Hawking.

Astronomers are certain of a black hole at the centre of the Milky Way galaxy, called Sagittarius A*. Estimates put the diameter at 44 million km and 4.31 million solar masses, it truly is a supermassive black hole! It has not been observed but its influence on nearby objects has been and the logical conclusion is a black hole is exerting the influence.

A massive black hole probably inhabits the centre of nearly every big galaxy. The heaviest yet measured is 17 billion times more massive than the sun.

Inside our galaxy there are roughly 100 million smaller black holes. They are between three and thirty times the mass of the sun. Fortunately there are none in our solar system otherwise it would cause chaos with gravity on Earth.  We would be thrown close to the sun and we would last not much longer than one year. The nearest black hole to Earth is estimated to be about 300 light years away.

This was a very interesting week of astronomy! Two more weeks to go of this course. More soon.

Astronomical Concepts – Week 5

The main topic this week was the Sun.

Here are a few facts about the sun:

  • It is a hot ball of glowing gas
  • It is a yellow dwarf, main sequence star
  • Approx. 150 million km from Earth (this distance is known as 1 astronomical unit (AU))
  • Formed 4.5 billion years ago
  • Formed from a giant cloud of spinning and collapsing gas
  • Light from the sun takes 8 minutes and 20 seconds to reach our eyes
  • Sunlight takes 170,000 years to get from the core to the sun’s surface


  • 91% Hydrogen
  • 8.9% Helium
  • 0.1% other elements such as oxygen, carbon, nitrogen, silicon, magnesium, neon, iron

The theory of how our sun and planets formed is called the Solar Nebula theory. Our solar system formed from the gravitational collapse of a large cloud of gas, 98% hydrogen and helium. As it collapses it spins and the centre becomes hot, where the protosun is located. It is colder on the outside of the spinning disc. As the cloud continues to collapse conservation of energy, momentum and angular momentum flatten it out. Further in towards the centre the resulting planets are warmer and further away the planets are colder.

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The image below shows the structure of the sun,


The temperature at the core is around 15 million celsius. The surface of the sun is around 5,500 celsius. In its core the sun is burning hydrogen and helium via nuclear fusion, this is what stars do and it is why they shine. The sun contains about 99.9% mass of the entire solar system and utterly dominates the gravity of the orbiting planets.


The sun also emits a solar wind, charged particles flowing outwards from the sun that causes space weather and on Earth causes the northern lights.

Our sun has been around for over 4.5 billion years but will not live forever. The graphic below indicates the life cycle of our sun and in about 5 billion years time something rather dramatic will happen!


So, in around 5 billion years time the sun will effectively run out of gas. The sun will begin to puff up in size and quite a lot bigger, around 30 times great in size, the Earth will literally be inside the sun. It will become a red giant. A red giant is red because its exterior has cooled from 9,000 to 3,000 Fahrenheit. This red giant stage will last for another 2 billion years. Eventually the sun will start to contract and become a bit larger than its original radius but give off 10 times as much energy than at present. This phase will last only 500 million years. Our sun will become a white dwarf and then a black dwarf.

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The left hand side is for stars like the sun, the right hand side is for stars that are much bigger than our sun.

We were lucky enough to be able to observe the sun from one of the observatory’s telescopes and through a h-alpha filter. It looked something like this,


This means to block out most types of light and view just a very narrow bandwidth focused on the hydrogen alpha spectral line. It means it is safe to observe the sun. The light occurs when a hydrogen electron falls from the third to the second lowest energy level. It is useful for observing prominences.

Did you know… NASA has a spacecraft orbiting the sun called the Solar Heliospheric Observatory (SOHO). The objective of the mission is to observe all aspects of the sun. It was launched in 1995 and is still going strong now. You can view the latest images of the sun on its webpage here. The image below is the sun taken with extreme ultraviolet imaging telescope (EIT 195) – 1.5 million kelvin.


Anyway, there is so much information online about the sun and it is so interesting. Keep reading and learning!

Next week …. more about stars!

Astronomical Concepts – Week 4

This week’s main topic was light.

James Maxwell

James Maxwell was a Scottish scientist who lived from 1831 to 1879. He is well known for the research he did on electromagnetism and light, building on the work of Michael Faraday. He produced a set of equations that explain the properties of magnetic and electric fields which helped to show that light was an electromagnetic wave. He was able to bring together well established laws of electricity and magnetism and with Faraday’s law they could imply that any disturbance in the electric and magnetic fields will travel out together in space at the speed of light.

He also described Saturn’s rings as numerous small particles, and this theory was proved later on in the 20th century by a space probe.

Einstein’s Photoelectric experiment

Einstein’s experiment showed that packets of light, called photons, contained a fixed amount of energy that depends on the light’s frequency. When a metal plate is exposed to light electrons are expelled. This is the photoelectric effect. The effect was discovered by German scientist Heinrich Hertz in 1887. His observations showed there was an interaction between light and matter. But Albert Einstein was needed to explain the theory further in 1905, his theory of light. Einstein said that light is a particle, called a photon. Einstein was awarded the Nobel Prize in physics in 1921 for his experiment.


Spectroscopy is a technique used to measure the light that is emitted or absorbed or scattered by materials. It breaks light into its component parts and this information can be used to identify and quantify those materials.


When light is absorbed or reflected by materials not all light behaves the same way. Only certain wavelengths of light are absorbed other get reflected. When you seperate the light that is passing through a sample you end up with an emission spectrum or absorption line.

An emission spectrum in the visible light range may look like this.


A spectrum like this would be created when material is given extra energy and that extra energy is later emitted as light energy.

An absorption spectrum would look like this.


A spectrum like this is created when light is passed through a gas or liquid or strikes a solid. Certain wavelengths of light will be absorbed by the material and later emitted in random directions. Most wavelengths will pass through the material without being absorbed. sun_spectrum

The image above shows a spectrum of our sun. From this spectra astronomers can tell what elements the sun is made from, for example hydrogen and helium. It is like a rainbow with holes, the holes are coming from the absorption of energy at a particular wavelength, at a particular colour, by the atoms in the cloud. This goes back to the energy levels of the atom, of only taking energy at very particular energies, as electrons move from one excited state to another excited state. So what you’re seeing is the absorption of photons by atoms. When energy is absorbed you are seeing the energy raising the energy level of an electron. So, we see the rainbow because the inside of the sun is hot and it emits a continuous thermal spectrum. The atoms in the outer layer of the sun absorb some of the energy and use it to promote electrons from a low energy level to a high energy level.


Astronomers also take pictures of light, usually through a filter. Astronomers want to see what the light looks like in red light or green light.

Astronomers also do timing with light, which means to measure the brightness or phase changes as things happen in time.

The combination of spectroscopy, imaging and timing can tell us all kinds of information from the thing we are looking at. We can tell hoe fast something is rotating, if it is moving towards or away from us, temperature, density and composition. Light is our spaceship, we can’t travel to stars and planets light years away but light does travel to us, it is the only way to get the information we need.

Doppler effect

Light waves from a moving source experience either a red shift or a blue shift in the lights frequency. A light source moving away from a stationary observer causes a shift towards the red end of the light spectrum, called a red shift. When the light source moves towards an observer the frequency shifts towards the blue end of the spectrum.


Why is the sky blue?

The sky is blue because atoms in our atmosphere scatter blue light more than they scatter red light. When we look towards the sun at sunset we see red and orange because the blue light has been scattered away from the line of sight. Blue is scattered more because it travels as small and short waves.

That’s enough about light, next week is all about the sun.

Astronomical Concepts – Week 3

The main theme of this week’s session was the outer part of our solar system. This includes:

  • Jupiter
  • Saturn
  • Uranus
  • Neptune
  • Pluto
  • Asteroids
  • Comets
  • Oort Cloud
  • Kuiper Belt

Oort Cloud

This has never been directly observed but it is believed to exist and it is an area of space on the edge of our solar system between 5,000 to 100,000 AU in distance, so over a vast area. The Oort Cloud consists of millions, perhaps billions of small icy bodies. Every now and then something might disturb one of these bodies and it will become a comet falling towards the Sun. It is named after Dutch astronomer Jan Oort who predicted its existence in 1950.


Kuiper Belt

The Kuiper Belt is another far away region of space that consists of rocky and icy bodies. It extends far beyond Neptune about 30 to 55 AU. It is also predicted to contain over a trillion comets. It takes comets about 200 years to orbit the sun and they travel in a similar plane to the planets. One of the largest and well known objects of the Kuiper Belt is the dwarf planet Pluto. In 2015 the New Horizon’s spacecraft flew past Pluto making it the first mission to a KBO. Another dwarf planet, named Eric, was found in 2005. It is slightly bigger than Pluto and has its own moon. At the time astronomers were considering making Elis the tenth planet, however, in 2006 the International Astronomical Union created a new class of planet called dwarf planets and Pluto and Elis were classified in this new category. The Belt was named after Gerard Kuiper in 1951.

Orbital resonance

A concept described by Paul was orbital resonance. An example to help describe this concept is playing on a push swing. A child can swing by itself at a natural frequency, but the frequency can change by use of an external force, someone else pushing the child on the swing. If the pushes are timed correctly the pushes will build up and the swing gets amplified. With planets and moons when two bodies orbit they exert a regular gravitational influence on each other. This is due to their orbital periods being related by a ratio of two small integers. An example is the 1:2:4 resonance of Jupiter’s moons Ganymede, Europa and Io.


These moons are all in resonance with Jupiter. Io completes exactly 4 orbits and Europa 2 in the same time it takes Ganymede to complete one orbit around Jupiter. During their orbits they sometimes lineup exactly and a gravitational tug is exerted with stretches their orbits into ellipses.

Another example of resonance is the 2:3 resonance of Pluto with Neptune. Pluto completes 2 orbits for every 3 orbits of Neptune around the sun.


Next week … light!

Astronomical Concepts – Week 2

The main topics this week were the solar system, gravity and the tidal effect. I have previously written on my blog about the solar system so for this entry I will just write about gravity and the tidal effect.

The two main theories of gravity come from Isaac Newton and Albert Einstein, both are used today, both are brilliant and vastly different. Gravity is one of the 4 main forces of nature, it works on grand scales, the great sculpture of the universe. Our Milky Way galaxy is locked in a gravitational embrace with Andromeda and in a few billion years the two galaxies will collide, just one example of the power of gravity. It holds galaxies together over billions of kilometres.

Gravity is the weakest of the four forces, yet it is so influential. The four fundamental forces of nature are gravity, weak, strong and electromagnetic. Well gravity is by far the weakest, certainly it is very weak here on Earth, but out there in the universe it is quite different. Stand on a planet more massive than ours and you would quickly notice the immense power of gravity. Stand on a neutron star and you would be ripped apart very quickly.


Newton realised that when objects fall to the Earth their must be a force acting on the object, reaching up and pulling it down. He stated that the force of gravity is always attractive, and affects everything with mass. Newton was also able to show that objects with different masses fall at the same rate because an object’s acceleration due to the force of gravity depends only on the mass of the object pulling it, such as a planet.

Newton’s cannon was a thought experiment that demonstrated his theory further. He imagined firing a cannon ball from the top of a mountain. Without the force of gravity acting on the cannon ball it would simply travel in a straight line. If gravity is present then the cannon ball’s path will depend on its speed. If it is slow moving it will fall down to the surface, if it is travelling fast enough it will go into orbit around the planet and if it reaches the escape velocity it will leave the orbit all togehter.


Einstein has a different approach. Einstein says that gravity is not a force but rather a property of space-time geometry. Objects in space, such as planets around a star are all attempting to travel in a straight line through space but that the curvature of the fabric of space means objects are constantly falling towards the mass exerting gravity. Einstein says when you are falling around an object you have cancelled out gravity. Astronauts on the International Space Station are weightless because they are continuously falling to Earth. There is gravity where they are, they are travelling at a speed to stay in orbit around the Earth. The astronauts are continually falling to the Earth but they never reach it, that is why they’re weightless. Being weightless means you are in free fall. When you are in free fall you cancel out gravity. Einstein’s elevator thought experiment explains his theory in more detail, read about it here.


Tidal forces are significant across our solar system. Here on Earth we experience tidal effects thanks to the moon. The Earth experiences two high tides, one on the side of the Earth closest to the moon as the moon pulls the water towards it and on the opposite side as the moon pulls the Earth away from it.

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An extreme case of tidal forces in the solar system is the heating of the moon Io around Jupiter. Jupiter is very massive so the effects on Io are huge,  Jupiter pulls Io inwards and the other moons away from Io pull it the other way, causing Io to distort in shape. This constant change results in lots of friction which in turn drives strong volcanic activity on the surface of Io. Io is the most volcanically active body in the solar system and its surface is constantly changing with large dark spots on the surface caused by collapsed volcanoes.

Our moon is also tidally locked, meaning we see the same side of the moon all the time. It spins once on its axis as long as it takes it to orbit the Earth once, so we always see the same face. The constant tugging from the Earth on the moon has caused this locking to happen.

So what is gravity?


Can’t wait for week 3 – the outer planets.