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 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.