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 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.