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,

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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.

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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,

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

Pulsars

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,

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The images are from the Einstein X-ray observatory.

Crab Nebula.

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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

Composition:

  • 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,

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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.

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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!

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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,

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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.

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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

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.

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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.

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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.

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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.

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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.

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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 1 Introduction

Thursday 13th October was the first week of my new astronomy course at Sydney Observatory. Like the first course I attended earlier this year it is presented by Dr Paul Payne. This course aims to expand on the first and build on some of the main concepts of astronomy, including the solar system, gravity, the theory of relativity, the Sun, stars and quantum theory.

This first week was an introduction to the course and focused on some important concepts: light, gas, nebula, the speed of light, atoms (particularly hydrogen), electrical fields, the electromagnetic spectrum and more. It certainly was a packed 2 and a half hours!

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Before we left we had time to observe the night sky through the observatory’s over 140 year old telescope, the oldest in Australia, to see great views of the surface of the Moon and Saturn. The views looked similar to the images below:

This was a great introduction to the course. As usual we had our lecture in the basement theatre and were treated to Paul’s 3D graphics and animations to help illustrate the concepts further. I left with a deeper knowledge of light, in particular how influential the wavelength is. Paul showed us many images of nebulae, including the famous Crab nebula, seen below:

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This is probably the most familiar shot of the nebula showing the bright blue, green and orange colours. The image below shows the Crab nebula in a variety of wavelengths. By looking at the nebula in different wavelengths it tells astronomers different information about the star.

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X-Rays for example are collected by the NASA Chandra space telescope, amongst other devices. Many things in space emit x-rays, such as black holes, neutron stars, binary star systems, supernova remnants, stars, the Sun and even some comets. Because X-Rays are absorbed by Earth’s atmosphere the Chandra telescope must orbit above it to an altitude of 139,000km. X-Rays are produced in the universe when matter is heated to millions of degrees. These temperatures occur when high magnetic fields, or extreme gravity, or explosive forces, hold sway. Chandra can also trace hot gas from an exploding star or even a black hole. Chandra can help to define the hot , turbulent regions of space to help us understand the origin, evolution and density of the universe. The image of the Crab above in x-ray shows blue colour and in the centre a pulsar can be seen. The star was first discovered in 1942 by Rudolf Minkowski and then later in 1968 the star was found to be emitting its radiation in rapid pulses, becoming one of the first pulsars to be discovered.

This was a great start to the course, highly engaging and interesting as always and I can’t wait for next week!

Innovation Lecture 2016 – Marita Cheng

On Wednesday September 14th I attended the Warren Centre Innovation Lecture 2016. This year’s speaker was Marita Cheng, founder of RoboGals and the 2012 Young Australian of the Year.

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This was the second time I have seen Marita speak, the first time being earlier this year at the Future Schools Expo in Sydney. Marita is an inspirational and engaging speaker. Her style is very natural, laid back with lots of humour. The venue for the lecture was the ballroom at the Westin Hotel in the Sydney CBD, a fitting venue for this eagerly anticipated event.

Marita was born in Cairns, Queensland and graduated from high school in 2006 in the top 0.2% of the nation. She has a Bachelor of Engineering in Mechatronics and Computer Science from Melbourne. Marita’s qualifications are clearly very impressive. She also spoke of her time at Singularity University, a benefit corporation that provides educational programs, innovative partnerships and a startup accelerator. She recently attended a 10 week course at Singularity and learnt from some amazing people in the fields of technology, robotics, computer science and more. You can see a day in the life at Singularity University filmed by Marita here. It was here that she gained valuable experience in setting up her own company, and through networking with other people she got the idea for Aipoly, an amazing app that helps blind people see by using artificial intelligence to identify objects using the camera and then the app speaks the object to the user. The app can identify hundreds of objects and colours and is soon to be able to understand complex scenes. This is another example of her incredible vision and for making the world a better place.

Marita is probably best known for founding the RoboGals education initiative. RoboGals is now a huge success with chapters in many countries and helping thousands of young people to be engaged in engineering and robotics. But in the early days just getting volunteers, and her friends, to help out was tough. Her story shows that with grit, determination and hard work anything is possible. Read more about RoboGals here.

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Marita now runs 2Mar Robotics, an innovative company that is dedicated to providing robotics solutions for people with disabilities. It is an inspirational initiative and she spoke candidly of the processes involved from setting up a company and getting her ideas off the ground, and hopefully to market. Marita and her team have designed some amazing machines to help people that really need them. Her ideas are rooted in making the world a better place,

“Engineering is all around us, so it’s important that the engineers who create our world are as diverse as the people who live in it.”

Marita Cheng

Marita’s story is engaging and inspirational and she has certainly inspired me to continue to deliver an engaging and exciting STEM program at my school and aim to inspire the next generation of female engineering superstars to make the world a better place to live in. In the world of STEAM – E is for engineering!

Exploring the Heavens – Final Week

For the final class the topic was Telescopes. We covered the history of telescopes and the different types. We were meant to go outside and use some telescopes, but due to the heavy Sydney rain this was not possible tonight. The advantage being we got to stay inside out of the cold and were able to ask Dr Payne more questions and learn more about astronomy, including about Edwin Hubble and the Hubble telescope, below:

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Telescopes come in many different forms including optical, radio, microwave, infrared, ultraviolet and x-ray. Telescopes are all about extending the capability of the first astronomical detector: the naked eye. The light we capture from telescopes is vital for us to explore and understand the universe.

The telescope was developed in the 17th century and was a Flemish invention. One of the first people to turn the telescope towards the stars was Galileo and in his life he built many telescopes. He made many discoveries, including:

  • The features of the moon such as valleys and craters
  • Jupiter’s moons which meant not everything circled the Earth
  • Many stars
  • Phases of Venus
  • Sunspots

The Galilean moons of Jupiter are: Io, Europa, Ganymede and Callisto.

His most powerful telescope magnified by 33x, meaning everything he saw through it appeared 33x larger. The telescope also captured lots of light so objects through the telescope appeared brighter. He was able to see 10x as many stars than were visible to the naked eye. Brightness is the most important feature of a telescope, not the magnifying power but the brightness.

Galileo built a refractor style of telescope which used lenses, but the problem with this style is that objects appear fuzzy. This problem is called chromatic aberration. This problem was solved by Isaac Newton. Instead of a lens he used a curved mirror and this corrected the fuzziness caused by the lens. This style of telescope is called a Newtonian. All large contemporary telescopes have mirrors as the main optical component. Newton’s original mirror was 2.5 cm in diameter, today they reach 10 m in diameter.

Refractor telescope

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This type of telescope is better if it is longer, objects appear closer and with more detail. The function of the objective lens is to form an image close to the opposite end of the tube. The distance from the objective lens to the image is called the focal length. The longer the focal length the larger the image produced. The function of the eyepiece is to enable the eye of the observer to have a closer look at the image. It is like a small magnifying glass. The more powerful, the more the telescope will magnify. The magnification of a telescope is equal to the focal length of the objective divided by the focal length of the eyepiece. For amateurs 40 to 150 is standard.

Reflector telescope

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A curved mirror replaces the objective lens. The primary mirror produces an image at its focal length and an eyepiece is used to examine that image. The observer looks into the side of the tube, thanks to the small secondary mirror. Most amateurs use reflecting telescopes. The larger the primary mirror the more light the telescope captured and the brighter the image through the eyepiece. Double the size of a telescope and the light collecting power increases by 4.

The diameter of a telescope matters more than magnification. About 15 cm is a good size to start with. A Newtonian is preferred as you can buy a larger telescope for the same price as a small refractor style telescope. A Newtonian will reveal nebulae that will not appear in the refractor for the same price.

Another type of reflecting telescope is the cassegrain, and they give a superior image to the Newtonian. They have a longer focal length and a completely enclosed tube.

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A good mount for your telescope is vital. A good mount is needed to help focus the lens and keep the telescope still. The example below is a Newtonian telescope on an equatorial mount.

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The mount has 2 axes of rotation and can be fitted with an automatic tracking device.

What can you see?

We are limited by the type of telescope we have and the atmosphere of the Earth. Light pollution is also a problem for city and town dwellers. It is best to take your telescope into the country to view many stars, Jupiter and Saturn. Depending on the size of the telescope, 20cm will ensure you can see the great red spot of Jupiter and the separation of the rings of Saturn. A 20cm reflector will also let you see Uranus, Neptune, but to see Pluto a 40cm instrument at least is required. With a 10cm reflector bright nebulae will be visible, but as wispy blue clouds.

The biggest radio telescope is currently being built in China at 500m in diameter. The next largest is the Aricebo in Puerto Rico and is 305m across.

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A radio telescope is like a giant ear listening for radio waves from space. Radio waves are a type of electromagnetic radiation similar to light. These signals are very weak, so the larger the telescope the more chance you have of picking up signals.

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This telescope will search for ancient signals of hydrogen, one of the building blocks of the early universe. It will also hunt for new stars, and in particular pulsars, rapidly rotating stars.

The dish is made from 4,500 triangular panels that have been carefully lowered into place. Each panel can be adjusted so the telescope can be moved to view different parts of the sky.

Read more about this amazing telescope here: http://www.bbc.co.uk/news/resources/idt-0192822d-14f1-432b-bd25-92eab6466362

The universe began with the big bang and astronomers are building more advanced machines to look further and further back into the past to see what the conditions were like when this happened. One of the main questions to answer is “What is the mean density of the universe?” This will determine how much gravity there is and the eventual fate of the universe. Will the universe keep expanding or will it eventually collapse?

Astronomers are slaves to light and we know that we can only observe about 10% of light from the universe. Or, actually that the universe only gives us 10% of its light. We can’t see dare energy and arm matter. It is thought that 68% of the universe is made from dark energy and 27% is dark matter. We know something is there because of the gravitational influence it has over nearby objects. Everything we know about the universe makes up about 5% of it.

More about dark energy and dark matter in another blog.

This was the final class and the whole course was amazing! Huge thanks go to Dr Paul Payne for his amazing lessons, 3D presentations and humour. Huge thanks also to everyone at Sydney Observatory for putting the course on. I will be back for more astronomy later in the year.

Here is a link to sign up for the next instalment of this course: https://maas.museum/event/astronomy-course-exploring-the-heavens/

Here is a link to Paul’s website: http://relativity.net.au/courses/

Exploring the Heavens – Week 4

The main focus for this week was ‘The Characteristics of Stars’. For this lass I was back to my usual Tuesday class time, and I had been looking forward to this session for quite some time.

The session started outside at the observatory waiting for the ISS to fly across the Sydney sky at 30,000 km/h. I had never seen this before so I was pretty excited to see a spaceship fly through the sky. It happened at 6.20pm and lasted for a couple of minutes. Incredible to think there are people inside the ISS doing science experiments at 30,000 km/h all day everyday.

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Then inside for another amazing 3D presentation by Dr Paul Payne about stars. Starting with the Sun, which produces an enormous amount of light and heat for billions of years. Stars are huge, 100,000 to 400,000 million kilometres across. The Sun is turbulent and the surface, photosphere, can have dark spots and bright patches that can flare up to 1,000x brighter.

Stars vary greatly in temperature, size and brightness. Stars do not burn, as this would not produce enough energy to keep them alive very long, they are nuclear power reactors where hydrogen is fusing to form helium and this processes keeps them alive for billions of years.

The most important property of a star is its mass as it determines everything about it – size, light, length of life. When a star dies it crushes itself to a fraction of its original size leaving behind either a white dwarf, neutron star or a black hole.

The formation of stars

Stars are formed from nebulae – an interstellar cloud of gas (mainly hydrogen and helium) and dust. The contracting ball of gas throws out rings of material to make planets and what was left is the central star. As the central star compressed it gets hotter and hotter and gravity keeps on compressing it even further. With a core temperature of about 10 million degrees atoms of hydrogen in the core move with such immense speeds that they collide and fuse together – called nuclear fusion. This reaction produces an enormous amount of energy and is a million times more efficient at generating energy that during the equivalent amount of coal.

Our Sun

Our Sun is stable and is currently fusing hydrogen at its core, a process that lasts for 90% of its life. Energy from the core radiates through the dense central layers and slowly makes its way to the surface in the form of x-rays and gamma rays. The journey could take a million years to happen. At the surface gas is not as dense so to expel the energy it uses a convection process. Gas below the surface moves in packets carrying heat energy. At the surface each packet radiates energy out into space, and we on Earth see this energy in the form of visible light.

The appearance of the Sun looks cellular, however, each cell is approx. 1000 km across. Small bright patches are where gas is radiating and darker veins are cooler areas where energy has fallen back into the Sun. The temperature of energy varies from 15 million degrees at the core to 5700 degrees at the photosphere. This is why the Sun looks yellow, because more yellow light is produced at this temperature than any other colour.

A sunspot is a region on the surface where the gas is cooler than its surroundings, approx. 4000 degrees cooler. The spot generates less light so appears darker. Galileo correctly recognised them and was able to measure the period of rotation of the Sun. The Sun does not rotate as a solid ball, each latitude rotates at a slightly different rate, for example the equator takes 27 days to rotate once and at a latitude of 40 degrees it takes 29 days.

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Solar eruptions are more frequent as the amount of sunspots increases. These eruptions are also called flares, where a small area may increase in intensity and temperatures of a million degrees may be generated so particles are exploded and hurled from the Sun that may even reach Earth and cause auroras visible near the poles.

The Life of the Sun

The Sun will fuse hydrogen for about 9 billion years. The leftover helium ash will build up the core. The Sun is stable during this time. It will eventually start running out of fuel at the core and it will prepare to die. It will take another billion years or so and its energy output will vary, called a variable star, where its brightness may vary.

The Death of the Sun

Eventually the core will collapse causing heat that will drive nuclear reaction faster resulting in the Sun swelling. It will grow to over 100x its present size and be 1000x brighter. The surface will cool and it will become a Red Giant. Finally the ignition of helium in the core, fusing to form carbon, nitrogen and oxygen, generating enormous power. It will glow with a brilliant red, something that happens to all stars prior to dying.

At this stage the star will produce heavier elements than helium up to iron. Elements heavier than iron when fused will not produce energy but absorb it, making the star very unstable. A planetary nebula is then produced, shells of gas are released cocooning itself in a cloud of its own material. They appear as faint blue discs surrounding dying stars. When the star tries to fuse iron the reaction absorbs energy from the core and the interior behaves more like a refrigerator than an oven. The core collapses, gravity compresses the star the size of the Sun to the size of Earth. It is extremely dense, a sugar cube sized peeve of its material would weigh 5 tonnes. The star is dead, it does not produce energy and it slowly cools off, it is now a white dwarf.

The power of stars

Stars are rated like light bulbs. Some stars produce 100,000x more power than our Sun. To measure the power we have to measure the brightness and how far away it is. If we know any two of power, brightness and distance we can find the third quantity.

Stars are hot!

The surface temperature of a star can be measured by its colour. Cool stars are red at 3000 degrees. Hot stars are blue at 15,000 degrees on the surface. We can see the colours of stars at night with the naked eye. We notice blue stars more as they generate more power. Examples are Sirius, Rigel. They are blue stars and are 8 and 815 light years away respectively. Examples of red stars, which do not last as long in the night sky, are Betelgeuse, Antares and Gamma Crucis.

Composition of stars

The composition of a star can be measured from the light from it. Telescopes equipped with a spectrometer can break the light into its different component features, called an absorption spectrum. Dark lines throughout the spectrum, like a barcode, reveal the elements, such as hydrogen and helium. The spectrum can also reveal the velocity as the lines will be red-shifted, meaning the star is moving away from us. The greater the velocity the greater the shift. If the star is moving towards us it will be blue-shifted. This shift in spectral lines is called the Doppler Effect.

An emission spectrum is produced by a glowing cloud of gas, like a nebula. Dark lines are now bright and the bright, rainbow continuous spectrum background is gone. Bright lines coincide with light lines and can measure all the properties listed for an absorption spectrum.

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A supernova is a very violent event in the universe when a massive star explodes. Elements heavier than iron are produced in the explosion. Gravity is the driving force behind the collapse of the core. The remnant is a compressed star smaller than a white dwarf, even 20 km across. A sugar cube sized piece can weigh 5,000,000 tonnes. This is known as a neutron star. It emits pulses of light, like a light house. Two beams of light are generated from the magnetic poles and as it rotates the beams are swept out into space. An example is the Crab Nebula Pulsar which flashes at 30x per second, meaning it rotates 30x per second.

Black Holes

A star with 3x the mass of our Sun can collapse to form a black hole. This is when the star pulls itself out of existence due to the amount of gravity it has. Nothing can escape a black hole, not even light. The star is reduced to a singularity into which all matter is pulled to. An envelope of space around it is called the event horizon, this is the point of no return. A typical size would be 10km. They are small and black so very hard to detect, we can only hope to detect their gravitational influence on objects around them. A famous black hole is in the binary system called Cygnus X1.

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Binary Stars

Most stars come in pairs. What appears as one point of light in the sky is actually the accumulation of two stars close together. These stars are in orbit around each other, this is called a binary system.

Next week… Telescopes.