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 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!