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 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.