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.
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 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.
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.
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.
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.
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.