If we were placed randomly somewhere in the universe, the chances are we would soon perish. Without the extremely specialized environment of Earth to sustain us every organ within our body would soon die in a matter of seconds. Living at the bottom of the Earth's atmosphere is the environment within which our bodily organs, and sensors, function normally. If we were placed randomly in the universe it would most likely be a void extremely distant from any stars, so distant that your eyes would be incapable of sensing any individual stars. If we imagine that we could float in this space, and experience the environment, we would be chilled to -270oC. There would be no air to breath, we would be floating in a near vacuum. We would be bombarded by only one proton every cubic centimeter (cc ) and also some more exotic particles particles like neutrinos, with 330 every cc.
At our location on Earth there are 3x1017 photons during daylight. Plenty for our eyes to sense the environment around us. Less noticeable would be the 65 billion neutrinos penetrating every cc per second, and 2x109 protons every cc. In physics all these things are described as particles. Discrete very small things. Protons and photons we can sense, neutrinos we cannot. It is this type of distinction that defines a large part of this course. There are aspects of the universe that are easy to measure and easy to understand. However there are even more aspects of the universe that are easy to measure and difficult to understand, and even more aspects of the universe the are difficult to measure and difficult to understand.
We live in a rich part of the universe. We can look up at night and see thousands of stars. We can sense them because stars are very good at producing light. There is a fundamental relationship between light and matter. Matter in stars can make light, and matter in our eyes can absorb light. Light is what communicates the nature of the universe to us. A large part of astronomy, and this course, is understanding that communication.
light and matter
The fundamental relationship between light and matter is described in classical physics by a field that protons and electrons generate. This is called the electric field and the reason we know it is there is because any two of these particles will influence each other at a distance. Two protons will repel each other, just like two electrons will repel. While an electron and a proton will attract. The electron and proton are said to have a charge, and it is this charge that generates the field. The charge on the electron is opposite to the charge on the protons, and so generates an opposite field. Besides generating the field the charge can feel the field. Like charges repel, opposite charges attract. It is important to remember that these terms we use; charge and electric field, are terms to help provide a description of what is observed in nature. Just because we have terms with a strict definition is does not necessarily mean we know what they are or how they are generated.
Throughout the nineteen century electric fields were studied and described mathematically with great precision. The electric field was found to be phenomenally strong compared to the other known field, called gravity. Gravity is generated by anything with mass. A very successful theoretical description of gravity was developed by Isaac Newton in the seventeen century. In this description anything that has mass generates a gravitational field and attracts anything else with mass. Matter is composed of electrons and protons and so electrons and protons generate both an electric and gravitational field. Let us try and imagine the different strengths of these two fields by imagining a completely empty universe except for a single electron and a proton separated by 1 metre. The electron and proton will attract each other with an electrical and gravitational attraction. If we could turn off the gravitational attraction and let the two particles attract each other utilizing simply their electric fields it would take about 0.07 seconds for the two particles to pull each other together. Now consider the same scenario with the electric fields turned off and the gravitation fields of the particles turned on. The particles would once again attract each other and come together under a gravitational pull. It would take over 7 times the current age of the universe for the particles to eventually meet. Equivalent to 105 billion years. The electrical attraction in matter is overwhelmingly more influential than the gravitational attraction.
Ironically, we generally do not notice electric fields because of their extraordinary strength. Both positive and negative charges work together to neutralize fields. A proton has a positive field that can be neutralized by attracting an electron. Once the electron is near the proton the negative field of the electron cancels the positive field of the proton. The two particle together behave like a neutral particle, a particle without an electric field. This is of course the nature of atoms. Atoms want to be neutral. Possessing equal number of protons and electrons. If an electron is lost them the strong net positive field reaches out to attract any nearby naked electron. As we saw earlier, even up to a metre away the electron will combine with the charged atom in less than a tenth of a second to neutralize the field. All matter tends to be neutral so we are neither attracted nor repelled electrically to matter that surrounds us. This neutralization of the electric field is so efficient that in our lives the weak gravitational field tends to dominate.
The simplest of all the atoms is the Hydrogen atom. A proton combined with an electron. The proton is about a 1000 times as massive as the electron. In terms of size, the proton is about 1.5 femtometres across, or 666 million would fit side by side across a millimetre. The electron is so small it is considered to be a point in physics. It is known to be at least 1000 times smaller than the proton.
The Hydrogen atom is about 0.1 nanometres across ( 1x10-10 m), so about 66 000 protons could fit across a hydrogen atom. An atom is mainly empty space. You are mainly empty space. It is the extremely strong electric fields that make you solid. The electrons and protons are held together so tightly that they are relucent to budge, creating the properties of a solid.
Hydrogen dominates when considering only the normal matter in the universe. By number, 94% of atoms are Hydrogen, 5.9% are Helium. The remaining 0.1% accounts for all the other elements. Oxygen coming in at 0.06%. You can explain much about the universe by considering only these three elements. However this explanation would not include the Earth or us.
In the nineteen century electric fields were known to be related to magnetic fields. By 1870 James Clarke Maxwell had described the relationship between these two fields with some wonderful equations. His equations revealed that a wave could be made within the fields that would propagate at the speed of light, 299 791.458 kilometers per second. Maxwell was certain he had a description for light.
Oscillating a charge generates an electromagnetic wave. Oscillate it at 4.4x1014 times a second ( Hertz ) and red light will be produced. Traveling at the speed of light the distance between the crests is about 680 nanometres ( nm ). This dimension is called the wavelength. The wave itself is not red but we sense that radiation as red. Oscillate it at 6.6x1014 Hertz, produces a wavelength of 450 nm, and we will sense it as violet. All the colours we sense lie within these extremes. The wavelengths are about 1/2000 of a millimetre in length. This is the radiation that the Sun is good at producing. The reason the Sun is good at producing light is because the Sun is very hot. At 5700oCit glows in visible light, producing more yellow than any other colour. The charged particles in the Sun are not oscillating but they are continually being accelerated through collisions and interactions with electric fields. The severity of these interactions is directly related to the temperature. Any charge that is accelerated will generate electromagnetic radiation. An oscillating charge has a very special, rhythmic acceleration.
This description of light was superceded by Einstein, in 1905, and brought in the era of modern physics. The more classical description developed by Maxwell is still used widely, particularly in engineering. Einstein's description is the photon description of light, where light is treated as a stream of particles, called photons. In the photon description of light each photon carries with it energy. The amount of energy is directly proportional to the frequency of the photon. The energy of a red photon is 2.9x10-19 Joules. One Joule is the amount of energy to lift 1kg vertically 10cm.
For practical reasons both the classical and modern descriptions of light are used in astronomy. This applies to all fields of classical and modern physics. Generally whichever theory works and is convenient to apply is used . Throughout this course both will be introduced and applied.
The Sun produces a broad range of photon energies, with the peak number occurring in the colour yellow. The temperature of a star can be measured by its colour. As the temperature goes up the frequency of the photons produced goes up. Blue stars are the hottest at about 20 000oC while red stars are the coolest at 3 000oC. The number of photons produced is also very sensitive to temperature. The number of photons going up sharply with temperature. Double the temperature and the number of photons produced goes up 16 times.
There is an absolute zero temperature, -273oC. This temperature is also the zero point for a temperature scale called Kelvin. 0o Kelvin is -273oC. Kelvin is the same as Centigrade with the zero point shifted. At absolute zero no electromagnetic radiation is produced. In between 0oK and the temperature of a red star, approximately 3000
At 300oK matter will tend to radiate in the infrared. Infrared is the same as light, just less energetic. At 300oK the radiation is ten times less energetic than red light. The photons have ten times less frequency and ten times longer wavelength. So just like a star, matter at 300oK glows. Your body temperature is approximately 300oK and so you are glowing in the infrared. Just like measuring the temperature of a star, the infrared you produce is used to measure your temperature. Most thermometers are infrared detectors that measure the intensity of infrared your body radiates. You tend to stick the thermometer in your ear to shield the detector from external infrared.
Observing the universe in infrared is useful because not everything is hot enough to produce light, like newly forming stars. Also infrared, with its longer wavelength, has penetrating powers. It enables observation of objects embedded in, and on the other side of, nebulae. In the images below you can compare a visible and infrared image of the Eagle nebula. Note that more starry objects can be seen in the infrared image. Of course the colours in the infrared image are false.
The usefulness of infrared in astronomy is well illustrated by the fact that the next large telescope in space will observe the universe in infrared. It is the James Webb Telescope supporting an 6.5 metre segmented mirror, to be launched in 2021.
In 1964 a microwave antenna was turned to the skies for the first time. The results were not unexpected by some. The universe glows in microwaves. From every direction there is a uniform microwave signal. The conclusion, the universe has a temperature and radiates in the electromagnetic spectrum corresponding to microwaves. The cosmic background microwave radiation peaks at a wavelength of 2mm. The peak wavelength corresponded to a temperature of about 3oK. This provided compelling evidence that the universe has been expanding from a hot Big Bang.
The electromagnetic spectrum extends to wavelengths of centimetres to metres in length. These radio wavelengths do not give us direct thermal information but can be used to tune into very low energy emissions of atoms and molecules. For example in 1961 the new built Parkes telescope turned its 64 metre dish to the skies to tune into the low energy, 21 cm, emissions of Hydrogen. It gave astronomer's a view of the invisible.
The energy required to make a radio photon is about ten thousand time less than light. So radio telescopes can pick up sources of low energy in the universe. As a result a radio image of an object can be very different to is optical counterpart.
Energies a little greater than that of visible light produce ultraviolet. The energies of these photons are enough to cause you damage, and too much exposure results in burns.
The sun obviously produces ultraviolet photons. Telescopes have been built to image the Sun in ultraviolet. Below is an image of the Sun recording only ultraviolet with a wavelength of 171nm. These photons have an energy more than twice that of the most energetic visible light.
Note the difference in the appearance of this image and the visible image above. Besides the colour, which is false in this image, the Sun appears more motley. Because is takes more energy to make ultraviolet it is only the more energetic parts of the Sun that can produce it.
At about a thousand times more energetic than visible light are x-rays.
It is common knowledge that you should not exposure yourself to too many xray photons. However the universe produces them in the more energetic regions. Like the region in the image below depicting a super dense neutron star.
the complete spectrum
Observing objects over multiple regions of the electromagnetic spectrum reveals many different environments within that object. From warm regions that can produce infrared, like newly forming stars, too high energy x-ray regions where gas is super-heated.
Much of what we see in the universe is in the form of gas. We live in a gas. We often describe the state of a gas by its temperature and pressure. The temperature is a measure of the average energy in the motion of each particle in the gas. This is called the kinetic energy of the particle. The pressure is a measure of the total kinetic energy per unit volume.
In the illustration below there are two gas chambers. The gas in both chambers is the same, however there are fewer particles in the right chamber. The chambers are separated by a barrier that can slide. Initially the barrier is locked. Thermometers at either end measure the temperature of the gas. The temperature is initially the same so the average kinetic energy of each particle in the two chambers is the same. Because there are more particles in the left chamber, the energy density is greater and so the pressure is greater.
The greater pressure is reflected in the the gas in the left chamber pushing harder on the barrier than on the right. If the barrier is set free to move then it will be pushed to the right. In so doing it will take energy from the left side and transfer it to the right. The temperature in the left chamber will go down and the temperature on the right go up. The barrier will eventually come to rest when the pressure on the left is the same as on the right.
These concepts are useful when considering how a star forms and maintains itself in equilibrium for millions or billions of years.
Hydrogen gas in the atmosphere of a star is mainly ionized, the electron stripped from the proton. The charges collide creating light. The energy of the light is dependent on the kinetic energy of the particles which is directly related to temperature. As illustrated earlier a star at 3000oK has enough energy to radiate mainly red light.
For a star of greater surface temperature the kinetic energy of the gas is greater, and the ability to make higher energy light is possible. A star with a surface temperature of 20 000oK will radiate with a blue colour.
non thermal colours
Not all colours are a direct result of temperature. The nebula below has multiple, discrete colours. The gas has been excited to emit characteristic colours associated with the chemical makeup of the nebula. The quantum physics lecture will explain these colours. By analysing the precise colours that are present astronomers can identify the elements that are in the nebula. This is an extremely powerful tool to measure the composition and environment present in a nebula.
Fortunately the universe is extremely transparent. Light can transverse the universe for billions of years to eventually be received by our telescopes. By analysing the composition and temperature of the millions of stars, like in the globular cluster depicted below, astronomers can deduce that the stars residing in them have been shining since the early universe.
On a larger scale stars and nebulae cluster into galaxies. Galaxies often take on a spiral shape and hold hundreds of billions of stars. When observing galaxies the light we receive has been traveling for millions or even billions of years.
Some of the more spectacular images from the Hubble space telescope are the Deep Field series of images. Every smudge on the image is a galaxy in the early universe. The light from the more distant specks has been traveling for almost thirteen billion years. Almost four three times the ago the Earth.
We now know that although the universe is rich in light associated with normal matter, made up of charges, there is an enormous amount of something in the universe that does not produce light. As a result it is very difficult to deduce what it is. Astronomy is in a state of flux. Revealing the nature of the dark matter may lead to a whole new physics.