Introduction
This is a talk that I recently gave to my Probus Club.
On a warm evening in early 1987, I drove out of Melbourne into the countryside with my teenage daughter. We wanted to get away from the bright lights of the city so that we could see the stars clearly.
This was the sight that greeted us.
The majestic band of the Milky Way was the most prominent feature, but we were really there to those two hazy patches to its right.
They are the Clouds of Magellan. Although they were well known to people living in the Southern Hemisphere, Europeans were not aware of them until the late 1400s and early 1500s. They are named after the Portuguese explorer Ferdinand Magellan.
What we were specifically looking for is shown in the next picture. It was the first supernova that could be seen without a telescope for over 300 years. I will explain what a supernova is later. The supernova is the star at the end of the arrow. A month or so before we saw it that star was only visible is very powerful telescopes, and a few months after our country trip it was again invisible to the naked eye.
There were plenty of other astronomical objects to view including this one.
The Southern Cross, the icon of our hemisphere.
The Big Bang ...
The star I particularly pointed out to my daughter is the one at the bottom left. It is called Alpha Centauri. I commented that it is the closest star to us (other than the Sun) and that the light left it four years ago. She expressed surprise, and asked how we know that.
(Note for pedants. Proxima Centauri is actually the closest, but it cannot be seen without a telescope!)
I am not sure that my explanation was clear to her. I will try to make it clear now.
I gave the talk in our regular meeting room, which has a great view out onto Port Philip bay. I turned to the Bay and continued with my talk.
Imagine we can see a ship anchored out in the bay. We might ask ourselves how far the ship is away. Luckily we have a surveyor with us. He takes a sight on the bow of the ship measures an angle of 90 degrees away from the ship and starts measuring our a distance in that direction, call that distance the baseline. After a few hundred metres has been measured the surveyor takes another angular measurement.
The surveyor draws a diagram like the one below. Distance equals baseline multiplied by tan of the angle.
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He then writes ...
Distance = baseline X tan(angle). ... into his notebook, does a calculation and reports the distance to the boat. This is not difficult maths, as it taught in Secondary School maths classes. If you are interested in investigating this problem further here is a link. |
We can use this same method to measure the distance to stars. We need a much larger baseline though. You might think that the diameter of the planet, 12,756 kms, would be enough ...
... but it is too small.
The largest baseline available to us is the radius of the Earth’s orbit around the sun ...
... 300 million kms.
Take pictures of the sky six months apart. When you compare the photos most of the stars will be in exactly the same position – because they are sooo far away. But some close start will be in slightly different positions. We can measure the change in positions of these stars and make a triangle like the one above. So we can determine the distance to some of the stars in the sky.
Using this method the distance to the nearest star – Alpha Centauri – is 41,320,000,000,000 kms. Obviously a km is not a useful measuring unit at these distances. One unit of distance that astronomers use is the Light Year. That is the distance that light travels in one year, which is 9.461 trillion kms. Divide the distance to Alpha Centauri in kms by the number of kms in a light year and you get the distance to Alpha Centauri as 4.36 light years.
So, we can measure the distances to the closest stars, what about the others. Imagine you are near a road at night. You want to cross the road. There is a car coming. On the whole car headlights are a similar brightness. So using the brightness of the car’s headlights we can get an idea how far away it is and whether it is close to cross.
A similar method is used to determine the how far away very distant stars are. If we know a star's intrinsic brightness - how bright it actually is - we can work out its distance away from us by comparing its intrinsic brightness to its apparent brightness, as seen from Earth.
The pioneer in this work was a woman - Henrietta Leavitt. She worked in an era when women were not as valued as they are now, and was not acknowledged publicly for her great contribution. For more details on Leavitt follow this link . Leavitt studied variable stars called Cepheid Variables.
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The calculation at left shows how to determine the distance to a star if its intrinsic brightness (L) is known and its brightness as seen from Earth (F) is measured. Light leaves a star in all directions, which means that it expands in a spherical shell.
The surface area of a sphere is
where r is the radius of the sphere. The amount of light reaching us from a star is the star's intrinsic brightness (L) divided by the area of a spherical surface with a radius the distance that the star is away from us - the first equation at the left. Here is a link to a video that further explains the derivation of the equation. |
For more information on these methods of measuring astronimical distances click on this link .
Using Cepheid's and other astronomical objects as described in the link above we can determine the structure of our local group of stars, the Milky Way Galaxy, as shown in the graphic below. Note that our solar system is just over half way out from the centre.
Here is what out galaxy would look like if we could view it from outside.
There are many hazy smudges (like the Magellanic Clouds) in the night sky and before methods to measure distances to stars were developed there were two points of view about them. Some astronomers thought the fuzzy patches were part of our local group of stars (the Milky Way) others thought that they were external to our galaxy. Powerful new telescopes that were developed in the early 20th Century resolved stars in these fuzzy patches. Cepheid Variable stars were found in them and their distances determined. These fuzzy patches were found to be outside our galaxy and often an enormous distance away.
The distance to the Large Magellanic Cloud was determined at about 158,000 light years.
The nearest large galaxy to our own is in the constellation Andromeda, and is about 2.5 million light years away.
So we can calculate the distances to other galaxies.
The next question is how are they moving?
I will start with a familiar issue, determining the velocity of a moving sound source.
Sound is waves in the air. When a sound source is moving towards you the sound waves are compressed and the pitch of the sound is increased. When a sound source is moving away from you the sound waves are stretched and the sound has a lower pitch.
In the diagram below the police car siren sounds a higher pitch as it approaches. When it is level with the listener the sound pitch drops to the actual siren pitch and then lowers in pitch as it moves away.
So we know the speed of sound, the pitch of the source and the pitch as it approaches and recedes.
It is possible to calculate the speed of the sound source (in this example the police car) using that information. There is a formula for this. A formula for an approaching vehicle is:
Where V_s is the speed of the source, C is the speed of sound, f_o is the (stationary ) frequency of the source and f is the measured frequency.
There is also a (slightly different) formula for a receding source.
Light is also a wave and it is possible to measure the speed of objects using changes in their light frequency. Red light is long frequency and blue light is short frequency. Yellow, orange and green are in between frequencies, as can be seen in the diagram below.
When an object is moving towards you the light frequency will compress and the light will shift in the blue direction (blue shifted). If a light source is receding then the light waves will be stretch and the light will be shifted in the red direction (red shifted).
A similar method to that used to determine the speed of a sound source can also be used to measure the speed of a light source, as described in the video below.
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So it is possible to measure the distance to galaxies and the speed that the galaxies are moving.
The photograph shows Hubble Hubble observing at 100 inch Mt Wilson Telescope. |
You might expect that about half of the galaxies would be moving towards us and about half moving away from us. But this is not Edwin Hubble found when he studied many galaxies in the 1920s. In 1927 he announced that virtually all galaxies are moving away from us and the further away a galaxy is the faster it is moving away, as shown in his plot, below.
The universe is expanding!
Two explanations for this phenomenon were developed.
The Big Bang – the most obvious explanation of the expanding universe is that we are living in the aftermath of a massive explosion. This was one of the explanations developed. It was called the Big Bang by Fred Hoyle, one of the theory’s opponents.
The Steady State – that the universe has always been expanding and that new matter comes into existence in the gaps left by the galaxies as they move apart, and over time new galaxies form.
There was for some time no evidence to distinguish between these two explanations until two guys came along in the mid-1960s who were working on a completely different problem. Satellites were a new phenomenon and Penzias and Wilson built a horn antenna in an attempt to improve satellite communication. The photograph below whows Penzias (on the right) and Wilson (on the left) with their antenna in the background.
They had a problem though. There was a distracting hiss in their instrument. They thoroughly checked the instrumentation, but could not find the source of the hiss. The news got around of their problems and they received a phone call from Robert Dicke that their discovery was a firm prediction of the Big Bang theory. It was nothing less than the afterglow of the Big Bang. This observation did not fit comfortably into Steady State cosmology. Relatively quickly most astronomers moved into the Big Bang camp.
When did the Big Bang occur? There are a number of different methods of determining this. The easiest to understand is to measure the expansion rate and then work that backwards to determine how long ago all matter in the universe was in one place.
The currently accepted age of the universe is 3.8 billion years ie 3,800,000,000 years. The short video below explains how this age is calculated.
... Star Dust ...
What did the Big Bang produce. You might expect that all of the elements that currently exist came out of the Big Bang, but that is not true. The Big Bang produced only hydrogen, helium and a little bit of lithium.
Only 9.5% of the elements that we are made from came out of the Big Bang. Where did all of the other elements that we are made of come?
The answer is stars – we are literally stardust!
Our planet is warmed and lit by light coming from the sun. Where does that energy come from. The answer is surprising. In its core the sun is millions of degrees hot, and hydrogen is being converted to helium, a process that produces energy.
The graphic below shows one of the process by which hydrogen nuclei produce helium.
The sun is a medium sized star, but in larger stars many more elements are produced, as shown by the graphic below. For a video describing the details on how this process occurs click on this link .
Evenutally the core of the star rapidly fills up with iron. This is a crisis for the star, as synthesising more elements from iron does not release energy, it requires more energy. So energy production suddenly stops in the star. The star has existed by two forces balancing each other - gravity trying to crush it and the energy produced in the core trying to push it apart.
Without energy coming from the core gravity wins and the star collapses. This collapse produces a great deal of energy which then blows the star apart, spreading all of the elements that have been formed out into the universe. This explosion also provides energy for the production of elements heavier than iron.
Some of these elements ended up in vast cosmic clouds - that consisted mainly of hydrogen - like the one in the photo below.
Four and a half billion years ago a cloud like that collapsed to form the Solar System. (Note the photo below is not to scale.)
The third planet from the sun is very unusual, as life has developed on it. There is clear evidence for life 3.6 billion years ago, and it is likely that life started earlier than that.
Here is a link to a video describing the latest ideas on how life formed on Earth.
There is no time here to consider the processes of how life evolved and diversified over billions of years, though if you are interested in this click on this link for a brief 36 minute exposision. Instead we will jump forward to just 260,000 years ago when a new creature arose on the plains of Africa. There was intelligence behind those eyes that looked up at the stars and made stories about them. Or species eventually explored the whole of the planet and then looked around for a new exploration challenge. What about that large ball in the night sky!
... and the Pale Blue Dot
So we return to the 1960 and the Apollo Space program. The first crewed Apollo craft to orbit the moon was Apollo 8. The graphic below shows its flight path.
One of the major tasks of Apollo 8 was to survey possible landing sites for the later missions, but it is most remembered for an event that took place during its fourth orbit. One of the Astronauts, Bill Anders said, "Oh. Look at that picture over there. There's the Earth comming up. Wow, that's pretty. Anders took a series of photos, one of which became one of the most celebrated photos ever taken - Earthrise.
The taking of the photo was without doubt one of the most profound events in the history of human culture, for at this moment we truly saw ourselves from a distance for the first time; and the Earth in its surrounding dark emptiness not only seemed infinitely beautiful, it seemed infinitely fragile. This wonderful image crystallised and cemented the sense of the planet's vulnerability.
Click on this link for a video that describes Apollo 8 and the taking of the Earthrise photo.
Eight years after Neil Armstrongs "... small step ..." in 1977, Nasa launched two space craft Voyager 1 and Voyager 2, to explore the outer planets.
Click on this link for a video describing the history, achievements and future of the Voyager space probes.
They gave us photographs of unprecedented detail of:
the majestic Jupiter
Saturn, with its beautiful rings
Uranus
and Neptune
Voyager 1 only visited Jupiter and Saturn, but Voyager 2 visited all four outer planets. When Voyager 1 had completed its mission Carl Sagan, an astrophysicist, writer and consultant to NASA, suggested that the space craft be turned around and take a picture of all of the planets including Earth. The photo of earth, shown below, was called the Pale Blue Dot.
In the photograph, the arrow points the the Earth, which appears as a pale blue dot.
I could describe this photograph, but instead I will leave that task to a much better wordsmith than me, Carl Sagan. To hear Sagan's wonderful, and uplifting description listen to the video below.
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