pp chain reaction in stars

Twinkle, twinkle little star,
How I wonder what you are.
Giant thermonuclear reaction;
Held by gravitational attraction.
Twinkle, twinkle little star,
You look so small ’cause you’re so far.

As you burn through constant fusion,
Your twinkle’s just an optical illusion.
That happens when your light gets near;
distorted by our atmosphere.
Twinkle, twinkle little star,
spreading light and heat so far.

As you use up fuel you’ll grow,
and give off a scarlet glow;
Maybe you’ll go supernova,
exploding elements all over.
Now I know just what you are;
and I know I’m made of stars.

Where does the Sun get its energy? A straightforward question but physicists struggled to find an answer until the 1920s, when Eddington suggested that nuclear fusion might be responsible.

A star is drawing on some vast reservoir of energy by means unknown to us. This reservoir can scarcely be other than the subatomic energy which, it is known exists abundantly in all matter; we sometimes dream that man will one day learn how to release it and use it for his service. The store is well nigh inexhaustible, if only it could be tapped. There is sufficient in the Sun to maintain its output of heat for 15 billion years. — Sir Arthur Stanley Eddington

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AH: the Hertzsprung Russell diagram

Our Sun is a typical yellow star, so its emission would be represented by the middle star in this image.

star_colors
image courtesy of kstars, kde.org – colour is exaggerated

The colour of a star also tells us something about the expected behaviour of a star, it’s lifetime, and destiny. This is achieved by plotting stars on a Hertzsprung-Russell diagram. More about HR diagrams here.

This video clip looks at how the stars are arranged on the HR diagram.

Hertzsprung Russell diagram from mr mackenzie on Vimeo.

While some HR diagrams use temperature along the x-axis, others use star classification.

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Hubble discovers our universe is expanding

edwin_hubble_with_pipe

In the 1920s, Edwin Hubble had access to the Hooker telescope on Mount Wilson, Los Angeles.  This was the largest telescope in the world at that time.  His first breakthrough was the discovery of a cepheid variable star in the Andromeda nebula.  This enabled him to calculate the distance to Andromeda and he quickly realised this was not a nebula but a galaxy outside the Milky Way.
This video follows his work.

Hubble – nebulae or galaxies? from mr mackenzie on Vimeo.

Hubble then turned his attention to other galaxies, looking for cepheid variable stars that would allow him to determine their distances from the Milky Way.  He used redshift to calculate their recession velocity and plotted a graph against distance.

hubble_plot

He found that the recession velocity (v) was directly proportional to distance (d).  We can express this relationship as

v~=~H_o~d

where H_o is the Hubble constant.  Astronomers agree that the current value of the constant is

H_o~=~72 kms^-1Mpc^-1.

Since this is a  SQA course, we need to convert into SI units – giving

H_o~=~2.3 * 10^-18~s^-1

In this video, Professor Jim Al-Khalili looks at Hubble’s work on the expanding universe.

Hubble’s discovery of the expanding universe from mr mackenzie on Vimeo.

Although he was American, Edwin Hubble transformed himself into a tea drinking, pipe smoking, tweed wearing Englishman during his time as a Rhodes Scholar at Oxford.  He probably wouldn’t approve of this last video.

Unfortunately, astronomers were not eligible for the Nobel Prize for Physics.  The rules have now been changed.

 

redshift

more redshift

 

and Yoker Uni’s video about Doppler and stuff

 

While redshift can be used to tell us about the recession velocity of (non relativistic) galaxies, we also need to find a way to measure the distance to these galaxies.  Astronomers have two main methods to measure these distances; parallax (more parallax here) and cepheid variable stars – there’s a Khan Academy video on cepheid variable stars.

using redshift to map the expanding universe from mr mackenzie on Vimeo.

special relativity

Special relativity is tricky get get your head round.  Let’s start with a video about the speed of light.

This video follows Einstein’s thought process as he worked through his special theory of relativity.

special relativity from mr mackenzie on Vimeo.

time dilation

A Tale of Two Twins from Oliver Luo on Vimeo.

another take on special relativity and the twins paradox

 

…and the Glesga Physics version

 

length contraction

This video has helpful examples to explain length contraction.

Sometimes it’s easier to imagine we’re a stationary observer watching a fast moving object go whizzing past.  For other situations, it’s better to put yourself into the same frame of reference as the moving object, so that everything else appears to be moving quickly, while you sit still.  The muon example in this video shows how an alternative perspective can work to our advantage in Special Relativity.

Another way to think about this alternative frame of reference is that it’s hard to measure distances when you yourself are moving really quickly.  Think about it, you’d get tangled up in your measuring tape like an Andrex puppy.

Screen Shot 2016-02-09 at 23.44.47

image: trotonline.co.uk

It would be far easier to imagine you’re the one sitting still and all the objects are moving relative to your position, as if your train is stationary and it’s everything outside that’s moving.  That keeps everything nice and tidy – including your measuring tape.  Got to love Einstein’s postulates of special relativity.

Screen Shot 2016-02-09 at 23.47.59

image: mirror.co.uk