Our Sun is a typical yellow star, so its emission would be represented by the middle star in this image.
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.
Some of you asked for revision questions ahead of the unit assessment. Down load these problems from older past papers. I will add a secoond post with the answers shortly.
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 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.
He found that the recession velocity (v) was directly proportional to distance (d). We can express this relationship as
where is the Hubble constant. Astronomers agree that the current value of the constant is
.
Since this is a SQA course, we need to convert into SI units – giving
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.
We’ve been looking at how the temperature of an object can affect it’s appearance. The attached handout recaps on our work in class and provides some examples of line spectra.
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.
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.
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.
Before the holidays, we performed a set of experiments to determine the latent heat of fusion and latent heat of vaporisation of water. Next week, we’ll review that work and think about possible reasons why our results were not closer to the “book” values.
We’ve started looking at telescopes and this BBC programme from 2 years ago featured the replacement for Hubble, the James Webb Space Telescope. The presenter, Maggie Aderin Pocock now hosts The Sky at Night on BBC4.
Please take some time to watch both parts of the film. Maggie covers the basic idea of satellites, how we achieve geostationary orbit, looks at examples of Earth observation and the manufacture of the James Webb’s massive 6.5m mirror. In part 1, around 17 minutes in, you’ll see satellite footage over the Highlands, can’t quite see Thurso though.
You can also use these notes to prepare for the test on Monday. The test will cover everything up to page 46 in the attached file, but you can skip pages 39 & 40 on Newton’s third law.