Measuring the Universe
- One astronomical unit is the distance from the earth to the sun – 1.5*1011m
- A light year is the distance travelled by light in a year. Worked out by multiplying the speed of light by the number of seconds in a year – 9.5*1015m
- For distances between starts and galaxies, parsecs are used.
Parsec
- This is measured using parallax – the angle through which stars appear to move when observed at different times during the year, due to earths orbit.
- Parallax angles are so small that they are measured in arc seconds – 1 arc second is 1/3600th of a degree.
- A parsec is defined as the distance to a star that gives a parallax angle of 1 arc second – 3.1*1016m.
Formation of stars
- Slightly denser areas of the universe have a slightly higher gravitational pull.
- This pulls in material, making it denser and giving it a greater gravitational pull, so it pulls in more material etc.
- This process is known as gravitational collapse.
- As it collapses it becomes hotter because it is losing potential energy and gaining kinetic energy (which is directly proportional to temperature).
This ball of gas and dust is known as a protostar. Eventually the core of the material becomes hot enough for hydrogen fusion to take place forming helium nuclei. This process is called hydrogen burning.
Once they are equal the star will stop collapsing and continue to burn for millions of years. It is now called a main sequence star.
The fusion creates radiation pressure which balances out the gravitational force.
Death of a star < 3 solar masses
- When it runs out of hydrogen to fuse, the star starts to collapse in on itself under its own gravity, heating up the core further.
- The material surrounding the core still has some hydrogen and its temperature now becomes hot enough to fuse the hydrogen – shell hydrogen burning.
- The core keeps contracting until is hot enough to fuse helium into carbon and oxygen – core helium burning.
- Core helium burning releases a lot of energy and creates lots of radiation pressure which pushes the outer layers of the star outwards forming a red giant.
- When the helium runs out the core will continue to collapse, however it doesn’t get hot enough for any more fusion to occur.
- Eventually electron degeneracy pressure stops it from contracting further and its outer layers spread out into space forming a planetary nebula, leaving behind a white dwarf.
Because the star is so big, its gravity will overcome the electron degeneracy pressure.
The electrons are forced to combine with protons to form a solid neutron core.
The outer layers of the star collapse and rebound against the solid neutron core in an explosion called a supernova.
All the remains of the star is the incredibly dense neutron core called a neutron star.
For the biggest and heaviest stars (>10 solar masses), gravitational collapse continues until it collapses into a singularity, forming a black hole.
You will need to know the evolution of stars in A2/A-level Physics exam! 👩🏫
Olbers Paradox
- Heinrich Olbers thought that the sky should always be brightly light, even at night.
- This is because the number of stars in imaginary shells surrounding the earth increases with distance.
- However the intensity of light from stars that reaches the earth decreases with distance according to the inverse square law 1/d2.
These two effects would cancel each other out, so the sky should always be bright.
However this can only occur if the universe is infinite, uniform and static.
So one or more of these has to be wrong for our universe.
These two effects would cancel each other out, so the sky should always be bright.
However this can only occur if the universe is infinite, uniform and static.
So one or more of these has to be wrong for our universe.
Redshift
- The spectra's of the light coming from stars is either shifted toward the red end of the spectrum (longer wavelength) or shifted towards the blue end of the spectrum (shorter wavelength).
- These effects are called redshift and blueshift.
- Stars/galaxies moving away from us are redshifted because the EM radiation they emit is ‘stretched out’.
- Stars/galaxies moving towards us are blueshifted because the EM radiation they emit is ‘compressed’.
Most galaxies show redshift, indicating that the universe is expanding.
The formula for calculating redshift is: ∆λ/λ=v/c
Where v is the speed of recession and c is the speed of light.
This can be used to calculate the speed at which galaxies are moving away from us.
Hubble's Law
- Hubble plotted a graph of galaxies speed of recession against their distance from earth.
- He found that that the further away galaxies were, the faster they were receding.
- The graph shows that distance from earth is directly proportional to the speed of recession.
- This is known as Hubble’s law.
v=Ho x
References:
http://abyss.uoregon.edu/~js/cosmo/lectures/lec05.html
https://www.wired.com/2003/07/alien-solar-system-much-like-ours/
This is the end of the topic! Well done!
Drafted by Cherry (Chemistry)