Astrophysics.
"The study of celestial objects, space, and the physical universe as a whole."
Table of contents:
1. Kepler's Laws.
2. Pondering Paradoxs.
1. Kepler's Laws
In the late 16th century, Tycho Brahe, a Danish astronomer said that in order to fully understand the orbits of the planets in relation to each other and the sun, a complete understanding of their movements must be established.
Throughout his life he compiled extensive notes and calculations on the various movements of the planets and this work gained him widespread fame in the scientific community. However, Johannes Kepler (one of Brahe's own employees) would soon overshadow him.
Following Brahe's death in 1601 Kepler, a mathematician by trade, studied the notes and calculations he left behind. From this, Kepler deduced 3 very simple but droopingly elegant laws of planetary motion:
1. The orbits of planets are always elliptical.
An ellipse is not an oval but a geometric shape where the locus of all its points are the sum of two fixed points or focuses (imagine a circle with two centres).
2. The radius vector from the sun to the planet sweeps out equal areas in equal intervals of time.
Planets do not travel at uniform speeds, they move faster when closer to the sun and slower when further away from the sun.
If we consider this and Kepler's first law we can deduce that although the planet moves slower when it is further from the sun, the elliptical nature of its orbit means it curves the same area of space out:
Area 1 is always equal to area 2 which in turn is always equal to any other area bounded by the sun and the movement of the planet in a give time t, provided t is a constant.
3. The squares of the periods of two planets are proportional to the cubes of the semi-major axes of their orbits.
This law is different from the first two because it describes the relationship between two planets orbiting the same star.
Orbital period (t) - the time it takes for the planet to complete one full orbit.
Semi-major axis (a) - half of the major axis (see figure 4.11) of an ellipse.
Proportional constant (k) – a constant term.
So t2∝ k a3.
2. Pondering Paradoxs.
Astronomy was the most active branch of science for many centuries - the night sky had enthralled astronomers for close to half a millenia. This gave them ample time to ponder over lots of paradoxs:
Olber's Paradox.
Heinrich Wilhelm Matthias Olbers was a German astronomer in the early 1800s. He discovered two asteroids but is best known for his proof of a non-infinite universe. The paradox that leads to this proof is as follows:
In a large enough forest, every line of sight ends in a tree as seen below. It follows then that every line of sight in the night sky should end in a star. Why is it then that the night sky is so dark?
Surely the entire night sky should be lit with starlight, with no dark gaps between stars in the same way there are no gaps between the trees of the forest. As this is obviously not the case - there are dark patches in the sky
Faint Young Sun Paradox.
3. The Universe at a Glance.
Whilst particle physics and quantum mechanics concern themselves on the very small, astrophysics is the study of the very big. It therefore follows that different laws have different significance on the wildly different scales. Most notable of these changes is the role gravity plays.
On the very small scale, gravity has almost no say whatsoever over the movement of particles because it is eclipsed so totally by the effects of the strong nuclear force and electromagnetism. Gravity is actually completely ignored when modelling particle interactions because of the tiny masses involved. However, once we zoom out and look the universe as a whole, gravity is the biggest player when it comes to the large scale and so is crucial to Astrophysics. Gravity governs the movement of planets, formation of stars, shape of solar systems and is perpetually fighting the expansion of the universe (see the section on dark energy and matter).
We now dive into the weird and wonderful parts of the universe, all shaped by gravity's helping hand:
The Many Breeds of Star.
Stars are officially defined as 'a luminous spheroid of plasma held together by self-gravity.' This does not do justice to the fierce struggle happening in every star in the night sky. The fight is simple: gravity vs thermonuclear pressure.
As seen in the diagram, all stars regardless of their ultimate destination start in nebulas. These stellar nurseries are giant revolving clouds of dust and gas that hang in space. Over time, gravity pulls them together to form small protostars which grow in mass as more dust and gas is pulled into them. As the grow, the pressure in their cores increases - gravity fights ever harder to force them to collapse.
At the core of the star, where the pressure is highest, nuclear fusion occurs (see the nuclear physics section) as the hydrogen atoms are pushed together. When they fuse into helium, they release energy known as thermonuclear pressure. This outward force balances the crushing inward force of gravity and so the star achieves equilibrium.
On the diagram this stars in equilibrium are the average star and massive star. The equilibrium stars come in many different colours, sizes and temperatures:
The Equilibrium Stars.
In the lifetime of a star, it spends most of it's time as a stable star - thermonuclear pressure is strong enough to stop the gravitational collapse. It is therefore logical that almost all the stars you see in the night sky are in their stable form. To distinguish between them, we use the Morgan–Keenan system (named after the 20th century astrophysicist W. W. Morgan, Philip Childs Keenan).
The MK system works on two principles: the star is first given a temperature rating from the letters O, B, A, F, G, K, M with M being the coldest (although still with a temperature of around 3000K) and O being the hottest with temperatures as high as 50,000K! This letter makes up the first part of the MK name.
After the O-M letter comes a number from 1-9. These numbers also relate to the temperature of the star by dividing each class into 10 sections. The lower the number the higher the temperature so a G0 star is hotter than a G6 star but both are colder than an A9.
Finally, the stars luminosity (how much light it gives off) is shown as a roman numeral from 0 (brightest) to VII (dullest). The number is based on the width of absorption lines in the spectra produced by the star:
0 - Hypergiants.
I - Supergiants.
II - Bright Giants.
III - Giants.
IV - Subgiants.
V - Main sequence stars.
VI - Subdwarfs.
VII - White dwarfs.
The larger the star, the more brightly it tends to shine, so the final part of the MK system - the roman numeral is also a good indicator of the size of the star.
Our sun (known as Sol) has an MK classification of G2 V. This makes it in the top 20% of hottest G-class stars and a main sequence star.