Astrophysics And Cosmology Flashcards
What is the universe
The universe is everything that exists - this includes plenty you can see, like stars and galaxies, and plenty that you can’t see, like microwave radiation, dark energy and dark matter.
What are galaxies
Galaxies, like our Milky Way galaxy, are clusters of stars and planets that are held together by gravity.
What is inside our galaxy (the Milky Way)
Inside the Milky Way is our Solar System, which consists of the Sun and all of the objects that orbit it. This includes the planets and their planetary satellites (including moons, artificial satellites and anything else that’s orbiting them), comets, and asteroids.
Name all the planets in our galaxy in order (closest distance from the sun first).
The planets (in order) are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The planets (and the asteroid belt) all have nearly circular orbits. Pluto is a dwarf planet beyond Neptune.
What is a planet?
A planet is an object in orbit around a star with three important characteristics:
• it has a mass large enough for its own gravity to give it a round shape (unlike the irregular shape of asteroids)
• it has no fusion reactions (unlike a star)
• it has cleared its orbit of most other objects (asteroids, etc.).
What is a planetary satellite (with example)
A planetary satellite is a body in orbit around a planet. This includes moons and man-made satellites.
What is a comet
They are small irregular bodies made up of ice, dust, and small pieces of rock. (Dirty snow balls)
Describe the orbit of comets
The orbits of the comets we see are highly elliptical.
They orbit the Sun about 1000 times further away than Pluto does (in the “Oort cloud”). Occasionally one gets dislodged and heads towards the Sun. It follows a new elliptical orbit, which can take millions of years to complete. Some comets (from closer in than the Oort cloud) follow a smaller orbit and they return to swing round the Sun more regularly.
What is astronomical units
One astronomical unit (AU) is defined as the mean distance between the Earth and the Sun.
What is the size of one AU
now know that 1 AU is equal to about 150 million km.
What is a light year
All electromagnetic waves travel at the speed of light, c, in a vacuum
(c = 3.00 x 10% ms-*). The distance that electromagnetic waves travel through a
vacuum in one year is called a light-year (ly). 1 ly is equivalent to about 9.5 × 10^13m.
How does observation of something light years away from you relate to the age of that object
If you see the light from a star that is, say, 10 light-years away then you’re actually seeing it as it was 10 years ago. The further away the object is, the further back in time you are actually seeing it. So when we look at the stars we’re looking back in time, and we can only see as far back as the beginning of the universe. This means we can work out the size of the observable universe
What is one arc second in degrees
There are 60 arcminutes in 1°
There are 60 arcseconds in each arcminute.
Therefore, 1 arcsecond = (1/3600)°
What can a parsec be defined as ?
The parsec is defined as the distance at which a radius of one AU subtends an angle of one arcsecond.
Since the radius of the Earth is a constant and theta (the angle subtended by the radius) is small what expression can be made about a parsec
Using trig: d = r/ tan(θ)
Since tan(θ) = θ (small angle approximation)
d= (constant) / θ
Where d is the distance in parsecs
Since the distance between the Earth and Sun is 1 AU and θ is measured in arc seconds
d = 1/p
What is a parsec in meters - explain how you got your answer
1 pc = 1AU / tan (1/3600) = 1.5x10^11 / tan(1/3600) = 3.1x10^16
What is the Stellar parallax?
Stellar parallax is a technique used to determine the distance to stars that are relatively close to the Earth, at distances less than 100 pc.
What is parallax (in relation to stars)?
Parallax is the apparent shift in the position of a relatively close star against the backdrop of much more distant stars as the Earth orbits the Sun.
View Kerboddle pg 384 or screen shot
In the equation relating to parallax : d= 1/p
What are d and p measured in ?
d is in parsec and p is the angle of parallax in arc seconds
How is a protostar formed?
Stars are born in a cloud of interstellar dust and gas, most of which was left when previous stars blew themselves apart in supernovae. The denser clumps of the cloud contract (very slowly) under the force of gravity.
When these clumps get dense enough, the cloud fragments into regions called protostars, that continue to contract and heat up.
How does a protostar eventually become main sequence
Eventually the temperature at the centre of a protostar reaches a few million degrees, and hydrogen nuclei start to fuse together to form helium.
As the star’s temperature increases and its volume decreases (remember, its contracting), the gas pressure increases.
There is also radiation pressure in the star - a pressure exerted by electromagnetic radiation on any surface it hits. It’s usually too tiny to notice, but becomes significant in stars because of the enormous amount of electromagnetic radiation released by fusion.
The combination of gas pressure and radiation pressure counteract the force of gravity, preventing the star from contracting further.
The star has now reached the MAIN SEQUENCE and will stay there, relatively unchanged, while it fuses hydrogen into helium.
What happens to a main sequence start when it begins to run out of fuel
When the hydrogen in the core runs out, nuclear fusion stops, and with it the outward pressure stops.
The core contracts and heats up under the weight of the star. The outer layers expand and cool, and the star becomes a RED GIANT.
The material surrounding the core still has plenty of hydrogen. The heat from the contracting core raises the temperature of this material enough for the hydrogen to fuse. This is called shell hydrogen burning.
(Very low-mass stars stop at this point. They use up their fuel and slowly fade away…)
The core continues to contract until, eventually, it gets hot enough and dense enough for helium to fuse into carbon and oxygen. This is called core helium burning. This releases a huge amount of energy, which pushes the outer layers of the star outwards.
When the helium runs out, the carbon-oxygen core contracts again and heats a shell around it so that helium can fuse in this region - shell helium burning.
What happens for the majority of a main sequence stars existence
Stars spend most of their lives as main sequence stars. The pressure (radiation and gas) produced from hydrogen fusion in their core balances the gravitational force trying to compress them. This stage is called core hydrogen burning.
What happens to low mass stars like the sun when the core cools
In low-mass stars, the core isn’t hot enough for any further fusion and so it continues to contract under its own weight. Once the core has shrunk to about Earth-size, electrons exert enough pressure (electron degeneracy pressure) to stop it collapsing any more
For stars below the Chandrasekhar limit, the helium shell becomes increasingly unstable as the core contracts.
The star pulsates and ejects its outer layers into space as a planetary nebula, leaving behind the dense core.
The star is now a very hot, dense solid called a WHITE DWARF, which will simply cool down and fade away.
What is the Chandrasekhar limit?
For stars with a core mass over about 1.4 times the mass of the sun the electron degeneracy pressure isn’t enough to counteract the gravitational force and the star collapses (going on to become a supernova ). The maximum mass for which the electron degeneracy pressure can counteract the gravitational force is called the Chandrasekhar limit.
Names each stage of the star cycle
Clouds of dust and gas
Protostar
Main sequence
Red giant
White dwarf
Which spend longer in the main sequence stage, high or low mass stars?
Stars with a large mass have a lot of fuel, but they use it up more quickly and don’t spend so long as main sequence stars.
What happens when the core a massive star runs out of fuel
If the star’s core is larger than the Chandrasekhar limit, electron degeneracy pressure can’t stop the core contracting. This happens when the mass of the core is more than 1.4 times the mass of the Sun. The core of the star continues to contract, and as it does, the outer layers fall in and rebound off the core, setting up huge shockwaves. These shockwaves cause the star to explode cataclysmically in a SUPERNOVA, leaving behind a NEUTRON STAR or (if the star was massive enough) a BLACK HOLE. The light from a supernova can briefly outshine an entire galaxy.
What’s happens when a massive star runs out out of hydrogen
When they are red giants the ‘core burning to shell burning’ process can continue beyond the fusion of helium, building up layers in an onion-like structure to become SUPER RED GIANTS (or red super giants). This fusion of helium causes the star to expand
For really massive stars, fusion can go all the way up to iron.
Nuclear fusion beyond iron isn’t energetically favorable, so once an iron core is formed then very quickly it’s goodbye star.
Note that is basically the same explanation as for a smaller star, only with more elements undergoing fusion
How do Neutron stars form
As the core of a massive star contracts, electrons get squashed onto the atomic nuclei, combining with protons to form neutrons and neutrinos (hence the name neutron star’). If a white dwarf’s core is 1.4 to 3 times the mass of the Sun then this is as far as the star can contract.
The core suddenly collapses to a neutron star, causing a supernova.
Neutron stars describe the properties of neutron starts
Neutron stars are incredibly dense (about 4 x 10’7 kgm^3). They’re also very small, typically about 20 km across, and they can rotate very fast (up to 600 times a second).
They emit radio waves in two beams as they rotate. These beams sometimes sweep past the Earth and can be observed as radio pulses rather like the flashes of a lighthouse. These rotating neutron stars are called PULSARS.
