Option F - Astrophysics (HL)
From KstructIB
Thanks to Tímea Garlati and a little help from Mephistophyles
[edit] F.5 Measuring radiation
[edit] F.5.1
The longer the period of light variation of a Cepheid, the greater is the luminosity. Observing the luminosity and the period the absolute magnitude can be determined. Comparing the absolute luminosity to the apparent luminosity we can calculate the distance to the Cepheid and surrounding stars. They serve therefore as 'distance markers' for distant stars where parallax can not be applied.
[edit] F.6 The expanding universe
[edit] F.6.1
see F.3.1
[edit] F.7 General relativity
[edit] F.7.1
'No observer can determine by experiment whether he or she is accelerating or is rather in a gravitational field.' For example, a pilot in fog, while making a steep turn can not tell whether he is accelerating by gravity or by the plane's thrust without looking at his instruments.
inertial mass = gravitational mass.
[edit] F.7.2
ANYONE???
The effect of gravity upon time can be visualized as a stretched rubber-sheet, representing the 3 dimensions of space and one of time that is deformed by gravity. The more massive the object, the greater the deformation.
Does it mean that we don't know whether we are accelerated through time or pulled backwards by gravity?
[edit] F.7.3
The effect of gravity upon time can be visualized as a stretched rubber-sheet, representing the 3 dimensions of space and one of time that is deformed by gravity. The more massive the object, the greater the deformation.
Does it mean that we don't know whether we are accelerated through time or pulled backwards by gravity?
EDIT: It is actually the fabric of space-time that can be visualized as a rubber-sheet. When massive celestial bodies such as planets are placed on this fabric, they bend it inwards. This curvature on the fabric can be defined as the effect of gravity. When smaller objects go into this curvature, they start spiraling around the central planet and get closer to the core as they sprial. You can visualize this motion as the water that spirals down the drain.
Therefore, we can say that gravity causes acceleration on the space-time fabric.
[edit] F.7.4
[edit] F.7.5
Light is bent by strong gravitational fields. This was observed when light from distant planet could be observed although the Sun stood in its way.
[edit] F.8 Stellar evolution
[edit] F.8.1
Stars undergo changes as their hydrogen is used up, first off they try to fuse the heavier elements, but this only works under certains circumstances and for a finite time. They either red giants after they go supernova, or they release a planetary nebula (nothing to do with planets, another form of expansion not as bad as going supernova). After going supernova stars become red giants, due to their large masses, or they decay into a neutron star. If they release a planetary nebula, they become a white dwarf or a brown dwarf, all depending on their mass.
(See also F.1.1)
[edit] F.8.2
Clouds of hydrogen and helium forms into 'Main sequence' stars.
Nuclear fusion takes place: H + H -> He , then He + He -> Be.
Further fusion takes only place in heavier stars, otherwise the pull of gravity forces the star to contract and cool to a red dwarf. If further fusion takes place the star becomes a red giant.
Small mass red giants : 1.4 solar masses (Chandraseka limit) can not withstand the pull of gravity, so it shrinks, becomes extremely hot, till it finally cools into a white dwarf.
Large red giants : Fuse until the formation of iron, but thereafter no fusion can take place without energy addition. So the star contracts, and heats up because of the large KE in the particles. When it can't be compressed further it explodes in a supernova. Such a great explosion may leave behind a very dense star made up of mostly neutrons, i.e. a neutron star.
Lighter neutron stars : With solar masses below 2-3 solar masses are thought to form Black holes because the gravitational force, which increases with mass, doesn't even allow light to leave the surface. Note, it is not a hole as such just a small extremely dense mass.
[edit] F.8.3
Pulsars are neutron stars that radiate energy at regular periods (See F.1.1)
Quasars suggest the existence of black holes, since the accelerated matter that black holes draw in could release its energy in the form large amounts of light.
X-ray is possibly produced as the accelerated matter due to Black holes is compressed and heated to millions of degrees.
[edit] F.8.4
Since black holes are really massive the 'stretched rubber sheet' of space-time becomes very deformed.
The Schwarzhild equation tells that Rs = 2GM/c2 meaning that there's a critical radius when a mass becomes a black hole. (G is the gravitational constant, M is mass and c is the speed of light). For Earth this radius is 1cm.
The event horizon is the boundary at which light can not escape from the black hole. The singularity is a spot in the middle of the event horizon to which the mass shrinks, since no known force can stop the contraction. Here all time, space, matter and energy end.
Wormholes should allow us to disregard the curvature of space-time and let us travel through space instantaneously.
Not exactly; wormholes allow us to travel faster between two points than would be possible over the plane of space-time. The problem with relativity in it is that it views space-time as a flat plane with holes or dents that represent large masses. But for wormholes to effectively work, space-time needs to be curved or even spherical (like a soccer ball). Also, if you appear on the other end faster than light could travel all the way around, you are effectively violating (in some complicated way) causality.
But how does this link to general relativity???
General relativity states space-time curves because of mass, yadda yadda. A singularity has such high mass that it can curve space time in on itself creating wormhole. Or it can cause a tear in the continuum, into which everything in the event horizon is eventually pulled, out of the universe itself.
not exactly, black holes have very high density areas called singularities, they are very curved, but they don't have infinite mass, but high enough to curve light into it, the event horizon is found using the The Schwarzhild equation (see above). The wormhole thing is something entirely different, it's not a rip in the fabric, more like a tunnel between two black holes. Einstein did some work on this and it led to a recent discovery at CalTech where they found it was theoretically possible to time travel, but you need to be REALLY small and REALLY fast. Anyhow, that should cover that, any more questions, just post them. The only thing on the syllabus as far as I know is the Schwarzhild equation.
