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Measuring radiation

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.

The expanding universe

F.6.1 see F.3.1

General relativity

F.7.1

‘No observer can determine by experiment whether he or she is accelerating or is rather in a gravitational field.’ -- a pilot can for example in fog, while making a steep turn not tell whether he is accelerating by gravity or by the plane’s thrust without looking at his instruments inertial mass = gravitational mass.

F.7.2

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?

F.7.3

F.7.4

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.

Stellar evolution

F.8.1

Stars undergo changes as their hydrogen starts to vanish. They become either red dwarfs immediately, or by first becoming red giants, then white dwarfs to finally also cool to red dwarfs, all depending on their sizes. See F.1.1

F.8.2

Clouds of hydrogen and helium forms into ‘Main sequence’ stars.

Nuclear fusion takes place:

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 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. (It is no hole just a small extremely dense mass)

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.

F.8.4

Since black holes are really massive the ‘stretched rubber sheet’ of space-time becomes very deformed.

The Schwarzhild equation tells that:

meaning that there’s a critical radius when a mass becomes a black hole.

• - gravitational constant,
• - mass,
• - 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. 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 to disregard the curvature of space-time and let us travel through space instantaneously.

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.

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