Tom’s Infinite Science Archive: Astrophysics
 Tom s Infinite Science Archive: Astrophysics -
Interesting Phenomena:
The Universe
Black holes In Spacetime
Journey Into A Black Hole
Quasars - Coming Very Soon
White holes and Worm Holes - Coming Very Soon
Neutron Stars and Pulsars - Coming Very Soon
Planetary Astronomy: Coming Soon -
Mercury
Venus
Earth
Mars
Jupiter
Jupiter
Saturn
Uranus
Neptune
Pluto
Other Celestial Objects: Coming Soon -
Galaxies
Stars
Comets/Meteors/Asteroids
Interesting Theories: Coming Soon -
The Big Bang
Quantum Theory
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Greetings and welcome.
Only a few of the subjects in Astrophysics are finished. I hope to get them going soon but I am very preoccupied at the moment. If I have not done them in what you would consider a long enough period of time then pleas Email me.
Helpful Information -
Astrophysics is a branch of astronomy that seeks to understand the birth, evolution, and end states of celestial objects and systems in terms of the physical laws that govern them. Astrophysics covers everything to do with stars from specialized aspects of Stellar Evolution to the way stars interact with their surroundings. The study of the sun is classed on its own. For each object or system under study, astrophysicists observe radiations emitted over the entire electromagnetic spectrum and variations of these emissions over time. This information is then interpreted with the aid of theoretical models. It is the task of such a model to explain the mechanisms by which radiation is generated within or near the object, and how the radiation then escapes. Radiation measurements can be used to estimate the distribution and energy states of the atoms, as well as the kinds of atoms, making up the object. The temperatures and pressures in the object may then be estimated using the laws of thermodynamics.
The Hertzsprung-Russell (H-R) diagram:
Named after its originators, Ejnar Hertzsprung and Henry Norris Russell, is a graph that separates the effects of temperature and surface area on stellar luminosities and enables us to sort the stars according to their diameters. Before we discuss the details of the H-R diagram (as it is often called), let us look at a similar diagram we might use to sort automobiles.
We can plot a diagram such as Figure 8-6 to show horsepower versus weight for various makes of cars. And in so doing, we find that in general the more a car weighs, the more horsepower it has. Most cars fall somewhere along the sequence of cars, running from heavy, high- powered cars to light, low-powered models. We might call this the main sequence of cars. But some cars have much more horsepower than normal for their weight- the sport or racing models-and the economy models have less power than normal for cars of the same weight. just as this diagram helps us understand the different kinds of autos, the H-R diagram helps us understand the kinds of stars.
The H-R diagram relates the intrinsic brightness of stars to their surface temperatures. We may plot either absolute magnitude or luminosity on the vertical axis of the graph, since both refer to intrinsic brightness. As you will remember from Chapter 6, spectral type is related to temperature, so we may plot either spectral type or temperature on the horizontal axis. Technically, only graphs of absolute magnitude versus spectral type are H-R diagrams. However, we will refer to plots of luminosity versus either spectral type or surface temperature by the generic term H-R diagram.
An H-R diagram. Roughly 90 percent of all stars are main sequence stars, including the sun-a G2 star with an absolute magnitude of about +5. Star diameters not to scale.
A point on an H-R diagram shows a star's luminosity and surface temperature. Points near the top of the diagram represent very luminous stars, and points near the bottom represents very faint stars. Points on the left represent hot stars, and points on the right represent cool stars. Notice that the location of a star in the H-R diagram has nothing to do with its location in space. Also, as we will see in later chapters, changes in a star's luminosity and surface temperature as it ages mean that its position on the H-R diagram changes, but this has nothing to do with the star's actual motion through space.
Dwarfs, Giants, and Supergiants:
The main sequence is the region of the H-R diagram running from upper left to lower right, which includes roughly 90 percent of all stars. These are the "ordinary" stars. As we might expect, the hot main-sequence stars are brighter than the cool main-sequence stars. The sun is a medium-temperature main-sequence star.
Just as sports cars do not fit in with the normal can in Figure 8-6, some stars do not fit in with the main. sequence stars in Figure 8-7. The giant stars lie at the upper right in the H-R diagram. These stars are cool, radiating little energy per square meter. Nevertheless, they are highly luminous because they have enormous surface areas, hence the name giant stars. In fact, we can estimate the size of these giants with a simple calculation. Notice that giants are about 100 times more luminous than the sun even though they have about the same surface temperature. Thus, they must have about 100 times rnore surface area than the sun. This means they must be a 10 times larger in diameter than the sun.
Near the top of the H-R diagram we find the super giants. These exceptionally luminous stars are 10 to 1000 times the diameter of the sun. Betelgeuse in Orion Is 2 supergiant. If it magically replaced the sun at the centre of our solar system, it would swallow up Mercury, Venus, Earth, and Mars, and would just reach Jupiter.
µ Cephei is believed to be over three times larger than Betelgeuse. It would reach nearly to the orbit of Uranus.
In the lower left of the H-R diagram are the economy models, stars that are very faint even though they are hot. Clearly such stars must be small. These are the white dwarf stars, about the size of Earth.
Spacetime:
Newtons Theory of Gravity ruled supreme for 250 years but was only a partial explanation of how the universe works. Scientists were shocked when Albert Einstein came along with his Theory of Relativity in fact, Einstein proposed two Theories of Relativity. The "Special Theory" of 1905 dealt with matter, energy and speed of light. The "General Theory" of 1915 concerned gravity. Instead of regarding gravity simply as a force, Einstein looked on it as a distortion of space itself. Where Einstein's predictions differ from Newton's, Einsteins General Theory has always proved more accurate.
Einstein's Right:
Mercury, the closest planet to the sun has a markedly oval orbit which does not return to the same starting point. The point of closest approach to the sun (perihelion) is always changing. Newton's theory of gravity cannot explain this unusual orbit, but Einstein's can. As Mercury follows the contours of warped space close to the sun's large mass, it's orbit naturally traces out a complex path in the shape of a rosette.
Dents in the fabric of space:
Einstein thought of empty space as being like a thin rubber sheet. If you place a heavy object, such as a billiard ball, on the sheet, it makes a dent. The sun, which is the most massive object in the solar system, warps the space around it, making a small dent, or gravitational well. Things moving through space follow the smooth path when they meet the indentation. Radio signals to and from the Viking Lander on Mars are delayed by the curvature of space near the sun - proving Einstein's theory to an accuracy of 0.001 percent.
Distortion in three dimensions:
To illustrate warped space, we usually draw space as having two dimensions like the image to the left. In reality space is three dimensional, and the cube on the left is how space would look without any objects in it. A massive object causes distortions bending the grid lines that map space (below-right). The natural path of objects through space is not a straight line, but a curved one as they follow the humps and hollows and start 'roll' towards more massive objects.
Any kind of spinning body tends to drag space around with it, but this effect is most noticeable close to massive objects. The space around a rotating black hole is rwisted as it is pulled around. Compare this (left) 3D structure with the diagram of the structure of space near a non-rotating massive object obove.
The gravitational well of a spinning black hole resembles a comic whirlpool (right) - any object coming within its attraction will be swirled around as it is sucked in. Outside the static limit, a spacecraft can move where it wants. Once in the ergosphere, though, it is inexorably draged around by the holes spin, but it could still escape if its engines were powerful enough. Within the outer event horizon it cannot get away even if its engines have infinite power.
Deeper and Deeper Dents:
While Newton regarded objects of increasing density as having increasingly higher escape velocities, Einstein saw them as making deeper "dents" in space.
Our Sun makes a relativity shallow dent. Objects "roll" towards it at moderate speeds.
A white dwarf, being denser, dents space far more noticeably. Objects roll quickly towards it as they approach the steep slope.
A neutron star creates a dent with very steep sides. Objects rolling in reach half the speed of light.
A Black Hole makes such a deep dent, that it forms a well. The sides of this gravitational well are so steep that even light can not escape. Once anything crosses the event horizon - the boundary where the escape velocity becomes equal to the speed of light - it is trapped inside forever.
1: Light rays approaching a black hole are bent around by steeply curved space.
2: Light can escape a black hole if it gives the hole a wide berth.
3: Rays that come closer may go into orbit around the black hole.
4: Light that comes dangerously close to the black hole is inevitably drawn in.
5: Schwarzschild radius.
6: Event horizon.
Once inside the event horizon, light spirals down the steep sides of the gravitational well.
Einstein Ring:
A black hole acts as a gravitational lens when it comes between an object and an observer. With a simulated black hole in front of a picture of Albert Einstein above left, the alignment is not perfect; most of the image is on one side, with a thin arc on the other center. With perfect alignment the image right becomes a ring known as an Einstein Ring.
The Time in Spacetime:
It is important to realize that for every sustained movement in spacetime, opposite directions of probability have been canceled. We observe the above forces, but what we do not observe is the battle between all the irregular directions of spacetime. For every step forward into a narrowing future there is an opposing move toward the past. A bit like Feynman's description of particles as taking every irregular path, this combined with the phrase, two steps forward, one step back.
Spacetime's time has no inherent direction beyond what the probabilities dictate. The unbalanced probabilities of a spacetime that is more positively dense than negative, guide and create what is to us a forward direction of time. The direction of time is strongest at the Big Bang, but the momentum of that direction continually decreases. It is actually the loss of any one definite direction of time, as spacetime becomes less dense and probabilities equalize, which causes the momentum of spacetime toward balance to slow. We observe that decrease in the momentum of time as a decrease in the rate that the cosmos expands and cools.
Units and Astronomical Data:
Units Used in Astronomy: |
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1 angsrtom (A) |
= 10-8 cm
= 10-10 m |
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1 astronomical unit (AU) |
= 1.495979 x 1011 m
= 92.95582 x 106miles |
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1 light-year (ly) |
= 6.3240 x 104 AU
= 9.46053 x 1015 m
= 5.9 x 1012 miles |
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1 parsec (pc) |
=206,265 AU
= 3.085678 x 1016 m
= 3.261633 ly |
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1 kiloparsec (kpc) |
= 1000 pc |
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1 megaparsec (mcp) |
= 1,000,000 pc |
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 Legal Notice:
The majority of the images of spacetime, black holes, quasars, etc. were illustrated by Luciano Corbella and were scanned and edited by myself.
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