Astronomy and Astrophysics: Steller Phenomena
Welcome to Astronomy and Astrophysics. Here, you can obtain information about neutron stars, black holes, including static, charged and rotating, the idea of spacetime, and more. I will try, to the best of my ability, to explain the more difficult concepts of these subjects, such as spacetime, and of the properties of black holes. Enjoy!
A REVIEW OF NEUTRON STARS AND RELATED PHENOMENA
When a star has ran out of 'fuel,' (Meaning that it no longer has enough energy left to burn) it bursts into a supernovae; sending matter through space and producing a very bright explosion. The leftover matter of the star collapses into a neutron star: its inner core made solely of neutrons. For this spectacular event to occur, the original star had to be about 3 solar masses; one solar mass being the weight of our sun. After the explosion and collapse take place, the resulting neutron star has about a 10 to 15 kilometer radius. It is very dense because of the former collapse, and it is now spinning very fast. This star does not glow, becuase it is formed from a dead star (Recall that the original star was out of energy, making it a 'dead star'). Neutron stars will come up later, in the section about black holes. But now that we're on the subject of neutron stars, another steller phenomena, that actually begins with neatron stars: pulsars.
A pulsar is a neutron star that sends forth radio waves, that pulse off and on, thus the name 'pulsar.' Because the star is spinning rapidly, it only appears to pulse, as veiwed from Earth. These radio waves are only emitted from the North and South magnetic poles of the neutron star. If the star is tilted, the radio waves fly out from the poles in a kind of sweeping motion. Astronomers pick up the waves as the beam moves across Earth, giving it a pulsing appearance.
AN INTRODUCTION TO SPACETIME
Spacetime is a rather difficult concept to explain, and harder to imagine. Why? Spacetime is one of the higher dimensions. Although I can't quite recall which it is, but spacetime combines time and space (kind of obvious, isn't it?). Before I explain more, know that time is not an absolute measurement, considering that time is only measurement of change or process. This may or may not help, but with that being said, spacetime it what the very Universe is made up of. As Kip Thorne states in his book "Black Holes and Time Warps," 'Your space is a mixture of my space and my time,' meaning that your space has influence on his space and his time. This fabric of our known world is absolute, meaning it depends on nothing. If you don't understand, you're certainly not the only one! Spacetime, along with any other higher dimension, is very hard to picture, because we live in a three dimensional world. Picturing any other dimension other than the first and second, is not an easy task! But I will try one last example: imagine a baseball sitting on top of a thin sheet of fabric. The ball sinks into the sheet, because of its weight. In the same way, a star sinks into space, because it is so dense. This does not fully show the properties of spacetime, but maybe you have a better idea now?
BLACK HOLES: THE THREE TYPES
Black holes are perhaps the most facinating and mysterious objects known to man. We seem to know so much about them, but nearly everything is thoery, since we cannot directly see a black hole yet. But hold that thought, I'll first go over black holes, then tell you why we can't see them.
Just how is a black hole formed? From a neutron star, actually. Recall the example about a ball sinking into a fabric sheet. If a neutron star that is too dense, gravitational forces will cause another collapse. This neutron star, depending on how dense it is, will continue to collapse, pushing more and more into space, like the baseball it pressure is applied to it. If too much pressure is put on the ball, the sheet will rip, just as space would. (It wouldn't actually rip, but it would be sort of torn into) Now, the neutron star is deep inside space, making the classic figure of a black hole.
Hold up a second...a hole in space? Is that possible? Apparently so; but what's under space, then? If you dug a hole in the ground, the surrounding material of the outside of the hole is dirt, right? What surrounds the outside of a black hole? A theoretical material called hyperspace. We will probably never know if hyperspace actually exists, but it seems to be a good theory for now...for more info on hyperspace, check out a book by Michio Kaku, called "Hyperspace."
Back to the formation of black holes. Once these events have taken place, there are three types of black holes that this one could be: a static hole, a charged hole, and a rotating black hole. Let's begin with the easiest one, the static hole. These have no charge and don't spin. Static black holes have what's called a photon sphere, which is the sphere that surrounds the black hole in which photons orbit. These photons have a very unstable orbit here. Then, as every hole has, a static hole has an event horizon, also called the critical circumference. After something passes the horizon, it can never escape. Why is this? Gravity; it is so strong here, that not even light can escape this force! Also, even more astonishing, time will actually stop here, becuase time it relative to gravity! Here's an example from Kip Thorne's book: say you tie a clock to the cieling of a room, then place another on the floor, with the exact same time as the former. Since gravity on Earth pushes downward, more pressure is put on the clock hanging from the cieling; thereby lower than the time on the clock on the floor. This process is called gravitational time dialation. Earth's time dialation is about 300 parts in a billion billion, mening that we would never live long enough to see time being slowed on Earth. Although it seems we could not prove this, laws of physics do, allowing us to put faith in this amazing theory.
Now, a black hole's time dialation is much faster, because gravity is so much stronger, having a more dramatic effect on time.
The last part of a static black hole is it's singularity. This is where all of the atoms of materials end up. (Since gravitational forces are so strong, everything that ends up in a black hole gets torn apart)
A charged black hole isn't too different from a static hole; the photon sphere is about the same, but the horizon somehow shrinks, and another one forms: producing an inner and outer horizon. This happens mostly because of the charges particals reacting to eachother and gravity. The more charge the black hole has, the smaller the outer horizon and the larger the inner horizon. If the charge's magnitude is equal to its mass, the horizons merge. If the charge of the hole is greater than its mass, the horizons vanish, leaving only the singularity and photon sphere. But both of these events are very unlikely, because a black holes mass is extreme. But if this does happen, the charges tend to rip apart atoms around it, trying to neutralize itself. According to some theories, the singularity may or may not be able to exist without the event horizons.
Lastly, a rotating black hole has what's called a boundary line, or static limit. Yet another name for this is the Ergosphere. This is the solid ellipsoid that swells out from the hole. The faster the hole spins, the further out the Ergosphere.
Surrounding the rotating hole is not one, but two photon spheres. Since this hole rotates, it sort of drags in space. For example, a whirl pool drags in water as it spins; a black hole drags in space as it spins. One of those spheres rotates with the rotation of the hole, the other goes against it. The photon sphere closest to the hole spins the fastest, making it the one going with the revolution of the actual hole. A rotating hole spins about 99.8 % of it's mass, as said by Thorne.
Having described the three types, one could find all the mathematical properties by using three numbers: the angular momentum, charge, and mass.
A QUICK DISCUSSION ON THERMODYNAMICS AND ENTROPY
Thermodynamics is the set of physical laws that govern the random statistical behavior of a large number of atoms. This simply means that thermodynamics is the behavior of atoms in a space as they react to heat; thus the name thermodynamics.
We will be looking at entropy, which, as stated in the second law of thermodynamics, can never decrease, only increase or remain the same. But what is entropy? It is the measure of the 'randomness' in a large collection of atoms, or the amount of 'randomness in the chosen area of space; and the increase of entropy means that things [inside the region of space] are continually becoming more and more random' (Kip Thorne). But what if we gathered all the air molecules from a room, with a certain amount of entropy. If we drop them into a black hole, the molecules would disappear, reducing the entropy from the Universe, and violating the second law of thermodynamics. To some, black holes cannot exist because they violate this law of entropy. But as Stephen Hawking once said: 'We'll just have to accept this violation, the properties of black holes require it.'
WORMHOLES AND WHITE HOLES
Is it inevitable that all matter will end up in the singularity?
Apparently not, according to some theoretical research. Commonly known as the Schwarzschild Wormhole (less commonly known as the Eistein-Rosen Bridge), this theory unites two black holes at the event horizons. Note: one is upright while the other is upside down. While the 'top hole' (the original black hole) sucks in matter, it would go through the wormhole and down the tube-like
figure, exiting through the white hole.
Now, let's concentrate on the white hole. A white hole has the oppisite effect on matter compared to the black hole. Rather than suck in matter, a white hole will spit out matter. It would make sense, right? Matter goes in one way and comes out the other? Actually, no. But before I explain why this is incorrect, let's suppose it is for a moment. What other kind of characteristics do white holes have? If matter would exit the white hole, that would mean that gravity is much different than that of a black hole. Remember how gravity has an effect on time; now think that since gravity is the opposite of strong it must be very weak, or there may be no gravity at all. Meaning that time could flow freely, that it is reversed! And for those who think ahead, what might the white hole be connected to? We know that a black hole is like a dent in spacetime.
_______________ ______
( hyperspace\ /<-black hole
(_______________/ \<-white hole
OR
______________ ______
hyperspace\ /<-black hole
______________/ \<-white hole
An existing white hole would prove that our universe either curves around itself, or is parallel, as shown in the diagram.
How could something like this exist? When a star collapses, it doesen't produce a wormhole. How can it? If it continues to collapse into a black hole, the imploding star would get smaller and smaller; eventually coming to a point in space called the singularity. The star would not implode and get smaller and then larger, as shown in the diagram of the wormhole.
Second, gravity is pushing down (inward) from the black hole, and gravity in the white hole is pushing upward (also inward, allowing the change in the flow of time. This presents a problem. If gravity is pushing downward and upward, the wormhole would be forced to tear apart, disconecting the black hole and white hole.
PROOF OF BLACK HOLES?
How do we know so much about black holes? We can't directly see them, and could never get close enough to study them with our current technology; how do we know for sure the even exist? Simple: because gravity is so strong around a black hole, is draws light from outside sources toward it.
star{*}----| {o} Earth
light rays->\/
black hole @
The diagram above shows exactly what happens: say there is a star nearby, the black hole will pull in the light from the star, making it appear that the star is so many light years away from where it really is. We look at star charts and can say this. We would se the star as coming from below us, when it's actually coming from the left side.
By studying this, we can theorize about black holes and related ideas, and perhaps one day in the far future, we can prove these theories.
I hope you've enjoyed my asronomy page, thanks for reading!
Email me if you have a question or comment, at:
astrophys719@yahoo.com
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