Eta carinae & the Homunuculus Nebulae (APOD 17 Jun 2008)

[doriaco]

My comment is more a question than an comment. Regarding the "red tinted debris" that is visible and "travelling fastest" of all. I have always wondered if such debris emited from the 'big bang' travelling fastest, and therefore furthest from the center, has not also cooled the fastest. If this is so would it not also be hardest to detect? And if this debris consists of a substantial amount of debris that has cooled into an outer shell of matter, virtually undetectable, but substaintial enough to create a gravitational pull (or warp in time/space) can this not be a answer to the dark matter problem?

[Chris Peterson]

My comment is more a question than an comment. Regarding the "red tinted debris" that is visible and "travelling fastest" of all. I have always wondered if such debris emited from the 'big bang' travelling fastest, and therefore furthest from the center, has not also cooled the fastest. If this is so would it not also be hardest to detect? And if this debris consists of a substantial amount of debris that has cooled into an outer shell of matter, virtually undetectable, but substaintial enough to create a gravitational pull (or warp in time/space) can this not be a answer to the dark matter problem?

There is no center to the Universe. Everything we can see is equally distant from the higher dimensional point of the Big Bang.

What "dark matter problem" are you referring to? Dark matter is a material that pervades space, and which we observe by its gravitational interaction with other matter. It sounds more like you are trying to find an alternate explanation for dark energy, the property theorized to be responsible for the increasing rate of growth of the expanding Universe.

[doriaco]

Chris,
Yes, that is exactly what I am thinking about. Please excuse my lack of proper vocabulary. I am neither scientifically trained nor as well informed as many on the site likely are. I am just interested and curious about such things. And the APOD prompted me to revisit this past thought of mine so I joined the forum hoping to learn something.
It does seem to make intuitive sense that the at the moment of the big bang some material was propelled first and therefore fastest (?) and since there is no (measurable) friction in the vacuum of space (other than gravity, that did not exist) this matter must be at the farthest edges of the universe. And while intuition does not always seem to coincide with the realities of the universe it, however, would also be a reasonable assumption to posit that this matter has also cooled first. As I mentioned in my initial question this cooled matter at such distances would seem to be problematic to detect. So my thought is that in a 'big bang' with no force being exerted on the occurence matter should have been expelled equally in all directions, in essence creating a ball or sphere of creation. If the outer reaches of the ball cool first and the collective mass of this matter is enough to exert a gravitational force on matter that already has a vector energy in the direction of the outer rim of this 'sphere' wouldn't this now 'dark energy' cause acceleration toward itself? And wouldn't all matter appear to be moving away from any particular point within the sphere unless it had both the same vector and velocity? Perhaps I don't know how to properly explain the thought I have, because upon rereading I'm not sure I've done a good job of it. I don't pretend to know what I am talking about but nontheless it is something I think about.

I didn't understand your first statement about everything being equally distant from the higher dimensional point of the big bang.

Thanks for the reply.

[billanderson]

What is the difference between this apparent explosion 165 years ago, and a nova or supernova?

[Chris Peterson]

It does seem to make intuitive sense that the at the moment of the big bang some material was propelled first and therefore fastest...

Ah, but intuition easily fails us when we start talking about cosmology. The problem is, no "material" was propelled outwards by the Big Bang. We tend to imagine it that way because we often mistakenly think of the Big Bang as an explosion. But really, it is the expansion of space itself, not the material that space contains. As far as we are able to observe, matter and energy are distributed fairly uniformly throughout the Universe. There is clumpiness on a variety of scales, but that's all.

I didn't understand your first statement about everything being equally distant from the higher dimensional point of the big bang.

The Big Bang took place in spacetime- a four dimensional domain. The origin of the Big Bang is a four-dimensional point, but we live in a Universe with just three spatial dimensions. In the same way that every point on the two-dimensional surface of an expanding sphere is equidistant from the center of that sphere, every point on the surface (volume) of our expanding three-dimensional universe is equidistant from its four-dimensional center. And we are no more able to observe that center than the two-dimensional inhabitants of the surface of a sphere can observe its center.

[Chris Peterson]

What is the difference between this apparent explosion 165 years ago, and a nova or supernova?

A nova is caused by the accretion of hydrogen from a companion star onto the surface of a white dwarf. When enough material builds up, it fuses, producing a sudden release of energy. Although material is lost, and may eventually contribute to a nebula, the total is very small (a fraction of a percent of a solar mass).

A supernova is caused when a large star has used up all its fuel, and can no longer maintain fusion. The outward pressure of its energy production no longer balances the inward force of gravity, and the star collapses on itself. A huge amount of energy is released, and nearly all the mass of the star is lost. These events produce nebulas, leaving behind only a core neutron star or black hole.

What happened to Eta Carina is not well understood. Similar events have been observed with other stars, one of which experienced a supernova just a few years later. It is probably some sort of failed supernova, possibly occurring in only part of the star (such as the surface). A significant fraction of the energy of a supernova is released, and enough material is lost to form a significant nebula. But the star itself survives as a fusing body.

[doriaco]

Thanks Chris. It's going to take a while for me to get my mind around all that, but eventually, I hope, I will. -George

[BMAONE23]

Is it possible for the event to have been caused by the direct collision of a larger star and a smaller star where the smaller star is obliterated and its mass can't be entirely absorbed by the gravity of the larger star?

[Chris Peterson]

Is it possible for the event to have been caused by the direct collision of a larger star and a smaller star where the smaller star is obliterated and its mass can't be entirely absorbed by the gravity of the larger star?

The likelihood of two stars colliding is so extremely small. I'd say that the fact we've observed a few of these false supernovas suggests that some other mechanism is needed to explain them.

[apodman]

The likelihood of two stars colliding is so extremely small.

Okay, why?

My guesses are:

1. Small cross-section to hit versus a lot of empty space.
2. Inertia, perturbations, and the gravitational soup of the cosmos keep stars whirling, not colliding.
3. Gravity saw to it that most collisions happened long ago.
4. A combination of some or all above.

But what is the actual reasoning?

[Chris Peterson]

The likelihood of two stars colliding is so extremely small.

Okay, why?

My guesses are:

1. Small cross-section to hit versus a lot of empty space.

That's the primary reason. You could build a star gun, and shoot stars into galactic cores or globular clusters, and those stars will come out the other side- probably along a different line due to gravitational perturbation, but you'd have to shoot millions to get a collision.

[astrolabe]

Hello Chris Peterson,

When the evolution of a star on it's way to becoming a neutron star or a BH sxhibits jets, does the star as it's losing mass, increase in rotation? It's apparent that Eta Car has jets, or it has gravitationally stripped the outer shells away from two angularly-equal companions, like a binary with three stars instead.

[starnut]

Thanks Chris. It's going to take a while for me to get my mind around all that, but eventually, I hope, I will. -George

Take a balloon. To us earthlings, the balloon is a three dimensional object with a two dimensional surface. Ignoring the stem, the surface of the balloon has no center and no edges. Now, pretend that the inside of the balloon represents the four dimensional spacetime where the big bang occurred and the surface represents our three dimensional universe. If you draw little pictures of galaxies on the surface and then blow the balloon up, the galaxies move farther apart, just like they do in our three dimensional expanding universe. Creatures in any of those galaxies will see that but they cannot see the center of the balloon. You could say that the air you blow into the balloon is like the dark energy driving the expansion of space.

Gary

[henk21cm]

1. Small cross-section to hit versus a lot of empty space.
2. Inertia, perturbations, and the gravitational soup of the cosmos keep stars whirling, not colliding.
3. Gravity saw to it that most collisions happened long ago.
4. A combination of some or all above.

Like Chris wrote, #1 is the main reason.

Your #2 might be true as well. In a galaxy most stars are moving in the same direction, with the same speed, like cars in a multiple carriage way. There are fast lanes, crawling lanes for the heavy trafic etc. Stars are so cleaver not to pass each other, neither in the same lane, nor change lanes. I came to this idea inspired by the very simple Monte Carlo simulations of colliding galaxies which i'm currently doing. (not my job, just for fun). In these galaxies stars are moving in a disordered way and as a result close encounters between stars are more common, of the order of once per thousand years. However in a stand alone galaxy the process is too unlikely to account for the more frequently occurring novae.

Your argument #3 is mainly true for planets in a solar system. Planets in highly elliptical orbits will have merged with their collision victims, or are destroyed. Star formation is a process different from planetary formation. The time scales for a stellar orbit are considerably longer than those of planetary orbits. The "Time will takes its toll" paradigm is more appropriate for planets (time scale order of 100 years) than for stars (time scale order of 100 Mega years).

[Animation]

accretion

I know this is off topic, but I've been wondering this semi-subconsciously for a long time. Why is the term accretion used, instead of accumulation? I mean this generally in astronomy.

I read the definition for both, and accretion seems to mean growth by natural or external addition. However, the definitions for accumulation are largely similar, though with less emphasis on the "natural" part, but without implying it is deliberate. Even one of the definitions mentioned the same definition of accretion and listed a link to accretion as a synonym.

So, I'm wondering why in Astronomy, the term accretion is largely used. Is there a subtle different that is important in Astronomy? Is there a historical or professional reason?

Thanks, and sorry for the de-rail.

Lewis

[Chris Peterson]

I know this is off topic, but I've been wondering this semi-subconsciously for a long time. Why is the term accretion used, instead of accumulation?

Mainly history, I think. English is a very rich language, and there are often multiple words that could be chosen. In science, at least, once a certain word is decided on, it's usually used exclusively. So while the dictionary gives the sort of definition that would have resulted in "accretion" being selected initially by astronomers, there is now an additional definition that specifically means material falling onto the surface of a star (although you would probably only find that in a scientific dictionary).

I have a sense of a slight distinction between "accretion" and "accumulation" that isn't reflected in the dictionary. To me, "accretion" carries a stronger sense of process, and more the sense of uniform accumulation. I think that sense comes from actual usage, both in astronomy and in geology (which uses the term to describe the process by which certain rocks form, as well as other things).

[Animation]

Thanks Chris!

Lewis

[henk21cm]

I read the definition for both, and accretion seems to mean growth by natural or external addition. However, the definitions for accumulation are largely similar, though with less emphasis on the "natural" part, but without implying it is deliberate.

G'day Lewis,

English is not my native language, so it is quite dangerous to say something about English to somebody for whom English is the native language. So i refer to two other languages. In Dutch and French the equivalent of accumulation has a specific meaning of: the intention, a pre-planned action. "Accumulation of power, by buying politicians". Accretion has the meaning of "growing", increasing by growth, a natural process, sometimes unvoluntary. "The accretion of gray hairs".

Your accretion disk of a nova is named similarly in Dutch: "Accretieschijf".

Hope this helps.

[Animation]

Thanks to you also!

Lewis

[iamlucky13]

Very nice bit of etymology Henk. Thank you.

Regarding star collisions, consider this. The sun has a radius of 1.4 million kilometers. The distance to the nearest star is 9.5 trillion kilometers. That's 7 million times as far.

Envision a target where the sun is the bullseye and the outer ring is the distance to the nearest star. Assuming you throw a star at a random location on the target, because the relative size of the bullseye is so small, your odds are only 1 in 200 trillion of actually hitting the sun.

To complicate things further, that's only the 2-D case. In actuallity, the odds of us seeing such a collision are further diminished because the universe is 4-dimensional. They might be slightly enhanced because I used the sun as a comparison, which is in a relatively empty region of our galaxy. Also, the fact that stars move in orbits affects the odds, too.

However, many astronomers still do believe they (very) occasionally happen.

[henk21cm]

Regarding star collisions, consider this. The sun has a radius of 1.4 million kilometers.

The suns diameter is 1.4 million km. About 10 lightseconds. The nearest star is 4.2 lightyears away. 1 y = 31 million s.

That's 7 million times as far.

13 million times. Your odds of 1 in 2E14 are correct. With just 2E11 stars in the galaxy, rather unlikely ever to happen in our lifetime, even in the denser populated areas of the galaxy. Gravity may favour the odds, perhaps with a factor 100. That is still a point of further investigation.

[Arramon]

That whole balloon theory is so overused. If our galaxies were all on a two-dimensional plane, aren't there 'sides' to the balloon? Kind of like the earth. We can't see one continent while standing upon another because of the curve of the earth, why is it that we see so many galaxies from our one vantage point, here on earth? The balloon has a center, because the walls of the balloon wrap around it and the inside is being filled (with whatever). You wouldn't have a spherical shape if that expands if there was no middle. That would be like blowing air into a balloon that didn't inflate, yet the surface area of the balloon kept getting bigger and bigger even though nothing pushed from within. =/

That doesn't make sense. People use the balloon theory to explain the distances of galaxies and why they are moving apart from eachother, but a balloon has a center (although the air comes from a hole on one end of the balloon). If the Big Bang were anything like that, wouldnt we just see galaxies that were visible within our own curve of the area we are on 'upon this supposed two-dimensional surface' if the universe is expanding as in this fashion? Or would the material of this 'balloon' effect be an invisible substance that is allowing us to see through the 'fourth dimensional plane' to the galaxies on the 'other side' of the balloon shape?

Also, not all galaxies are moving away from eachother, and on vast scales, clumping is happening in a manner that is nothing near what an expanding balloon does on its surface to 'inked' in points that would move away from eachother if the 'balloon' expanded. Its more like galaxies are organizing into super structures that resemble human designs more than a balloon design.

http://www.spacedaily.com/images/cosmol ... k-1024.jpg

That has no resemblance to an expanding 'balloon' universe. More like one giant brain. =b

[Animation]

That whole balloon theory is so overused. If our galaxies were all on a two-dimensional plane, aren't there 'sides' to the balloon?

You must have missed that "Mirror, Mirror" episode of Star Trek.

Anyway, you said it yourself. It is an analogy. The comparison is only superficially appropriate. The analogy can help you visually understand something not entirely unlike what you are trying to understand. However, it breaks down.

The first place it breaks down is the fact that a balloon is just a shape with surface which is no more or less in the same dimension in actuality as the interior of the balloon; so yeah, the balloon surface actually has sides. What you are supposed to grasp from the analogy I believe is more akin to a 2-dimensional universe mapped over a sphere, so the meaning of what the thickness or sides are ... would probably have some other meaning.

So, yeah, by definition, its an analogy, and it breaks down at some point.

but a balloon has a center

The analogy should make you realize that we exist in a way that makes the curvature not normally apparent. The center and its characteristics exist in a different way entirely such that they never quite map like you'd expect.

wouldnt we just see galaxies that were visible within our own curve of the area we are on 'upon this supposed two-dimensional surface'

Well, an energy analogous to light (but existing in the 2D analogy universe mapped over a sphere) would exist within and be limited to the 2D universe, same as most everything else. So, your idea of not being able to see past the horizon doesnt even apply, as the universe would have to have thickness in the mapped plane, which it doesnt. No, this "light" would travel "straight" in this 2D universe ... meaning that by going "straight" it could travel to anywhere on the sphere within the "2D on a sphere" analogy universe.

I think maybe something similar to that may be part of why they say space-time is curved? Or maybe it isnt. I *AM* a totally un-educated person on the subject. I just seem to remember that when light in our 3D universe is traveling "straight" ... it really isnt.

Anyway, it is an analogy, and so will break down at some point. I could be totally off-base, but I think you are treating the analogy literally. I'm not saying my understanding is complete either. I'm just a newb.

Flame away!

Lewis

[Chris Peterson]

That whole balloon theory is so overused.

No, it is often misused. It provides an excellent analogy, ala Flatland, of the 3D appearance of a 4D universe.

If our galaxies were all on a two-dimensional plane, aren't there 'sides' to the balloon?

Because galaxies are not on a two-dimensional plane. They are on (or in) a three-dimensional volume.

That doesn't make sense. People use the balloon theory to explain the distances of galaxies and why they are moving apart from each other, but a balloon has a center (although the air comes from a hole on one end of the balloon). If the Big Bang were anything like that, wouldnt we just see galaxies that were visible within our own curve of the area we are on 'upon this supposed two-dimensional surface' if the universe is expanding as in this fashion?

The balloon is 3D, but its surface is 2D. A 2D inhabitant of that surface would not easily be able to tell that he lived on a curved surface. He would be able to see as far as light time allowed; nothing would be invisible because of the curvature since light would also follow the surface. We appear to live in a 4D universe, but only see its three spatial dimensions, which are a surface (volume) expanding outwards from a point that doesn't lie in 3D space. We can see as far as light time permits, aka the visible Universe.

Or would the material of this 'balloon' effect be an invisible substance that is allowing us to see through the 'fourth dimensional plane' to the galaxies on the 'other side' of the balloon shape?

You're taking the analogy too far. It's an analogy of geometry; there need be no substance, either of the balloon or its interior.

Also, not all galaxies are moving away from each other, and on vast scales, clumping is happening in a manner that is nothing near what an expanding balloon does on its surface to 'inked' in points that would move away from eachother if the 'balloon' expanded.

Again, the analogy is intended to explain how space expands. There is structure in the Universe because gravity is also operating, and it is stronger than the expansion of space over moderate to short distances. Instead of painting dots on the balloon, imagine its surface is covered with little magnets representing galaxies. Close galaxies would stay close because of mutual attraction, even as space grows around them.

[apodman]

And the dipoles in the little magnets drive the "Mirror, Mirror" effect.

(To boldly go where no analogy has gone before.)