The Milky Way: A Tourist's Guide

[Jamie]

What a difference a couple of years make. Buzz Lightyear has given up on the whole "infinity and beyond" thing, instead focusing on his own backyard. (Actually, the front yard of his owner, Andy, and the toys that get stolen therefrom.)

In the same time frame, astronomers have undergone a similar transition of thinking.

Once looking far off into the deep black in search of answers to black holes, the big thinkers of space have returned, if not to Earth, at least to the cosmic backyard, solidifying the theory that there's a big ol' gravity monster just 26,000 light-years away, in the very center of our home galaxy. (One light-year is equal to 5.88 trillion miles, or 9.46 trillion kilometers.)

The revelation has researchers scrambling to study what is by far the closest black hole around. It also raises a simple question: What do we really know about our own galaxy?

In seven separate articles in the January 7 issue of the journal Science, all of this new thinking is discussed.

A virtual tour: First stop, the halo

Were an intergalactic traveler to approach the Milky Way edge-on, she would first encounter a vast but sparsely populated galactic halo, with a few lone stars and about 170 star clusters, plus various clouds of gas, all of which does not rotate in sync with the rest of the galaxy.

A less obvious aspect to this halo would be an incredible amount of dark matter, stuff that can't be seen but is thought to comprise the bulk of the Milky Way's mass.

Our dauntless explorer would find that the stars in the halo are mostly ancient -- 12 billion years old and older. They would not appear to rotate around the galactic center in any organized manner, though gravitationally bound -- as such, the stars in the halo are moving every which way.

These old stars and clusters, and the streaming gas clouds, tell many stories. For one, they may represent the age of the galaxy itself.

They are also evidence of galaxy gobbling. Researchers are now convinced that the Milky Way has built itself up over the eons, in part, by swallowing smaller galaxies. Streams of stars and clouds of gas represent the remnants of this galactic feasting.

Recent evidence also shows that dwarf galaxies are still being pulled in. The spherical galactic halo, constituting the outermost reaches of the galaxy, is the Ellis Island for all these incoming galaxies.

"Direct and indirect evidence now available of such events leaves no doubt that the accretion of satellites and fragments into the galaxy's halo has been a continuing process since early in its formation," writes Roland Buser in one of the Science articles.

Ongoing process

The feast continues. Streams of gas extending in front of and behind two nearby galaxies, the Large and Small Magellanic clouds, show that the galaxies are being distorted as they pass through our galaxy.

The two galaxies are expected to spiral into the Milky Way for the next 10 billion years.

Another nearby galaxy, the Sagittarius dwarf, is also orbiting our own, stretched into a long, odd shape by the Milky Way's gravity. The tiny Sagittarius, one-thousandth the size of our galaxy, has only about 750 million years to go before it is swallowed whole.

And at a meeting of the American Astronomical Society next week, scientists will present further evidence that miniature galaxies, which may be leftovers from the earliest days of the universe, are raining into the Milky Way continuously, providing fuel for new star formation.

Next stop: The spiral disk

Next, our traveler would encounter a more heavily populated, fairly flat disk of gas and stars known as the thick disk. Like the outer bands of a hurricane, this pancake of stuff would be seen spiraling slowly around the center of the galaxy.

Further in, the visitor would find a flatter disk of stars, like a thin pancake nestled inside the thicker one. The stars in this thin disk rotate even more rapidly around the galactic center, hence the more compact nature of this inner pancake. If our traveler knows her celestial coordinates, she might run across our own sun soon after entering this thin disk.

Further in is yet another disk, known as the "extreme disk." Stars and clouds of gas are moving even more rapidly. The stars here are younger, ranging in age from 1 billion to 10 billion years old.

Researchers suspect the galaxy formed out of a simple thin disk, building itself up via mergers with smaller galaxies and gradually developing the more complex spiral that is nestled inside the present halo. None of these regions is entirely distinct, but instead they all overlap.

Approaching the middle now, feeling a greater pull of gravity, the explorer would find a "central bulge" of stars, all obscuring the unexplained center of our galaxy, to which the far more numerous stars are bound ever more strongly.

Finally, the black hole

At the center of our Milky Way is, almost certainly, a gigantic black hole from which our traveler, however intrepid, would want to stay clear. Researchers have suspected the matter-gobbling monster, called Sagittarius A*, for 25 years, but only recently have observations confirmed its existence -- though still not by any direct methods.

"In the last two years, scientists have firmed up evidence for this super-sized black hole, which has swallowed enough gas and dust to equal the mass of more than two million suns," writes Erik Stokstad in another Science article.

Stokstad notes that while it is huge, our black hole is small compared to others, which can be billions of times as massive as the sun.

The evidence for the black hole involves measurements of stars that orbit at incredible speeds -- 50 times faster than Earth orbits the sun -- in a tightly packed area of the central bulge. The gravity required to keep these stars in such a fast, tight orbit is calculable, and the tiny area into which it must fit indicates that it has to be a black hole, experts say.

But researchers have yet to figure out how the black hole works and where all the matter and energy goes once it is sucked inward. Seeing things in the center of our galaxy is difficult -- it's all obscured from our view by the central bulge -- and little evidence exists showing the outflow of high-energy radiation that has been seen coming from other black holes.

How the black hole evolved

Researchers suspect the Milky Way's black hole formed early on, when a large cloud of gas collapsed on itself, Stokstad writes. It probably started out small, growing over billions of years while gradually feeding on inflowing clouds of gas.

Periodically, stars would be sucked in and swallowed in dramatic fashion. A star would be ripped apart and a flash of radiation would briefly outshine all the rest of the stars in the Milky Way combined.

Are we safe from this monster? Quite, Stokstad writes. While tremendous, the gravity of a black hole is not strong enough to pull in stars from the far reaches of its galaxy.

Still, we know so little

If it seems as though scientists are painting a pretty complete picture of the galaxy, consider this: The movements of stars tell researchers that they have yet to find about 95 percent of the mass in the Milky Way.

Much of the dark matter, as researchers call it, is thought to exist in a vast sphere that surrounds the visible portions of the galaxy. It could be composed of ancient dwarf stars that can't be seen or, as some suggest, it might exist in the form of small, elementary particles that aren't yet understood.

By figuring out the composition and role of dark matter, and understanding how the central black hole works, researchers will begin to put some of the final pieces of our galactic puzzle together.

The Chandra X-ray Observatory, launched last year, is one of three new telescopes that will probe the Milky Way for clues to all this.

Eventually, dust

While our home galaxy has become what it is by picking off smaller galaxies, it appears we are due for a dose of our own medicine.

Right now, the Andromeda Galaxy and the Milky Way are rushing toward each other at more than 310,000 miles (500,000 kilometers) per hour, explains Robert Irion in another Science article.

The pace will pick up as the 2.5 million-light-year gap closes.

"The momentum of the galaxies will carry them past each other initially, but the gravitational attraction of their dark matter halos will doom them to coalesce," Irion writes. The result won't be pretty. Andromeda is twice as big as the Milky Way and it will "distort our galaxy beyond recognition."

Irion says we've got between 1 billion and 3 billion years before our beloved spiral becomes a wrecked ellipse.

Meanwhile, colliding gas clouds will form new suns. And our own sun, along with the nine planets, might be cast off into virtual nothingness, so far away from the activity that the night sky on Earth would no longer be full of stars.

Worse, we could be flung inward, where the night would be bright as day from all the exploding stars, and where the intense radiation might render life on our formerly hospitable world impossible.

For either scenario to have any effect on Earth, our planet would have to still be around when the galactic merger takes place. And that's not certain.

Many experts think our own sun may have already exploded by then, turning Earth and any remaining Earthlings back into dust anyway. Or, as other theories suggest, our sun might have 6 billion years left to it.

By Robert Roy Britt

Senior Science Writer

[OXO]

Very interesting read that Jamie thanks for posting it mate

[lunator]

Jamie

Good find, very interesting.

Cheers

Ian

[Astroman]

"But researchers have yet to figure out how the black hole works and where all the matter and energy goes once it is sucked inward. Seeing things in the center of our galaxy is difficult -- it's all obscured from our view by the central bulge -- and little evidence exists showing the outflow of high-energy radiation that has been seen coming from other black holes."

"Many experts think our own sun may have already exploded by then, turning Earth and any remaining Earthlings back into dust anyway. Or, as other theories suggest, our sun might have 6 billion years left to it."

Um, I hate to always be doing this, but some of this stuff is just wrong. Plus, this article was apparently written in 2000, since Chandra was launched on July 23, 1999.

A couple things in the first paragraph I quoted;

Black holes do not "suck-in" material! They fall in if and only if they are close enough or on a trajectory that intercepts the event horizon. It's a subtle distinction, but important.

Our view of the core is NOT obscured by the central bulge. It is obscured well before the edge of that structure by intervening dust and gas. In the visual range, we can see only about 1/3 of the way to the core, or ~6,000 ly. Infrared, radio and x-rays get us closer, with x-rays beeing used to gather the data about the rapidly orbiting stars mentioned. Hence, Chandra, RXTE, MMXT and so on.

One paragraph I didn't quote says our central BH is by far the closest. This is also not true. There are stellar mass BH's all around us. Cygnus X-1 for instance. Cassiopiea-A too and others. Vela-A and so on.

The other paragraph I quoted suggests, or outright states, that our Sun will have "exploded" in ~6 billion years. This is false. Sol does not contain enough mass to explode. It will slowly expire, expand and end up as a plaentary nebula. The expansion is thought to begin in only about 1 million years or so. We won't have to survive massive radiation, as we will have been living inside the star for eons before the end.

I'd like to know the year the Science magazine articles were published so I can check them for a better tome.

No offense, SkyGuy.

[Jamie]

No offence taken mate.

If i post anything like this and you wish to question what has been posted

you are more than welcome.

At least that way i get to read another point of view.

Here is a link to the said article for you to investigate further.

Let me know what you come up with.

http://www.space.com/news/milky_way_000104.html

[OXO]

The Theory of Black Holes and the mechanics is still incomplete and nothing has been proven has it?

James

[Jamie]

To be quite honest all the talk about black holes and all the other

subjects that get scrutinised are just that "someones theories"

Some just seem more acceptable than others.

Or maybe we just choose to believe the ones that we think we understand.

(kind of hard to believe in anything you don't know anything about or understand)

[Astroman]

Thanks for the link, SkyGuy. I should have figured it was Space.com. It's typical of their writing style. Posted January, 2000, we've learned a lot about the BH at the MW core since then. That, and a lot of other things.

James: The theories concerning BH's are not complete, as you say. But there is a LOT of evidence that confirms them. I believe I posted a link about them in the Links section, or maybe the Learning Zone, I'm not sure. But there's quite a lot of evidence in favor of the basic precepts. Lots has been "proven", but still a lot remains to be confirmed.

[OXO]

James: The theories concerning BH's are not complete, as you say. But there is a LOT of evidence that confirms them. I believe I posted a link about them in the Links section, or maybe the Learning Zone, I'm not sure. But there's quite a lot of evidence in favor of the basic precepts. Lots has been "proven", but still a lot remains to be confirmed.

Thanks Astroman, i must confess my reading of the Theory's attached to BH's stopped after seeing so many of them change with what seemed within the hour, my faith in some aspects of Astronomy's Theoretical concerns is pretty down beat by this and i take a lot of it with a pinch of salt but thats just me not saying iam right or wrong hehe. Until something is Fact then i cant take a lot them the seriously :?.

James

[Astroman]

Actually, the fact that the theory changes and adjusts to observations and studies should bolster your confidence in it, James. That's what the scientific method is all about. You can't say something is "just a theory and not a fact", because a real theory is based on as many facts as possible. And these facts are observable and repeatable and must point to the conclusion of the theory in question. If any parts of a theory don't fit, they are discarded and vice versa. If new evidence points in a different direction, that evidence is incorporated and the theory adapts.

Check out this link about the scientific method from the University of Rochester, NY. It does a better job of explaining than I do.

http://teacher.pas.rochester.edu/phy_labs/AppendixE/AppendixE.html

[OXO]

I'll check it later thanks Astroman just got some stuff to do at the moment..

Cheers

James

[Astroman]

I'm not sure perpetual motion has been a theory since the 19th century, Damo. Understanding of friction has kind of killed that one.

Ok James. The link basically just expands on what I wrote here. The basic steps are, observation of a phenomenon, formation of a hypothesis, hypothesis leads to prediction of other, related phenomena and using experiments to test predictions.

[OXO]

Hi Astroman,

I thought i knew the concept behind how people come to there conclusions with regards there theory's and i do, the problem is still the fact the theory's change so often as does the evidence from what i have seen example someone says one thing another will say something completely differant thats gone on since the age of science both theory's will hold there own in a court of science with the Mathematics and evidence they present. Now this is were i begin to wonder well someone else is going to chip in and say something differant to the two before and it goes on and on. Of course theres some theory's without which the future of our understanding of all things science wouldn't of come to hand without them i guess thats because of the inquisitive nature of humanity and i can fully understand that. So what iam saying is that with some many theory's to one answer i loose my faith in them for the reasons i have stated..

James

[Kaptain Klevtsov]

Depends what you mean WRT "power machines" Damo. Your fridge is such an animal. If you measure how much electrical power you put in and the amount of heat that comes off the coil on the back you will find that there is more coming out than you put in. The "extra" bit comes out of your fridge, that's why it's cold inside.

A lot of hokum has been thrown out over the last couple of hundred years but some of it needs to be reassessed.

To make a device that gives out "free" energy would be great but it is not theoretically possible. What is possible is to use energy more efficiently. In the case of a heat pump (fridge) you can use say 5kW to attempt to cool down your garden. You will then get about 7kW of heat to either throw away or to heat your house. That kind of "free" energy is not only possible, but being sold commercially even as we speak.

Captain Chaos

[Kaptain Klevtsov]

I know its not self sufficient, my point was that if you cool "outside" then you get more heat to heat your house than you pay for. And that IS free energy.

BUT it is nowhere close to the first or second kind of perpetual motion machine. If you close the loop, ie put it around "outside" as well as the heat pump, you will get zero gain. The clever bit is in moving the heat from where you dont want it (outside) to where you do want it (inside).

Captain Chaos

[Kaptain Klevtsov]

I think if we take the machine out of the equation for one moment, we can get a better understanding of why some theories can only remain a theory.

Say we go to deepest darkest space and by some quirk of feat find a complete vacuum with absolutely no gravity...... If we set a disc spinning we would expect it to spin forever, due to no outside forces able to take away any 'work' that was initially given to it.

But we can not say that it would do this because we can neither measure the disk nor live for an infinite amount years. (Maybe we could live forever, but how would we know if we can live forever if we can't come to a conclusion?)

And no magnetic field, we don't want eddy currents to mess with it.

To measure the spinning over a short space of time compared to "forever" would give a theoretical result as you say, but we need to have these theories as a basis for making stuff in the day to day world. The "theory" only gets upgraded to "law" after many observations have been made and they all correlate with the theory. In this case you can go back to Newton's laws. They have been messed with a little since Einstein, but the law says that the spinning disk will keep on spinning until something makes it do otherwise. If it changes velocity, then we need to find out what changed it's velocity, it cannot change velocity without any outside influence.

Current physical laws that are used in everyday things are based on ways of defining the obvious, Newton started some stuff off, but was standing on the shoulders of giants when he did so.

In short, we have to make assumptions based on what we see and what is scientifically "obvious", Newton did that, got the T shirt (well probably a new wig or something) and got the detail wrong. It doesn't mean that we use E=MC2 to work out how to aim a shot at pool, as he was as near as makes no matter in this.

It's only your theory that you are even reading this, it doesn't really exist outside your conciousness, neither does the rest of the universe. In fact you probably don't exist, really.

Captain Chaos

Like the man said, Deep and dark