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Just Wild About Harry
Q&A with Harry Ferguson
Astronomer, Space Telescope Science Institute

We asked for your feedback and boy did you deliver! During our first question and answer forum, more than 150 astronomy buffs submitted their questions to astronomer Dr. Harry Ferguson of the Space Telescope Science Institute (STSI). To help answer some of the great questions submitted by "Mysteries of Deep Space" viewers, Dr. Ferguson has enlisted the aid of several of his peers at the STSI, all of whom were responsible for planning and analyzing the Hubble Deep Field (HDF) observations. In addition to Harry Ferguson, respondents in this forum include:

And don't forget Dr. Ferguson himself, who was one of the architects of the ground-breaking HDF observations, taken with the Hubble Space Telescope in December 1995. Dr. Ferguson specializes in the study of very faint, distant galaxies. He received his undergraduate degree in Astrophysics from Harvard University in 1981, his PhD from Johns Hopkins in 1990, and worked as a postdoctoral fellow at Cambridge University before joining the staff at the STSI in 1993.

In the following forum, questions posed by visitors to this Web site appear in plain text, while the experts' answers appear in bold text. Each response also contains the initials of the scientist who answered the question.

Tom Kasprovich
Granite City, Illinois

I just finished watching the PBS special on deep space. My question concerns the farthest galaxies we are able to see. We are seeing them as they were say 12 billion years ago. Where are those galaxies now. They are obviously in a different location now and we won't see the light from that location till far in the future.

That's exactly right, Tom. We can only see those galaxies as they were. We can only speculate about what those galaxies look like now. Presumably, they look like the galaxies that are right around us (only, say, a billion light years away). But we really haven't got a good handle on how the distant galaxies evolved into the ones we see nearby. We have to piece the puzzle together by studying galaxies at intermediate distances.

By the way, the aliens or whatever on those galaxies we see in the Hubble Deep Field, see our galaxy as it was 12 billion years ago. So all we have to do is figure out some way to ask them what the Milky Way looked like :)

Best Regards,

Name and City Withheld



There is probably a lot of stuff out there we haven't seen yet. However, it's not a matter of light not having time to reach us from places that are more distant. Because we can see the microwave background all around us, we know that radiation from the era before stars and galaxies formed has already reached the earth. What we are missing is both the very small galaxies and possibly the individual isolated stars that formed when the universe was very young. And, for very distant galaxies, we are seeing them as they were, not as they are. So we are missing the chronology of evolution, and have to try to piece it together by studying the properties of galaxies at the whole range of distances from very nearby to 15 billion light years away.

Because stars have finite lifetimes, it is unlikely that the universe will ever reach a phase where the whole night sky is filled with stars.

The universe encompasses all of what exists in the dimensions that we can sense (space and time). There could be other universes, or things "outside" our universe that are completely disconnected from our set of dimensions. I don't know how we would ever find out, but it is food for thought.

Best Regards,

Mark Guthrie
Victoria Texas

Dr. Ferguson,

After watching the first episode last night, I'm troubled by what I believe is a simple question. If we know the known universe started with the big bang, where's the starting point? There has to be a "ground zero" doesn't there? If we do know where it all started, how far are we from the center?

Respectfully Submitted,
Mark Guthrie

Everything in the universe was part of the big bang. There is no center.

The analogy that only partially works in helping to visualize this is to take a look at a globe. Imagine you are a two-dimensional being. You live only on the surface of the globe. You cannot move in toward the center of the globe, and you cannot leave its surface. In fact, the only dimensions you can sense at all are along the surface. So, now, where is the center of your world? Put another way, where is the center of the SURFACE of the earth?

Now imagine that the globe is actually a balloon and someone starts to blow it up. All the continents start to move apart. The ones that are further away from where you are standing move away faster. Your whole world is expanding. Yet it doesn't have a center in the two dimensions you are familiar with.

The universe works the same way, but in three dimensions.

Best Regards,

Bryan Evans
White Rock, B.C.

Dr. Ferguson, I have a question re the big bang that has been bothering me for a long time. If everything began with a tremendous explosion with everything accelerating away from everything else at fantastic speed there, then all the matter of the universe should be gathered in a thin band like the skin of a bubble. So if we look back towards the origin of the big bang, there should be a large void until we see the other side of the bubble where the matter that has been travelling out from the big bang is located (the other side of the bubble) and if we use that distance in order to ascertain the age of the universe, should we not divide that number by 2. Hope I've explained myself clearly enough for you to understand my delima. Thank you

The big bang isn't really an explosion. It is a uniform expansion of space. A better analogy than an explosion is the rubber sheet that Margaret Geller showed in the first episode of the Mysteries of Deep space. When she stretched the rubber sheet, all the stars got further apart. Another analogy is those little dehydrated sponges the size of a pill that expand to a much larger size when put in water. Unlike the sponge and the rubber sheet, the universe has no center. All points in space are equivalent.

Best Regards,

Mike Walker

Dr Ferguson,
At the unveiling of the Hubble Deep Field photographs, a couple of the astronomers were seen extrapolating the results of the Deep Field observations to the whole sky, coming up with a massive 50 billion observable galaxies from Earth (assuming an uninterrupted view, of course).

Is that extrapolation a valid one? Assuming the Universe was created from the Big Bang - the galaxies must all be rushing outwards from a single point, and as it is highly unlikely that our own galaxy is located in the center, I would have thought that there should be more observable galaxies if we look back towards our point of origin, and fewer if we look in the direction we are moving. Is that the case.

Secondly, do we know how fast the Universe is expanding? If it is only at a fraction of the speed of light, then surely we should only be able to see as far as the distance light has travelled since the Universe came into being, and that distance would be less than 90% of the way back to the Big Bang as mentioned on the program. If that is not the case, what is the theoretical limit to our observations? Could be actually see the Big Bang itself if we had a big enough telescope?

When astronomers talk about an expanding Universe, I expect most people, myself included, think of something like expanding ripples on a lake surface and make assumptions based on that model. I suspect reality is more complex, but I do not have a good idea as to what it can be!

Mike Walker

As one of the astronomers shown in that picture, I can't give you an unbiased answer. I believe the extrapolation is valid insofar as we knew what fraction of sky we observed, and simply had to multiply by the ratio of the area of the whole sky to the area of our tiny pencilbeam. However, a lot of subtleties are glossed over in going from the number 50 billion, which is the number of galaxies we could see if we observed to the same depth with Hubble over the whole sky and portions of our own galaxy weren't in the way (incidentally, it would take about 600,000 years of observations to do this). To estimate the total number of galaxies in the universe, we would also have to guess how many there are that are fainter than ones we detected, and how many were too distant to see. We can make educated guesses, but that's about it.

There is no center to the universe, and no identifiable point of origin of the big bang. The expansion of the universe is uniform, like the stretching of the rubber sheet shown in the show. Think of yourself living on the surface of a balloon covered with a bunch of dots (galaxies) spaced at random. As the balloon inflates, all the dots move apart. However whatever direction you look, you will see about the same number of dots.

It's a bit hard to answer your second question, since we are in a sense "inside" the big bang. There are portions of the universe that are disconnected from us, in the sense that it would take longer than the age of the universe for light to reach us from these distances. However, the real reason we cannot see back to the big bang no matter how large a telescope we build is that our view is blocked. Right after the big bang and up through the first 300,000 years or so, matter in the universe was so hot that it only existed as a soup of protons, neutrons, electrons, photons (light), and a few other subatomic particles. Under these conditions, a beam of light (what we call a photon) could not travel far before hitting one of the particles. When such a collision occurs the light is scattered in another direction (or in some cases, actually creates a new particle of matter). With the universe in such a state, one can only "see" things which are happening out to a distance equal to the typical path length between these matter-photon collisions - which at that time was a very, very small distance. It was not unlike being in a very thick fog. Thus when we look back at this fog, we can only see a very small depth into it.

This fog shows up as very faint microwave and radio radiation (a fraction of the "snow" on your television set comes from the cosmic microwave background). This radiation comes from every direction, because, as I said, we are essentially inside the big bang. The radiation from different parts in the sky is uniform to one part in 100,000. However, small non-uniformities have been picked up. The COBE satellite was the first to detect the ripples on the microwave background. Satellites are in the works now to map the signal in detail. The nature of the fluctuations will provide an extremely good probe of the physics in the early universe. So even though we can't see the big bang itself, we can learn about what was going at very early times.

Best Regards,

Colin Glassey
San Jose, CA

Are there any good models for the distribution of elements within our own galaxy? Current theories of stellar evolution suggest that all the elements beyond Hydrogen (and Helium) are "produced" in various types of stellar explosions. Does this then imply that the suns close to the center of our own galaxy are "poor" in the heavier elements, since they are much older than those suns in the spiral arms?

There are models. It is debatable whether there are any "good" models. Most of the models make very simple assumptions for how primordial gas fell into the galaxy as it was forming, how and when stars formed out of the gas, and what range of masses these stars had. The models also make assumptions for how the chemical elements get distributed as different generations of stars form and die. In fact, we have only a rudimentary understanding of these different physical processes, so the models are only as good as our guesses.

Interestingly, the stars near the center of our galaxy and other galaxies are very rich in heavier elements, but you are right they are in general much older than suns in the spiral arms. The favored explanation is that the stars in the center formed rapidly, enriching the gas around them to high levels within about a billion years. The stars that produce Oxygen, Magnesium, and Silicon live only about 10 million years, so there is time for many generations of them to pass. Interestingly, these stars don't produce much iron. That comes later from the explosions of coalescing binary stars (according to stellar-evolution theory). So this model of the formation of the center of our galaxy predicts that the Oxygen to Iron ratio in stars there should be higher than in the sun. This is exactly what is observed. That doesn't mean the model is right, but it might be on the right track.

Best Regards,

Vini Falciano
Rochester, NY

Dr. Ferguson,
Is there a better model to understanding the images we receive from space (billions of light years in the past)?

By the time we get this "light" from distant galaxies, it is already millions of years old (which would imply from a "smaller" universe closer to the Big Bang), but yet these galaxies are already spread across the . . . universe, as it were.

Why don't we see the beginning? (Since I don't know the numbers, you can relay the Math. I can handle it!

I tried the time vs. 3D concept. I'm just not getting' it!

The furthest back we can look is to a time when the universe was about 300,000 years old. Earlier than that, the universe was made up of subatomic particles, and light couldn't travel very far before being scattered off in another direction.

There are all sorts of analogies to the expanding universe (living on the surface of an inflating balloon is a good two-dimensional analogy). However, none of them are wonderfully satisfactory, I agree.

Best Regards,

Howard Alter
Toronto, Ontario, Canada

Dear Dr. Ferguson,
During this fascinating program one of the scientists involved in the Hubble Deep Field observations described having discovered galaxies up to 14 billion light years away, the image therefore being of the universe at its infancy. How, I wonder, did these particular galaxies come to be so far from the Milky Way - from the explanation given of the Big Bang, 14 billion years ago galaxies must have been much closer together; indeed, only a billion years before they had been part of the same mass. It would appear, therefore, that our galaxy and those distant ones must have been travelling away from each other at practically the speed of light to be 14 billion light years apart after only 15 billion years. Please help me with my confusion about this.

Howard Alter

Yes, the relative motion of our galaxy relative to the most distant seen in the Hubble Deep Field is nearly the speed of light.

Best Regards,

Name and City Withheld

1. Is a Galaxy a group of planets or early form of solar system?

2. We see movies in theaters about outer space and the types of space craft is awesome; even though it is just a movie, these are cool ideas and the question that comes to mind is why can't someone build a much faster space shuttle so that it wouldn't take years to visit other planets??

3. All joking a side, is it possible for someone to buy$$$ a planet? The Moon for Example?

One of my future goals someday is to become an astronaut and the mysteries that it holds encourages me to keep this dream alive. One draw back however is the speed that the shuttle moves. Man, if only there would be a faster shuttle!!

Thanks for your time!

1) Galaxies are huge assemblages of stars and planets. Our galaxy has about 100 billion stars. It probably has even more planets (our own solar system has nine, and we only have one star).

2) Even if you could build a space shuttle that could travel close to the speed of light, it would still take quite a long time to speed up and slow down (otherwise you kill the astronauts), so a trip to even the nearest star is something that would take generations -- and lots of $$.

3) Can I interest you in some beachside property around Alpha-Centauri?

Best Regards,

David W. Smith
Conshohocken, PA

How is the size and distance to various space objects determined?

There are lots of different ways. None are very precise. The most direct way is to measure "parallax," that is the change in the position of a star when the earth is on one side of the sun, compared to when the earth is on the other side of the sun. This can only be done for fairly nearby stars. For distant objects like galaxies, astronomers try to find "standard candles," such as certain types of variable stars which are known to have a fairly constant brightness from studies of our own galaxy. By comparing the brightness of the stars in the distant galaxy to the brightness of the same type of stars in our own galaxy, we can get the distance.

Best Regards,

Jake Tapia
San Francisco, CA

Your program said that there are over 50billion estimated galaxies in space...I was wondering...When you look up into the night sky with the naked eye, are most of the lights up there stars...or galaxies?? I am starting to think that a star is never alone...but always included in a galaxy...I hope I do not sound to un (space) educated....Thanks

Most of the objects you can see with your plain eye are stars in our own galaxy. If you go to the southern hemisphere and look up on a dark night, you will see our two nearest neighboring galaxies, the "Magellanic Clouds." These are actually "dwarf galaxies" orbiting around the Milky Way. Each contains only a few billion stars. In the northern hemisphere, you can make out our nearest big neighbor, the Andromeda galaxy, if you know where to look. It's hopeless to try this in the city; you need a really dark sky. The Andromeda galaxy is 2.5 million light years away.

There may be some stars out there that are not included in galaxies. In fact, we think we have found some with Hubble. However, they were most likely born in a galaxy and thrown off in an interaction with another galaxy. The densest concentration of such intergalactic stars is in the Virgo Cluster, 60 million light years away.

Best Regards,

Mickey Finn
Mesa Arizona

If one would blow up a balloon & then let it go in the vacuum of space what would happen to the balloon?


Because there is no pressure outside the balloon, it would suddenly expand, and then, since latex is not all that strong, it would (soundlessly) pop.

Best Regards,

Brian Graham
Columbia, SC

1. Astronomers who use the famous red shift as a means of determining distance of a stellar object must rely on the quality of the (visual) spectrum data they receive. There lies the crux of the problem-- how reliable is the faint light emitted by a galaxy ninety percent to the edge of the observable universe?

If astronomers get a photon per second (or less), how can a shift be determined with any degree of accuracy? It's the equivalent of using a very powerful telephoto lens to photograph a person 20 miles distant. Sure, you might recognize certain general features, but what about skin color, etc.?

This may be the best means we have at the moment, but isn't the red shift measure progressively limited, the closer to the creation point we focus? Put another way, without any other yardstick, how accurate can we say the red shift is?

2. What was the cause of the surface aberration found on the Hubble mirror, and how was a corrective lens designed, given that the aberration probably could not be measured by any physical means short of removing the mirror and returning it to earth?

3. Is optical astronomy still the most comprehensive avenue of information for science, or has it become only a small portion of the research done today?

1) The redshift itself can be measured very precisely if you can get enough photons. The features we use to measure redshifts are emission and absorption lines of common elements like Hydrogen and Oxygen. Unless the physics of atoms changes over time, the comparison of the positions of the spectral lines to their positions in laboratory measurements on earth yields a redshift. It is not out of the question that physics on the scale of atoms evolves as the universe evolves. However, so far, every test we have been able to do indicates that the universe is well-behaved when it comes to atoms.

You are absolutely right that you need to get lots of photons to measure a redshift. Even the largest telescopes on earth can only measure redshifts for the brightest 150 or so of the 3000 galaxies detected in the Hubble Deep Field. For galaxies fainter than that, we can estimate an approximate redshift from their color. But that is a very imprecise estimate. It is fairly useful, though, if you want to divide up the galaxies we see in the images into those that are closer than half-way across the universe and those that are further away. These sorts of imprecise redshift estimates are actually teaching us a lot about how and when galaxies form.

2) The Hubble mirror was originally polished to the wrong figure (it had what is known as "spherical aberration," which means all the rays of light hitting different portions of the mirror did not come to the same focus. The cause of this disaster was a washer that was put in backwards in a piece of test equipment that was used during the fabrication of the mirror. Precise measurements of the distorted images of stars taken with the flawed mirror were used to determine the nature of the aberration and the correction needed.

3) Optical astronomy is only a small portion of research being done today. It has the widest public appeal, for obvious reasons, but much of our information about the universe comes from telescopes that observe in the radio, infrared, or x-ray regions of the spectrum.

Best Regards,

Kirk Payne
Redmond, OR

Does anyone really know what gravity is or can we just describe its effects? Could a understanding of the mechanics of gravity possibly force a new theory of relativity?

We can just describe its effects. It is hoped that something along the lines of string theory will lead to a real understanding of what gravity is.

Best Regards,

Rick Satcher
Gainesville, FL

What are your current views on the hubble constant and will there ever be a consensus on it? Also, why are astronomers getting such different results? Please keep in mind that I'm only an amateur complicated math theories please!!! Thank you, Rick.

I suspect that the Hubble constant will come in around 65 km per second per Megaparsec, when all the dust settles.

The dust actually is settling a bit already, I think. Most of the recent measurements have been between 50 and 80. The reason there is disagreement is that it is quite tricky to measure the distance to a distant galaxy. For example, one way to estimate the Hubble constant is to measure Cepheid variable stars in a distant galaxy. To be able to see Cepheids, the galaxy has to be closer than about 100 million light years. However, even at that great distance, random motions of galaxies are a fair fraction of their motions due to the overall expansion of the universe. So you have to use the galaxy with the Cepheids to calibrate some other distance measurement that will allow you to estimate the distance to galaxies that are too far away to see individual stars. For the galaxy that has the Cepheids, you have to worry that there might be dust in front of them, or that they might be a little different than Cepheids in our own galaxy. These are just a few of the issues. The reason there is such heated disagreement is that astronomers don't even agree on which issues are important and which aren't.

Best Regards,

Robert Griffin
Ft Thomas, Kentucky

The big bang theory does not account for 'spin'. Only an outward release from centrifugal forces will allow such motion and orbit on a narrow path. If the matter to make planets is manufacture in suns, this centrifugal force is developed as the ever complexing molecules rise and fall to be compressed until finally spewed forth as molten liquid.Only a 'black hole'could compress the energy represented by temperature into the stuff of galaxies. Quasars and empty black holes would be failed, or forming galaxies. Saying that the universe is any number of billions of years old is false, it has no age we could calculate.

You are right that the big bang theory does not account for spin. The spin of astronomical objects like galaxies and solar systems probably came about as the result of tidal interactions between gas clouds that existed as these entities were forming.

Most of the available evidence supports a finite age to the universe. The concept of age does get a bit tricky in curved space time, but if the big-bang theory is right, the age is calculable once you define what it is you want to call the age.

Best Regards,

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