Astronomy

The Year 2012

Question:

With all this talk about 2012, I was wondering if there was any truth to it scientifically. One theory I have heard is that during this date the planets will be aligned in a certain way that will cause the earth to shift positions causing natural disasters everywhere and resulting in the end of civilization. I’ve also heard something about a black hole and the earth going through it causing the same results. Is there any truth to this? Can we actually forecast the position of the planets and other elements in space in the future and the impact this can have on earth? Thank you

Answer:

There is no truth to the idea that there is going to be some astronomical disaster sometime in 2012. This sort of thing pops up every few years; the current one is helped by a recently-released movie that picks up the theme. But to the best of our knowledge, there is no great calamity about to occur.

We can forecast the positions of the planets in our solar system very well using Newton’s law of gravity, and can do even better using Einstein’s version. If there were a black hole with the potential to do harm to the Earth within our solar system, we would know about it -- it would throw these calculations off and we wouldn’t be able to navigate spacecraft to the other planets as well as we can!

Models of the past solar system suggest that the gravity of the other planets has affected the Earth’s orbit, but to a small extent. These kinds of changes happen gradually, over thousands or millions of years, not in one big event on one particular day.

The only astronomical event that is likely to cause major problems on Earth is the impact of an asteroid or comet. Astronomers are working hard on finding all of these “potentially hazardous objects” and, again, as far as we know nothing big is going to hit the Earth in 2012.

- Pauline Barmby

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Earth's Orbit

Question:

Is it true that it would be disastrous if the earth deviated from its orbit by a few millimeters ?

Answer:

In answer to your question, my colleagues and I all agree that the earth is already subject to oscillations in its orbit. It seems that the greatest effects caused by these oscillations have been the ice ages. Therefore, the effects can be quite serious, but not completely devastating.

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How does a black hole work and suck up things?

Question:

How does a black hole work and suck up things?

Answer:

Black holes are places with such strong gravity that nothing can escape from them, not even light. That’s why they’re called “black”.

What does it mean for something to “escape” from something else? Imagine throwing a ball up in the air -- it falls back down. Now imagine throwing it harder -- it will still fall down, but after spending a longer time in the air. If you could throw a ball very fast -- 11 kilometres per second -- then it wouldn’t come back down and we’d say it had escaped.

The escape velocity of an object in the universe, like the Moon, or the Sun, depends on how massive the object is, and also how big around it is. Objects with a lot of mass packed into a small size have very high escape velocities. A black hole is an object formed when a lot of matter collapses into a very small region, and its escape velocity is equal to the speed of light. Since Einstein’s theory of relativity tells us that nothing can go faster than light, nothing can escape a black hole.

This doesn’t mean that black holes are giant vacuum cleaners that go around sucking up everything though. The escape velocity from an object goes down as you get further from its centre. It’s only close to the black hole that the escape velocity is greater than light speed. Outside this distance, the black hole acts like any other object that has gravity (stars, planets, etc) and it’s perfectly possible for something else to orbit a black hole safely and not fall in. We can actually see stars doing just this around the black hole in the centre of the Milky Way.

There are lots of great websites about black holes if you want to learn more. I like this one: http://hubblesite.org/explore_astronomy/black_holes/

- Pauline Barmby

Answer 2:

Most black holes are dead stars which have run out of energy sources so that gravity has caused the star to collapse inside its own “event horizon”.  Think of the event horizon as the ’surface of no return’.  If you are inside a black hole’s event horizon, you must move faster than the speed of light to escape the black hole’s gravity.  Nothing with mass can travel faster than light, so nothing escapes, including light.  Hence the name “black hole”.

A black hole’s gravity is so strong that it significantly bends the geometry of space and time near it.  (The facts that gravity tells space how to curve, and space tells matter and light how to move, were fundamental predictions of Einstein’s Theory of General Relativity.)

If something is close enough to a black hole, it can be drawn inside the event horizon, never to be seen again.  But if you are far enough away from a black hole, its gravity is almost indistinguishable from a normal star of the same mass.  What if the Sun were to magically suddenly turn into a black hole? That can’t happen, I promise, but if it did, other than getting dark and cold, the Earth would continue to orbit just as it does now.  We’d feel the same gravitational field.  We would not be sucked in.  A black hole is not a cosmic vacuum cleaner that will suck in everything in the Universe.

Clear skies,

- Dr. Jaymie Matthews

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Big Bang

Question:

Which way do I have to turn my head to look toward the Big Bang?

Answer:

Any direction is equally good. Although many people take the words “Big Bang” to mean that there was a big explosion that happened at a specific place and time, the important idea of the Big Bang is that space itself is expanding and cooling. So there was no centre at which some explosion happened.

The radiation left over from when the universe was much hotter is called the cosmic microwave background, and it can be detected in any direction you point your radio telescope.

- Pauline Barmby

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Is there a boundary to the universe?

Question:

Is there a boundary to the universe, and is it possible to speculate about what is beyond it? Space without matter, absolute vacuum, etc.

Answer:

To the best of our knowledge, there is no boundary. Einstein’s theory of general relativity tells us that spacetime can be either infinite in extent or finite but with no boundary. How can it be finite but unbounded?

Imagine a sphere -- it has a finite surface area, but the surface has no edge or boundary. The universe can be like this too, but in 4 dimensions instead of two, which is much harder to imagine.

There are ideas which say that the universe is just one of many universes embedded in a higher-dimensional space (the “multiverse”) but even in that case there don’t have to be boundaries between universes, any more than there are boundaries between left/right and up/down.

These are tricky concepts to convey in a short answer. If you’d like to know more, there are lots of excellent books on the topic of cosmology, the study of the universe. An annotated list is here:

http://www.astro.ucla.edu/~wright/cosmobib.html

- Pauline Barmby

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Universe

Question:

Radiation from the very edge of the universe tells us something about the nature of the objects that sent it 3.7 billion years ago from the time of the Big Bang. Can we speculate about what remains at the edge today, at this moment in time, or have the universe’s earliest galaxies already collapsed and disappeared? In other words, if we just got the letter, does the post office it came from even still exist?

Answer:

By looking out into the universe, we look back in time. The universe is likely more or less the same everywhere, so we can imagine that the regions of the universe we see 13 billion years ago at this present moment look very similar to what our local universe looks like now. Using physics and our present knowledge of how galaxies evolve over billions of years, however, we can extrapolate how those very young galaxies may look today. That said, many of the most massive stars emitting light from long ago likely no longer exist because they would have exhausted their hydrogen fuel well before their light reached us. (New stars in the galaxy would have formed in the mean time, however.)

- Dr. James Di Francesco

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How old is the universe?

Question:

How old is the universe? If light from the infancy of the universe is just getting to us now, at some point, matter or at least the building blocks of matter, had to be going a lot faster than light. If light is faster than all matter, then are we looking… at a reflection, essentially; because we would always be racing up and falling farther behind the big bang explosion and its light. If we are internal to the big bang explosion, then is the light always racing away from us? If we are external to the big bang explosion, did the explosion generate enough power, in frictionless space to get us to go faster than light and only the gravity of the big bang atom, and now universe slowing us down enough to see the light?

Answer:

The universe is approximately 13.75 billion years old. We get this number by looking at the Cosmic Microwave Background (or CMB), which is the light left over from the Big Bang, and also the edge of the Visible Universe. The CMB tells us that the Universe was once in a hot dense state, and the fact that it’s cooler and much less dense now tells us that it’s expanding. The first observational evidence for an expanding Universe was found by Edwin Hubble who noticed that galaxies which were further away also appeared to be moving away from us more quickly. This implied that all these galaxies must have originated at one point, the Big Bang Singularity.

Now, it is indeed true that no object (or particle) can travel faster than the speed of light, but it is possible for space itself to grow faster than the speed of light, since space is not comprised of matter. One way to think about it is that galaxies in space aren’t actually rushing away from us with the expansion of the universe, but they’re in fact stationary in space and that the space itself is expanding faster than the speed of light. Einstein’s theory of Relativity tells us that objects travelling in space cannot go faster than the speed of light relative to each other, but it doesn’t place any limits on how quickly space itself can expand.

Because the speed of light is finite, when we look at objects in space, we’re seeing them as they were when the light first left them. So if a galaxy is 2.5 million light-years away, then we’re looking at it as it was 2.5 million years ago. In addition, because the Universe is expanding, the light will also be stretched so that its colour becomes redder (we call this cosmological redshift). This means that the light from the CMB, which when it was first emitted was in the visible range of light, is now, 13.75 billion years later, in the microwave range of light (essentially radio waves). This light from the infancy of the Universe has been hitting us since it was emitted, but the wavelength of this light has been progressively getting redder as the Universe expanded and as light from further pieces of the Universe was able to reach us.

- Ilana MacDonald

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Why does Venus rotate backwards?

Question:

Why does Venus rotate backwards?

Answer:

Short answer: No one really knows.

Longer answer: There are a couple of leading theories on this issue:

(1) One might expect that a planet will spin in the same direction as it travels because it formed from a disk of material that was rotating in that direction early in the Solar System. However, in the late stages of planet formation, the planets experience impacts with fairly large bodies called ‘planetesimals’, and if one of these planetesimals hits the planet at a glancing angle, it can knock it over (technical term: apply a torque) thereby changing its spin axis. If this is the case, then the final spin direction of a planet will be related to the way it was knocked about in the last stages of planet formation. This theory is also used to explain why Uranus’ spin axis is tilted perpendicular to most of the other planets (its spin axis is in the plane of the solar system) and why the other planets in the solar system have a variety of different spin axis angles (for example, Earth’s spin axis is tilted about 23 degrees from the plane of the solar system).

(2) A planet has to conserve its total angular momentum, which is directly related to its net spin axis. The net spin axis is made up of the spin axis of its core (the iron part of the planet) plus its mantle (the rocky part of the planet) plus its atmosphere. Because Venus is believed to have a liquid core (like the Earth does) and it has a thick atmosphere, its possible for friction forces to exchange angular momentum between the core and the mantle or between the atmosphere and the mantle. This can result in changing the spin axis of the mantle by changing the spin axes of the core and/or atmosphere. So it might be that interactions between the different layers of Venus have resulted in tilting the planet’s mantle so that the mantle is spinning retrograde. Its the mantle spin axis that we equate with the planet’s spin axis since that is the part that we see rotating. In order for this theory to work, it helps that Venus is a slow rotator (a day on Venus is 117 Earth days!).

- Sabine Stanley

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Speed of Light

Question:

My question concerns the speed of light versus the expansion of the universe. When we look at distant galaxies and stars, we see them as they were many millions or billions of years ago. If the universe is expanding today, can we assume that it was smaller back when the light from these distant galaxies first emerged? And if so, would that light not have already reached us since the distance between us and it was shorter back then? I believe the answer is no, so does that mean the universe expands faster than the speed of light, so that the increase in distance between us and the light happens at a greater rate than the speed of light itself?

Answer:

You've got the right idea here. Let's think about light leaving a distant object to travel to us. If the universe were not expanding the amount of time the light would take in years would be equal to the distance in light-years. So we see the object as it was in the past (this is true even for nearby stars in our own Milky Way galaxy which is not expanding). For some very far-away objects, even the age of the universe is not long enough for the light to have had time to reach us.

The universe is expanding, although not necessarily at a constant rate.

So, as you say, that means that light has to catch-up with the increasing distance between us and distant objects. This happens even if the distance between us and distant objects is increasing more slowly than the speed of light -- the light still has to cover the "extra" distance. But it is also possible for the distance to increase faster than light speed; this is not a contradiction of Einstein's special relativity.

A great reference for these kinds of questions is Ned Wright's cosmology tutorial:

http://www.astro.ucla.edu/~wright/cosmolog.htm

- Pauline Barmby

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Moon facts

Question: I am writing a novel about a civilization on the moon, and I need some help with some basic facts! Is the north pole of the moon the side that gets continuous sunlight, and where crater Plaskett is?

Answer: There are places near both lunar poles that receive nearly continuous sunlight. There is usually more interest in the South Pole because there is a greater total area of crater interiors that *never* see sunlight, going down to the bottoms of many craters. Water ice could survive in these perpetual shadows. The same is true for the lunar North Pole, but the total shadowed area is smaller.

Plaskett is near the North Pole of the Moon, but on the Far Side of the Moon.

Question: What is the average temperature during the day and then the night at crater Plaskett?

Answer: The Diviner instrument on the Lunar Reconnaissance Orbiter has measured the temperatures in the shadowed regions are as low as shown that the interiors of some of these shadowed regions are as cold as -248 C (only 25 degrees above absolute zero!). Polar regions that receive nearly constant sunlight have an average temperature of about -170 C (100 degrees above absolute zero) due to the shallow angle at which sunlight strikes the surface.

(For comparison, at the lunar equator, the night-to-day changes in temperatures are about -150 C to +100 C.

Question: Are there season changes like on Earth with different temperatures year round?

Answer: Not really. The reason for seasons on Earth is the tilt (or inclination) of our planet's rotation axis relative to the plane of its orbit. That tilt is 23.5 degrees. The tilt of the Moon's rotation axis is only 1.5 degrees, so there are no significant seasonal variations of the surface temperature as the Earth-Moon system orbits the Sun.

Question: What would a calendar look like for the moon? Is there 27 days for a full axis?

Answer: The synodic month (about 29.5 days... from one Full Moon to next Full Moon) would be the length of a "day" on the Moon.

The only definition of a "month" for the Moon would be the cycle of phases of the Earth as seen from the Moon. This is the same as the synodic month defined above. So the day and the month on the Moon would have the same length.

The year on the Moon would be essentially the same as the year on Earth. The Earth-Moon system makes one complete orbit around the Sun about every 365.25 days. That's essentially the length of the year on the Moon. The point that orbits the Sun is the centre of mass of the Earth-Moon system (i.e., the "balance" point of their mutual orbital motions around each other). There may be a slight difference in the strict definition of the year for the Moon, in terms of when it returns to the same point in the sky as seen from the Sun. It depends on how accurately you want to define your calendar for someone living on the Moon.

-Dr. Jaymie Matthews

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The shape of our galaxy

Question:

A TV show (and the NASA Web site) showed an image of our galaxy with all of these beautiful planets. Why is our galaxy arranged in a flat plane? Why isn’t it spherical with the Sun in the centre, for instance?

Answer:

In some sense, the Milky Way Galaxy is flat and spherical at the same time. The stars and gas we can see are distributed in a flattened disk with a central bulge, resembling a flying saucer) but the invisible dark matter that makes up most of the mass of our Galaxy is in a much larger sphere with us near the centre.

First, why do the stars and gas orbit in nearly the same plane?

The answer to that question is essentially the same as the answer to the question "Why do all the planets in the Solar System orbit in nearly the same plane?"  And that answer is: Angular momentum.

The gas that formed our Galaxy and other galaxies was once thinly distributed through space. Vast gas clouds began to collapse under their own weight, as gravity concentrated the gas into galaxies of stars. Those collapsing clouds began to form stars and concentrate into galaxies.  The clouds had some spin (i.e., angular momentum), which is conserved in nature.  As a cloud gets smaller, without gaining any mass, it spins faster to conserve angular momentum. (A figure skater (with skates on ice and almost no friction) knows this instinctively during a pirouette, spinning faster as she draws her arms and legs closer to her torso.) Gas continues to freefall towards the centre of the baby galaxy along the axis of spin, but centripetal force at the spin equator resists gravity. (Think of being on a fast-spinning amusement park ride where you are pressed up against your seat.) The result is a flattened spinning disk of gas and stars.

On a smaller scale, this is the scenario for the formation of the Solar System, and why the orbits of the planets do not resemble the cartoon image of electron orbits in an atom.

The Milky Way birth story is more complicated than this, and not fully understood.  Some of the gas condensed into stars before it had a chance to flatten, so that's why our Galaxy has a spherical "halo" of stars that are the oldest in the Galaxy (possibly in the Universe).  And like a human biography, the story doesn't end at birth.  Galaxies can swallow up smaller galaxies, growing and changing their appearance through "galactic cannibalism".  So some galaxies end up looking more like eggs than flying saucers.

Galaxies can also crash into one another, although there is no actual physical contact given the immense distances between the stars in a galaxy. Gravity is what can turn two colliding galaxies into a new galaxy.  Unlike a car crash, where higher speed means more damage, a slow galaxy crash leads to the most spectacular results, allowing gravity time to modify the orbits of the stars and gas.

But most of the mass of our Galaxy is not in the form of stars and gas we can see.  It is "dark matter", which does not interact much with normal matter except through its gravitational effects (which is how dark matter was first recognised). The dark matter in our Galaxy is distributed in a vast sphere, with what we see as the Milky Way just a tiny disk at its centre, where it settled under the gravity of the dark matter halo. To some theoreticians, the more practical definition of a spiral galaxy like the Milky Way is "a huge sphere of dark matter with a small smattering of stars and gas at the centre".

So depending on how you look at it, the Milky Way is bright and flat, with the Sun near its outer edge, or the Galaxy is much larger, dark and spherical, with the Sun not too far from the centre (or the bottom of a deep gravitational well).

-Dr. Jaymie Matthews

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Size of the universe

Question: 

Do we still consider the universe as being infinite or limited to 13.7 billion light years in space. Also, if we return to the Big Bang of 13.7 billion years ago, what was the size of the universe and was it always infinite? Can we not use the same reasoning with time?

Answer:

There are several different "sizes" we can talk about for the universe.  One is the size of the observable universe --- the distance from which light has had time to reach us since the Big Bang. You might think that if the universe is 13.8 billion years old (the new best measurement) then this size would be 13.8 billion light-years. But in fact it's larger than this because the universe has always been expanding, and the furthest observable distance is about 47 billion light years.

The size of the observable universe has been getting larger since the Big Bang. But our best current knowledge indicates that the entire universe is infinite, and always has been. We don't think the same is true for time: when we look out into space, we look back into time because light doesn't reach us instantly. We know that conditions were different in the past -- for example, the universe was hotter and denser -- and this is an important part of the argument that time is not infinite, but had some kind of special initial point.

- Pauline Barmby

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Are black holes visible?

Question:

Are black holes visible? I am wondering because I thought that astronauts and their ships may not be safe if they cannot identify where these are.

Answer:

By definition, black holes are invisible since their gravity is so strong that even light cannot escape their surface. Black holes are extremely dense objects with a central point called a "singularity", where all the mass of the black hole is contained. What we normally call the edge of a black hole is an invisible boundary called the "event horizon", which is the distance from the singularity where an object would have to be travelling at the speed of light to escape the black hole's gravity. That means that if you get close enough to a black hole, nothing can escape, not even light!

This doesn't mean that it's impossible to detect a black hole. In fact, there are a few ways to detect black holes indirectly. If it is in a binary system with another star, it is possible to see the effects of the black hole's gravity on its companion. So, if you see a star orbiting something in space, but that something is invisible, chances are it's orbiting a black hole. The other way to detect a black hole is to detect the light coming from matter falling into it. For example, if a black hole and a massive star are in close orbit around each other, material from the star might start falling into the black hole. As the gas from the star approaches, it forms a rapidly-rotating disk around the black hole, called an accretion disk, and because the gas is spinning so quickly, it heats up and emits a very energetic form of light called X-rays. Many of the bright X-ray sources studied by astronomers are caused by these accretion disks.

As for astronauts wandering into a black hole, it's my opinion that we don't have much to worry about. A common misconception is that black holes are cosmic vacuum cleaners, but if you're far enough away, a black hole will have the same gravitational force as any object of the same mass. That is, if the Sun were to suddenly change into a black hole of the same mass, the Earth's orbit wouldn't change. And so, a black hole in our galaxy won't have any extra gravitational force just because it's a black hole, and an astronaut wouldn't have anything to worry about unless he or she approached the black hole's event horizon.

In addition, the black holes that have been detected so far are thousands of light years away, which is much much further than any astronaut has ever travelled. Currently, space travel is limited to our own solar system, the edge of which is passed the Oort cloud (a cloud of debris orbiting the Sun where comets come from), just under two light-years away. The closest black hole we know of is Cygnus X-1, and it's just over 6000 light-years away. In comparison, the closest system of stars, Alpha Centauri, is just over 4 light-years away. With these kinds of distances, the chances of running into a black hole, even when we have the technology to travel to the closest stars, are very slim.

- Ilana MacDonald

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The Big Bang cloud

Question:

Where does the Big Bang cloud come from? 

Answer:

The Big Bang is our current best theory of how the Universe began and why we currently observe its expansion. The Theory states that the Universe began in a very small and dense state and has been expanding and cooling ever since. We have many lines of evidence to support this, starting with Edwin Hubble's discovery in 1929 that most galaxies appear to be moving away from us, and that the further away they are, the faster they're receding. Another piece of evidence was found in 1964 when Arno Penzias and Robert Wilson discovered a signal from the beginning of the Universe called the Cosmic Microwave Background (CMB).

The fact that we know that the Universe began according to the Big Bang Theory doesn't mean that we know exactly how it started or where everything in the Universe came from. There is still a lot of speculation as to what happened in the earliest moments of the Universe, but we do know a few things about what happened early in its history. We think that at the very beginning, the Universe expanded rapidly from a very dense and even state in a process we call Inflation about 0.0000000000000000000000000000000000001 seconds after its birth. From this, the first particles were created (quarks, electrons, and other elementary particles), and the Universe was so hot that particles were constantly jumping in and out of existence. At 0.000001 seconds, the Universe cooled enough that the first protons and neutrons were able to form, and when it was just a few minutes old, the first Helium atoms were made. For the next 300,000 years or so, the Universe was in a hot plasma state, that is, the electrons of all the atoms were floating freely, until the Universe cooled enough that they were able to recombine with their nuclei to make a neutral gas. This was when the CMB was emitted.

Just about every atom we see today in the Universe was created in those first few seconds after the Big Bang. The gas collapsed to form stars and galaxies, then planets and even humans. We are all made from the atoms that first formed in the Big Bang! And so, even though we don't know exactly where the stuff that first made up the Universe came from, we do know how the first gases formed and how they were transformed to become everything in the Universe

- Ilana MacDonald

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