The most distant thing your eye can see

When it comes to seeing faint things, our eyes aren't that bad. Yes, some other animals use tricks to be able to see fainter things than we can (like reflective linings to the eyes, or more sensitive rods in the eyeball). But, given time to adjust to the dark, our eyes can see stars when only a dozen or so photons of light are hitting our eyeball every second. That's not too bad! So, how far our unaided see?

On a clear, moonless night, most people with decent vision can see the Andromeda Galaxy (Messier 31), and under excellent conditions, many people have been able to pick out the Pinwheel Galaxy (Messier 33, a.k.a. the Triangulum Galaxy. Andromeda is about 2 million light-years away, and the Triangulum Galaxy may be 3 million light-years away.

This sounds really far, but, in astronomical terms, they are neighbor galaxies. Andromeda, Triangulum, and the Milky Way galaxies are all part of the "Local Group" of galaxies. The Local Group is kind of like a solar system of galaxies. All the galaxies in the Local Group are bound by gravity and are very slowly orbiting one another. In a few billion years, we may even collide with the Andromeda Galaxy.

Some eagle-eyed people looking on pristine nights from mountain-top sites have been able to see Messier 81, a galaxy about 12 million light-years away. Messier 81 is not part of the Local Group of galaxies, but is instead the heavyweight core of the closest group of galaxies outside our own. But this is a little like being able to see the house across the street in a large metropolis -- not too impressive. The most distant galaxies the Hubble Space Telescope can see are a thousand times further away than Messier 81.

However, last Wednesday morning, March 19, 2008, humans had a chance to see an event with their own eye that was seven billion light-years away, or halfway across the visible Universe. At 6:12am and 49 seconds Universal Time (2:12 am Eastern Daylight Time), the Swift gamma-ray satellite detected a burst of gamma rays coming from the direction of the constellation Bootes (near the Big Dipper). Robotic telescopes on the ground instantly moved to this portion of the sky, where they found a point that brightened from invisibility to magnitude 5.5 in only 20 seconds. It stayed bright for about 30 seconds, and then started to fade away to the nothing from which it came.

For those who don't speak magnitudes, a magnitude of 5.5 is bright enough for the human eye to see on a drak, clear night, but just barely. Later analysis determined that the light these robotic telescopes saw came from a galaxy nearly 7 billion light-years away.

The short of the story is that, had you been lucky enough to have been outside, had your eyes adapted to the darkness, and knew where to look, you would have been able to see a faint spot of light coming from a galaxy halfway across the visible Universe last Wednesday morning, but only for about 30 seconds. As far as we know, no human knowingly saw this event -- the "flash" was too faint to notice unless you knew right where to look, and the 30 second time interval was not long enough for anyone to have been able to learn about the gamma ray burst, consult their star charts, look, and be certain what they were seeing.

Still, it is exciting to think that, in some circumstances, it might be possible for a human with no equipment other than their eyeball to see something that far away. Gamma ray bursts happen about once a day, and while this is the first burst to have gotten this bright, a flash like this probably happens every few years. We've just never had the technology before. Maybe some day in the next decade or two, someone will get lucky enough to catch a few photons from en explosion that happened billions of years ago with nothing but their eye.

How things go boom in the night

Image credit: F. Roepke & W. Hillebrandt

In the past few days, I've talked about the controversy of what types of stars explode as Type Ia supernovae. But the mystery surrounding these explosions is not limited to what explodes, but also how.

When a white dwarf explodes, it is undergoing an uncontrolled nuclear reaction, turning carbon and oxygen into silicon, iron, nickel, cobalt, and many other elements. In a matter of a second or so, a sun's worth of carbon and oxygen is burned. But there is a question as to how it burns. Burning comes in two flavors. In a deflagration, or what you would call a normal flame, a flame moves more slowly than the sound speed. A good example is a wildfire, which can race across the ground at 60+ miles per hour (more than 100 km/hr), but even seemingly explosive burning, like gunpowder or an automobile engine, is still just a deflagration.

In a detonation, the flame moves faster than the speed of sound. Most high explosives do this sort of explosion. Detonations are more destructive than deflagrations, which is why plastic explosives are used in modern warfare instead of kegs of black powder.

In white dwarfs, the speed of sound is very high. In Earth's atmosphere, it is around 770 mi/hr at Earth's surface. Sound moves faster through dense material. For example, the average speed of sound through the Earth (as measured by earthquakes) is about five miles per second, or 18,000 miles per hour! Even so, this means it can take nearly half an hour for a sound wave to go through the Earth. White dwarfs, on the other hand, are much denser yet. Although the size of the Earth, sound waves can travel all the way through a white dwarf in less than two seconds at a whopping speed of over 14 million miles per hour! So, a white dwarf can completely burn in a matter of a couple of seconds and still not be a detonation.

How can we tell how a white dwarf burns? By what it leaves behind. In a deflagration, the white dwarf burns pretty thoroughly, leaving behind almost no carbon. The same is true of deflagrations on the Earth -- in your car's engine, you burn all of the gas you put into the piston. In a gun, you burn almost all of the gunpowder. Otherwise, you are just wasting fuel! In a detonation, pieces of unburned material are sent flying. So, for a white dwarf, we would expect to see lots of carbon and oxygen left if it detonates.

When we look at the Type Ia supernovae, we usually see a little bit of carbon and oxygen -- not much, but some. Neither the detonation or deflagration models really work! The truth seems to lie somewhere in between -- most of the white dwarf burns "gently," but somehow, somewhere a detonation explosion occurs, adding some extra energy and keeping all of the carbon from burning.

The problem is, it really is difficult for theorists to explain why one type of burning would turn into another type. So, at meetings such as the one last week, scientists argue very heatedly about their latest models. This isn't work that I do, so I am not qualified to make a guess as to which models are more likely to be correct.

The picture above is a model of a white dwarf deflagrating ("gently burning"). The yellow-orange is the unburned white dwarf, while the grey/white indicates where the flame has already burned the white dwarf. You can see that, as the burning continues, the heat causes the white dwarf to get larger. Soon after the phases pictured above, the flame reaches the surface, and the ashes are spewed into space at a speed of 10,000 miles per second!

Things that go boom in the night, part 3

Image credit: NASA/Dana Berry, Sky Works Digital

This update is a few days later than promised; I happened to get involved in some discussions during breaks in our conference, and by the evening, I was too tired to go online. So, although the conference is over, I'll continue to try to bring things up to date.

So, remember that we were talking about Type Ia supernovae, some of the most intriguing explosions in the Universe. There are two suspects for objects that form these particular explosions. The first, discussed last time, involves a white dwarf pulling mass from a companion star until it reaches the all-critical "Chandrasekhar mass," about 40% larger than the sun's mass, at which the white dwarf would undergo a thermonuclear explosion. Today we talk about the second mechanism, the double degenerate model, so called because it involves two white dwarfs, and white dwarfs are also known as "degenerate stars," because when you pack the mass of the sun into something the size of the Earth, the matter enters a state called degeneracy.

The way the double degenerate model works is fairly simple. If you have two white dwarfs orbiting each other, over time they will get closer and closer. This is because under Einstein's Theory of General Relativity, the objects radiate "gravitational waves," which carry energy away from the system. When the white dwarfs get close enough, they will merge together. If their total mass is over the Chandrasekhar mass, the combined white dwarf will explode!

Of course, the devil is in the details. As the white dwarfs get close, if one is more massive than the other, the bigger one will rip the smaller one to shreds. We'd get the white dwarf version of Saturn, in which a white dwarf (with the mass of the sun squeezed into a ball the size of the Earth) would be surrounded by rings with a mass of half the sun or more, still in that weird state of degeneracy. This is really hard to study from theory, and we certainly have never observed a star in this state. Maybe the remnants of the second star will fall onto the bigger star quickly, which would cause an explosion. But if they don't fall on quickly enough, the system may cause big winds that will blow the smaller star's remnants away.

Perhaps a bigger problem is that we only know of a few white dwarfs in double degenerate systems, and only one of these is above the magic "Chandrasekhar limit." For these to cause Type Ia supernovae, there should be dozens in our Milky Way that we should be able to see. Maybe we are there and haven't found them yet. But maybe these aren't the parents of Type Ia supernovae.

So, in short, we really don't know what causes Type Ia supernovae. This is really disturbing, since we use these explosions to study the distant Universe. But it is a mystery I think we can solve, given some more time.

Over the next few days, I'll summarize a few other controversies from the conference that are quite interesting.