Date: August 5, 2009
Title: What Do We See When We Look Up?
Podcaster: Wayne Harris-Wyrick
Organization: Oklahoma City Astronomy Club and Kirkpatrick Planetarium. Check out their website.
Description: What do we detect when we look into space? Very little, actually. Almost all of what we know about the Universe comes from analysis of a very limited set of phenomena. Yet we have managed to extrapolate a lot of knowledge from that limited array of what we detect and measure from space.
Bio: Wayne Harris-Wyrick is the Director of the Kirkpatrick Planetarium, a part of Science Museum Oklahoma, where he has taught astronomy to countless thousands of kids and families for more than 30 years. Harris-Wyrick writes a monthly astronomy column covering all aspects of this amazing universe. Wayne can be contacted at wwyrick@sciencemuseumok.org. Visit the Museum’s site at www.sciencemuseumok.org.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by the Oklahoma City Astronomy Club. Founded in 1958, our club has a long and distinguished history. We are also members of the Southwestern Region of the Astronomical League and host the Okie-Tex Star Party which is consistently rated as one of America’s Top Ten Star Parties. More information on our club can be found at okcastroclub.com.
Transcript:
Astronomers suffer a frustrating fate. They must wait patiently for the Universe to come to them. And what does come is a mere trickle: various forms of light across the electromagnetic spectrum, some strange subatomic particles called neutrinos, elementary atomic particles lumped together under the name of cosmic rays, rocks that we traveled to the Moon to get plus some interplanetary dust particles captured by high-flying aircraft, and meteorites that have accidentally landed on Earth. That’s it. All of our direct knowledge of the Universe comes from just those items. And of those, only three – light, cosmic rays, and neutrinos – are known to have come from beyond our solar system, although certain meteorites do contain what appears to be extrasolar material.
Light contains a fantastic amount of information about its source. By studying the specific intensity of light at each of the different wavelengths, from the longest radio waves to the most energetic gamma rays, we can determine the composition, temperature, mass, rotation, magnetic field strength and orientation, motion, and many other physical characteristics. That’s pretty amazing considering that ALL of the electromagnetic radiation that penetrates our atmosphere and gets through Earth’s magnetic fields, save that from our nearest star, the Sun, arrives with such a minimal energy density that it is completely harmless to all living things.
Cosmic rays emanate from some of the most energetic events in the Universe. They are composed of parts of atoms, mostly protons and electrons, and they almost never actually reach the ground. We have to study them via rockets and high altitude balloons. We often detect not the cosmic ray itself, but so-called secondary showers. These occur when the cosmic ray particle hits an atom in our atmosphere and generates a shower of secondary particles created from the energy of the collision. And because they are charged particles, we have great difficulty even determining their origin since the Milky Way’s magnetic field curves them in their flight from points unknown to our detectors. Supernovae generate prodigious cosmic rays, but the most energetic are still a mystery. Perhaps the same collapse of massive stars into black holes that create gamma ray bursts also accelerate protons to these fantastic energies.
Neutrinos are produced inside stars as a part of the nuclear reactions that convert hydrogen into all the other elements in our universe. They are also known to come from supernova explosions. In fact, it was the detection of neutrinos from supernova SN 87A a short time after the electromagnetic radiation arrived that helped clinch the observation that neutrinos have a small amount of mass. That observation explained a long-standing puzzle regarding neutrinos produced deep in our sun. By stellar physics theories, there should have been three times more neutrinos coming from the sun then we could find. When the supernova neutrinos were analyzed, we realized that they are not massless as had been assumed. They possessed just enough mass to allow them to change among the three varieties of neutrinos as they traveled from the sun to Earth, thus accounting for that mystery.
Meteorites come from various sources, all within our solar system. Most are fragments of various members of the asteroid belt between Mars and Jupiter. As asteroids collide, small fragments scatter. Some get knocked off with such energy they fall in towards the sun. Or a fragment may pass too close to a much larger asteroid. In a form of cosmic billiards that NASA uses regularly in interplanetary flights, the smaller fragment receives a gravity assist that changes its orbit, possibly sending it in towards the sun.
When such a fragment, called a meteoroid, passes close enough to Earth, our gravity reels them in. They enter our atmosphere at speeds up to one hundred fifty thousand miles per hour, creating tremendous friction as they rub against the molecules of our atmosphere. The friction heats the air to incandescent, much like what makes a fluorescent lamp glow. We see that streak of light as a “meteor”. The hunk of rock moves at supersonic speeds; air can’t move away from a rock traveling so quickly. If it is small enough, the meteoroid is essentially burned to dust.
But if the meteoroid is large enough, it will shatter. Air pressure builds up in front of the rock and can’t move away. This pressure breaks the rock and slows down the fragments. If the initial piece entering our atmosphere is large enough, the splintered bits will survive to reach the ground as meteorites. Many are found on the ice sheets of Antarctica, where the dark rocks stand out against the white ice. Just as many land elsewhere, but they are simply not as obvious.
The study of these accidental meteorites plus the specks of interplanetary dust collected high in our atmosphere date the formation of our solar system to just over 4.6 billion years ago. They also tell us that the collapse of the cloud of gas and dust from which our solar system formed, the pre-solar nebula, was likely triggered by the blast of a nearby supernova. This supernova was a sister star to our sun, one created with at least ten times more mass than our sun. Massive stars live fast and furious and explode as supernovae at the end of their short lives. We find the debris from that supernova scattered among the atoms and molecules of meteorites and the interplanetary dust particles.
Apollo astronauts collected eight hundred and forty two pounds of rocks from the surface of the Moon. It was the study of these rocks, in particular the ratio of various elements and isotopes that taught us how the Moon formed. Some elements were absent or very scarce in the Moon compared to Earth. This means the Moon could not have formed from the exact same material that Earth did. Yet the ratios of oxygen and other isotopes indicates that at least some aspects of the Moon’s composition was identical to Earth, and HAD to have come from the same part of the solar nebula. This led to the currently accepted origin of the Moon.
It is the result of a collision between the young, still molten Earth and a body the size of Mars in an irregular orbit around the sun. Part of Earth’s crust and that of the interloper blasted out into orbit around Earth and eventually coalesced into the Moon, thus accounting for similar composition between the Moon and Earth’s crust. At the same time, the heavy iron and other siderophilic elements from the colliding body sank to join with Earth’s nascent core, explaining the lack of iron and related elements in the Moon.
Virtually everything else we know about the Universe comes from studying the electromagnetic spectrum, of which the visible light we see is but a tiny portion. By studying the light from the points of light we see in the night sky, we know that they are stars, like our sun, at great distances from us. Our sun is not a special star: it is not bigger or brighter or hotter, it is just closer. If you moved our sun as far away as the average distance of the stars we see at night, it would not even be visible without a telescope. As stars go, our sun is rather mediocre. In fact, the large majority of the stars you see at night with your eyes are bigger and brighter than our sun! There are even more stars that are smaller and fainter, but most of those can’t be seen without a telescope.
By studying the light and other radiation we see in our telescopes, we know the Universe started 13.7 billion years ago in a sudden and extremely powerful event known as the Big Bang. We know that the first stars began to shine several hundred million years later, followed not long after, on a cosmic scale, by the formation of the first crude galaxies. Almost everything we know about the size, motion, composition, and history of the Universe comes from studying light in all of its forms.
The next time you look up at the night sky and wonder at the universe, contemplate just how much we have learned from that short list of stuff that we have actually received from the cosmos.
365 Days of Astronomy
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