August 17, 2011 at 10:14 am | Leave a Comment
Get Your Straight Skinny Right Here
I used to study comets for a living. They’re iceballs, mixed with a little dirt. They’re pretty small as solar system bodies go — often not more than a couple of miles (or kilometers across). They orbit the Sun just like planets do, and once you know a comet’s orbit (or any solar system object’s orbit), you can predict it pretty well. They don’t suddenly change their orbits without reason (see Kepler’s laws of Planetary Motion, which apply to comets and asteroids in general (see discussion under “First Law”) as well, to understand why).
To really “get” what a comet is and does, the next time it snows in your neighborhood, take a handful of snow and mix it with some dirt. If it doesn’t snow, then go get a snowcone or get some chipped ice and mix it with dirt to make an iceball.
Heft it in your hand. Look at it. It’s not very dangerous on its own, is it? Common sense tells you that it doesn’t have much mass, it doesn’t have a strong gravitational pull. If you could build a snowball maybe a mile or two across and put it into orbit around the Sun, you’d have a comet. Most comets are made of water ice, with traces of other ices mixed in (carbon dioxide ice, methane ice, stuff like that that we know the physical principles of). They orbit the Sun, often in very long orbits that take them out beyond the orbits of Mars, or Jupiter or even Neptune. There are many, many comets and each one has the same basic makeup and long orbits. I findthem fascinating because of what they are and where they came from, and what they tell us about the solar system’s history.
The true value of comets is really what they tell us about the conditions in which they formed. that’s what always kept me interested in the comets we studied. Each one carries a treasure trove of chemical information about the elements in and conditions prevailing in the early solar system. In the original solar nebula, the cloud of gas and dust from which the Sun and planets formed, gases such as hydrogen, oxygen, nitrogen and so forth were pretty abundant. So were grains of dust and water and other molecules. Because space temperatures are cold, many of the molecules existed as frozen ices.
As the conditions at the center of the nebula warmed up (where the Sun was forming), the hot bright radiation of the protosun destroyed any icy material that existed nearby. Only the icy materials and gases in the far reaches of the solar system (mostly out beyond Jupiter,where the temperatures were cold enough to support icy objects) survived.
Comets come from a reservoir of icy chunks that has existed beyond Neptune’s orbit since the very earliest epochs of solar system history. All these objects — collectively grouped as Oort Cloud objects — orbit the Sun, but at very huge distances. And, as I mentioned above, they carry the chemical evidence of what it was like in the early solar nebula. That makes each comet a treasury of information.
So, how do comets get to the inner solar system? Their orbits are changed by entirely normal and scientifically understandable circumstances. Since they’re small, it doesn’t take much to nudge a cometary nucleus from its orbit into a slightly different orbit — one that takes it closer to the Sun. The most logical and commonsense suspects for such gravitational nudges would be nearby planets (dwarf or otherwise), or possibly a passing star (and yes, stars can do that) at the very edges of the solar system. Spacecraft (alien or otherwise) would not be big enough to nudge a cometary nucleus, but a close pass with a body the size of Pluto, for example, would.
Anyway, once nudged, the cometary nucleus is on a new orbit — and often times that orbit is one that takes it in toward the Sun and through the orbital paths of other planets and asteroids. As a comet gets closer to the Sun, it feels more of the Sun’s gravitational pull, and—at that point, you can see how Kepler’s laws really do work—a comet’s orbit is shaped by the gravitational tug of the Sun and any planetary bodies it flies close to. If it happened to get close to Earth, it might be affected by that, for example.
This is all very natural and, if you understand what orbits are and how they evolve over time due to natural forces, then you “get” what comets do. They’re frozen chunks of ice and dust, following paths set in motion a long time ago
So, there’s this comet called Elenin doing its closest pass to the Sun during its elliptical orbit. It’s doing what all things in orbit around the Sun do—which is completely normal and nothing to be worried about. Its path will take it close enough so that we could spot it, but not close enough that it’s going to do anything to us. Even if it passed really close to Earth, its mass is so small and its body so inconsequential that nothing would happen. Really.
Comet Elenin as seen by STEREO spacecraft, August 6, 2011. From Earth, presently the comet is a faint smudge of light in deep sky exposures. By late August comet Elenin could be visible to the naked eye as a dim "fuzzy star" with a tail.
So, here’s the skinny on Elenin’s appearance in our skies. On October 16 of this year, it will be approximately 22 million miles (35 million kilometers) from Earth. That is 90 times the distance between Earth and the Moon (which lies around 238,000 miles (~333,000 kilometers) away). It’s probably not going to be very bright in the sky, and you may need binoculars to see it. So, it’s not really the brightest comet to come into the inner solar system. Certainly many amateur and not a few professionals will take a look at it, and measure its tail and gas out put to help understand its chemical makeup. But, that’s about it. Another entirely normal cometary appearance in the solar system.
There are a LOT of people out there, posting on the Web about how Elenin is going to blot out the Sun, or align with some other celestial body and cause trouble for Earth in some other way. Some of the stuff I’ve read even invokes unknown aliens, UFO fleets (that nobody except the Bravo Sierra vanguard can see), suddenly appearing and disappearing mysterious spacecraft, and other ad hoc fantasies. It’s like reading about the Bermuda Triangle or voodoo economics—lots of Bravo Sierra, few (if any) provable facts.
It really is all nonsense. There’s no other polite way to put it. These fantasies are written by people who haven’t taken the time to learn the basic laws of physics and Kepler’s motions. It’s kind of like reading financial news from people who don’t understand how money works or soccer stories written by people who don’t know the rules of the game.
How an object as small as Elenin could blot out the Sun from a distance of 22 million miles makes me laugh. This is a really small comet. If you were looking directly at the Sun (never a good idea though—since it would burn your retinas in a few seconds, so don’t even think about it) and the comet passed between us and the Sun, I doubt you’d even see the difference. That is, if you could see at all after staring at the Sun that long. Do you really want to trust your eyesight to idiots on the Web who post such nonsense? So, why trust their “scientific knowledge”? That’s right. You wouldn’t.
You probably should read all the nonsense though—it’s always good fine-tune your Bravo Sierra Detector(TM), especially as we head into an election year in the United States. And, in these tough economic times, a little laughter at silliness can be a good thing, as long as you know it’s silliness. I know that logic and the laws of science are sometimes less enticing and entertaining than out-and-out nonsense.
Before you do wade through the Web-enabled fantasies about this comet, arm yourself with some scientific facts. Check out the Comet Elenin FAQ, written by people who know the science of comets. The more you know, the less likely it is you’ll be taken in by purveyors of Bravo Sierra.
August 16, 2011 at 9:17 am | Leave a Comment
The Lion’s Treasure
The spiral galaxy NGC 3512, as seen by the European Southern Observatory's Very Large Telescope in Chile. The galaxy is about 35 million light-years away and spans an area of space 50,000 light-years across. Courtesy ESO.
I like to look at pictures of galaxies. They always trigger in my mind questions about what kind of life must exist on planets around their stars. Since we’ll likely never know (at least in our lifetimes) about beings in other galaxies, it’s a great way to think about starry empires and the civilizations that support them. As a science person, I look at galaxies and immediately start cataloguing in my mind their structures, which gives me some idea of their evolutionary history— that is, what they went through to get to the shape we see them in—and also how much star-forming activity is occurring in the arms. Galaxies are treasure houses of star formation, star death, and the materials for new stars, planets, and life.
This galaxy, NGC 3521, is what astronomers call a “flocculent” spiral galaxy. That means its spiral arms are fluffy with clouds of gas and they are dotted with star-forming regions. The areas where new stars are being born are mostly blue in color because of the hot young ultraviolet-bright stars they contain. The reddish areas contain older stars, and the dark lanes are those clouds of gas and dust. There could be stars forming inside those clouds and in a few tens of thousands of years, bright blue splotches will light up more regions of the dust lanes.
The core of this galaxy is really quite compact—meaning tightly bound together. There are likely millions of stars in that little region, and possibly a black hole. All in all, this is a busy galaxy—bustling with star formation, and with the creation of many stars, very likely it has a population of planets in orbit around some of those stars. If so, I expect that this treasure of a galaxy in Leo also has some life forms in it—maybe some of them are looking at pictures of our galaxy and wondering the same things about the Milky Way.
August 11, 2011 at 10:01 am | Leave a Comment
And An Answer to One of Them
Last week aboard the good ship Corinthian II, I was sitting out on the deck having a little lunch and chatting with some of fellow passengers about fascinating topics in astronomy. It’s always interesting to hear what fascinates people about space and astronomy and I’m always happy to answer questions about those topics.
One of the questions that comes up frequently (and did in the conversation I had that afternoon) is “What will happen to the Sun?” Most of the time, people really ARE interested in the science behind the Sun’s existence and I”m happy to oblige them with the executive summary of end-times astrophysics for our star.
And it IS (or will be) an astrophysical event. Each thing that will happen to the Sun can be figured out by applying the laws of physics, of gravity, gas laws, and other scientific knowledge. No mysterious death rays or aliens figure into these, because those “actors” don’t usually follow the laws of physics (or of normality, as far as I can tell). And yeah, there are all these crazy ideas out there floating around about how the Aztecs or Mayans or the Illuminati or the Pleiadians or some other alien race has predicted the Sun will go wonky next year, or that the death beam from the center of the Milky Way will cream us all at a predetermined time. However, nothing that anybody can dream up after a couple of beers (or surfing through weird Web sites) is as interesting as what will really happen to the Sun.
The Necklace Nebula, a recently discovered planetary nebula in the constellation Sagitta. This is a Hubble Space Telescope image. Courtesy NASA, ESA, and the Hubble Heritage Team (STScI/AURA).
So, what WILL happen? Take a gander at this image here to the left. It’s a planetary nebula — essentially what’s left over after a sun-like star loses most of its mass to space. The star doesn’t blow itself to smithereens — that’s what stars that are many times more massive than the Sun do when THEY die. No, stars like the Sun go to their fates more gently (for stars). The short story is that it huffs off its outer atmosphere over long periods of time, and then what’s left collapses to become a white dwarf. So, the Sun — in about 6-7 billion years, could look something like this.
This image actually shows what happens when two stars are involved in a planetary nebula. A pair of stars orbiting very close together are at the heart of this nebula (called PN G054.2-03.4). About 10,000 years ago one of the aging stars ballooned to the point where it enveloped its companion star. This caused the larger star to spin so fast that much of its gaseous envelope expanded into space. Due to centrifugal force, most of the gas escaped along the star’s equator, producing a dense ring. The embedded bright knots are the densest gas clumps in the ring.
The stars are furiously whirling around each other, completing an orbit in a little more than a day. (For comparison, Mercury, the closest planet to the Sun, takes 88 days to orbit the Sun.)
An artist's-eye view of what the Sun and solar system will look like in a few billion years as the Sun ages and dies. Courtesy ESA.
The Sun’s planetary nebula will be a glowing cloud of gas and dust, heated by radiation from the leftover white dwarf. It will light up the clouds and highlight the clumpiness in the nebula.
What people are really wanting to know when they ask that question about the Sun dying is what will happen to Earth. Sad to say, the prognosis for our little oasis in space isn’t good at that point. Life will have been crisped in the heat of the expanding outer atmosphere of the Sun–since it will swell up to become a red giant in the process of dying. The oceans will boil away. What ever is left could be a cinder. I say “could” because it’s possible that the Sun’s stellar wind will be very strong, which could cause the orbits of the planets to drift outwards. So, our planet might escape the fiery death part–at least for a while.
How does star death for the Sun happen? Look at what the Sun does. It goes about its daily business of turning hydrogen into helium in its core. It has been doing this for billions of years, like all stars do. The heat and pressure of the burning in this nuclear furnace is enough to keep the outer layers of the Sun from collapsing in. This is what’s happening now–the Sun we enjoy is in equilibrium–meaning the heat and pressures in the core balance the gravitational tendencies of the outer layers to want to fall in to the center.
But, in a few billion years, our star will start to run out of hydrogen in its core and lose the core pressure that holds up the other layers. At that point, all they will collapse under the pull of gravity, and what’s left of the hydrogen will heat up. Fusion (the hydrogen-to-helium process) will resume. This time, however, the outer layers–particularly the outer atmosphere–will swell up a few hundred times larger and be cooler and redder than the Sun we know today.
At that point, our lovely yellow star will become a red giant. And, in that swelling, it will likely smother the inner planets. If the dying Sun has a strong-enough and mass-loaded stellar wind, that could push the planets out a bit, and that’s where astronomers speculate the Earth could escape being turned into a crispy planetary critter. It’s hard to tell at this point what would really happen, but the ultimate fate of Earth and the inner planets isn’t going to be like we know it today.
So, that’s the answer to the question, “What will happen when the Sun dies?” It’s a stellar process that occurs throughout the universe, and we understand more about it by studying the planetary nebulae whose remains chart the future of our own star.
August 10, 2011 at 8:30 am | Leave a Comment
Our Link to the Celestial Realm
I just got back from a lecture gig on board a study-cruise program in the Mediterranean. We visited Italy, Greece, Turkey, and the Greek Islands, and the theme of the educational program was “Gods and Heroes”. My job was to talk to people about the ancient connections to the sky that the Greeks and Romans (in particular) had in ancient times. In contrast to today, when we get all our time information from gadgets like watches and cell phones, our calendars hang on the wall, and we don’t even think about whether it’s night or day, ancient people had no mechanical means of understanding the sky. They used it, but they didn’t really get what it was they were seeing. At least, not in the scientific sense that we do today.
Yes, they were incredible observationalists and their charts were the basis of astrology, which was the predecessor to the science of astronomy. But, the ancients only really had what they could see with the naked eye. They had a one-dimensional view of the sky — things appeared in it, sometimes they seemed to move against the backdrop of stars, and there was no scientifically rigorous study of the celestial realm. People weren’t stupid back then — they just had different priorities (like survival) and belief systems (“things in the sky must be gods and goddesses”).
A Greek sundial dating back to 3 BC. Used by permission; link: http://en.wikipedia.org/wiki/File:SunDialAiKhanoum.jpg
So, the Sun, for example, became more than a bright shiny thing in the sky. People imagined that it had a purpose, it had a reason for being there — and, being humans — they endowed it with human and superhuman powers. They created a god/goddess in their own image and then made it bigger, mightier, and mysterious.
Of course, we all have a link to the Sun today — it’s what helped us devise the units of day and night. It provides warmth and stimulates life processes. So, it’s no surprise that such a powerful influence on the planet would have been an object of worship among ancient people. Heck, some people worship the Sun today — I often refer to them as the “SPF 50 Cult”.
It didn’t take long for people to figure out that the Sun could be used as a timekeeper, a way to mark the passage of days. Along with the Moon’s cycle of phases — it became the basis for what eventually have become the various world calendars. The ancients used the sky as a tool to capture time, to define their daily lives, and as a place for their gods/goddesses/heroes/villains. In a sense, it provided not just orderliness, but entertainment and a sense of cultural unity (“We all believe in Ra, the Sun God, therefore we are people of the Sun God”.)
Today, of course, we know the Sun is a star. There’s nothing mysterious about it. There are certainly parts of its processes that scientists are still working to understand and explain — that’s the nature of the science of solar physics. But, as a former deity — well, the Sun’s WAY bigger than that. It’s more real to us as a star and we have a very good idea of how stars work (mechanically and astrophysically). And, we know where it lies in the hierarchy of stars that make up the stellar city we live in — the Milky Way.
Still, it was a lot of fun to go back to the “ancient world” for ten days and look at things like the Sun and Moon and planets through the eyes of those who lived in those times. The work they did in charting the sky, the apparent motions of the Sun, Moon, planets, and stars, were the cartographica basis for the sky charts we use today. And, if certain “natural philosophers” among the ancient peoples around the world hadn’t looked up and wondered “What is that we see?” about sky objects, we wouldn’t have the science of astronomy to help us understand the universe in the straightforward, rigorous way we do today.