It's the end of the world as we know it

Image Credit: Dirk Terrell

There is a Robert Frost poem quoted far too often by astronomers, Fire and Ice, that begins:

Some say the world will end in fire, Some say in ice...

The poem very naturally leads into a discussion of the fate of the Earth when our sun dies. And, since a recent news story picked up on some new research regarding the Earth's fate, I thought I might as well use the poem myself for an easy-to-write blog post.

The sun is the key to all life on Earth. It provides us with energy, with warmth, and even with protection from some dangerous cosmic radiation. Every day we see the sun rise, every day we see it set, and for eons, it seems that the sun has not changed.

Astronomically speaking, this view is almost correct. The sun does change slightly from year to year due to its sunspot cycle, but its brightness is amazingly constant. This is good for life on Earth -- if the sun were to get much brighter or much fainter, we would experience drastic climate changes.

The source of the sun's constancy is its nuclear furnace, buried deep in the core of the sun. Here nuclear fusion is going on, as the sun burns its giant supply of hydrogen into helium. (We've actually detected subatomic particles from these fusion reactions called neutrinos; in 2002 Raymond Davis and Masatoshi Koshiba were awarded the Nobel Prize in Physics for discovering neutrinos coming from the sun). The sun has enough hydrogen to power itself for nearly ten billion years, and the sun has only been shining for about 4.6 billion years, so there's a long way for the sun to go!

However, when the sun starts to run out of fuel, it begins to realize that there is a problem. Gravity will pull the helium ash from the sun's nuclear reaction inward, heating it up. And this extra heat will cause the sun's outer layers to swell up, past the orbit of Mercury and toward the orbit of Venus. As the sun's outermost layers swell outwards, they will cool, turning the sun from a soft yellow to a robust red color. The sun has become a red giant. The Earth will still be around, but the extreme brightness of the red giant sun will boil our oceans away and perhaps even melt the Earth's crust -- not a pleasant place to be!

Eventually, gravity squeezes the helium enough that it ignites nuclear fusion, with three helium atoms burning to form one carbon atom. This new source of energy will keep the red giant sun (which actually will shrink a bit at this point) going for another billion years or so, but when it runs out of helium, the sun will swell up again, this time bloating out to Earth's orbit. And after a short time, the sun will blow off its outer layers, spewing half of its mass out into space, and leaving behind a white dwarf star. If the Earth survives, it will go from being hellishly hot to bitterly cold, as the feeble light of the white dwarf sun slowly fades away.

So, what will happen to the Earth? The red giant sun will reach out beyond Earth's orbit, but the sun will also be losing a lot of mass. This reduces its gravitational pull, allowing the Earth to move a little further away from the sun. Our best calculations put the Earth right on the edge of the red giant sun, with some people saying the Earth will be swallowed by the sun, and some saying the Earth will escape. Fire or ice?

So, back to the news article. Two astronomers, Robert Smith of the University of Sussex and Klaus-Peter Schroeder of the University of Guanajuata, modeled the red giant Sun using some of the most up-to-date physics available. They also included some physics that they had previously neglected, such as the aerodynamic drag of the sun's atmosphere on the Earth. The result of their new calculations: the Earth will be swallowed by the red giant sun and vaporized. The Earth will end in fire.

So, is this the final word on the subject? I don't think so. Although these models are some of the most sophisticated yet used to determine the fate of the Earth, there are some things we still don't fully understand about the death throes of stars. If the sun loses more mass than these models predict, the Earth may move far enough away from the red giant sun to avoid being swallowed. And we don't really know exactly how much material the sun will lose and when -- these details could make all the difference!

I don't know about you, but I don't want the Earth to be swallowed by the sun, even though all life on Earth will have been extinct for a couple billion years. Call me sentimental. But, like any science, it doesn't matter how badly I want something, we have to let the facts speak for themselves. And, at least for now, the facts spell doom for our planet.

The end of a heroic spacecraft

Image Credit: NASA / JPL

The ancient Greek epic "The Odyssey" follows the adventures of Odysseus (known in Latin as Ulysses), who wanders the Mediterranian for ten years after the Trojan War, trying to get home to his wife and son. Today, it was announced that the modern Ulysses, a spacecraft studying the sun, will soon fall silent and be doomed to wander the Solar System for aeons to come.

Ulysses is a space probe studying the sun. It is unique in that it used Jupiter's gravity to get into an orbit that goes over the north and south poles of the sun. Without a little muscle from Jupiter, it isn't possible to launch a spacecraft into such an orbit -- all of our other solar probes orbit the sun at its equator, just as the Earth and all the planets do.

Ulysses was launched by the space shuttle Discovery in 1990, and after an 18-month trip to Jupiter, began its mission. It takes Ulysses 6 years to orbit the sun once, and it is most of the way through its third trip around the sun. Since the sun goes through a cycle of activity every 11 years, Ulysses has seen one and a half complete solar cycles.

The Ulysses mission was designed not to take pictures of the sun (we can do that from Earth!), but to study the sun's radiation and wind and magnetic fields from vantage points that the Earth can never see. Ulysses has studied how the sun's wind and magnetic field vary from the equator to the poles, information we could only guess at before. We've learned that the solar wind (a stream of particles coming out of the sun) is much faster near the sun's poles, moving along at over 450 miles per second! Here at the sun's equator, the wind blows at a much slower 200 miles per second. Ulysses also found that the sun's magnetic field is not much stronger near the poles than at the equator; a simple magnetic field would be twice as strong at the poles. This tells us that the sun's magnetic field is quite complicated. And, among many other discoveries, Ulysses found that the sun's magnetic field is very good at protecting us from cosmic rays, powerful radiation from the Galaxy and beyond. no matter what direction they are coming from.

Is there any saving Ulysses? No. Ulysses is powered by a radioactive power plant -- although the spacecraft is studying the sun, it is too far away from the sun for solar panels to work very well. In the power plant, radioactive fuel decays, creating heat that is converted to electricity. But as the fuel decays, the energy released steadily drops. Within the next month or two, the energy released will drop below the levels needed to keep the spacecraft pointed at the Earth.

So, although Odysseus was able to find his way back home after 20 years abroad, the Ulysses spacecraft will never get to come home. It will soon fall silent, continuing to orbit the sun for ages to come.

My congratulations to the Ulysses team on a long and successful mission!

Thats right, woodchuck-chuckers - it's GROUNDHOG DAY!

Image Source: Melissa Farlow, National Geographic

Well, okay, Groundhog Day is tomorrow. But, as I won't be blogging tomorrow, I wanted to get a word in edgewise on this most noblest of astronomical holidays.

For those of you who may live in a country not blessed with such a glorious holiday, let me explain the day. On February 2, the legendary groundhog, a furry woodland animal that looks exceptionally cute but is not always the most friendly of the beasts, awakens from its winter hibernation and sticks its head out of the ground. If the groundhog sees its shadow, it is spooked (not being among the most clever creatures) and runs back in its hole to sleep for another six weeks. If it doesn't see its shadow, it happily scampers about, and we get an early spring.

Of course, this legend has little to do with reality. The fact is that, whether or not the groundhog sees its shadow, there are another six weeks of winter. Look at your calendar, count 6 1/2 weeks into the future, and you'll see the first day of spring. Now count backward 6 1/2 weeks, and you'll see the first day of winter. Groundhog Day is halfway between the winter solstice and the spring equinox. And that is not coincidence!

These days halfway between an equinox and a solstice are called the cross-quarter days. They used to play a large role in pagan cultures (and still do among neo-pagan religions). We even have "holidays" on or near three of the four cross-quarter days: Groundhog Day, May Day, and Halloween.

What does a cross-quarter day mean for us? Not too much -- most of the northern hemisphere is in the middle of a cold winter, hoping that spring will come soon, and the southern hemisphere is enjoying summer vacation, and hoping that summer lasts a little longer. Around now you will probably start to notice the days getting noticeably longer pretty quickly. Around the winter solstice, the length of the day stays pretty constant for a while, but as the Earth continues to speed around our sun, the sun will appear to rise higher in the sky (for those of us living in the north), the days will get longer, and, eventually, the temperatures will get a bit warmer.

I make a big deal out of days like this, I realize. It's because the seasons are one of the most obvious links we have to the larger Universe around us, and one that most Americans don't understand. And it's a hard picture to grasp -- we are on a tiny ball of a planet that spins once a day and goes in circles around our parent sun once a year, which itself is orbiting the Milky Way galaxy every 250 million years, and our galaxy which is moving around our neighbor galaxies and slowly falling toward a cluster of galaxies 60 million light-years away. We're in constant motion, most of it extraordinarily slow by galactic standards. But we can sense some of it -- our planet's spinning, and its motion around the sun -- and these motions intimately affect our lives. Once we can grasp that, we've started to open our minds to a much larger universe around us.

The sun in 3-D!

Today's Astronomy Picture of the Day is our first ever three-dimensional picture of the sun. You'll need some red-blue 3D glasses to see it (I got mine from a McDonald's Happy Meal a few years ago, so they don't need to be expensive!).

Why do we need a 3-D view of the sun? From the Earth, the sun looks like a flat disk. We know it is spherical, and when we see a sunspot or activity of the sun, we can make a decent guess as to where it is based on our model of the sun. But many things stick out from the surface of the sun, like prominences, flares, and the solar corona. And when we see a solar flare or coronal mass ejection, it is a bit of a guess if these storms are headed for the Earth.

With the newSTEREO spacecraft, it should be possible to make better predictions as to if a solar storm is going to hit the Earth. And, since solar storms can disrupt radio, damage satellites, endanger astronauts, and even cause power outages, these are important advances in predictions of solar storms.

So, grab your 3D glasses and enjoy a safe look at the sun online!

Do cosmic forces shape your life?

Today, this story from Space.com suggests that cosmic forces shape life on Earth. No, I am not talking about astrology, the pseudo-science that claims (falsely!) the stars control our destinies.

The article claims that, in the fossil record, there appears to be a period of about 62 million years to major changes in life on Earth, and that this time period matches the period of one cycle in the sun's orbit around the Milky Way.

To go further, we need to understand how the sun orbits the Milky Way. The Milky Way is mostly flat, shaped like a Frisbee or a dinner plate, though it does bulge a little in the middle. We call this Frisbee part of the Milky Way the disk. The sun lies about two thirds of the way out from the center of the Milky Way, and for the most part traces a circle about the middle.

However, the sun also moves up and down, like a bobber in water. This is because the disk of the Milky Way isn't empty, but consists of stars and gas, each of which has its own gravity that pulls and tugs on the sun and our solar system. The amount of time it takes the sun to bob up and down once is about 62 million years, about the same length of time as the changes in life on Earth.

Now, when the sun moves up out of the disk (or "north"), it is moving in the same direction that our whole galaxy is moving. When the sun moves back down (or "south"), it is moving in the opposite direction. The scientists in this study claim that, when the sun is moving north, it moves out of the Milky Way's magnetic field, which helps to shield cosmic rays coming from other galaxies. If this is true, then the increased cosmic rays could spur changes in life on Earth.

Is this true? Maybe, but I remain skeptical. We know very little about the shape of the magnetic field of the Milky Way, and how far it extends into space. We know even less about how much cosmic radiation this magnetic field shields us from. And we don't know exactly how long it takes the sun to bob up and down once, or how much this bobbing might vary from one bob to the next.

But this is how science is done. The new study proposes an explanation for something we observe. Now it needs to make a prediction about something we don't yet know the answer to. If, in the future, that prediction holds up, then the hypothesis that the sun's bobbing motion affects life on Earth is made a little stronger. If that prediction fails, then the hypothesis is less likely to be true.

Once in the past, a scientist named Milutin Milankovich proposed that the Earth's ice ages were due to natural cycles in the exact shape of Earth's orbit. He predicted that, as the ice ages were more precisely dated, these cycles should match ice ages exactly. The idea was radical and not accepted by a lot of scientists until it was found that the timing of the ice ages follows these cycles with very high precision. Now, Milankovich's theory is well accepted by many climate scientists and planetary scientists.

So, it may end up that many extinctions and changes of life on Earth are due to this bobbing motion. Similar hypotheses have been made before, but these were not able to make testable predictions, a vital part of any scientific theory. We shall see!

Let the sun shine in

In one of this week's installments of the comic strip Mallard Fillmore, the strip claims thatglobal warming is being caused by the sun, and it cites two sources -- National Geographic news, and the Danish National Space Center. But reader beware. I haven't found a reference in National Geographic or on their website that claims the sun is causing global warming (I did find one article that claims the opposite), but that doesn't mean that a news story to that effect isn't buried somewhere. Meanwhile, the Danish Science Center does report that the sun is responsible for some global warming, but on the same page, they go on to say "That there exists a significant contribution from solar activity variations to global temperature increase does not, however, exclude other contributions to the rising global temperature, natural as well as human."

So, does the sun contribute to global warming? Yes, to some extent -- the latest numbers I hear are that the sun may cause, at most, 25% of observed global warming. But that leaves 75% to be accounted for, and the answer is unambiguously that people are responsible for most, if not all, of that. In February, scientists released a report that states that global warming is occurring and that humans are responsible. Among climate scientists, there is no significant debate about these facts. Yes, you will always be able to find some scientists who disagree. But the vast majority of scientists agree that humans are the primary cause of global warming. In fact, if anything, the February report likely understates the evidence for human impact on the climate. This is because the scientists had to get unanimous approval of politicians before issuing the report, including politicians from governments that ignore or downplay global warming, such as the U.S. and China.

Today, a second in a series of reports on global warming was released. This report details how scientists believe global warming will impact the environment, including impacts on humans. Yes, much in this report is speculation, but it is informed speculation based on nearly 29,000 separate data sets, climate models, research reports, etc. This isn't idle speculation -- it is a prediction. And again, political pressures forced the report to be watered down from what scientists truly believe.

The facts are that global warming is occurring, people are the primary cause, and the effects of ignoring global warming are dire for people as well as the environment. But let us be positive -- we can stop it, and without massive, sudden changes in our lifestyles. In fact, the sooner we start to change, the less drastic any human action has to be! The more we put off changes, the more drastic we will have to change in the future.

History shows that humans can change and make positive impacts on the environment. When I was in high school, the ozone hole was a big scare. Chemicals used by humans were causing a hole to develop in the ozone layer, and that hole was growing from year to year. We banded together as a species and stopped most production of those chemicals, and now the ozone hole is stabilized, and in some years appears to be shrinking. In the 60s and 70s, smog was a major problem in most major U.S. cities, but emissions controls on cars and industry has greatly reduced (though not eliminated) the problem. We have also eliminated or reduced many of the most pressing threats to our water supply through the Clean Water Act. And when certain pesticides were threatening many birds and other animals with extinction, we eliminated most of that threat. In none of these cases did we have to give up our way of life -- we made changes, sure, but the economy was not plunged into an endless depression, most of us in the U.S. still have our own cars, and our food supply has not been decimated by insects.

The same can be true for global warming. There are changes that we can and must make that will cost money and will be inconvenient, but, in the grand scheme of things, these changes are minor. First and foremost, we must reduce our use of fossil fuels and change to become completely dependent on renewable resources. This can be a gradual change, occurring over decades, but we must start that change. Energy efficiency can start with very simple changes. Replace normal incandescent light bulbs with compact fluorescents. During the summer, let the temperature in your house be one or two degrees warmer. If you have a use for a truck or SUV, that's fine. But also own a smaller car that you use when you don't need the carrying capacity of a larger vehicle. Shut down your computer when you are not using it.

These steps are small and won't solve the problem, but they are a beginning. We can be reasonable about our response to global warming -- we don't have to regress back to Stone Age technology. But we do have to get started. And the sooner we get started, the less dramatic the impact on our lives will be. Let's give the poor sun a break, admit that we are causing major and negative changes in the Earth's climate, and start working on solutions.

Added on April 9: The web site for the UN taskforce on global warming, the Intergovernmental Panel on Climate Change, is here.