Tuesday, August 26, 2014

New Evidence that We Live in a Space Bubble

Many people enjoy talking about their neighborhood, and how well they've come to know it. But do you know much about the "neighborhood" that our Sun and its planets hang out in? New evidence from an instrument launched aboard a NASA rocket has confirmed that our cosmic neighborhood really is a big bubble.

We have known for some time that our solar system sits inside a region which is emptier than the typical neighborhood in the Milky Way Galaxy. This "Local Bubble" (as it is called) is about 300 light years across, meaning light would take 300 years to cross it. Our diagram shows the bubble and some of the bright stars that are located in it. (The stars that looked brightest to our ancestors are the ones that wound up getting names. Many of the names we use today are Arabic translations of ancient Greek names.)

The Sun is actually located in a region that has a bit more loose gas and dust that the bubble. We call our slightly denser region the "local fluff." You can see the local fluff and other slightly denser regions inside the big bubble in yellow on our picture.

But what made this bigger bubble that we sit inside? Our best idea was that the violent explosions of giant stars ("supernovae") carved out this emptier region perhaps 10 or 20 million years or so ago. Some astronomers were not convinced of the exploding-stars origin of our bubble and thought that we might be fooled by some ways that our Sun's wind (the flow of atomic particles boiling off our star's surface) is interacting with the material in the fluff.

A new instrument, which looked for x-rays that come from collisions of atoms in deep space, was launched in 2012 aboard a rocket and got to spend five minutes above the Earth's atmosphere, where cosmic x-rays can be monitored. The data collected from those five minutes (!) was enough for the astronomers to confirm that the violence of exploding stars was the major contributor to the signs of the local bubble.

The exploding stars were not close enough to the Sun and the Earth to hurt life on our planet significantly -- after all, life on Earth thrived earlier than 10 million years ago and still thrives today. But it's clearer and clearer to astronomers that exploding stars have a lot to do with the geography and chemical makeup of our Galaxy. They not only made the bubble we find ourselves in, but we also know that it is the death of these stars that recycles the elements they get to make in their hot centers. This enriches the neighborhood with atoms like carbon, oxygen, and nitrogen -- the chemical building blocks of life as we know it. (Just about every atom of calcium in your bones, for example, was actually once inside a star that later exploded.)

By the way, our cosmic bubble is by no means unique. Other bubbles have been discovered near and far, blown by countless other star explosions over the 13 billion-year history of the Milky Way Galaxy.

For more technical details about this research, see the nice Science@NASA newsletter at: http://science.nasa.gov/science-news/science-at-nasa/2014/26aug_localbubble/

Sunday, August 17, 2014

Clouds Moving Across a Lake on Saturn's Moon Titan

Recently released images from the Cassini probe in the Saturn system show fresh clouds moving slowly above a giant lake on Saturn's moon Titan.  The lake itself is not water, but probably liquid methane and ethane (methane is what we call natural gas on Earth and ethane is an ingredient in fuel).

Now you might say, big deal, so there are clouds on top of a lake. I can see that anytime on Earth.  But the big difference is that the temperature on this distant moon is so cold, that water there is always frozen harder than rock.  So lakes, rivers, rain drops, and clouds can only form from substances that can stay liquid and gas at very cold temperatures.  Methane plays the same role on Titan that water plays on Earth -- it can be solid ice, flowing liquid, and gas in the atmosphere.  The clouds we see are condensations of droplets of methane. (As always, click on the picture to see it bigger.)

Even just having the images is a real tribute to our technology.  When we first got close-up images of Titan from space missions that flew by, its surface was not visible.  The entire giant moon is covered with a thick haze.  Light cannot make it through such haze (as people in Los Angeles on bad air days know all too well :-)), so we equipped the Cassini spacecraft with special instruments that can "see" infrared (heat rays) and radar -- waves that do get through the thick atmosphere of Titan.

The lake above which we see the new clouds is called Ligeia Mare (the Ligeian Sea), the second largest known body of liquid outside the Earth.  It's so big that if you walked around it, you'd walk 1240 miles.  Winds on Titan are pushing the clouds over the lake at speeds of about 7 to 10 miles per hour.

A large storm in 2010 cleared the atmosphere of Titan of most clouds, and astronomers who follow the weather on this distant moon have been waiting for the summer cloud patterns in Titan's northern hemisphere to return.

NASA's imaging wizards even put together a little movie of the cloud motions, which you can see at: http://photojournal.jpl.nasa.gov/archive/PIA18420.gif

The movie is not going to win any Oscars, but as you watch the few frames with different amounts of detail showing, remind yourself that you are watching images taken 900 million miles from Earth by a fragile spacecraft that has been orbiting through the Saturn system for almost 10 years.

By the way, if you want to see the shape of Lake Ligeia, see the image below, taken with radar, that shows it especially well:

Sunday, August 10, 2014

An Eclipse of the Sun from Mars

Our Moon is not the only astronomical body that can eclipse the Sun. In August of last year, the cameras aboard the Curiosity rover on Mars caught one of the little moons of Mars, Phobos, making an eclipse.

In the photo, you can see Phobos go across the face of the Sun. Note that the moon's shape is that of a potato (not a sphere).

Phobos is really a small moon (we believe it's an asteroid that Mars captured long ago) -- it is only about 16 miles wide in its longest dimension. The reason it covers so much of the Sun in the picture is that it orbits very close to Mars. It's only 3700 miles above the surface of the red planet.  (Compare that to the 240,000-mile distance of our own Moon!)

If you stood on the surface of Mars, Phobos would be a dim light in the night sky, rising in the west and setting in the east, and taking about 4 hours to go from horizon to horizon. (In other words, Phobos orbits Mars faster than Mars spins or rotates!)

The three black and white images of Phobos crossing the Sun were taken on Aug. 21, 2013 three seconds apart. Not a bad feat of photography by the team running the rover on its mission in Gale crater!

If you want to see Phobos better, below is a remarkable close-up photo of the little moon, taken with the Mars Reconnaissance Orbiter spacecraft, and photo processed to make the color differences more intense:

I was reminded of eclipses because I've just come back from an astronomy meeting where we discussed another August eclipse. In August 2017, our Moon will eclipse the Sun completely. The total eclipse will be visible in only one country -- the United States -- and in a band only 150 miles or so wide. Everyone else in North America will see a "partial eclipse" -- a nice bite taken out of the Sun. We estimate that 500 million people will be able to see the partial eclipse. This could become one of the most dramatic opportunities and challenges for astronomy education in our time. But we'll talk more about that in future blog posts, as the time gets closer.

Wednesday, August 6, 2014

Close-up View of a Comet

Today the Rosetta spacecraft closed in on Comet 67P (the P stands for periodic comet, meaning it comes around again and again every six and a half years. ) It's also known as Comet Churyumov–Gerasimenko (C-G). Look at the amazing image our cameras sent back, taken from a distance of only 177 miles.

The comet and the spacecraft are currently still about half way between the orbit of Jupiter and the orbit of Mars.  In this exciting rendezvous, Rosetta will match course with the icy comet and then stay with it as it gets closer and closer to the Sun and its ice begins to "sublimate" (turn from ice directly to a vapor in the vacuum of space.) 

We hope to stay with the comet for a year, as it goes inward and then swings back outward again, watching the Sun's light and heat playing with the comet all the while. In November, part of Rosetta will actually land on the icy surface of Comet 67P.

But for now, just enjoy the weird and wonderful image.   The comet's shape is definitely not symmetrical.  Is it two ice pieces that stuck together in an ancient collision?  Have past encounters with the Sun's heat resulted in this odd shape?

The comet is 2.5 miles wide, and you can see details as small as 17 feet (5.3 meters) across on our picture.   Can you see the individual "boulders" or "ice rocks" sitting on the comet's surface?

If you'll pardon the expression, what a cool picture!

By the way, click on the photo to see it bigger and with more detail.

Wednesday, July 30, 2014

A Special Event near San Francisco This Sunday

I would like to invite all of you who live in or near the San Francisco area to a very special event this Sunday, at the Hyatt Regency Hotel near the San Francisco Airport, in the town of Burlingame. I will be moderating an afternoon discussion with three of my favorite astronomers (all excellent public speakers.)

Our topic will be "Beyond Earth: Planets, Life, and Intelligence in the Cosmos" and we will discuss the search for planets around other stars, for life on the worlds in our own solar system, and for intelligent life elsewhere in the universe. Geoff Marcy is considered the foremost planet hunter on our planet, having discovered more planets orbiting other stars than any other human being. Jill Tarter has headed the search for intelligent radio signals from ET for many years, and is the scientist whom Jody Foster was playing in the film "Contact." Chris McKay is one of the top Mars experts in the world and explores places on Earth that most resemble Mars.

The program is sponsored by the non-profit Astronomical Society of the Pacific, which is celebrating its 125th anniversary this year. The illustrated talks and panel will go from 1:30 to 5:30 pm at the Hyatt Hotel, 1333 Bayshore Highway, Burlingame, CA 94010. It's all free and open to the public, but the sponsors hope each attendee will make a donation of $10 to help pay for the expenses at the hotel. We expect big crowds, so come a little early to get a seat.

For more information and to RSVP, go tohttp://www.astrosociety.org/universe2014

With almost 2,000 planets now known to orbit other stars, and with many stars having more than one planet (just like our Sun does), this is the time that topics that seemed like science fiction only a few years ago are becoming an exciting part of science. Hear the latest from those at the forefront of this work.

If you know people living near San Francisco who might be interested, perhaps you can help me by sending this information on to them.

Sunday, July 20, 2014

Strange Shape to a Comet We are About to Visit

On Wednesday, August 6, the European spacecraft Rosetta is going  to have a close encounter of the best kind with a comet (a chunk of cosmic ice mixed with dirt.)  It's called Comet 67P (the P stands for periodic comet, meaning it comes around again and again every six and a half years.)  Its more informal name is Comet Churyumov–Gerasimenko, after the two astronomers who discovered it on a 1969 photograph. 

In mid-July when our photo was taken, the spacecraft was still more than 7000 miles from the comet, but it was already becoming clearer in the camera.  And, as you can see, 67P/C-G is weird looking. Instrument project manager project manager Carsten G├╝ttler said,“The images faintly remind me of a rubber ducky with a body and a head." 

What could cause the icy body of this comet, which is roughly two and a half miles across, to look like this.  One possibility is that it is really two comets stuck together, something we have seen in other comets (such as 8P and 103P). Or maybe it was one comet that broke apart into pieces when it got too close to the gravity pull of a big planet like Jupiter, and this odd fragment is all that's left.

Another possibility is that early in its life the comet got hit by other chunks of cosmic ice or rock, carving out big pieces of it and leaving great rounded valleys behind.

We should learn more when we get closer to this ancient icy visitor, and especially when part of Rosetta attempts a landing.  Stay tuned. 

In the meantime, here is a great animation of the images from this past week, showing the comet spinning in the majestic darkness of 
space: http://sci.esa.int/rosetta/54356-rotating-view-of-comet-67pc-g-on-14-july-2014/ 

Sunday, July 13, 2014

Finding an Invisible Star Corpse

An international team of astronomers recently reported a wonderfully strange discovery.  They found the faint, cold "corpse" of a dead star, not by detecting any light from it, but through its gravity grip on another star corpse with which it shares a system.  It's quite a story!

To follow it, we need to take a brief excursion into the gruesome deaths of stars.  All stars collapse at the end of their lives, but stars with different amounts of mass (stuff) die differently.  Less massive stars (like our Sun) eventually die by collapsing into a ball not much bigger than a planet -- such a white-hot but tiny corpse is called a "white dwarf."  They start out as really hot, but cool off as they shine away their light and heat into the darkness of space.

More massive stars have a more complicated death in store for them. They die in a sudden event, where the core of the star collapses catastrophically, while the rest of the star blows up in a giant explosion called a "supernova."  We have discussed such explosion in several posts. 

What interests us today, however, is the tiny core of the star which is super-squozen (that's a technical term!) by the star's death.  It's typically not much bigger than your average suburban town, and called a "neutron star" -- because in it all the atoms lose their identity and become neutrons.

Such a neutron star, surrounded by an "atmosphere" of materials from the messy explosion of the rest of the star, can sometimes be detected by faint pulses of radio waves it gives off, and is then called a "pulsar."  (A watch company "borrowed" that name, but we astronomers had it first.)

So now here's the discovery.  Astronomers using radio telescopes first discovered a pulsar in the constellation Aquarius, indicating that a neutron star was revealing itself to us.  But the pulsar showed signs of wobbling, as if something with strong gravity was orbiting around it and pulling on it.  Careful measurements reveal that the orbiting object is a white dwarf, and it takes only two and a half days to revolve around the neutron star. (Remember that Earth takes 365 days to go around the Sun, so these two star corpses are in an intimately close dance.)

To their amazement, when other astronomers tried to find the light or heat of the white dwarf companion, they COULDN'T -- not even with the biggest telescopes.  That white dwarf must be so old that it has cooled down below the level where we can find it (at its distance of about 900 light years.)  This makes it the coldest, dimmest star corpse ever found.  If you are into star corpses, and we astronomers really are -- this is a big deal.  It's remarkable that with today's technology, we can know this white dwarf is there not by its light, not by its heat, but just by its pull on another dead star nearby. 

Note: Our image is just a painting, not any kind of photograph.  It shows the pulsar on the left (with two beams of radio waves coming from it) and the white dwarf on the right.  Although we can see light coming from the white dwarf in the painting, in real life the star is so faint and far away, no light from it is detectable with today's instruments.