Wednesday, December 24, 2014

An Amazing Image of Saturn's Rings for the Holidays



Carolyn Porco, the dynamic leader of the Imaging Team for the Cassini Mission (exploring Saturn and its neighborhood), has sent a remarkable holiday greeting card from Saturn and I wanted to share it with you.

What we are looking at is a view of the inner part of Saturn's complicated ring system (mainly the so-called B ring.) We are seeing about 750 miles of the white rim of the B ring, beyond which is the dark emptier space called the Cassini Division. You can see a number of thin "ringlets" in that division toward the upper part of our picture.

But what is really stunning is what the long shadows cast by the Sun reveal at the edge of the bright main ring. You can see peaks or thin mountains of ice particles, sticking up more than a mile and a half (2 km) above the plane of the rings. Compare those heights with the typical height of the ring system, which is about 10 meters (or about 30 feet) in most places.

Here is what Carolyn says about the image, "Cassini scientists believe that this is one prominent region at the outer edge of the B ring where large bodies, or moonlets, up to a kilometer or more in size, are found. It is possible that these bodies significantly affect the ring material streaming past them, and force the particles upward, in a "splashing" manner."

She continues, "This image [taken in 2009] and others like it are only possible around the time of Saturn's equinox, which occurs every half-Saturn-year, or about every 15 Earth years. The illumination geometry that accompanies equinox lowers the sun's angle to the ring plane and causes structures jutting out of the plane to cast long shadows across the rings."

The rings of Saturn are made of billions and billions (as Carl Sagan used to say) of individual chunks of ice -- mostly water ice. The particles range in size from smaller than a sand grain to larger than a modern SUV. We have known for some time that the interaction of moons and rings in the Saturn system can create a wide range of artistic patterns using the ring particles as their raw materials. But these tall icicles or icy towers are among the most spectacular we have ever seen.

Happy Holidays. May 2015 bring all of us more amazing views of our cosmic environment and increase our understanding that the differences that separate us on Earth are so trivial compared to the truly alien nature of what's out there.


[Click on the image to make it bigger and see even more detail!]


Tuesday, December 16, 2014

You Could Name a Crater on the Planet Mercury




The team of scientists and educators behind the Messenger spacecraft orbiting the planet Mercury is giving the public the chance to nominate artists, writers, or composers whose names deserve to be on one of five craters on the hot little planet closest to the Sun.

There have been a number of publicity campaigns recently that deal with naming things, but this one is the real thing. By international agreement, the naming of worlds and features in space is done by a special committee of the International Astronomical Union (the U.N. of astronomers.) But the committee is happy (when there is no duplication or veering from tradition) to honor the wishes of the discoverers. And the Messenger team, having discovered many new craters on the wonderful close-up images they have obtained, is happy to give the public a chance to participate.

Also by agreement, Mercury's huge population of impact craters is going to be named only for well-known composers of music, serious artists, and writers of renown. The most famous people in these categories (Beethoven, Van Gogh, Tolstoy) already have craters named for them. So now, it's time to find less well-known people from a wide range of cultures to be honored. The Messenger team will submit their 15 favorite entries to the committee to select the names of five newly discovered craters. You can see photos of the craters and get all the details on the contest website: http://namecraters.carnegiescience.edu/home

You are encouraged to check first to see if your favorite candidates don't already have a crater. I was pleased to see that Jean Sibelius, one of my favorite composers, was already on the list. On the other hand, Alexander Scriabin, who took romantic music in amazing new directions, and wanted to combine sound and light long before there were laser shows, still hasn't got a crater to his name anywhere in the solar system. Maybe some of us have to nominate him.

The rules say anyone you nominate has to have been publicly known for at least 50 years and dead for at least three. But beyond that, you are encouraged to get -- pardon the expression -- creative. The contest starts today and is open for only one month. There is no prize, only the satisfaction of knowing that you have helped put someone whose work you really enjoy on another world.


To see all the people who have given their names to features in the solar system, you can search at: http://planetarynames.wr.usgs.gov/AdvancedSearch  (putting the last name into the field called "Feature Name.")

Wednesday, December 10, 2014

The Lake that Built a Mountain on Mars




NASA’s instrument-laden Curiosity Rover is now at the foot of Mt. Sharp, its ultimate target on Mars. New results from the rover have confirmed and filled in our ideas about what the 3-mile high mountain is doing in the middle of 96-mile wide Gale Crater. And, most important, the new observations give us even more confidence in the notion that ancient Mars was very different from Mars today – it may well have had a much thicker atmosphere, flowing rivers, and occasionally full lakes of water.

Mt. Sharp is an interesting mountain, in that it seems to be built up out of layer after layer of sediment. This material might have been carried to the center of the crater by either water or winds, scientists thought. The new results indicate that both water and wind may have had a role in building Mt. Sharp over the millennia.

We already knew that one or more dry river channels end at Gale Crater, making it likely that long ago, water probably flowed into the crater. Over long periods of time, the crater lake may have formed and evaporated again and again. Rivers flowing over the red sands of Mars would have carried quite a bit of sediment into the crater and would have deposited this material in its center. As the central mountain began to accumulate, the next river flood would lap up against it. The resulting waves could carry material higher than the river’s original level, thus building up the mountain further.

When the lake was dry, the big wind storms, that other instruments have shown us are a regular feature of Mars weather, could have added wind blown sand to the top of the mountain. So Mt. Sharp could have grown during both wet and dry periods.

Curiosity is only at the bottom of the mountain right now, but it has the ability to climb up to higher layers over the coming months. Scientists are very interested in what the layers higher up might reveal to us about how Mt. Sharp grew and how the climate on ancient Mars changed as time passed.

Our top picture, taken on Mars on Nov. 2, shows some of the layers building up at the bottom of Mt. Sharp in a formation scientists have nicknamed "Whale Rock." For a version of this picture that shows scale, see: http://photojournal.jpl.nasa.gov/figures/PIA19081_fig1.jpg 


The picture below is an earlier "selfie" that the Curiosity rover took showing its instruments and cameras at work. Click on either photo and you'll see a larger version, full of amazing detail -- straight from the surface of Mars!

Sunday, November 30, 2014

A Rogue Black Hole Escaping its Galaxy


Astronomers have announced the possible discovery of a big black hole that has escaped the galaxy of stars that gave birth to it, a phenomenon that had been predicted in theory, but not observed.

Galaxies (great islands of stars) that share a neighborhood in space can sometimes collide. If each has a big black hole at the center -- as many galaxies do -- usually the two black holes collide and merge. But, under the right conditions, the black holes can engage in a "game of pool" (billiards for those with more refined tastes.)

One of the big black holes can "recoil" or "rebound," and wind up being shot out of the combined galaxies. This is a pretty unusual circumstance, and astronomers have been searching for an example. Now, it looks like they may have found one.

The project involved observations with several telescopes, including the Swift satellite and the giant Keck telescope in Hawaii. The galaxy that remains is called Markarian 177 (a name that comes from a catalog of disturbed galaxies made by an Armenian astronomer.) The object that may be the escaped black hole has a number from the Sloan Digital Sky Survey catalog -- SDSS1133.

The galaxy is about 90 million light years away in the bowl of the Big Dipper. As shown on the accompanying image, the rogue object is about 2600 light years from the center of the galaxy.

As black hole fans are now probably thinking as they read this, it's not the actual black hole we are seeing on this highly magnified Keck infra-red image. Black holes are black and hard to see against the black of space. The SDSS object is a black hole surrounded by gas and dust that the black hole is "eating." It is the ring of "food" that we see glowing on the picture.

There is a small possibility that the SDSS object is something else entirely, such as an exploding star, although there is evidence from earlier images against that interpretation. Astronomers plan to use the Hubble Space Telescope to get even more information about this mysterious pair of objects.

If the black hole interpretation holds up, this will be a new creature in the "astrophysical zoo" to add to the many weird things we have been discovering lately. Rogue black holes are already in use in science fiction (such as Allen Steele's novel "Spindrift") and may now join the real universe as well.

Thursday, November 20, 2014



The Movie Interstellar: The Science Behind the Story

Lots of my students are asking questions about the new film Interstellar.  I have been hearing about the movie for some time; the studio even sent me a “teachers’ guide” that was not especially useful, but piqued my interest.  Most exciting for me was the news that the scientific advisor (and one of the producers of the film) was Prof. Kip Thorne of Caltech, who is arguably the world’s expert on black holes and wormholes -- and designed the science behind the “galactic subway system” in Carl Sagan’s novel and film Contact.

I wrote to Dr. Thorne and he told me that he had indeed been involved with Interstellar, and had, in fact written a book, entitled The Science of Interstellar, to explain the complicated science that was the basis for some of the more intriguing scenes and events in the film.   The book also tells the story of the film, which was always supposed to be strong on science, and, at one time, was set to be directed by Stephen Spielberg.  But that original deal fell apart and it took a while to bring Christopher Nolan, the film’s current director, into the project.

Dr. Thorne wasn’t the only scientist involved in the long period before the movie went from an idea to a completed production.  Yesterday, I was talking with Dr. Frank Drake, the father of the scientific search for extra-terrestrial intelligence, and he told me that he was on one of the early panels of scientists brought together (with Stephen Spielberg) to make sure the plot stayed close to real science and possible science.

With Christopher Nolan and his brother Jonathan (a writer of screenplays) involved, the film evolved from the script Dr.Thorne and producer Lynda Obst had first envisioned, to include much more about a future Earth facing ecological disaster.  Still, the original thought, to portray black holes and worm holes as accurately as modern science can make it, was high on everyone’s list of priorities.

A black hole is a place where the death and collapse of a huge star has produced such strong gravity, that space itself is “warped” -- and nothing, not even light can escape.  Near a black hole, time proceeds more slowly than in the rest of the universe, and this change in the flow of time becomes a major plot element in the movie.  Both the existence of black holes and their strange effect on time have been demonstrated by many experiments and are well established.

A wormhole is more speculative, but something Einstein himself thought a bit about.   It’s a place where a black hole or some other unusual feature of space and time becomes a tunnel (or short cut) from one place in space to another place very far away, or even to another time.  In a wormhole, the dangerous effects you would feel falling into a black hole are somehow kept at bay, so that a spaceship can go through it to elsewhere or elsewhen.

Both wormholes and black holes get major roles in Interstellar.  The astronauts in the movie first use a wormhole to get from our Milky Way Galaxy to another galaxy far far away.   Then, they wind up in the new galaxy in a location where there is a massive, spinning black hole, with planets around it.  I won’t give anything else away, except to say that the special effects showing the wormhole and the gargantuan black hole were composed from calculations made by Dr. Thorne and his team at Caltech, fed directly to the computers of the special effects team for the movie – and they are truly SPECTACULAR.

For more, I recommend reading Dr. Thorne’s book (pictured in this post) and going to see the movie – especially if you are a black hole fan.  Dr. Thorne also has a web page with some animations at: https://interstellar.withgoogle.com/transmissions

Monday, November 10, 2014

Landing on a Comet: High Adventure in Space


This Wednesday, a European landing craft called "Philae" will attempt humanity's first landing on a fast-moving comet (a complex chunk of ice approaching from the depths of space.)

After a 10-year, 4-billion mile journey to Comet Churyumov-Gerasimenko (C-G for short), the Rosetta spacecraft (which is now orbiting the comet) will drop a probe about the size of a kitchen range from a height of 13 miles.  Taking some 7 hours to slowly land on its icy target, Philae will be moving at only 2 miles per hour at the end.  Still, the gravity of the two-mile wide comet is so low, it could bounce off and move away or simply fall over and roll.   To prevent this, the designers have equipped Philae with harpoons to grab the comet and with legs that have rotating screws in them to hold on to the ice for dear life.

Our robot representatives have only landed on six worlds so far: the Moon, Mars, Venus, Saturn's moon Titan, and two asteroids.  None of those landings were quite as difficult and strange as this one.  It takes radio signals from Rosetta about half an hour to get back to Earth even at the speed of light.  Thus the European Space Agency controllers can't help Philae if it gets into trouble.  Its own computer software will have to make the decisions that will lead to its survival or loss. 

For a complete picture of Comet C-G, see my August 6 post.  The photo accompanying today's post is a fantasy montage, showing the Philae lander safely on the comet's icy, boulder-strewn surface.  The full comet has a weird L-shaped structure, as if two oval comets had somehow stuck together at a weird angle. Because its shape is so complex, its gravity is not simple either, making landing even more challenging. But if Philae lands, its 10 instruments will give us our first-ever up-close look at one of the chunks of ancient ice that are building blocks left over from the early days of our solar system. 

Keep your fingers and toes crossed that the landing succeeds.  For a basic animation of what the spacecraft will do and what instruments it carries, please see: http://www.youtube.com/watch?v=szKZ77MbF9Q  

A fuller documentary about the Rosetta mission, see: http://www.youtube.com/watch?v=cArihDTnOZg 

Sunday, October 26, 2014

A Dark Spot on the Sun Seething with Energy


Many of you in North America who saw the partial eclipse of the Sun last Thursday may have noticed a nice dark spot under the eclipsed part of the Sun.  The full "active region" (that the visible spot is part of) is bigger in area than the giant planet Jupiter and has become the largest such feature on the Sun in 24 years.

With the kind permission of the photographers, I am posting here a beautiful close-up of the area, by Randall Shivak and Alan Friedman.  Look how wonderfully complex it is!

The darker regions on the Sun are called sunspots; they look darker because they are slightly cooler than the Sun's visible surface layer. The Sun is made entirely of seething hot gas, where the atoms are so hot, they easily lose their electrons.  This makes our Sun highly magnetic, and as it spins, its magnetic zones get all twisted up.  It's those twisted regions that appear to us as active regions.

Astronomers have already observed some "flares" -- sudden releases of extra energy -- from this region, but they haven't been sent in our direction in space.  The region is now facing the Earth as our Sun does its slow rotation.  So we may get some extra high -energy particles coming our way in days to come (or not -- weather from the Sun is as hard to predict as weather on Earth.)

For a nice movie of this region "crackling" with magnetic energy as the Sun rotates, see: http://apod.nasa.gov/apod/ap141022.html

Wednesday, October 15, 2014

A Comet Passes by Mars; An Eclipse of the Sun

 

On Thursday afternoon, Oct. 23, there will be a partial eclipse of the Sun, visible from the U.S. Different amounts of the Sun's area will be covered by the Sun. (For example, 40% of the Sun will be covered from the San Francisco region.) I have put question-and-answer introductions to the eclipse at: http://www.astrosociety.org/society-news/2014-eclipse-fact-sheets/

You can consult the national sheet for the degree the Sun will be eclipsed in your area. Here's wishing everyone clear skies. And please remember, do NOT look at the Sun without proper protection. Follow the instructions on the sheets or go to an astronomy place (observatory, college, planetarium) near you to join an eclipse viewing session.

This Sunday, Oct. 19th, a comet will pass closer to Mars, than any comet has passed to the Earth in recorded history. Comet Siding Spring (named after the observatory where it was discovered) will pass within only 87,000 miles of Mars. An armada of telescopes near Mars and Earth will try to get a good glimpse of it. From its orbit, this appears to be what you might call a "virgin" comet, making its first entry to the inner solar system. It is coming from the distant reservoir of ancient "icy chunks" that surround our solar system in a giant cloud.

A comet is just such an icy chunk that comes close enough to the Sun so that the Sun's energy and wind begin to evaporate the ice and loosen the dust frozen inside it for billions of years. These comets thus contain some of the original material from which our solar system was formed some 5 billion years ago. 

To learn more about NASA's plans for observing this Mars visitor, see: http://science.nasa.gov/science-news/science-at-nasa/2014/09oct_cometprep/


For a while, NASA scientists worried that dust from the comet may damage some of the spacecraft around Mars, but recent calculations are showing that the path the comet is taking means that its dust is not likely to pose a serious threat.  But we'll see Sunday.  Perhaps there will be faint "shooting stars" (dust burning up in the thin atmosphere) on Mars.

Sunday, October 5, 2014

HBO is Showing "Einstein and Eddington" Now


In 2008, the BBC made a biopic (or dramatization) of events in Albert Einstein's and Arthur Eddington's life and work, and how they intersected. It was cosponsored by HBO and I just found out that HBO is currently showing this film (you can see it in the "On Demand" section of your cable TV offerings if you subscribe to HBO.) It may not last long, and it is not available on DVD yet in the U.S.

I recommend it with some enthusiasm (it's moving, and fun to watch), but also many reservations (the science and history are not always accurate, or -- to be charitable -- are twisted or changed in the interests of higher drama.)

I think everyone has heard of Einstein, but Arthur Eddington was an astronomer and physicist in England in the early part of the 20th century who contributed a lot to our understanding of how stars work. He was also a key member of the eclipse expedition in 1919 that tested if Einstein's crazy new theory of gravity, space, and time -- the general theory of relativity -- was correct. Measurements during that eclipse, and especially during a later eclipse made by a team from the Lick Observatory, established that Einstein was right and that the universe was more complex and beautiful in its inner workings than earlier scientists had imagined.

The film begins and ends around the eclipse expedition, but then goes back in time to set the scene. Many historical details are wrong or skipped over -- Elsa was divorced with two kids when she re-encountered Einstein in Berlin, the famous image of Einstein sticking out his tongue was later in his life, Eddington didn't have to tell Einstein about Mercury, etc. But such details don't matter to most viewers, and sometimes mixing things up a bit helps move the story along. And the flavor of the excitement around relativity is well characterized, with the two main actors doing a nice job in portraying the scientists and their personalities.

If you get HBO, or have a friend who does, and have a chance to see it, I recommend the film for everyone except historians of 20th century science, who will have a fit about those details. (The same production team also did a biopic about Stephen Hawking, called just "Hawking" and I gather you can find that film in segments on YouTube.)


Above you see the two scientists as portrayed in the movie, below you see the two of them from real life.
(For movie and TV fans, I can't resist adding: Gollum plays Einstein, Dr. Who plays Eddington.)



Sunday, September 28, 2014

A Baby Galaxy with a Grown-up Black Hole Inside



Using the Hubble and Gemini telescopes, astronomers have found a mystery --  a tiny galaxy that has a huge black hole in its center. That hungry black hole has eaten enough material to make 20 millions Suns!

The baby galaxy is really small -- its diameter is only 300 lightyears.  It's crowded in that little space; it contains about 140 million stars.  (Compare it with our Milky Way Galaxy, which stretches over 100,000 lightyears, and contains at least 200 billion stars.  Yet our central black hole has eaten only about 4 millions sun's worth of material.)  How could such a baby galaxy have such a big black hole?

Astronomers are no longer surprised to discover giant black holes at the centers of most galaxies.  Where a galaxy is most crowded (in its middle) is where a black hole (a star corpse with enormous gravity) has the most "food" to eat and can therefore grow.  But, in general, we have found that the larger a galaxy, the larger the monster black hole at its center.  So finding a baby galaxy sporting such a big black hole comes as a huge surprise.

A clue to this mystery comes from the name of the baby galaxy -- its awkward designation is "M60-UCD1."  UCD stands for ultra-compact dwarf (galaxy), which makes sense.   But M60 refers to the 60th entry in Charles Messier's catalog of fuzzy sky objects published in the 1780's.  That Messier catalog features some of the brightest and easiest to see galaxies and nebulae.   There is no way a tiny faint baby galaxy would have made his list!

It turns out that our baby galaxy is orbiting the much larger and brighter galaxy called M60.  In our picture, you see M60, a huge, blimp-shaped "grown-up" galaxy, which has its own super-massive black hole at the center.  You can see our baby galaxy in the inset of the photo. (You may need to click on the image to see it well.)

The fact that our baby galaxy is a "satellite" of the big galaxy may explain the mystery of its small size and big black hole.  The discoverers suggest that in the distant past, our baby was actually a big galaxy, with many more stars (explaining how it got its big black hole.)  But it had a "close encounter" with M60.  The gravity of the big galaxy stripped away its outer stars, leaving the "victim" of this encounter much smaller. 

If M60 took away and absorbed the outer layers of its neighbor, that would make M60 a cosmic cannibal.  That sounds awful, but in recent years astronomers have begun to realize that just about every big galaxy has grown to its present size by cannibalizing some of its smaller neighbors.  

It's a dog-eat-dog world out there among the galaxies, and the big bullies really get to throw their weight around.  Our little galaxy was once a more regular member of the galaxy club, but it lived in a rough neighborhood and got really beaten up by the local gravity bully.  Now it's a mere shadow of its former self.  

Sunday, September 21, 2014

Happy Fall Equinox on Monday and Other Sky Events


Monday morning (Sept. 22) is the "autumnal" or fall equinox for those of us in the Northern Hemisphere. On that day, the length of the day and the night are roughly the same ("equinox" means equal night). We are moving from the summer, when the days were longer, toward the winter when our nights are longer -- and our parts of the planet have fewer hours to heat up from sunlight.

The equinox is sometimes called the official start of fall and throughout history there have been festivals and beginnings celebrated at the time. It so happens that Foothill College, where I teach, starts its fall quarter on the equinox this year, so I will be welcoming several hundred new students to my classes with a happy equinox greeting (and many who don't know the term will be looking at me as if I were a crazy person!)

The fall quarter will bring many interesting events to fans of astronomy. Tonight, Sunday, the MAVEN spacecraft will be inserted into orbit around Mars, so it can begin to study how little Mars, with its lower gravity, lost its atmosphere over the millennia, and how its remaining air layers interact with the radiation and wind from the Sun.

October 8th, we will have a total eclipse of the Moon (these are much more common than total eclipses of the Sun.) Alas, for those of us in North America, this will be a middle of the night eclipse! The full Moon will go dark as the Earth's shadow falls on it, but most of us will be in bed, deeply asleep, when it happens. For example, in San Francisco, the eclipse happens from about 2:30 am to 5:30 am, so only dedicated night owls will be watching it. (If you miss it, don't worry, there will be two lunar eclipses visible in 2015.)

On October 19th, a newly discovered comet will come closer to the planet Mars than any known comet has come to Earth, and so our robot "representatives" around Mars will be keeping their cameras out for that pass.

On October 23rd, in the afternoon, much of North America will witness a partial eclipse of the Sun in the afternoon. Many astronomy organizations will be planning eclipse parties, with safe viewing options. I will do a full post about the eclipse when we get closer to the time.

And, down on Earth, October 25th, the opening day of the 2014 Bay Area Science Festival, I will be giving a free public talk on Mt. Tamalpais about the "Top Tourist Sights of the Solar System: Where Bill Gates' Great-Granddaughter will go for Her Honeymoon." See: http://wonderfest.org/top-tourist-signs-of-the-solar-system/

Wednesday, September 3, 2014

A Beautiful Image with a Nice Ring To It



Here is a dramatic image of a dying star, courtesy of amateur astronomer and master photographer Robert Gendler. Called the "Ring Nebula," this cloud of of expelled material surrounds a star somewhat like our Sun, but further along in its life cycle.

Usually, regular telescopes only show the inner glowing part that you see in bluish green in the center. But Dr. Gendler has combined the visible-light image with fainter, cooler infra-red information to show how the star has expelled material not just once, but many times. You can see shell after shell surrounding the star. Like a dying man in those old Victorian novels, who coughs and coughs for months before death releases him, this star has been "coughing up" its outermost layers, as it adjusts to the final internal collapse. After the expanding shells have moved away, what will be left is a dense, hot "star corpse" astronomers call a white dwarf.

The image pixels come from the Hubble, Subaru, and Large Binocular Telescopes. By all means click on the picture and look at the larger version.

The Ring Nebula (a favorite astronomical object for newlyweds) is about 2000 lightyears away in the constellation of Lyra. It is perhaps the best known example of a "planetary nebula." (The name comes from their fuzzy appearance in early telescope; the expanding shell of gas has NOTHING to do with planets.) Astronomers also call it M57 (the 57th entry in Charles Messier's catalog of fuzzy objects in the sky.) If you search for M57 on the web you can learn a lot more about it; or just enjoy the weirdly wonderful picture.


Robert Gendler's other astronomy images can be found at: http://www.robgendlerastropics.com/  

To see a larger version of the amazing Hubble Space Telescope image of this object (which is at the center of our picture), go to: http://hubblesite.org/newscenter/archive/releases/2013/13/image/b/format/large_web/ 

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.

Friday, July 4, 2014

Cosmic Fireworks



For those of you who don't get to see fireworks today (July 4), here is an image of cosmic fireworks for your enjoyment. The glowing ring on our picture (taken with the Hubble Space Telescope) shows material ejected long ago from a star whose total explosion we saw in 1987. Recently, earlier debris from the star has been hit by the fastest moving material from the star's final explosion. The shock of collision has set knots of thicker material in the ring to glow like a necklace of sparklers.

The dramatic explosion at the end of the life of a star is called a "supernova," and this particular supernova is located in one of our closest neighbor galaxies, the Large Magellanic Cloud, about 168,000 light years away. We wish it were closer, but a visible supernova is a rare event. This was actually the first supernova explosion we have been treated to since the invention of the telescope in 1609-1610. It's been given the extremely clever name Supernova 1987A (the first such explosion seen in 1987.)

The huge star whose explosion we saw in 1987 had been losing some of its mass in earlier periods of its life and was surrounded by a cloud of its own debris. Later a wind of hot particles had blown from the star as it went through a different stage of its development. That wind had made a cavity in the gas around the star.

Now the material from the star's actual explosion, which has been moving for 27 years at great speed, has reached the inner walls of this cavity and is colliding with thicker blobs of the older material. That's the glowing ring we see so clearly in the Hubble image.

If you want to see a time lapse movie made of Hubble images taken each year as the ring started lighting up, you can see it at:
http://en.wikipedia.org/wiki/SN_1987A#mediaviewer/File:SN1987a_debris_evolution_animation.gif

Happy fireworks day!

Tuesday, July 1, 2014

A Wonderfully Scientific Science Fiction Film


Last night, thanks to the Wonderfest organization in San Francisco, I got a chance to see a science fiction film that was in theaters for so short a time that if you blinked, you probably missed it. It's called "Europa Report" and it is the story of a crewed expedition to Jupiter's intriguing moon Europa. This world, one of the four large moons of Jupiter discovered by Galileo, is an ice-covered ball that may well have a large liquid ocean under its surface.

You can see Europa's cracked surface on the picture I have posted above. (This is actually a montage of images from NASA's Galileo spacecraft, with additional image processing by Professor Ted Stryk to bring out more detail.) One possibility is that the dark cracks we see are places where material from deeper inside the Moon (perhaps even from the liquid ocean) is seeping up toward the surface.

In the movie (see the poster below), future astronauts (from a private space company) go to Europa to search for evidence of life. The complex story is told by interweaving the view from cameras in their suits and in their cabins, with footage they recorded for sending back to Earth and interviews with the mission controllers, who eventually lose contact with the ship. What the surviving astronauts eventually find is far more than the micro-organisms scientists currently hope might exist deep under the ice of Europa.

The film, by Ecuadorian director Sebastian Cordero, is now available on DVD and various internet film services. If you enjoy somewhat complicated but scientifically reasonable space stories, I commend it to your attention. If not, spend a little time Googling other images of Europa and check out one of the strangest worlds with which we have the pleasure of sharing the solar system.

Photo: A Wonderfully Scientific Science Fiction Film

Last night, thanks to the Wonderfest organization in San Francisco, I got a chance to see a science fiction film that was in theaters for so short a time that if you blinked, you probably missed it.  It's called "Europa Report" and it is the story of a crewed expedition to Jupiter's intriguing moon Europa.  This world, one of the four large moons of Jupiter discovered by Galileo, is an ice-covered ball that may well have a large liquid ocean under its surface.

You can see Europa's cracked surface on the other picture I have posted with this report.  (This is actually a montage of images from NASA's Galileo spacecraft, with additional image processing by Ted Stryk to bring out more detail.) One possibility is that the dark cracks we see are places where material from deeper inside the Moon (perhaps even from the liquid ocean) is seeping up toward the surface.

In the movie, future astronauts (from a private space company) go to Europa to search for evidence of life.  The complex story is told by interweaving the view from cameras in their suits and in their cabins, with footage they recorded for sending back to Earth and interviews with the mission controllers, who eventually lose contact with the ship. What the surviving astronauts eventually find is far more than the micro-organisms scientists currently hope might exist deep under the ice of Europa.

The film, by Ecuadorian director Sebastian Cordero, is now available on DVD and various internet film services.  If you enjoy somewhat complicated but scientifically reasonable space stories, I commend it to your attention.  If not, spend a little time googling images of Europa and check out one of the strangest worlds with which we have the pleasure of sharing the solar system.

Wednesday, June 18, 2014

Water Found on Largest Asteroid



Astronomers working with the Herschel space observatory have discovered water vapor coming from the largest asteroid in the asteroid belt, the one called Ceres (pronounced like "series"). By strict definition, Ceres is so big and round that it is no longer considered an asteroid, but is now designated dwarf planet number one (having been discovered on Jan. 1, 1801, long before Pluto).

Ceres is about 600 miles across and takes 4.6 years to orbit the Sun (Mars takes 1.9 years, while Jupiter takes 12 years.) The Hubble Space Telescope images of it (see one attached here) show lighter and darker areas. The water vapor is not evenly distributed around Ceres and there is more of it when Ceres is closer to the Sun, so there may be some ice that is sublimating (going from frozen form to vapor) in parts of Ceres.

At Ceres' distance from the Sun, ice on the surface would have all sublimated long ago, so this must be vapor coming from a deeper (frozen) layer, which was a surprise. Some astronomers think there may be enough water ice under the surface of Ceres to make an ocean. This is the first time water vapor has been detected in the asteroid belt, although we have seen it coming from a moon of Jupiter's and a moon of Saturn's. There may be plumes or "geysers" of water vapor coming from parts of Ceres, perhaps like the ones on Saturn's satellite Enceladus.

We will know a lot more about conditions on Ceres next March and April, when the Dawn spacecraft arrives for a rendezvous with Ceres and provides images with unprecedented detail of this intriguing member of our solar system. (Dawn gave us lots of great information about Vesta, the second largest asteroid, before it departed for Ceres in 2012.) Isn't it great how nature continues to surprise and delight us as we explore our cosmic neighborhood?

Sunday, June 8, 2014

An Eerie Astronomical Photo


Click on the photo to make it bigger ^

Here is a gorgeous image taken by master astro-photographer Travis Rector (of the University of Alaska), showing a region of cosmic raw material (glowing gas and dark dust.) This is like one of those inkblot tests that psychologists are fond of -- what shape you see in the picture may be highly individual.

What we are actually looking at is a small part of a larger region where star birth is going on right now. Known by its catalog number of IC1396, this "emission nebula" is set to glow by the brilliant light of new stars that have already been born in this region and are shining with adolescent energy.

IC1396 is a cloud of loose gas and dark dust (the dust really isn't that different from what you find under your desk when you are too busy thinking cosmic thoughts to clean up regularly.) The dust is shaped into long filaments, when the light of energetic stars pushes the less dense parts of the dust away from the stars (in this case, in the down direction on this photo.) Only the thickest regions of dust remain after a while, making long, dark tendrils like we see here.

IC 1396 is about 3000 light years away in the constellation of Cepheus. You can see the entire nebula (much larger than the small part seen in our photo) here:
http://apod.nasa.gov/apod/ap120805.html

So what did the dark shape remind YOU of?

Monday, June 2, 2014

Stars That Eat Planets for Lunch


In the last couple of weeks, astronomers have announced the discovery of two stars that show evidence of having eaten some of their own Earth-like planets. Today came the announcement of the discovery of a star with planets that is growing larger and larger and is going to be eating its inner planets in the next couple of hundred million years.

These kinds of star "cannibal activity" are not that rare and should not cause us undue distress. It's business as usual in the Milky Way Galaxy, but what's new is that we are getting direct evidence for activities that earlier we only predicted from theory.

The two stars that already finished their meal a long time ago are known by their catalog numbers, HD20781 and HD20782. These stars belong to the same star system, and formed from the same original cloud of cosmic raw materials. This is the first "binary star" system where astronomers have discovered planets around each star. One has two Neptune-sized planets close by, the other has a Jupiter-sized planet whose orbit is not a nice circle but a stretched oval-shape.

Astronomers took a look at what the two stars are made of -- something we can learn by looking at the details in the colors of light from them. Every different element leaves its own "fingerprints" in the colors of light. Because the two stars formed at the same time, from the same "cosmic womb," we have more information about what they were like at the beginning.

According to Trey Mack, of Vanderbilt University, a graduate student who did the detailed analysis, both stars show evidence of elements in their atmospheres that were most likely not there when the two stars formed. Instead they are tell-tale signs of the stars having eaten planets made of rock. One star seems to have eaten the equivalent of 20 Earths, while the other consumed "only" ten Earths earlier in its history.

Astronomers have thought for a while that big "bully" planets like Jupiter and Neptune can, under the right circumstances, move inward and force smaller planets (like our Earth) to approach their stars until they are consumed. We are fortunate that this did not happen in the case of our solar system, and the Earth has a stable, undisturbed orbit which has allowed this blog's readers to evolve here. But it's getting clearer and clearer that not all star system will have the same history as ours did.

The other work deals with an act of cannibalism that happens late in the life of a star. All stars, at some point, have a "life-crisis" when their first fuel for making energy is all used up and the star has to adjust by briefly swelling up into a red-colored giant. The news is that astronomers have discovered that one of the stars around which the Kepler space mission has found three planets is on its way toward this period of swelling up. The star is known as Kepler 56, and the way in which it has started to swell up is telling us that it will eventually swallow its two inner planets, leaving only its outermost planet as survivor.

The devastation will not occur until more than a hundred million years have gone by, so it's not an immediate tragedy. We have seen many stars that swell up like this, but none of them have been known to possess planets. Kepler 56's worlds are therefore the first planets known to be orbiting other stars whose doom we can now predict. (By the way, in case you are worried, our own Sun will also have such a crisis and will swell up. But this is not happening for another five or six billion years, so I am not ready yet to include this eventuality on my home insurance policy.)


The image, by the way, is an artist's impression of an Earth-like planet being torn apart and then "eaten" by a Sun-like star.

Monday, May 26, 2014

Astrology, Astronomy, and Jetology



Every once in a while, I am mistakenly introduced as an astrologer instead of an astronomer.  There's a big difference, as I am quick to point out.  Astrology is an ancient superstition, left over from thousands of years ago, when most people saw celestial objects as gods or symbols of the gods, and not as physical bodies whose characteristics our instruments can now explore.

When, in the 1980's, it was revealed that President Reagan's White House schedule was for years vetted by a San Francisco astrologer, I was interviewed by the media as a critic of astrology.  I tried to come up with an analogy that would make sense to the news media and the public for why scientists had a hard time believing in the idea that your personality or destiny would be governed by the location of the Sun, Moon, and planets in the sky at the moment of your birth. I suggested a new form of "cosmic wisdom" called JETOLOGY, where your personality or fate could be explained by the position of all the jumbo jets at the time you were born. When people sniggered at the notion of jetology, I could then ask why astrology sounds any better.

I am reminded of this because, for those of you who are in the San Francisco Bay Area, I am preparing to be one of the featured speakers at SkeptiCAL, a one-day conference Saturday, May 31 in Oakland, on how to confront claims of the paranormal or the "too good to be true" by using common sense skeptical thinking.  See: http://www.skepticalcon.com/

Years ago, I wrote a short "Astrology Defense Kit" for science-oriented people who meet astrology believers.  You can read it at: https://www.researchgate.net/publication/253783518_Your_Astrology_Defense_Kit?ev=prf_pub 

If you want to see a wonderful short video on how it can be that the vague pronouncements that make up most astrological readings are taken seriously, I recommend the following episode of "Trick of the Mind" by British magician and skeptic, Derren Brown: https://www.youtube.com/watch?v=haP7Ys9ocTk

Ah, skeptical thinking -- if we had more of it during our years of education, during political campaigns, or when we view advertisements, our country would be so much better off.

Wednesday, May 14, 2014

A Possible Sister Star that Formed with Our Sun



Astronomers now know that stars, like people, tend to be sociable. Stars are often born and hang out in groups -- double stars, triple stars, star associations, and star clusters.  Yet our Sun is a single star, surrounded only by its family of planets and moons, but no sibling star to keep it company.  Was it always this way?

In recent years, it's become clearer that our Sun could well have formed in a loose group of thousands of stars, when it first "clumped" out of the gas and dust (raw material) of the Galaxy some 5 billion years ago.  If our mother cluster was indeed just loosely held together by the mutual gravity of the stars, in all that time, the stars could have drifted apart -- as many families do in the busy course of life. 

So we have been on the lookout for the Sun's now far-away sisters. This past week, a team of astronomers, headed by Ivan Ramirez of the University of Texas, announced that they might have found our first long-lost sibling. 

There are perhaps as many as 400 billion stars in our Milky Way Galaxy.  How can we possibly find our sisters in that huge and anonymous crowd?  After all, there are no star birth certificates on file at county offices and no little tags that hang around a star's neck.  The search involved two factors: First they looked for stars that have the same chemical make-up as the Sun (they consist of the same proportion of elements) -- which you'd expect from stars born in the same"womb."  And then they searched for stars whose motion could be calculated backwards in time and would have placed them close to us 5 billion years ago. 

Sorting through 30 possible candidates identified by other groups of astronomers after painstaking work, Ramirez' group came up with exactly one star that fit all the criteria.  Called by its catalog number, HD 162826, it's 110 light years away now, in the constellation box called Hercules.  (See the map.)  It has 15 % more mass than the Sun, and so it is a little brighter and a little hotter than our star. (In astronomy jargon, it's a Type F star, while our Sun is Type G.)

It turns out that HD 162826 has been searched for planets for the last 15 years, and so far, no planets have turned up.  But the kind of search it has been subjected to can't find small (Earth-like) planets. So the we have a long way to go before we can decide it the star has a planetary system around it or not.

Future surveys, allowing us to analyze the make-up of fainter stars, could well turn up other family members out there.  For now, astronomers will keep a far closer eye on this one possible sister star and follow it as it goes about its life.  Who knows, one day, someone from there may just respond to the equivalent of a cosmic blog post.