Sunday, September 28, 2014
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
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
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
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
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
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
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.