Bubbly Burp Tracks Giant Black Hole's Last Meal

Observations with a number of telescopes, including the Hubble, have now dated a kind of burp in the eating habits of the giant black hole at the center of our Milky Way Galaxy. It appears that about six million years ago, the central black hole "ate" a large cluster of stars and the neighborhood around it was energized by the process of the meal.

Black holes are regions where material (starting with dead stars) has collapsed so much, that nothing, not even light, can get out. The black hole at the center of our Milky Way now includes enough material to make more than 4 million stars like the Sun! It's what we call a "supermassive black hole" and lurks in the middle of our Galaxy like a giant speed trap for unwary stars or star groups that get too close.

As material is in the process of being "swallowed" by the black hole, it glows with desperate radiation, just before it falls in and disappears from view. A great bubble produced by the black hole as it ate its last serious meal has now been tracked by astronomers with much greater precision.

Just like the sound of the burp your uncle makes (after a heavy Thanksgiving meal) can travel through the dining room, so the bubble from this last meal can be seen traveling through the Galaxy like a giant expanding shell. The shell was probe by the Hubble as the light of distant objects raced through it and astronomers were able to measure the speed of the bubble's motion.

Six million or so years ago, it appears that a large clump of stars or gas (the raw materials of stars) was consumed by the black hole, making two "Fermi bubbles" in the Galaxy. Since then, the black hole has only been "snacking" -- tearing apart and eating an occasional star or random bit of gas. But no serious meal has made a big bubble from the mouth of the black hole since then.

Our diagram shows how in the six million years since that meal, the bubble has expanded at speeds of two million miles per hour and made a giant bubbles north and south of the black holes that extend for tens of thousands of light years. That kind of puts your uncle's last burp into perspective!

A Hidden Baby Galaxy in our Cosmic Neighborhood

An international team of astronomers, led by researchers at Japan’s Tohoku University, has just reported finding the faintest satellite galaxy ever seen orbiting our home galaxy, the Milky Way.  All stars are born in great islands or groupings of stars called galaxies.

Big galaxies like the Milky Way are surrounded by smaller “baby galaxies” (or satellite galaxies), some of which collide with it over cosmic times.  About 50 such galaxies are currently known to orbit our Milky Way – with the two “Magellanic Clouds” (discovered by Ferdinand Magellan’s crew) being the most famous of them.

Because many of the smallest galaxies are very faint, they are hard for us to make out.  Remember, we are inside the Milky Way, and so (as we try to look outwards) we always have to observe through the stars and star clusters of our own galaxy.  The faint baby galaxies can be hard to tell apart from clusters or groups of stars in the Milky Way itself. (This is why it’s hard to get a good photo of the Milky Way; we are inside it and so it’s like trying to take a selfie from inside your kidney.  The view is not so clear.)

Still, using the giant Subaru telescope (whose mirror is more than 24 feet wide), the team was able to find the faintest baby galaxy ever found, which is being called Virgo I (since we see it in the constellation of Virgo.)  At an estimated distance of 280,000 lightyears from us, Virgo I was much fainter than earlier surveys for our neighbor galaxies were able to reveal.

The whole Virgo I “dwarf galaxy” is only about 248 lightyears wide.  Compare that to the 100,000 lightyear diameter of the Milky Way! The Magellanic Clouds are estimated to be 7,000 and 14,000 light across.  So you can see that Virgo I really is just a baby. See the tiny smudge it makes on our accompanying image.

But if one such baby galaxy has escaped our notice until now, chances are many others like it may also be out there.  Some of our theories predict that major galaxies like the Milky Way should be surrounded by many more dwarf galaxies that we have seen so far.  Virgo I leads astronomers to think that more may be out there -- just waiting for bigger telescopes and more observations before they are discovered. 

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.

Black Hole at Center of the Milky Way is Found to be Cooking its Dinner

Astronomers working with the European Herschel Space Observatory have discovered really hot gas in the vicinity of the monster black hole at the center of our Galaxy. Over the years, many lines of evidence have shown us that there is a black hole with enough material to make 4 million Suns at the heart of the Milky Way. The new observations, made using infrared (or heat) rays, show that gases such as water vapor and carbon monoxide have been heated to about 1000 degrees Centigrade within a lightyear of the black hole.

While energy from nearby stars may also be heating this inner gas, the astronomers can't account for so much heat from stars alone. They think that great streamers of gas heading toward the black hole may be colliding and the shock waves from the collisions may be significant contributors to the heating. Some of the streamers of gas will someday be "eaten" by the black hole. In other words, like many a hungry diner, the black hole appears to be "cooking" its dinner in anticipation of eating it.

In fact, other observations have recently shown a cloud of gas weighing as much as several Earths, falling to its doom much closer to the black hole. This cloud may be consumed by the black hole as soon as the end of 2013. When such clouds actually spiral inward to their doom, they heat up a lot at the end. The last thing we observe from them before they fall into the black hole (and are no longer visible) is a "burp" of x-rays. Several x-ray telescopes in space are prepared to record such burps when they happen.

If you are cooking a barbecue this Memorial Day Weekend (a holiday in the U.S.), you can enjoy the idea that some serious cooking may also be going on at the center of our Galaxy. The center region is 26,000 lightyears away from us, so none of this poses the least danger to planet Earth and its cooks.

(By the way, to see one of the lines of evidence for the existence of the monster black hole, we recommend a great new movie made from observations by astronomer Andrea Ghez' group at UCLA. The movie shows the whirling orbits of stars very close to the black hole, being pulled around by the enormous gravity of the black hole. Check it out at:

http://www.astro.ucla.edu/~ghezgroup/gc/images/media/ghezGC_comp3-18_H264_864.mov

Note that each second of the movie shows two years of star motion. It's enough to make you dizzy.)

Giant Black Hole at the Center of Our Galaxy (and a Video)

A new observatory in space has caught the giant black hole at the center of our home galaxy in the process of having a snack. We know that the at the very heart of the Milky Way there lurks a black hole with as much material inside as 3 - 4 million Suns. When material falls into such a black hole, just before it enters, it gets very hot by friction and gives off (not light but) a glow of x-rays.

Once material falls into a black hole, then no light or x-rays can escape from it. But just before any such stuff goes to its doom, we do see a flare of x-rays from it. It is just this sort of flare that the NuSTAR satellite detected in July (just about a month after it was launched). It wasn't really a full meal for the black hole, merely an afternoon snack, but the temperature of the glowing gas cloud before it fell in was an astounding 180 million degrees Fahrenheit.

The so called "super-massive" black hole at the center of our Galaxy is a relatively small one, compared to some of the really massive black holes in the centers of other galaxies.  Some of these have billions (thousands of millions) of times the mass of our Sun contained within them.

If you are already a black hole fan, such observations will probably just confirm your view of how weird black holes are. But if you are not sure what black holes are, I might recommend a little video that the SETI Institute made of me during a family event there. It's a brief, friendly explanation of just what black holes are and why falling into one is a "once-in-a-lifetime experience." You can see it at:
http://www.youtube.com/watch?v=7DX_cc-IjpY