Monthly Archives: March 2011

black hole, black holes, chandra x-ray telescope, x-ray astronomy

Blobs in Space

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Often Turn Out to be Pretty Interesting

his composite image shows the central region of the spiral galaxy NGC 4151, dubbed the "Eye of Sauron" by astronomers for its similarity to the eye of the malevolent character in "The Lord of the Rings". Courtesy Chandra X-ray Telescope.

You just have to know what you’re looking at when you see something like this appear in the astronomy news of the day. Sometimes I laughingly refer to pictures like this as “blobs in space”. But, as I know — and you’re about to read– a further look at them gives you a peek at something you didn’t expect.And, you get to learn something new about things like galaxy cores and black holes, in the bargain.

This set of “blobs” is dubbed the “Eye of Sauron” because astronomers studying it decided this central region of the spiral galaxy NGC 4151 kind of looked like the object of the same name in the “Lord of the Rings” movie. The pupil of this “eye” shows x-rays (colored blue) streaming from a central object embedded in the core of the galaxy.

The yellow spots out in the corners of the eye are actually visible-light data (called “optical”) showing the location of positively charged hydrogen gas (also known as H II, which means it is energized in some way).  These are regions where star formation has occurred in the recent past.

The red blobs are clouds of neutral hydrogen gas, which show up in radio wavelengths.  They are part of a larger structure that is being distorted by gravitational interactions with other parts of the galaxy. And, some of it is material falling INTO the center of the galaxy, toward the pupil.

So, now that we know what the pieces and parts are, what does they mean?  Recent work shows that the blue-labeled x-ray emission is probably coming from an outburst powered by a supermassive black hole at the heart of the galaxy.  That black hole region is colored white in the image.

Evidence for the idea that some kind of action is tossing out x-rays from the center comes from the elongation of the x-ray emissions running from the top left to the bottom right and details of the x-ray spectrum. There are also signs of interactions between a central source and the surrounding gas, particularly the yellow arc of H II emission located above and to the left of the black hole.

Astronomers have posed a couple of  different scenarios to explain the x-ray emission. One possibility is that the central black hole was growing much more quickly about 25,000 years ago (in Earth’s time frame) and the radiation from the material falling onto the black hole was so bright that it stripped electrons away from the atoms in the gas in its path. X-rays were then emitted when electrons recombined with these ionized atoms.

The second possibility also involved a hefty flow of material into the black hole in relatively recent times. The energy released by material flowing into the black hole in an accretion disk created a vigorous outflow of gas from the surface of the disk. That outflox directly heated gas in its path to  temperatures hot enough to permit the emission of x-rays.  Unless the gas is confined somehow, it would expand away from the region in less than 100,000 years. In both of these scenarios, the relatively short amount of time since the last episode of high activity by the black hole may imply such outbursts occupy at least about 1% of the black hole’s lifetime.

It’s amazing what you can learn when you look at blobs in space through the viewpoint of astronomers. They use not just astronomy and physics, but also chemistry and astrophysics to explore and understand the processes and events that occur in the hearts of galaxies like this one, which lies some 43 million light-years away from us. Thanks to tools like the Chandra X-ray Telescope, the Isaac Newton Group of  Telescopes on La Palma, and the National Radio Astronomy Observatories, images like this can be obtained, explained, and shared with everybody.  We all win when that happens. For more information on this image and to see other wavelength views of the “Eye of Sauron” visit the Chandra X-ray Telescope’s story about these observations.

comet, European Space Agency, international halley watch

Halley’s Comet

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Remembering the Flybys

The nucleus of Comet Halley as seen by the Giotto spacecraft in March 1986. Courtesy ESA.

It’s hard to believe that 25 years have passed since Comet Halley swung around our way in its 75.3-year orbit. Right about now it’s heading out to the farthest point in its orbit — around 32.6 astronomical units from the Sun.  That’s farther away than Neptune’s average distance. In a few years, it will reach its most distant point (called aphelion) and then start its inward journey to round the Sun again in 2061.

In mid-March of 1986, a small armada of spacecraft flew near and through Comet Halley’s tail. One of those missions was the Giotto probe, which was nearly destroyed by its close passage to the comet.  But, it returned the first images ever seen of a comet’s nucleus and changed how we viewed these dirty snowballs.

The spacecraft was the European Space Agency’s first deep-space mission, and this year the agency has posted a “remembrance” of the night when the spacecraft approached the comet. Giotto was built to a design that drew on the Geos Earth-orbiting research satellites. It was fitted with shielding to protect it from the ‘sand-blasting’ it endured as it sped through the comet’s tail. The mission was originally conceived as a joint project with NASA, the Tempel-2 Rendezvous–Halley Intercept mission. When the United States pulled out after budget cuts, ESA decided to forge on, finding Japan and Russia willing to contribute their own missions. Together, they sent a flotilla, with the Russian missions serving as pathfinders to guide Giotto to its dangerous encounter.

There WAS another mission set to go to the comet — it was called Spartan Halley, or more technically, Spartan 203.  It was equipped with ultraviolet detectors to observe the glowing gases in the plasma tail of the comet. It was set for launch on Space Shuttle Challenger, and was lost when the shuttle was destroyed in the January 28, 1986 accident.

Comet Halley was a milestone of comet science in many ways. I was part of a team that studied the plasma tail of the comet as it traversed our point of view during the months of closest approach to and movement away from the Sun. We used images from the International Halley Watch, a ground-based effort undertaken by hundreds of observers to study the comet throughout the months it was visible to us on Earth. The images we were most interested in stretched from mid-1985 to mid-1986, the months when the plasma tail was “turned on” and we were able to see structure in it. The comet itself had been spotted in an image as early as 1982, but its tail structure had not yet formed, since it was too far from the Sun to do so.

Comet Halley as imaged by Bill Liller from Easter Island on March 8, 1986. The plasma tail is the lower, bluish portion of the tail.

We began studying the images (or at least my part of the project) began in 1988, when I went to work studying those images under the aegis of the Large-Scale Phenomenon Network of the International Halley Watch. My job was to take the images we had selected and pinpoint the exact location of the comet’s nucleus against the backdrop of the sky.  Of course, it was tough to SEE the actual nucleus, so we had to approximate the location very carefully and then use stars to triangulate the position.  From there, we could then figure out the relative position and distance of structures in the plasma tail. That, in turn, told us something about the speed and “loading” of the solar wind, since the solar wind directly affects a comet’s plasma tail.

In the following years after Halley’s appearance, we studied other comets, among them deVico, Borrelly, D’Arrest, Encke, Honda-Mrkos-Padjuakova, Mueller, and others. The goal was to observe the plasma tails as they turned on and were affected by their interactions with the solar wind. It was a great deal of work that added to the comet literature, and I’m pleased to have been a small part of it.  It’s just hard to believe it was 25 years ago!