Milky+Way

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Rubric: [[file:Space Exploration Adventure Rubric.doc]], [[file:Space Exploration Adventure Rubric.pdf]]


Milky Way Milky Way By Dylan Johnson The Milky Way galaxy is made up of stars and nebulas. In the center of the galaxy is a massive black hole. A black hole is a place in space where gravity is so strong because matter has been squeezed into a tiny space. The gravity is so strong that not even light could get through. The Milky Way consists of about 400 billion stars and a gas and dust arrangement into 3 components, the halo, the disk, and the nuclear budge. The halo consists of the oldest stars known. The disk is flattened rotating system that contains the sun and other stars. The nuclear budge is in the center of the galaxy.
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 * __ http://casswww.ucsd.edu/archive/public/tutorial/MW.html __**

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 * __ Information on the Milky Way __**

Milky Way: A Crowded Neighborhood
The center of the Milky Way is a crowded neighborhood and not always a calm one, as seen in this image from NASA's Chandra X-ray Observatory. In addition to the supermassive black hole at the center, the area is filled with all sorts of different inhabitants that affect and influence one another. The image shows three massive star clusters: the Arches (upper right), Quintuplet (upper center), and the GC star cluster (bottom center), which is near the enormous black hole known as Sagittarius A. The massive stars in these clusters can themselves be very bright, point-like X-ray sources, when winds blowing off their surfaces collide with winds from an orbiting companion star. The stars in these clusters also release vast amounts of energy when they reach the ends of their lives and explode as supernovas, which, in turn, heat the material between the stars. The stars near the Galactic Center also can emit X-rays as stellar corpses -- either in the form of neutron stars or black holes in binary systems -- and are also seen as point-like sources in the Chandra image. GREENBELT, Md. - Using NASA, Japanese, and European X-ray satellites, a team of Japanese astronomers has discovered that our galaxy’s central black hole let loose a powerful flare three centuries ago.

The finding helps resolve a long-standing mystery: why is the Milky Way’s black hole so quiescent? The black hole, known as Sagittarius A* (pronounced "A-star"), is a certified monster, containing about 4 million times the mass of our Sun. Yet the energy radiated from its surroundings is billions of times weaker than the radiation emitted from central black holes in other galaxies.

"We have wondered why the Milky Way’s black hole appears to be a slumbering giant," says team leader Tatsuya Inui of Kyoto University in Japan. "But now we realize that the black hole was far more active in the past. Perhaps it’s just resting after a major outburst."

The new study, which will appear in the Publications of the Astronomical Society of Japan, combines results from Japan’s Suzaku and ASCA X-ray satellites, NASA’s Chandra X-ray Observatory, and the European Space Agency’s XMM-Newton X-ray Observatory.

The observations, collected between 1994 and 2005, revealed that clouds of gas near the central black hole brightened and faded quickly in X-ray light as they responded to X-ray pulses emanating from just outside the black hole. When gas spirals inward toward the black hole, it heats up to millions of degrees and emits X-rays. As more and more matter piles up near the black hole, the greater the X-ray output.

Four X-ray satellites imaged a small region in the gas cloud Sagittarius B2, and saw pockets brighten and fade over the course of nearly 12 years. These light echoes are caused by varying X-ray output from our galaxy’s central black hole. **Credits:** ASCA and Suzaku: JAXA; Chandra: NASA/CXC; XMM-Newton: ESA. [|> Larger ASCA image] [|> Larger Chandra image] [|> Larger XMM-Newton image] [|> Larger Suzaku image] These X-ray pulses take 300 years to traverse the distance between the central black hole and a large cloud known as Sagittarius B2, so the cloud responds to events that occurred 300 years earlier. When the X-rays reach the cloud, they collide with iron atoms, kicking out electrons that are close to the atomic nucleus. When electrons from farther out fill in these gaps, the iron atoms emit X-rays. But after the X-ray pulse passes through, the cloud fades to its normal brightness.

Amazingly, a region in Sagittarius B2 only 10 light-years across varied considerably in brightness in just 5 years. These brightenings are known as light echoes. By resolving the X-ray spectral line from iron, Suzaku’s observations were crucial for eliminating the possibility that subatomic particles caused the light echoes.

"By observing how this cloud lit up and faded over 10 years, we could trace back the black hole’s activity 300 years ago," says team member Katsuji Koyama of Kyoto University. "The black hole was a million times brighter three centuries ago. It must have unleashed an incredibly powerful flare."

This new study builds upon research by several groups who pioneered the light-echo technique. Last year, a team led by Michael Muno, who now works at the California Institute of Technology in Pasadena, Calif., used Chandra observations of X-ray light echoes to show that Sagittarius A* generated a powerful burst of X-rays about 50 years ago -- about a dozen years before astronomers had satellites that could detect X-rays from outer space. "The outburst three centuries ago was 10 times brighter than the one we detected," says Muno.

The galactic center is about 26,000 light-years from Earth, meaning we see events as they occurred 26,000 years ago. Astronomers still lack a detailed understanding of why Sagittarius A* varies so much in its activity. One possibility, says Koyama, is that a supernova a few centuries ago plowed up gas and swept it into the black hole, leading to a temporary feeding frenzy that awoke the black hole from its slumber and produced the giant flare.

Launched in 2005, Suzaku is the fifth in a series of Japanese satellites devoted to studying celestial X-ray sources and is managed by the Japan Aerospace Exploration Agency (JAXA). This mission is a collaborative effort between Japanese universities and institutions and NASA Goddard.


 * __ The Halo __**
 * The Halo consists of the oldest stars known, including about 146 [|Globular Clusters], believed to have been formed during the early formation of the Galaxy with ages of 10-15 billion years from their H-R Diagrams. The halo is also filled with a very diffuse, hot, highly-ionized gas. The very hot gas in the halo produces a [|gamma-ray halo].**
 * Neither the full extent nor the mass of the halo is well known. Investigations of the gaseous halos of other spiral galaxies show that the gas in the halo extends much further than previously thought, out to hundreds of thousands of light years. Studies of the rotation of the Milky Way show that the halo dominates the mass of the galaxy, but the material is not visible, now called [|dark matter.]**
 * __ The Disk __**
 * The disk of the Galaxy is a flattened, rotating system which contains the Sun and other intermediate-to-young stars. The sun sits about 2/3 of the way from the center to the edge of the disk (about 25,000l.y. by the most modern estimates). The sun revolves around the center of the galaxy about once every 250 million years. The disk also the galaxy about contains atomic (HI) and molecular (H2) gas and dust.**
 * Since the Earth lies in the disk of the Milky Way, dust prevents us from determining the large scale structure of the Galaxy's spiral pattern beyond a few thousand light-years. Radio observations have detailed the structure of the gas in the spiral arms, but it is still not known if our galaxy is a normal spiral like our neighbor Andromeda, or a barred spiral like shown to the left. The bulge of the galaxy is slightly elongated in the direction of the Sun, which may be due to a bar. **
 * __ The Center of the Galaxy __**
 * What lies at the** **[|center of our Galaxy]? Again, dust obscures the visible light from us and we must use radio and infrared observations to elicit the nuclear properties of the Galaxy. A census shows us that the Galactic Center region is an unusually crowded place, even in this visible-light [|Map of Central region]. At radio wavelengths, where we can peer down to the very center, we see the complex strctures shown in the [|1-meter wavelength radio map] made by NRL astronomers which is shown below. The map shows a region about 2000 light-years on a side; the center of the Milky Way coincides with the source marked Sag A (or Sagittarius A), which is actually three sources, a yound supernova remnant on the east side, an unusual ionized hydrogen region on the west side, and a very compact source called Sagittarius A* at the very center.**


 * Although there is no lack of fascinating questions about the Galactic Center, recent interest has been focused on the question of the possibility that a massive black hole exists at the center of the central star cluster. The presence of very high velocities in the stars and gas near the center of the galaxy has suggested to astronomers for a long time that a massive black hole might be present, providing a strong enough gravitational pull to keep the stars and gas in orbit. [|Andrea Ghez], a professor at UCLA, used the Keck 10-meter telescope at infrared wavelengths to measure the velocities of 20 stars that lie close to the galactic center over a three-year period. She found the stars are orbiting at speeds up to 1000 km/s (//3 million miles per hour//)! Observation done by scientists at the Max-Planck Institut in Germany have confirmed these results. This large gravitational acceleration requires an object with a mass 2.5 million times that of our Sun.**
 * The stars are located near Sagittarius A*, the radio source lying near the center of the galaxy. From its radio signal alone, Sgr A* did not have to be particularly massive, since its emission is not very powerful. Using the VLBA (Very Long Baseline Array) radio telescope, astronomers studied the motion of Sgr A*; they found a velocity of less than 20 km/sec for Sgr A* itself. This means it is very unlikely that Sgr A* is a single star or group of stars. Only a very massive object could remain stationary under the conditions exisiting at the center of the galaxy. The evidence is mounting that Sag A* is indeed a black hole of 2-3 million times the mass of the sun. Astronomers speculate that the Black Hole is being "fed" by gas from the molecular ring, or supernova remnant. By consuming less than about 1% of the mass of a star each year, releasing gravitational potential energy, Sag A* can easily account for the high-energy phenomena near the galactic center.**

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