JJJuuulllyyy 222000000999 IIIYYYAAA DDDiiissscccooovvveeerrryyy GGGuuuiiidddeee This Month’s Theme: BBBlllaaaccckkk HHHooollleeesss Featured Activity: BBBlllaaaccckkk HHHooollleee SSStttaaarrr MMMaaappp Featured Observing Object: TTThhheee MMMiiilllkkkyyy WWWaaayyy GGGaaalllaaaxxyyy The International Year of Astronomy is a global celebration of astronomy and its contributions to society and culture, highlighted by the 400th anniversary of the first use of an astronomical telescope by Galileo Galilei. Join us as we look up! http://astronomy2009.us The Astronomical Society of the Pacific increases the understanding and appreciation of astronomy by engaging scientists, educators, enthusiasts and the public to advance science and science literacy. http://www.astrosociety.org July's Topic: Black Holes Before the bright lights of cities outshone the wonders of the night sky, the Milky Way was a common sight for most of the world. Legends from various cultures described this region as a road, a river, or a flock of birds. The Greeks and Romans thought the haze looked like milk, which is where the name Milky Way comes from. When Galileo pointed his small telescope towards this band of light, he didn't see milk or birds. "The Milky Way is nothing else but a mass of innumerable stars planted together in clusters." We know now that the Milky Way Galaxy is indeed a collection of gas, dust and billions of stars, including our Sun. But there is more to the galaxy than stars, gas, and dust. There are also black holes. In the dense center of the Milky Way, there is good evidence that a supermassive black hole exists with over 2 million times the mass of our Sun. Scientists have good evidence that most large galaxies contain giant black holes in their centers. But we can't point a telescope at a black hole and see it directly. We only see their effect on the things around them, like stars and gas. In fact, the mass of a black hole is determined using physics developed by Galileo’s contemporary, Johannes Kepler. There is a smaller, more common type of black hole with only a few times the mass of our Sun. These are the remains of giant stars that end their lives in a supernova explosion. If there is enough mass left at the core of the star after the explosion, it collapses to a point, creating a region of gravity so strong that not even light can escape. This makes finding black holes a little bit tricky. But scientists love a challenge. As it turns out, when material falls into a black hole, it gets heated up to millions of degrees releasing high energy radiation like x-rays before it is lost forever. So searching for sources of x-rays is another way to detect black holes, and this is how NASA's Chandra Observatory studies the black hole at the center of our galaxy. Scientists are also using x-rays to research the behavior of black holes with the Suzaku and XMM- Newton space observatories. The Hubble Space Telescope has even discovered medium sized black holes. With future generations of telescopes, like the James Webb Space Telescope, astronomers will be able to go deeper in their understanding of black holes and the role of the black holes in galaxy evolution. While we can't see black holes directly, you can use the star maps in this guide to find out where they are located in the night sky. Learn more about Black Holes from NASA. Find more activities featured during IYA 2009. chandra explores “downtown” in the milky way The word galaxy comes from the Greek word meaning “milky circle” or, more familiarly, “milky way.” The white band of light across the night sky that we call the Milky Way was poetically described long before Galileo. But with his small telescope, what he discovered was a multitude of individual stars, “so numerous as almost to surpass belief.” Today we know that the Milky Way is our home galaxy - a vast rotating spiral of gas, dust, and hundreds of billions of stars. The Sun and its planetary system formed in the outer reaches of the Milky Way about 4.5 billion years ago. In the center of the Galaxy is the bar-shaped Galactic bulge which harbors a supermassive black hole with a mass equal to that of about 3 million suns. Surrounding the central bulge is a relatively thin disk of stars about 2,000 light years thick and roughly 100,000 light years across. Giant clouds of dust and gas in the disk and bulge absorb starlight and give the Galaxy its patchy appearance. The Milky Way is home to generations of stars past. Many stars become small, dense white dwarfs after a bloated ‘red giant’ phase. Other, more massive stars explode as supernovas, enriching the Galaxy with heavy elements manufactured in their cores, and leaving behind either neutron stars or black holes. The Galaxy’s bright stellar disk is embedded in a faint disk of old stars which is about 3 times thicker than the thin disk. Surrounding the thick Galactic disk is an extremely faint halo that contains the oldest stars in the Galaxy. The Galactic halo is dominated by dark matter, a still mysterious form of matter that cannot be observed directly. “There is a way on high, conspicuous in the clear heavens, called the Milky Way, brilliant with its own brightness.” Ovid (43 BC - 17 AD), Metamorphoses CHANDRA MOSAIC OF THE GALACTIC CENTER HTTP://CHANDRA.HARVARD.EDU Chandra X-ray Observatory chandra explores “downtown” in the milky way X-ray Thread DB00-58DB00-58 Arches SNRSNR 0.9+0.10.9+0.1 Cluster 1E 1743.1-2843 Sagittarius B2 Sagittarius A 1E 1740.7-2942 Sagittarius C Quintuplet SagittariusSagittarius B1B1 Cluster DB00-6 ColdCold GasGas CloudCloud DB01-42 & Radio Arc THE GALACTIC CENTER: A panoramic X-ray view, covering a 900-by-400 light-year swath, shows that the center Most of the action in our Milky Way takes place in its crowded center, the bustling “downtown”, so to speak, of of the Galaxy is a teeming and tumultuous place. There are supernova remnants: SNR 0.9-0.1, Sagittarius A our Galactic metropolis. With Chandra’s keen X-ray vision, scientists are trying to determine how this relatively East, and probably the X-ray Thread. There are many bright X-ray sources, which astronomers believe are binary systems — or pairs of orbiting objects — that contain a black hole or a neutron star (the 1E sources). There are small patch of Galactic real estate affects the evolution of the Galaxy as a whole. For example, Chandra sees hundreds of unnamed point-like sources that scientists think are solo neutron stars or white dwarfs, which all large quantities of extremely hot gas apparently escaping from the center. This outflow of gas -- enriched with light up the region. In addition, the massive stars in the Arches and other star clusters (the DB sources) will soon elements like iron, carbon, and silicon from the frequent destruction of stars -- is distributed into the rest of the explode to produce more supernovas, neutron stars, and black holes. Galaxy. These elements are crucial to the formation of stars and planets, including Earth. Additional telescopes have also found other exotic members of this cosmic zoo. Infrared and radio observations fi nd giant molecular clouds (Sagittarius A, B1, B2, and C, and the Cold Gas Cloud near the Radio Arc) where stars Dust and gas produced by millions of form. Normally too cool to be detected in X-rays, the edges of these clouds have been heated, allowing Chandra massive stars makes it diffi cult for to see their X-ray glow. optical telescopes to see into this region. However, other wavelengths can reveal certain features in the Galactic Center. All this commotion takes place in a diffuse cloud of hot gas that shows up as extended X-ray emission. This A composite of images made at X-ray diffuse X-ray glow gets brighter toward the Galactic Center. Sagittarius A (Sgr A), the bright blob in the center, (blue), infrared (green), and radio (red) X-RAY INFRARED is composed of three main parts: Sgr A East, Sgr A West, and Sgr A*. Sgr A East is the remnant of a supernova shows the relation between hot gas (X- that stirred things up about 10,000 years ago. Sgr A West is a spiral-shaped structure of gas that may be falling ray), cool gas and dust (infrared) and high energy electrons trapped in the toward Sgr A*, the supermassive black hole that marks the center of the Milky Way Galaxy. Sgr A* contains about magnetic fi eld in the Galactic Center 3 million times the mass of the Sun, and is gaining weight daily as it pulls in more material. (radio). Because it is only about 25,000 light years from Earth, the center of our Galaxy provides an excellent Coordinates (J2000): RA 17h 45m 23s Dec -29°01’17” Observation Time: 94.2 hours total laboratory to learn about the cores of Constellation: Sagittarius Color Code: Energy RADIO X-RAY, INFRARED & RADIO other galaxies. Observation Date: July 2001 (30 separate pointings) Instrument: ACIS Credits (Left-Right, Top-Bottom): Chandra broadband X-ray (NASA/UMass/D. Wang et al.), Infrared (MSX), Radio (VLA/NRL/N. Kassim) CHANDRA More on The milky way X-RAY CENTER How can you take a picture of our Galaxy if we are in it? Since our Solar System is embedded within our Galaxy, we can only show an artist’s representation of what it looks like from the outside.