20TH ANNIVERSARY EDITION
Astronomy
AMATEUR
FEBRUARY 2012
YOU ARE HERE
THE DEEPEST LOOK INTO THE UNIVERSE
THE HUBBLE TELESCOPE
+ 20 YEARS OF EXPLORATION
THE SOLAR ISSUE
Astronomy
AMATEUR
APRIL 2012
OUR SUN & the
10 MOST
INFLUENTIAL
APOD
IMAGES OF ALL TIME
ALSO IN THIS ISSUE:
SOLAR FLARE WATCH CAPTURING AURORA ECLIPSES
SUN 2012 THIS EXTREME POWERHOUSE MAKES UP 99.8 % OF THE SOLAR SYSTEM.
SEPT 22
JUNE 20
Autumn Equinox
Summer Solstice
MAR 20
Spring Equinox
Mass (in kg):
JUNE 4
Annular Solar Eclipse
DEC 21
NOV 13
Winter Solstice
Total Solar Eclipse
1,988,920,000,000,000,000,000,000,000,000 0.33 4.87 5.98 0.65
}
• 10^24 kg
1.8986
• 10^27 kg
5.6846
• 10^27 kg
8.68
• 10^26 kg
1.024
• 10^26 kg
CONTENTS
Hubble 20th Anniversary Edition
MARCH 2012
FEATURES 24
2012 A HISTORIC YEAR FOR SOLAR ECLIPSES Two solar exlipses, one total and one annual, and the last transit of Venus until 2117 make this stellar year for observers.
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ASK ASTRO
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HOW ROBITC TELESCOPES ARE CHANING ASTRONOMY Automated observatories run telescopes themselves, allowing astronomers to focus on results.
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CREATE ULTIMATE OBSERVING CHARTS FOR FREE
EXPLORE THE SKIES 18
SKYWATCH MOON PHASES 2012
COSMIC DEBRIS 16
MYSTERIOUS BLOBS FROM OUTER SPACE MONTHLY TOP APOD BREAKDOWN
17
4
STUNNING STAR GAZING WITH YOUR TABLET
FEBRUARY 2012 AMATEUR ASTRONOMY
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THE ULTRA-VIOLET UNIVERSE
20
CAPTURING AURORA
COVER STORIES 50 BRAND NEW
GALAXIES BY HUBBLE
Famous obersvations on galaxies that will forever stand the test of time.
56 MYSTERIES OF THE MILKY WAY
COLUMNS 12
STRANGE UNIVERSE
13
OBSERVING BASICS
14
SECRET SKY
15
IMAGING THE COSMOS
Mapping out our home galaxy and uncovering its strange objects with help from Hubble.
IN EVERY ISSUE HUBBLE ANNIVERSARY 63
JAMES T. WEBB: FLYING IN HUBBLE’S SHADOW
68 70
HUBBLE REVEALS SOME OF THE OLDEST GALAXIES KNOWN
72
AFTER HUBBlE REPAIRS, NEW IMAGES FROM SPACE
SEEING WHAT THE HUBBLE SEES IN IMAX 3D
6
THIS MONTH IN ASTRONOMY
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BEAUTIFUL UNIVERSE
10
LETTERS
11
WEB TALK
16
ASTRO NEWS
74 75
NEW PRODUCTS
78
READER GALLERY
82
THE COSMIC GRID
AVERTISER INDEX
FEBRUARY 2012 AMATEURASTRO.COM 5
BLOBS FROM OUTER SPACE
MONTHLY TOP APOD BREAKDOWN
Mysterious Space Blob Glows from Within Observations from the European Southern Observatory’s Very Large Telescope have shed light on the power source of a rare vast cloud of glowing gas in the early Universe. The observations show for the first time that this giant “Lymanalpha blob”—one of the largest single objects known—must be powered by galaxies embedded within it. A team of astronomers has used ESO’s Very Large Telescope (VLT) to study an unusual object called a Lyman-alpha blob [1]. These huge and very luminous rare structures are normally seen in regions of the early Universe where matter is concentrated. The team found that the light coming from one of these blobs is polarised [2]. In everyday life, for example, polarised light is used to create 3D effects in cinemas [3]. This is the first time that polarisation has ever been found in a Lymanalpha blob, and this observation helps to unlock the mystery of how the blobs shine. “We have shown for the first time that the glow of this enigmatic object is scattered light from brilliant galaxies hidden within, rather than the gas throughout the cloud itself shining.” explains Matthew Hayes (University of Toulouse, France), lead author of the paper. Lyman-alpha blobs aresome of the biggest objects in the Universe: gigantic clouds of hydrogen gas that can reach diameters of a few hundred 6
FEBRUARY 2012 AMATEUR ASTRONOMY
Primordial Cloud of Hydrogen is Centrally Powered. (Credit: ESO)
FEB. 25, 2012 / Sombrero Galaxy in Infrared This floating ring is the size of a galaxy. In fact, it is part of the photogenic Sombrero Galaxy, one of the largest galaxies in the nearby Virgo Cluster of Galaxies. The dark band of dust that obscures the mid-section of the Sombrero Galaxy in optical light actually glows brightly in infrared light.
thousand light-years (a few times larger than the size of the Milky Way), and which are as powerful as the brightest galaxies. They are typically found at large distances, so we see them as they were when the Universe was only a few billion years old. They are therefore important in our understanding of how galaxies formed and evolved when the Universe was younger. But the power source for their extreme luminosity, and the precise nature of the blobs, has remained unclear. The team studied one of the first and brightest of these blobs to be found. Known as LAB-1, it was discovered in 2000, and it is so far away that its light has taken about 11.5 billion years to reach us (redshift 3.1). With a diameter of about 300 000 light-years it is also one of the largest known, and has several primordial galaxies inside it.
and evolved when the Universe was younger. But the power source for their extreme luminosity, and the
FEB. 3, 2012 / INSIDE THE EAGLE NEBULA The first identified compact galaxy group, Stephan’s Quintet is featured in this eye-catching image constructed with data drawn from the extensive Hubble Legacy Archive. About 300 million light-years away, only four of these five galaxies are actually locked in a cosmic dance of repeated close encounters.
FEB. 6, 2012 / The mysterious rings of supernova 1987a What’s causing those odd rings in supernova 1987A? Twenty five years ago, in 1987, the brightest supernova in recent history was seen in the Large Magellanic Cloud. At the center of the above picture is an object central to the remains of the violent stellar explosion.
Varying Arrays of Visual Scores The Hubble’s discoveries transformed science, with its abilities to show the universe in unprecendented detail has turned astronomical conjectures into concrete certainties.
STUNNING STAR GAZING WITH YOUR SMART PHONE OR TABLET The International Year of Astronomy Awarded product, Star Walk is an Education app that allows users to easily locate and identify 20,000+ objects in the night sky. The 360-degree, touch control star map displays constellations, stars, planets, satellites, and galaxies currently overhead from anywhere on Earth. Highly praised and the winner of a 2010 Apple Design Award, the latest update allows users to enjoy unprecedented eye candy and interactivity of the star map, achieved with the new camera and high resolution of the new device. BY JOHN SMITH Virginia-based mobile software developer Vito Technology Inc., makers of the business app Task2Gather, has come up with a useful and fun star map that uses your iPhone’s current location in order to provide a picture of the sky located right above you. Star Walk also allows the user to search the map by stars, constellations, or planets. Selecting one of the above will bring up the chosen object on Star Walk’s screen. The user can then select an object to see more information on it like its size and coordinates. All of this is pretty awesome already, thanks especially to the great graphics and intuitive interface. But the time-lapse function is what makes Star Walk truly great. Selecting the time button on the screen will show the current date and time and an according picture of the stars and planets. Using a scroll bar, the user can scroll through different times, causing the image of the sky to change accordingly. This looks like a time-lapse movie of the stars, going through day and night mode and changing star positions. Star Walk is well developed with great graphics and an interface that is easy to navigate. A great map for anyone wanting to learn a little more about the constellations or as a reference when looking up at the sky at night Star Walk is your realistic guide for star gazing. Whenever you admire the starry night sky, Star Walk helps you to recognize anonymous heavenly bodies. Just find them on your interactive star map and tap “i” to retrieve useful information, such as celestial coordinates of these heavenly bodies. Don’t be afraid to get overloaded by tons of specific terms. Star Walk is not for scientific use but for learning something new playfully. The cutting-edge graphics and smooth moving/scaling make your work with Star Walk pleasant and easy. Star Walk covers about 9000 stars, planets and constellations, in a number of languages.
You can also use Star Walk for iPad as a time machine, allowing you to view the sky based on how it looks during specific time frame. It could be last night, or as long as two years ago. The apps even lets you view astronomical events such as solar eclipses and meteor showers. Use GPS to get located automatically or choose manually any location. New York, Sydney or Paris - you can observe the sky from wherever you like. Having chosen the location you can watch the starry sky on your iPhone as if you were there physically. But there is more... You can choose time. Actually that means that you can virtually travel through the time, future or past. This is a chance to see the stars and constellations through the eyes of Galileo. Of course, you can also choose a date from the future and watch the sky as it will be seen by our descendants over centuries.
SKY WATCH Southern Hemisphere What’s that on the Sun? Over the past two weeks, one of the most energetic sunspot regions of recent years crossed the face of the Sun. Active Region 1429, visible above as the group of dark spots on the Sun’s upper right, blasted out several solar flares and coronal mass ejections since coming around the edge of the Sun almost a month ago. Fast moving particles from these solar explosions have impacted the Earth and been responsible for many colorful auroras seen over the past two weeks. The picturesque foreground features trees and birds near Merida, Spain, where the above image was taken about a week ago. Although AR 1429 has continued to rotate to the right and gone around the limb of the Sun -- as seen from the Earth -monitoring of the region will be continued by one of the STEREO satellites, however, which is orbiting the Sun well ahead of the Earth.
The Planets It was visible around the world. The sunset conjunction of Jupiter and Venus was visible last week almost no matter where you lived on Earth. Anyone on the planet with a clear western horizon at sunset could see them. This week the two are still notable, even though Jupiter has sunk below the brighter Venus. And if you look higher in the sky you can see Mars as well. Pictured above, a creative photographer traveled away from the town lights of Szubin, Poland to image a near closest approach of the two planets almost a week ago. The bright planets were separated only by three degrees and his daughter striking a humorous pose. A faint red sunset still glowed in the background. Although this conjunction is drawing to a close, another conjunction between Venus and Jupiter will occur next May.
Western Hemisphere As stated above, Venus and Jupiter are this month’s two brightest planets. Shortly after sunset on February 20, they dominate the sky above the western horizon and this snowy landscape. In clear and transparent skies over Cherry Springs State Park, Pennsylvania, USA, they are also seen immersed in Zodiacal light. The extended, diffuse, triangular glow is sunlight scattered by dust along the plane of the ecliptic. Brighter near the horizon, the Zodiacal glow angles upward, first to Venus and then to Jupiter hugging the ecliptic as they orbit the Sun. Fading even further, the glow stretches toward the lovely Pleides star cluster near the top of the frame. Following their appearance in this Zodiacal skyscape, the coming days will see Venus and Jupiter sharing the early evening sky with a young crescent Moon. The two bright planets are even headed for a close pairing or conjunction, separated by about 3 degrees on March 13.
Jan Fe uary b Ap ruar 15 Ma ril 14 y 14 Jun y 14 Jul e 12 y Au 11 Se gust Oc ptem 10 No tober ber 8 De vemb 7 cem er be 6 r5
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NE MO W ON
Jan Fe uary b Ma ruar 20 Ap rch 3 y 26 r Ma il 28 0 Jun y 27 Jul e 25 y Au 26 Se gust p No tem 24 De vemb ber 2 cem er 3 be 21 r2 1
FEBRUARY 2012 AMATEUR ASTRONOMY
MO
ON
FU MO LL ON
20
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THE ULTRAVIOLET UNIVERSE NOW YOU SEE IT, NOW YOU DON’T
Ultraviolet light falls beyond the limits of what we can see
NASA’s Galaxy Evolution Explorer found a tail behind a galaxy called IC 3418. The starstudded tail can be seen on the left, as detected by the space telescope in ultraviolet light. The tail has escaped detection in visible light, as shown by the image on the right, taken by a visible-light telescope on the ground. This tail was created as the galaxy plunged into gas in a family of galaxies known as the Virgo cluster.
ultraviolet calls to mind images of sore skin resulting from
all the blue and of the spectrum. In human terms the world
overexposure to the Sun, an indication of the high energy of this form of light.
The hottest stars in the Universe are brightest in ultraviolet light. The dusty clouds that give birth to these massive, luminous objects are in turn sculpted and shaped under the onslaught of the high energy photons they emit. Ultraviolet light shows to where the action in the star formation — amongst the young, the massive and the hot stars.
FEBRUARY 2012 AMATEURASTRO.COM 9
With your camera mounted on a sturdy tripod, use a wide-angle lens, such as a 24mm or 28mm (50mm will suffice if that is all you have). A zoom lens should be used at its widest setting. A digital cameras should be set, if possible, to ISO 800; otherwise, ISO 400. Set the f-stop at the lowest setting possible(f/3.5 or lower). Focus at the infinity mark on the lens. Add something to the foreground of the shot for depth: a tree, house, person, your telescope, etc.
FEBRUARY AURORA BOREALIS
Using a hand-held cable release to trip the shutter, take exposures of 20 to 30 seconds each.
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FEBRUARY 2012 AMATEUR ASTRONOMY
Illuminated by an eerie greenish light, this remarkable little planet is covered with ice and snow and ringed by tall pine trees. Of course, this little planet is actually planet Earth, and the surrounding stars are above the horizon near Östersund, Sweden. The pale greenish illumination is from a curtain of shimmering Aurora Borealis also known as the Northern Lights. The display was triggered when a giant solar coronal mass ejection (CME) rocked planet Earth’s magnetosphere on January 24th and produced a strong geomagnetic storm. Northern hemisphere skygazers will also recognize the familiar orientation of stars at the left, including the Pleiades and Hyades star clusters and the stars of Orion. Increasing solar activity has caused recent auroral displays to be wide spread, including Aurora Australis, the Southern Lights, at high southern latitudes.
What’s that in the sky? An aurora. A large coronal mass ejection occurred on our Sun, throwing a cloud of fast moving electrons, protons, and ions toward the Earth. Although most of this cloud passed above the Earth, some of it impacted our Earth’s magnetosphere and resulted in spectacular auroras being seen at high northern latitudes. Pictured right is a particularly photogenic auroral corona captured last night above Grotfjord, Norway. To some, this shimmering green glow of recombining atmospheric oxygen might appear as a large eagle. This round of solar activity is not yet over!
FEBRUARY 2012 AMATEURASTRO.COM 11
MYSTERIES OF THE MARVELOUS MILKY WAY From its furthest dark-matter spattered reaches, to the black hole at its core, our galaxy still holds myriad mysteries. By Stephen Battersby Take 10 billion tonnes of antimatter and 10 billion tonnes of matter, and stir. Our galaxy mixes up one of these explosive cocktails every second, resulting in a warm inner glow of gamma radiation. Each photon created in this process carries an energy exactly equivalent to the annihilated mass of an electron and its antimatter counterpart, a positron. But what could be pumping out so many positrons? Most galactic radiation –; including visible light, ultraviolet, infrared, X-rays, radio waves and gamma rays of other energies –; comes predominantly from the Milky Way’s flat disc, where brilliant, short-lived new stars are formed. But the annihilation gamma rays come mainly from the galaxy’s much smaller, bulging centre. “It is a unique case,” says Nikos Prantzos of the Institute of Astrophysics in Paris, France.
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FEBRUARY 2012 AMATEUR ASTRONOMY
There are some odd suggestions for this odd origin. The positrons could be created by decaying dark matter or blobs of exotic quantum matter called Q-balls; or they could be spat out by microscopic black holes or a tangle of cosmic strings, snags in the structure of space-time. Or it might be something more familiar. Supernova explosions in the Milky Way’s disc create radioactive isotopes that emit positrons as they decay, and neutron stars and black holes can make antimatter when they feast on material from a sibling star. Some of these objects also inhabit the galactic bulge, although only enough to account for a fraction of the gamma-ray emission seen there. The picture changes if the galaxy’s magnetic field can funnel positrons from the disc into the central bulge. That
100,000 Lightyears Our Sun
depends on the field’s basic shape, something we could learn from observations of how radio waves from distant sources are polarised. It also depends on whether positrons can travel tens of thousands of light years before being annihilated. That will be much harder to work out, because it depends on the small-scale details of magnetic fields and interstellar gas beyond the power of our telescopes. If positrons can travel so far, another possibility opens up. “They could come from an event that happened long ago in the central black hole,” says Prantzos. Starved of fuel, our galaxy’s black hole is currently quiet (see “Mystery object: Sagittarius A*”). Millions of years ago, though, it might have flared up, pumping out positrons that have since pushed out through the central bulge, creating a spherical halo of annihilation. Without a more detailed picture of the gamma-ray emissions, for now the truth remains out there. Step out into a clear night, far from the city lights, and you’ll see a creamy streak of stars splashed across the sky: the “Milky Way” that has come to stand for our island universe. We see it as we stare through the flat, star-dense disc of our galaxy where we are also quartered. But what does our home look like from outside? The short answer is we’re not sure. Our telescopes unveil other galaxies in majestic detail, but introspection is much trickier. We think we live in a spiral galaxy of the sort we see scattered throughout the cosmos, but our lowly viewpoint in the galactic disc means we struggle to trace how its arms are furled, or even count how many there are. Interstellar dust doesn’t help: it blocks our view over distances of more than a few thousand light years, so we cannot map out distant spiral arms by their stars. We instead trace clouds of hydrogen atoms, which emit radio waves with
a characteristic wavelength of about 21 centimetres. This long-wave radiation penetrates the dust, and by measuring the change in its wavelength –; its Doppler shift –; we can work out a cloud’s speed towards or away from us. Comparing that with the ways in which different parts of the galaxy should rotate allows us to pinpoint a cloud’s distance. The resulting tentative maps suggest that the galaxy is a complicated, messy, many-armed spiral (see diagram, below). But even that sketch is arguable. For a start, the galaxy’s rotation is not precisely known, and individual clouds need not follow the average motion; different models produce different maps. And when we look towards or away from the galactic centre, where the clouds are moving almost sideways relative to us, the Doppler method is no help in determining their distance. “Arms can only be identified in segments,” says galaxy mapper Robert Benjamin of the University of Wisconsin-Whitewater. “The task of piecing them together is left to the discretion of the astronomer.” A parallel mapping effort, which suffers from similar limitations, uses radio emission from carbon monoxide gas that hangs around parts of the galaxy. Since 2008, this method has revealed more details of the galaxy’s structure, including what seem to be new arm segments. Better landmarks may be interstellar clouds where molecules of water or methanol act as lasers, amplifying a narrow line of microwave emission to produce bright pinpoints. These “masers” are so well localised that we can see how their position shifts as Earth orbits the sun, and thus triangulate their distance from us precisely. There are too few of them to map out the galaxy on their own, but they can be used to test the results of other methods. Maser range-finding could finally reveal the true face of the Milky Way. The Milky Way and Andromeda are siblings: two great spirals that dominate our local group of galaxies. They have about the same total mass, and we used to think they were near-twins. Not any more. “As we look in more detail, we see that they are quite different,” says Alan McConnachie of the Herzberg Institute of Astrophysics in Victoria, Canada. Andromeda is the favourite child. It is brighter, with a wider disc of stars. The black hole at its heart is more than a hundred times as massive as ours. And while our galaxy is strewn with about 150 of the bright galactic baubles known as globular clusters, Andromeda boasts more than 400.
FEBRUARY 2012 AMATEURASTRO.COM 13
V838 MONOCREOTIS In February 2002, this previously undistinguished star, about 20,000 light years away, briefly achieved a luminosity a million times that of our sun. The following month it happened again. And in April. It was first assumed to be a nova –; a white dwarf that pulls gas off a companion until it triggers a thermonuclear explosion on its surface. But novas don’t happen three times in quick succession and then go quiet. Was it a rarely seen flare-up near the end of a giant star’s life? The screams of two stars colliding? Or did one star swallow three giant planets? What is certain is that the triple burst of light was reflected off nearby dust to surround the object with rapidly changing light shells, making it a true cosmic beauty Pacing out their slow circuits over a billion years or more, 26 small galaxies are known to orbit the Milky Way. That might seem an impressive band of loyal followers. Astrophysicists, though, think we should have an army. That expectation is based on the prevailing model of how dark matter helps to form galaxies. Dark matter’s composition is unknown, but it is thought to outweigh ordinary matter by five to one. In simulations of the early universe, the gravitational pull of cold clumps of dark matter draws in ordinary gas to form the first galactic building blocks. The theory works well on large scales, reproducing the spongy pattern of galaxies and voids seen across the cosmos. On smaller scales, however, the simulations show that around every large spiral galaxy, dark matter clumps should sculpt thousands of dwarf galaxies. One possible explanation for the discrepancy is that dark matter is not cold and clumpy,
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FEBRUARY 2012 AMATEUR ASTRONOMY
but a hotter gas of lightweight particles that forms small blobs less easily. Or perhaps dark matter doesn’t exist at all: if the strength of gravity were to change at long range, that would do much that dark matter does without requiring so many dwarfs. Tinkering with gravity is controversial. A less radical idea is that all those small dark-matter blobs do exist, but we just can’t see them. Because their gravitational grip is weak, the gas could have been pushed out before many stars could form; a few giant stars may have blasted it out with their fierce heat and explosive deaths, for example. This theory is hard to check in simulations, says astrophysicist James Binney of the University of Oxford, because it depends on details such as the local gas density. “That has very fine structure which is way beyond the resolution of any simulation.” If the idea is right, though, it has a startling implication. Thousands of dark galaxies are arrayed around the Milky Way: a legion not lost, but invisible.
BLUE STRAGGLERS The Milky Way’s dense globular clusters are spherical swarms of red, lightweight and ancient stars, most of them more than 10 billion years old. A few globular-cluster stars, however, shine in blue-white light –; suggesting something anomalously hot, young and bright. We now think these “blue stragglers” are just as old as their companions, but have somehow been rejuvenated. Some may have sucked gas from a neighbouring star, compressing their central nuclear engine to make them burn faster and hotter. Others may be the offspring of stellar mergers, two cool red stars fusing to make blue.
S2 S2 is a fast, intense, blue-white star that frankly has some explaining to do. It orbits within a whisker of the galaxy’s central black hole, Sagittarius A* (see above), swinging by at a speed of up to 5000 kilometres per second, or nearly 2 per cent of the speed of light. At this distance, the black hole’s gravity should shred gas clouds before they can condense into new stars. And although a star might migrate inwards from more tranquil breeding grounds, S2 is a bright young thing no more than about 10 million years old, whose lifetime seems too brief for such a trek. The monster at the center of our Galaxy is about to get fed. Recent observations by the Very Large Telescopes indicate that a cloud of gas will venture too close to the supermassive black hole at the Galactic center. The gas cloud is being disrupted, stretched out, heated up, and some of it is expected to fall into the black hole over the next two years. In this artist’s illustration, what remains of the blob after a close pass to the black hole is shown in red and yellow, arching out from the gravitational death trap to its right. The cloud’s orbit is shown in red, while the orbits of central stars are shown in blue. The infalling nebula is estimated to contain setimes the mass of our Earth, while the central black hole, thought to correspond to the radio source Sagittarius A*, contains about four million times the mass of our Sun. Once it falls in, nothing is expected to be heard from the doomed gas ever. .
SAGITTARIUS A* At the very heart of the Milky Way is a region known as Sagittarius A. This region is known the be the home of a supermassive black hole with millions of times the mass of our own Sun. And with the discovery of this object, astronomers have turned up evidence that there are supermassive black holes at the centers most
spiral
and elliptical galaxies. The best observations of Sagittarius A*, using Very Long Baseline Interferometry radio astronomy have determined that it’s approximately 44 million km across (that’s just the distance of Mercury to the Sun). Astronomers have estimated that it contains 4.31 million solar masses. Of course, astronomers haven’t actually seen the supermassive black hole itself. Instead, they have observed the motion of stars in the vicinity of Sagittarius A*. After 10 years of observations, astronomers detected the motion of a star that came within 17 light-hours distance from the supermassive black hole; that’s only 3 times the distance from the Sun to Pluto. Only a compact object with the mass of millions of stars would be able to make a high mass object like a star move in that trajectory. The discovery of a supermassive black hole at the heart of the Milky Way helped astronomers puzzle out a different mystery: quasars. These are objects that shine with the brightness of millions of stars. We now know that quasars come from the radiation generated by the disks of material surrounding actively feeding supermassive black holes. Our own black hole is quiet today, but it could have been active in the past, and might be active again in the future. Some astronomers have suggested other objects that could have the same density and gravity to explain Sagittarius A, but anything would quickly collapse down into a supermassive black hole within the lifetime of the Milky Way.
FEBRUARY 2012 AMATEURASTRO.COM 15
Way’s centre, thought to hold a huge black hole four million times the sun’s mass. In some galaxies, such a black hole would be a fearsome source of radiation, blazing in light and X-rays as it feasted on nearby gas.Not so in our galaxy. That is partly because Sagittarius A* has a much scantier supply of gas, but even so it is faint, and seems unusually inefficient at converting gas into heat and light. Some clues as to why might come next year, when a nearby gas cloud looks set to plunge into our listless giant’s maw. We can see the fiery brilliance of supernovae from more than halfway across the universe, billions of light years away. So why are we missing them in our own backyard? Comparison with similar spiral galaxies suggests that the Milky Way should host about three stellar explosions per century, but in the past millennium and a bit we have seen only five or six. All of these have been within about 15,000 light years of us, while the Milky Way’s disc is 100,000 light years across. We are missing the more distant explosions for a simple reason: our lowly viewpoint means most of the galactic disc is hidden behind interstellar dust. As the blast wave of a supernova ploughs out into space, it energises particles to emit radio waves, which are not absorbed by intervening dust. Radio astronomers are now finding evidence of many of these supernova remnants. In 2008, one near the centre of our galaxy, called G1.9+0.3, was calculated to be the youngest known. Its light probably reached Earth about 110 years ago –; perhaps marking Queen Victoria’s funeral, had it not been for the black veils of dust in between. There is still a dearth of remnants from the past 2000 years, but these explosions occur in complex regions of star formation where ionised gas clouds are also emitting radio waves, confusing the picture. “I think they’re out there,” says David Green of the University of Cambridge, who discovered G1.9+0.3 in the 1970’s “We have
just not found them yet.” Now you see them, now you don’t. Among the many faint companions of the Milky Way are two shining exceptions. The large and small Magellanic clouds are by far the largest of our dwarf-galaxy entourage. They are complex and active places where bright young stars are being born and others die, as the most recent nearby supernova, which occurred in the Large Magellanic Cloud in 1987, revealed. That is all well and good –; except that these galaxies might not be our satellites at all. Observations in 2006 by the Hubble Space Telescope showed that these two dwarfs are crawling across the sky at a little shy of a millionth of a degree per year. That might not sound speedy, but the galaxies are more than 150,000 light years away, so it translates to a velocity of more than 100 kilometres per second. That might be too fast for the clouds to be held by the Milky Way’s gravity. Whether it is too fast depends on our galaxy’s total mass. Most of the Milky Way’s mass is thought to reside in its surrounding halo of dark matter, which stretches far beyond the bright disc of stars. The best way to estimate the mass of the whole galaxy is by following the motions of other, smaller satellite galaxies that are more conclusively in orbit, to see how they respond to our gravity. But these objects are fainter and slower than the Magellanic clouds, making them even harder to catch in motion. Our best estimates of the Milky Way’s mass are somewhere between one trillion and three trillion times that of the sun. That allows three possibilities. If the mass is near the top end, then we should easily hold onto the Magellanic clouds, and they have probably orbited the Milky Way a couple of times since it formed. If it is somewhere in the middle, the clouds are likely to be making their first close approach to us. Over a few hundred million years they will begin to head further away, but they will return to us eventually, like a comet on an elongated orbit. If the Milky Way is right at the lightweight end of the scale, however, the Magellanic clouds are just passing, and we will have to wave goodbye to this charismatic pair of space tourists.gas could have been pushed out before many stars could form; a few
Norma Arm 3 Kiloparsec Arm
Central Bar
S2
Globular Cluster Blue Stragglers
Sagittarius A
Scutum-Crux Arm Centarus Arm
V838 Monocreotis
Sagittarius Sagittarius-Carina Arm
10,000 Lightyears
Cygnus Arm Outer Arm
Our Sun
Orion Local Arm
PROPERTIES OF MILKY WAY GALAXY Diameter of Galaxy: 90,000 Lightyears Classification: SBbc Number of Stars: 200 Billion Mass of the Galaxy: 1 Trillion Solar Masses Length of Central Bar: 25,000 Lightyears
Distance of the Sun from center: 26,000 Lightyears Thickness of Galaxy at Sun: 2000 Lightyears Velocity of Sun around Galaxy: 220 km/s Orbital Period of Sun around Galaxy: 225 million years
Neutral Hydrogen
FEBRUARY 2012 AMATEURASTRO.COM 17
BRA D
GALA XYS NEW
BY
HUBBLE
Over
the past two decades, the world
has followed Hubble’s saga, watching as image after image revealed another part of the cosmos that would otherwise be invisible to our eyes. We absorbed the discoveries that the pictures sparked and witnessed the astronaut missions that brought the telescope new technology and extended its life. Not long after the Hubble achieves its 20th anniversary, the Hubble Space telescope will soon be pegged by its successor, the James T. Webb Telescope, (set to be launched in 2014). The Hubble telescope dominates in unveilling decades of wild discoveries of our universe, and our conception of time. The telescope is designed to study optical light wavelengths, giving clues into observing extremely old light, as we look deep into the universe, because the f arther away an object is away, the further we see back in time. More than 30,000 objects have come under the telescope’s gaze. Each observation slotting another piece into the cosmic puzzle of the universe.
What’s lighting up the Cigar Galaxy? M82, as this irregular galaxy is also known, was stirred up by a recent pass near large spiral galaxy M81. This doesn’t fully explain the source of the red-glowing outwardly expanding gas, however. Recent evidence indicates that this gas is being driven out by the combined emerging particle winds of many stars, together creating a galactic superwind.. The right photographic mosaic
highlights a specific color red light strongly emitted by ionized hydrogen gas, showing detailed filaments of this gas. The filaments extend for over 10,000 light years. The 12-million light-year distant Cigar Galaxy is the brightest galaxy in the sky in infrared light, and can be seen in visible light with a small telescope towards the constellation of the Great Bear (Ursa Major).
Why is this cluster of galaxies so jumbled? Far from a smooth distribution, Abell 2744 not only has knots of galaxies, but the X-ray emitting hot gas (colored red) in the cluster appears distributed differently than the dark matter. The dark matter, taking up over 75 percent of the cluster mass and colored blue in the above image, was inferred by that needed to create the distortion of background galaxies by gravitational lensing. The jumble appears to result from the slow motion collision of at least four smaller galaxy clusters over the past few billion years. The above picture combines optical images from the Hubble Space Telescope and the Very Large Telescope with X-ray images from the Chandra X-Ray Observatory. Abell 2744, dubbed Pandora's cluster, spans over two million light years and can best be seen with a really large telescope toward the constellation of the Sculptor.
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Two galaxies are squaring off in Corvus and here are the latest pictures. But when two galaxies collide, the stars that compose them usually do not. That's because galaxies are mostly empty space and, however bright, stars only take up only a small amount of that space. During the slow, hundred million year collision, one galaxy can still rip the other apart gravitationally, and dust and gas common to both galaxies does collide. In this clash of the titans, dark dust pillars mark massive molecular clouds are being compressed during the galactic encounter, causing the rapid birth of millions of stars, some of which are gravitationally bound together in massive star clusters.
This floating ring is the size of a galaxy. In fact, it is part of the photogenic Sombrero Galaxy, one of the largest galaxies in the nearby Virgo Cluster of Galaxies. The dark band of dust that obscures the midsection of the Sombrero Galaxy in optical light actually glows brightly in infrared light. The above image, digitally sharpened, shows the infrared glow,
recently recorded by the orbiting Spitzer Space Telescope, superposed in false-color on an existing image taken by NASA’s Hubble Space Telescope in optical light. The Sombrero Galaxy, also known as M104, spans about 50,000 light years across and lies 28 million light years away. M104 can be seen with a small telescope in the direction of the constellation Virgo.
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ULTRA HUBBLE
DEEP FIELD
What did the first galaxies look like? To help answer this question, the Hubble Space Telescope has just finished taking the Hubble Ultra Deep Field (HUDF), the deepest image of the universe ever taken in visible light. Pictured above, the HUDF shows a sampling of the oldest galaxies ever seen, galaxies that formed just after the dark ages, 13 billion years ago, when the universe was only 5 percent of its present age. The Hubble Space Telescope’s NICMOS and new ACS cameras took the image. Staring nearly 3 months at
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the same spot, the HUDF is four times more sensitive, in some colors, than the original Hubble Deep Field (HDF). Astronomers the world over will likely study the HUDF for years to come to better understand how stars and galaxies formed in the early universe, making this image the most important picture ever taken by the Hubble telesope. Right, is an image showcasing Hubble’s ability to not only rely on optical light, but also UV light, offering the chance for more mysteries of the universe to be uncovered.
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