More evidence that the Milky Way has four spiral arms

Astronomers have been arguing over just how many spiral arms our galaxy exhibits. Is the Milky Way a four or two-armed spiral galaxy? Astronomers had often assumed the Milky Way was potentially a four-armed spiral galaxy, but comparatively recent observations from NASA’s Spitzer telescope implied the galaxy had two spiral arms. In 2013, astronomers mapped star forming regions and argued they had found the two missing arms, bringing the total number of arms back to four.

The case for a four-armed Milky Way may have just gotten stronger.

A team of Brazilian astronomers used star clusters embedded in their natal clouds to trace the galaxy’s structure. «Our results favour a four-armed spiral galaxy, which includes the Sagittarius-Carina, Perseus, and outer arms,» remarked the group from the Universidade Federal do Rio Grande do Sul.

«Despite efforts aimed at improving our understanding of the galaxy’s structure, questions remain. There is no consensus regarding the number and shape of the galaxy’s spiral arms.», noted lead author D. Camargo. He added that the sun’s location within the obscured disc of the galaxy was a principal factor hindering our understanding of the Milky Way’s broader structure. In other words, we do not have a bird’s eye view of our galaxy.

The team remarked that young embedded clusters are excellent tracers of the galaxy’s structure, «The present results indicate that the galaxy’s embedded clusters are predominantly located in the spiral arms.» They noted that star formation may occur after the collapse and fragmentation of giant molecular clouds found within spiral arms, and consequently the young embedded star clusters that subsequently emerge are excellent probes of galactic structure as they have not displaced far from their birthplace.

The team used data from NASA’s WISE infrared telescope to identify young clusters still embedded in their natal clouds, which are often encompassed by significant dust. Infrared stellar light is less obscured by dust than visible light, giving the astronomers an unprecedented view. Indeed, the group discovered 7 new embedded clusters, several of which (designated Camargo 441-444) may belong to a larger aggregate that resides in the Perseus arm. They suggested that a giant molecular cloud was compressed by the spiral arm which may have triggered star formation in several clumps, and numerous star clusters with similar ages emerged (an alternative or concurrent scenario is sequential formation).

The team also used near-infrared data from the 2MASS survey to determine distances for the star clusters, once the objects were identified in the WISE images. A primary goal of their work was to establish accurate fundamental cluster parameters, which would bolster any resulting conclusions concerning the galaxy’s overall structure. An innovative algorithm was therefore adopted to minimize contamination by foreground and background stars along the sight-line, which may otherwise appear as cluster members and degrade the reliability of any distant estimates.

«The embedded clusters in the present sample are distributed along the Sagittarius-Carina, Perseus, and outer arms.», concluded the team. They likewise noted that the search for new embedded clusters throughout the entire galaxy must continue unabated, since such targets may foster our understanding of the galaxy’s structure.

Read more at: http://phys.org/news/2015-05-evidence-milky-spiral-arms.html#jCp

Delta Cephei’s hidden companion around star        

To measure distances in the universe, astronomers use cepheids, a family of variable stars whose luminosity varies with time. Their role as distance calibrators has brought them attention from researchers for more than a century. While it was thought that nearly everything was known about the prototype of cepheids, named Delta Cephei, a team of researchers at the University of Geneva (UNIGE), the Johns Hopkins University, and the European Space Agency (ESA), have now discovered that this star has a hidden companion. They have published an article about the discovery in The Astrophysical Journal.

Delta Cephei, prototype of the cepheids, which has given its name to all similar variable stars, was discovered 230 years ago by the English astronomer John Goodricke. Since the early 20th century, scientists have been interested in measuring cosmic distances using a relationship between these stars‘ periods of pulsation and their luminosities (intrinsic brightness), discovered by the American Henrietta Leavitt. Today, researchers from the Astronomical Observatory of UNIGE, Johns Hopkins University and the ESA show that Delta Cephei is, in fact, a double star, made up of a cepheid-type variable star and a companion that had thus far escaped detection, probably because of its low luminosity. Yet, pairs of stars, called binaries, complicate the calibration of the period-luminosity relationship, and can bias the measurement of distances. This is a surprising discovery, since Delta Cephei is one of the most studied stars, of which scientists thought they knew almost everything.

A secret companion

As the scientists from Geneva and Baltimore measured Delta Cephei’s pulsations with the Hermes spectrograph, installed at the Mercator telescope based on La Palma, one of the Canary Islands, an unexpected signal was detected. Using high-precision Doppler spectroscopy (developed and used for researching exoplanets), the researchers discovered that the speed with which Delta Cephei approaches the sun is not constant, but changes with time in a characteristic fashion. This change of speed can only be explained by the presence of another star orbiting around Delta Cephei. In other words, there is a secret companion, whose existence we did not suspect. By combining their own observations with data from the scientific literature, the researchers determined the orbit of the two stars and observed that the mass of the companion is low, around 10 times lower than the mass of Delta Cephei. «We were shocked: despite all the attention Delta Cephei was given over the years, we were lacking an essential piece of information,» states Richard Anderson, researcher at UNIGE at the time of the discovery, first author of the article and now a researcher at Johns Hopkins University, in the United States.

According to the scientists, the data collected in the framework of ESA’s Gaia space mission will enable precise measurement of Delta Cephei’s orbit. The presence of the companion must consequently be taken into account when the Gaia team determines the Delta Cephei’s distance. «Although our study does not challenge the cosmic distance ladder as a whole, improving the precision of every one of its rungs will eventually benefit cosmology,» explains Richard Anderson. «This discovery reminds us that something is always to be learned. If even one of the closest cepheids is keeping such secrets , who knows what we will discover about the ones furthest from us!»

A turbulent past?

Due to its eccentric orbit, Delta Cephei varies in distance from its companion. This suggests a very dynamic evolution, since the two stars are approaching each other to within twice the Earth-sun distance every six years, which is a small distance for a supergiant star such as Delta Cephei, whose radius is 43 times larger than that of the sun. Delta Cephei’s eccentric orbit thus points to interactions between the two stars due to tidal forces that occur when they are close to each other. This could help the interpretation of other astronomers’ work in the past, who reported a strange circumstellar environment for which no definitive explanation has been found yet. This study is likely to inspire further research aiming to better understand the evolution of Delta Cephei, since binarity is an essential property to consider for interpreting the evolution of a star. Studying the evolution of Cepheids is particularly interesting, since it helps to improve the understanding of the structure and evolution of stars in general. «We are waiting for the results from new measurements taken with the Hermes spectrograph and the observations from Gaia. These will allow us to precisely trace the possibly turbulent past of Delta Cephei,» the astronomers say. «It is a fascinating adventure!»

Read more at: http://phys.org/news/2015-05-delta-cephei-hidden-companion.html#jCp

Jupiter’s moon Europa is brimming with water.                                                   

Jupiter’s moon Europa is brimming with water. Although it is thought to be mostly made up of rocky material, the moon is wrapped in a thick layer of water – some frozen to form an icy crust, some potentially pooled in shallow underground lakes or layers of slush, and vast quantities more lurking even deeper still in the form of a giant subsurface ocean.

This false-colour image from NASA’s Galileo spacecraft shows a disrupted part of Europa’s crust known as Conamara Chaos. The long criss-crossing grooves etched into the shattered chunks of ice are a perfect example of «chaos terrain» – a feature seen most prominently in our Solar System on Europa, Mars and Mercury.

Although the exact ways chaos regions form are not completely understood, in the case of Europa scientists have a few ideas. One possibility is fast-moving impactors that smash through the moon’s brittle crust. As a liquid layer lies immediately beneath the crust, the shards are more mobile and can refreeze in different configurations, creating a fractured terrain with young scars carved into the icy plains.

Many chaos regions have small impact craters clustered nearby. In the case of Conamara Chaos, for example, a large 26 km-diameter crater named Pwyll lies 1000 km to the south, and a handful of smaller, 500 m-diameter craters are scattered throughout the region, likely formed by lumps of ice thrown up by the impact that created Pwyll.

Another suggestion is that Europa harbours an intricate system of shallow subsurface lakes. Instead of an object slamming into the Jovian moon, a lake system could influence and stress the crust from below to cause the thin ice sheets to fracture and collapse.

This patch of Europa’s crust takes on an iridescent appearance in this false-colour image, which strongly enhances subtle colour differences present in the scene. Areas of blue and white stand out distinctly from areas of rusty orange and bronze. This colouration is thought to be caused by material from Pwyll: when the crater formed it threw up a blanket of fine ice particles that settled over parts of Conamara Chaos, colouring parts of the landscape in dark blue (coarser particles of ice), light blue (smaller particles) and white (very fine particles). The bronze patches are regions of ice that have been stained by minerals from beneath the disrupted crust.

Although astronomers have studied Europa intensively, the only way to confirm the structure and composition of the moon is to probe its shell and interior with a space probe. ESA’s JUpiter ICy moons Explorer (Juice) mission aims to do just that when it arrives in the Jovian system in 2030. Alongside detailed studies of Jupiter itself, Juice will explore and characterise three of the gas giant’s potentially habitable icy moons: Ganymede, Europa and Callisto. The mission is in development, on track for launch in 2022.

North is to the top of the picture and the Sun illuminates the surface from the right side of the frame. The image is centred at 9ºN / 274ºW, and covers an area of some 70 km by 30 km. The image combines data taken by Galileo’s Solid State Imaging system during three orbits through the Jovian system in 1996 and 1997.

Read more at: http://phys.org/news/2015-05-image-chaos-watery-europa.html#jCp

  Comet Lovejoy does the polar plunge                                                                                                                                  

Lots of towns hold a polar plunge fundraising event in the winter. Duluth, Minnesota’s version, where participants jump in Lake Superior every February, might just be the coldest. Comet Lovejoy’s a season behind, but sure enough, it’s following suite, diving deep into the dark waters of the north celestial pole this month.

I dropped in on our old friend last night, when it glowed only 8° from the North Star. In 8×40 binoculars, the comet was faintly visible as a hazy blob of light with a brighter center. Not a sight to knock you over, but the fact that this comet is still visible in binoculars after so many months makes it worthwhile to seek out. Moonless skies for the next 10-11 nights means lots of opportunities.

Unless a new comet is discovered, Lovejoy will continue to remain the only «bright» comet visible from mid-northern latitudes for some time. There’s a tiny chance Comet C/2014 Q1 PanSTARRS will wax bright enough to see in twilight in early July, but it will be very low in the northwestern sky at dusk and visible for a few nights at most. Only C/2013 US10 Catalina offers the chance for a naked eye / binocular appearance, when it re-emerges from the solar glare in the latter half of November in the morning sky.

Southern hemisphere observers have more to smile about with Comet C/2015 G2 MASTER currently flaunting its fluff at magnitude +6.6 or just under the naked eye limit. They’ll also get a far better view of C/2014 Q1 PanSTARRS come this July and August.

Through a telescope, Lovejoy still shows off a round, 6 arc minute diameter coma (one-fifth as wide as a full moon) and a denser, brighter core highlighted by a starlike false nucleus. We call it false because the true comet nucleus, probably no more than a few kilometers across, hides within a dusty cocoon of its own making. Only spacecraft have been able to get close enough for a clear view of comet nuclei. Each shows a unique and usually non-spherical shape because comets aren’t massive enough for their own self-gravity to crush them into spheres the way larger moons and planets do. If you’re a single object and big, being spherical comes naturally. Just face the North Star (Polaris) to begin tracking Comet Lovejoy. The map shows the sky facing north around 10:30 p.m. local time in mid-May. Stars are plotted to magnitude +8. Click for a larger version. Credit: Chris Marriott’s SkyMap

In my 15-inch (37-cm) telescope a faint wisp of a tail poked from the coma to the north. Looking at the map, you can see the comet’s headed due north through Cepheus toward Polaris, the North Star. Each passing night, it draws closer to the sky’s celestial pivot point, missing it by just 1° on the evenings of May 27 and 28. Closest approach to the north celestial pole, which marks the spot in the sky toward which Earth’s north polar axis currently points, occurs on May 29 with a separation of 54 arc minutes or just under a degree.

Finding Polaris is easy. Just draw a line through the two stars at the end of of the Big Dipper’s Bowl toward the horizon. The first similarly bright star you run into is the North Star. Using the map, you can navigate from Polaris to the fuzzy comet with either binoculars or telescope. Wide view of the sky facing north in mid-May around 10:30 p.m. local time. Use the Pointer stars in the Big Dipper to point you to Polaris and from there to the comet. Credit: Chris Marriott’s SkyMap Comet Lovejoy will be closest to the imaginary point in the sky called the north celestial pole on May 29. Polaris lies 0.75° from the pole and describes a small circle 1.5° in diameter around it each day. Credit: Stellarium Read more at: http://phys.org/news/2015-05-comet-lovejoy-polar-plunge.html#jCp

Galaxy NGC 3923 Galactic onion

The glowing object in this image is an elliptical galaxy called NGC 3923. It is located over 90 million light-years away in the constellation of Hydra. NGC 3923 is an example of a shell galaxy where the stars in its halo are arranged in layers. Finding concentric shells of stars enclosing a galaxy is quite common and is observed in many elliptical galaxies. In fact, every tenth elliptical galaxy exhibits this onion-like structure, which has never been observed in spiral galaxies. The shell-like structures are thought to develop as a consequence of galactic cannibalism, when a larger galaxy ingests a smaller companion. As the two centres approach, they initially oscillate about a common centre, and this oscillation ripples outwards forming the shells of stars just as ripples on a pond spread when the surface is disturbed. NGC 3923 has over twenty shells, with only a few of the outer ones visible in this image and its shells are much more subtle than those of other shell galaxies. The shells of this galaxy are also interestingly symmetrical, while other shell galaxies are more skewed. A version of this image was entered into the Hubble’s Hidden Treasures image processing competition by contestant Judy Schmidt. Credit:

Advanced Alien Civilizations Still Science Fiction — For Now                                                                                                A false-color image of the mid-infrared emission from the Great Galaxy in Andromeda, as seen by Nasa’s WISE space telescope.               Credit: NASA/JPL-Caltech/WISE Team

A wide-ranging search of faraway galaxies has turned up no obvious signs of advanced alien civilizations. A team of scientists dug through observations made by NASA’s Wide-field Infrared Survey Explorer (WISE) spacecraft, hunting for telltale heat signatures coming from 100,000 galaxies— a strategy suggested by theoretical physicist Freeman Dyson back in the 1960s. «Whether an advanced spacefaring civilization uses the large amounts of energy from its galaxy’s stars to power computers, spaceflight, communication or something we can’t yet imagine, fundamental thermodynamics tells us that this energy must be radiated away as heat in the midinfrared wavelengths,» study co-author Jason Wright, of Pennsylvania State University, said in a statement. «This same basic physics causes your computer to radiate heat while it is turned on.» [13 Ways to Hunt Intelligent Alien Life] The team found no smoking guns during this pilot study, known as the Glimpsing Heat from Alien Technologies Survey (G-HAT). «Our results mean that, out of the 100,000 galaxies that WISE could see in sufficient detail, none of them is widely populated by an alien civilization using most of the starlight in its galaxy for its own purposes,» Wright said in the statement.                                                                                                                                    A false-color image of the mid-infrared nebula surrounding the nearby star 48 Librae. Credit: Roger Griffth (Penn State) / IPAC (NASA/JPL-Caltech «That’s interesting because these galaxies are billions of years old, which should have been plenty of time for them to have been filled with alien civilizations, if they exist,» he added. «Either they don’t exist, or they don’t yet use enough energy for us to recognize them.» That’s not to say the G-HAT team found nothing interesting, intriguing or odd in their hunt for heat signatures. Indeed, about 50 of the galaxies had unusually high levels of midinfrared radiation. Follow-up studies could help determine if this heat is being generated by natural processes, or if it could be a sign of intelligent aliens, the researchers said. «As we look more carefully at the light from these galaxies, we should be able to push our sensitivity to alien technology down to much lower levels, and to better distinguish heat resulting from natural astronomical sources from heat produced by advanced technologies,» Wright said. «This pilot study is just the beginning.» The new study was published last month in The Astrophysical Journal Supplement Series. The WISE spacecraft launched in December 2009 and scanned the entire sky in infrared light twice before being placed in hibernation in 2011. The observatory was reactivated in 2013 to hunt for comets and asteroids that could pose a danger to Earth.                                                                                                                                                 http://www.space.com/29362-search-intelligent-aliens-wise-mission.html