Astronomy & Science

Astrónomos de Caltech desvelan una lejana protogalaxia conectada a la red cósmica

Ciencia Kanija -

Artículo publicado por Kimm Fesenmaier el 5 de agosto de 2015 en Caltech

Un equipo de astrónomos liderados por Caltech ha descubierto un gigantesco disco de gas giratorio a 10 000 millones de años luz de distancia, una galaxia en formación que recibe un suministro de gas primordial frío procedente del Big Bang. Usando el instrumento Cosmic Web Imager (CWI), diseñado y construido en Caltech, y situado en el Observatorio Palomar, los investigadores pudieron tomar imágenes de la protogalaxia, y hallaron que está conectada a un filamento del medio intergaláctico, la red cósmica compuesta de gas difuso y que zigzaguea entre las galaxias y se extiende por todo el universo.

El hallazgo proporciona el apoyo observacional más sólido, hasta el momento, para lo que se conoce como modelo de formación de galaxias de flujo frío. Este modelo sostiene que, en los inicios del universo, un gas relativamente frío fluía desde la red cósmica directamente hacia las galaxias, avivando una rápida formación estelar.

El artículo que describe los hallazgos y cómo fueron posibles gracias al CWI, actualmente se encuentra en línea, y se publicará en el ejemplar impreso del 13 de agosto de la revista Nature.

Protogalaxia

Protogalaxia

“Ésta es la primera prueba sólida sobre cómo se forman las galaxias”, dice Christopher Martin, profesor de Física en Caltech, investigador principal en CWI, y autor principal del nuevo artículo. “Aunque las simulaciones y el trabajo teórico han subrayado cada vez más la importancia de los flujos fríos, carecíamos de pruebas observacionales de su papel en la formación de galaxias”.

El disco protogaláctico que ha identificado el equipo se extiende a lo largo de unos 400 000 años luz, unas cuatro veces más grande que nuestra Vía Láctea. Está situado en un sistema dominado por dos cuásares, colocándose el más cercano de los mismos, UM287, de tal modo que su emisión se lanza como el flash de una cámara, ayudando a iluminar el filamento de red cósmica que alimenta de gas a la protogalaxia.

El año pasado, Sebastiano Cantalupo, entonces en la Universidad de California en Santa Cruz (ahora en el ETH Zurich) y sus colegas, publicaron un artículo, también en Nature, anunciando el descubrimiento de lo que pensaban que era un gran filamento cerca de UM287. La característica que observaron era más brillante de lo que debería haber sido si realmente fuese un único filamento. Parecía que hubiese algo más allí.

En septiembre de 2014, Martin y sus colegas, incluyendo a Cantalupo, decidieron continuar las observaciones del sistema usando CWI. Como espectrógrafo de campo integral, CWI permitió al equipo recopilar imágenes alrededor de UM287 en cientos de distintas longitudes de onda simultáneamente, revelando los detalles de la composición, distribución de masa, y velocidad del sistema.

Martin y sus colegas se centraron en un rango de longitudes de onda alrededor de una línea de emisión en el ultravioleta, conocida como línea Lyman-alfa. Dicha línea, una huella del gas hidrógeno atómico, se usa normalmente en astronomía como indicador de la materia primordial.

Los investigadores recopilaron una serie de imágenes espectrales, las cuales combinaron para formar un mapa el múltiples longitudes de onda de una zona del cielo alrededor de los dos cuásares. Estos datos delinearon las áreas donde el gas emitía en la línea Lyman-alfa, e indicaron la velocidad a la que se movía el gas respecto al centro del sistema.

“Las imágenes muestran claramente que existe un disco en rotación, puede verse que un lado se mueve acercándose hacia nosotros, mientras que otro se aleja. Y también puede apreciarse que hay un filamento que se extiende más allá del disco”, dice Martin. Las medidas indican que el disco está girando a una tasa de unos 400 kilómetros por segundo, algo más rápidamente que la velocida de rotación de la Vía Láctea.

“El filamento tiene una velocidad más o menos constante. Básicamente, está canalizando gas hacia el disco a una ritmo fijo”, señala Matt Matuszewski (PhD ’12), científico instrumental en el grupo de Martin, y coautor del artículo. “Una vez que el gas se fusiona con el disco dentro del halo de materia oscura, es distribuido por el gas en rotación y la materia oscura del halo”. La materia oscura es una forma de materia que no podemos ver, y que se cree que forma el 27% del universo. Se piensa que las galaxias se forman dentro de grandes halos de materia oscura.

Las nuevas observaciones y medidas proporcionan la primera confirmación directa de lo que se conoce como modelo de flujo frío de formación de galaxias.

Esto modelo, fuente de un acalorado debate desde 2003, se opone al viejo modelo estándar de formación de galaxias. El modelo estándar decía que, cuando colapsan los halos de materia oscura, atraen una gran cantidad de materia normal en forma de gas junto con ellos, calentándola a una temperatura extremadamente elevada. El gas, entonces, se enfría muy lentamente, proporcionando un flujo regular, aunque lento, de gas frío que puede formar estrellas en las galaxias en crecimiento.

Este modelo parecía correcto hasta 1996, cuando Chuck Steidel, y el profesor de Astronomía en Caltech Lee A. DuBridge, descubrieron una lejana población de galaxias que producían estrellas a un ritmo muy alto sólo 2000 millones de años tras el Big Bang. El modelo estándar no puede proporcionar el prodigioso suministro de combustible para estas galaxias en rápida formación.

El modelo de flujo frío proporcionó una solución potencial. Los teóricos sugirieron que el gas relativamente frío, suministrado por los filamentos de la red cósmica, fluye directamente hacia las protogalaxias. De esto modo puede condensarse rápidamente para formar estrellas. Las simulaciones demuestran que cuando cae el gas, contiene una gran cantidad de momento angular, o giro, y forma grandes discos de rotación.

“Ésta es una predicción directa del modelo de flujo frío, y es exactamente lo que vemos, un gran disco con mucho momento angular que podemos medir”, comenta Martin.

Phil Hopkins, profesor ayudante de astrofísica teórica enCaltech, que no estuvo implicado en el estudio, encuentra el nuevo descubrimiento “muy convincente”.

“Como evidencia de que existe una protogalaxia conectada a la red cósmica, y que podemos detectarla, es realmente apasionante”, comenta. “Por supuesto, ahora querremos saber un millón de cosas sobre qué está haciendo realmente el gas que cae a las galaxias, por lo que estoy seguro de que habrá continuación del estudio”.

Martin señala que el equipo ya ha identificado dos discos adicionales que parecen estar recibiendo gas directamente de filamentos de la red cósmica del mismo modo.

Brown Dwarfs Form Like Stars

Sky&Telescope -

Recent radio observations support the idea that brown dwarfs form like full-fledged stars do.

Brown dwarfs, which bridge the gap between stars and planets, have been an exciting target for astronomers since their discovery in the mid-1990s. Since that time, we’ve observed and classified hundreds of these objects, but the details of how they form still remain an active area of research. The answer to a simple question, "Do brown dwarfs form in a method similar to stars, or do they form more like planets?" has eluded astronomers for decades.

forming brown dwarf

Artist's conception of a very young, still-forming brown dwarf, with a disk of material orbiting it and jets of material ejected outward from the poles of the disk.
Credit: Bill Saxton / NRAO / AUI / NSF

The general picture of star formation is relatively clear. Massive clouds of gas, millions of times more massive than the Sun, collapse due to gravity. As the collapse ensues, clumps within the cloud begin to pull in material more rapidly than other, less dense regions. These overdensities create immense pressures and temperatures inside themselves (reaching over 25 million degrees Fahrenheit!). Eventually, the conditions for nuclear fusion are reached, and a star is born.

During this time, the star is still accreting material at a much smaller rate, spinning rapidly, and producing large magnetic fields. Thanks to the conservation of angular momentum, the rapid rotation also forms a disk out of the protostar’s birth cloud, in which smaller gravitational collapses may happen, resulting in planets.

But it’s unclear if brown dwarfs arise the same way. A group of astronomers led by Oscar Morata (Academia Sinica, Taiwan) has published results in the July 1st Astrophysical Journal that might go a long way in answering that question.

Brown dwarfs are often referred to as "failed stars." Spanning a mass range of about 13 to 80 times that of Jupiter, brown dwarfs lack sufficient pressures and temperatures in their cores to ignite nuclear fusion, the process that generates starlight. They start off with surface temperatures about one-third that of our Sun, and most of that heat is generated as the cloud that forms the brown dwarf collapses under its own gravity. This makes the youngest brown dwarfs the hottest and brightest members of their class. As time passes they slowly cool, like fireplace embers — but over billions of years.

The team targeted young brown dwarfs, because they haven't had a chance to cool off and are thus intrinsically brighter than older ones. The team was searching for jets, which astronomers commonly see coming from protostars but have not observed often from forming brown dwarfs.

Morata and his team imaged 11 proto-brown dwarfs using the Karl G. Jansky Very Large Array (VLA) radio telescopes in New Mexico. These objects are still in the process of forming and are still gravitationally accreting gas and dust. All of the young brown dwarfs sit in a star-forming region in Taurus about 450 light-years away, with an age of only about 1 million years.

Four of the young brown dwarfs showed radio emission due to jets, a hallmark of young, more massive stars. Jets are usually seen in young stars that are just forming, when the stellar magnetic fields are still very strong and the star is spinning rapidly. These magnetic fields can trap charged particles, supplied by strong protostellar winds, which emit radio waves as they accelerate around the fields. Observers have found that with normal stars, the strength of the magnetic fields, the amount of radio waves produced by particles spiraling through the jets, and the overall amount of proto-starlight produced are all related, with more massive (and therefore brighter) stars producing stronger magnetic fields and jets.

By comparing the strength of the radio waves emitted by the proto-brown dwarfs to the overall amount of light these forming objects produce, the team was able to show that these young brown dwarfs display the same behavior as their more massive stellar cousins.

“This is the first time that such jets have been found coming from brown dwarfs at such an early stage of their formation, and shows that they form in a way similar to that of stars," said Morata in an NRAO press release. "These are the lowest-mass objects that seem to form the same way as stars," he added.

This exciting discovery shows that brown dwarfs are more similar to stars than to planets. It also builds on past results, including computer simulations of star formation that not only also produced brown dwarfs, but a ratio of brown dwarfs to stars similar to that observed in our galaxy. Those simulations assumed that brown dwarfs formed in a similar way to stars, and the detection of jets further confirms this scenario. With new arrays such as ALMA coming online, we can expect to see more results on jets around brown dwarfs in the future.

 

Reference: O. Morata et al. "First Detection of Thermal Radiojets in a Sample of Proto-Brown Dwarf Candidates." Astrophysical Journal. July 1, 2015.

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Cómo la gravedad mató al gato de Schrödinger

Ciencia Kanija -

Artículo original publicado por Elizabeth Gibney el 17 de junio de 2015 en Nature News

Los teóricos defienden que un espacio-tiempo curvado evita las superposiciones cuánticas de objetos a gran escala.

Si el gato del famoso experimento mental de Erwin Schrödinger se comportase de acuerdo con la teoría cuántica, podría existir en varios estados a la vez: tanto vivo, como muerto. La explicación habitual que dan los físicos a por qué no vemos estas superposiciones cuánticas — en gatos o en cualquier otro aspecto del mundo cotidiano — es la interferencia con el entorno. Tan pronto como un objeto cuántico interactúa con una partícula aislada o pasa a través de un campo, cae en un estado, colapsando en nuestra visión clásica y cotidiana.

schrodinger cat

Pero incluso si los físicos pudiesen aislar completamente un gran objeto en una superposición cuántica, de acuerdo con investigadores de la Universidad de Viena, seguiría colapsando en un estado – al menos, en la superficie de la Tierra. “En algún lugar del espacio interestelar podría ser que el gato tuviese una oportunidad de conservar la coherencia cuántica pero, en la Tierra, hay pocas esperanzas de que estos suceda”, dice Igor Pikovski. La razón, afirma, es la gravedad.

La idea de Pikovski y sus colegas, plasmada en un artículo publicado en Nature Physics el pasado 15 de junio1, es, actualmente, sólo un argumento matemático. Pero los experimentadores esperan poner a prueba si la gravedad realmente colapsa las superposiciones cuánticas, dice Hendrik Ulbricht, físico experimental en la Universidad de Southampton, en el Reino Unido. “Esta idea es nueva y genial, y estoy listo para tratar de verla en experimentos”, comenta. Ensamblar la tecnología para realizarlos, no obstante, puede llevar hasta una década, señala.

Los aficionados al cine que hayan visto la película Interstellar ya están familiarizados con el principio básico que subyace al trabajo del equipo de Viena. La Teoría General de la Relatividad de Einstein afirma que un objeto extremadamente masivo provoca que un reloj que se encuentre cerca avance más lentamente, debido a que el potente campo gravitatorio estira el tejido del espacio-tiempo (que es la razón por la que un personaje de la película envejece sólo una hora cerca de un agujero negro, mientras que en la Tierra han transcurrido siete años). A una escala más sutil, una molécula situada cerca de la superficie de la Tierra experimenta un envejecimiento sensiblemente menor que una situada ligeramente más lejos.

Debido al efecto de la gravedad en el espacio-tiempo, el equipo de Pikovski se dio cuenta de que la varianza en la posición de una molécula también influirá en su energía interna (las vibraciones de las partículas en el interior de la molécula, que evolucionan con el tiempo). Si se colocase una molécula en una superposición cuántica de dos lugares, la correlación entre la posición y la energía interna pronto provocaría que se aplicara la decoherencia a la molécula, tomando un único camino, sugieren. “En la mayor parte de situaciones, la decoherencia se debe a algo externo; aquí, se ve como la vibración interna interactúa con el movimiento de la propia partícula”, añade Pikovski.

Un límite práctico

Nadie ha visto por el momento este efecto, debido a que otras fuentes de decoherencia — tales como campos magnéticos, radiación térmica y vibraciones — son, normalmente, mucho más fuertes y provocan que el sistema cuántico colapse mucho antes de que la gravedad sea un problema. Pero los experimentadores están deseando intentarlo.

Markus Arndt, físico experimental también en la Universidad de Viena, ya ha puesto a prueba si pueden observarse superposiciones cuánticas en objetos grandes, aunque no del tamaño de un gato (ver engordando al gato de Schrödinger). Envía grandes moléculas a través de un interferómetro de materia-onda, un sistema que da a cada molécula una elección entre dos rutas distintas. En la visión clásica, una molécula viaja a lo largo de un único camino; una molécula cuántica pasa de manera efectiva a través de ambas rutas a la vez e interfiere consigo misma para crear un patrón ondulatorio característico.

Una configuración similar podría usarse para poner a prueba la capacidad de la gravedad para destruir el comportamiento cuántico: comparando un interferómetro vertical, en el cual la superposición se perdería pronto debido al estiramiento del tiempo en uno de los caminos respecto a otro, con una configuración horizontal, donde podría mantenerse la superposición. Arndt, que ha puesto a prueba el efecto para moléculas de hasta 810 átomos2, señala que las moléculas grandes serían buenas para poner a prueba el efecto gravitatorio debido a que contienen muchas partículas que contribuyen a la energía interna. Pero los investigadores no sólo tienen que suprimir el entorno externo para reducir otros efectos de decoherencia, sino que también tendrían que aumentar la separación entre las dos rutas de micrómetros a metros, o usar moléculas 1 millón de veces más masivas. “Ciertamente, es un gran desafío”, dice Arndt.

Si el efecto de la gravedad limita el comportamiento cuántico en la Tierra, las pruebas sobre realidad cuántica en objetos grandes pueden, finalmente, tener que moverse al espacio, dice Angelo Bassi, físico en la Universidad de Trieste en Italia. “Pero desde un punto de vista profundo y fundamental, esto no es nada nuevo”, señala. Un campo gravitatorio simplemente es otro elemento del entorno con el que interactuar, por lo que aplicarlo no explica si el comportamiento cuántico derivaría en una realidad clásica si se mitigase la influencia gravitatoria — por ejemplo, realizando el experimento en el espacio, libre de influencia gravitatoria.

El efecto descrito por Pikovski y sus colegas tampoco dice nada sobre la gravedad cuántica: una teoría que unificaría la gravedad y la mecánica cuántica en una única descripción, algo en lo que trabajan muchos investigadores. “Es un efecto interesante, pero sigue siendo física cuántica aplicada a la relatividad general clásica. De este modo, no modifica nuestra visión del mundo”, concluye Bassi.

Referencias

Nature doi:10.1038/nature.2015.17773

1- Pikovski, I., Zych, M., Costa, F. & Brukner, Č. Nature Phys. http://dx.doi.org/10.1038/nphys3366 (15 June 2015).

2- Eibenberger, S. et al Phys. Chem. Chem. Phys. 15 14696–14700 2013

Gigantic Protogalaxy in the Cosmic Web

Sky&Telescope -

Astronomers have found that a massive filament of gas in the early universe actually seems to be a humongous, galaxy-forming disk.

Galaxy formation is a complicated affair. It can involve big smashups and nonchalant snacking, but as I explain in my feature article in S&T’s September issue, that’s only a slice of the story. One way that galaxies grow — and possibly the predominant way in the early universe — is from cold gas funneled like a pipeline into wells of dark matter.

cold-gas accretion simulation

This snapshot from a computer simulation shows a protogalaxy (central disk) growing by accreting cold gas from cosmic filaments (blue streams) in the early universe. Over time, hot gas (red) chokes off the filamentary inflow. The protogalaxy shown here is only one-fifth as wide as the disk recently discovered in the early universe, but the overall properties (given the scaling up) are similar.
Oscar Agertz

These dark matter wells are dense filaments in the weblike cosmic structure, along which galaxies form. Computer simulations suggest that cold-gas accretion was a big deal in the universe’s first couple billion years. After that, the halo of gas around a forming galaxy generally grew so hot that it choked off the pipeline (although not necessarily entirely). But observing cold-gas accretion in action is tough, because the gas is fairly diffuse and faint.

Last year, astronomers detected a large, bright filament of gas called UM 287 shining at us from about 11 billion years ago. It’s bathed in the glow — well, vicious radiation — of a nearby quasar, whose supermassive black hole spawns an ultraviolet beam that illuminates the filament and makes it fluoresce. At the time, the team estimated that the cosmic filament was about 10 times more massive than expected, given simulation results.

But it turns out the filament isn’t too massive, and for an interesting reason.

Christopher Martin (Caltech) and colleagues took a second look with the Palomar Cosmic Web Imager, a high-tech spectrograph they built and installed on the 5-meter (200-inch) Hale Telescope on Palomar Mountain in California. The spectrograph homed in on a particular wavelength called Lyman-alpha, which comes from cold neutral hydrogen that’s been irradiated by ultraviolet light. Shifts in the Lyman-alpha line toward redder or bluer wavelengths indicate the gas is moving along our line of sight. By analyzing the filament’s spectra, the team discovered that one part of the filament is moving toward us, while another section is moving away from us. In other words, the filament isn’t merely a filament: it’s a fuel line feeding a gigantic disk.

Big Disk in the Early Universe

The disk is huge: it’s about 400,000 light-years across, or three to four times the size of the Milky Way’s spiral disk. The rotational velocity suggests it’s sitting in a halo of 10 trillion solar masses’ worth of dark matter, 10 times larger than the halo our galaxy inhabits, the team reports August 5th in Nature. There’s even a hint of star formation in its center, but the team isn’t sure of that yet.

“Overall, it’s hard to say with certainty that they’re definitely seeing a cold-flow disk — as opposed to some other phenomenon that just happens to look like a cold-flow disk,” says Kyle Stewart (California Baptist University), whose team has simulated the growth of these objects. “But when you look at all the observable properties of cold-flow disks from the simulations to determine what they should look like in the real universe, in my opinion, it’s amazingly similar to what these authors have just observed.”

One notable parallel is the disk’s high rotation speed: about 500 km/s (1 million mph), twice that of the Milky Way’s disk. That supports the idea that the disk is forming from cold gas channeled in via the filament it's grafted onto, and not from hot gas falling in from all sides.

You can think of the difference like two ways of filling a tub with water. The dark matter well is like a colossal, circular tub. The tub is spinning slowly, because the dark matter’s angular momentum was conserved as gravity collapsed it into a big clump. If infalling gas were hot and flowing in from all sides, the gas would spin slowly with the dark matter, like water gently flowing into a big rotating bowl. But if you shoot the water into the tub from a hose, it’ll whisk around the tub’s sides much faster than the tub itself is spinning. Theorists think the cold flow shoots in with a lot more spin to it because the cold stream is concentrated, and thus more susceptible to the dark matter’s tidal forces, Martin explains.

Since it’s so massive so early in the universe, the disk likely grew into an elliptical, Martin suggests. Depending on what coalesced around it, maybe today it’s even sitting at the center of a big galaxy cluster.

 

Reference: D. C. Martin et al. “A giant protogalactic disk linked to the cosmic web.” Nature. Online August 5, 2015.

Want to know more about how galaxies grow? Pick up a copy of Sky & Telescope's September 2015 issue and read our in-depth feature.

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Let’s Get Serious About Ceres

Sky&Telescope -

With Ceres coming alive in the latest Dawn images, why not step outside and see it with your own eyes? We also look at the history of its discovery and suggest a way to honor its discoverer, Giuseppe Piazzi.

The "other" dwarf planet

598-mile-wide Ceres spends its time in the inner asteroid belt between Mars and Jupiter. It's covered in craters, lined with fissures and also hosts a cluster of enigmatic white spots that may be ice inside 57-mile-diameter Occator Crater (top).
NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

With the Dawn spacecraft now spiraling down to its third and closest-yet science orbit around Ceres, what better time to see this small, unique world with your own eyes? As amateur astronomers, it's a thrill to behold an object of such intense scrutiny from our own backyards. That's even more the case when spectacular images are available to feed our imaginations, turning a pinprick of light into a rumpled sphere overrun with icy craters and vaporous white spots.

Ceres is only 16 minutes away as light flies, or 182.3 million miles from Earth, and shines at magnitude +7.6. It was a tad closer and brighter in late July, but it also rose at a less convenient hour.

In mid-August, the dwarf planet comes into view not long after nightfall. With one caveat. From mid-northern latitudes, it's rather low in the southern sky. Still, if I can see it from northern Minnesota in binoculars, chances are, you can, too.

The Moon steps away from the evening sky this week, making for ideal viewing conditions. To observe Ceres, find a location with an open view to the south. My southern sky's far from ideal, but a gap in the tree line lets me sample celestial objects as far south as –35° declination for up to a half hour. Ceres will arc slowly westward in eastern Sagittarius near the Capricornus border the next two months fading a bit to magnitude +8.0 by late August.

Ceres Climbs Out of Bed

Looking south-southeast on August 7 from Denver around 10:30 p.m. local time, you'll find Ceres 15° high in Sagittarius. The asterism of four 4.5-4.8 magnitude stars I've dubbed the "Cross" will point you in its direction. Start with this map and then use the more detailed version below.
Source: Stellarium

The maps will help you get there. Ceres looks exactly like a star in 8x40 or similar binoculars. As you watch evening to evening, you'll see the dwarf planet amble west in retrograde motion, much like the other outer planets do around the time of their opposition.

Looks like we've got luck on our side the next few days as Ceres pairs up with a slightly fainter 7.7 magnitude star. The chance encounter provides an opportunity to easily spot Ceres' westward journey — watch them separate in the coming nights.

Archer to Cross to Ceres

Detailed map showing Ceres' path in eastern Sagittarius August 5 - September 9, 2015 around 10 p.m. CDT. Stars are shown to magnitude 8 with a few relevant ones labeled with magnitudes. Click to download a large version. First locate the Handle of the Teapot (far right), then sweep to the east in your binoculars to "The Cross". It points to a 6.2 magnitude star. This is your "jumping off" point to locate the asteroid. 
Source: Chris Marriott's SkyMap

The Discovery of Ceres

Giuseppe Piazzi discovered Ceres quite by accident on the first evening of the 19th century (January 1, 1801) while making stellar position measurement from his observatory in Palermo, Italy. Unbeknownst to him at the time, a group of 24 astronomers headed up by Baron Franz Xaver von Zach, a German Hungarian astronomer, were preparing to make a systematic search for a hypothetical "missing planet" predicted by the Titius-Bode law between Mars and Jupiter. Fittingly, they named their group the“Himmels Polizei” (Celestial Police).

"Something Better Than a Comet"

Italian Catholic priest and astronomer Giuseppe Piazzi around 1807 with his new "star", the dwarf planet Ceres.
Public Domain

Despite Piazzi's respected reputation, he had not been invited to become a member. No matter. That night around 8 p.m. he spotted a tiny new "star" in the "shoulder of Taurus" (below the Pleiades near the border with Aries) and announced it to the press the same day as a comet.

Piazzi kept track of the object through the the 23rd, observing it on a total of 13 nights. The more he looked, the more convinced he became it might be something more than a comet. Here's an excerpt from a letter Piazzi sent to astronomer Barnaba Oriani in 1801, sharing his observations:

"I have announced this star as a comet, but since it shows no nebulosity, and moreover, since it had a slow and rather uniform motion, I surmise that it could be something better than a comet."

Piazzi's observation echoes that of William Herschel just 20 years earlier when Herschel chanced across what he thought at first was a new comet, but turned out to be Uranus, the first new planet discovered since antiquity.

By the time Piazzi's data reached other astronomers in the spring of 1801, Ceres was near conjunction and too close to the Sun to observe. Astronomers worked to create a set of ephemerides they could use to track it down when it reappeared in the morning sky that summer. Unfortunately, Piazzi had observed it along too short of an arc (just 3°) and computing an accurate orbit wasn't possible.

By August, astronomers were getting desperate with some starting to wonder if the new planet even existed. Enter 24-year-old mathematician Carl Friedrich Gauss. He took it the orbit problem to heart and wrestled an accurate solution in a little more than a month. Using Gauss's new ephemerides, von Zach finally spotted Ceres on December 7, 1802 . . . and the rest is history. For more fascinating reading on the discovery of Ceres, click HERE.

Topographic map of Ceres

Craters are the dominant landform on Ceres, and as of July 2015 about a dozen had received official names. Color coding indicates altitude, from a low (dark blue) about 7½ km (5 miles) above the surface mean to highs (red) about 7½ km above it.
NASA / JPL / UCLA / MPS / DLR / IDA

Given Piazzi's wonderful discovery, you might expect to see his name on a large crater or other prominent feature on the dwarf planet. Yet if you peruse a recent map with formally approved names, he's nowhere to be found!

Maybe the IAU Working Group for Planetary System Nomenclature, responsible for the naming of features on solar system bodies, has a special spot in mind, but I wonder. The group has approved two themes for use on Ceres — gods and goddesses of agriculture and vegetation from world mythology” for craters and names of agricultural festivals for other features.

Ceres on the Move

1 Ceres' track across Sagittarius and Capricornus August 5 - December 3, 2015. Click to download a large version.
Source: Chris Marriott's SkyMap

Hmmm. That doesn't seem to leave room for Giuseppe unless the group allows exceptions similar to to Venus' Maxwell Montes (after Scottish physicist James Clerk Maxwell), the only male presence among a myriad of mythological goddess. It, along with Alpha and Beta Regio, were first seen in Arecibo radar images in the 1960s before the naming convention was adopted.

Sure, you'll find a Piazzi crater on the Moon and even asteroid 1000 Piazzia, but there's also a Tombaugh Crater on Mars and the asteroid 1604 Tombaugh, named for Pluto's discoverer. His name now graces (pending approval) the icy plains of Pluto's Tombaugh Regio.

it only seems right to honor Ceres' discover with a prominent feature on the dwarf planet itself. How about those bright, white spots or the 3-mile-high pyramid mountain? I think you'd agree that Piazzi's magnificent orb not only deserves but a look with your own eyes but his name, too.

Read all about Dawn's arrival at Ceres and what it will do there in the April 2015 issue of Sky & Telescope.

The post Let’s Get Serious About Ceres appeared first on Sky & Telescope.

Shedding New Light on Near-Earth Asteroids

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Students captured some amazing videos of near-Earth asteroids this past month, demonstrating a powerful tool for learning about some of our nearest celestial neighbors.

Astronomers have been bouncing radar beams off asteroids since the close passage of 1566 Icarus in 1968. Now, students participating in a specialty summer school carried out a detailed series of observations of asteroid 2015 HM10 using the Green Bank Telescope (GBT), a single 100-meter radio dish, and NASA’s Deep Space Network radar transmitter at Goldstone, California.

The student team had a close target: the asteroid passed just 440,000 kilometers (1.1 times the Earth-Moon distance) from our planet on July 7th. But the near pass posed a challenge too, since the asteroid moved quickly across the sky.

Capturing an Asteroid with Radar NRAO Summer School Students

Student Participants in the NRAO Single Dish Summer School Program.
K. O’Neil / NRAO / AUI / NSF

The students were participating in a specialty program that gives astronomy graduate students and postdocs an opportunity to gain practical experience in single-dish radio astronomy, and in particular a special technique called bistatic radar.

This technique helps paint a picture of a targeted asteroid in unprecedented detail. Usually, radio astronomers use a single radio dish to transmit and receive radar. But the reflected radar signal is typically much weaker than the transmitted signal.

“In bistatic observations, we use one telescope to transmit and another to receive,” says Marina Brozovic (NASA / JPL). “It is to our advantage to receive with a larger telescope, because a larger surface area means stronger signal.” In the case of the asteroid observations, the gigantic GBT served as the receiver for the radar transmitted from Goldstone. “We can double the signal strength if we receive at GBT,” Brozovic adds. “As such, GBT has been a great asset in enhancing radar observations.”

The bistatic technique is especially useful for exceptionally close asteroid passages (those that pass within twice the distance to the Moon), because it becomes difficult to switch a single antenna between the send and receive mode due to a short light travel time.

At 80-meters across on its longest axis, the asteroid 2015 HM10 is slightly larger than a Boeing 777-300ER airliner from nose-tip to tail. It orbits the Sun once every 4.06 years, and the July 7th passage was the asteroid’s closest pass to Earth this century. The team’s project captured 42 radar images with a resolution of about 3.75 meters per pixel, showing the asteroid spinning every 22 minutes.

2015 HM10 on closest approach

Video animation of 2015 HM10 during closest approach.
NASA / JPL-Caltech / NRAO / AUI / NSF

Meanwhile, NASA scientists applied the same bistatic radar technique in order to image another asteroid, 2011 UW158, on July 19th as it passed 2.4 million kilometers from Earth. Images with a resolution of 7.5 meters per pixel revealed parallel ridges on the elongated asteroid.

Watching a Tumbling Space Peanut

Just last week, astronomers released new images of asteroid 1999 JD6 using the same technique. The rock passed 4.5 million miles from Earth on July 24th (19 times the distance between the Earth and Moon), and the images clearly show a two-lobed contact binary asteroid spinning around every 7.5 hours. About 15% of the near-Earth asteroids that scientists have resolved display a dumbbell shape , and the Rosetta mission discovered the same double-lobed structure on Comet 67P/Churyumov-Gerasimenko.

Tumbling Space Peanut

A sequence of images of ‘peanut asteroid’ 1999 JD6 on closest approach.
NASA / JPL-Caltech / GSSR

In addition to asteroids’ shape, radar observations reveal their spin, surface features, and sometimes they even reveal surprises, such as the tiny moon found orbiting the asteroid 1998 QE2 during its passage 5.8 million km from Earth on May 29, 2013. The more recent radar images of asteroid 2004 BL86 on January 27, 2015, also revealed a previously detected moon-like companion.

2004 BL86 asteroid and moon

This video, captured earlier this year, shows asteroid 2004 BL86 and its previously detected moon.
NASA / JPL-Caltech / NRAO / AUI / NSF

And there’s much more in store for characterizing asteroids in this way. “We have a busy observing year ahead of us, and [NASA] will again be joining forces with our colleagues at Green Bank Telescope on at least several asteroids,” Brozovic says.

The next one on the list? NEA (413577) 2005 UL5, a 300-meter asteroid that will approach Earth within 6 lunar distances in late November — this asteroid, Brozovic adds, will be one of the best targets this year.

Expect to see more amazing asteroid images in the near future.

What role did asteroids play in previous extinctions on Earth? Could we divert a threatening asteroid? Could we ever mine asteroids? These are among Astronomy's 60 Greatest Mysteries.

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Closest Rocky Exoplanet Discovered

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Super-Earth HD 219134b is just 21 light-years away, orbiting a nearby orange star that you can see from your backyard.

HD 219134 in Cassiopeia

See HD 219134 for yourself - it's an orange, 5th-magnitude star in Cassiopeia. Cassiopeia is upside down in this image, with her feet pointing to the top left corner. (See more exoplanet-hosting stars.)
NASA / JPL-Caltech / DSS

If Cassiopeia in her celestial seat were to raise her head from her looking glass, she would see HD 219134, a 5th-magnitude, orange, -type star 21 light-years from Earth. And if she peered closely, Cassiopeia might make out the system of four planets orbiting this nearby star.

Of course, Cassiopeia is mythical and doesn’t see a thing, but Fatemeh Motalebi (University of Geneva, Switzerland) and colleagues do. Using  the ESO 3.6-meter telescope in the Canary Islands and the Spitzer Space Telescope, the astronomers discovered the signature of a transiting rocky planet. The planet itself is never seen, but its crossing briefly dims the star by a minuscule amount that was first detected as part of the HARPS-N survey, then confirmed in Spitzer follow-up observations.

Two additional super-Earths and a giant planet haven’t transited (yet), but Spitzer revealed their presence by the gravitational tugs they exert on their parent star.

Jackpot: A Nearby Rocky Super-Earth HD219134 Light Curve

HD 219134b transits its star every 3 days, briefly dimming its star's brightness very slightly. First spotted as part of the HARPS-N survey, Spitzer observations confirmed the transit and pinned down the planet's mass and radius.
NASA / JPL-Caltech

Here are the vital stats: The innermost planet, “b,” is a super-Earth between 4 and 5 times the mass of Earth and about 1.6 times the size, which whips around (and in front of) its star every 3 days.

Those numbers put the super-Earth’s density somewhere between 4.7 and 7.0 g/cm3. For reference, Earth’s density is 5.5 g/cm3 (that’s the highest density for any planet in the solar system), and models suggest that rocky super-Earths would literally pack on the pounds, becoming denser as they grow larger. So there’s little doubt this planet is rocky, and like Earth, its atmosphere is probably skin-thin in comparison to its bulk.

But that doesn’t mean astronomers can’t study the planet’s atmosphere. The authors are optimistic about attempting to obtain high-resolution spectra — as the planet transits, a sliver of starlight would shine through the planet’s atmosphere and might reveal the presence of molecules such as water. Whether or not the team detected anything, the attempt would guide future observations with James Webb Space Telescope or ground-based megatelescopes.

Companion Worlds

Two of the rocky planet’s companions might also be super-Earths. At minimum, they have 2.7 and 8.7 times Earth’s mass — “at minimum” because these planets haven’t transited (yet), and calculating mass via an object’s gravitational tug is a matter of perspective. If these planets also transit, then those minimum masses are pretty close to their real masses. But if they turn out to have inclined orbits, their masses could be much larger. They orbit every 6.8 and 46.8 days, respectively, so future missions such as CHEOPS (scheduled for a December 2017 launch) will be on the lookout for potential transits.

Farther out, at twice the Earth-Sun distance, a giant planet orbits the same star every three years.

The Kepler mission made clear that super-Earths are surprisingly common in the universe, if not in our own solar system. So even if it’s not habitable, HD 219134b will make an excellent test case for understanding these unique worlds.

Reference:
Fatemeh Motalebi et al. "The HARPS-N Rocky Planet Search I. HD219134b: A transiting rocky planet in a multi-planet system at 6.5 pc from the Sun." Accepted for publication in Astronomy & Astrophysics.

Astronomers are only beginning to learn about the weird weather on alien worlds. Read up on what we know so far in the May 2014 issue of Sky & Telescope.

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Deep Sky with your DSLR

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Getting started in astrophotography has never been easier.
By Michael A. Covington in the Sky & Telescope June 2012 issue.

Pipe Nebula

Astrophotography with digital single-lens reflex (DSLR) cameras spans all facets of amateur astrophotography. Today’s camera models have much lower noise than in the past and more features useful to amateurs. In the accompanying text, Michael A. Covington explains how you can capture spectacular images like the one above of the Pipe Nebula in Sagittarius.
Michael A. Covington

There's no question that digital single-lens reflex cameras (DSLRs) are the most versatile cameras available today. No other device can go from shooting your children’s birthday party in the backyard to recording distant galaxies through a telescope without needing any modifications. DSLRs have truly thrust open the door of astro-imaging to anyone with an interest in shooting the night sky.

Several factors make DSLRs good for astronomy. Most of the cameras are designed to use the same lenses as their 35-mm film precursors, and they have relatively large sensors compared to their point-and-shoot counterparts. The common APS-C size CMOS sensors in many consumer DSLRs are about 65% as big as 35-mm film, and about as large as many mid-range astronomical CCD cameras.

Unlike film, the CMOS sensor in a DSLR has no reciprocity failure; it never forgets a photon. (Well, hardly ever.) A 2-minute exposure is long enough to capture a respectable image of the Orion Nebula. In 10 minutes with a modest telephoto lens, you can record 16th-magnitude stars. And DSLRs don’t require you to bring along a computer when you’re shooting the night sky.

There are many DSLR cameras available for purchase, but Canon manufactures the most popular ones for astrophotography. It is the only DSLR maker that has actively cultivated the astronomy market, at one time even marketing a DSLR specifically for astrophotographers (the 20Da). This model has long since been discontinued, but Canon has incorporated many of its best features useful for astrophotography into current models.

Other DSLRs from Nikon and Pentax are similar, differing mainly in the way functions are accessed and image files are formatted. Virtually all current DSLRs block the astronomically important far-red end of the visible spectrum where hydrogen gas fluoresces. To increase their camera’s red sensitivity, many astrophotographers have had their cameras modified. They remove the camera’s infrared-blocking filter and replace it with a filter that transmits more of the red light from hydrogen emission. Those daring individuals willing to tinker with their cameras can purchase a modified infrared-blocking filter and do the work themselves, or you can send your camera to Hap Griffin (www.hapg.org) or Hutech (www.hutech.com) to have the filter replaced for you. Once the camera is modified, you’ll have to set a custom color balance to shoot pleasing daytime images.

Orion's Belt

Most DSLR cameras have filters that block the far-red end of the spectrum to produce natural-color daylight images. Left: Unfortunately, these stock filters block much of the reddish nebulosity in the Milky Way, as seen in this image of Orion’s Belt. Right: When the stock filter is replaced with a custom filter, the resulting image of Orion’s Belt reveals much more nebulosity.
Alan Dyer (2)

If you’re shopping for a new DSLR with astrophotography in mind, one particular feature worth seeking out is known as “Live View.” This feature allows you to turn on the sensor and view a live video on the camera’s rear LCD screen. This makes focusing your lens or telescope a breeze compared to other methods. If you don’t have Live View, you’ll need some form of focusing aid, or you can confirm focus by taking short 5-second exposures and immediately viewing them on the rear screen.

Shooting Stars

Once you’ve picked your camera, there are a few additional accessories you’ll need to start shooting the night sky. The first is a device that will let you shoot long exposures without touching the camera. You can make single exposures up to 30 seconds by pressing the shutter release on the camera, and setting a delay so the vibration from pressing the button will have died away before the shutter opens. For longer exposures, you can use a special cable release with a built-in intervalometer. It allows you to program a series of long exposures and eliminates the need for a delay between images. Versatile, inexpensive cable release intervalometers are made by Phottix (www.phottix.com) and other accessory makers, and are easy to find on Amazon.com or ebay.com. Make sure you select the proper model for your particular camera.

You’ll also want a tripod for your first foray into DSLR astrophotography. Even if your primary goal is to shoot close ups of deep-sky objects through your telescope, shooting simple camera-on-tripod shots will help familiarize you with the functions of your camera that you’ll use for all types of deep-sky astrophotography. The tripod also comes in handy for shooting conjunctions, wide-field photos of the Milky Way, and meteor showers — popular targets for all astrophotographers.

DSLR accessories

Because DSLRs look and feel a lot like 35-mm film cameras, the way you attach one to your telescope is similar. To start shooting the sky with a DSLR, you’ll need a few accessories, such as an intervalometer (far left) that can automatically shoot multiple long exposures, and a T-ring adapter (bottom right center) for your particular model.
S&T / Sean Walker

Under a starry, moonless sky, put your camera on your tripod. Use a wide-angle lens at its widest f/stop (lowest f/number) and focus manually on a bright star using live focus, if the feature is available with your camera. Zoom in on the live-focus view to help achieve the sharpest focus. Set the ISO speed to 1600 and expose for 30 seconds. You’ll get a picture that shows plenty of stars and possibly some of the brighter deep-sky objects.

A few nights of practice will familiarize you with your camera’s features that are beneficial for astrophotography, such as mirror lock-up, noise reduction, and programming sequences of exposures on your intervalometer.

If you long for deeper exposures with round stars, you can “piggyback” your camera on top of your telescope, photographing the sky through your camera lens while using the telescope to track. With this method, you’ll find that the standard 18-55-mm zoom lens that comes bundled with many DSLRs isn’t very good for astronomy; it’s slow (usually no faster than f/4.5) and less sharp than many fixed-focal-length lenses. Also, being a zoom, it may shift focal length or focus as the telescope tilts to track the sky.

Fixed-focal-length lenses are better suited for astrophotography. You can of course buy superb telephoto lenses from Canon, Sigma, and other makers. Here’s a useful tip: adapters are available to convert old manual-focus Olympus, Nikon, Pentax, Contax/Yashica, and screwmount lenses to work on your Canon EOS or Nikon DSLRs. Because autofocus doesn’t work for deep-sky astrophotography, you can use old manual-focus lenses that are much less expensive than the newest lenses on the market. These adapters are available from Fotodiox (www.fotodiox.com).

If you want to try your hand at shooting objects through your telescope, you’ll need an adapter. This usually consists of a T-ring and an adapter that couples your camera to your telescope in place of an eyepiece. With this setup, you can immediately take photographs of the Moon using your telescope as a camera lens. To take pictures of deep-sky objects, you can experiment and make exposures of 5 seconds or more to test how long your telescope mount will track before stars appear as streaks. Even most high-end telescope mounts require an autoguider or other special measures to compensate for errors in the mount’s gears, wind buffeting, or other variations in tracking.

Image Processing Stars and Omega Centauri

At high ISO settings, DSLRs are far more sensitive than the best films of the past. This 5-second exposure with a Canon 40D at ISO 1600 and a 50-mm f/2.8 lens captures 6th magnitude stars and the bright globular cluster Omega Centuri at lower right.
Michael A. Covington

No matter what kind of astrophotography you’re doing, the image that comes out of the camera isn’t usually the finished product. A short, fixed-tripod exposure often appears too dark, while a 2- or 3-minute guided exposure is likely to look too bright because of light pollution. That’s normal. Any automatic white balance in your camera is not usually applicable to deep-sky astrophotos because the subject is too faint for the camera’s computer to make an accurate judgment. You can adjust all of these settings with image-processing software. Although some programs that come with digital cameras offer rudimentary adjustment abilities, I highly recommend acquiring a software program specifically geared toward DSLR astrophotography. These programs are necessary to get the most out of your images.

Some popular programs available for DSLR astrophotography include MaxIm DL (www.cyanogen.com), ImagesPlus (www.mlunsold.com), Nebulosity (www.starklabs.com), and DeepSkyStacker (http://deepskystacker.free.fr). Most are available as a trial before purchase, and DeepSkyStacker is freeware.

Once you’ve chosen your processing software, two steps will immediately make your images better. The first is dark-frame calibration. When you take an exposure longer than a few seconds with most DSLRs, you’ll see a random scatter of colored pixels — red, green, and blue specks — indicating places where the sensor has what are known as “hot pixels.”

Processing DSLR images

Every deep photograph recorded with a DSLR requires some adjustment. The author shot this picture of Comet Holmes from his urban backyard on November 15, 2007. Note the reddish background sky (left), which was easily corrected using image-processing software (right). This piggybacked 3-minute exposure was taken with a Canon 40D at ISO 400 using a 300-mm f/5 lens.
Michael A. Covington

The best way to get rid of hot pixels is to subtract a dark frame, a picture taken with no light reaching the sensor, but in all other respects just like the original, with the same exposure time, ISO setting, and sensor temperature. The dark frame will have the same hot pixels, so subtracting it from your picture will remove them. Most DSLRs can do this for you automatically if you turn on your camera’s long-exposure noise-reduction function. Then, after you take a long exposure, the camera will immediately take another one just like it with the shutter closed, perform the subtraction automatically, and record the resulting image. Although this feature is handy and guarantees that the dark frame matches all the settings of the original exposure, it takes up precious time that you could be using to record images.

The alternative is to take one or more dark frames manually with the lens cap on and subtract them later with software. Preferably, take several — at least a half dozen — so that software can average them to eliminate random noise. One set of dark frames can serve for pictures of several celestial objects taken on the same evening with the same exposure time and ISO setting.

Messier objects in Sagittarius

Stacking multiple exposures helps to reveal faint nebulosity. Combining eight 4-minute exposures shows the Messier objects (from top to bottom) M21, M20, and M8 in Sagittarius.
Michael. A Covington

Besides dark-frame calibration, the second technique that will make your images smoother is to shoot multiple exposures of your target and combine them. This technique, known as stacking, has many advantages. You can get an hour’s exposure without needing an hour of perfect guiding. If you have guiding problems, you can take many exposures and simply toss out the poorly tracked ones. You can also avoid reaching the sky fog limit because no single exposure is excessively long. Stacking algorithms in image-processing programs can automatically discard airplane trails, random hot pixels, and other large discrepancies between the images you are combining. And when you stack multiple exposures, the noise level in the stacked image is reduced proportional to the square root of the number of exposures you combine.

There are limits, of course. You can’t stack 3.6 million 1/1000-second exposures and get the equivalent of an hour-long exposure. The individual exposures have to be long enough for a useful image. That is typically 5 to 10 minutes, unless tracking limitations compel you to go shorter.

Stacking and dark-frame subtraction are also best done with raw files, not JPEGs, because JPEGs have been stretched nonlinearly for display, and data is thrown away in the compression process that makes JPEG files small compared to raw format. After dark-frame calibration and stacking, you can then adjust the brightness, contrast, and color balance of your image and perhaps sharpen it if necessary before saving the final version.

As you improve at recording data, you can learn many additional tricks and techniques to squeeze even more out of your images. But the tips in this article will put you well on your way to taking great astrophotos while avoiding many pitfalls.

Astrophotography.inddMichael Covington is an avid astrophotographer and author
of Digital SLR Astrophotography, which is available at
www.covingtoninnovations.com/dslr.

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