Posted on by Fraser Cain

Where is the Sun?

Map of the Milky Way. Image credit: Caltech

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I’m sure you know that we live in the Milky Way galaxy, but where is the Sun located? And how did astronomers figure out where the Sun is located, since we’re living inside the galaxy?

The Milky Way is a grand spiral galaxy, which astronomers think has four major spiral arms: Perseus, Cygnus, Scutum-Crux, Sagittarius. Some astronomers think we might actually just have two arms, Perseus and Sagittarius. The Sun is located in the inner rim of the Orion Arm, which is thought to be an offshoot of the Sagittarius Arm. The Sun is located about 26,000 light-years away from the center of the galaxy.

Before telescopes, the Milky Way just looked like a bright area in the sky, but when Galileo first turned his telescope on the region in 1610, he realized that it was actually made up of faint stars. The astronomer Immanuel Kant correctly guessed that this might be a cloud of stars held together by gravity, like the Solar System.

The famous astronomer William Herschel attempted to map out the stars in the Milky Way to get a sense of the galaxy’s size and shape, and determine the Sun’s position in it. From Herschel’s first map, it appeared the Sun was at the center of the Milky Way. It was only later on that astronomers realized that gas and dust was obscuring our view to distant parts of the galaxy, and that we were actually in the outer region of the Milky Way.

The astronomer Harlow Shapley accurately determined where the Sun is in the MIlky Way in the early 20th century by noticing that globular clusters were uniformly located above and below the Milky Way, but they were concentrated in the sky towards the constellation Sagittarius. Shapely realized that many globular clusters must be blocked by the galactic core. He created one of the most accurate maps of the Milky Way.

It wasn’t until the 20th century, with the development of larger and more powerful telescopes that astronomers could see the shape of other spiral galaxies, located millions of light-years away. In 1936, Edwin Hubble used cepheid variables as yardsticks to measure the distances to many galaxies, and prove conclusively that the Universe was filled with galaxies, each with as many stars as our own Milky Way.

Here’s an article from Universe Today about how the Milky Way might actually just have two spiral arms, and the largest picture ever taken of the Milky Way.

Here’s an article about the Great Debate that Harlow Shapley had about the nature of the Milky Way. And here’s Shapley’s obituary, published in Nature in 1972.

We have recorded an episode of Astronomy Cast just about the Sun called The Sun, Spots and All.

Reference:
NASA’s Imagine the Universe!

Posted on by Nancy Atkinson

Hubble NICMOS Instrument Experiences Anomaly

NICMOS

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A cooling system for the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) science instrument aboard the Hubble telescope experienced an anomaly during a restart, causing the instrument to go into safe mode. After a couple of additional restarts, the problem still persists, and a decision was made for NICMOS to “stand down” while engineers study the anomaly and allow the cooling system to warm up, which may take a couple of weeks. In the short term, this will affect planned science observations, and engineers are hoping to avoid any long term complications. At this point, if the problem cannot be fixed from the ground, it is unclear how it might affect the upcoming servicing mission, scheduled for an Oct. 10 launch.

New software was uploaded last week to the computer that controls Hubble’s five science instruments to get the telescope ready for the upcoming servicing mission (SM4). Installation of the software requires putting all of the telescope’s science instruments into safe mode configuration for a short period of time.

About six hours after the system was reactivated, at about 4 a.m. EDT on Sept. 11, the NICMOS anomaly was seen. The cooling system put itself into safe mode after seeing too high a speed in the circulator pump operation. After studying data, flight controllers modified operating protection parameters and attempted a restart of the system on Sunday, Sept. 14. The circulator system again indicated a high speed violation so the system was returned to safe mode.

Engineers believe the ice particles in the cooling loop could be causing the problem. With some small adjustments in start-up procedures, engineers think the cooling system can be successfully reactivated. The flight team tried another restart Monday evening (9/15). The anomaly was still seen after that restart, so the Hubble Project’s plan now is to stand down from any additional attempts to restart. Engineers will study the anomaly while waiting until the cooling system has been allowed to warm somewhat, which may take several weeks.

The impact to planned NICMOS science operations involves approximately 70 exposures from three guest observer programs and additional exposures from two NICMOS internal calibration programs. Additionally, all NICMOS science has been removed from this week’s observation schedule. Sixty-one orbits of NICMOS science were scheduled for the week between September 15 and September 21.

The servicing mission already has a jam-packed schedule, and its uncertain if any last minute additions to the mission would be possible.

Source: NASA

Posted on by Astronomy Cast

Podcast: The Search for the Theory of Everything

Einstein and Relativity
Albert Einstein

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At the earliest moments of the Universe, there were no separate forces, energy or matter. It was all just the same stuff. And then the different forces froze out, differentiating into electromagnetism, the strong force and the weak force. Today we’ll look at the problem that has puzzled physicists for generations: is there a single equation that explains all the forces we see in the Universe. Is there a theory of everything?

Click here to download the episode.

Or subscribe to: astronomycast.com/podcast.xml with your podcatching software.

The Search for the Theory of Everything show notes.

Posted on by Nancy Atkinson

Do All Galaxies Have Tentacles?

This Hubble Space Telescope image of two spiral galaxies shows an interesting feature on the smaller galaxy. Silhouetted in front of the larger background galaxy is a small galaxy, and tentacles of dust can be seen extending beyond the small galaxy’s disk of starlight. These dark, dusty structures appear to be devoid of stars, almost like barren branches. They are rarely so visible in a galaxy because there is usually nothing behind them but darkness. But here, with the backdrop of the larger galaxy they are illuminated. Astronomers have never seen dust this far beyond the visible edge of a galaxy, and they don’t know if these dusty structures are common features in galaxies.

The background galaxy is 780 million light-years away, but the distance between the two galaxies has not yet been calculated. Astronomers think the two are relatively close, but not close enough to actually interact. The background galaxy is about the size of the Milky Way Galaxy and is about 10 times larger than the foreground galaxy. Understanding a galaxy’s color and how dust affects and dims that color are crucial to measuring a galaxy’s true brightness. By knowing the true brightness, astronomers can calculate the galaxy’s distance from Earth.

Most of the stars speckled across this image belong to the nearby spiral galaxy NGC 253, which is out of view to the right. Astronomers used Hubble’s Advanced Camera for Surveys to snap images of NGC 253 when they spied the two galaxies in the background. From ground-based telescopes, the two galaxies look like a single blob. But the Advanced Camera’s sharp “eye” distinguished the blob as two galaxies, cataloged as 2MASX J00482185-2507365. The images were taken on Sept. 19, 2006.

Source: Hubblesite

Posted on by Nancy Atkinson

Our Sun May Have Migrated Over Time

Computer simulation showing the development and evolution of the disk of a galaxy such as the Milky Way. Credit: Rok Roškar

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When you stir cream in your coffee or tea, does the swirl stay the same or does it change as it spins in your cup? As galaxies form and swirl, the motions and eddies may actually cause stars to move within the galaxy. A long-standing scientific belief holds that stars tend to hang out in the same general part of a galaxy where they originally formed. But some astrophysicists have recently questioned whether that is true, and now new simulations show that, at least in galaxies similar to our own Milky Way, stars such as the sun can migrate great distances. If this is true, it could change the entire notion that there are parts of galaxies – so-called habitable zones – that are more conducive to supporting life than other areas.

“Our view of the extent of the habitable zone is based in part on the idea that certain chemical elements necessary for life are available in some parts of a galaxy’s disk but not others,” said Rok RoÅ¡kar, a doctoral student in astronomy at the University of Washington. “If stars migrate, then that zone can’t be a stationary place.”

RoÅ¡kar is lead author of a paper describing the findings from the simulations, published in the Sept. 10 edition of the Astrophysical Journal Letters. If the idea of habitable zone doesn’t hold up, it would change scientists’ understanding of just where, and how, life could evolve in a galaxy, he said.

Using more than 100,000 hours of computer time on a UW computer cluster and a supercomputer at the University of Texas, the scientists ran simulations of the formation and evolution of a galaxy disk from material that had swirled together 4 billion years after the big bang. Watch a simulation video.

The simulations begin with conditions about 9 billion years ago, after material for the disk of our galaxy had largely come together but the actual disk formation had not yet started. The scientists set basic parameters to mimic the development of the Milky Way to that point, but then let the simulated galaxy evolve on its own.

If a star, during its orbit around the center of the galaxy, is intercepted by a spiral arm of the galaxy, scientists previously assumed the star’s orbit would become more erratic in the same way that a car’s wheel might become wobbly after it hits a pothole.

However, in the new simulations the orbits of some stars might get larger or smaller but still remain very circular after hitting the massive spiral wave. Our sun has a nearly circular orbit, so the findings mean that when it formed 4.59 billion years ago (about 50 million years before the Earth), it could have been either nearer to or farther from the center of the galaxy, rather than halfway toward the outer edge where it is now.

Migrating stars also help explain a long-standing problem in the chemical mix of stars in the neighborhood of our solar system, which has long been known to be more mixed and diluted than would be expected if stars spent their entire lives where they were born. By bringing in stars from very different starting locations, the sun’s neighborhood has become a more diverse and interesting place, the researchers said.

The findings are based on a few runs of the simulations, but the scientists plan to run a range of simulations with varying physical properties to generate different kinds of galactic disks, and then determine whether stars show similar ability to migrate large distances within different types of disk galaxies.

Source: University of Washington

Posted on by Nancy Atkinson

Dark Matter Halos? How About Disks, Too

A composite image shows a dark matter disk in red. From images in the Two Micron All Sky Survey. Credit: Credit: J. Read & O. Agertz.

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Scientists are trying to understand the invisible and hypothetical ‘dark matter’ – the stuff that we know exists by inference of its gravitational influence on the matter we can see. The most common held notion of dark matter is that it exists in ‘halos’ or clumps that surround galaxies. But a new study predicts that galaxies like our own Milky Way, also contain a disk of dark matter. Using the results of a supercomputer simulation, scientists from the University of Zurich and the University of Central Lancashire say that if dark matter in fact resides as a disk within a galaxy, it could allow physicists to directly detect and identify the nature of dark matter for the first time.

Physicists believe dark matter makes up 22% of the mass of the Universe (compared with the 4% of normal matter and 74% comprising the mysterious ‘dark energy’). But, despite its pervasive influence, no-one is sure what dark matter consists of.

This ‘standard’ theory of dark matter is based on supercomputer simulations that model the gravitational influence of the dark matter alone. The new work includes the gravitational influence of the stars and gas that also make up our Galaxy.

Stars and gas are thought to have settled into disks very early on in the life of the Universe and this affected how smaller dark matter halos formed. The team’s results suggest that most lumps of dark matter in our locality merged to form a halo around the Milky Way. But the largest lumps were preferentially dragged towards the galactic disk and were then torn apart, creating a disk of dark matter within our Galaxy.

“The dark disk only has about half of the density of the dark matter halo, which is why no one has spotted it before,” said lead author Justin Read. “However, despite its low density, if the disk exists it has dramatic implications for the detection of dark matter here on Earth.”

The Earth and Sun move at some 220 kilometres per second along a nearly circular orbit about the center of our Galaxy. Since the dark matter halo does not rotate, from an Earth-based perspective it feels as if we have a ‘wind’ of dark matter flowing towards us at great speed. By contrast, the ‘wind’ from the dark disk is much slower than from the halo because the disk co-rotates with the Earth.

“It’s like sitting in your car on the highway moving at a hundred kilometres an hour”, said team member Dr. Victor Debattista. “It feels like all of the other cars are stationary because they are moving at the same speed.”

This abundance of low-speed dark matter particles, the science team says, could be a real boon for researchers because they are more likely to excite a response in dark matter detectors than fast-moving particles. “Current detectors cannot distinguish these slow moving particles from other background ‘noise’,” said Prof. Laura Baudis, a collaborator at the University of Zurich and one of the lead investigators for the XENON direct detection experiment, which is located at the Gran Sasso Underground Laboratory in Italy. “But the XENON100 detector that we are turning on right now is much more sensitive. For many popular dark matter particle candidates, it will be able to see something if it’s there.”

If so, its possible that the dark disk could be directly detected in the very near future.

Sources: Monthly Notices paper, Royal Astronomical Society

Posted on by Nancy Atkinson

Newest Mission to Mars: MAVEN

Why do planets like Mars have a different atmosphere than Earth? Credit: NASA

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Did Mars once have a thick atmosphere? Could the climate on the Red Planet have supported water and possibly life in the past? These are the questions NASA hopes to answer in great detail with the newest orbiter mission to Mars. Called the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft, the $485 million mission is scheduled for launch in late 2013. MAVEN is part of the Mars Scout Program, which is designed to send a series of small, low-cost, principal investigator-led missions to the Red Planet. The Phoenix Mars Lander was the first spacecraft selected in this program. “This mission will provide the first direct measurements ever taken to address key scientific questions about Mars’ evolution,” said Doug McCuistion, director of the Mars Exploration Program at NASA Headquarters in Washington.

Evidence from orbit and the planet’s surface points to a once denser atmosphere on Mars that supported the presence of liquid water on the surface. As part of a dramatic climate change, most of the Martian atmosphere was lost. MAVEN will make definitive scientific measurements of present-day atmospheric loss that will offer clues about the planet’s history.

“The loss of Mars’ atmosphere has been an ongoing mystery,” McCuistion said. “MAVEN will help us solve it.”

The science team will be led from the University of Colorado at Boulder, and its Laboratory for Atmospheric and Space Physics. The principal investigator for the mission is Bruce Jakosky from UC Boulder. “We are absolutely thrilled about this announcement,” said Jakosky. “We have an outstanding mission that will obtain fundamental science results for Mars. We have a great team and we are ready to go.”

Artist depiction of the MAVEN spacecraft. Credit: NASA
Artist depiction of the MAVEN spacecraft. Credit: NASA

Lockheed Martin of Littleton, Colo., will build the spacecraft based on designs from NASA’s Mars Reconnaissance Orbiter and 2001 Mars Odyssey missions.
MAVEN was evaluated to have the best science value and lowest implementation risk from 20 mission investigation proposals submitted in response to a NASA Announcement of Opportunity in August 2006.

After arriving at Mars in the fall of 2014, MAVEN will use its propulsion system to enter an elliptical orbit ranging 90 to 3,870 miles above the planet. The spacecraft’s eight science instruments will take measurements during a full Earth year, which is roughly equivalent to half of a Martian year.
MAVEN’s instrument suites include a remote sensing package that will determine global characteristics of the upper atmosphere, and the spacecraft will dip to an altitude of 80 miles above the planet. A particles and fields payload contains six instruments that will characterize the solar wind, upper atmosphere and the ionosphere – a layer of charged particles very high in the Martian atmosphere.

The third instrument suite, a Neutral Gas and Ion Mass Spectrometer will measure the composition and isotopes of neutral and charged forms of gases in the Martian atmosphere

During and after its primary science mission, the spacecraft may be used to provide communications relay support for robotic missions on the Martian surface.

More information on MAVEN.

Sources: NASA, UC Boulder

Posted on by Tammy Plotner

Just Another Harvest Moon…

September 26, 2007 APOD - Saguaro Moon by Stefan Seip

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Actually, the official time was September 15 at 5:13 a.m. EDT, but missing the exact time isn’t going to stop a little beauty from happening tonight. What else can I say except “Come a little bit closer.. Hear what I have to say. Just like children sleepin’… We could dream this night away. But there’s a full moon risin’… Let’s go dancin’ in the light. We know where the music’s playin’… Let’s go out and feel the night…”

I love Harvest Moon time – mainly because there’s so much folklore and legend attached to it. Here in the heartland, we associate it with tractors in the field, working late into the night gathering the harvest by the light of our nearest astronomical neighbor. It’s a romantic and fanciful thought – especially since modern tractors just combine the stuff down with headlights approximately bright enough to land a Boeing 747. However, it’s still a lot of fun to think about old cultures like the Norse folks who believed the Moon granted them Loki’s blessing for plenty. I’ve heard it called the Singing Moon, too… A time for rest after harvest, sit around, sing some songs, smoke a peace pipe. Or, you can celebrate with the Celtics. They called it the Wine Moon, eh? No matter what your choice may be, the whole object is to be mellow.

But, hey! What’s mellow without a little science behind it? What constitutes the Harvest Moon most of all is that it’s closest to Equinox. Just this little tidbit and the change of seasons ought to give you a clue of what’s going on. Most of time during the year, the Moon comes along about 50 minutes later each night, but as the tilt of our Earth is gradually changing, that time is a bit shorter – by around 20 minutes for several evenings in a row. Why? The answer is easy enough. The ecliptic – or plane of Earth’s orbit around the sun – makes a narrow angle with respect to the horizon in the evening in autumn.

Is it really more orange or yellow than normal? How about larger? Oh, yes. You want those science facts, don’t you? Sure! Why not… Oftentimes we perceive the Harvest Moon as being more orange than at any other time of the year. The reason is not only scientific enough – but true. Coloration is caused by the scattering of the light by particles in our atmosphere. When the Moon is low, as it is now, we get more scattering effect and it truly is a deeper orange. The very act of harvesting itself also produces dust and oftentimes that color will last through the whole night. As for larger? Well, that’s just an illusion. Everyone knows the Moon looks larger on the horizon, but did you know this is a psychological phenomenon and not a physical one? Prove it to yourself by looking at the rising Moon upright…it looks larger, doesn’t it? Now stand on your head, or find a comfortable way to view it upside down…now how big is it?

Go on out tonight and enjoy the Harvest Moon… “Harvest moon… I see the days grow shorter. I feel the nights grow cold. Harvest moon… Young people feeling restless. Old people feeling old. Harvest moon… I sense the darkness clearer. I feel the presence here. Harvest moon… A change in the weather. I love this time of year. Harvest Moon….”

Posted on by Fraser Cain

The Earth Goes Around the Sun

The Geocentric View of the Solar System
An illustration of the Ptolemaic geocentric system by Portuguese cosmographer and cartographer Bartolomeu Velho, 1568 (Bibliothèque Nationale, Paris)

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In ancient times, everyone thought the Earth was the center of the Universe – it was obvious to anyone who just looked up. The Sun, Moon, stars and planets were thought to be attached to crystal spheres that turned around us. We now know that the Earth goes around the Sun, but how do we know this?

In astronomy, putting the Sun at the center of the Solar System is known as heliocentrism, while putting the Earth at the center is called geocentrism. As astronomers put in more and more time studying the heavens, they realized that this model didn’t match reality. The Sun didn’t follow an exact path every day, and the planets didn’t move how they were supposed to.

It wasn’t until the 16th century that the Polish astronomer Copernicus developed a model that placed the Sun at the center of the Solar System.

Until that point, astronomers had developed very complicated models that tried to explain the motions of the planets. At times they appeared to move backwards in the sky, and then go forwards again. Astronomers had developed the thought that there were spheres within spheres that could explain these motions. Copernicus simplified things, and showed that all the planets were orbiting the Sun, and the strange motions of the planets was then easy to understand as the Earth caught up and then passed them in orbit.

In 1610, Galileo used his first rudimentary telescope to observe that Venus went in phases just like the Moon. This went against the theory that everything orbited the Earth, and was further evidence that it goes around the Sun. Galileo also observed how Jupiter has 4 major moons that orbit it. This broke the previous believe that all objects orbit the Earth.

More precise measurements followed, and Johannes Kepler created his three laws that explained that the planets were actually following elliptical orbits around the Sun. He was the first astronomer to accurately predict a transit of Venus, where the planet was seen to pass directly in front of the Sun.

The motion of the Earth as it goes around the Sun is well calculated today. Space agencies use these calculations to launch spacecraft to explore the other planets in the Solar System. If everything went around the Earth, we’d know by now.

References:
NASA: Heliocentric Solar System
NASA Earth Observatory: Planetary Motion