ABAA News:Talk on Stellar Parallax & Distance measurement to Stars

Last Sunday at ABAA there was an interesting talk and discussion on Stellar Parallax. I had asked Jayanth if he could send a small write up of the talk that he gave on the "Stellar Parallax & Distance measurement to Stars" so that it can be made available to all our blog readers. In his busy schedule Jayanth did manage to find some time to write a summery of the talk on Sunday. Here is the Article and Photos of the session.

What is Parallax?

To visualize this, See any object close to you say a few centimeters away, and look at it alternatively with your left eye closed and then with the right eye. You will notice that the position of the object seems to be different with reference to a far off background objects. You will notice that the object shifts from one position to other alternatively when you see the same once with your left and righ eye. The reason for this is not the object shifting but shifting the place you watch the object from. Your right eye and left eye.

The angles at which your eyes have to look obviously depend on the distance of the object and the distance between your two eyes. nose. The angle gets smaller with larger object to eye distance and smaller eye to eye distance. Therefore the difference in angles can be used to measure the distance to the object if you know the distance between your two eyes!. This difference in angles is called theparallax. The distance between your eyes is called the baseline; the bigger the baseline is, the bigger the parallax is, for the same distance of the observed object to the baseline.

The stars are very far away from us and their parallax is so small that the short baseline from one eye to another is too small to detect or even measure. In order to measure this small parallex of stars, we need a larger base line. A large base line is available which is the diameter of earths orbit = 2 Astronomical Units (AU) = 300 million KM.arth. Simply look where (at which angle) a star is in summer in the sky, and then look again where (at which angle) the same star is in winter in the sky, determine the difference in angles, and you have the parallax for this star. Now you can compute the distance to the star. The following picture should illustrate this:

Here in this calculations Parsec (parallax second) is the distance of a star that would have a parallax of two arc seconds, or, equivalently, the distance from our sun at which the angle between earth and sun (1 AU) would be one arc second. Using this unit t is useful because for large distances involved, distance is inversely proportional to the parallax. Hence a star with a parallax of x arcseconds is 2/x parsec away. This way we can calculate, one parsec is approximately 206,000 AU, or 3.09 × 10¹³kilometers, or 3.26 light years.

The closest star is 4.2 light years away which means parallax of the closest star  will be less than one parsec. In order to measure such small angles, instruments with great precision are needed. Parallax to a star 61 Cygni was measured by astronomer and mathematician Wilhelm Nessel in the year 1838 from Konigsberg observatory and the value was 0.314 arc seconds. The modern value is 0.292 arc Seconds (total angles) which computes the distance to this star close to 10 light years.

Due to the precision of measurements required by this method and the atmospheric distortions limiting the accuracies, 1989 the satellite Hipparcos was launched which has the capability of measuring parallax down to 1 milli arc second. This means Satellite Hipparcos was capable of using Parallax method to estimate distance to near by stars up to 1600 light years. Another satellite named GAIA  launched in 2010 is  able to measure even more distant stars, up to 32000 light years.

By using radio waves instead of visible light, one can measure even smaller parallaxes, even down on earth, without the need for satellites. Using Very Long Baseline Interferometry (Two radio telescopes with a large distance between them which is usually on different sides of the earth) improves the measurement of angles greatly. The precision is in the order of 100 micro arc second.

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Huge tornadoes discovered on the Sun

Solar tornadoes several times as wide as the Earth can be generated in the solar atmosphere, say researchers in the UK. A solar tornado was discovered using the Atmospheric Imaging Assembly telescope on board the Solar Dynamic Observatory (SDO) satellite. A movie of the tornado will be presented at the National Astronomy Meeting 2012 in Manchester on Thursday 29th March.

"This is perhaps the first time that such a huge solar tornado is filmed by an imager. Previously much smaller solar tornadoes were found my SOHO satellite. But they were not filmed," says Dr. Xing Li, of Aberystwyth University.

Dr. Huw Morgan, co-discover of the solar tornado, adds, "This unique and spectacular tornado must play a role in triggering global solar storms."

The Atmospheric Imaging Assembly saw superheated gases as hot as 50 000 – 2 000 000 Kelvin sucked from the root of a dense structure called prominence, and spiral up into the high atmosphere and travel about 200 000 kilometres along helical paths for a period of at least three hours. The tornadoes were observed on 25 September 2011.

The hot gases in the tornadoes have speeds as high as 300,000 km per hour. Gas speeds of terrestrial tornadoes can reach 150km per hour.

The tornadoes often occur at the root of huge coronal mass ejections. When heading toward the Earth, these coronal mass ejections can cause significant damage to the earth’s space environment, satellites, even knock out the electricity grid.

The solar tornadoes drag winding magnetic field and electric currents into the high atmosphere. It is possible that the magnetic field and currents play a key role in driving the coronal mass ejections.

SDO was launched in February 2010. The satellite is orbiting the Earth in a circular, geosynchronous orbit at an altitude of 36,000 kilometres. It monitors constantly solar variations so scientists can understand the cause of the change and eventuallyhave a capability to predict the space weather.

Credit: RAS

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Billions of Rocky Planets in the Habitable Zones around Red Dwarfs in the Milky Way

A new result from ESO’s HARPS planet finder shows that rocky planets not much bigger than Earth are very common in the habitable zones around faint red stars. The international team estimates that there are tens of billions of such planets in the Milky Way galaxy alone, and probably about one hundred in the Sun’s immediate neighborhood. This is the first direct measurement of the frequency of super-Earths around red dwarfs, which account for 80% of the stars in the Milky Way.

Artist’s impression of sunset on the super-Earth world Gliese 667 Cc

This first direct estimate of the number of light planets around red dwarf stars has just been announced by an international team using observations with the HARPS spectrograph on the 3.6-metre telescope at ESO’s La Silla Observatory in Chile. A recent announcement, showing that planets are ubiquitous in our galaxy used a different method that was not sensitive to this important class of exoplanets.

The HARPS team has been searching for exoplanets orbiting the most common kind of star in the Milky Way — red dwarf stars (also known as M dwarfs). These stars are faint and cool compared to the Sun, but very common and long-lived, and therefore account for 80% of all the stars in the Milky Way.

“Our new observations with HARPS mean that about 40% of all red dwarf stars have a super-Earth orbiting in the habitable zone where liquid water can exist on the surface of the planet,” says Xavier Bonfils (IPAG, Observatoire des Sciences de l'Univers de Grenoble, France), the leader of the team. “Because red dwarfs are so common — there are about 160 billion of them in the Milky Way — this leads us to the astonishing result that there are tens of billions of these planets in our galaxy alone.”

The HARPS team surveyed a carefully chosen sample of 102 red dwarf stars in the southern skies over a six-year period. A total of nine super-Earths (planets with masses between one and ten times that of Earth) were found, including two inside the habitable zones of Gliese 581 and Gliese 667 C respectively. The astronomers could estimate how heavy the planets were and how far from their stars they orbited.

By combining all the data, including observations of stars that did not have planets, and looking at the fraction of existing planets that could be discovered, the team has been able to work out how common different sorts of planets are around red dwarfs. They find that the frequency of occurrence of super-Earths in the habitable zone is 41% with a range from 28% to 95%.

On the other hand, more massive planets, similar to Jupiter and Saturn in our Solar System, are found to be rare around red dwarfs. Less than 12% of red dwarfs are expected to have giant planets (with masses between 100 and 1000 times that of the Earth).

As there are many red dwarf stars close to the Sun the new estimate means that there are probably about one hundred super-Earth planets in the habitable zones around stars in the neighbourhood of the Sun at distances less than about 30 light-years.

"The habitable zone around a red dwarf, where the temperature is suitable for liquid water to exist on the surface, is much closer to the star than the Earth is to the Sun," says Stéphane Udry (Geneva Observatory and member of the team). "But red dwarfs are known to be subject to stellar eruptions or flares, which may bathe the planet in X-rays or ultraviolet radiation, and which may make life there less likely." 

One of the planets discovered in the HARPS survey of red dwarfs is Gliese 667 Cc. This is the second planet in this triple star system (see eso0939 for the first) and seems to be situated close to the centre of the habitable zone. Although this planet is more than four times heavier than the Earth it is the closest twin to Earth found so far and almost certainly has the right conditions for the existence of liquid water on its surface. This is the second super-Earth planet inside the habitable zone of a red dwarf discovered during this HARPS survey, after Gliese 581d was announced in 2007 and confirmed in 2009.

“Now that we know that there are many super-Earths around nearby red dwarfs we need to identify more of them using both HARPS and future instruments. Some of these planets are expected to pass in front of their parent star as they orbit — this will open up the exciting possibility of studying the planet’s atmosphere and searching for signs of life,” concludes Xavier Delfosse, another member of the team.

Credit: ESO

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AstroNews: Jupiter helps Halley’s Comet give us more spectacular meteor displays

The dramatic appearance of Halley's comet in the night sky has been observed and recorded by astronomers since 240 BC. Now a study shows that the orbital influences of Jupiter on the comet and the debris it leaves in its wake are responsible for periodic outbursts of activity in the Orionid meteor showers. The results will be presented by Aswin Sekhar at the National Astronomy Meeting in Manchester on Tuesday 27^th March.

The nucleus of Halley's Comet, taken by the Halley Multicolour Camera on board ESA's Giotto spacecraft in 1986. Credit MPS Germany/H.U. Keller/ESA

Halley's comet orbits the Sun every 75-76 years on average. As its nucleus approaches the Sun, it heats up and releases gas and dust that form the spectacular tail. This outgassing leaves a trail of debris around the orbit.

When the Earth crosses Halley's path, twice per orbit, dust particles (meteoroids) burn up in the Earth’s atmosphere and we see meteor showers: the Orionids in October and the Eta Aquariids in May. Previous research has suggested that Orionid meteoroids have at times fallen into 'resonances' with Jupiter's orbit – a numerical relationship that influences orbital behaviour. Sekhar's new study suggests that Halley itself has been in resonances with Jupiter in the past, which in turn would increase the chances of populating resonant meteoroids in the stream. The particles ejected during those times experience a tendency to clump together due to periodic effects from Jupiter.

 Image of 2007 Orionids, showing Orion constellation in the backdrop. Credit: S. Quirk   

"This resonant behaviour of meteoroids means that Halley's debris is not uniformly distributed along its orbital path. When the Earth encounters one of these clumps, it experiences a much more spectacular meteor shower than usual," said Sekhar, of Armagh Observatory.

Sekhar has modelled Halley’s orbital evolution over more than 12 000 years into the past and 15 000 years into the future. The model suggests that from 1404 BC to 690 BC, Halley was trapped in a 1:6 resonance with Jupiter (in which Halley completed one orbit for every six orbits of Jupiter around the Sun). Later, from 240 BC to 1700 AD, the comet’s orbit had a 2:13 relationship with Jupiter’s orbit. Debris deposited during these two periods can be directly attributed to heightened activity in the Orionid meteor showers in some years. Sekhar’s work suggests that the unusual Orionid outburst observed in 1993 was due to 2:13 resonant meteoroids ejected from Halley around 240 BC. He predicts that the next similar display of meteors from this 2:13 resonance will be in 2070 AD.

"The real beauty of this area of science lies in the convergence of cometary physics and orbital dynamics. The close correlation between historical records from ancient civilisations and the predictions using modern science make it even more elegant," said Sekhar. He added, "There are enough unsolved problems pertaining to Halley and its meteor streams to keep us occupied till the next apparition of the comet in 2061!"

Credit: RAS

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Brightest and Biggest planets close to each other

We have noticed from past few days that Jupiter and Venus are moving close to each other in the evening sky. On March 14^th they will be closest to each other and the will set at same time around 9pm. After sunset in the twilight just look above western horizon and bright Venus is easy to spot, next to Venus, towards south will be Jupiter. They will just 3degrees apart from each other making it a wonderful sight to watch.  Hope clouds will not play a spoil sport and let us see this wonderful sight. If you miss on 14^th don’t worry, next couple of days the planets will be close to each other, so keep watching and clear skies

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Astro News: Could Venus be shifting gear?

ESA’s Venus Express spacecraft has discovered that our cloud-covered neighbour spins a little slower than previously measured. Peering through the dense atmosphere in the infrared, the orbiter found surface features were not quite where they should be.
Using the VIRTIS instrument at infrared wavelengths to penetrate the thick cloud cover, scientists studied surface features and discovered that some were displaced by up to 20 km from where they should be given the accepted rotation rate as measured by NASA’s Magellan orbiter in the early 1990s.

These detailed measurements from orbit are helping scientists determine whether Venus has a solid or liquid core, which will help our understanding of the planet’s creation and how it evolved.

If Venus has a solid core, its mass must be more concentrated towards the centre. In this case, the planet’s rotation would react less to external forces.

The most important of those forces is due to the dense atmosphere – more than 90 times the pressure of Earth’s and high-speed weather systems, which are believed to change the planet’s rotation rate through friction with the surface.

Earth experiences a similar effect, where it is largely caused by wind and tides. The length of an Earth day can change by roughly a millisecond and depends seasonally with wind patterns and temperatures over the course of a year.

In the 1980s and 1990s, the Venera and Magellan orbiters made radar maps of the surface of Venus, long shrouded in mystery as well as a dense, crushing and poisonous atmosphere. These maps gave us our first detailed global view of this unique and hostile world.

Over its four-year mission, Magellan was able to watch features rotate under the spacecraft, allowing scientists to determine the length of the day on Venus as being equal to 243.0185 Earth days.

However, surface features seen by Venus Express some 16 years later could only be lined up with those observed by Magellan if the length of the Venus day is on average 6.5 minutes longer than Magellan measured.

This also agrees with the most recent long-duration radar measurements from Earth.

“When the two maps did not align, I first thought there was a mistake in my calculations as Magellan measured the value very accurately, but we have checked every possible error we could think of,” said Nils Müller, a planetary scientist at the DLR German Aerospace Centre, lead author of a research paper investigating the rotation.

Scientists, including Özgur Karatekin of the Royal Observatory of Belgium, looked at the possibility of short-term random variations in the length of a Venus day, but concluded these should average themselves out over longer timescales.

On the other hand, other recent atmospheric models have shown that the planet could have weather cycles stretching over decades, which could lead to equally long-term changes in the rotation period. Other effects could also be at work, including exchanges of angular momentum between Venus and the Earth when the two planets are relatively close to each other.

“An accurate value for Venus’ rotation rate will help in planning future missions, because precise information will be needed to select potential landing sites,” noted Håkan Svedhem, ESA’s Venus Express project scientist.

While further study is needed, it’s clear that Venus Express is penetrating far deeper into the mysteries of this enigmatic planet then anyone dreamed.

News source:ESA



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Galileo's discovery that changed our views

On January 7^th 1610, Galileo Galilei made an observation through his small telescope that changed our understanding of the universe. Using his self made refractor telescope with an aperture just one inch in diameter with a magnification of 15X, Galileo pointed his telescope to the bright planet Jupiter. The planet Jupiter showed up in the narrow of  15minutes of arc  field of the telescope, accompanied by three satellites, Io was observed on 8^th January as his crude instrument was unable to separate Io and Europa the previous night.

There was no previous records of observations made of Jupiter through telescope and Galileo was in uncharted territory. Galileo first took these 4 objects as stars and continued his observations. The movement of Jupiter with respect to these “stars” made Galileo to think that maybe Jupiter has a retrograde loop. As he continued to track Jupiter, he observed that Jupiter did not move much with respect to other stars apart from the 4 “stars” that were close to Jupiter. One more puzzling observation was that these 4 “stars” always stayed close to the planet.

After several nights of observations Galileo came to the conclusion that these 4 objects were carried along by Jupiter and they are moons of Jupiter. This had a profound impact on our way of looking at the universe. This observation gave the Copernican theory more ground.

Galileo gave the moons of Jupiter roman numbers I,II,III and IV. The names suggested by Simon Marius is what we use now. Io (I), Europa (II), Ganymede (III) and Callisto (IV). The moons are called Galilean moons in honour of Galileo.

Here is a copy of Galileo notes translated

Credit: NASA

Centuries later we sent spacecraft to Jupiter and also took pictures of the Jovian moons. Here are close up photos of the moons and also few facts about them.

Io

Credit: NASA/JPL/University of Arizona

Io is the closest of the Galilean moons which orbits the planet at a distance of 421000km. Io is little bigger than our earth's moon. Io has a radius of 1821km, earth's moon has a radius of 1738km. Io goes around Jupiter in 1.7days. Io is the most volcanically active moon in our solar system. One of the reason for this high activity is that Io is affected by gravitation of Jupiter and high tidal forces make molten interior escape through the gaps created from the distortions caused by the tidal forces.

Europa

  Credit: NASA/JPL/Ted Stryk 

Europa is the second moon from Jupiter and is little smaller than our moon. The radius of Europa is around 1560km. It orbits the planet at a distance of 670000km and completes one rotation in 3.5days. The surface is covered with frozen salt water and scientists think that with the eccentric orbit of Europa there may be places where it may be possible for water to exist.

Ganymede

  Credit: NASA/JPL/Ted Stryk 

Ganymede is the largest moon in the solar system, larger than the planet Mercury. The radius of Ganymede is 2631km and that of Mercury is 2439km. Ganymede orbits the planet Jupiter 1070000km and completes on orbit every 7.15days. This is the only moon to be known to have magnetosphere and also its been thought the moon may have a salt water ocean that is trapped under the ice surface.

Callisto

Credit: NASA/JPL/Ted Stryk

Callisto is the last of Galilean moons and also the farthest from the planet Jupiter. Callisto orbits the planet at a distance of 1882000km and completes on rotation around the planet in 16.6days. The moon Callisto is also the third largest moon in the solar system with a radius of 2410km almost the size of Mercury. Callisto is among the most cratered objects in solar system. Scientists think that the core could have frozen as it formed and no geological activity has taken place from the past 4 billion years.

Jupiter is easy to spot these days. Just after sunset if we look you, Jupiter will be shining bright in the Zenith. No star in the region is as bright as Jupiter. The moons of Jupiter can easily be seen even with a pair of binoculars. If you don't have any optical instruments please contact any astronomy clubs near you and they will surely help you. If you are in Bangalore you can visit us at ABAA on Sunday evening and use our telescopes to observe Jupiter.

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