Soal-soal Latihan

Beberapa soal astronomi sebagai bahan latihan dan diskusi dengan teman2:

1. If the Earth rotated in the opposite sense (clockwise rather than counterclockwise), how long would the solar day be?
2. Suppose that the Earth’s pole was perpendicular to its orbit. How would the azimuth of sunrise vary throughout the year? How would the length of day and night vary throughout the year at the equator? at the North and South Poles? where you live?
3. You are an astronaut on the moon. You look up, and see the Earth in its full phase and on the meridian. What lunar phase do people on Earth observe? What if you saw a first quarter Earth? new Earth? third quarter Earth? Draw a picture showing the geometry.
4. If a planet always keeps the same side towards the Sun, how many sidereal days are in a year on that planet?
5. If on a given day, the night is 24 hours long at the North Pole, how long is the night at the South Pole?
6. On what day of the year are the nights longest at the equator?
7. From the fact that the Moon takes 29.5 days to complete a full cycle of phases, show that it rises an average of 48 minutes later each night.
8. What is the ratio of the flux hitting the Moon during the first quarter phase to the flux hitting the Moon near the full phase?
9. Titan and the Moon have similar escape velocities. Why does Titan have an atmosphere, but the Moon does not?
   

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Astronomers Found The Most Distant Cosmic Object

The redness of the afterglow is indicative of the event's distance

Astronomers have confirmed that an exploding star spotted by Nasa's Swift satellite is the most distant cosmic object to be detected by telescopes.

In the journal Nature, two teams of astronomers report their observations of a gamma-ray burst from a star that died 13.1 billion light-years away.

The massive star died about 630 million years after the Big Bang.

UK astronomer Nial Tanvir described the observation as "a step back in cosmic time".

Professor Tanvir led an international team studying the afterglow of the explosion, using the United Kingdom Infrared Telescope (UKIRT) in Hawaii.

Swift detects around 100 gamma ray bursts every year

He told BBC News that his team was able to observe the afterglow for 10 days, while the gamma ray burst itself lasted around 12 seconds.

The event, dubbed GRB 090423, is an example of one of the most violent explosions in the Universe.

It is thought to have been associated with the cataclysmic death of a massive star - triggered by the centre of the star collapsing to form a "stellar-sized" black hole.

"Swift detects something like 100 gamma ray bursts per year," said Professor Tanvir. "And we follow up on lots of them in the hope that eventually we will get one like this one - something really very distant."

Another team, led by Italian astronomer Ruben Salvaterra studied the afterglow independently with the National Galileo Telescope in La Palma.

Little red dot

He told BBC News: "This kind of observation is quite difficult, so having two groups have the same result with two different instruments makes this much more robust."

"It is not surprising - we expected to see an event this distant eventually," said Professor Salvaterra.

"But to be there when it happens is quite amazing - definitely something to tell the grandchildren."

A GAMMA-RAY BURST RECIPE Models assume GRBs arise when giant stars burn out and collapse During collapse, super-fast jets of matter burst out from the stars Collisions occur with gas already shed by the dying behemoths The interaction generates the energetic signals detected by Swift Remnants of the huge stars end their days as black holes

The astronomers were able to calculate the vast distance using a phenomenon known as "red shift".

Most of the light from the explosion was absorbed by intergalactic hydrogen gas. As that light travelled towards Earth, the expansion of the Universe "stretches" its wavelength, causing it to become redder.

"The greater that amount of movement [or stretching], the greater the distance." he said.

The image of this gamma ray burst was produced by combining several infrared images.

"So in this case, it's the redness of the dot that indicates that it is very distant," Professor Tanvir explained.

Before this record-breaking event, the furthest object observed from Earth was a gamma ray burst 12.9 billion light-years away.

"This is quite a big step back to the era when the first stars formed in the Universe," said Professor Tanvir.

"Not too long ago we had no idea where the first galaxies came from, so astronomers think this is a profound moment.

"This is... the last blank bit of the map of the Universe - the time between the Big Bang and the formation of these early galaxies."

Data from two powerful telescopes confirmed the result

And this is not the end of the story.

Bing Zhang, an astronomer from the University of Nevada, who was not involved in this study, wrote an article in Nature, explaining its significance.

The discovery, he said, opened up the exciting possibility of studying the "dark ages" of the Universe with gamma ray bursts.

And Professor Tanvir is already planning follow-up studies "looking for the galaxy this exploding star occurred in."

Next year, he and his team will be using the Hubble Space Telescope to try to locate that distant, very early galaxy.

Source: BBC News

James Webb Space Telescope

The James Webb Space Telescope (JWST) is a planned infrared space observatory, the partial successor to the aging Hubble Space Telescope. The JWST will not be a complete successor, because it will not be sensitive to all of the light wavelengths that Hubble can see. The main scientific goal is to observe the most distant objects in the universe, those beyond the reach of either ground based instruments or the Hubble. The JWST project is a NASA-led international collaboration with contributors in fifteen nations, the European Space Agency and the Canadian Space Agency.

Current plans call for the telescope to be launched on an Ariane 5 rocket in June 2014, on a five-year mission (10 year goal). The JWST will reside in solar orbit near the Sun-Earth L2 point, which is on a line passing from the Sun to the Earth, but about 1.5 million km farther away from the Sun than is the Earth. This position, which moves around the Sun in exact orbital synchrony with the Earth, will allow JWST to shield itself from infrared from both Sun and Earth, by using a single radiation shield positioned between the telescope and the Sun-Earth direction.

Orbit
To avoid swamping the very faint astronomical signals with radiation from the telescope, the telescope and its instruments must be very cold. Therefore, JWST has a large shield that blocks the light from the Sun, Earth, and Moon, which otherwise would heat up the telescope, and interfere with the observations. To have this work, JWST must be in an orbit where all three of these objects are in about the same direction. The answer was to put JWST in an orbit around the Earth-Sun L2 point.

The L2 orbit is an elliptical orbit about the semi-stable second Lagrange point. The Earth-Sun L2 point, about which the Webb telescope will orbit, is 1.51 million km from the Earth, which is about 3.92 times farther away from Earth than is the moon. This distance underscores how much more difficult the Webb telescope would be to service, after launch.


In the case of JWST, the three bodies involved are the Sun, the Earth and the JWST. Normally, an object circling the Sun further out than the Earth would take more than one year to complete its orbit. However, the balance of gravitational pull at the L2 point (in particular, the extra pull from Earth as well as the Sun) means that JWST will keep up with the Earth as it goes around the Sun. The combined gravitational forces of the Sun and the Earth can hold a spacecraft at this point, so that in theory it takes no rocket thrust to keep a spacecraft in orbit around L2.

Optics
Although JWST has a planned mass half that of the Hubble, its primary mirror (a 6.5 meter diameter gold-coated beryllium reflector) has a collecting area which is almost six times larger. As this diameter is much larger than any current launch vehicle, the mirror is composed of 18 hexagonal segments, which will unfold after the telescope is launched. These mirrors are currently being developed by Axsys Technologies in Cullman, Alabama. Sensitive micromotors and a wavefront sensor will position the mirror segments in the correct location, but subsequent to this initial configuration they will only rarely be moved; this process is therefore much like an initial calibration, unlike terrestrial telescopes like the Keck which continually adjust their mirror segments using active optics to overcome the effects of gravitational and wind loading.



Source : universe today and wikipedia

Astronomers Find Most Distant Supermassive Black Hole Yet

Astronom dari Universitas Hawaii telah berhasil mengamati sebuah galaksi raksasa yang mengelilingi sebuah supermasif black hole terjauh. Galaksi tersebut berjarak 12,8 milyar tahun , nampak sebesar galaksi Bimasakti dan memiliki sebuah supermasif black hole yang mengandung sedikitnya 1 milyar kali massa dibandingkan massa Matahari.

Pengetahuan tentang galaksi induk dari sang supermasif black hole sangat penting untuk memahami misteri bagaimana galaksi dan black hole telah berevolusi bersama. Hingga saat ini, proses pembelajaran galaksi induk dari alam semesta yang jauh sangat sulit akibat sinarnya terhalang oleh black hole.

Asal muasal dari supermsif black hole masih merupakan masalah yang belum terpecahkan dan penemuan baru ini dapat membuka jalan baru untuk menginvestigasi evolusi bersama galaksi-black hole pada awal terbentuknya alam semesta.

Model yang disukai saat ini membutuhkan beberapa black hole berukuran sedang untuk bergabung. Galaksi induk yang ditemukan dalam penelitian ini menyediakan sumber black hole berukuran sedang tersebut. Setelah membentuk supermasif black hole, black hole ini akan terus berkembang karena kemampuan gravitasinya untuk menarik massa dari objek di sekelilingnya. ENergi yang dilepaskan dalam proses ini berkontribusi atas munculnya sinar terang yang diemisikan dari daerah di sekeliling black hole.

Artikel dari peneliti dapat di-download di sini

Sumber: Universe today

Mars, methane and mysteries

Mars may not be as dormant as scientists once thought. The 2004 discovery of methane means that either there is life on Mars, or that volcanic activity continues to generate heat below the martian surface. ESA plans to find out which it is. Either outcome is big news for a planet once thought to be biologically and geologically inactive.

The methane mystery started soon after December 2003, when ESA’s Mars Express arrived in orbit around the red planet. As the Planetary Fourier Spectrometer (PFS) began taking data, Vittorio Formisano, Istituto di Fisica dello Spazio Interplanetario CNR, Rome, and the rest of the instrument team saw a puzzling signal. As well as the atmospheric gases they were anticipating, such as carbon monoxide and water vapour, they also saw methane.

“Methane was a surprise, we were not expecting that,” says Agustin Chicarro, ESA Mars Lead Scientist. The reason is that on Earth much of the methane in our atmosphere is released by evolved life forms, such as cattle digesting food. While there are ways to produce methane without life, such as by volcanic activity, it is the possible biological route that has focused attention on the discovery.

The Mars Express detection of methane is not an isolated case. While the spacecraft was en route, two independent teams of astronomers using ground-based telescopes started to see traces of methane. After five years of intensive study, the suite of observations all confirmed the discovery and presented planetary scientists with a big puzzle.

Methane is thought to be stable in the martian atmosphere for around 300 years. So, whatever is generating the methane up there, it is a recent occurrence. In January 2009, a team led by Michael Mumma of NASA’s Goddard Space Flight Center published results that the methane they saw in 2003 was concentrated in three regions of the planet. This showed that the methane was being released at the present time and was being observed before it had time to distribute itself around the planet.

Things then took a strange turn. Instead of taking 300 years to disappear, the methane had almost entirely vanished by early 2006. Clearly something unusual is going on at Mars. “We thought we understood how methane behaved on Mars but if the measurements are correct then we must be missing something big,” says Franck Lefèvre, Université Pierre et Marie Curie, CNRS, Paris and a member of Mars Express’s SPICAM instrument team.

Together with his colleague François Forget, Mars Express Interdisciplinary Scientist in charge of atmospheric studies and also of Université Pierre et Marie Curie, CNRS, Paris, Lefèvre has investigated the disappearance using a computer model of Mars’ climate. “We have tackled the problem as atmospheric physicists, without worrying about the nature of the source of the methane,” he says.

In results published last week they found that, while their computer model can reproduce atmospheric species such as carbon monoxide and ozone, it is unable to reproduce the behaviour of the methane. “Something is removing the methane from the atmosphere 600 times faster than the models can account for,” says Lefèvre. “Consequently, the source must be 600 times more intense than originally assumed, which is considerable even by Earth’s geological standards.”

To remove methane at such a rate, suspicion falls on the surface of the planet. Either the methane is being trapped in the dust there or highly reactive chemicals such as hydrogen peroxide are destroying it, as was hinted by the Viking missions in the 1970s. If the latter, then the surface is much more hostile to organic molecules (those containing carbon) than previously thought. This will make searching for traces of past or present life much tougher and future rovers will have to drill below the martian surface to look for signs of life.

To help get to the bottom of the methane mystery, ESA and the Italian space agency (ASI) are to hold a three-day international workshop in November. The assembled scientists will discuss the results and plan strategies for the future study of methane. At the workshop, the Mars Express PFS team hopes to present a global map of martian methane. “We have made the PFS mapping a priority over the last few months,” says Olivier Witasse, ESA Project Scientist for Mars Express.

In July, ESA agreed with NASA to launch joint missions to Mars. The topic of methane is of such importance that it will be most likely addressed in these future missions. “Understanding the methane on Mars is one of our top priorities,” says Witasse.

However the methane is eventually explained, it makes Mars a more fascinating place than even planetary scientists dreamed.

Source: ESA

Astronomer Found Planetary Nebula Around Heavy Stars

Lead image caption: An optical image from the 0.6-m University of Michigan/CTIO Curtis Schmidt telescope of the brightest Radio Planetary Nebula in the Small Magellanic Cloud, JD 04. The inset box shows a portion of this image overlaid with radio contours from the Australia Telescope Compact Array. The planetary nebula is a glowing record of the final death throes of the star. (Optical images are courtesy of the Magellanic Cloud Emission Line Survey (MCELS) team).

Planetary nebula – the glowing gaseous shells thrown off by stars during the latter stages of their evolution – were thought to only form around stars the size of our Sun or smaller. Although astronomers had predicted these shells should form around "heavier" stars, none had ever been detected. Until now. An international team of scientists have discovered a new class of object which they call “Super Planetary Nebulae,” found around stars up to 8 times the mass of the Sun.

“This came as a shock to us,” said Miroslav Filipovic from the University of Western Sydney “as no one expected to detect these object at radio wavelengths and with the present generation of radio telescopes. We have been holding up our findings for some 3 years until we were 100% sure that they are indeed Planetary Nebulae”.

The team surveyed the Magellanic Clouds, the two companion galaxies to the Milky Way, with radio telescopes of the Commonwealth Scientific and Industrial Research Organisation (CSIRO) Australia Telescope National Facility. They noticed that 15 radio objects in the Clouds match with well known planetary nebulae observed by optical telescopes.

The new class of objects are unusually strong radio sources and are associated with larger original stars (progenitors), up to 8 times the mass of the Sun. The nebular material around each star may have as much as 2.6 times the mass of the Sun.

Filipovic's team argues that the detections of these new objects may help to solve the so called “missing mass problem” – the absence of planetary nebulae around central stars that were originally 1 to 8 times the mass of the Sun. Up to now most known planetary nebulae have central stars and surrounding nebulae with respectively only about 0.6 and 0.3 times the mass of the Sun but none have been detected around more massive stars.

Some of the 15 newly discovered planetary nebulae in the Magellanic Clouds are 3 times more luminous than any of their Milky Way cousins. But to see them in greater detail astronomers will need the power of a coming radio telescope – the Square Kilometre Array planned for the deserts of Western Australia.

Source: Universetoday

Hubble Ultra Deep Field in 3-D

Pernahkan Anda mendengar tentang Hubble Ultra Deep Field Image? Jika belum, silakan simak sedikit penjelasan dari wikipedia.

The Hubble Ultra Deep Field, or HUDF, is an image of a small region of space in the constellation Fornax, composited from Hubble Space Telescope data accumulated over a period from September 24, 2003 through January 16, 2004. It is the deepest image of the universe ever taken, looking back approximately 13 billion years, and it will be used to search for galaxies that existed between 400 and 800 million years after the Big Bang.The HUDF image was taken in a section of the sky with a low density of bright stars in the near-field, allowing much better viewing of dimmer, more distant objects. The image contains an estimated 10,000 galaxies.

Located southwest of Orion in the Southern-Hemisphere constellation Fornax, the image covers 11.0 square arcminutes. This is just one-tenth the diameter of the full moon as viewed from Earth, smaller than a 1 mm by 1 mm square of paper held 1 meter away, and equal to roughly one thirteen-millionth of the total area of the sky. The image is oriented such that the upper left corner points toward north (-46.4°) on the celestial sphere.

Contents
The HUDF is the deepest image of the universe ever taken and it will be used to search for galaxies that existed between 400 and 800 million years after the Big Bang (redshifts between 7 and 12). The star near the center of the field is USNO-A2.0 0600-01400432 with apparent magnitude of 18.95.

The field imaged by the ACS contains over 10,000 objects, the majority of which are galaxies, many at redshifts greater than 3, and some that probably have redshifts between 6 and 7. The NICMOS measurements may have discovered galaxies at redshifts up to 12.

Scientific results

1. High rates of star formation during the very early stages of galaxy formation, under a billion years after the Big Bang.
2. Improved characterization of the distribution of galaxies, their numbers, sizes and luminosities at different epochs, allowing investigation into the evolution of galaxies.
3. Confirmation that galaxies at high redshifts are smaller and less symmetrical than ones at lower redshifts, showing the rapid evolution of galaxies in the first couple of billion years after the Big Bang.

(untuk informasi lebih lengkap tentang HUDF, silakan lihat di artikel ini dan untuk peta dari HUDF, silakan klik link ini.)

Berikut juga ditampilkan sebuah video untuk visualisasi HUDF.

Semoga bermanfaat.

Soal Latihan Astrofisika

Selamat mencoba beberapa soal yang dapat Anda download lewat link berikut:

Soal Latihan Astrofisika - 15 Juni 2009

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