The Voyager Golden Record

The Voyager Golden Record carries a small sample of uranium on its cover, with a built-in clock for a message engineered to last around a billion years        

Each of the two Voyager spacecraft, launched in 1977, carries a phonograph record. The records hold sounds and images chosen to represent Earth, assembled by a committee led by Carl Sagan. The records themselves are the part most people know about. The cover is the part worth a closer look. Electroplated onto the aluminium cover of each record is an ultra-pure sample of uranium-238. It sits in a small area about two centimetres across, and it is there to serve a single function. It is a clock. The Golden Record Pioneers 10 and 11, which preceded Voyager, both carried small metal plaques identifying their time and place of origin for the benefit of any other spacefarers which might find them in the distant future. NASA placed a more ambitious message aboard Voyager 1 and 2, a kind of time capsule, intended to communicate a story of our world to extraterrestrials. The Voyager message is carried by a phonograph record, a 12-inch gold-plated copper disk containing sounds and images selected to portray the diversity of life and culture on Earth. The Golden Record Cover is an easily recognized drawing of the phonograph record and the stylus carried with it. The stylus is in the correct position to play the record from the beginning. Written around it in binary arithmetic is the correct time of one rotation of the record, 3.6 seconds, expressed in time units of 0,70 billionths of a second, the time period associated with a fundamental transition of the hydrogen atom. The drawing indicates that the record should be played from the outside in. 

Below this drawing is a side view of the record and stylus, with a binary number giving the time to play one side of the record. The information in the upper right-hand portion of the cover is designed to show how pictures are to be constructed from the recorded signals. The top drawing shows the typical signal which occurs at the start of a picture. The picture is made from this signal, which traces the picture as a series of vertical lines, similar to ordinary television. Picture lines 1, 2 and 3 are noted in binary numbers, and the duration of one of the "picture lines," about 8 milliseconds, is noted. The drawing immediately below shows how these lines are to be drawn vertically, with staggered "interlace" to give the correct picture rendition. Immediately below this is a drawing of an entire picture raster, showing that there are 512 vertical lines in a complete picture. Immediately below this is a replica of the first picture on the record to permit the recipients to verify that they are decoding the signals correctly. A circle was used in this picture to ensure that the recipients use the correct ratio of horizontal to vertical height in picture reconstruction. The drawing in the lower left-hand corner of the cover is the pulsar map previously sent as part of the plaques on Pioneers 10 and 11. It shows the location of the solar system with respect to 14 pulsars, whose precise periods are given. 

The drawing containing two circles in the lower right-hand corner is a drawing of the hydrogen atom in its two lowest states, with a connecting line and digit 1 to indicate that the time interval associated with the transition from one state to the other is to be used as the fundamental time scale, both for the time given on the cover and in the decoded pictures. The reasoning is straightforward. Uranium-238 decays into a chain of daughter products at a fixed, known rate. A finder who measures how much uranium-238 remains, against how much of the daughter material has accumulated, can calculate how long the decay has been running. The figure is the time elapsed since the sample was placed on the spacecraft. It tells the finder how old the record is. The uranium is not the only timekeeping device on the record cover. It is one of two, and the two are meant to be checked against each other. The cover also carries a pulsar map, the same basic diagram used earlier on the Pioneer 10 and 11 plaques. It shows the position of the Sun relative to 14 pulsars, with each pulsar’s rotation period written in binary. Pulsars spin down slowly and predictably over time. A finder who knows how fast those 14 pulsars are spinning when the record is found, and compares that to the periods recorded on the cover, can also work out roughly how much time has passed. So the cover gives a finder two independent ways to date the record. One is the decay of the uranium. The other is the slowing of the pulsars. If both methods point to a similar launch epoch, a finder can have more confidence in the answer. The redundancy is deliberate.

Electroplated onto the record's cover is an ultra-pure source of uranium-238 with a radioactivity of about 0.00026 microcuries. The steady decay of the uranium source into its daughter isotopes makes it a kind of radioactive clock. Half of the uranium-238 will decay in 4.51 billion years. Thus, by examining this two-centimeter diameter area on the record plate and measuring the amount of daughter elements to the remaining uranium-238, an extraterrestrial recipient of the Voyager spacecraft could calculate the time elapsed since a spot of uranium was placed aboard the spacecraft. This should be a check on the epoch of launch, which is also described by the pulsar map on the record cover. The  Voyager Golden Records  are two  phonograph records  which were included aboard both  Voyager spacecraft. The records contain sounds and images selected to portray the diversity of life and culture on Earth, and are intended for any intelligent  extraterrestrial life  form who may find them. The records are a sort of  time capsule . Although neither Voyager spacecraft is heading toward any particular star,  Voyager 1  will pass within 1.6  light-years ' distance of the star  Gliese 445 , currently in the constellation  Camelopardalis , in about  40,000 years . The spacecraft will be encountered and the record played only if there are advanced space-faring  civilizations  in  interstellar  space, but the launching of this  'bottle' into the cosmic 'ocean'  says something very hopeful about life on this planet. The choice of isotope is the whole point. Uranium-238 has a half-life of about 4.5 billion years, meaning that after that span roughly half of any given quantity has decayed. It's is a slow clock, and a slow clock is what a message of this kind needs.

A fast-decaying isotope would be useless here. If the half-life were measured in years or centuries, the sample would have decayed away to almost nothing long before any plausible finder encountered it, leaving nothing to measure. A half-life of billions of years means the clock stays readable across the kind of timescales the Voyager spacecraft will actually be drifting. The Voyager record is often described as a message built to last around a billion years, and the figure is worth handling carefully. It is an estimate of the physical survival of the record itself, not a guarantee and not a precise prediction. The record is gold-plated copper in an aluminium cover, mounted on the spacecraft body. In interstellar space it faces erosion mainly from micrometeoroid impacts and cosmic rays, both of which act slowly. Estimates of how long the record’s grooves would remain physically readable run to a timescale of that order, hundreds of millions to a billion-plus years. These are estimates of material durability, and they carry wide uncertainty. The uranium clock is well matched to that lifespan. With a half-life of about 4.5 billion years, the uranium-238 sample will still be measurable, still a working clock, long after a billion years have passed. The clock is designed to outlast the message it dates. 

The selection of content for the record took almost a year. Sagan and his associates assembled 115 images and a variety of natural sounds, such as those made by surf, wind, thunder and animals (including the songs of  birds  and  whales ). To this they added musical selections from different cultures and eras, spoken greetings in 55 ancient and modern languages, other human sounds, like footsteps and laughter (Sagan's), and printed messages from US president  Jimmy Carter  and  UN   Secretary-General   Kurt Waldheim. The Golden Record also carries an hour-long recording of the brainwaves of  Ann Druyan . During the recording of the brainwaves, Druyan thought of many topics, including Earth's history, civilizations and the problems they face, and what it was like to fall in love. The  pulsar  map and hydrogen molecule diagram are shared in common with the  Pioneer plaque. The 115 images are encoded in analogue form and composed of 512 vertical lines. The remainder of the record is audio, designed to be played at 16⅔ revolutions per minute. The drawing indicates that the record should be played from the outside in. A circle was used in this picture to ensure that the recipients use the correct ratio of horizontal to vertical height in picture reconstruction. Color images were represented by three images in sequence, one each for red, green, and blue components of the image. A color image of the spectrum of the sun was included for calibration purposes. 

The record also includes the inspirational message  Per aspera ad astra  in  Morse code. The collection of images includes many photographs and diagrams both in black and white, and color. The first images are of scientific interest, showing mathematical and physical quantities, the  Solar System  and its planets,  DNA , and human  anatomy  and  reproduction . Care was taken to include not only pictures of humanity, but also some of animals, insects, plants and landscapes. Images of humanity depict a broad range of cultures. These images show food, architecture, and humans in portraits as well as going about their day-to-day lives. Many pictures are annotated with one or more indications of scales of time, size, or mass. Some images contain indications of  chemical composition . All measures used on the pictures are defined in the first few images using physical references that are likely to be consistent anywhere in the  universe. The Voyager spacecraft were not aimed at any recipient, and the distances between stars are such that neither craft will pass close to another star for tens of thousands of years at least. The record cover is built the way it is not because a finder is expected, but so that the information is complete and self-checking if, against very long odds, one ever exists. This is the quiet logic of the uranium spot. It is two centimetres of metal added to a cover, on the assumption that the most useful thing you can give a hypothetical finder, along with the message, is an honest way to know how old the message is.

Voyager 1 was launched in 1977, passed the orbit of  Pluto  in 1990, and left the  Solar System  (in the sense of passing the  termination shock) in November 2004. It is now in the  Kuiper belt . In about 40,000 years, it and  Voyager 2  will each come to within about 1.8  light-years  of two separate stars:  Voyager 1  will have approached star  Gliese 445 , located in the constellation  Camelopardalis ; and  Voyager 2  will have approached star  Ross 248 , located in the constellation of  Andromeda . In March 2012,  Voyager 1  was over 17.9 billion km from the Sun and traveling at a speed of 3.6  AU  per year (approximately 61,000 km/h (38,000 mph)), while  Voyager 2  was over 14.7 billion km away and moving at about 3.3 AU per year (approximately 56,000 km/h (35,000 mph)). Voyager 1  has entered the  heliosheath , the region beyond the termination shock. The termination shock is where the solar wind, a thin stream of electrically charged gas blowing continuously outward from the Sun, is slowed by pressure from gas between the stars. At the termination shock, the solar wind slows abruptly from its average speed of 300–700 km/s (670,000–1,570,000 mph) and becomes denser and hotter. On 12 September, 2013, NASA announced that  Voyager 1 had left the heliosheath and entered  interstellar space, although it still remains within the Sun's gravitational sphere of influence.

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Milky Way ate another galaxy

 Scientists think they have found bones of a galaxy called Loki, ate by The Milky Way        

An unusual collection of stars may represent the remnants of a dwarf galaxy that the Milky Way devoured about 10 billion years ago. Astronomers have dubbed the ancient galaxy Loki. The finding could change the current understanding of how the Milky Way evolved in the distant past. An unusual collection of stars may represent the remnants of a dwarf galaxy that the Milky Way devoured. The finding could change the current understanding of how the Milky Way evolved in the distant past. The vast Milky Way spans about 100,000 light-years and contains anywhere between 100 billion and 400 billion stars. A light-year is the distance light travels in one year, which is 5.88 trillion miles (9.46 trillion km's).  It grew over time starting about 12 billion years ago by merging with a multitude of dwarf galaxies. But the original size and mass of the Milky Way remain an open question, driving scientists to search for evidence of the galaxies it consumed to determine its history and evolution. To identify those missing puzzle pieces, astronomers have now zeroed in on a cluster of metal-lacking stars detected oddly close to the galactic disk. 

By studying the chemistry of these stars and their motion close to the galactic disk, the researchers found that the stars' home galaxy, nicknamed "Loki," might have merged with our galaxy about 10 billion years ago. Studying these stars could be "very important" in understanding the history of the Milky Way and the universe itself, lead author of the study Federico Sestito, an astrophysicist at the University of Hertfordshire in the U.K, said. Sestito added that Loki may have been among "the very first small galaxies formed in the young universe." The Milky Way's Central Molecular Zone (CMZ) surrounds our galaxy's supermassive black hole and may share characteristics with the dense and chaotic galaxies of the early universe. Massive galaxies are not born whole. They are assembled over billions of years through mergers with smaller galaxies, which are sometimes absorbed. In the early universe, shortly after the Big Bang, matter clumped into clouds of gas which collapsed into the first primitive galaxies. These small systems then fell into one another, merged and gradually built up into the large structures we see today. The astronomers are interested in these stars near the disk, a massive rotating pancake-like region containing much of the Milky Way’s stars, because the first stars in the universe were comprised of hydrogen and helium, which fused heavier elements together in their cores before exploding and unleashing the heavy elements which enriched future generations of stars. Metal-poor stars are often associated with ancient dwarf galaxies, which the Milky Way might have consumed over time to grow to its current massive state, and remnants of these cosmic meals might be hiding deep within the galaxy. The metal-poor composition of such ancient stars close to the galactic disk suggests that the Milky Way once made a rather large meal of another galaxy early in its history, and it could represent a critical, previously overlooked building block of our galaxy.

In the study, astronomers identified 20 old, very-metal-poor stars orbiting unusually close to the galactic disk, the flat, rotating region of the Milky Way where most stars, including the sun, reside, and examined whether a past merger might explain what they were seeing. The very first stars which formed in the universe were made of hydrogen and helium. It was only inside those early stars that hydrogen and helium fused into heavier elements, which astronomers call metals. These stars, when they eventually exploded, enriched the surrounding gas with those metals. Each successive generation of stars was therefore born from material slightly more enriched than the last. As these small galaxies collided and merged, their stars, gas and dark matter became part of the growing young Milky Way. Because of this, computer simulations suggest that stars from the earliest mergers are expected to be found deeper inside the Milky Way today, while stars from galaxies which merged later are more likely to be scattered farther out in the galactic halo, a vast, spherical region that extends beyond the bright disk. However, very few metal-poor stars have been found in the inner regions of the Milky Way to test this idea. So, when the team identified 20 metal-poor stars orbiting close to the galactic disk, they wondered whether the stars could be remnants of an ancient merger.

The Milky Way is suspected to have merged with up to a dozen or more dwarf galaxies over its 12-billion-year history. Astronomers are like the detectives of the universe, searching the cosmos for clues of its origins, and very-metal-poor, or VMP, stars are a powerful tool in that quest, said Dr. Cara Battersby, associate professor of physics at the University of Connecticut, who did not participate in the study. “VMP stars have been around for billions of years, holding within them clues to the formation of the Universe’s earliest generations of stars,” Battersby said. Studying the metal-poor stars’ composition and motion can unlock details about the conditions and dynamics of the early universe, she added. The search for metal-poor stars in the Milky Way has largely centered on the plentiful range of old stars in the galaxy’s stellar halo, so named because it’s a large, round diffuse cloud that surrounds the galactic disk. Some astronomers believe evidence of more ancient mergers could be found deeper inside the Milky Way, such as in its disk. An abundance of young, metal-rich stars, as well as a plethora of dust, crowded within the galactic disk has made it hard to spot metal-poor stars there, said Dr. Federico Sestito.

The team observed existing catalog of metal-poor stars using a powerful spectrograph at the Canada-France-Hawaii Telescope, which revealed their chemical abundances. Using precise positional data from the Gaia space telescope, they calculated the stars' distances and how they orbit in our galaxy. Sestito said that "a mixture of information from the chemistry and the orbits of these stars" nudged them to examine the stars' origin. Rather than drifting through the halo of the galaxy where ancient, metal-poor stars have been mostly observed, these stars were tracing paths close to the Milky Way's disk within just 6,500 light-years from the sun. "Usually, stars in the disk are metal-rich and younger, like the sun," he said, "while our stars [in the study] are old and very metal-poor (like in dwarf galaxies)." Additionally, some of these stars were found moving in the same direction as the Milky Way's rotation, while others traveled in the opposite direction. But these two groups did not show any difference in their chemical abundances. Explaining how a single in falling galaxy could leave stars moving in opposite directions was also challenging. The answer came from computer simulations of galaxy formation. If the merger happened early enough, when the young Milky Way was still lightweight and had not yet settled into a spinning disk, the in falling galaxy would have had enough freedom to scatter its stars in all directions.

"The early merging history of a large galaxy might be very chaotic, with various smaller systems merging together and dispersing their stars with many different orbits," Sestito explained. This scenario could produce both prograde and retrograde orbits, placing the merger event around 3 billion years after the Big Bang. As a result, the simulations showed that a single dwarf galaxy swallowed by the young Milky Way more than 10 billion years ago, could have scattered its stars into exactly the orbital pattern observed today. The models also helped estimate the total mass of this galaxy to be around 1.4 billion solar masses. The exact age of the stars is hard to pin down, but their chemical composition suggests they are older than 10 billion years, Sestito said, and all of them are located roughly 7,000 light-years from our solar system. The stars also have similar chemical compositions, suggesting they all came from the same metal-poor dwarf galaxy. Gaia's mapping shows how 40,000 stars, all located within 326 light-years of the solar system, will move in the next 400,000 years. Eleven of the stars were in a prograde orbit, or moving in the same direction of the galactic disk, while nine were on a retrograde orbit, or moving in the opposite direction, possible remnants of a dwarf galaxy gobbled up by the Milky Way. The study authors believe the accreted, or hijacked, stars, simply remained as part of our galaxy, getting knocked around and ending up in different orbital patterns, Battersby said.

“If the Loki scenario is correct, then a system merged with our galaxy could deposit its stars into both prograde and in the opposite direction,” Sestito said. “This can be allowed only if the merger event happened when our Milky Way was still infant/smaller and its gravitational potential was weaker than nowadays. Cosmological simulations suggest that this could have happened no later than 3 or 4 billion years from the Big Bang.” Dr. Hans-Walter Rix, director of the department of galaxies and cosmology at the Max Planck Institute for Astronomy in Germany, said what was most impressive about the study was “how they use the detailed chemical element abundances as a fingerprint to identify a common birth origin of these stars in a now-shredded satellite galaxy, even though some of the stars are going the right way around and some the wrong way.” “Similarly, our accreted stars gave us some hard time in understanding their origin,” Sestito said. “At first it was not easy to reconcile the fact that an accreted system can disperse its stars in both prograde and opposite orbits.” Another explanation for the stars could be that they stem from more than one merger event with the Milky Way, he said. Because researchers are still in the early stages of exploring the chemical signatures of the lowest-metallicity stars in the Milky Way disk, it remains plausible that these stars belong to a subgroup of stars or substructure within the Milky Way, Chiti noted. "I'm looking forward to what future work mapping the chemistry of large samples of very metal-poor stars in the Milky Way disk may show," he said. To confirm the nature of Loki, the team would need to observe its stars and other non-Loki targets with the same telescope setup to better understand the differences between this system and other parts of the Milky Way halo.

The Milky Way has grown through galactic cannibalism, or when a large galaxy eats a small galaxy and uses immense gravitational force to absorb its stars and gas. The leftover shreds of such meals enable astronomers to assemble the galaxy’s “eating history,” said Dr. Alexander Ji, assistant professor in the department of astronomy and astrophysics at the University of Chicago. There are lots of little mergers happening all the time, but the really big meals can change the growth history of the Milky Way,” Ji said. One such transformative event occurred as the Milky Way merged with the Gaia-Sausage-Enceladus galaxy between 8 billion and 10 billion years ago. “We think it helped ‘reset’ the Milky Way from its early turbulent phase to the more stable growing disk that it has today,” Ji said. The new study suggests that the Milky Way merging with the Loki galaxy was almost on the scale of the Gaia-Sausage-Enceladus event. But the evidence is largely hidden because Loki’s remnants are hard to find near the Milky Way’s disk, Ji added. “If this is real, it would indicate that we are missing a major part of our Milky Way’s formation history, and we might need to revisit our current picture to see the impact of such an event,” Ji said. Ji doubts Loki is a previously unknown galaxy, given that possible discoveries of merger events often turn out to be extensions of known systems, but he noted the study authors included appropriate caveats in their work. “It’s an interesting new possibility worth pursuing, and I expect there will be people looking to test whether Loki is real with larger datasets,” Ji said. With upcoming advanced spectroscopic facilities, astronomers will be able to observe hundreds of stars with available high-quality data on their trajectories and chemical abundances. The hidden systems in the inner regions of the galaxy could hold clues to the primitive galaxies of the young universe, though detecting them in the crowded disk would be challenging in our universe.

Muhammad (Peace be upon him) Name