Unexpectedly Heavy Stars from Long Ago Puzzle Astronomers (SPACE.com)

Ancient stars found in the outer reaches of our Milky Way are surprisingly chock full of some of the heaviest chemical elements, which could have formed in the galaxy's early history, a new study reveals.

When astronomers found abnormally large amounts of heavy elements like gold, platinum and uranium in some of the oldest stars in the Milky Way they were puzzled, because an abundance of very heavy metals is typically only seen in much later generations of stars.

To investigate this mystery, researchers observed these ancient stars over the course of several years using the European Southern Observatory's fleet of telescopes in Chile. They trained their telescopes on 17 "abnormal" stars in the Milky Way that were found to be rich in the heaviest chemical elements.

The results of the study are detailed in the Nov. 14 issue of the Astrophysical Journal Letters.

"In the outer parts of the Milky Way there are old 'stellar fossils' from our own galaxy's childhood," the study's lead author Terese Hansen, an astrophysicist at the Niels Bohr Institute at the University of Copenhagen, said in a statement. "These old stars lie in a halo above and below the galaxy's flat disc. In a small percentage — approximately 1-to-2 percent of these primitive stars — you find abnormal quantities of the heaviest elements relative to iron and other 'normal' heavy elements." [Top 10 Star Mysteries]

Hansen and her colleagues calculated the orbital motions of the stars, which led to an important clue about what kind of mechanisms must have created the heavy elements in the stars.

According to the researchers, there are two possible theories to explain these ancient stars, both centered around supernova explosions, when massive stars run out of fuel and collapse in energetic bursts.

Shortly after the universe was created, it was dominated by light elements like hydrogen and helium. As clouds of these gasses clumped together and collapsed in on themselves under their own gravity, the first stars were formed.

At the heart of these stars, hydrogen and helium merged together and formed the first heavy elements like carbon, nitrogen and oxygen.

When these massive stars died in supernova explosions, they spread the newly formed elements as gas clouds into space. These gas clouds eventually collapsed in on themselves again to form new stars containing the heavier elements. Throughout this process, the newer generations of stars become richer and richer in heavy elements.

After a few hundred million years, all of the known chemical elements existed. But the very early stars contained only a thousandth of the amount of heavy elements that are seen in the sun and other stars today. Hansen and her colleagues suggest that some early stars may have been in close binary systems. In such a twin star system, when one star went supernova, it would have coated its companion star with a thin layer of heavy elementslike gold and uranium.

"My observations of the motions of the stars showed that the majority of the 17 heavy-element-rich stars are in fact single," Hansen said. "Only three belong to binary star systems — this is completely normal, 20 percent of all stars belong to binary star systems. So the theory of the gold-plated neighboring star cannot be the general explanation."

Another theory is that early supernovas could shoot jets of these elements in different directions, dispersing them into the surrounding clouds of gas that eventually formed some of the stars we see today in the Milky Way.This scenario could help explain how many of the old stars became abnormally rich in heavy elements, the researchers said.

"In the supernova explosion the heavy elements like gold, platinum and uranium are formed and when the jets hit the surrounding gas clouds, they will be enriched with the elements and form stars that are incredibly rich in heavy elements," Hansen said.

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New Look at Exploding Stars Provides Cosmic Yardstick (SPACE.com)

Nola Taylor Redd, SPACE.com Contributor
Space.com Nola Taylor Redd, Space.com Contributor
space.com – Thu Aug 11, 2:14 pm ET

In universe spanning more than a billion light-years, distance can't be measured with a ruler. To judge how far away objects are, astronomers must rely on other objects whose properties are already known — such as certain kinds of exploding stars called supernova.  

New research is shedding light on the identity of one of these "standard candles," so-called because their brightness is standard enough that their true distance can be deduced from it.

Astronomers are hoping that analyzing one specific type of supernova explosion will give them a better understanding of how frequently it differs from another type. That, in turn, should allow for even more precise measurements of distance in the universe.

One dwarf or two

When a compact, dying star known as a white dwarf orbits another star closely enough, its strong gravitational pull can ultimately rip its partner apart. But the massive survivor can pack only so much material onto its surface. When its critical point is reached, it explodes as a Type 1a supernova.  

These events can be divided into two categories. One involves only the single white dwarf and its victim. The other involves two white dwarfs, with one destroying the other.  New research, published in the Aug. 12 issue of the journal Science, takes a look at just how commonplace the single-white-dwarf version of a Type 1a supernova may be. [Video: Supernovas – Destroyers and Creators]

When two white dwarfs are orbiting one another and the smaller one moves too close, it is almost instantly torn apart, creating a disk to orbit its destructive companion.

Almost immediately, the disk falls onto the remaining star, pushing it over the critical mass threshold and causing an explosion.

But when the second star in a pair isn't a white dwarf, things move slower. The stars don't get as close, and tidal forces manage to pull away only some of the gas from the near side of the second star. The white dwarf feeds on the material until it eventually reaches the critical mass, exploding as a supernova.

"Both models agree that the explosion is an accreting white dwarf," the lead author of the study, Assaf Sternberg at the Weizmann Institute of Science in Israel, told SPACE.com via email. "The disagreement is on the origin of the accreted material."

It is this material that interested Sternberg and his team. When the destroyed star is a white dwarf, the material is quickly consumed, but when it is not, traces of the gas linger even after the explosion.

The international team of astronomers used the Keck telescope in Hawaii  and the Magellan telescope in Chile to study the sodium in gas clouds around 41 Type 1a supernovas. Sodium is an element found in most stars but not in white dwarfs.

From the sample taken, the team determined that at least 24 percent of the explosions did not involve white dwarfs as the companion.

This number was a lower limit: Half or even all of the pairings could involve only one white dwarf star. The researchers couldn't specifically target which explosions contain white dwarfs and which do not. Instead, they looked for a distribution. They found more systems with sodium than would be found if there were an equal number of double-white-dwarf and single-white-dwarf systems.

Judging distances

Josh Simon, of the Carnegie Institute, explained how this event helps determine distances in the universe.

"If you know that the light bulb is 60 watts, then you can figure out how far away the light is from you by measuring how bright it looks," he told SPACE.com by email.

But the second star in the set could be a number of things. Simon likened the different pairings to light bulbs of varying wattage.

"You can't tell the difference between a 50-watt bulb nearby, a 60-watt bulb a bit further away, or a 100-watt bulb even farther away than that," Simon said.

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'Dog Days' of Summer End With Dog Star's Sky Return (SPACE.com)

The so-called "Dog Days" of summer in the Northern Hemisphere officially came to an end this week, when the Sirius, the bright Dog Star, returned the night sky.

Everyone talks about "Dog Days" but few may know what the expression actually means. Some might suggest it signifies hot, sultry days "not fit for a dog." (If during you live in Dallas or Oklahoma City this summer, you may probably more than agree with this particular definition.) Others, meanwhile, may say it's the weather in which dogs go mad.

But the actual Dog Days, or "Canicular" days as they're known, are defined as the period from July 3 through Aug. 11 when the Dog Star, Sirius, rises in conjunction (or nearly so) with the sun. As a result, the classical Greek and Roman belief was that the combination of the brightest luminary of the day (the sun) and the brightest star of night (Sirius) were responsible for the extreme heat that is experienced during the middle of the northern summer. Other effects, according to the ancients, were droughts, plagues and madness.

A more sensible view was put forward by the astronomer Geminus around 70 B.C. He wrote: "It is generally believed that Sirius produces the heat of the Dog Days, but this is an error, for the star merely marks a season of the year when the sun’s heat is the greatest." [The 9 Hottest Places on Earth]

The sky map of for the star Sirius here shows where it currently appears in the predawn sky.

Dog star's night sky legacy

In ancient Egypt, the New Year began with the return of Sirius. It was, in fact, the "Nile Star" or the "Star of Isis" of the early Egyptians. Interestingly, some 5,000 years ago, this star's "heliacal rising" (appearing to rise just prior to the sun) occurred not in August, as is the case today, but rather on or around June 25. 

When the ancient Egyptians saw Sirius rising just before the sun, they knew that the "Nile Days" were at hand. Its annual reappearance was a warning to people who lived along the Nile River. The star always returned just before the river rose, and so announced the coming of floodwaters, which would add to the fertility of their lands. People then opened the gates of canals that irrigated their fields. 

Priests, who were the calendar keepers, sighted the first rising of the Dog Star from their temples. At the temple of Isis-Hathor at Denderah is a statue of Isis, which is located at the end of an aisle lined by tall columns. A jewel was placed in the goddess’ forehead. [Skywatching Events for August 2011]

The statue was oriented to the rising of Sirius, so that the light from the returning Dog Star would fall upon the gem. When the priests saw the light of the star shining upon the gem for the first time, they would march from the temple and announce the New Year.  In the temple appears the inscription: "Her majesty Isis shines into the temple on New Year’s Day, and she mingles her light with that of her father Ra on the horizon."

The Dog Star returns this week

This week, just before sunrise, Sirius can again be glimpsed rising just above the southeast horizon for those living in mid-northern latitudes. At more southerly latitudes, Sirius is already conspicuous, twinkling above the horizon at dawn. 

Sirius is the brightest star of the constellation Canis Major, the "Greater Dog" in Latin. According to Burnham's Celestial Handbook other names for it include "The Sparkling One" or "The Scorching One."

The star appears a brilliant white with a tinge of blue, but when the air is unsteady, or when it is low to the horizon as it is now, it seems to flicker and splinter with all the colors of the rainbow. At a distance of just 8.7 light-years, Sirius is the fifth-nearest known star. Among the naked-eye stars, it is the nearest of all, with the sole exception of Alpha Centauri. 

So regardless of how hot your local weather is, or has been, this appearance of Sirius — a star we most associate with the winter season — now rising just ahead of the sun, is a subtle reminder that the hottest part of the year is now behind us and a promise that a change toward cooler weather is only weeks away. 

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.

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Exploded Star's Guts Shining Bright Again, Photo Shows (SPACE.com)

The glowing entrails of an exploding star, thought to have faded over time, now appear to be lighting up again, a new Hubble Space Telescope photo reveals.

NASA released the new Hubble image of the well-known star explosion, called Supernova 1987A, today (June 10). The photo shows the closest supernova explosion witnessed in almost 400 years. This has allowed astronomers to study it in unprecedented detail as the outburst evolves.

In the latest study of Supernova 1987A a team of astronomers announced that the debris from the explosion that had faded over the years is brightening. This suggests that the star explosion, which is located 165,000 light-years away in the Large Magellanic Cloud (a close neighbor of our own Milky Way galaxy),  is turning into a so-called supernova remnant. [See Hubble's new photo of Supernova 1987A]

The research is detailed in the June 9 edition of the journal Nature.

Supernovas typically transition into remnants when the exploded material starts to fade, but the brightness increases due to interactions between the debris cloud and surrounding gas. This cosmic shift is usually difficult for astronomers to study, but due to the relatively close proximity of the Large Magellanic Cloud, astronomers have been able to make detailed observations of Supernova 1987A periodically from 1994 to 2009.

"Supernova 1987A has become the youngest supernova remnant visible to us," said Robert Kirshner of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. "It's only possible to see this brightening because SN 1987A is so close and Hubble has such sharp vision."

Kirshner leads a long-term study of SN 1987A using the Hubble Space Telescope. Since its launch in 1990, Hubble has provided a continuous record of the changes in the supernova.

In the new Hubble image, SN 1987A is surrounded by a ring of material that blew off the star thousands of years before it exploded. The ring extends about one light-year (about 6 trillion miles or 9.5 trillion km) across. Inside that ring, the star's guts are rushing outward in an expanding debris cloud.

Most of the light from the supernova comes from radioactive decay of elements that were created in the explosion. This light fades over time, but the brightening of SN 1987A's debris suggests that a new power source is lighting it.

The debris of 1987A is beginning to impact the surrounding ring, which is creating powerful shock waves that produce X-rays that can be observed by NASA's Chandra X-ray Observatory. Those X-rays are illuminating the supernova debris and the heated shock waves are making it glow. This same process powers well-known supernova remnants in our own galaxy, like Cassiopeia A.

Furthermore, since SN 1987A is still young, astronomers can study the remnants of the explosion to decode its history.

Eventually, that history will be lost when the bulk of the expanding stellar debris impacts the surrounding ring and shreds it, researchers said. But, until then, SN 1987A offers astronomers the opportunity to watch as a supernova changes, they added.

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