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spaceexp · 5 years ago

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Hubble Celebrates its 30th Anniversary with a Tapestry of Blazing Starbirth

ESA - Hubble Space Telescope logo. April 24, 2020

Tapestry of Blazing Starbirth

Hubble Space Telescope’s iconic images and scientific breakthroughs have redefined our view of the Universe. To commemorate three decades of scientific discoveries, this image is one of the most photogenic examples of the many turbulent stellar nurseries the telescope has observed during its 30-year lifetime. The portrait features the giant nebula NGC 2014 and its neighbour NGC 2020 which together form part of a vast star-forming region in the Large Magellanic Cloud, a satellite galaxy of the Milky Way, approximately 163 000 light-years away. The image is nicknamed the “Cosmic Reef” because it resembles an undersea world.

Wide-field view of NGC 2014 and NGC 2020 in the Large Magellanic Cloud (Ground-based Image)

On 24 April 1990 the Hubble Space Telescope was launched aboard the space shuttle Discovery, along with a five-astronaut crew. Deployed into low-Earth orbit a day later, the telescope has since opened a new eye onto the cosmos that has been transformative for our civilization.

Zooming Into the Cosmic Reef

Hubble is revolutionising modern astronomy not only for astronomers, but also by taking the public on a wondrous journey of exploration and discovery. Hubble’s seemingly never-ending, breathtaking celestial snapshots provide a visual shorthand for its exemplary scientific achievements. Unlike any other telescope before it, Hubble has made astronomy relevant, engaging, and accessible for people of all ages. The mission has yielded to date 1.4 million observations and provided data that astronomers around the world have used to write more than 17 000 peer-reviewed scientific publications, making it one of the most prolific space observatories in history. Its rich data archive alone will fuel future astronomy research for generations to come.

Pan Across the Cosmic Reef

Each year, the NASA/ESA Hubble Space Telescope dedicates a small portion of its precious observing time to taking a special anniversary image, showcasing particularly beautiful and meaningful objects. These images continue to challenge scientists with exciting new surprises and to fascinate the public with ever more evocative observations. This year, Hubble is celebrating this new milestone with a portrait of two colourful nebulae that reveals how energetic, massive stars sculpt their homes of gas and dust. Although NGC 2014 and NGC 2020 appear to be separate in this visible-light image, they are actually part of one giant star formation complex. The star-forming regions seen here are dominated by the glow of stars at least 10 times more massive than our Sun. These stars have short lives of only a few million years, compared to the 10-billion-year lifetime of our Sun.

Cosmic Reef for Fulldome

The sparkling centerpiece of NGC 2014 is a grouping of bright, hefty stars near the centre of the image that has blown away its cocoon of hydrogen gas (coloured red) and dust in which it was born. A torrent of ultraviolet radiation from the star cluster is illuminating the landscape around it. These massive stars also unleash fierce winds that are eroding the gas cloud above and to the right of them. The gas in these areas is less dense, making it easier for the stellar winds to blast through them, creating bubble-like structures reminiscent of brain coral, that have earned the nebula the nickname the “Brain Coral.” By contrast, the blue-coloured nebula below NGC 2014 has been shaped by one mammoth star that is roughly 200 000 times more luminous than our Sun. It is an example of a rare class of stars called Wolf-Rayet stars. They are thought to be the descendants of the most massive stars. Wolf-Rayet stars are very luminous and have a high rate of mass loss through powerful winds. The star in the Hubble image is 15 times more massive than the Sun and is unleashing powerful winds, which have cleared out the area around it. It has ejected its outer layers of gas, sweeping them around into a cone-like shape, and exposing its searing hot core. The behemoth appears offset from the centre because the telescope is viewing the cone from a slightly tilted angle. In a few million years, the star might become a supernova. The brilliant blue colour of the nebula comes from oxygen gas that is heated to roughly 11 000 degrees Celsius, which is much hotter than the hydrogen gas surrounding it.

3D Animation of the Cosmic Reef

Stars, both big and small, are born when clouds of dust and gas collapse because of gravity. As more and more material falls onto the forming star, it finally becomes hot and dense enough at its centre to trigger the nuclear fusion reactions that make stars, including our Sun, shine. Massive stars make up only a few percent of the billions of stars in our Universe. Yet they play a crucial role in shaping our Universe, through stellar winds, supernova explosions, and the production of heavy elements.

Hubble Space Telescope (HST)

“The Hubble Space Telescope has shaped the imagination of truly a whole generation, inspiring not only scientists, but almost everybody,” said Günther Hasinger, Director of Science for the European Space Agency. “It is paramount for the excellent and long-lasting cooperation between NASA and ESA.” More information: The Hubble Space Telescope is a project of international cooperation between ESA and NASA. This image was taken with the Telescope’s Wide Field Camera 3. Links: Hubblecast 128: 30 Years of Science with the Hubble Space Telescope: https://www.spacetelescope.org/videos/heic2007a/ Hubblecast 129: Hubble’s Collection of Anniversary Images: https://www.spacetelescope.org/videos/heic2007b/ Hubble 30th Anniversary Press Package: http://www.spacetelescope.org/static/archives/releases/pdf/heic2007a-ESA-press-packet30th.pdf Hubble30 Webpage: https://www.spacetelescope.org/Hubble30/ Call for Happy Birthday Wishes: Make Hubble a Birthday Cake!: https://www.spacetelescope.org/announcements/ann2005/ Call for Artistic Creations: Let’s Say Thank-You to Hubble!: https://www.spacetelescope.org/announcements/ann2007/ HubbleSite release: https://hubblesite.org/contents/news-releases/2020/news-2020-16 Link to Space Scoop: http://www.spacescoop.org/en/scoops/2016/happy-birthday-hubble/ Images of Hubble: https://www.spacetelescope.org/images/archive/category/spacecraft/ Images, Animation, Text Credits: NASA, ESA/STScI/ESA/Hubble/Bethany Downer/Digitized Sky Survey 2. Acknowledgement: Davide De Martin/Videos: ESA/Hubble, NASA, STScI/Digitized Sky Survey 2, L. Calçada. Music: Astral Electronic/NASA, ESA, G. Bacon, J. DePasquale, L. Hustak, J. Olmstead, A. Pagan, D. Player, and F. Summers (STScI). Music: "Cosmic Reef" by J. DePasquale (STScI). Greetings, Orbiter.ch Full article

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wonders-of-the-cosmos · 7 years ago

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Blue supergiant star

Blue supergiant stars are hot luminous stars, referred to scientifically as OB supergiants. They have luminosity class I and spectral class B9 or earlier.

Blue supergiants (BSGs) are found towards the top left of the Hertzsprung–Russell diagram to the right of the main sequence. They are larger than the Sun but smaller than a red supergiant, with surface temperatures of 10,000–50,000 K and luminosities from about 10,000 to a million times the Sun.

Formation

Supergiants are evolved high-mass stars, larger and more luminous than main-sequence stars. O class and early B class stars with initial masses around 10-100 M☉ evolve away from the main sequence in just a few million years as their hydrogen is consumed and heavy elements start to appear near the surface of the star. These stars usually become blue supergiants, although it is possible that some of them evolve directly to Wolf–Rayet stars.

Expansion into the supergiant stage occurs when hydrogen in the core of the star is depleted and hydrogen shell burning starts, but it may also be caused as heavy elements are dredged up to the surface by convection and mass loss due to radiation pressure increase.

Blue supergiants are newly evolved from the main sequence, have extremely high luminosities, high mass loss rates, and are generally unstable. Many of them become luminous blue variables (LBVs) with episodes of extreme mass loss. Lower mass blue supergiants continue to expand until they become red supergiants. In the process they obviously must spend some time as yellow supergiants or yellow hypergiants, but this expansion occurs in just a few thousand years and so these stars are rare. Higher mass red supergiants blow away their outer atmospheres and evolve back to blue supergiants, and possibly onwards to Wolf–Rayet stars. Depending on the exact mass and composition of a red supergiant, it can execute a number of blue loops before either exploding as a type II supernova or finally dumping enough of its outer layers to become a blue supergiant again, less luminous than the first time but more unstable. If such a star can pass through the yellow evolutionary void it is expected that it becomes one of the lower luminosity LBVs.

The most massive blue supergiants are too luminous to retain an extensive atmosphere and they never expand into a red supergiant. The dividing line is approximately 40 M☉, although the coolest and largest red supergiants develop from stars with initial masses of 15-25 M☉. It isn't clear whether more massive blue supergiants can lose enough mass to evolve safely into a comfortable old age as a Wolf Rayet star and finally a white dwarf, or they reach the Wolf Rayet stage and explode as supernovae, or they explode as supernovae while blue supergiants.

Properties

Because of their extreme masses they have relatively short lifespans and are mainly observed in young cosmic structures such as open clusters, the arms of spiral galaxies, and in irregular galaxies. They are rarely observed in spiral galaxy cores, elliptical galaxies, or globular clusters, most of which are believed to be composed of older stars, although the core of the Milky Way has recently been found to be home to several massive open clusters and associated young hot stars.

The best known example is Rigel, the brightest star in the constellation of Orion. Its mass is about 20 times that of the Sun, and its luminosity is around 117,000 times greater. Despite their rarity and their short lives they are heavily represented among the stars visible to the naked eye; their immense brightness is more than enough to compensate for their scarcity.

Blue supergiants have fast stellar winds and the most luminous, called hypergiants, have spectra dominated by emission lines that indicate strong continuum driven mass loss. Blue supergiants show varying quantities of heavy elements in their spectra, depending on their age and the efficiency with which the products of nucleosynthesis in the core are convected up to the surface. Quickly rotating supergiants can be highly mixed and show high proportions of helium and even heavier elements while still burning hydrogen at the core, and these stars show spectra very similar to a Wolf Rayet star.

While the stellar wind from a red supergiant is dense and slow, the wind from a blue supergiant is fast but sparse. When a red supergiant becomes a blue supergiant, the faster wind it produces impacts the already emitted slow wind and causes the outflowing material to condense into a thin shell. In some cases several concentric faint shells can be seen from successive episodes of mass loss, either previous blue loops from the red supergiant stage, or eruptions such as LBV outbursts.

Astronomers suggest that blue supergiant stars may be the most likely sources of ultra-long GRBs. These stars hold about 20 times the sun's mass and may reach sizes 1,000 times larger than the sun, making them nearly wide enough to span Jupiter's orbit.

Source

Rigel

Fourth image: Hubble Space Telescope image of nebula M1-67 around Wolf–Rayet star WR 124. (source).

Fifth image: Rigel and the IC 2118 nebula which it illuminates. (source).

Sixth image: Spectrum of a B2 star. (souce).

Seventh image: Star cluster NGC 3572 and its surroundings. (source).

Eighth image: Orion constellation (The star Rigel is at the top right of the image. by: Joseph Brimacombe)

images of star comparisons. (nasa).

#bluesupergiantstar #star #estrela #space #espaço #astrophysics #astrofisica #astronomy #astronom #universe #universo #rigel #hertzsprung-husselldiagram

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sciencespies · 5 years ago

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The mass inflow and outflow rates of the Milky Way

https://sciencespies.com/space/the-mass-inflow-and-outflow-rates-of-the-milky-way/

The mass inflow and outflow rates of the Milky Way

This illustration shows a messy, chaotic galaxy undergoing bursts of star formation. Credit: ESA, NASA, L. Calçada

According to the most widely accepted cosmological models, the first galaxies began to form between 13 and 14 billion years ago. Over the course of the next billion years, the cosmic structures now observed first emerged. These include things like galaxy clusters, superclusters and filaments, but also galactic features like globular clusters, galactic bulges, and supermassive black holes (SMBHs).

However, like living organisms, galaxies have continued to evolve ever since. In fact, over the course of their lifetimes, galaxies accrete and eject mass all the time. In a recent study, an international team of astronomers calculated the rate of inflow and outflow of material for the Milky Way. Then the good folks at Astrobites gave it a good breakdown and showed just how relevant it is to our understanding of galactic formation and evolution.

The study was led by ESA astronomer Dr. Andrew J. Fox and included members from the Space Telescope Science Institute’s (STScI) Milky Way Halo Research Group, the ESA’s Association of Universities for Research in Astronomy (AURA), and multiple universities. Based on previous studies, they examined the rate at which gas flows in and out of the Milky Way from surrounding high-velocity clouds (HVC).

Since the availability of material is key to star formation in a galaxy, knowing the rate at which it is added and lost is important to understanding how galaxies evolve over time. And as Michael Foley of Astrobites summarized, characterizing the rates at which material is added to galaxies is crucial to understanding the details of this “galactic fountain” model.

In accordance with this model, the most massive stars in a galaxy produce stellar winds that drive material out of the galaxy disk. When they go supernova near the end of their lifespans, they similarly drive most of their material out. This material then infalls back into the disk over time, providing material for new stars to form.

“These processes are collectively known as stellar feedback, and they are responsible for pushing gas back out of the Milky Way,” said Foley. “In other words, the Milky Way is not an isolated lake of material; it is a reservoir that is constantly gaining and losing gas due to gravity and stellar feedback.”

In addition, recent studies have shown that star formation may be closely related to the size of the supermassive black hole (SMBH) at a galaxy’s core. Basically, SMBHs put out a tremendous amount of energy that can heat gas and dust surrounding the core, which prevents it from clumping effectively and undergoing gravitational collapse to form new stars.

Artist’s view of the Milky Way with the location of the Sun and the star-forming region at the opposite side in the Scutum-Centaurus spiral arm. Credit: Bill Saxton, NRAO/AUI/NSF; Robert Hurt, NASA

As such, the rate at which material flows in and out of a galaxy is key to determining the rate of star formation. To calculate the rate at which this happens for the Milky Way, Dr. Fox and his colleagues consulted data from multiple sources. Dr. Fox told Universe Today via email:

“We mined the archive. NASA and ESA maintain well-curated archives of all Hubble Space Telescope data, and we went through all the observations of background quasars taken with the Cosmic Origins Spectrograph (COS), a sensitive spectrograph on Hubble that can be used to analyze the ultraviolet light from distant sources. We found 270 such quasars. First, we used these observations to make a catalog of fast-moving gas clouds known as high-velocity clouds (HVCs). Then we devised a method for splitting the HVCs into inflowing and outflowing populations by making use of the Doppler shift.”

In addition, a recent study showed that the Milky Way experienced a dormant period roughly 7 billion years ago, which lasted for about 2 billion years. This was the result of shock waves that caused interstellar gas clouds to become heated, which temporarily caused the flow of cold gas into our galaxy to stop. Over time, the gas cooled and began flowing in again, triggering a second round of star formation.

After looking at all the data, Fox and his colleagues were able to place constraints on the rate of inflow and outflow for the Milky Way:

“After comparing the rates of inflowing and outflowing gas, we found an excess of inflow, which is good news for future star formation in our galaxy, since there is plenty of gas that can be converted into stars and planets. We measured about 0.5 solar masses per year of inflow and 0.16 solar masses per year of outflow, so there’s a net inflow.”

However, as Foley indicated, HVCs are believed to live for periods of only about 100 million years or so. As a result, this net inflow cannot be expected to last indefinitely. “Finally, they ignore HVCs that are known to reside in structures (such as the Fermi Bubbles) that don’t trace the inflowing or outflowing gas,” he adds.

Since 2010, astronomers have been aware of the mysterious structures emerging from the center of our galaxy known as Fermi Bubbles. These bubble-like structures extend for thousands of light-years and are thought to be the result of SMBH’s consuming interstellar gas and belching out gamma rays.

Artist’s impression of the “Fermi Bubbles” around the Milky Way. Credit: NASA’s Goddard Space Flight Center

However, in the meantime, the results provide new insight into how galaxies form and evolve. The study also bolsters the new case made for “cold flow accretion,” a theory originally proposed by Prof. Avishai Dekel and colleagues from the Hebrew University of Jerusalem’s Racah Institute of Physics to explain how galaxies accrete gas from surrounding space during their formation.

“These results show that galaxies like the Milky Way do not evolve in a steady state,” Dr. Fox summarized. “Instead they accrete and lose gas episodically. It’s a boom and bust cycle: When gas comes in, more stars can be formed, but if too much gas comes in, it can trigger a starburst so intense that it blows away all the remaining gas, shutting off the star formation. Thus, the balance between inflow and outflow regulates how much star formation occurs. Our new results help to illuminate this process.”

Another interesting takeaway from this study is the fact that what applies to our Milky Way also applies to star systems. For instance, our solar system is also subject to the inflow and outflow of material over time. Objects like “Oumuamua and the more recent 2I/Borisov confirm that asteroids and comets are kicked out of star systems and scooped up by others regularly.

But what about gas and dust? Is our solar system and (by extension) planet Earth losing or gaining weight over time? And what could this mean for the future of our system and home planet? For example, astrophysicist and author Brian Koberlein addressed the latter issue in 2015 on his website. Using the then-recent Gemini meteor shower as an example, he wrote:

“In fact, from satellite observations of meteor trails, it’s estimated that about 100-300 metric tons (tonnes) of material strikes Earth every day. That adds up to about 30,000 to 100,000 tonnes per year. That might seem like a lot, but over a million years, that would only amount to less than a billionth of a percent of Earth’s total mass.”

However, as he goes on to explain, Earth also loses mass on a regular basis through a number of processes. These include radioactive decay of material in the Earth’s crust, which leads to energy and subatomic particles (alpha, beta and gamma-rays) leaving our planet. A second is atmospheric loss, in which gases like hydrogen and helium are lost to space. Together, these add up to a loss of about 110,000 tonnes per year.

On the surface, this would seem like a net loss of about 10,000 or more tonnes annually. What’s more, microbiologist/science communicator Dr. Chris Smith and Cambridge physicist Dave Ansell estimated in 2012 that the Earth gains 40,000 tonnes of dust a year from space, while it loses 90,000 a year through atmospheric and other processes.

Data gathered from 1994-2013 on small asteroids impacting Earth’s atmosphere and disintegrating to create very bright meteors, called bolides. Credit: NASA

So it may be possible that Earth is getting lighter at a rate of 10,000 to 50,000 tonnes a year. However, the rate at which material is being added is not well constrained at this point, so it is possible that we might be breaking even (though the possibility that Earth is gaining mass seems unlikely). As for our solar system, the situation is similar. On the one hand, interstellar gas and dust flows in all the time.

On the other hand, our sun—which accounts for 99.86 percent of the solar system’s mass—is also shedding mass over time. Using data gathered by NASA’s MESSENGER probe, a team of NASA and MIT researchers concluded that the sun is losing mass due to solar wind and interior processes. According to Ask an Astronomer, this is happening at a rate of 1.3245 x 1015 tonnes a year, even though the sun is expanding simultaneously.

That’s a staggering number, but the sun has a mass of about 1.9885×1027 tonnes. So it won’t be winking out anytime soon. But as it loses mass, its gravitational influence on Earth and the other planets will diminish. However, by the time our sun reaches the end of its main sequence, it will expand considerably and could very well swallow Mercury, Venus, Earth and even Mars completely.

So while our galaxy may be gaining mass for the foreseeable future, it looks like our sun and Earth itself are slowly losing mass. This should not be seen as bad news, but it does have implications in the long run. In the meantime, it’s kind of encouraging to know that even the oldest and most massive objects in the universe are subject to change like living creatures.

Whether we’re talking about planets, stars, or galaxies, they are born, they live and they die. And in between, they can be trusted to put on or lose a few pounds. The circle of life, played out on the cosmic scale.

Join us on Facebook or Twitter for a regular update.

Explore further

Image: Hubble spots a swarm of stars

More information: Andrew J. Fox, et al. The Mass Inflow and Outflow Rates of the Milky Way. arXiv:1909.05561v1 [astro-ph.GA]: arxiv.org/abs/1909.05561

Source Universe Today

Citation: The mass inflow and outflow rates of the Milky Way (2019, October 4) retrieved 4 October 2019 from https://phys.org/news/2019-10-mass-inflow-outflow-milky.html

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sciencebulletin · 5 years ago

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Innovative model provides insight into the behavior of the black hole at the center of our galaxy

Like most galaxies, the Milky Way hosts a supermassive black hole at its center. Called Sagittarius A*, the object has captured astronomers' curiosity for decades. And now there is an effort to image it directly. Catching a good photo of the celestial beast will require a better understanding of what's going on around it, which has proved challenging due to the vastly different scales involved. "That's the biggest thing we had to overcome," said Sean Ressler, a postdoctoral researcher at UC Santa Barbara's Kavli Institute for Theoretical Physics (KITP), who just published a paper in the Astrophysical Journal Letters, investigating the magnetic properties of the accretion disk surrounding Sagittarius A*. In the study, Ressler, fellow KITP postdoc Chris White and their colleagues, Eliot Quataert of UC Berkeley and James Stone at the Institute for Advanced Study, sought to determine whether the black hole's magnetic field, which is generated by in-falling matter, can build up to the point where it briefly chokes off this flow, a condition scientists call magnetically arrested. Answering this would require simulating the system all the way out to the closest orbiting stars. The system in question spans seven orders of magnitude. The black hole's event horizon, or envelope of no return, reaches around 4 to 8 million miles from its center. Meanwhile, the stars orbit around 20 trillion miles away, or about as far as the sun's nearest neighboring star. "So you have to track the matter falling in from this very large scale all the way down to this very small scale," said Ressler. "And doing that in a single simulation is incredibly challenging, to the point that it's impossible." The smallest events proceed on timescales of seconds while the largest phenomena play out over thousands of years. This paper connects small scale simulations, which are mostly theory-based, with large-scale simulations that can be constrained by actual observations. To achieve this, Ressler divided the task between models at three overlapping scales. The first simulation relied on data from Sagittarius A*'s surrounding stars. Fortunately, the black hole's activity is dominated by just 30 or so Wolf-Rayet stars, which blow off tremendous amounts of material. "The mass loss from just one of the stars is larger than the total amount of stuff falling into the black hole during the same time," Ressler said. The stars spend only around 100,000 years in this dynamic phase before transitioning into a more stable stage of life. Using observational data, Ressler simulated the orbits of these stars over the course of about a thousand years. He then used the results as the starting point for a simulation of medium-range distances, which evolve over shorter time scales. He repeated this for a simulation down to the very edge of the event horizon, where activity takes place in matters of seconds. Rather than stitching together hard overlaps, this approach allowed Ressler to fade the results of the three simulations into one another. "These are really the first models of the accretion at the smallest scales in A* that take into account the reality of the supply of matter coming from orbiting stars," said coauthor White. And the technique worked splendidly. "It went beyond my expectations," Ressler remarked. The results indicated that Sagittarius A* can become magnetically arrested. This came as a surprise to the team, since the Milky Way has a relatively quiet galactic center. Usually, magnetically arrested black holes have high-energy jets shooting particles away at relativistic speeds. But so far scientists have seen little evidence for jets around Sagittarius A*. "The other ingredient that helps create jets is a rapidly spinning black hole," said White, "so this may be telling us something about the spin of Sagittarius A*." Unfortunately, black hole spin is difficult to determine. Ressler modeled Sagittarius A* as a stationary object. "We don't know anything about the spin," he said. "There's a possibility that it's actually just not spinning." Ressler and White next plan to model a spinning back hole, which is much more challenging. It immediately introduces a host of new variables, including spin rate, direction and tilt relative to the accretion disc. They will use data from the European Southern Observatory's GRAVITY interferometer to guide these decisions. The team used the simulations to create images that can be compared to actual observations of the black hole. Scientists at the Event Horizon Telescope collaboration—which made headlines in April 2019 with the first direct image of a black hole—have already reached out requesting the simulation data in order to supplement their effort to photograph Sagittarius A*. The Event Horizon Telescope effectively takes a time average of its observations, which results in a blurry image. This was less of an issue when the observatory had their sights on Messier 87*, because it is around 1,000 times larger than Sagittarius A*, so it changes around 1,000 times more slowly. "It's like taking a picture of a sloth versus taking a picture of a hummingbird," Ressler explained. Their current and future results should help the consortium interpret their data on our own galactic center. Ressler's results are a big step forward in our understanding of the activity at the center of the Milky Way. "This is the first time that Sagittarius A* has been modeled over such a large range in radii in 3-D simulations, and the first event horizon-scale simulations to employ direct observations of the Wolf-Rayet stars," Ressler said. Provided by: University of California - Santa Barbara More information: Sean M. Ressler et al. Ab Initio Horizon-scale Simulations of Magnetically Arrested Accretion in Sagittarius A* Fed by Stellar Winds. The Astrophysical Journal (2020). DOI: 10.3847/2041-8213/ab9532 Image: Sagittarius A*. This image was taken with NASA's Chandra X-Ray Observatory. Credit: Public domain Read the full article

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mitchbattros · 7 years ago

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Astronomers Blown Away By Historic Stellar Blast

Imagine traveling to the Moon in just 20 seconds! That's how fast material from a 170 year old stellar eruption sped away from the unstable, eruptive, and extremely massive star Eta Carinae.

Astronomers conclude that this is the fastest jettisoned gas ever measured from a stellar outburst that didn't result in the complete annihilation of the star. The blast, from the most luminous star known in our galaxy, released almost as much energy as a typical supernova explosion that would have left behind a stellar corpse. However, in this case a double-star system remained and played a critical role in the circumstances that led to the colossal blast. Over the past seven years a team of astronomers led by Nathan Smith, of the University of Arizona, and Armin Rest, of the Space Telescope Science Institute, determined the extent of this extreme stellar blast by observing light echoes from Eta Carinae and its surroundings. Light echos occur when the light from bright, short-lived events are reflected off of clouds of dust, which act like distant mirrors redirecting light in our direction. Like an audio echo, the arriving signal of the reflected light has a time delay after the original event due to the finite speed of light. In the case of Eta Carinae, the bright event was a major eruption of the star that expelled a huge amount of mass back in the mid-1800s during what is known as the "Great Eruption." The delayed signal of these light echoes allowed astronomers to decode the light from the eruption with modern astronomical telescopes and instruments, even though the original eruption was seen from Earth back in the mid-19th century. That was a time before modern tools like the astronomical spectrograph were invented. "A light echo is the next best thing to time travel," Smith said. "That's why light echoes are so beautiful. They give us a chance to unravel the mysteries of a rare stellar eruption that was witnessed 170 years ago, but using our modern telescopes and cameras. We can also compare that information about the event itself with the 170-year old remnant nebula that was ejected. This was a behemoth stellar explosion from a very rare monster star, the likes of which has not happened since in our Milky Way Galaxy." The Great Eruption temporarily promoted Eta Carinae to the second brightest star visible in our nighttime sky, vasty outshining the energy output every other star in the Milky Way, after which the star faded from naked eye visibility. The outburst expelled material (about 10 times more than the mass of our Sun) that also formed the bright glowing gas cloud known as the Homunculus. This dumbbell-shaped remnant is visible surrounding the star from within a vast star-forming region. The eruptive remnant can even be seen in small amateur telescopes from the Earth's Southern Hemisphere and equatorial regions, but is best seen in images obtained with the Hubble Space Telescope. The team used instruments on the 8-meter Gemini South telescope, Cerro Tololo Inter-American Observatory 4-meter Blanco telescope, and the Magellan Telescope at Las Campanas Observatory to decode the light from these light echoes and to understand the expansion speeds in the historical explosion. "Gemini spectroscopy helped pin down the unprecedented velocities we observed in this gas, which clocked in at between about 10,000 to 20,000 kilometers per second," according to Rest. The research team, Gemini Observatory, and Blanco telescope are all supported by the U.S. National Science Foundation (NSF). "We see these really high velocities all the time in supernova explosions where the star is obliterated." Smith notes. However, in this case the star survived, and explaining that led the researchers into new territory. "Something must have dumped a lot of energy into the star in a short amount of time," said Smith. The material expelled by Eta Carinae is travelling up to 20 times faster than expected for typical winds from a massive star so, according to Smith and his collaborators, enlisting the help of two partner stars might explain the extreme outflow. The researchers suggest that the most straightforward way to simultaneously explain a wide range of observed facts surrounding the eruption and the remnant star system seen today is with an interaction of three stars, including a dramatic event where two of the three stars merged into one monster star. If that's the case, then the present-day binary system must have started out as triple system, with one of those two stars being the one that swallowed its sibling. "Understanding the dynamics and environment around the largest stars in our galaxy is one of the most difficult areas of astronomy," said Richard Green, Director of the Division of Astronomical Sciences at NSF, the major funding agency for Gemini. "Very massive stars live short lives compared to stars like our Sun, but nevertheless catching one in the act of a major evolutionary step is statistically unlikely. That's why a case like Eta Carinae is so critical, and why NSF supports this kind of research." Chris Smith, Head of Mission at the AURA Observatory in Chile and also part of the research team adds a historical perspective. "I'm thrilled that we can see light echoes coming from an event that John Herschel observed in the middle of the 19th century from South Africa," he said. "Now, over 150 years later we can look back in time, thanks to these light echoes, and unveil the secrets of this supernova wannabe using the modern instrumentation on Gemini to analyze the light in ways Hershel couldn't have even imagined!" Eta Carinae is an unstable type of star known as a Luminous Blue Variable (LBV), located about 7,500 light years from Earth in a young star forming nebula found in the southern constellation of Carinae. The star is one of the intrinsically brightest in our galaxy and shines some five million times brighter than our Sun with a mass about one hundred times greater. Stars like Eta Carinae have the greatest mass-loss rates prior to undergoing supernova explosions, but the amount of mass expelled in Eta Carinae's 19th century Great Eruption exceeds any others known. Eta Carinae will probably undergo a true supernova explosion sometime within the next half-million years at most, but possibly much sooner. Some types of supernovae have been seen to experience eruptive blasts like that of Eta Carinae in only the few years or decades before their final explosion, so some astronomers speculate that Eta Carinae might blow sooner rather than later. The Gemini Observations utilized the Gemini Multi-Object Spectrograph on the Gemini South telescope in Chile and used a powerful technique called Nod and Shuffle that enables greatly improved spectroscopic measurements of extremely faint sources by reducing the contaminating effects of the night sky. The new results are presented in two papers accepted for publication in the Monthly Notices of the Royal Astronomical Society. Read the full article

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castastro-blog · 8 years ago

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WR 30a is a massive spectroscopic binary in the constellation Carina. The primary is an extremely rare star on the WO oxygen sequence. WR 30a was discovered in a photographic survey in the constellation Carina using the Curtis-Schmidt Telescope at the Cerro Tololo Inter-American Observatory. It was listed as MS4 out of nine new discoveries, classified only as "WR::". WR 30a was entered into the sixth catalogue of galactic WR stars at the last minute with the designation WR 29a and a spectral class of "WR + ABS". A review of Wolf-Rayet stars in 1984 reported that WR30a had a right ascension greater than WR 30 and should correctly be numbered 30a rather than 29a. The name was corrected in the seventh edition of the catalogue. Still in 1984, WR 30a was studied spectroscopically and assigned a WC4 class. Another 1984 study noted dilution of some emission lines, and suggested the presence of a binary companion of approximate spectral type O4. The WO spectral classification had already been defined, but neither paper considered WR 30a to show sufficiently high excitation lines or strong oxygen lines to merit that classification. A WO spectral class was eventually assigned, with relatively weak Oviemission but confirmed by the lack of Ciiiemission. A WO5 class was temporarily assigned to account for the unusually low excitation, but it was confirmed at WO4 when quantitative criteria for the WO sub-classes were defined. The identification of the companion remained only as an approximate O4 until 2001, when detailed spectroscopy assigned an O5((f)) class. This is based on the existence of narrow Niii emission lines at 463.4 - 464.1 nm, and the identification of strong Heii absorption at 468.6 nm. The luminosity class could not be determined with certainty, but a supergiant can be ruled out and the line widths suggest a giant class is most likely. WR 30a is a close spectrocopic binary containing a WO4 star and a non-supergiant O5 star. They orbit each other every 4.916 days. Although spectral lines from both stars can be detected and orbital radial velocity variations measured, the orbit is still poorly known. The primary has highly broadened emission lines which are difficult to measure accurately, and the secondary has a relatively low orbital speed due to its high mass. Measurements of different spectral lines and different portions of line profiles lead to different results. Some components of the spectrum are produced by stellar winds not moving at orbital velocity with the stars. The stars do not eclipse each other, but they are deformed by the gravity and show small brightness variations during the orbit. These brightness variations are regular and consistent over long periods, so the orbital period is known accurately. The inclination can be estimated from the mass function and the colliding winds. The eccentricity is small and the most accurate model of spectral line profile variations during the orbit gives an eccentricity of 0.2. The semi-major axis of the orbit is 35.4 R☉, with the WO star moving in an ellipse of semi-major axis 30 R☉ and the more massive O companion in an ellipse of semi-major axis 5.4 R☉. The separation of the stars varies from 28 R☉ to 42 R☉. Although the hot secondary star produces what would typically be considered a fast stellar wind, it is entirely overpowered by the wind from the primary star. The shock front where the winds collide is approximately a cone around the O star with a half angle of 50°. The apex of the shock cone is estimated to lie at 25 R☉ from the WO stars and 10 R☉from the O star. 10 R☉ is comparable to the radius of a typical non-supergiant O5 star so that its own wind is forced back against the surface of the star. WR30a is one of the very few known oxygen-sequence Wolf-Rayet stars, just four in the Milky Way galaxy and five in external galaxies. Modelling the atmosphere gives a luminosity around 195,000 L☉. It is a very small dense star, with a radius less than the sun's but with a mass nearly 10 solar masses. Very strong stellar winds, with a terminal velocity of 4,500 kilometers per second are causing WR 30a to lose over 10−5 M☉/year. For comparison, the Sun loses (2-3) x 10−14 solar masses per year due to its solar wind, several hundred million times less than WR 30a. The secondary star has an O5 spectral class. It is not a supergiant, but could be a giant star. Some helium lines and nitrogen emission is detected in the spectrum, indicating the mixing of fusion products to the surface and a strong stellar wind. The secondary star is visually over 10 times brighter than the primary and over five times more massive, although the primary dominates the appearance of the spectrum. Researchers are careful to avoid ambiguity about the star defined as the primary and typically refer to the components as "WR" and "O". WR 30a is a very strong x-ray source. This is expected for a colliding-wind binary, but the source of the x-rays has not been conclusively determined. They may have a thermal or non-thermal origin. WO Wolf-Rayet stars are the last evolutionary stage of the most massive stars before exploding as supernovae, possibly with a gamma-ray burst. It is very likely that WR 30a is on its last stages of nuclear fusion, near or beyond the end of the helium burning stage. Single-star evolutionary models of the WO component of WR 30a suggest it started life as a rapidly rotating 120 M☉ star which has now lost over 90% of its mass. We present an SDSS i image with partial spectra & lightcurves of the rare oxygen rich (WO) Wolf-Rayet class. The WO class is a very brief evolutionary period for the most massive stars before reaching the SN stage. They possess extremely hot temperatures in excess of 100,000 K and mass-loss rates typical of 1 M⊙ every 10^5 years with the fastest wind speeds of any star at rates of 4,500 km/s. Of the 300+ known Wolf-Rayets in our galaxy and the Magellanic Clouds, only 5 belong to the WO class, while more may exist, they lay behind optically thick nebulae and along the galactic plane which makes it difficult to detect them.

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