subatomiconsciousness:

How/Why black holes emit X-rays

Many stars are observed to be in binary systems, where two stars are orbiting each other (as the Earth orbits the sun). Another thing to know is that, the more massive a star is the faster it uses up its nuclear fuel (mostly hydrogen); therefore the sooner it “dies”.

If we happen to have a binary star system, and the more massive of the two stars explodes as a supernova and it leaves behind a neutron star or a black hole, then it will result in a binary star system with a normal star and a compact object orbiting each other. All these things working out is rare, but there are over a billion stars in the galaxy, so even rare things happen fairly often.

Now, imagine that the “normal star” then runs out of its fuel. The first thing it will do is expand as it enters its “red giant phase”, as our Sun will about 4,000,000,000 years from now. Then, some of the star’s outer atmosphere will spill over onto the black hole. It will eventually fall in, and in the process become very hot. We can observe this hot gas with X-ray telescopes, so we call this an X-ray binary.

As far as the significance of the X-ray emission, it is to let us observe the effects of the black hole, and therefore learn something about it. Black holes do not emit light, in fact they are so dense that they trap it. Therefore, the best way to learn about them is by observing the material they affect. Observing X-rays from an X-ray binary is one effective way of doing this.

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How white dwarfs mimic black holes 

(white dwarfs : When a star like our sun gets to be very old, after another seven billion years or so, it will no longer be able to sustain burning its nuclear fuel. With only about half of the its mass remaining, it will shrink to a fraction of its radius and become a white dwarf star. read more about white dwarfs here)

The research by Professor Phil Charles, Professor Malcolm Coe and postgraduate student Liz Bartlett has appeared in The Astrophysical Journal that is devoted to recent developments, discoveries, and theories in astronomy and astrophysics.

The Southampton Physics and Astronomy team are part of a global collaboration - with colleagues in Taiwan, South Africa, Poland, Australia and Italy - that has revealed that bright X-ray flares in nearby galaxies, once assumed to indicate the presence of black holes, can in fact be produced by white dwarfs.

They made the discovery by detecting a dramatic, short-lived X-ray flare that was picked up by an X-ray telescope on the International Space Station. Using optical telescopes in South Africa and Chile, the Southampton astronomers showed that the flare, called XRF111111 as it happened on 11 November, 2011, was located in the Small Magellanic Cloud. These Magellanic Clouds are between 160,000 and 200,000 light years away and are the nearest satellite galaxies to the Milky Way.They are visible to the naked eye from the Southern Hemisphere.

The flare from XRF111111 was so luminous that astronomers initially thought it was likely to be a black hole producing X-rays but further research by Phil and his team revealed that its X-ray temperature was so low that it had to be a white dwarf instead.

[…]

“We think that this incredible X-ray flash was not due to accretion onto a black hole but was instead due to a nova explosion on a white dwarf that took place close to a hot massive star. This was something that we, as astronomers, have never seen before. “This surprising result shows that, in the right circumstances, white dwarfs are capable of mimicking black holes, the most luminous objects we know of.”

Read more 

picture : binary white dwarf stars credits to : Tod Strohmayer (GSFC), CXC, NASA, Illustration: Dana Berry (CXC)  

Can Living Planets Exist Around Dead Stars? A news release last week reported what may seem self-evident to most planet hunters: white dwarf stars are lousy places to go looking for inhabited worlds. However, we’ve learned that exoplanets are so eclectic, that we should never say never.

(via discoverynews)

Effects of Einstein’s Elusive Gravity Waves Observed

Image: Two white dwarfs similar to those in the system SDSS J065133.338+284423.37 spiral together in this illustration from NASA. Credit: D. Berry/NASA GSFC

Locked in a spiraling orbital embrace, the super-dense remains of two dead stars are giving astronomers the evidence needed to confirm one of Einstein’s predictions about the Universe.

A binary system located about 3,000 light-years away, SDSS J065133.338+284423.37 (J0651 for short) contains two white dwarfs orbiting each other rapidly — once every 12.75 minutes. The system was discovered in April 2011, and since then astronomers have had their eyes — and four separate telescopes in locations around the world — on it to see if gravitational effects first predicted by Einstein could be seen.

According to Einstein, space-time is a structure in itself, in which all cosmic objects — planets, stars, galaxies — reside. Every object with mass puts a “dent” in this structure in all dimensions; the more massive an object, the “deeper” the dent. Light energy travels in a straight line, but when it encounters these dents it can dip in and veer off-course, an effect we see from Earth as gravitational lensing.

Einstein also predicted that exceptionally massive, rapidly rotating objects — such as a white dwarf binary pair — would create outwardly-expanding ripples in space-time that would ultimately “steal” kinetic energy from the objects themselves. These gravitational waves would be very subtle, yet in theory, observable.

What researchers led by a team at The University of Texas at Austin have found is optical evidence of gravitational waves slowing down the stars in J0651. Originally observed in 2011 eclipsing each other (as seen from Earth) once every six minutes, the stars now eclipse six seconds sooner. This equates to a predicted orbital period reduction of about 0.25 milliseconds each year.*

“These compact stars are orbiting each other so closely that we have been able to observe the usually negligible influence of gravitational waves using a relatively simple camera on a 75-year-old telescope in just 13 months,” said study lead author J.J. Hermes, a graduate student at The University of Texas at Austin.

Based on these measurements, by April 2013 the stars will be eclipsing each other 20 seconds sooner than first observed. Eventually they will merge together entirely.

Although this isn’t “direct” observation of gravitational waves, it is evidence inferred by their predicted effects… akin to watching a floating lantern in a dark pond at night moving up and down and deducing that there are waves present.

“It’s exciting to confirm predictions Einstein made nearly a century ago by watching two stars bobbing in the wake caused by their sheer mass,” said Hermes.

As of early last year NASA and ESA had a proposed mission called LISA (Laser Interferometer Space Antenna) that would have put a series of 3 detectors into space 5 million km apart, connected by lasers. This arrangement of precision-positioned spacecraft could have detected any passing gravitational waves in the local space-time neighborhood, making direct observation possible. Sadly this mission was canceled due to FY2012 budget cuts for NASA, but ESA is moving ahead with developments for its own gravitational wave mission, called eLISA/NGO — the first “pathfinder” portion of which is slated to launch in 2014.

The study was submitted to Astrophysical Journal Letters on August 24. Read more on the McDonald Observatory news release here.

*The difference in the eclipse time is noted as six seconds even though the orbital period decay of the two stars is only .25 milliseconds/year because of a pile-up effect of all the eclipses observed since April 2011. The measurements made by the research team takes into consideration the phase change in the J0651 system, which experiences a piling effect — similar to an out-of-sync watch — that increases relative to time^2 and is therefore a larger and easier number to detect and work with. Once that was measured, the actual orbital period decay could be figured out.

futurist-foresight:

Astronomy: An interesting look at the stars in the observable universe.

astrodidact:

Our sun is so incredibly small in comparison to many of the other types of stars in the observable universe. If our sun were any larger and brighter, we would need to be s lot further away than 94 million miles that’s for sure!!

White Dwarf Stars Cool

Credit: H. Richer (UBC) et al., WFPC2, HST, NASA

Explanation: Diminutive by stellar standards, white dwarf stars are also intensely hot … but they are cooling. No longer do their interior nuclear fires burn, so they will continue to cool until they fade away. This Hubble Space Telescope image covers a small region near the center of a globular cluster known as M4. Here, researchers have discovered a large concentration of white dwarf stars (circled above). This was expected - low mass stars, including the Sun, are believed to evolve to the white dwarf stage. Studying how these stars cool could lead to a better understanding of their ages, of the age of their parent globular cluster, and even the age of our universe.

Will This Be The End Of The Earth?

Astronomers have found four nearby white dwarf stars surrounded by disks of material that could be the remains of rocky planets much like Earth — and one star in particular appears to be in the act of swallowing up what’s left of an Earthlike planet’s core.

The research, announced today by the Royal Astronomical Society, gives a chilling look at the eventual fate that may await our own planet.

Astronomers from the University of Warwick used Hubble to identify the composition of four white dwarfs’ atmospheres, found during a survey of over 80 such stars located within 100 light-years of the Sun. What they found was a majority of the material was composed of elements found in our own Solar System: oxygen, magnesium, silicon and iron. Together these elements make up 93% of our planet.

In addition, a curiously low ratio of carbon was identified, indicating that rocky planets were at one time in orbit around the stars.

Since white dwarfs are the leftover cores of stellar-mass stars that have burnt through all their fuel, the material in their atmosphere is likely the leftover bits of planets. Once held in safe, stable orbits, when their stars neared the ends of their lives they expanded, possibly engulfing the innermost planets and disrupting the orbits of others, triggering a runaway collision effect that eventually shattered them all, forming an orbiting cloud of debris.

This could very well be what happens to our Solar System in four or five billion years.

“What we are seeing today in these white dwarfs several hundred light years away could well be a snapshot of the very distant future of the Earth,” said Professor Boris Gänsicke of the Department of Physics at the University of Warwick, who led the study. ”During the transformation of the Sun into a white dwarf, it will lose a large amount of mass, and all the planets will move further out. This may destabilise the orbits and lead to collisions between planetary bodies as happened in the unstable early days of our solar systems.”

One of the white dwarfs studied, labeled PG0843+516, may even be actively eating the remains of an once-Earthlike world’s core.

The researchers identified an abundance of heavier elements like iron, nickel and sulphur in the atmosphere surrounding PG0843+516. These elements are found in the cores of terrestrial planets, having sunk into their interiors during the early stages of planetary formation. Finding them out in the open attests to the destruction of a rocky world like ours.

Of course, being heavier elements, they will be the first to be accreted  by their star.

“It is entirely feasible that in PG0843+516 we see the accretion of such fragments made from the core material of what was once a terrestrial exoplanet,” Prof. Gänsicke said.

It’s an eerie look into a distant future, when all of Earth could be just some elements detected in a cloud.

^Want the full report? Click on the source link :)

I can’t quite put into words how much this makes me giddy with amazement. The universe is constantly upgrading my degree of respect & bewilderment. Enjoy this & share it with everyone.

NASA’s Stereo Spots a New Nova

While on duty observing the Sun from its position in solar orbit, NASA’s STEREO-B spacecraft captured the sudden appearance of a distant bright object. This flare-up turned out to be a nova — designated Sagittarii 2012 — the violent expulsion of material and radiation from a re-igniting white dwarf star.

Unlike a supernova, which is the cataclysmic collapse and explosion of a massive star whose core has finally fused its last, a nova is the result of material falling onto the surface of a white dwarf that’s part of a binary pair. The material, typically hydrogen and helium gas, is drawn off the white dwarf’s partner which has expanded into a red giant.

Eventually the white dwarf cannot contain all of the material that it has sucked in from its neighbor… material which has been heated to tremendous temperatures on its surface as it got compressed further and further by the white dwarf’s incredibly strong gravity. Fusion occurs on the dwarf’s outermost layers, blasting its surface out into space in an explosion of light and energy.

This is a nova — so called because, when witnessed in the night sky, one could suddenly appear as a “new star” in the heavens — sometimes even outshining all other visible stars!

An individual nova will soon fade, but a white dwarf can produce many such flares over time. It all depends on how rapidly it’s accreting material (and how much there is available.)

Over the course of 4 days, Sagittarii 2012 reached a magnitude of about 8.5… still too dim to be seen with the unaided eye, but STEREO-B was able to detect it with its SECCHI (Sun Earth Connection Coronal and Heliospheric Investigation) instrument, which is sensitive to extreme ultraviolet wavelengths.

The video above was made from images acquired from April 20 – 24, 2012.

It’s not known yet how far away Sagittarii 2012 is but rest assured it poses no threat to Earth. The energy expelled by a nova is nowhere near that of a supernova, and although you wouldn’t want to have a front-row seat to such an event we’re well away from the danger zone.

What this does show is that STEREO-B is not only a super Sun-watching sentinel, but also very good at observing much more distant stars as well!

the-star-stuff:

White Dwarfs

A white dwarf is the remnant of an average-sized star that has passed through the red giant stage of its life. After the star has used up its remaining fuel. At this point the star may expel some of its matter into space, creating a planetary nebula. What remains is the dead core of the star. Nuclear fusion no longer takes place. The core glows because of its residual heat. Eventually the core will radiate all of its heat into space and cool down to become what is known as a black dwarf. White dwarf stars are very dense. Their size is about the same as that of the Earth, but the contain as much mass as the Sun. They are extremely hot, reaching temperatures of over 100,000 degrees.

Illustration By Gary’s TT

(via theuniverseishuge)