White dwarf star

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We just got our first complete look at an exploding white dwarf star

For the first time ever, scientists have captured the full process of a nova explosion — the moments before, during and after the blast.

Nova explosions are different from supernovas because the star isn’t completely destroyed. Novas happen in two-star systems, when a white dwarf star has been sucking in mostly hydrogen gas from a close neighboring star. The extra hydrogen explodes, but the explosion only happens on the star’s surface. Apparently, white dwarfs have something in common with bears.

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Zombie white dwarf star caught destroying an orbiting planet    

More than 570 light years away in the constellation Virgo, a disintegrating planet orbits around a white dwarf — the leftovers of a yellow star after it died. The cause of the planet’s demise is the zombie star itself; the white dwarf is extremely dense, and its enormous gravitational pull is tearing the rock apart, creating an enormous cloud of dust and debris that follows the planet on its orbit.

This dance of the dead was observed by NASA’s Kepler spacecraft last year. The discovery of the planetary system is the first of its kind. It helps to confirm what many scientists have suspected for years: that planets can actually orbit white dwarfs.

— Loren Grush for The Verge

Ancient White Dwarfs in the Milky Way

A white dwarf, also called a degenerate dwarf, is a stellar remnant composed mostly of electron-degenerate matter. They are very dense; a white dwarf’s mass is comparable to that of the Sun, and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored thermal energy. White dwarfs are thought to be the final evolutionary state of all stars whose mass is not high enough to become a neutron star. Over 97% of the stars in the Milky Way will eventually become white dwarfs.

Credit: NASA/Hubble

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White Dwarfs and Neutron Stars

What happens to a star after it dies depends entirely on the mass it contains. If the star has a low to medium mass (anything less than 8 solar masses) then at the end of its life it will transform into a white dwarf. If a star is massive (8-20 solar masses) then it will turn into a neutron star.

When a red giant starts to fuse helium to carbon and oxygen but lacks the mass to generate the core temperatures required to fuse carbon, an inert mass of carbon and oxygen will build up at the core. Towards the end of the stars nuclear fusion stage, it will shed its outer layers in the form of ionized gas forming a planetary nebula. The core that is left behind is the white dwarf typically about the size of earth. This is made up of electron degenerate matter which forms because the white dwarf lacks its previous ability to create an internal pressure meaning gravity squashes the mass much closer together. The reason this is happens is because under normal circumstances electrons with the same spin can’t occupy the same energy level, and there’s only two ways an electron can spin (this is known as the Pauli Exclusion Principle). In a normal gas this isn’t a problem because there aren’t enough electrons to fill the energy levels. In a “degenerate” gas however, all its energy levels filled. For a white dwarf to be forced smaller by gravity, it would have to make electrons go where they couldn’t go thus white dwarfs survive through quantum mechanical principles that prevent their collapse further. There are other unusual properties as well, white dwarfs with greater masses are actually smaller because gravity has to force the electrons closer together to maintain the outward pressure. However there is a limit to how much mass a white dwarf can have and it’s about 1.4 solar masses.

Neutron stars are incredibly dense (and one of my favourite things ever) with a typical one being about 20km and containing 1.4 solar masses. A teaspoon would weigh about a billion tonnes on a neutron star that’s how dense they are. They are also composed entirely of neutrons as the force of gravity is so great that it has caused the electrons and protons to merge into neutrons. The power from the supernova that created the star causes it to spin up to 43,000 times a minute gradually slowing over time. The neutron stars which are still spinning emit electromagnetic radiation that we can detect when it’s pointing towards earth (much like a lighthouse). These neutron stars are known as pulsars. The magnetic axis of the pulsar is what determines the direction the beam will fire off in. However this is not necessarily the same as the rotational axis and this misalignment is what causes pulsars to appear to pulse. There are currently three different types of pulsars that astronomers are aware of. The first is rotation powered pulsars which the radiation given off is caused by a slowing down of the rotation of the star. Accretion powered pulsars occur when the gravitational potential energy that falls onto the neutron star causes X-Ray’s that can be received from Earth. Finally there is magnetars where the radiation is caused by an extremely strong magnetic field losing energy.

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This is the second week of Deep, Dark, Space month!

This week is Redd the Red dwarf hearing a tale about zombie stars!

http://www.space.com/26748-supernova-explosions-zombie-stars.html

https://www.nasa.gov/press/2014/august/nasa-s-hubble-finds-supernova-star-system-linked-to-potential-zombie-star/

http://www.universetoday.com/90854/new-nasa-mission-hunts-down-zombie-stars/

Diamond Star.

In 2004, astronomers discovered a star composed entirely of diamond, measuring 4,000 km across and 10 billion trillion trillion carats. 50 light years from Earth, the diamond star is classified as a crystallized white dwarf, the hot core that remains after a star burns out. Only recently have scientists been able to study the contents of the white dwarf, and they’ve confirmed that the crystallized carbon interior of the star is, in fact, the galaxy’s largest diamond.

sciencealert.com
Astronomers have discovered the first star with an almost pure oxygen atmosphere
Everything else is gone.
By Peter Dockrill

Scientists have identified a new kind of star that up until this point had only been considered hypothetically: an ancient sun that has lived so long, its outermost layer is now composed almost entirely of pure oxygen.

When relatively small stars – those with less than 10 times the mass of our Sun – get close to the end of their lifespan, they shed their outer layers and become what are called white dwarfs. Under high gravity, the heaviest elements descend into the star’s dense core, while lighter elements such as hydrogen and helium rise to the surface.

At least, that’s what usually happens. This star, dubbed SDSS J124043.01+671034.68, bucks the trend, with astronomers discovering its outer atmosphere is essentially greater than 99.99 percent oxygen. Only traces of other elements have been detected, including neon, magnesium, and silicon, but as for the hydrogen and helium you’d expect to find dominating the surface, there’s no sign.

It’s a puzzle for the team that found it, led by Souza Oliveira Kepler from the Federal University of Rio Grande do Sul in Brazil. “What happened to all these light elements?” he told William Herkewitz at Popular Mechanics. “How did they all get stripped away?”

Continue Reading.

NGC 5307

NGC 5307 is a planetary nebula located about 10,000 light years away towards the constellation Centaurus. It is about half a light year across and shows a symmetrical, spiral shape. This could be caused by a wobbling jet of gas being expelled from a central white dwarf star.

Planetary nebula last only around 10,000 years. They are formed from dying stars, which cast off their outer shells of material as they run out of fuel. Over time, the material will expand and the nebula will grow as ultraviolet radiation from the remains of the central star illuminate the surrounding gas.

Image from HubbleSite, information from HubbleSite and NASA.