Would A White Dwarf Outlive A Neutron Star?

This multiwavelength composite shows the supernova remnant IC 443, also known as the Jellyfish Nebula. Fermi GeV gamma-ray emission is shown in magenta, optical wavelengths as yellow, and infrared data from NASA's Wide-field Infrared Survey Explorer (WISE) mission is shown as blue (3.4 microns), cyan (4.6 microns), green (12 microns) and red (22 microns). Image credit: NASA/DOE/Fermi LAT Collaboration, Tom Bash and John Fox/Adam Block/NOAO/AURA/NSF, JPL-Caltech/UCLA

This multiwavelength composite shows the supernova remnant IC 443, also known as the Jellyfish Nebula. Fermi GeV gamma-ray emission is shown in magenta, optical wavelengths as yellow, and infrared data from NASA's Wide-field Infrared Survey Explorer (WISE) mission is shown as blue (3.4 microns), cyan (4.6 microns), green (12 microns) and red (22 microns). Image credit: NASA/DOE/Fermi LAT Collaboration, Tom Bash and John Fox/Adam Block/NOAO/AURA/NSF, JPL-Caltech/UCLA

Originally posted at Forbes!

This is an interesting question, because most of the time when we talk about the lifetime of a star, we mean the part before the creation of a white dwarf or a neutron star.  Typically, a star has a “lifetime” when it is burning hydrogen into helium, gradually consuming its central reservoir of hydrogen, producing light as it does so.

It’s using this kind of definition for “lifetime” that we say our home star Sol is middle aged. Our Sun is a little over 4.5 billion years old, about halfway through its estimated ~9 billion years of hydrogen burning. Once hydrogen burning is complete, our Sun, along with all other stars of its mass, will go through an elaborate end-of-life transformation, which involves shedding most of the outer layers of the star. In the case of the Sun, we’ll produce a planetary nebula out of the discarded outer surface of the star, leaving only the hot nucleus of the star behind as a white dwarf.

This colourful bubble is a planetary nebula called NGC 6818, also known as the Little Gem Nebula. A version of the image was submitted to the Hubble’s Hidden Treasures image processing competition by contestant Judy Schmidt. Image credit: ESA/Hubble & NASA; acknowledgement: J. Schmidt (geckzilla.com)

This colourful bubble is a planetary nebula called NGC 6818, also known as the Little Gem Nebula. A version of the image was submitted to the Hubble’s Hidden Treasures image processing competition by contestant Judy Schmidt. Image credit: ESA/Hubble & NASA; acknowledgement: J. Schmidt (geckzilla.com)

If the star were a bit larger, it would explode as a supernova instead of creating a planetary nebula and white dwarf. The star has to sit within a very specific range of masses in order to create a neutron star - too large, and the gravitational weight of the star can compress the core of the star down into a black hole, instead of stopping at a neutron star. If the star is too light, it won’t supernova at all, and you’re left with a white dwarf instead of a neutron star.

Whether you have a white dwarf or a neutron star, both are considered stellar remnants. They’re what remains of a star, after fusion has stopped in the core of the star, and gravity has dealt with the unstable star. Neither the white dwarf or the neutron star will be generating any new heat in their cores, or will be able to do much else, beyond sit there, without some kind of external influence.

A dying star is throwing a cosmic tantrum in this combined image from NASA's Spitzer Space Telescope and the Galaxy Evolution Explorer (GALEX). In death, the star's dusty outer layers are unraveling into space, glowing from the intense ultraviolet radiation being pumped out by the hot stellar core. This object, called the Helix nebula, lies 650 light-years away, in the constellation of Aquarius. Image credit: NASA/JPL-Caltech

A dying star is throwing a cosmic tantrum in this combined image from NASA's Spitzer Space Telescope and the Galaxy Evolution Explorer (GALEX). In death, the star's dusty outer layers are unraveling into space, glowing from the intense ultraviolet radiation being pumped out by the hot stellar core. This object, called the Helix nebula, lies 650 light-years away, in the constellation of Aquarius. Image credit: NASA/JPL-Caltech

A white dwarf, on its own, will sit in place and glow, gradually losing energy to the deep dark of space, until the white dwarf has lost enough heat to match the background temperature of the universe, at about 3 degrees Kelvin. If the white dwarf could achieve this, we would call the resulting object a black dwarf, but so far, none have ever been observed. This lack of black dwarfs isn't a surprise - the amount of time a white dwarf is predicted to need in order to get to black dwarf status is longer than the Universe has been around. There quite literally hasn’t been enough time in the entire universe for a white dwarf to cool.

Neutron stars are also high temperature objects, and all they can do is cool, but they come equipped with frankly astoundingly strong magnetic fields, and so their cooling process is a little more involved. We spot many neutron stars in the night skies by watching them beam X-rays away from themselves. Combined with the extremely rapid rotation that a lot of neutron stars have, neutron stars are often picked out by watching them pulse in brightness, like a high energy radiation lighthouse.

Without some kind of external factor coming into play, neither a white dwarf or a neutron star will cease to exist in our universe, so in that regard a neutron star and a white dwarf are tied for longevity. There are external factors you could invoke to dispose of a white dwarf or a neutron star - these usually come by way of a second star, which donates mass to the white dwarf or to the neutron star. If the white dwarf gains enough material from devouring its neighbor star, it can explode in a supernova, detonating itself into oblivion. A neutron star can also gather enough mass to itself so that it collapses into a black hole, imploding inwards on itself. However, there's no reason for the white dwarf to always do this faster than a neutron star, because the key factor here is how quickly the star can steal material from its companion. The speed of the theft depends on the orbit of the companion, not whether or not you have a white dwarf instead of a neutron star.

Without such a stellar donor, both of these stellar remnants will simply hang out in the Universe, slowly losing their heat, until they have matched the background temperature of the Universe.

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What happens when one star in a binary turns into a red giant?

When you have a binary star system with the stars close together, what happens when one of the stars starts to turn into a red giant?


Binary stars are actually pretty common (about a third of the stars in our galaxy are in some kind of binary), although the very close pairs are a little less common.

For the majority of the lifetime of the stars in a binary, both stars just spin around each other, burning their own hydrogen and orbiting tranquilly. However, unless the two stars are exactly the same size when they’re formed (not generally the case), then one star will run out of hydrogen before the other, and will transform into a red giant star.

Red giants are pretty fuzzy stars, only loosely held together by their own gravity. For some scale, our own Sun will become a red giant in its future. It will expand from its current (quite large) radius of 695,500 kilometers to something 200 times that large. But it won’t have gained any material to grow that much bigger - it just spreads out what’s already there.

If the star is on its own, this newfound wispiness doesn’t change much. But if there’s another star nearby, the other star can begin to tug on the outer layers of the red giant, pulling some of the outer layers towards itself. (This is the same thing that the moon does to our oceans.) Because the red giant is so big, the outer layers are only weakly tied to the centre of the star. If the companion star in the binary is close enough or massive enough, it can begin to tear the outer layers of the red giant away, and pull them towards itself. This will change the shape of the red giant from a diffuse sphere into a diffuse teardrop, with the point of the teardrop facing the companion star.

What the companion star does with that surface material depends on what kind of star the companion is. By the time one star has aged its way to being a red giant, its companion is in one of two states, which will be dictated by how massive the companion was when it began its life.

If the companion started out with a lower mass than its red giant partner, it will still be burning hydrogen in its core when the red giant begins to form. As it siphons gas off of the red giant, this companion star will grow in mass. Depending on how rapidly the red giant is growing, and how quickly the companion is siphoning material, the lower mass star can actually grow so large that it becomes more massive than the red giant. After having so much of its material drawn away from itself and onto its neighbor, the red giant will slow its donation of mass. This can result in a fairly stable configuration - the red giant, having lost a good amount of its atmosphere to its neighbor, will continue to slowly bleed gas into its neighbor’s gravitational well, and the neighbor will continue burning hydrogen until it has exhausted its own resources.

The other option for these binaries is if the red giant was the smaller of the two stars when they started their lives. This means that the star now turning into a red giant took much longer to reach the end of its life than its neighbor. (The bigger your mass, the shorter your life, if you’re a star.) So the companion star has already gone through its death throes, and can be one of a number of interesting stellar remnants.

The main options for your stellar companion in this case are: a white dwarf, a neutron star, or a black hole.



If your red giant is pouring gas down onto a white dwarf, you will eventually trigger some kind of explosion: either a nova or a supernova. A nova is a thermonuclear detonation on the surface of a white dwarf, and can recur multiple times, as it’s just a surface explosion. This kind of behavior makes these binaries fairly noticeable, because the brightness of the star will flare to many times its original brightness. A supernova, on the other hand, will detonate the entire white dwarf, blasting itself apart, and leaving nothing behind (also quite noticeable). This kind of supernova occurs when the white dwarf gains too much material to be stable (these stars are balancing gravity against an electron’s unwillingness to be pushed too close to another electron), and some trigger in the core sparks a runaway burning of material.

If the other object is a black hole or a neutron star, you’ll wind up with what’s called an X-ray binary, for the somewhat boring reason that it produces a lot of X-rays. For these objects, as the gas from the red giant is pulled off of the red giant star, it gets pulled into a very thin disk, surrounding the black hole or neutron star. The disk forms because it’s very hard for gas to lose a lot of momentum all at once and plunge straight down onto the black hole, but as a result, the gas winds up heating up to an incredible temperature before it makes it all the way to the neutron star or black hole. This heat causes the X-ray glow, and keeps the disk itself almost invisible in optical light.

So there you have it! A binary system of stars with one red giant will result in the companion tearing the outer layers of the red giant star away. From there, you wind up growing the object nearby, or causing a nova, a supernova, or the creation of a lot of X-rays.

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