How Do We Spot White Dwarf Stars Orbiting Red Giants?

Hi, I just read your article on the red giant/white dwarf binaries and loved it! Thanks for the post! My question is, do we have any recorded examples of a RG/WD binary? I sifted through about 20 descriptions of popular binary systems, figuring it would be in a list with that title, but didn’t see any.
New ultraviolet images from NASA’s Galaxy Evolution Explorer shows a speeding star that is leaving an enormous trail of “seeds” for new solar systems. The star, named Mira (pronounced my-rah) after the latin word for “wonderful,” is shedding material that will be recycled into new stars, planets and possibly even life as it hurls through our galaxy. Mira, also known as Mira A, is not alone in its travels through space. It has a distant companion star called Mira B that is thought to be the burnt-out, dead core of a star, called a white dwarf. Mira A and B circle around each other slowly, making one orbit about every 500 years. Astronomers believe that Mira B has no effect on Mira’s tail. Image credit: NASA/JPL-Caltech

New ultraviolet images from NASA’s Galaxy Evolution Explorer shows a speeding star that is leaving an enormous trail of “seeds” for new solar systems. The star, named Mira (pronounced my-rah) after the latin word for “wonderful,” is shedding material that will be recycled into new stars, planets and possibly even life as it hurls through our galaxy. Mira, also known as Mira A, is not alone in its travels through space. It has a distant companion star called Mira B that is thought to be the burnt-out, dead core of a star, called a white dwarf. Mira A and B circle around each other slowly, making one orbit about every 500 years. Astronomers believe that Mira B has no effect on Mira’s tail. Image credit: NASA/JPL-Caltech

Originally posted at Forbes!

There are lots of red giants and white dwarf stars orbiting each other out in our galaxy, and probably just as many in every other galaxy as well. Any set of binary stars usually forms at the same time, but since the lifetime of a star is tied very closely to its mass, if the two stars aren’t the exact same mass, then they reach the ends of their lives at different times.

A red giant star hasn’t completely finished its lifetime as a star just yet - it’s no longer burning hydrogen in its core the way our star is, but it’s still going - just more loosely held together, and burning helium instead of hydrogen. A white dwarf, on the other hand, has already reached the end of its pathway. White dwarfs are formed after the star runs out of helium to burn, and the outer layers of the star are lost into the formation of a planetary nebula. Only a central nugget of the former star remains after it loses its grip on its outer layers, which remains as a white-hot ball of atoms, slowly cooling to the temperature of the void of space. This is also the fate of our star, several billion years from now.

So a binary star system with a red giant and a white dwarf is a system where one star has already lost the majority of its mass into a planetary nebula, and the other star, being of a different mass, has progressed through its evolution at a different speed, and hasn’t come to the end of its helium burning phase just yet. If the stars are far enough apart, they will simply continue to orbit each other, until the red giant star reaches the end of its life and either creates its own planetary nebula and white dwarf, or explodes as a supernova, leaving behind a black hole or neutron star.

A team of researchers pointed the telescope at GK Persei, an object that became a sensation in the astronomical world in 1901 when it suddenly appeared as one of the brightest stars in the sky for a few days, before gradually fading away in brightness. Today, astronomers cite GK Persei as an example of a "classical nova," an outburst produced by a thermonuclear explosion on the surface of a white dwarf star, the dense remnant of a Sun-like star. This new image of GK Persei contains X-rays from Chandra (blue), optical data from NASA's Hubble Space Telescope (yellow), and radio data from the National Science Foundation's Very Large Array (pink). Image credit: X-ray: NASA/CXC/RIKEN/D.Takei et al; Optical: NASA/STScI; Radio: NRAO/VLA

A team of researchers pointed the telescope at GK Persei, an object that became a sensation in the astronomical world in 1901 when it suddenly appeared as one of the brightest stars in the sky for a few days, before gradually fading away in brightness. Today, astronomers cite GK Persei as an example of a "classical nova," an outburst produced by a thermonuclear explosion on the surface of a white dwarf star, the dense remnant of a Sun-like star. This new image of GK Persei contains X-rays from Chandra (blue), optical data from NASA's Hubble Space Telescope (yellow), and radio data from the National Science Foundation's Very Large Array (pink). Image credit: X-ray: NASA/CXC/RIKEN/D.Takei et al; Optical: NASA/STScI; Radio: NRAO/VLA

If the stars are close enough together, some other fun things start to happen. White dwarfs can begin to steal away some of the red giant star’s gas, pulling it in towards itself and growing in mass. After enough time, and after enough material has been thieved from its companion, the white dwarf can explode in either a nova (which is a surface explosion, which doesn’t destroy the star) or in a supernova (which is a complete detonation of the white dwarf). Not all supernovae are triggered this way; some are triggered by two white dwarfs colliding, and some are triggered by very high mass stars reaching an explosive end to their life. But novae seem to be exclusively the land of large companions and white dwarfs, and given that the white dwarf has already aged its way to the end of its lifetime, that means many of the companions will be red giants.

A stunning amount of energy is unleashed when a star goes nova. Image credit: NASA

A stunning amount of energy is unleashed when a star goes nova. Image credit: NASA

There’s at least one very well known red giant/white dwarf system in the night sky of the northern hemisphere. Unfortunately, it is found in the constellation Cetus, which has no particularly bright stars, but you can point your face in its direction if you choose. Mira, the star at the top of the page, is a red giant star plowing through the thin gas between stars in our galaxy, producing a shock wave and a long tail. But hiding in these images is a very faint white dwarf, visible to us in X-rays, pulling material in towards itself. If you want to go hunting for white dwarfs lurking around red giants, try looking for stars which have exploded in novae.

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What happens when Betelgeuse explodes?

If Betelgeuse explodes right now, could we see it with naked eye? It is over 400 light years away, so you might think that people would see it long after it actually happens?
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Originally posted @ Medium!

Betelgeuse is already one of the brightest stars in the night sky, sitting somewhere around the 8th or 9th brightest star in the night sky. (These lists don’t include the Sun, which is somewhat obviously always the brightest object in the sky.) It sits in the constellation Orion, along with a number of other bright stars, and makes up the left hand shoulder of the warrior. It looks visibly orange in the night sky, and is classified as a red supergiant star, in the later stages of its life. It’s also one of the few stars that’s close enough for us to resolve in more detail than a point source, and the pictures are pretty fun.

If Betelgeuse were to go supernova right now — as in, if you could break physics and travel to the star instantaneously to check on it — you’re absolutely correct to think that it would take us quite a while to notice. Betelgeuse is about 600 light years away from our solar system, so the light traveling from Betelgeuse has about 600 years of travel before it will reach us. If the star had physically exploded in 2015, we wouldn’t spot the light from that explosion until 2615. We’re constantly observing this star (and pretty much everything in the Universe) as it was, a significant period of time ago. This is also why astronomers say that in studying the night sky, we study the past. The more distant the object, the further in the past we observe. 600 light years, in the grand scheme of things, is pretty close; we’re still dealing with our local neighborhood inside our own galaxy.

Supernovae are incredibly bright phenomena. At the brightest point of the explosion, a supernova can outshine the whole galaxy it lives in. A single star has managed to, for a short time, be a brighter source of light than the several billion other stars in its galaxy combined. This is tremendously bright. Supernovae do have a “rising time” of about a week, when the star is increasing in brightness — it stays at its peak brightness for a few days, and then slowly declines into obscurity over a period of a couple of weeks.

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But how bright would Betelgeuse specifically be? We can do some math to work this out, making the assumption that Betelgeuse explodes as a Type II supernova. The exact style of supernova is still up for a bit of debate, depending on the exact rotation speed and mass loss of the star over the next hundred thousand years. Regardless of the exact method of its explosion, all the supernovae options for this star have a peak brightness of approximately the same value, so for a quick calculation that’s good enough to determine what we’d see with the naked eye.

There are two ways of measuring brightness in the astronomy world; the first is absolute magnitude, which is the brightness of the star, as it would be measured from a fixed distance. (It’s arbitrary, but the fixed distance chosen is 10 parsecs, or about 33 light years.) This is trying to get to a measure of intrinsic brightness — as though we could line up everything in the sky at equal distance from us, and compare them to each other that way. We can’t actually measure the brightness of a star this way, but we can apply some corrections based on the distance to the star to get to it. The absolute magnitude of a Type II supernova is around -17. Because astronomers have the worst conventions in the world (for largely “historical reasons”), negative numbers mean brighter objects. The sun has an absolute magnitude of 4.83, which, once we translate out of “magnitudes”, means that the sun is 500 million times fainter than the supernova, when measured at the same distance. This huge difference in relative brightness is why a supernova can outshine an entire galaxy.

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The other method of measuring brightness is a bit more straightforward. It’s the apparent brightness — i.e., how bright does it appear to us as viewed from the Earth. In this frame of reference, more distant objects will always appear fainter, regardless of how intrinsically bright they are. Because Betelgeuse is still fairly distant from us, the apparent brightness would be significantly less than the absolute magnitude. Based on the distance to Betelgeuse, we can work out that the apparent magnitude of the peak of the explosion would be -10. The sun, in apparent magnitude, is the brightest thing in our sky, and is checking in at an apparent magnitude of -26.74. Once again translated out of magnitudes, this means that the Sun as seen from the Earth is a whopping ~5 million times brighter than Betelgeuse’s explosion, so our supernova certainly won’t be anywhere near as bright as our sun in the daytime. That’s not to say you wouldn’t be able to see it — it would definitely be bright enough to see during the daytime, as long as you were looking in the right direction. (After all, you can still see Venus in the daytime, if you know where to look!)

Nighttime will be a different story. The brightness of Betelgeuse’s supernova is about the same as the quarter moon. It would also be about 16 times brighter than the brightest supernova known to have been seen from earth, which occurred in 1006, and was recorded by a number of early civilizations. (An image of what remains of that supernova is shown below.)

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It was said that the supernova in 1006 was bright enough to cast a shadow at night. Betelgeuse, being significantly brighter, would likely also cast shadows — which, if you think about the brightness of a quarter moon, would make sense!

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All that said, Betelgeuse isn’t expected to explode for another 100,000 years or so. We do expect a few supernova in our galaxy every few hundred years, so there are a number of stars that are nearing the ends of their lifetimes within our galaxy. It’s hard to predict exactly when a star will transition from “close to the end of its life” to “exploding in the next week”, so while we expect that none of these will be exploding in the next little while, it’s difficult to predict which one of the stars will be the first to go. In the mean time, we can take wonderful pictures of the more nearby stars, like the one below taken by Hubble, and watch them cast off their outer layers at an incredible rate.

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Will an exploding sun affect our galaxy?

Hubble Space Telescope image of supernova 1994D in galaxy NGC 4526.  Image Credit:  NASA/ESA, The Hubble Key Project Team and The High-Z Supernova Search Team

Hubble Space Telescope image of supernova 1994D in galaxy NGC 4526. Image Credit: NASA/ESA, The Hubble Key Project Team and The High-Z Supernova Search Team

One star exploding won’t do very much! Our galaxy has a few hundred billion stars, plus extra mass in the form of dark matter. If one of those stars explodes, that’s one trillionth of the mass of the whole galaxy exploding. It’s not a big enough event to have a noticeable affect on the galaxy as a whole - one star can only put out so much energy to try and rattle the galaxy.

The explosion can certainly change the visual appearance of the galaxy, if the star is big enough. A supernova can often outshine the entire galaxy the star is living inside - this means as you look at the galaxy, a bright star will suddenly appear, and then gradually fade from view. A supernova in our galaxy was recorded in China in 1006; it was apparently bright enough in the night sky that people were easily able to see at night, and was even visible during the day. However, for a supernova to occur, you need the star to be at least 8 times the size of our sun.

Our sun will not go supernova - it will have a somewhat gentler, although still quite dramatic end of life. It’s predicted to become a planetary nebula, with a white dwarf star as all that remains of our Sun. The process of becoming a planetary nebula doesn’t produce intense shock waves the same way that a supernova does, so while it creates wonderful pictures, it won’t even affect the local area around the sun very much, let alone the whole galaxy.

What an exploding star does do for any galaxy is provide all the heavier elements to the gas hanging about within a galaxy. Then, the next time a star forms in that area, it will have those elements to work with. Any element heavier than iron - for instance, all the gold and silver we have on earth - was formed in an older star’s supernova.

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