[NOTE: Whenever I write about actual cosmic events that might possibly affect us on Earth, I get scared emails from some folks. So let me be up front: there are no stars close enough to Earth to hurt us should they explode. Nothing I write in this post changes that; I’m talking about a star that can go supernova that’s closer than I thought any was, but still much too far away to do much to us. So don’t panic. But do please enjoy the over-the-topness of what happens when a star explodes. Because it’s cool.]
On May 13 I tweeted this one: BAFact: A supernova has to be less than about 75 light years away to hurt us. No star that close can explode, so we’re OK. The distance may actually be somewhere between 50 – 100 light years, and it depends on the kind of exploding star, but I have to keep these factoids to about 110 characters to tweet them. Nuance is at a premium.
I got so many replies about that one that I decided to do a theme week, and stick with supernovae. The next day I tweeted this: BAFact: The nearest star that can go supernova is Spica – it’s 260 light years away, so we’re safe, and I linked to a video I did a few years back this.
A few minutes later I got a tweet from Nyrath, saying that he thought the nearest star that could explode was IK Pegasi, 150 light years away.
I looked this up, and here’s the thing: he’s right! I had never heard of IK Peg, so I didn’t even know it existed. And it turns out it is the nearest star that can explode, though technically it probably isn’t.
And you know when I say something weirdly oxymoronic like that there must be a good story here, right? Mwuhahahaha. Yes. yes, there is. Stick with me; this is long, but also awesome.
It’s been known for a while that IK Peg is a weird star (you can read quite a bit about it on the ESO website, though the formatting is a bit messed up). It looks like an A-type star — that is, more massive, hotter, and bigger than the Sun. It’s not nearly enough to explode — stars need to be at least 8 times the Sun’s mass to do that, and this star is only about 1.7 times heftier than the Sun.
It pulsates, getting brighter and dimmer on a pretty rapid timescale: each cycle only takes about an hour. A lot of stars do this, but typically when one does it means it’s nearing the end of its life. In a few dozen million years it’ll swell up into a red giant, blow out a strong wind that’ll strip its outer layers away (creating a gorgeous planetary nebula), and eventually retire as a white dwarf; small, dense, and hot, cooling slowly over billions of years.
Except… there’s a monkey in the wrench. The star isn’t alone.
It has a companion. And this is where things get interesting.
Some new research just released asks a question near and dear to me: are there thousands of spinning white dwarfs in our galaxy, just waiting to explode as they gradually slow their rotation?
The answer is very probably yes. Let me be clear, as I always must be when covering topics like this: we’re not in any real danger from these things. Space is vast, and supernovae are few. If these things were that volatile we wouldn’t be here to talk about them in the first place.
But it’s still a very cool scientific question, and actually a fairly simple concept. Here’s how it works.
Imagine a binary system of two stars like the Sun, orbiting each other. One star nears the end of its life, swells up into a red giant, and blows off its outer layers. After a few millions years, all that’s left is its core: a dense, hot ball called a white dwarf. The size of the Earth but with the mass of a star, white dwarfs are pretty weird. They have incredibly strong gravity, which wants to crush them down even further, but they are supported by the electric repulsion of electrons, which is a pretty mighty force. It’s an uneasy truce.
It’s made even uneasier by the other star. It too eventually swells up, and can start to dump matter onto the dwarf (like in the picture above). If enough mass piles up, the immense gravity of the dwarf can induce nuclear fusion. Sometimes the material explodes, flaring in brightness, and we get a nova. Other times, if enough matter piles up — making the total mass of the white dwarf a bit more than 1.4 times that of the Sun — the ignition of fusion can cause a runaway reaction in the star, disrupting it entirely. The white dwarf tears itself apart, and you get one of the biggest and most violent explosions in the Universe: a supernova.
But there’s a hitch. Read More
It’s kind of amazing that with nearly 500 planets discovered orbiting other stars, we’re still finding ones that are really weird. Massive planets orbiting so close to their stars they are practically plowing through the stellar atmosphere; hot spots on the planet not aligned with their stars; planets orbiting so far out it’s a struggle to understand how they got there.
And now we can add the planets NN Serpentis c and d to that list.
Lying about 1500 light years from Earth, NN Ser is a binary star — most stars in the sky are part of multiple systems, so that in itself isn’t all that odd. But NN Ser is weird: it’s a very dinky red dwarf orbiting very close to a white dwarf. And by very close, I mean really close: they’re separated by only 600,000 km (360,000 miles), which isn’t much farther apart than the Earth and the Moon!
I’ll get back to the stars in a sec. The planets found (named c and d because the two stars are a and b, according to the naming conventions) are Jupiter-scale beasts, with masses of about 6 and 2 times Jupiter’s, orbiting the binary stars at a distance of roughly 825 and 450 million km (500 million and 270 million miles).
Those numbers don’t seem too odd; lots of planets have been found with similar characteristics. But when you take a closer look at the system…
Deep in the heart of a globular cluster orbiting an elliptical galaxy, it looks very much as if a massive black hole is in the process of tearing apart and devouring the remnant of an old star. And how do we know we’re witnessing this violent stellar demise? Black holes are messy eaters.
The discovery comes from the Chandra Observatory, a telescope in space designed to detect X-rays. This high-energy form of light can only be generated by violent events, things like exploding stars, strong magnetic fields, or extremely hot objects. Astronomers (including Jimmy Irwin, an old friend I went to grad school with!) using Chandra detected an unusually bright source of X-rays coming from a globular cluster — a tightly packed collection of stars — belonging to NGC 1399, a galaxy 65 million light years away. In the picture above (a combination of Chandra X-ray images and optical images from the huge Magellan telescopes in Chile), the galaxy is the bright blob on the right, and the new object — called a ULX for Ultra Luminous X-ray source — is marked with the red lines.
We know black holes exist in globular clusters, so that’s nothing new. We also know stars are so jam-packed in globulars that it’s not only possible but relative common (on a cosmic scale) for these stars to interact gravitationally. When a star gets too near a black hole, it can have matter pulled from its surface, which falls into the black hole. As it plummets to its death, it can first pile up just outside The Point of No Return, whipping madly around the hole, and heating up so violently it can emit X-rays.
That sort of thing has been seen before. What’s new here is that first, the type of X-ray emission seen from this event indicates that the star isn’t simply giving up matter slowly to the black hole; it’s actually getting torn apart, physically shredded by the vast gravity of the black hole. Second, what the astronomers have seen is that the emission is rich in the element of oxygen, but oddly missing hydrogen. Hydrogen is the most common element in the Universe, and all normal stars are almost entirely made of the stuff (the Sun is, for example). Not seeing it means the star getting eaten up by the black hole is most likely a white dwarf, the dense remnant of a dead star’s core. After a lifetime of fusing hydrogen into helium, there typically isn’t any hydrogen left in a star’s core. Once the star dies, the remaining core becomes a white dwarf, devoid of hydrogen but also commonly rich in oxygen.
So not only is this possibly the first time a black hole has been caught in the act of viciously ripping a star apart, the star itself is a bit of an oddball.
And there’s more, too. Looking at spectra taken of the object reveals how fast the material is moving as it orbits the black hole, and that in turn tells us how massive the black hole is. What astronomers found is that this particular black hole must have a mass of a thousand times that of the Sun! Because of the way black holes form, it’s common to see them have a few times the mass of the Sun, or even as much as 20 or so. We also see truly gigantic ones with millions or billions of times the Sun’s mass. But it’s recently been theorized that intermediate-mass black holes exist as well, with hundreds or thousands of times our Sun’s mass. Observations have been tantalizing about these objects, and this new evidence from Chandra adds to the idea that middle-weight black holes exist.
I think observations like this are very exciting. When a new type of object is suspected, or even found, we usually get incremental supporting evidence for them. But it’s rare to get a twofer: not only does this support the existence of intermediate mass black holes, but we caught one in the act of violently tearing apart its dead neighbor. In a hundred years or so, the last morsels of the white dwarf will fall into the black hole, never to be seen again. Not even crumbs will be left, so it’s pretty cool we were able to see this when we did.