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.