Sunday, July 13, 2014
Finding an Invisible Star Corpse
An international team of astronomers recently reported a wonderfully strange discovery. They found the faint, cold "corpse" of a dead star, not by detecting any light from it, but through its gravity grip on another star corpse with which it shares a system. It's quite a story!
To follow it, we need to take a brief excursion into the gruesome deaths of stars. All stars collapse at the end of their lives, but stars with different amounts of mass (stuff) die differently. Less massive stars (like our Sun) eventually die by collapsing into a ball not much bigger than a planet -- such a white-hot but tiny corpse is called a "white dwarf." They start out as really hot, but cool off as they shine away their light and heat into the darkness of space.
More massive stars have a more complicated death in store for them. They die in a sudden event, where the core of the star collapses catastrophically, while the rest of the star blows up in a giant explosion called a "supernova." We have discussed such explosion in several posts.
What interests us today, however, is the tiny core of the star which is super-squozen (that's a technical term!) by the star's death. It's typically not much bigger than your average suburban town, and called a "neutron star" -- because in it all the atoms lose their identity and become neutrons.
Such a neutron star, surrounded by an "atmosphere" of materials from the messy explosion of the rest of the star, can sometimes be detected by faint pulses of radio waves it gives off, and is then called a "pulsar." (A watch company "borrowed" that name, but we astronomers had it first.)
So now here's the discovery. Astronomers using radio telescopes first discovered a pulsar in the constellation Aquarius, indicating that a neutron star was revealing itself to us. But the pulsar showed signs of wobbling, as if something with strong gravity was orbiting around it and pulling on it. Careful measurements reveal that the orbiting object is a white dwarf, and it takes only two and a half days to revolve around the neutron star. (Remember that Earth takes 365 days to go around the Sun, so these two star corpses are in an intimately close dance.)
To their amazement, when other astronomers tried to find the light or heat of the white dwarf companion, they COULDN'T -- not even with the biggest telescopes. That white dwarf must be so old that it has cooled down below the level where we can find it (at its distance of about 900 light years.) This makes it the coldest, dimmest star corpse ever found. If you are into star corpses, and we astronomers really are -- this is a big deal. It's remarkable that with today's technology, we can know this white dwarf is there not by its light, not by its heat, but just by its pull on another dead star nearby.
Note: Our image is just a painting, not any kind of photograph. It shows the pulsar on the left (with two beams of radio waves coming from it) and the white dwarf on the right. Although we can see light coming from the white dwarf in the painting, in real life the star is so faint and far away, no light from it is detectable with today's instruments.