Cosmic relationship: a star ate its spouse
Unlike the solitary Sun, about half of the stars in our Milky Way galaxy are in a “long-term relationship” with another star, orbiting each other, called a binary system.
This week, researchers described one such system, with the pair orbiting each other every 51 minutes in the fastest such orbital period known for a class of binary stars. In this drama, one star eats another, writes Reuters.
The two stars are located at a distance of about 3,000 light years from Earth in the direction of the constellation Hercules. A light year is the distance light travels in a year, 9.5 trillion km.
The system belongs to a class of binary stars known as “cataclysmic variables,” in which a star similar to our Sun orbits a so-called white dwarf, basically the hot and compact core of a burned-out star. Variable simply means that their combined brightness changes over time as seen from Earth. A cataclysm means that this luminosity changes dramatically — in some cases by a factor of 10,000 or more.
Over millions of years, the distance between these two stars has shrunk so much that they are now closer to each other than the Moon is to Earth.
“Imagine if the moon zipped across the sky 10 times a night. That’s the kind of speed we are talking about,” said MIT astrophysicist Kevin Burdge, lead author of the study, published last week in the journal Nature.
However, closeness does not mean that they are kind to each other – the white dwarf mercilessly sucks the material out of its partner.
This larger star is about the same temperature as the Sun, but has shrunk to 10% the diameter of the Sun — the size of Jupiter, the largest planet in our Solar System. The white dwarf has a mass of about 56% that of our Sun, but it is tightly packed, and its diameter is about 1.5 times that of Earth.
The researchers used data from the Palomar Observatory in California and telescopes in Hawaii and the Canary Islands.
Most stars are composed mainly of hydrogen, with smaller amounts of helium and other elements. The larger of the two stars in this aging binary system is extremely helium-rich, not only because its companion has had a snack of hydrogen from its outer layers, but also because it has plenty of the element in its core through the slow process of fusing hydrogen atoms into helium in its fusion cauldron.
This binary system periodically flashes on and off in part because the larger star is physically deformed into a teardrop, rather than spherical, shape by the white dwarf’s gravitational tug.
There are more than a thousand known cataclysms, although only a dozen of them have orbital periods of less than 75 minutes. Although the 51 minutes of this binary system is fast, it is not a record compared to other classes of binary systems. The fastest known orbital period among binary systems is only 5 minutes and 21 seconds when two white dwarfs orbit each other.