The latest wow-find from astronomers has snuck up on us by stealth. A small red dwarf star discovered in November 2013 by German astronomer Ralf-Dieter Scholz looked, first off, to be pretty ordinary.
Conceptual picture I made of a red dwarf with companion using my trusty Celestia installation.
It even had a boring name: WISE J072003.20−084651.2, courtesy of being found lurking in data collected by the WISE Satellite. It is estimated to be 19.6 light years distant – in our neighbourhood, as stars go, but not exceptionally close. It has around 86 times the mass of Jupiter, making it a M9.5 class red dwarf, one of the smallest possible.
It is orbited – at the equivalent distance of Venus – by a ‘brown dwarf’ companion, a body with 65 times the mass of Jupiter. This world is warmed to near-luminescence by gravitational compression and – potentially – deuterium fusion, but isn’t massive enough to trigger hydrogen-1 fusion and light up like its star.
Yawn. Red dwarfs are the most common stars around. Proxima Centauri, the closest star to the Sun, is a prime example. In fact, of the 60 stars known within 16.3 light years, 46 are red dwarfs. We’re finding lots in our neighbourhood lately because many are so cool and dim – by stellar standards – they’re invisible even to high-powered telescopes. It takes satellites with sensitive infra-red detectors to pick them up. Brown dwarfs are also appearing to these instruments – singly, or orbiting stars that (wait for it) are often red dwarfs.
Since 2013, though, Scholz’s Star has rung alarm bells. First was its proper motion – the way it tracked tangentially across the sky, relative to other stars. Eric Mamajek of the University of Rochester in New York led a team looking into that, using data collected by the South African Large Telescope and the Las Campanas Observatory’s Magellan telescope in South America. They discovered the proper motion was very slow. But the star itself had very high radial velocity – its actual speed. Around 83 kilometres a second, in fact – four times the usual velocity of stars in this part of the galaxy.
This added up to a star that was travelling fast – but which from our viewpoint didn’t appear to be moving. That’s no paradox – imagine you’re looking up a straight road at a car disappearing into the distance. It’s moving fast, but from where you’re standing, it isn’t moving left or right (‘proper motion’). That’s because it’s moving directly away – and that’s true of Scholz’s Star.
Comparison between stars and brown dwarfs. Not strictly to scale. Public domain, NASA/JPL/Caltech.
Mamajek and his team ran 10,000 mathematical simulations to find out how close it had been. And – just announced this month – they discovered that, some 70,000 years ago, Scholz’s Star skimmed past our solar system. With a closest approach of just 0.82 light years – some 52,000 times the distance of Earth from the Sun – it banged through the outer fringes of the Oort cloud, the icy cloud of debris left over from the formation of our solar system, which extends out to a light year or so.
The star was far too small and far too distant to affect the orbits of the Sun’s planets. Here on Earth, the Moon has 2,000,000,000,000,000 times the tidal effect exerted by Scholz’s Star at closest approach. But it will have perturbed some of the the ice-and-dirt clusters of the Oort cloud. Passing stars are thought to do this every so often, and it’s thought that Scholz’s Star was far from the most serious. Some material will probably have been lobbed sunwards, and will still be on their way in – meaning there will be a small scattering of comets arriving in about 2,000,000 years. Yah, we’re talking about astronomy here – which means having a barrel full of zeroes by your desk.
Did ancient humans see the star? If we draw a circle around the Sun at 100,000 times Earth’s distance and plot Scholz’s Star’s path through it, we find the star took around 10,000 years to traverse that line. Back then it had huge ‘proper motion’ by stellar standards – enough to move across the sky by the angular width of the full Moon in 26 years.
But even at closest approach Scholz’s Star would have been around 11.8 magnitude and thus utterly invisible to the naked eye. But the thing about red dwarfs is that they’re often magnetically unstable – and emit huge flares. In some cases that can increase the brightness of the star, briefly, about 400 times. That would have been enough to make it visible as it tracked through our skies – intermittently. The jury’s out on Scholz’s Star, but Mamajek has speculated that it probably did flare regularly.
Yup, Scholz’s Star was probably winking at us. Which is kind of cool. And begs a question – what would happen if a star came even closer? More soon.
Copyright © Matthew Wright 2015