New Scientist
A famous supernova observed in 1604, known as Kepler’s Star, may have originated from another galaxy. Recent analysis of its motion shows it moving faster and differently than nearby stars, suggesting it came from a smaller galaxy swallowed by the Milky Way. Such “alien supernovae” are rare but possible.
Was a famous supernova an alien invader from another galaxy?
Author: Alex Wilkins
ONE of the most famous exploding stars ever recorded by humanity may have been an invader from another galaxy, according to an analysis of its movements.
In 1604, astronomers saw a new, incredibly bright star appear in the sky, outshining any other. German astronomer Johannes Kepler, who also derived some of the first laws of planetary motion, observed it for a year to track its brightness as it faded, and the star was later named after him.
Astronomers later realised that the sudden brightness and subsequent dimming meant Kepler’s Star must have been a supernova – the explosion of a dying star. In the 1970s, they found that the explosion’s leftover bubble of expanding gas – the supernova remnant – was moving away from the galactic centre of the Milky Way at high speed. “It was known for a few decades that it has this peculiar motion,” says Ping Zhou at Nanjing University in China. “It’s escaping the galaxy and it’s moving towards us at a high speed.”
Now, Zhou and her colleagues have reanalysed Kepler’s Star’s movements and concluded it is likely it came from another galaxy, making it the first example of what they call an alien supernova.
Zhou and her team first reconstructed Kepler’s Star’s motion using past measurements of the gas in its remnant. They then compared this with its neighbouring stars’ movements, using data from the Gaia space telescope, which tracks billions of stars across the Milky Way. They found that Kepler’s Star would have been moving much more quickly and in a different direction of the sky than its neighbours, which are all likely to have had a similar origin to each other, implying that Kepler’s Star came from a smaller satellite galaxy that merged with the Milky Way, says Zhou.
They also calculated how often these alien supernovae might happen, based on how many smaller galaxies the Milky Way has gobbled up and the rate at which their stars form. They found there should be a few alien supernovae every 60,000 years, which corresponds to around 1 per cent of the supernovae we see in the Milky Way (arXiv, doi.org/pk86).
“The idea that some supernovae in our galaxy should come from stars that belonged to satellite dwarf galaxies that had been swallowed by the Milky Way is sound,” says Or Graur at the University of Portsmouth in the UK.
The rate of alien supernovae also makes sense, but whether Kepler’s Star itself is one of these is less convincing, says Graur, because the authors relied on older measurements of it from the 1970s and 1990s. “You can see the huge difference in precision that comes from switching from the 1970s measurements to the ones from the 1990s,” says Graur.
“We should not assume that the old data is wrong,” says Zhou. “They are measured with larger uncertainty, but this is still relevant. At only 400 years old, it is evolving very quickly. If there is any historic data of it, it is still very valuable.”
Credits: TCA, LLC.