See our youngest neutron star
ASTRONOMERS have finally detected strong evidence that a stellar explosion seen 33 years ago produced a neutron star, offering a great opportunity to study the youngest such object ever detected.
It was a February night in 1987 when an astronomer colleague telephoned me and uttered the word “supernova”. Immediately I was filled with excitement. He suggested that I go outside and take a look at the Large Magellanic Cloud (LMC), one of the nearest galaxies to our own Milky Way galaxy, clearly visible to the unaided eye as a small “cloud’’ to the south.
And there it was: a star where none had been seen before. It was the brightest object in that part of the sky. A star had exploded in the LMC, and it had become the first supernova (exploding star) to be clearly visible to the unaided eye since 1604.
There are two basic types of supernova. One type is when a star called a white dwarf explodes after a neighbouring star dumps so much material on to it that its mass is tipped above the limit possible for a white dwarf. The type seen in 1987 is called a “core-collapse’’ supernova, in which a massive star’s iron core can no longer support itself. The collapse results in a massive explosion in which much of the star’s material is blasted off into space, but the core remains.
That core can become either a neutron star, which is a very dense object composed of subatomic particles called neutrons packed together, or it can become a black hole, in which the core collapses into a point called a singularity, leaving a volume around it from which light cannot escape.
Astronomers later identified the star on earlier images (before the explosion), and it turned out to be one with the catalogue number Sanduleak -69°202. Based on their knowledge of this star and the observations of the explosion, they expected that the remaining part of the star would be a neutron star, but until recently they had not detected one.
Neutron stars can often be identified because of their pulsations of radiation that are detected as they spin around and “beam’’ this radiation regularly in our direction. These are called pulsars, the first of which was detected in 1967 by Jocelyn Bell in England.
There was an early rush to try to detect such pulses coming from that direction in space, but none has been detected for certain.
However, the recent results are very exciting. A team of astronomers using the array of telescope “dishes’’ known as the Atacama Large Millimeter/ submillimeter Array (ALMA) in northern Chile detected a “blob’’ of warm dust that is likely to be heated by the energy of a nearby neutron star.
An initial announcement was made in 2019, and a further paper published this year has shown that a neutron star is the most likely explanation.
It’s hardly a temperature that anyone would consider “warm’’. The dust is glowing at the particular wavelengths used by ALMA, which indicates that its temperature is about -240C. This is only about 33C above the level that scientists call absolute zero — the temperature at which there is no thermal energy at all.
However, it is the right temperature to strongly suggest the presence of the expected neutron star.
Though we currently see that region of space only 33 years after the star exploded, the event actually happened a very long time ago: 168,000 years in the past. This is because the object is 168,000 light years away, so it has taken that long for the light to reach us. However, this doesn’t matter to astronomers, who are effectively looking back in time to the period of the explosion and its immediate aftermath.
We are long overdue for a supernova to become visible in our own galaxy. The 1604 event was the most recent. It is known as “Kepler’s Star’’, because of the observations of it made by the famous astronomer and mathematician Johannes Kepler.
We don’t know when the next naked-eye one will happen, but if it is a relatively nearby star such as Betelgeuse in the constellation of Orion, it will be dazzlingly bright.