Warm ‘blob’ betrays location of missing neutron star

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The neutron star that remained after we saw a massive star explode as a supernova in 1987 seems to have finally been found – thanks to a warm cloud of dust called ‘the blob’.

Supernova 1987A was an important milestone in astronomy. For the first time, astronomers today have been able to closely follow the explosion of a star. What failed, however, was to detect the neutron star that you would expect as a remnant of such an explosion: a super-compact ‘mini star’ with a diameter of only a few dozen kilometers. Now, 33 years later, it seems that astronomers have finally succeeded with the ALMA radio telescope collection.

Just don’t count on a photo or the like that shows the long-awaited object in all its glory. What ALMA astronomers reported last fall was a warm dust cloud amid the supernova remnants. However, ‘warm’… The cloud in question, called the blob by the astronomers, had a temperature of -243 degrees Celsius. But since the surrounding dust is even 11 to 16 degrees colder, that asked for an explanation. A new theoretical study by Swiss astronomer Dany Page and colleagues now confirms that the best possible explanation is a neutron star that warms up the dust.

Youngest neutron star to date

The blob isn’t exactly where the supernova explosion itself happened – but that’s not surprising either. Indeed, it is expected that during the violence of the explosion, the resulting neutron star was thrown away at a speed of several hundred kilometers per second to where the blob has now been seen.

Initially, however, the ALMA astronomers thought that the neutron star must be brighter than it could have been if it were to be able to warm up the dust cloud. Dany and colleagues can also reassure their colleagues on that point. A neutron star like this, which is only 33 years old – and thus the youngest ever seen – should be so bright that it was able to do the job.

The blob (left) is a relatively warm dust cloud in the remnant of supernova 1987A (right). Image: ALMA (ESO / NAOJ / NRAO), P. Cigan and R. Indebetouw; NRAO / AUI / NSF, B. Saxton; NASA / ESA

Other options all seem less likely. For example, estimates of the mass of the star and the course of the supernova indicate that the remnant must be a neutron star and not a black hole. Furthermore, neutrinos were measured in 1987 from the supernova: particles that you would expect to see when forming a neutron star, but not at a fresh black hole.

Another scenario that the astronomers have considered is that radioactive material has heated the cloud. However, as Page and colleagues write: it really does seem that the cloud was heated by one object in one place – and not by radioactive material that has spread over the entire area.

Increasingly transparent

Those who are not yet convinced and still really want a photo, will have to be patient. And for once, it’s not because we need a telescope that hasn’t been built yet. Almost literally, the dust from the explosion has not yet settled; the surrounding piece of space is still opaque. It will gradually become more transparent as the decades go by. And then hopefully we will someday really be able to see what has become of the star that we saw with great violence in 1987.

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