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In the orbit of two giants

Eta Carinae, approximately 7500 light-years from Earth, has everything that an astronomer could want. First, there’s the nebula surrounding Eta Carinae. The so-called Homunculus Nebula is still growing. It has the shape of two opposing cones, whose tips originate in Eta Carinae, and measures more than 0.5 light-years from end to end. From the propagation rate of up to 700 km/s, the existence of the nebula can be traced back to an outburst in the 1840s.

Second, it is not just a single star, but a binary system consisting of two blue giants. The primary star has a mass of 100 solar masses and is thus one of the most massive stars in the Milky Way. But even the secondary star is not a lightweight. It is 30 times heavier than our home star.

Both stars also orbit each other once every 5.5 years at a very close distance. Sometimes they come as close as the Sun and Mars, then move as far apart as the Sun and Uranus. At a cosmic scale, however, that is still just a stone’s throw away, and thus they inevitably each hurl large portions of their mass at each other in the form of dense, supersonic stellar winds made from charged particles. In this way, in only about 5000 years, the primary star loses as much mass as our Sun has in total. The secondary star propels a stellar wind moving at about eleven million kilometer per hour (corresponds to at least one percent of the speed of light).

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How does a star simply vanish?

PHL 293B, also known as HL 293B, the Kinman dwarf galaxy, A2228-00, or SDSS J223036.79-000636.9, is a small, not especially bright galaxy 75 million light-years from the Sun. It belongs to a class of so-called “blue compact dwarf galaxies.” These normally consist of several large, young star clusters containing hot, massive stars. The brightest of these are blue – thus the designation of the galaxies and their color.

PHL 293B is no different. Between 2001 and 2011, astronomers observed that the dwarf galaxy was dominated by a blue giant, a “luminous blue variable” (LBV), which shines approximately 2.5 million times brighter than the Sun. Such stars, as you might suspect from their name, are unstable and dramatic changes in their spectrum and brightness are the rule, not the exception. These variations leave behind specific traces that can be identified by scientists.

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Too heavy to be a neutron star, too light to be a black hole

Sometimes (always?), new research instruments like the Ligo-Virgo gravitational wave detector collaboration not only provide long expected answers to old questions, but also create completely new questions too. Take, for example, GW190412, which is the designation given to the latest conundrum, for which physicists can thank Ligo-Virgo. It refers to a gravitational wave burst that reached Earth on 14 August 2019. From the measured data, the researchers determined that a relatively lightweight object and a significantly more massive object must have merged together to form a black hole with a mass of now 25 solar masses.

There’s no question about the nature of the heavier object, which was determined to have a mass of 23 solar masses and thus had to be a black hole. But the smaller object, at 2.6 solar masses, was too heavy to be a neutron star, but also too light to be a black hole. The fate of a star is determined by its original mass. When it dies and explodes, all that remains is either a neutron star with a mass of maximum 2.5 solar masses (called the Tolman-Oppenheimer-Volkoff limit) or a black hole that is significantly more massive. The previously lightest known stellar black hole had a mass of 5 solar masses. It had seemed that there could be no objects within this rather large gap.

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