Astronomers spotted a new star reaching 25,000 km per second around Sagittarius A*, while a separate study models that collapsing dark stars could be the dominant source of the nanohertz gravitational-wave background.
According to a peer-reviewed paper in Nature, the GRAVITY+ instrument has spotted S301, a star reaching 25,000 km per second, or about 8 percent of the speed of light, around the Sagittarius A* black hole at the centre of the Milky Way. The star completes its orbit in 8.7 years and passes within 1.78 billion km of the 4 million solar mass black hole at its closest approach. This distance is about 12 times the distance between the Earth and the Sun.[1]
Pinning down the orbit of the star, which is 2 billion times fainter than Betelgeuse, required combining the light of four 8-metre telescopes. The research team notes that the star passes close enough to feel the black hole's rotation. No supermassive black hole's spin has been measured directly so far; however, its next close approach in 2031 is expected to allow the first direct measurement of a supermassive black hole's spin using new tools like the Extremely Large Telescope.[1]
A separate modelling study by researchers at Colgate University examines the scenario in which the first supermassive black holes grew from hypothetical dark stars, which are held up by dark-matter heating rather than fusion. The calculation, published in Physical Review D, indicates that a comoving density of even one per thousand for these hypothetical objects could make them the dominant source of the nanohertz gravitational-wave signal measured by pulsar timing arrays in a chain stretching from primordial density fluctuations to accretion-driven growth; the signal amplitude is set by a trade-off between seed density and the assumed host halo mass.[2]
