Astronomers detect fastest known star in Milky Way

• Astronomers have discovered the fastest known star in the Milky Way, orbiting Sagittarius A* black hole at its centre. 

• The star, named S301, was detected with the European Southern Observatory’s Very Large Telescope Interferometer (ESO’s VLTI).

• Because S301 comes so close to Sagittarius A*, it is the first star known that could be used to directly measure the rotation of a black hole.

• During its closest passage to the black hole, the star travels at around 25,000 kilometres per second — 100,000 times faster than a commercial plane, or over 8 per cent of the speed of light — making it the record holder for the fastest star in the Milky Way. 

• The star is traveling in a highly elliptical path around Sgr A*, taking about 8.7 years to complete an orbit.

• Black holes are extraordinarily dense objects with gravity so strong that not even light can escape. 

• Sgr A* possesses about 4 million times the Sun’s mass and is located about 26,000 light-years from Earth. 

• A light-year is the distance light travels in a year, 9.5 trillion km.

How astronomers found S301?

• Finding S301, which appears two billion times fainter than Betelgeuse (the orange star in the constellation Orion) in the sky, was no easy feat. 

• The team used the VLTI, a facility at ESO’s Paranal Observatory in Chile and its GRAVITY instrument, now known as GRAVITY+ following an infrastructure upgrade. 

• The VLTI’s superpower lies in its ability to combine the light from four 8-metre telescopes to create a ‘virtual’ telescope with 15 times the spatial resolution of a single 8-metre telescope.

• With GRAVITY, and later with GRAVITY+, the team managed to catch a first glimpse of the new star in spring 2023 and have followed it since to constrain its orbit. 

• They could also trace S301’s orbital history back to 2017, finding that it last made its closest approach to the central black hole in early 2023.

What is a black hole?

• A black hole is born when a large star collapses in on itself. Far from being a “hole”, they are instead incredibly dense objects with a gravitational pull so strong that nothing, not even light, may escape them. As they suck in matter such as gas, dust and space debris, they form an accretion disk — a churning mass of super-accelerated particles that are among the brightest objects in the Universe — around them.

• Scientists generally believe that there are two types of black holes. The more common stellar black holes — up to 20 times more massive than the Sun — form when the centre of a very big star collapses in on itself.

• Supermassive black holes are at least a million times bigger than the Sun and their origins are uncertain.

European Southern Observatory

• The European Southern Observatory (or formally the European Organisation for Astronomical Research in the Southern Hemisphere), is an intergovernmental organisation.

• It was established in 1962 by 16 Member States.

• ESO brings together over 750 staff from more than 30 countries, and countless more collaborators worldwide.

• The organisation is led by the Director General who is appointed by the ESO Council. 

• ESO’s telescopes are currently located in the Chilean Atacama Desert. 

• ESO operates three observing sites in Chile — La Silla, Paranal and Chajnantor. 

• ESO’s headquarters is situated in Garching, close to Munich in Germany.