S55 (also known as S0–102) is a star that is located very close to the centre of the Milky Way, near the radio source
Sagittarius A*
Sagittarius A* ( ), abbreviated Sgr A* ( ), is the supermassive black hole at the Galactic Center of the Milky Way. It is located near the border of the constellations Sagittarius and Scorpius, about 5.6° south of the ecliptic, ...
, orbiting it with an orbital period of 12.8 years. Until 2019, when the star
S62 became the new record holder, it was the star with the shortest known period orbiting the black hole at the centre of the Milky Way. This beat the record of 16 years previously set by
S2. The star was identified by a
University of California, Los Angeles
The University of California, Los Angeles (UCLA) is a public land-grant research university in Los Angeles, California. UCLA's academic roots were established in 1881 as a teachers college then known as the southern branch of the California S ...
team headed by
Andrea M. Ghez. At its
periapsis
An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion.
General description
There are two apsides in any elli ...
, its speed reaches 1.7% of the
speed of light
The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit ...
. At that point it is 246
astronomical units
The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbits ...
(34 light hours, 36.7 billion km) from the centre,
while the black hole radius is only a small fraction of that size (the
Schwarzschild radius
The Schwarzschild radius or the gravitational radius is a physical parameter in the Schwarzschild solution to Einstein's field equations that corresponds to the radius defining the event horizon of a Schwarzschild black hole. It is a characteris ...
is about ).
It passed that point in 2009 and will be there again in 2022.
Its position in the sky has been monitored from 2000 to 2012 using the
W. M. Keck telescope and from 2002 to 2016 with the
VLT. One complete orbit has been observed. From Earth's current perspective, it travels in a clockwise direction. Having observed two stars orbiting through complete periods around the centre (S55 and S2), the
gravitational potential
In classical mechanics, the gravitational potential at a location is equal to the work (energy transferred) per unit mass that would be needed to move an object to that location from a fixed reference location. It is analogous to the electric ...
of SgrA* could be established.
[ It is also possible that there is a great deal of ]dark matter
Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not a ...
around the orbits of these stars.gravitational redshift
In physics and general relativity, gravitational redshift (known as Einstein shift in older literature) is the phenomenon that electromagnetic waves or photons travelling out of a gravitational well (seem to) lose energy. This loss of energy ...
should become observable.
References
External links
*
* Includes position measurements and images.
{{DEFAULTSORT:S0-102
Sagittarius (constellation)