Image of the galactic center showing the position of S2
Credit: ESO / MPE / Gillessen et al.
Epoch J2000.0 Equinox J2000.0 (ICRS)
|Right ascension||17h 45m 40.0442s|
|Declination||−29° 00′ 27.975″|
|Spectral type||B0-2 V|
|Period (P)||16.0518 yr|
|Semi-major axis (a)||0.12540 ± 0.00018″|
|Eccentricity (e)||0.88466 ± 0.00018|
|Inclination (i)||133.818 ± 0.093°|
|Longitude of the node (Ω)||227.85 ± 0.19°|
|Periastron epoch (T)||2018.37974 ± 0.00015|
|Argument of periastron (ω)|
|66.13 ± 0.12°|
[CRG2004] 13, [GKM98] S0–2, [PGM2006] E1, [EG97] S2, [GPE2000] 0.15, [SOG2003] 1, S0–2.
S2, also known as S0–2, is a star in the star cluster close to the radio source Sagittarius A* (Sgr A*), orbiting it with a period of 16.0518 years, a semi-major axis of about 970 au, and a pericenter distance of 17 light hours (18 Tm or 120 au) – an orbit with a period only about 30% longer than that of Jupiter around the Sun, but coming no closer than about four times the distance of Neptune from the Sun. The mass when the star first formed is estimated by the European Southern Observatory (ESO) to have been approximately 14 M☉. Based on its spectral type (B0V ~ B3V), it probably has a mass of 10 to 15 solar masses.
Its changing apparent position has been monitored since 1995 by two groups (at UCLA and at the Max Planck Institute for Extraterrestrial Physics) as part of an effort to gather evidence for the existence of a supermassive black hole in the center of the Milky Way galaxy. The accumulating evidence points to Sgr A* as being the site of such a black hole. By 2008, S2 had been observed for one complete orbit. In 2020, partway through its next orbit, the GRAVITY collaboration released an analysis showing full agreement with Schwarzschild geodesics.
A team of astronomers, mainly from the Max Planck Institute for Extraterrestrial Physics, used observations of S2's orbital dynamics around Sgr A* to measure the distance from the Earth to the galactic center. They determined it to be 7.94 ± 0.42 kiloparsecs, in close agreement with prior determinations by other methods.
The designation S0–2 was first used in 1998. S0 indicates a star within one arc-second of Sgr A*, indicating the galactic centre, and S0–2 was the second closest star seen at the time of the measurements. The star had been catalogued simply as S2 a year earlier, the second of eleven infrared sources near the galactic centre, numbered approximately anti-clockwise. It is a coincidence that the star is numbered "2" in both lists; other catalogs number it differently.
The highly eccentric orbit of S2 will give astronomers an opportunity to test for various effects predicted by general relativity and even extra-dimensional effects. These effects reached a maximum at closest approach, which occurred in mid-2018. Given a recent estimate of 4.31 million M☉ for the mass of the Sgr A* black hole and S2's close approach, this makes S2's the fastest known ballistic orbit, reaching speeds exceeding 5,000 km/s (11,000,000 mph, or1⁄60 the speed of light) and acceleration of about 1.5 m/s2 (almost one-sixth of Earth's surface gravity).
The motion of S2 is also useful for detecting the presence of other objects near to Sgr A*. It is believed that there are thousands of stars, as well as dark stellar remnants (stellar black holes, neutron stars, white dwarfs) distributed in the volume through which S2 moves. These objects will perturb S2's orbit, causing it to deviate gradually from the Keplerian ellipse that characterizes motion around a single point mass. So far, the strongest constraint that can be placed on these remnants is that their total mass comprises less than one percent of the mass of the supermassive black hole.
2018 pericentre passage
In 2018, S2 made its closest approach to Sgr A*, reaching 7650 km/s or almost 3% of the speed of light, while passing the black hole at a distance of just 120 AU or about 1400 times its Schwarzschild radius. S2 reached its pericenter on May 19, 2018, while its velocity in the line of sight from Earth peaked in April, and later hit its minimum in late August and early September.
Independent analyses by the GRAVITY collaboration (led by Reinhard Genzel) and the KECK/UCLA Galactic Center Group (led by Andrea Ghez) revealed a combined transverse Doppler and gravitational redshift up to 200 km/s/c, in agreement with general relativity predictions.
In 2012, a star called S0–102 (or S55) was found to be orbiting even closer to the Milky Way's central supermassive black hole than S0–2 does. At one-sixteenth the brightness of S0–2, S0–102 was not initially recognized because it required many more years of observations to distinguish it from its local infrared background. S0–102 has an orbital period of 11.5 years, even shorter than that of S0–2. Of all the stars orbiting the black hole, only these two have their orbital parameters and trajectories fully known in all three dimensions of space. The discovery of two stars orbiting the central black hole so closely with their orbits fully described is of extreme interest to astronomers, as the pair together will allow much more precise measurements on the nature of gravity and general relativity around the black hole than would be possible from using S0–2 alone.
A still closer star S62 has since been discovered with an orbital period of 9.9 years.
Artist's annotation of S2 passing supermassive black hole at center of Milky Way, confirming gravitational red shift
- Lists of stars
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- on YouTube
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Media files used on this page
This is a mosaic image, one of the largest ever taken by NASA's Hubble Space Telescope, of the Crab Nebula, a six-light-year-wide expanding remnant of a star's supernova explosion. Japanese and Chinese astronomers recorded this violent event in 1054 CE, as did, almost certainly, Native Americans.
The orange filaments are the tattered remains of the star and consist mostly of hydrogen. The rapidly spinning neutron star embedded in the center of the nebula is the dynamo powering the nebula's eerie interior bluish glow. The blue light comes from electrons whirling at nearly the speed of light around magnetic field lines from the neutron star. The neutron star, like a lighthouse, ejects twin beams of radiation that appear to pulse 30 times a second due to the neutron star's rotation. A neutron star is the crushed ultra-dense core of the exploded star.
The Crab Nebula derived its name from its appearance in a drawing made by Irish astronomer Lord Rosse in 1844, using a 36-inch telescope. When viewed by Hubble, as well as by large ground-based telescopes such as the European Southern Observatory's Very Large Telescope, the Crab Nebula takes on a more detailed appearance that yields clues into the spectacular demise of a star, 6,500 light-years away.The newly composed image was assembled from 24 individual Wide Field and Planetary Camera 2 exposures taken in October 1999, January 2000, and December 2000. The colors in the image indicate the different elements that were expelled during the explosion. Blue in the filaments in the outer part of the nebula represents neutral oxygen, green is singly-ionized sulfur, and red indicates doubly-ionized oxygen.
Author/Creator: ESO, European Southern Observatory, Licence: CC BY 4.0
Artist's impression of "the oldest star of our Galaxy": HE 1523-0901
- About 13.2 billion years old
- Approximately 7500 light years far from Earth
- Published as part of Hamburg/ESO Survey in the May 10 2007 issue of The Astrophysical Journal
Author/Creator: Me, Licence: Copyrighted free use
SVG replacement for File:Spaceship and the Sun.jpg. A stylized illustration of a spaceship and the sun, based on the description of the emblem of the fictional Galactic Empire in Isaac Asimov's Foundation series ("The golden globe with its conventionalized rays, and the oblique cigar shape that was a space vessel"). This image could be used as a icon for science-fiction related articles.
This view of the rising Earth greeted the Apollo 8 astronauts as they came from behind the Moon after the fourth nearside orbit. Earth is about five degrees above the horizon in the photo. The unnamed surface features in the foreground are near the eastern limb of the Moon as viewed from Earth. The lunar horizon is approximately 780 kilometers from the spacecraft. Width of the photographed area at the horizon is about 175 kilometers. On the Earth 240,000 miles away, the sunset terminator bisects Africa.
This is a montage of planetary images taken by spacecraft managed by the Jet Propulsion Laboratory in Pasadena, CA. Included are (from top to bottom) images of Mercury, Venus, Earth (and Moon), Mars, Jupiter, Saturn, Uranus and Neptune. The spacecraft responsible for these images are as follows:
- the Mercury image was taken by Mariner 10,
- the Venus image by Magellan,
- the Earth and Moon images by Galileo,
- the Mars image by Mars Global Surveyor,
- the Jupiter image by Cassini, and
- the Saturn, Uranus and Neptune images by Voyager.
- Pluto is not shown as it is no longer a planet. The inner planets (Mercury, Venus, Earth, Moon, and Mars) are roughly to scale to each other; the outer planets (Jupiter, Saturn, Uranus, and Neptune) are roughly to scale to each other. PIA 00545 is the same montage with Neptune shown larger in the foreground. Actual diameters are given below:
- Sun (to photosphere) 1,392,684 km
- Mercury 4,879.4 km
- Venus 12,103.7 km
- Earth 12,756.28 km
- Moon 3,476.2 km
- Mars 6,804.9 km
- Jupiter 142,984 km
- Saturn 120,536 km
- Uranus 51,118 km
- Neptune 49,528 km
Author/Creator: ESO/M. Kornmesser, Licence: CC BY 4.0
This artist’s impression shows the path of the star S2 as it passes very close to the supermassive black hole at the centre of the Milky Way. As it gets close to the black hole, the very strong gravitational field causes the colour of the star to shift slightly to the red, an effect of Einstein’s general theory of relativity. In this graphic the colour effect and size of the objects have been exaggerated for clarity.
Author/Creator: Cmglee, Licence: CC BY-SA 3.0
Inferred well-determined orbits of 6 stars around supermassive black hole candidate Sagittarius A* at the Milky Way galactic centre based on data from "SINFONI in the Galactic Center: Young Stars and Infrared Flares in the Central Light-Month" by Eisenhauer et al, The Astrophysical Journal, 628:246-259, 2005. Note: For the comparison image, the scale is assumed to be 7940 AU/arcsecond (1" in radians × 7.94 kpc in AU) or 7.94 AU/pixel in SVG. At this scale, the distance between the Sun and Proxima Centauri (its nearest star) is 33.8 times (268 000 AU ÷ 7940 AU/arcsecond) the height of the chart.
Author/Creator: ESO/MPE/S. Gillessen et al., Licence: CC BY 4.0
Image of the galactic centre. For the interferometric GRAVITY observations the star IRS 16C was used as a reference star, the actual target was the star S2. The position of the centre, which harbours the (invisible) black hole known as Sgr A*,with 4 million solar masses, is marked by the orange cross.
Author/Creator: European Southern Observatory, Licence: CC BY 4.0
The S2 (star) orbiting the center of the Milky Way Galaxy. This is a screenshot from a video created by the European Southern Observatory, and features the last moment of the video where S2 has fully completed its clockwise orbit.