If you like your Science Settled or your History served with a thick Gradualist Gloss then you should stop reading now and retreat to your preferred Safe Space.
The mainstream narrative implies the Milky Way is “our” galaxy.
The Milky Way is the galaxy that contains our Solar System.
The Milky Way is a barred spiral galaxy that has a diameter usually considered to be about 100,000–120,000 light-years but may be 150,000–180,000 light-years.
The Milky Way is estimated to contain 100–400 billion stars, although this number may be as high as one trillion.
There are likely at least 100 billion planets in the Milky Way.
The Sun lies close to the inner rim of the Milky Way’s Orion Arm, in the Local Interstellar Cloud or the Gould Belt, at a distance of 7.5–8.5 kpc (25,000–28,000 light-years) from the Galactic Center.
The Sun is contained within the Local Bubble, a space of rarefied hot gas, possibly produced by the supernova remnant Geminga.
The distance between the local arm and the next arm out, the Perseus Arm, is about 6,500 light-years
Sagittarius A is located at the rotational centre of “our” galaxy.
The Solar System is located within the disk, about 27,000 light-years from the Galactic Center, on the inner edge of one of the spiral-shaped concentrations of gas and dust called the Orion Arm.
The Galactic Center is the rotational center of the Milky Way.
The estimates for its location range from 7.6 to 8.7 kiloparsecs (about 25,000 to 28,000 lightyears) from Earth in the direction of the constellations Sagittarius, Ophiuchus, and Scorpius where the Milky Way appears brightest.
In 1958 the International Astronomical Union (IAU) decided to adopt the position of Sagittarius A as the true zero co-ordinate point for the system of galactic latitude and longitude.
In the equatorial coordinate system the location is: RA 17h 45m 40.04s, Dec −29° 00′ 28.1″ (J2000 epoch).
Sagittarius A or Sgr A is a complex radio source at the center of the Milky Way.
It is located in the constellation Sagittarius, and is hidden from view at optical wavelengths by large clouds of cosmic dust in the spiral arms of the Milky Way.
It consists of three components, the supernova remnant Sagittarius A East, the spiral structure Sagittarius A West, and a very bright compact radio source at the center of the spiral, Sagittarius A*.
These three overlap: Sagittarius A East is the largest, West appears off-center within East, and A* is at the center of West.
However, another candidate for “our” galaxy was discovered in 1994 called the Sagittarius Dwarf Spheroidal Galaxy.
The Sagittarius Dwarf Spheroidal Galaxy (Sgr dSph), also known as the Sagittarius Dwarf Elliptical Galaxy (Sgr dE or Sag DEG), is an elliptical loop-shaped satellite galaxy of the Milky Way.
It consists of four globular clusters, the main cluster having been discovered in 1994.
Sgr dSph is roughly 10,000 light-years in diameter, and is currently about 70,000 light-years from Earth, travelling in a polar orbit (i.e. an orbit passing over the Milky Way’s galactic poles) at a distance of about 50,000 light-years from the core of the Milky Way (about 1/3 the distance of the Large Magellanic Cloud).
In its looping, spiraling path, it has passed through the plane of the Milky Way several times in the past.
Using volumes of data from the Two-Micron All Sky Survey (2MASS), a major project to survey the sky in infrared light led by the University of Massachusetts, the astronomers are answering questions that have baffled scientists for decades and proving that our own Milky Way is consuming one of its neighbors in a dramatic display of ongoing galactic cannibalism. The study published in the Astrophysical Journal, is the first to map the full extent of the Sagittarius galaxy and show in visually vivid detail how its debris wraps around and passes through our Milky Way.
Sagittarius is 10,000 times smaller in mass than the Milky Way, so it is getting stretched out, torn apart and gobbled up by the bigger Milky Way.
A new infra red digital survey of the entire sky was made in 2003.
Teams from the universities of Virginia and Massachusetts used a supercomputer to sort through half a billion stars to create a — NEW STAR MAP showing our Solar System (yellow circle) to be at the exact nexus crossroads where two galaxies are actually joining.
Scientists Now Know: We’re Not From Here!
Viewzone – Dan Eden – June 2007
Further discoveries by astrophysics teams from both the University of Virginia and the University of Massachusetts Amherst, drawing upon the 2MASS Two-Micron All Sky Infrared Survey data, revealed the entire loop-shaped structure.
In 2003 with the aid of infrared telescopes and super computers, Steven Majewski, Michael Skrutskie, and Martin Weinberg were able to help create a new star map, picking out the full Sagittarius Dwarf presence, position, and looping shape from the mass of background stars and finding this smaller galaxy to be at a near right angle to the plane of the Milky Way.
The above images are taken from two animations made from the same simulation of a satellite being torn apart by the Milky Way’s tidal field.
The simulations followed the satellite’s evolution for several billion years.
In the both images, the Milky Way is represented in blue in the center, with the satellite orbiting around it.
The satellite itself appears much larger than it really is in these animations because the images were colored to emphasize the structure of the debris.
The one on the left has been color coded to show the density of stars being stripped from the satellite.
The one on the right has been color coded with the time when particles are lost from the satellite (ie the particles that are lost the first time the satellite goes around the Galaxy are shown in red, and each subsequent time in green, blue, yellow….)
The Milky Way as a Cannibal – Kathryn V. Johnston
Three newly-discovered streams arcing high over the Milky Way Galaxy are remnants of cannibalized galaxies and star clusters. The streams are between 13,000 and 130,000 light-years distant from Earth and extend over much of the Northern sky.
Two of the newly discovered streams are almost certainly the remains of ancient star clusters… The third stream is spread over a much larger region of the sky, and is most likely the scattered remains of a dwarf galaxy.
Rings Around the Galaxy (Annotated) – 30 May 2007
The M54 globular cluster is at the centre of Sagittarius Dwarf Spheroidal Galaxy.
Sgr dSph has four known globular clusters with one, M54, apparently residing at its core.
Messier 54 (also known as M54 or NGC 6715) is a globular cluster in the constellation Sagittarius. It was discovered by Charles Messier in 1778 and subsequently included in his catalog of comet-like objects.
Previously thought to belong to our galaxy at a distance from Earth of about 50,000 light-years, it was discovered in 1994 that M54 most likely belongs to the Sagittarius Dwarf Elliptical Galaxy (SagDEG), making it the first globular cluster formerly thought to be part of our galaxy reassigned to extragalactic status, even if not recognized as such for nearly two and a quarter centuries.
Modern estimates now place M54 at a distance of some 87,000 light-years, translating into a true radius of 150 light-years across.
Deciding which of these two galaxies is “our” galaxy is not as straightforward as originally envisioned because the Solar System is currently located in the Galactic Interchange area between the Milky Way and the Sagittarius Dwarf Spheroidal Galaxy.
If the Solar System belongs to the Milky Way then we may experience a bumpy ride as incoming objects from the Sagittarius Dwarf Spheroidal Galaxy are “gobbled up by the bigger Milky Way”.
On the other hand.
If the Solar System belongs to the Sagittarius Dwarf Spheroidal Galaxy then we may experience a bumpy ride as we are “gobbled up by the bigger Milky Way”.
Based on its current trajectory, the Sgr dSph main cluster is about to pass through the galactic disc of the Milky Way within the next hundred million years, while the extended loop-shaped ellipse is already extended around and through our local space and on through the Milky Way galactic disc, and in the process of slowly being absorbed into the larger galaxy, calculated at 10,000 times the mass of Sgr dSph.
At first, many astronomers thought that Sgr dSph had already reached an advanced state of destruction, so that a large part of its original matter was already mixed with that of the Milky Way. However, Sgr dSph still has coherence as a dispersed elongated ellipse, and appears to move in a roughly polar orbit around the Milky Way as close as 50,000 light-years from the galactic core. Although it may have begun as a ball of stars before falling towards the Milky Way, Sgr dSph is now being torn apart by immense tidal forces over hundreds of millions of years. Numerical simulations suggest that stars ripped out from the dwarf would be spread out in a long stellar stream along its path, which were subsequently detected.
However, some astronomers contend that Sgr dSph has been in orbit around the Milky Way for some billions of years, and has already orbited it approximately ten times. Its ability to retain some coherence despite such strains would indicate an unusually high concentration of dark matter within that galaxy.
In 1999, Johnston et al. concluded that Sgr dSph has orbited the Milky Way for at least one Gya and that during that time its mass has decreased by a factor of two or three. Its orbit is found to have galactocentric distances that oscillate between ~13 and ~41 kpc with a period of 550 to 750 million years. The last perigalacticon was approximately fifty million years ago. Also in 1999, Jiang & Binney found that it may have started its infall into the Milky Way at a point more than 200 kpc away if its starting mass was as large as ~1011M☉.
The models of both its orbit and the Milky Way’s potential field could be improved by proper motion observations of Sgr dSph’s stellar debris. This issue is under intense investigation, with computational support by the MilkyWay@Home project.
A simulation published in 2011 suggested that the Milky Way may have obtained its spiral structure as a result of repeated collisions with Sgr dSph.
If you were once comforted by the thought that a Milky Way was “the sweet you can eat between meals” then you are probably disconcerted to discover that the Milky Way might be slowly eating away at the Earth between meals.
The Milky Way bar is a chocolate bar manufactured and distributed by the Mars confectionery company. The American version of the Milky Way bar is made of chocolate-malt nougat topped with caramel and covered with milk chocolate and sold as the Mars bar everywhere else. The Milky Way is available everywhere else as well, but it is a different chocolate bar, with a character all of its own.
In 1935, the slogan was “The sweet you can eat between meals.”
Therefore, in the interests of Science, let’s dig a little deeper… to be continued.