Milky Way

## General Information

The Milky Way galaxy, which encompasses the Solar System, gets its name from how it appears in the night sky from Earth: a fuzzy strip of light made up of stars that are too close together to be seen individually. The Milky Way's disk-shaped structure is seen from within, giving the impression that it is a band when viewed from Earth. Using his telescope, Galileo Galilei separated the band of light into individual stars for the first time in 1610. The majority of astronomers believed that the Milky Way held all of the stars in the universe until the early 1920s. Edwin Hubble's observations after the Great Debate of 1920 between astronomers Harlow Shapley and Heber Doust Curtis shown that the Milky Way is but one galaxy among many.

The Milky Way is a barred spiral galaxy that is only around 1,000 light-years thick at the spiral arms, with an estimated D25 isophotal diameter of 26.8 ± 1.1 kiloparsecs. According to recent calculations, there might be a dark matter region with a diameter of over 2 million light-years that also includes a few bright stars. In addition to having several satellite galaxies, the Milky Way is a member of the Local Group of galaxies, which is a subcluster of the Virgo Supercluster, which is a subcluster of the Laniakea Supercluster.

It is thought to have between 100 and 400 billion stars and at least that many planets. The Solar System is situated on the inner margin of the Orion Arm, one of the spiral-shaped concentrations of gas and dust, around 27,000 light-years from the Galactic Center. A bulge and one or more bars that extend from the bulge are formed by the stars in the innermost 10,000 light-years. The Galactic Center is a supermassive black hole with a mass of 4.100 million solar masses, also known as Sagittarius A\*, a strong radio emitter. At a speed of around 220 km/s, stars and gasses circle the Galactic Center at a variety of distances. The Milky Way's constant rotating speed appears to defy Keplerian dynamics principles and implies that a large portion of its mass remains unseen by telescopes because it neither emits nor absorbs electromagnetic radiation. We refer to this hypothetical material as "dark matter". At the Sun's radius, the spinning period is around 212 million years.

About 600 km/s is the average speed of the Milky Way as a whole, measured in relation to extragalactic frames of reference. Since the oldest stars in the Milky Way are almost as ancient as the universe, they most likely originated not too long after the Big Bang's Dark Ages.

### History

Aristotle writes in Meteorologica that the Greek philosophers Anaxagoras and Democritus believed that certain stars get light from the Sun but have solar rays conceal their brilliance, and that this is how the Milky Way is made up of the starlight that is not immediately visible owing to Earth's shadow. Because of its outermost position in the atmosphere, which constitutes its vast circle, Aristotle himself thought that the Milky Way was a result of stars burning that did not evaporate and was instead a component of Earth's upper atmosphere, along with the stars. According to him, the Milky Way Galaxy's milky appearance results from Earth's atmospheric refraction. Olympiodorus the Younger, a Neoplatonist philosopher, challenged this theory, claiming that if the Milky Way were sublunary, it should look differently at different times and locations on Earth and exhibit parallax, which it does not. He believes the Milky Way to be heavenly. Later on, the Muslim world would be influenced by this concept.

The Milky Way is said to be "a collection of countless fragments of the nature of nebulous stars," according to the Persian astronomer Al-Biruni.Avempace, an Andalusian astronomer, suggested that although the Milky Way was composed of several stars, it looked to be a single, continuous picture within Earth's atmosphere. He supported this theory with the observation that Jupiter and Mars coincided in 1106 or 1107. The Milky Way was described as "a myriad of tiny stars packed together in the sphere of the fixed stars" by Ibn Qayyim al-Jawziyya.

Galileo Galilei's discovery in 1610 that the Milky Way is made up of a vast number of dim stars using a telescope provided evidence that the Milky Way is made up of many stars. Galileo also came to the conclusion that Earth's atmospheric refraction was the cause of the Milky Way's appearance. Immanuel Kant conjectured in a book from 1755—drawing off earlier research by Wright—that the Milky Way may be a revolving mass of many stars bound together by forces of gravity similar to those in the Solar System, but on a far grander scale. From our vantage point inside the disk, the resultant starry structure would seem like a band in the sky. A further theory put out by Wright and Kant suggested that some of the nebulae that may be seen in the night sky could really be individual "galaxies" that resemble our own. Though the phrase "island universes" persisted until the 1930s, Kant referred to both the Milky Way and the "extragalactic nebulae" as such.

William Herschel made the first attempt in 1785 to characterize the Milky Way's structure and the Sun's location within it by meticulously counting the stars in the various visible sky areas. He sketched up a Milky Way figure, showing the Solar System in the center.

With the use of a newly built telescope, Lord Rosse was able to differentiate between elliptical and spiral-shaped nebulae in 1845. In some of these nebulae, he was also able to distinguish individual point sources, supporting an earlier hypothesis of Kant's.

Jacobus Kapteyn discovered in 1904 that star proper motions were not random, as was previously thought, and that stars could be split into two streams that moved almost oppositely. Later on, it was discovered that Kapteyn's data had been the first indication of our galaxy's rotation, which eventually helped Bertil Lindblad and Jan Oort discover galactic rotation.

Heber Curtis made the discovery of the nova S Andromedae inside the Great Andromeda Nebula in 1917. Looking through the picture archive, he discovered an additional 11 novae. These novae were found to be, on average, 10 magnitudes fainter than those found in the Milky Way by Curtis. Consequently, he calculated an estimated distance of 150,000 parsecs. He developed an interest in the "island universes" theory, according to which spiral nebulae are separate galaxies. The nature of the Milky Way, spiral nebulae, and the universe's dimensions were the topics of the Great Debate between Heber Curtis and Harlow Shapley in 1920.

In the early 1920s, Edwin Hubble used the 2.5 m Hooker telescope at Mount Wilson observatory to definitively resolve the dispute. He was able to take astronomical images that resolved the outer regions of certain spiral nebulae as groups of individual stars because to the new telescope's ability to gather light. In order to determine the distance to the nebulae, he was also able to pinpoint a few Cepheid variations. He discovered that the Andromeda Nebula is too far away to be a component of the Milky Way, at 275,000 parsecs from the Sun.

### Properties

#### Astrography

By measuring the parallax of a billion stars, the ESA spacecraft Gaia calculates distances and is now mapping the Milky Way, with four map releases scheduled for 2016, 2018, 2021, and 2024. "Transformational" data has been described for Gaia. According to estimates, Gaia has increased the number of stars seen from around 2 million in the 1990s to 2 billion. It has increased the observable volume of space by an accuracy factor of 1,000 and a radius factor of 100.

##### Locations

The Sun is situated between the Split linear structures and the Radcliffe wave, close to the inner edge of the Orion Arm, inside the Local Fluff of the Local Bubble. According to Gillessen's analysis of star orbits around Sgr A\*, the Sun is situated at a distance of approximately 27.14 ± 0.46 kly from the Galactic Center. Boehle used a stellar orbit study to get a lesser value of 25.64 ± 0.46 kly. At this moment, the Sun is located 5–30 parsecs above, or north of, the Galactic disk's center plane. The Perseus Arm, the arm after the local arm, is located around 2,000 parsecs away. The galactic habitable zone of the Milky Way is where the Sun and the Solar System are situated.

In a sphere 15 parsecs in radius from the Sun, there are around 208 stars brighter than absolute magnitude 8.5, resulting in a density of one star per 69 cubic parsecs, or 2,360 cubic light-years. However, the density of stars within 5 parsecs of the Sun is around one star per 8.2 cubic parsecs, or one per 284 cubic light-years, since there are 64 known stars in this region. This shows that there are considerably more faint stars than bright stars: there are 15.5 million stars brighter than visible magnitude 14 but only 500 stars brighter than apparent magnitude 4 in the entire sky.

The direction in which the Sun moves through space in the Milky path is known as the solar apex, or the apex of the Sun's path. The Sun is moving through space in the approximate direction of the star Vega, which is located close to the constellation Hercules. This motion is angled about 60 sky degrees in the direction of the Galactic Center. It is anticipated that the Sun's orbit around the Milky Way is generally elliptical, with additional disturbances coming from non-uniform mass distributions and the Galactic spiral arms. Furthermore, the Sun traverses the Galactic plane around 2.7 times for each orbit. This is quite similar to the operation of a non-drag force term simple harmonic oscillator. Until recently, it was believed that these oscillations corresponded with times of mass extinction of Earth's lifeforms. No association was found in a reanalysis based on CO data, examining the impacts of the Sun's transit through the spiral structure.

A single circle around the Milky Way takes the Solar System around 240 million years to complete. Thus, over its lifespan, the Sun is estimated to have completed 18–20 orbits and 1/1250 revolutions since the beginning of humankind. With respect to the Milky Way, the Solar System orbits at a speed of around 220 km/s, or 0.073% the speed of light. At 84,000 km/h, the Sun travels through the heliosphere. The Solar System takes around 1,400 years to travel one light-year at current pace, or eight days to reach one astronomical unit (AU).Following the ecliptic, the Solar System is traveling toward the zodiacal constellation Scorpius.

##### Quadrants

One of the four circular sectors that make up the Milky Way's divide is referred to as a "quadrant of the Milky Way" or a galactic quadrant. Within the field of astronomy, the galactic coordinate system is used to define the galactic quadrants, with the Sun serving as the system's origin.

Ordinals are used to describe quadrants; examples include "first galactic quadrant," "second galactic quadrant," and "third quadrant of the Milky Way." The quadrants are as follows as viewed from the north galactic pole, with 0° representing the ray that passes through the galactic center after beginning at the Sun:

quadrant
 2nd
 3rd
 4th
(360° ≅ 0°)

when viewed from north of the Galactic Center, the galactic longitude (ℓ) increases counter-clockwise; when viewed from south of the Galactic Center, ℓ increases clockwise.

#### Size

One of the two biggest galaxies in the Local Group is the Milky Way, albeit it is unclear how big its galactic disk is and how much of an impact it has on the isophotal diameter. Regarding an extrapolation of the exponential disk with the scale length of the inner disc, it is estimated that the significant bulk of stars in the galaxy lie within the 26 kpc diameter, and that the number of stars beyond the outermost disc drastically decreases to a very small number.

Astronomers employ a variety of techniques to determine a galaxy's size, and the outcomes of these techniques might vary from one another. The most often used technique is the D25 standard, which is the isophote at which a galaxy's photometric brightness in the B-band exceeds 25 mag/arcsec2. A 1997 estimate by Goodwin et al. modeled the link between the surface brightnesses of Cepheid variable stars in 17 other spiral galaxies and the distribution of these stars inside the Milky Way. By assuming an exponential disc to represent the galactic disk and adopting a central surface brightness of 22.1±0.3 B-mag/arcsec−2, this resulted in an isophotal diameter for the Milky Way of 26.8 ± 1.1 kiloparsecs.

This is much less than the isophotal diameter of the Andromeda Galaxy and somewhat lower than the average isophotal size of the galaxies, which is 28.3 kpc. The study comes to the conclusion that the Milky Way and the Andromeda Galaxy are normal, conventional spiral galaxies rather than extraordinarily massive ones, as was previously commonly assumed. They are also among the biggest known galaxies. In order to compare the Milky Way's relative physical scale, imagine that the Solar System, including Neptune, were as big as a US quarter. In that case, the Milky Way would be roughly equivalent to the longest north-south line in the contiguous United States. An even earlier research, conducted in 1978, estimated the Milky Way's diameter to be around 23 kpc.

According to a 2015 paper, there is a ring-shaped star filament known as the Triangulum–Andromeda Ring that wraps around the Milky Way at a diameter of at least 50 kpc. This filament, along with Monoceros Ring, is thought to be primarily the result of disk oscillations and may be a component of the Milky Way's outer disk, which would explain why the stellar disk has grown to this size. Later, in 2018, a more recent article partially disproved this theory and backed the conclusion that the RR Lyrae stars' velocity dispersion was larger and that the Monoceros Ring, A13, and TriAnd Ring were stellar overdensities that were thrown off from the main stellar disk.

Another 2018 study found that disk stars are most likely present between 26 and 31.5 kpc from the Galactic Center, or possibly even further, well beyond the previously thought to be the abrupt drop-off of the disk's stellar density, which means that few or no stars are expected to be above this limit, with the exception of stars that are part of the galactic halo's older population.

A 2020 research estimated that the diameter of the Milky Way is 584 ± 122 kpc, with the dark matter halo's edge located at about 292 ± 61 kpc. The thickness of the Milky Way's star disk is likewise predicted to be as much as 1.35 kpc.

#### Mass

Stars and planets make up a very minor portion of the Milky Way's overall mass, which is estimated to be between 890 billion and 1.54 trillion times that of the Sun. Depending on the methodology and data utilized, estimates of the Milky Way's mass vary. 5.8×1011 solar masses is the low end of the estimation range, which is little less than that of the Andromeda Galaxy. 2009 measurements with the Very Long Baseline Array revealed that stars at the furthest edge of the Milky Way had velocities of up to 254 km/s.

This indicates that the Milky Way is more massive, approximately equaling the mass of the Andromeda Galaxy at 7×1011 M☉ inside 160,000 ly of its core, because orbital velocity relies on the total mass inside the orbital radius. A 2010 study of the radial velocity of halo stars revealed that 7×1011 M☉ of mass is contained within 80 kiloparsecs. The estimated mass of the Milky Way is 8.5×1011 M☉, which is less than half of the mass of the Andromeda Galaxy, according to a 2014 research. The Milky Way's mass is estimated to be 1.29×1012 M☉ in 2019.

Dark matter, an unidentified and unseen kind of stuff that interacts gravitationally with regular matter, appears to make up a significant portion of the Milky Way's mass. It is hypothesized that a dark matter halo extends somewhat consistently beyond one hundred kiloparsecs from the Galactic Center. Dark matter masses range from 1-1.5×1012 M☉, according to mathematical simulations of the Milky Way. Studies conducted in 2013 and 2014 show that the mass can vary from 8×1011 M☉ to 4.5×1012 M☉. In contrast, it is believed that the combined mass of all stars in the Milky Way is between 4.6 and 6.43 x 10^10^10^ M☉.

The interstellar medium, which surrounds stars, has 90% hydrogen and 10% helium by mass. Of the hydrogen, two thirds is found in the atomic form, while the other third is found as molecular hydrogen. The interstellar gas mass in the Milky Way is equivalent to 10% to 15% of the galaxy's total star mass. An extra 1% of the gas's mass is made up of interstellar dust.

Within a radius of around 39.5 kpc, researchers revealed in March 2019 that the virial mass of the Milky Way Galaxy is 1.54 trillion solar masses, nearly twice as much as previously determined. This suggests that dark matter accounts for about 90% of the galaxy's mass.

Astronomers revealed in September 2023 that the Milky Way Galaxy's virial mass is just 2.06 1011 solar masses—a tenth of the mass found in earlier research. The mass was calculated using information from the Gaia satellite.