# Stars/Galaxies/Milky Way

The Milky Way is a name for the galaxy we live in.
It is a member of the Local Group of galaxies.

## Milky Way theory

Here's a theoretical definition:
Def. a large spiral galaxy that includes the Sun is called the Milky Way.
Def. any "galaxy, considerably smaller than the Milky Way, that has only several billions of stars" is called a dwarf galaxy.
Def. a "faint galaxy, devoid of gas, having a higher than normal proportion of dark matter; especially those that orbit the Milky Way and Andromeda is called a dwarf spheroidal galaxy.
Def. originating "outside the Milky Way galaxy" or "outside of a galaxy" is called extragalactic.

## Sources

The center of the galaxy is in the direction of Sagittarius, and the Milky Way "passes" (going westward, Earthview, to the right) through Scorpius, Ara, Norma, Triangulum Australe, Circinus, Centaurus, Musca, Crux, Carina, Vela, Puppis, Canis Major, Monoceros, Orion & Gemini, Taurus, Auriga, Perseus, Andromeda, Cassiopeia, Cepheus & Lacerta, Cygnus, Vulpecula, Sagitta, Aquila, Ophiuchus, Scutum, and back to Sagittarius.
In the diagram at right, spherical triangles are shown for deriving the relationship between equatorial and galactic coordinate systems. The yellow plane is the celestial equator, P and P' are its celestial poles. The green plane is the galactic plane, G and G' are its galactic poles. B is the direction towards the galactic center, which is now used as a starting point for measuring galactic longitude. Prior to 1959, the starting point was the intersection of the galactic and equatorial planes (point C).

## Backgrounds

By comparing the soft X-ray background with the distribution of neutral hydrogen, it is generally agreed that within the Milky Way disk, super soft X-rays are absorbed by this neutral hydrogen.
The ROSAT image at the right is an Aitoff-Hammer equal-area map in galactic coordinates with the Galactic center in the middle of the 0.25 keV diffuse X-ray background.

## X-rays

The 0.75 keV diffuse X-ray background map at the right from the ROSAT all-sky survey in the same projection as the SXRB and neutral hydrogen. The image shows a radically different structure than the 0.25 keV X-ray background. At 0.75 keV, the sky is dominated by the relatively smooth extragalactic background and a limited number of bright extended Galactic objects.
The second image at the right is a composite image (optical plus X-ray) of the central region of our Milky Way galaxy.
"The Milky Way fills the background of the image [at second right] with countless yellowish older stars. Some of them appear fainter and redder because of the dust in NGC 6559."
The third image at the right is a Chandra X-ray Observatory image of the Galactic Central region.
At the lower right is a composite image. "In celebration of the International Year of Astronomy 2009, NASA's Great Observatories -- the Hubble Space Telescope, the Spitzer Space Telescope, and the Chandra X-ray Observatory -- have produced a matched trio of images of the central region of our Milky Way galaxy. Each image shows the telescope's different wavelength view of the galactic center region, illustrating the unique science each observatory conducts."
"In this spectacular image, observations using infrared light and X-ray light see through the obscuring dust and reveal the intense activity near the galactic core. Note that the center of the galaxy is located within the bright white region to the right of and just below the middle of the image. The entire image width covers about one-half a degree, about the same angular width as the full moon."
"Although best known for its visible-light images, Hubble also observes over a limited range of infrared light [Figure 2 (middle frame of poster) at third lower right]. The galactic center is marked by the bright patch in the lower right. Along the left side are large arcs of warm gas that have been heated by clusters of bright massive stars. In addition, Hubble uncovered many more massive stars across the region. Winds and radiation from these stars create the complex structures seen in the gas throughout the image.This sweeping panorama is one of the sharpest infrared pictures ever made of the galactic center region."
"Spitzer's infrared-light observations provide a detailed and spectacular view of the galactic center region [Figure 1 (top frame of poster) lowest left]. The swirling core of our galaxy harbors hundreds of thousands of stars that cannot be seen in visible light. These stars heat the nearby gas and dust. These dusty clouds glow in infrared light and reveal their often dramatic shapes. Some of these clouds harbor stellar nurseries that are forming new generations of stars. Like the downtown of a large city, the center of our galaxy is a crowded, active, and vibrant place."
The fourth image at the right is a 400 by 900 light-year mosaic of several Chandra X-ray Observatory images of the Galactic center region.
At top left is an image of the Galactic central region using the Chandra X-ray Observatory.
"X-rays detected by Chandra expose a wealth of exotic objects and high-energy features [Figure 3 (bottom frame of poster)]. In this image, pink represents lower energy X-rays and blue indicates higher energy. Hundreds of small dots show emission from material around black holes and other dense stellar objects. A supermassive black hole -- some four million times more massive than the Sun -- resides within the bright region in the lower right. The diffuse X-ray light comes from gas heated to millions of degrees by outflows from the supermassive black hole, winds from giant stars, and stellar explosions. This central region is the most energetic place in our galaxy."
The second image at left is a "400 by 900 light-year mosaic of several Chandra images of the central region of our Milky Way galaxy ... [It] reveals hundreds of white dwarf stars, neutron stars, and black holes bathed in an incandescent fog of multimillion-degree gas. The supermassive black hole at the center of the galaxy is located inside the bright white patch in the center of the image. The colors indicate X-ray energy bands - red (low), green (medium), and blue (high)."

## Opticals

Of the Local Group, “[i]ts two dominant galaxies, the Milky Way and Andromeda (M31), are separated by a distance of ~700 kpc and are moving toward each other with a radial velocity of about -117 km s$^{-1}$ (Binney & Tremaine 1987, p. 605).” making Andromeda one of the few blueshifted galaxies. The Andromeda Galaxy and the Milky Way are thus expected to collide in about 4.5 billion years, although the details are uncertain since Andromeda's tangential velocity with respect to the Milky Way is only known to within about a factor of two. A likely outcome of the collision is that the galaxies will merge to form a giant elliptical galaxy. Such events are frequent among the galaxies in galaxy groups. The fate of the Earth and the Solar System in the event of a collision are currently unknown. If the galaxies do not merge, there is a small chance that the Solar System could be ejected from the Milky Way or join Andromeda.

## Visuals

The image at the right is a visible image of the Galactic Central region from the Hubble Space Telescope.

## Reds

In the image on the right, the Milky Way is viewed by H-Alpha Sky Survey.

## Infrareds

Huge, cold clouds of gas and dust in our own galaxy, as well as in nearby galaxies, glow in far-infrared light. This is due to thermal radiation of interstellar dust contained in molecular clouds.

## Submillimeters

"The centre of our Galaxy, the Milky Way, lies 27,000 light years from Earth. [The Submillimetre Common User Bolometer Array] SCUBA shows us [in the image above] an exotic region of gas clouds, bubbles, and threads, shaped by stars, supernovae, and magnetic fields. The view is blocked at optical wavelengths by the intervening dust. Credit: D. Pierce-Price et al."
"This is a colour composite image [at right] of the central region of our Milky Way galaxy, about 26 000 light years from Earth. Giant clouds of gas and dust are shown in blue, as detected by the LABOCA instrument on the Atacama Pathfinder Experiment (APEX) telescope at submillimetre wavelengths (870 micron). The image also contains near-infrared data from the 2MASS project at K-band (in red), H-band (in green), and J-band (in blue). The image shows a region approximately 100 light-years wide."
Terahertz radiation is emitted as part of the black body radiation from anything with temperatures greater than about 10 kelvin. While this thermal emission is very weak, observations at these frequencies are important for characterizing the cold 10-20K dust in the interstellar medium in the Milky Way galaxy, and in distant starburst galaxies. Telescopes operating in this band include the James Clerk Maxwell Telescope, the Caltech Submillimeter Observatory and the Submillimeter Array at the Mauna Kea Observatory in Hawaii, the BLAST balloon borne telescope, the Herschel Space Observatory, and the Heinrich Hertz Submillimeter Telescope at the Mount Graham International Observatory in Arizona. The Atacama Large Millimeter Array, under construction, will operate in the submillimeter range. The opacity of the Earth's atmosphere to submillimeter radiation restricts these observatories to very high altitude sites, or to space.

## Radios

At right is a radio image of the central region of the Milky Way galaxy. The arrow indicates a supernova remnant which is the location of a newly-discovered transient, the bursting low-frequency radio source GCRT J1745-3009.

## Superluminals

"In far-distant quasars and galaxies, millions or even billions of light-years away, the gravitational energy of supermassive black holes is capable of accelerating "jets" of subatomic particles to speeds approaching that of light. The VLA has observed such jets for many years. In some of these jets, blobs of material have been seen to move at apparent speeds greater than that of light -- a phenomenon called superluminal motion. The apparent faster-than-light motion actually is an illusion seen when a jet of material is travelling close to -- but below -- the speed of light and directed toward Earth."
"In the Spring of 1994, Felix Mirabel from Saclay, France, and Luis Rodriguez, from the National Autonomous University in Mexico City, were observing an X-ray emitting object called GRS 1915+105, which had just shown an outburst of radio emission. This object was known to be about 40,000 light-years away, within our own Milky Way Galaxy -- in our own cosmic neighborhood. Their time series of VLA observations, seen in this image, showed that a pair of objects ejected from GRS 1915+105 were moving apart at an apparently superluminal speed. This was the first time that superluminal motion had been detected in our own Galaxy."
"This surprising result showed that the supermassive black holes at the centers of galaxies -- black holes millions of times more massive than the Sun -- have smaller counterparts capable of producing similar jet ejections. GRS 1915+105 is thought to be a double-star system in which one of the components is a black hole or neutron star only a few times the mass of the Sun. The more-massive object is pulling material from its stellar companion. The material circles the massive object in an accretion disk before being pulled into it. Friction in the accretion disk creates temperatures hot enough that the material emits X-rays, and magnetic processes are believed to accelerate the material in the jets."
"Since Mirabel and Rodriguez discovered the superluminal motion in GRS 1915+105, several other Galactic "microquasars" have been discovered and studied with the VLA and the VLBA. In 1999, NRAO astronomer Robert Hjellming turned the VLA toward a bursting microquasar within 24 hours of a reported X-ray outburst. Working with X-ray observers Donald Smith and Ronald Remillard of MIT, Hellming found that this object is a microquasar only 1,600 light-years away, making it the closest black hole to Earth yet discovered."
"Microquasars within our own Galaxy, because they are closer and thus easier to study, have become invaluable "laboratories" for revealing the physical processes that produce superfast jets of material. For discovering this new class of celestial object, Mirabel and Rodriguez received the prestigous Bruno Rossi Prize of the American Astronomical Society in 1997."
"On the far side of our Galaxy, gas clouds explode away from a small black hole. This might seem peculiar, as black holes are supposed to attract matter. But material falling toward a black hole collides and heats up, creating an environment similar to a quasar that is far from stable. In the [at second right] time-lapse sequence, micro-quasar GRS1915 expels bubbles of hot gas in spectacular jets. These computer enhanced radio images show one plasma bubble coming almost directly toward us at 90 percent the speed of light, and another moving away. Each of the four frames marks the passage of one day. Originally detected on October 29th, these bubbles have now faded from view."

## Astrochemistry

"Spectra of the helium 2.06 µm and hydrogen 2.17 µm lines ... confirm the existence of an extended region of high-velocity redshifted line emission centered near [Sgr A$^*$/IRS 16]."

## Zincs

The "column density weighted metallicity of [Complete Optical and Radio Absorption Line System] CORALS [damped Lyα systems] DLAs, [⟨(Zn/H)$_{DLA}$⟩] = −0.88 ± 0.21 in the redshift interval 1.86 < z$_{abs}$ < 3.45, is only marginally higher than that of a control sample from the recent compilation by Kulkarni et al., [⟨(Zn/H)$_{DLA}$⟩] = −1.09 ± 0.10."

## Nebulas

The "forms divide themselves naturally into two groups:
* those [nebulae] found in or near the Milky Way and
* those in moderate or high galactic latitudes."

## Galaxy stars

"Astronomers typically infer important properties of globular clusters by looking at the light of their constituent stars. But they have to be very careful when they observe objects like Messier 56, which is located close to the Galactic plane. This region is crowded by “field-stars”, in other words, stars in the Milky Way that happen to lie in the same direction but do not belong to the cluster. These objects can contaminate the light, and hence undermine the conclusions reached by astronomers." Bold added.
"Astronomers have found that the majority of clusters with this type of chemical makeup lie along a plane in the Milky Way’s halo. This suggests that such clusters were captured from a satellite galaxy, rather than being the oldest members of the Milky Way's globular cluster system as had been previously thought."
At left the "NASA/ESA Hubble Space Telescope image shows a compact and distant globular star cluster that lies in one of the smallest constellations in the night sky, Delphinus (The Dolphin). Due to its modest size, great distance and relatively low brightness, NGC 7006 is often ignored by amateur astronomers. But even remote globular clusters such as this one appear bright and clear when imaged by Hubble’s Advanced Camera for Surveys." The visual portion is centered at 606 nm (blue), a visual + infrared is green, and the infrared is centered at 814 nm (red).
"NGC 7006 resides in the outskirts of the Milky Way. It is about 135 000 light-years away, five times the distance between the Sun and the centre of the galaxy, and it is part of the galactic halo. This roughly spherical region of the Milky Way is made up of dark matter, gas and sparsely distributed stellar clusters." Bold added.
"Like other remote globular clusters, NGC 7006 provides important clues that help astronomers to understand how stars formed and assembled in the halo. The cluster now pictured by Hubble has a very eccentric orbit indicating that it may have formed independently, in a small galaxy outside our own that was then captured by the Milky Way."
"Although NGC 7006 is very distant for a Milky Way globular cluster, it is much closer than the many faint galaxies that can be seen in the background of this image. Each of these faint smudges is probably accompanied by many globular clusters similar to NGC 7006 that are too faint to be seen even by Hubble."
"This image was taken using the Wide Field Channel of the Advanced Camera for Surveys, in a combination of visible and near-infrared light. The field of view is a little over 3 by 3 arcminutes."
"In [the image on the right is a] stereographic projection, the Milky Way curves around the entire image in an arc, with the newly discovered river of stars displayed in red and covering almost the entire southern Galactic hemisphere."
This "cluster of stars formed in our galaxy. Since then, that cluster has whipped four long circles around the edge of the Milky Way. In that time, the Milky Way's gravity has stretched that cluster out from a blob into a long stellar stream. Right now, the stars are passing relatively close to Earth, just about 330 light-years away."
"The river, which is 1,300 light-years long and 160 light-years wide, winds through the Milky Way's vast, dense star field. But 3D-mapping data from Gaia, a European Space Agency spacecraft, showed that the stars in the stream moved together at roughly the same speed and in the same direction."
"Identifying nearby disk streams is like looking for the proverbial needle in a haystack. Astronomers have been looking at, and through, this new stream for a long time, as it covers most of the night sky, but only now realize it is there, and it is huge, and shockingly close to the sun."
"Finding things close to home is very useful, it means they are not too faint nor too blurred for further detailed exploration, [an] astronomer's dream."

## Galactic astronomy

"The space distribution of stars and the chemical elements in the Milky Way Galaxy are discussed along with the large-scale structure and stellar content of galaxies, the solar motion, the stellar residual-velocity distribution, and the rotation of galaxies."

## Intergalactic medium

The existence of hot coronal gas within the Milky Way halo is one of the major discoveries of the ROSAT mission.
For no galactic corona, the gas pressure above some height (H) above the galactic disk is small compared to that in the disk. The galactic halo region is that region at a distance greater than H. When the only source of energy and mass for the halo is the galactic disk, gas streams into the halo until the pressure gradient is such that the pressure in the halo approaches the disk pressure, on a timescale of about 5 x 10$^7$ yr.
The X-ray luminosity (L$_x$) of the galactic coronal cloud is 1.5 x 10$^{40}$ erg s$^{-1}$.

## Galaxies

The Milky Way sits in a large, flat array of galaxies called the Local Sheet, which bounds the Local Void. The Local Void extends approximately 60 megaparsecs, beginning at the edge of the Local Group. It is believed that the distance from Earth to the centre of the Local Void must be at least 23 megaparsecs (75 Mly).
"Observations of 120,000 galaxies [bolster] the Milky Way’s loner status".
The "Milky Way has far fewer neighbors than it should. There [is] a rise in density about 1 billion light-years out, suggesting the Milky Way resides in an abyss about 2 billion light-years wide."
"If you don’t account for the void effects, you could mistake this relationship to indicate that there is too much expansion."

## Galactic coordinates

In the artist's conception of the Milky Way at right is a grid over the concept. The galactic coordinate system is a celestial coordinate system in spherical coordinates, with the Sun as its center, a primary direction aligned with the approximate center of the Milky Way galaxy, and a fundamental plane approximately in the galactic plane. It has a right-handed convention, meaning that coordinates are positive toward the north and toward the east in the fundamental plane.

## Classical history

The classical history period dates from around 2,000 to 1,000 b2k.
Democritus "lived at Abdère 300 years before the Christian era [2300 b2k]. In a short fragment quoted by Plutarch, he declares that the Milky Way is an agglomeration of small stars too far away to be perceived singly."

## Astromathematics

Astronomical radiation mathematics is the laboratory mathematics such as simulations that are generated to try to understand the observations of radiation astronomy.

## Hypotheses

* The Milky Way may have formed by the engulfing of nearby galaxies.