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## 1. Galaxies

Galaxies are clouds of stars with millions or billions of stars and often also with gas clouds and so-called dark matter. There are uncountably many other galaxies beyond that of our own. You can see the closest ones with your own eyes, without using a telescope. These are the Small Magellanic Cloud (in the constellation Tucana - the Toucan), the Large Magellanic Cloud (in the constellation Dorado - the Goldfish), and the Andromeda Nebula (in the constellation Andromeda). Just like our Galaxy is made up of stars and other things with lots of empty space in between, so the Universe is made up of galaxies with lots of empty space in between. Many of the Messier objects are galaxies, for example M 31 (the Andromeda Nebula). You can see a list of them (with links to pictures) at //www.seds.org/messier/objects.html#galaxy.

Galaxies exist in many shapes and sizes. Edwin Hubble devised the classification scheme that is still used today to indicate the type of the galaxy. In Hubble's scheme there are four major types of galaxies: elliptical galaxies (indicated with the type letter E), spiral galaxies (S), barred spiral galaxies (SB), and irregular galaxies (Irr).

Elliptical galaxies have the shape of a rugby ball and look from all sides as an ellipse or circle. They usually contain no gas and no internal structure. The largest galaxies are elliptical galaxies. The type E is subdivided into subclasses that are indicated with a number. The smaller the number, the flatter the galaxy. E0 is flattest, and E9 the most spherical.

Spiral galaxies look like a flat disk and have spiral arms that wind their way from the middle to the edge of the disk. Spiral galaxies contain lots of gas clouds as well as stars. The subclasses of type S and Sa, Sb, and Sc. Subtype a has tightly wound spiral arms and a relatively large core. Going towards subtype c, the arms get less tightly wound and the core gets smaller. If a galaxy is between, for example, Sb and Sc, then it is sometimes classified as Sbc.

Barred spiral galaxies are like spiral galaxies but have a sort of bar running through the center. Spiral arms often sprout from the ends of the bar. Barred spiral galaxies have the same subtypes a, b, and c as ordinary spiral galaxies.

Irregular galaxies do not fit into any of the other classes and have, as their name suggests, an irregular shape. Many small galaxies are irregular galaxies.

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The difference between, for example, type SBa and type Sc is that SBa is a barred spiral and Sc an ordinary spiral, that SBa has more tightly wound spiral arms and a more prominent core than Sc.

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## 2. The Milky Way

The Milky Way is the galaxy that the Sun belongs to. The Milky way looks like a band of weakly shining clouds in the sky, that are always in the same place between the stars, and thus move with the stars along the sky. You can see the Milky Way only from dark places far from the light of cities. From the Netherlands it is hard to see the Milky Way, because almost everywhere there are one or two cities nearby that produce light pollution. The best time to see the Milky Way is when it crosses the sky straight overhead at midnight. The time of the year at which this happens depends on your latitude, but it is roughly at the beginning of January and the beginning of July.

The Milky Way passes through the following well-known constellations (and through others as well): the Swan, Cassiopeia, the Centaur, the Scorpion, the Archer.

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## 3. Milky Way and Galaxy

"The Galaxy" is the same as one of the meanings of "the Milky Way", namely the total of all gas clouds and stars, including the Sun, that are bound together by gravity and move through the Universe together, separate from other similar galaxies. That meaning is relatively young and dates to about 1930.

The other ― much older ― meaning of "Milky Way" is the weakly shining band in the sky. That meaning dates back more than 2000 years. At that time people didn't yet know that that band is made up of the light of countless stars that are too dim to see with the unaided eye, so it was logical to consider the Milky Way and the stars as two entirely separate things. That that weakly shining band is made up of countless stars became clear only after the invention of the telescope around 1608.

And that the Milky Way doesn't fill up the entire Universe but that there are more galaxies like our own, and that we therefore need a word for such a thing, has been known only since about 1930.

So the confusion that can arise between "Milky Way" as the name for only the weakly shining band in the sky, and "Milky Way" as the name for the entire galaxy is due to the meaning of "Milky Way" having been expanded in the course of time, when the nature of the Milky Way became clear.

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## 4. The Central Line of the Milky Way

The central line of the Milky Way is the equator of the galactic coordinate system as defined by the IAU. This coordinate system is described, e.g., at //en.wikipedia.org/wiki/Galactic_coordinates#In_terms_of_equatorial_coordinates.

The equatorial coordinates of the galactic equator referred to the standard equinox of B1950.0 can be found from the following formulas (based on Chapter 12 of [Meeus]:

\begin{align} \tan y \| = \frac{\tan(l - 123°)}{\sin(27.4°)} \\ α \| = y + 12.25° \\ \sin δ \| = \cos(27.4°) \cos(l - 123°) \end{align}

In these formulas, $$α$$ is the right ascension, $$δ$$ the declination, and $$l$$ the galactic longitude. These formulas are part of the definition of the galactic coordinate system, so they are exact. However, modern star atlases are usually referred to the standard equinox of J2000.0 instead of the one of B1950.0, so you'll have to correct for the precession between 1950 and 2000 if you wish to know the coordinates of the galactic equator relative to J2000.0.

There appear to be quite a few web sites that discuss or at least mention this coordinate transformation. Type "galactic coordinates 192.25" into your favorite search engine to find a couple.

Note: before 1959 an older galactic coordinate system was used, that had a slightly different equator.

I've found several references (such as the Wikipedia article) that confirm that it was the IAU that defined this new galactic coordinate system, but I have not found the exact IAU publication that contains it. Many IAU publications are not freely available to the general public.

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The Milky Way is what we can see of the galaxy that we are in, which is also called the Milky Way or Milky Way Galaxy. Our Galaxy has a diameter of about 100,000 lightyears. The center of the Milky Way lies in the direction of the constellation of the Archer (Sagittarius) at a distance of about 25,000 lightyears from the Sun, but is hidden from our sight by thick clouds of gas and dust. The Sun takes about 200 million years to orbit around the center of the Galaxy.

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## 5. The Plane of the Milky Way

The Milky Way has the shape of a fat disk of stars and clouds of gas, and we are in that disk. The "plane of the Milky Way" is a plane that divides the disk of the Milky Way into a lower part and an upper part (as if you cut a pancake so that you get two pancakes that are half as thick).

An equator is usually a flat boundary of zero thickness that divides an object into two parts that are equal in some natural sense. The disk of the Milky Way is slightly warped, so its vertical middle makes a surface that is slightly warped, too. For convenience we'll call that surface the "equator of the Milky Way", though it is not perfectly flat.

The Sun is currently a few dozen lightyears to the north of that plane, but it is not entirely clear how many, exactly, which means that not everyone agrees where the plane of the Milky Way runs near the Sun. That is not very surprising, because the plane of the Milky Way is not marked in space by strange stars or anything like that. Finding the plane of the Milky Way is similar to finding the center line of a long but narrow forest without sharp borders. Most people will agree about roughly where the center line is, but where it is exactly is not so clear.

Some people say that the Sun is now 20 lightyears north of the plane of the Milky Way, others say 45 lightyears, and //www.ingenta.com/isis/searching/Expand/ingenta?pub=infobike://klu/astr/2003/00000288/00000004/05123578 says 34.6 lightyears. The Sun wiggles around the plane of the Milky Way and passes through the plane about every 35 million years or so.

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Gamma rays are emitted by extremely hot or otherwise extremely energetic materials, and also during certain radioactive processes where one type of matter is changed into another. So, if there is very little matter in some region, then you cannot expect many gamma rays from that region. The highest densities of material in the Milky Way are found in the disk of the Milky Way, so it is to be expected that more gamma rays come from the disk of the Milky Way than from regions outside of the disk. The density decreases gradually over a couple of hundreds of lightyears when you move vertically away from the vertical middle of the disk, so I expect the emission of gamma rays to also decrease gradually over such distances above or below the equator.

An equator is not a separate object, and can (almost) never be easily detected from natural evidence in the area around the equator. For example, you cannot tell in a picture of some region near the equator on Earth exactly where the equator runs. The area looks similar on both sides of the equator, and doesn't suddenly change as you cross the equator. Likewise, it is not easy to tell where the equator of the Milky Way is, exactly, and there is no sudden increase or decrease of gamma ray emission or reception when you cross that equator.

As far as we know, the Sun has always been inside the disk of the Milky Way, and has crossed the equator many times in the past, so any sudden increases or decreases in reception of gamma rays must have been related to specific gamma ray sources, rather than to the exact location of the Sun relative to the equator of the Milky Way.

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## 6. The Milky Way Galaxy is a Spiral Galaxy

The Milky Way Galaxy is a spiral galaxy. That means that the flat disk of the Milky Way contains a number of arms that wind their way from the center to the edge. You can see a picture of the structure of the Milky Way (as discovered so far) at //www.ras.ucalgary.ca/CGPS/where/plan/.

If you start at the center and move outward past the Sun, then you encounter the following arms: the Norma arm, the Scutum-Crux Arm, the Sagittarius Arm, the Local Arm, and the Perseus Arm. There is also a piece of arm called the Outer Arm, but that seems to be a part of the Norma Arm. These arms (except for the Local and Outer Arms) are named for the constellations that contain them (as seen from Earth). The Local Arm that the Sun happens to be in is not a full arm.

At the end of 2003, Australian astronomers from CSIRO reported (//www.atnf.csiro.au/news/press/spiralarm/) that they discovered a new piece of spiral arm at 15 - 20 kpc from the center. It is very well possible that this is yet another part of the Norma Arm, at yet greater distance from the center than the Outer Arm, which also seems part of the Norma Arm.

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## 7. The Name of the Milky Way

The ancient Greek astronomers thought that the Milky Way looked like a river of milk running through the sky, and that's where both "Milky Way" and "Galaxy" come from. Galaxy comes from the Greek word for milk.

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## 8. The Discovery of the Milky Way

A very long time ago there were no cities, and no city lights. The people of that time could see even very dim stars and the Milky Way every cloudless night. So, it is likely that people have been aware of the Milky Way for many thousands of years, and we can't tell who discovered the Milky Way first.

It took a very long time, however, before we discovered the true nature of the Milky Way. After the invention of the telescope in 1609 people saw that the Milky Way contained very many apparently dim stars that could not be seen individually without a telescope. In 1755, the philosopher Immanuel Kant proposed that some of the nebulas visible in the sky could be separate Milky Ways at large distance, instead of small nebulas within one giant Milky Way that filled all of the Universe. Only in 1923 did Edwin Hubble (for whom the Hubble Space Telescope is named) prove that some of those nebulas indeed lie far outside of our own Milky Way galaxy, and that our Milky Way does not fill the whole Universe. Nowadays we know that our Milky Way is but a typical spiral galaxy, in a backwater of the Universe.

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## 9. The Discovery of the Center of the Milky Way Galaxy

The discovery of the center of the Milky Way was a lengthy process involving many people. Until the beginning of the 20th century we didn't know where the center of the Milky Way is. Many people then believed that the Milky Way filled the whole Universe; at least, nobody had proved otherwise. Yet, there were clues that the Milky Way wasn't the same in all directions. The observations of nebulae that were collected by J. Herschel (1864) and Mr. Dreyer (1888) showed that globular clusters have a great preference for the side of the sky where the constellation of the Archer is. In addition, the Milky Way does not appear equally bright in all directions. It is brightest near the constellation of the Archer, and least bright near the constellation of Perseus. This was shown especially by O. Boeddicker in Ireland (1892) and C. Easton in Dordrecht in the Netherlands (1893). In 1922, Freundlich and Von der Pahlen discovered that stars of spectral type B showed a curious velocity distribution that depended on their position along the Milky Way. In 1927, J.H. Oort of Leiden in the Netherlands showed that this curious distribution could be explained if you assumed that the Milky Way rotates around a center that was about 6000 pc away in the direction of the constellation of the Archer. (I summarized much of this history from [Pannekoek].) Nowadays, with many more measurements of various kinds, astronomers think that the center is more like 8000 pc away.

All of the gas and dust between us and the center of the Milky Way absorbs so much of the light that we can't see all the way to the center on ordinary photographs. (The total absorption is some 27 magnitudes!) Radio waves, infrared radiation, and gamma rays are much less troubled by such absorption, so we can record those kinds of radiation coming from the center, but only since about the 1950s (radio) and the 1980s (infrared, gamma). Around 1990 it became clear that the compact radio source Sagittarius A* (often abbreviated to Sgr A*) is in the exact center of the Milky Way and is associated with a black hole of 3.6 million solar masses. That radio source was discovered in February of 1974 by Bruce Balick and Robert Brown. The name Sagittarius A* was first used by Robert Brown in 1982 and has since then become the standard name for that object (according to [Goss]).

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## 10. The Direction of the Center of the Milky Way

The center of the Milky Way (between the constellations of the Archer, the Scorpion, and the Snake Bearer) is about 5 degrees away in the sky from the path that the Sun takes (the ecliptic), so the Sun is always at least 5 degrees away from the center of the Milky Way, and the line through space from the Earth via the Sun to the center of the Milky Way always has a bend of at least 5 degrees in it.

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The equatorial coordinates of the direction towards the center of the Milky Way Galaxy are (relative to the equinox of 2000.0): right ascension 17h46m, declination −28°56'. Things that don't move relative to the stars (such as the stars themselves) always rise at the same sidereal time and always set at the same sidereal time, as seen from a fixed location. For the center of the Milky Way, those sidereal times are listed in the following table, in the columns "Rise" and "Set".

90° north never never
80° north never never
70° north never never
60° north 16:39 18:53 02:41 23:01
50° north 14:31 21:01 04:20 21:22
40° north 13:37 21:55 05:09 20:33
30° north 13:00 22:32 05:42 20:00
20° north 12:32 23:00 06:08 19:34
10° north 12:08 23:24 06:30 19:12
11:46 23:46 06:51 18:51
10° south 11:24 00:08 07:12 18:30
20° south 11:00 00:32 07:34 18:08
30° south 10:32 01:00 08:00 17:42
40° south 09:55 01:37 08:33 17:09
50° south 09:01 02:31 09:22 16:20
60° south 06:53 04:39 11:01 14:41
70° south always never
80° south always never
90° south always never

The transformation of sidereal time to clock time is explained on the Time Page.

When the Milky Way goes straight over your head, then the center line of the Milky Way makes a right angle with the poles of the Milky Way, so then those poles must be on the horizon. The North Pole of the Milky Way has right ascension 12h51m and declination +27°08' (compared to the equinox J2000.0). That point is on the horizon at the two sidereal times that are listed in the preceding table in the column "Straight Overhead".

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## 12. The Neighbors of the Milky Way Galaxy

Our Milky Way Galaxy is part of a group of galaxies that is called the Local Group. The nearest neighbor galaxy that is about as large as our own Galaxy is the Andromeda Nebula (M 31). The Galaxy also has a number of smaller neighbors, which probably have not yet all been discovered. The best-known small neighbors are the Large Magellanic Cloud and the Small Magellanic Cloud. You can find a list on the UniverseFamilyTree Page about galaxy clusters.

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## 13. Thousands of Millions of Galaxies

If we assume that the Universe looks everywhere pretty much the same (against which we have no indications), then there have to be thousands of millions of galaxies in the Universe. A typical galaxy has a mass of about 100 thousand millions times that of the Sun (measured from the motions in and around the galaxy, which depend on all mass, including invisible mass). If we divide the total mass of the visible Universe (around 4 × 1022 solar masses or 9 × 1052 kilograms) by the typical mass of a galaxy, then we get an estimate of about 400 thousand million galaxies in the visible Universe.

Only a very small fraction of all of those galaxies has been investigated. Even the Sloan Digital Sky Survey (www.sdss.org), the largest systematic census of objects in the Universe, will count "only" about 100 million objects, among which an estimated one million galaxies.

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## 14. Active Galaxies

The leading models for active galaxies assume that there is a large black hole at the very center of such a galaxy and that a large amount of energy is released by material from the surroundings that falls into the black hole. See //en.wikipedia.org/wiki/Active_galaxy and //imagine.gsfc.nasa.gov/docs/science/know_l1/active_galaxies.html. For the Active Galaxies Newsletter, see //www.ast.man.ac.uk/~rb/agn/.

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## 15. The Great Wall

There are many very large concentration of galaxies. One, at about 300 million to 500 million lightyears from Earth in the direction of the constellations from at least Leo to Herculus, is known as the "Great Wall". However, its full extent is not yet known, and it is not everywhere very well separated from neighboring concentrations of galaxies. The part we've seen so far measures about 200 by 600 by 20 million lightyears in size. See //www.angelfire.com/id/jsredshift/grtwall.htm and //adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1992ApJ...384..396R.

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## 16. Lonely Galaxies

The galaxies in the Universe are so far apart because they formed out of very much larger clouds of gas and dust that collapsed and shrunk because of their own gravity. All of the gas and dust from a very large region of space concentrated into a single galaxy that filled only a very small part of the region of space, so the rest of the space ended up empty.

If each galaxy ended up where the center of the cloud used to be, and if two of those clouds touched each other, then the distance between the two galaxies that formed out of them would still be many times their size, because those galaxies are so much smaller than the original clouds.

Also, the Universe is expanding, so galaxies are further apart now than they were thousands of millions of years ago.

Space between galaxies is mostly empty. There may be a few stars that have escaped from a galaxy, and there are a few protons and electrons here and there, but all in all there is so little matter there that a similar situation in a laboratory on Earth would be called a good vacuum.

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## 17. Colliding Galaxies

Galaxies are usually millions of lightyears apart, but yet collisions between galaxies do occasionally occur. A galaxy is mostly empty space, so it is very unlikely that stars from both galaxies will collide. What happens, exactly, when two particular galaxies collide depends on the mass of the galaxies, on the smallest distance between them during the collision, on the relative velocity, and on the orientation of the galaxies. It may be that the two galaxies will merge into a single galaxy, but it may also be that they merely have a slight change of direction. It may be that one of the galaxies loses stars to the other one, or that both lose stars to each other. Often, tidal forces cause large groups of stars to be ejected into the Universe, and those form (temporary) tails to the galaxies. It also often happens that the collision generates a pressure wave that courses through the galaxies and causes star formation. Such a galaxy will contains unusually many young, bright stars for a few million years.

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## 18. How to Tell if a Point of Light is a Galaxy or a Star

One can identify the nature of a point of light by its spectrum and its brightness. The spectrum of a galaxy is the combination of the spectra of all of its bright components, mostly the stars, but sometimes also with a large contribution from an Active Galactic Nucleus (AGN, for example such as in quasars).

The spectrum of an AGN looks very different from that of a star, so the difference is quite easy to spot if you can measure the spectrum.

Because the spectrum of a normal galaxy (without an AGN) is the combination of the spectra of stars of many different types, it usually does not look exactly like the spectrum of any particular type of star, and from that you can tell that it is not the spectrum of a single star.

A galaxy is very much bigger than a single star, so a galaxy must be very far away indeed to appear in the sky as just a point of light. At such great distances, the galaxy is likely to show a large redshift of its spectrum (because of Hubble's Law and the expansion of the Universe), which means that all spectral lines are shifted to longer wavelengths compared to the same spectral lines in the spectrum of a nearby star. The redshift indicates the distance, and the distance together with the apparent brightness indicates how much light the object emits, and that is very much greater for a galaxy than for a single star.

A small object in our Solar System appears as a point of light in our sky, and such an object can be told apart from a star or galaxy by its spectrum, too, for example because the spectrum reveals the temperature of the object, and objects in our Solar System (other than the Sun itself) are much cooler than the Sun and other stars are. Such objects also reflect sunlight, which carries the temperature signature of the Sun, so one has to be careful to check for another contribution from the object itself, with a much lower temperature.

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## 19. Moving Galaxies

We have direct and indirect evidence that galaxies move. Indirect evidence is that many galaxies look like they have been disturbed by the gravity of something else, and that must then have been something with a similar amount of mass, and other galaxies are good candidates for that. Sometimes such an other galaxy is close by the disturbed galaxy, but sometimes there is no other galaxy nearby, and then that galaxy must have been closer in the past than it is today, so it must have moved.

The direct evidence is the Doppler shift of the spectral lines in the light coming from those galaxies. Just like the pitch of a siren gets higher when the ambulance drives towards you, and gets lower when the ambulance drives away from you, so does the frequency of light waves increase or decrease as the source of the light moves towards you or away from you.

The light coming from galaxies has many spectral lines of which we know what their real frequencies are, so we can determine their speed along the line of sight.

Determining the speed of galaxies at right angles to the line of sight (i.e., across the sky) is a lot more difficult than determining the speed along the line of sight. Of most galaxies of which we know their speed along the line of sight we do not know their speed across the sky.

We do not have a preferred position in the Universe, so the spread of speeds of galaxies around the local average is about as great at right angles to the line of sight as it is along the line of sight.

The typical speed of a galaxy relative to its neighbors is of the order of 10 to 100 km/s.

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Last updated: 2021-07-19