Galaxies - with particular reference to our very own Milky Way #
Among the hundreds of thousands of galaxy clusters in the universe, we live at the outskirts of the Virgo supercluster, in one of many small groups of galaxies. Our group of galaxies, the Local Group, contains two large spirals – Andromeda (M31) and our own Milky Way – with a few dozen smaller galaxies in two subgroups around them.
So you might say that our cosmic address would be something like:
The two Magellanic Clouds are the largest of the satellite galaxies to the Milky Way, at distances of ~ 50 kiloparsecs (kpc) that is 150,000 light years (ly) away. We cannot see the shape of our own Galaxy, but we have a good idea of what it looks like from looking at other nearby spirals. From face on, we can clearly see the spiral arms, and the fact that the colours of the stars are very different in the disk and the bulge. Spirals viewed from the side show the characteristic “flying saucer” shape, with an extremely flat disk (cut by dark dust lanes) and a central bulge.
So you might say that our cosmic address would be something like:
- Earth
- The solar system
- The milky way
- The Local Group
- The Virgo Supercluster [0]
The two Magellanic Clouds are the largest of the satellite galaxies to the Milky Way, at distances of ~ 50 kiloparsecs (kpc) that is 150,000 light years (ly) away. We cannot see the shape of our own Galaxy, but we have a good idea of what it looks like from looking at other nearby spirals. From face on, we can clearly see the spiral arms, and the fact that the colours of the stars are very different in the disk and the bulge. Spirals viewed from the side show the characteristic “flying saucer” shape, with an extremely flat disk (cut by dark dust lanes) and a central bulge.
Schematic of the Milky Way showing the stellar disk (light blue), the thick disk (dark blue), stellar bulge (yellow), stellar halo (mustard yellow), dark halo (black), and globular cluster system (filled circles): from Bland-Hawthorn & Freeman 2005. Secondary source: Origins: From the Big Bang to Life, 23 March 2011.
The Milky Way has several different regions which are quite distinct:
Another way of looking at the situation as a whole is to imagine the Milky Way viewed edgewise as a fried egg sunny side up: a bright dense yolk of stars comprising the galactic centre, around which the galaxy’s spirals form a thin saucer known as the galactic disc. An evanescent halo of old stars spin through the entire galactic disk, and about 30 known dwarf galaxies spin through the outermost regions of the halo[1].
The thin disk extends about 1000 light years above and below the Galactic plane, and is responsible for about 90% of the light of the Galaxy. The thick disk is about 3.5 times as thick, but contains much older, fainter stars so contributes much less light. It was not until the orbits of large numbers of stars were determined that these two components could be properly distinguished. The orbits of stars in these different regions are very different. Thin-disk stars have circular orbits almost completely in the plane. Thick-disk stars have orbits that take them up to 1 kpc, or 3000 light years (ly) above and below the plane. Halo stars are on highly eccentric orbits that take them plunging through the disk and out into the halo:
- the bulge: consists of old stars;
- the halo, which contains very few stars (most of the halo is made up of dark matter; lying within the halo are the globular clusters, roughly spherical collections of up to a million stars, which appear to be the oldest objects in the Galaxy); and
- the spiral disk, which can be separated into the thin disk and the thick disk.
Another way of looking at the situation as a whole is to imagine the Milky Way viewed edgewise as a fried egg sunny side up: a bright dense yolk of stars comprising the galactic centre, around which the galaxy’s spirals form a thin saucer known as the galactic disc. An evanescent halo of old stars spin through the entire galactic disk, and about 30 known dwarf galaxies spin through the outermost regions of the halo[1].
The thin disk extends about 1000 light years above and below the Galactic plane, and is responsible for about 90% of the light of the Galaxy. The thick disk is about 3.5 times as thick, but contains much older, fainter stars so contributes much less light. It was not until the orbits of large numbers of stars were determined that these two components could be properly distinguished. The orbits of stars in these different regions are very different. Thin-disk stars have circular orbits almost completely in the plane. Thick-disk stars have orbits that take them up to 1 kpc, or 3000 light years (ly) above and below the plane. Halo stars are on highly eccentric orbits that take them plunging through the disk and out into the halo:
A curious feature of the Milky Way disc is that it has a pronounced warp at its outer rim, now thought to be caused substantially by gravitational disturbances caused by the Large and Small Magellanic Clouds as they pass through the galaxy’s dark matter. The warp represents a wave motion captured at one moment in time. The wave has three distinct components corresponding to three different frequencies of the disc, as though the galaxy were a giant gong. The gravity of the Magellanic Clouds, aided by dark matter, acts as the gong.[2]
About 30 known dwarf galaxies spin through the outermost regions of the halo. On average, typical dwarf galaxies contain only a few billion stars, far fewer than the 200 to 400 billion stars in the comparatively gigantic Milky Way[3]. The composition of stars in the different populations are also different from each other. The stars of the halo are very metal-poor, those of the thin disk metal-rich and young, while the stars of the thick-disk have intermediate metallicity and age[4].
The Milky Way appears to have relatively fewer satellite galaxies than it should. Gravitational modelling suggests it should be orbited by literally hundreds of them. However, recent searches using the Sloan Digital Sky Survey (SDSS) have revealed that the satellites are out there all right, though composed almost entirely of dark matter, making them difficult to detect[5]. In fact, over the last 15 years Sloan has built up a database or more than 80 million stars within the Milky Way along with information on their distances, colours and other characteristics spread over one quarter of the sky. Many of these stars emanated from dwarf galaxies orbiting the Milky Way. [6]
Enter the Vela supercluster
Another curious circumstance is that, based on gravitational calculations predicated upon our galactic neighbours, the Milky Way – be it remembered a collection of 100 billion or so stars with a mass of 400 billion suns - hasn't quite been travelling in the direction or with the speed that astronomers might have expected as it meanders its way through the universe. The reason for this has hitherto been a puzzle, but it is now thought that the difference may be accounted for by a recently discovered massive supercluster of galaxies in the near vicinity known as the Vela supercluster whose gravity might explain the difference between the measured motion of the Milky Way and the motion predicated upon the gravitational pull of the other known galaxies in the nearby vicinity.
At cosmic scales galaxies form intricate structures, known as clusters and superclusters, and understanding how these form helps our understanding of how galaxies are born, live and die – and contributes to our overall cosmological knowledge about the formation and fate of our universe. Our own galaxy is part of the Laniakea supercluster, which is quite puny compared to the Vela supercluster. Normal-sized clusters of galaxies are in near-gravitational equilibrium and are relatively stable. Superclusters, however, are the biggest gravitationally bounded structures in the universe. They are much messier than smaller clusters and are still collapsing in on themselves, having not yet reached gravitational equilibrium.
The Vela supercluster, perhaps the largest structure ever discovered, is “mind-bogglingly huge”: spanning across 370 million light years at an average distance of 840 million light years from Earth. It probably contains up to 100,000 galaxies, each with hundreds of billions of stars. Professor Matthew Colless from the Australian National University explains that gravity-wise we are literally “falling into” these other massive things in our nearby universe - the Virgo cluster and beyond that the Coma cluster and the Great Attractor, and now it would seem the massive Vela supercluster as well – and the sum of all that falling comprises our motion. If the Vela supercluster is the final piece in the puzzle explaining the motion of the Milky Way - two new Australian surveys (the Taipan optcial survey and the Wallaby radio survey) will help confirm its size - our cosmological model of the universe has one less loose end.
More recent developments [7]
Using advanced radio and optical telescopes, including the Bar and Spiral Structure Legacy (BeSSeL) Survey, and very long baseline interferometry (VLBI), which forms an integral part of the VERA (VLBI Exploration of Radio Astrometry) project, and the Very Long Baseline Array, astronomers now believe that:
With VLBI, astronomers are now also able to survey very great distances by observing the parallax effect, whereby a nearby object seen against a distant background will appear at different positions when viewed from different vantage points, and achieve resolutions about 40x better than the sharpest images from the Hubble Space Telescope.
About 30 known dwarf galaxies spin through the outermost regions of the halo. On average, typical dwarf galaxies contain only a few billion stars, far fewer than the 200 to 400 billion stars in the comparatively gigantic Milky Way[3]. The composition of stars in the different populations are also different from each other. The stars of the halo are very metal-poor, those of the thin disk metal-rich and young, while the stars of the thick-disk have intermediate metallicity and age[4].
The Milky Way appears to have relatively fewer satellite galaxies than it should. Gravitational modelling suggests it should be orbited by literally hundreds of them. However, recent searches using the Sloan Digital Sky Survey (SDSS) have revealed that the satellites are out there all right, though composed almost entirely of dark matter, making them difficult to detect[5]. In fact, over the last 15 years Sloan has built up a database or more than 80 million stars within the Milky Way along with information on their distances, colours and other characteristics spread over one quarter of the sky. Many of these stars emanated from dwarf galaxies orbiting the Milky Way. [6]
Enter the Vela supercluster
Another curious circumstance is that, based on gravitational calculations predicated upon our galactic neighbours, the Milky Way – be it remembered a collection of 100 billion or so stars with a mass of 400 billion suns - hasn't quite been travelling in the direction or with the speed that astronomers might have expected as it meanders its way through the universe. The reason for this has hitherto been a puzzle, but it is now thought that the difference may be accounted for by a recently discovered massive supercluster of galaxies in the near vicinity known as the Vela supercluster whose gravity might explain the difference between the measured motion of the Milky Way and the motion predicated upon the gravitational pull of the other known galaxies in the nearby vicinity.
At cosmic scales galaxies form intricate structures, known as clusters and superclusters, and understanding how these form helps our understanding of how galaxies are born, live and die – and contributes to our overall cosmological knowledge about the formation and fate of our universe. Our own galaxy is part of the Laniakea supercluster, which is quite puny compared to the Vela supercluster. Normal-sized clusters of galaxies are in near-gravitational equilibrium and are relatively stable. Superclusters, however, are the biggest gravitationally bounded structures in the universe. They are much messier than smaller clusters and are still collapsing in on themselves, having not yet reached gravitational equilibrium.
The Vela supercluster, perhaps the largest structure ever discovered, is “mind-bogglingly huge”: spanning across 370 million light years at an average distance of 840 million light years from Earth. It probably contains up to 100,000 galaxies, each with hundreds of billions of stars. Professor Matthew Colless from the Australian National University explains that gravity-wise we are literally “falling into” these other massive things in our nearby universe - the Virgo cluster and beyond that the Coma cluster and the Great Attractor, and now it would seem the massive Vela supercluster as well – and the sum of all that falling comprises our motion. If the Vela supercluster is the final piece in the puzzle explaining the motion of the Milky Way - two new Australian surveys (the Taipan optcial survey and the Wallaby radio survey) will help confirm its size - our cosmological model of the universe has one less loose end.
More recent developments [7]
Using advanced radio and optical telescopes, including the Bar and Spiral Structure Legacy (BeSSeL) Survey, and very long baseline interferometry (VLBI), which forms an integral part of the VERA (VLBI Exploration of Radio Astrometry) project, and the Very Long Baseline Array, astronomers now believe that:
- the Milky Way is spinning at 236 kilometers per second, which is about eight times the speed at which Earth orbits the sun. Based on these parameter values, the sun circles the Milky Way every 212 million years. "To put this in perspective, the last time our solar system was in this part of the Milky Way, dinosaurs roamed the planet".
- Utilising the BeSSeL Survey and the VERA project, astronomers have amassed about 200 parallax-based distance measurements for young hot stars across around one third of the Milky Way, revealing four spiral arms that are continuous over great distances with a bright central bar and a reasonable degree of symmetry, as per the graphics below.
- These features make our galaxy appear fairly normal, but it certainly is not typical. About two thirds of spiral galaxies exhibit bars, so in this way the Milky Way is in the majority. Yet its possession of four clearly defined and fairly symmetric spiral arms makes it stand out from most other spiral galaxies, which have fewer, messier arms.
- Our sun, located almost exactly in the galaxy’s midplane but far from its centre about two thirds of the way out, is very close to a fifth feature called the Local arm, which seems to be an isolated fragment of a spiral arm that wraps around less than a quarter of the Milky Way. Over its short length, though, it has amounts of massive star formation.
- The Perseus arm is thought to be one of the two dominant arms in the Milky Way, the other being the Scutum-Centaurus–Outer-Scutum-Centaurus arm.
With VLBI, astronomers are now also able to survey very great distances by observing the parallax effect, whereby a nearby object seen against a distant background will appear at different positions when viewed from different vantage points, and achieve resolutions about 40x better than the sharpest images from the Hubble Space Telescope.
Source:,Xing-Wu
Zheng and Mark J. Reid, Bar and Spiral Structure Legacy (BeSSeL) Survey (a VLBA Key Science
Project), Nanjing University, and Center for Astrophysics | Harvard &
Smithsonian (Milky Way chart and illustration).
# In the formulation of this page, I am indebted to Dr Helen Johnston's Continuing Education course, Origins: From the Big Bang to Life, March 2011, Sydney University Physics Department (CCE, Origins). Her permission to publish in this form is gratefully acknowledged.
[0] Neil deGrasse Tyson Cosmos: A Spacetime Odyssey (2014).
[1] Anna Frebel, ‘Four starry lights’. Scientific American, December 2012, 50, 53.
[2] Leo Blitz, “The Dark side of the Milky Way”, Scientific American, Special Collector’s Edition, op cit, August 2013, 50ff, esp 5.
[3] Anna Frebel, Ibid.
[4] To an astronomer, every element other than hydrogen and helium is called a metal, so oxygen and carbon are described as metals. Only hydrogen and helium were made in the Big Bang, so the first generation of stars, made from the primordial gas of hydrogen and helium, was metal-free.
[5] Blitz, op cit, 50 at 56.
[6] http://www.smh.com.au/technology/sci-tech/hidden-vela-supercluster-of-galaxies-has-been-dragging-the-milky-way-through-the-universe-20161221-gtfk0j.html
[7] This is a n edited summary of the article "New view of the Milky Way", by Xing-Wu Zheng and Mark J. Reid which appeared in the April 2020 edition of the Scientific American, pp 24-31
# In the formulation of this page, I am indebted to Dr Helen Johnston's Continuing Education course, Origins: From the Big Bang to Life, March 2011, Sydney University Physics Department (CCE, Origins). Her permission to publish in this form is gratefully acknowledged.
[0] Neil deGrasse Tyson Cosmos: A Spacetime Odyssey (2014).
[1] Anna Frebel, ‘Four starry lights’. Scientific American, December 2012, 50, 53.
[2] Leo Blitz, “The Dark side of the Milky Way”, Scientific American, Special Collector’s Edition, op cit, August 2013, 50ff, esp 5.
[3] Anna Frebel, Ibid.
[4] To an astronomer, every element other than hydrogen and helium is called a metal, so oxygen and carbon are described as metals. Only hydrogen and helium were made in the Big Bang, so the first generation of stars, made from the primordial gas of hydrogen and helium, was metal-free.
[5] Blitz, op cit, 50 at 56.
[6] http://www.smh.com.au/technology/sci-tech/hidden-vela-supercluster-of-galaxies-has-been-dragging-the-milky-way-through-the-universe-20161221-gtfk0j.html
[7] This is a n edited summary of the article "New view of the Milky Way", by Xing-Wu Zheng and Mark J. Reid which appeared in the April 2020 edition of the Scientific American, pp 24-31