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The Whirlpool Galaxy is also known as Messier 51a, and it’s one of the most familiar galaxies. If you’ve seen a picture of a galaxy captured by the Hubble Space Telescope, chances are you were looking at the Whirlpool Galaxy. That’s because this galaxy, located about 23 million light-years away in the constellation Canes Venatici, is aligned almost perfect face on. We have beautiful view of the Whirlpool Galaxy’s entire structure, from its spiral arms to its dense galactic core.
The Whirlpool Galaxy is fascinating for another reason as well. It has a companion galaxy to one side called NGC 5195. The two galaxies interact through gravity, and this gives astronomers a chance to study what happens when galaxies collide.
Astronomers have calculated that the Whirlpool Galaxy measures about 38,000 light-years across, with a mass of about 160 million times the mass of the Sun. This makes the galaxy smaller and less massive than our own Milky Way.
You can see the Whirlpool galaxy with a good pair of binoculars, or a small backyard telescope; although, you’ll want a bigger telescope to see the spiral structure and detect the companion galaxy NGC 5195. To find the Whirlpool Galaxy, located the easternmost star in the Big Dipper. Then go about 3.5 degrees to the southeast. On a dark night you should be able to see a fuzzy spot where the galaxy is.
Astronomers think that NGC 5195 first passed through the main disk of the Whirlpool Galaxy about 500 to 600 million years ago, and then made another disk crossing about 50 to 100 million years ago.
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Gravity is a funny thing, it really wants to keep everything together. The Moon orbits the Earth because of gravity, and the Earth travels around the Sun. The Sun is captured by the Milky Way’s gravity, and even the Milky Way is held together in a loose group of galaxies called the Local Group. And even the Local Group is part of the Virgo Supercluster.
Galaxy groups form the largest structures in the Universe. Our Local Group, for example, contains about 50 galaxies; most of which are smaller dwarf galaxies. The three large spiral galaxies in the Local Group are Andromeda, the Milky Way, and the Triangulum Galaxy. The rest are mostly satellite galaxies to these three large galaxies.
A typical galaxy group has around 50 galaxies, and contains a total mass of about 10 trillion times the mass of the Sun. Galaxy clusters are even larger, and can contain up to 1000 galaxies., with a mass of 100 trillion to 1000 trillion suns.
The largest structures in the Universe are the superclusters. These can contain hundreds of galaxy groups and clusters and measure hundreds of light-years across. We live in a relatively small example called the Virgo Supercluster, which contains at least 100 galaxy groups and clusters, and measures about 110 million light-years across. And the Virgo Supercluster is just one of millions of such supercluster galaxy groups in the Universe.
At the very largest scales in the Universe, the superclusters form long filaments that cross even larger voids in the Universe. The matter is held together in filaments that resemble a foam.
We have written many articles about galaxies for Universe Today. Here’s an article about a galaxy group smash up.
During the month of May, the “Wolf” rises and prowls the skies after midnight. Lupus was one of the 48 original constellations listed by the first century astronomer Ptolemy and on its western border is a Wolf-Rayet planetary nebula – IC 4406 – which contains some of the hottest stars known to be in existence. What exactly lay inside this 1900 light year distant torus-shaped cloud of dust? Then let’s really step inside this Hubble dimensional visualization by Jukka Metsavanio and take a closer look…
Whenever we present a dimensional visualization it is done in two fashions. The first is called “Parallel Vision” and it is much like a magic eye puzzle. When you open the full size image and your eyes are the correct distance from the screen, the images will seem to merge and create a 3D effect. However, for some folks, this doesn’t work well – so Jukka has also created the “Cross Version”, where you simply cross your eyes and the images will merge, creating a central image which appears 3D. As we learned some time ago, it might not always work for all people, but there are a few other tricks you can try. Now sit back and prepare to be blown away…
IC 4406 Cross by JP Metsavainio
The rectangluar appearance of planetary nebula, IC 4406, isn’t such a great mystery. We know from looking at a great number of objects that our point of view affects how we see things and we realize we’re seeing this incredible structure almost in the plane of its equator. Astronomers believe the entirety of the nebula is shaped like a prolate spheroid – where the polar diameter is greater than the equatorial diameter. Why such an unusual shape? Quite probably because IC 4406 is believed to be bipolar. No. It’s not going to freak out on you… It simply means this planetary nebula has an axially symmetric bi-lobed appearance. This may be the beginnings or the endings of the evolutionary stages of all planetary nebulae – but it does have its quirks.
While the function that shapes this structure isn’t exactly clear to astronomers, many believe it may belong to the physical process known as bipolar outflow – continuous highly energetic streams of gas emanating from the poles of a star. What types of stars? Again, it isn’t always clear. Bipolar outflow can occur with protostars where a dense, concentrated jet produces a supersonic shock fronts. More evolved young stars, such as T-Tauri types, also produce bow shocks visible at optical wavelengths that we refer to as Herbig-Haro objects. Evolved stars produce spherically-symmetric winds (called post-AGB winds) that are focused into cones and eventually become classic planetary nebula structures. There is even speculation that these outflows may be impacting with interstellar dust surrounding the star or supernova remnants. But… what exactly causes these beautiful structures we see inside?
According to C.R. O’Dell: “This progression begins with dark tangential structures showing no alignment with the central star and location near the main ionization front. At the end of the progression in the largest nebulae, the knots are located throughout much of the ionized zone, where they are photoionized on the side facing the central star and accompanied by long tails well aligned radially. This modification of characteristics is what would be expected if the knots were formed near or outside the main ionization front, obtaining densities high enough to lead to their being only partially ionized as they are fully illuminated by the Lyman continuum (Lyc) radiation field. Their expansion velocities must be lower than that of the main body of the nebular shell. Their forms are altered by exposure to the radiation field from the star, although it is not clear as to the relative role of radiation pressure acting on the dust component vis-à-vis ionization shadowing.”
However, there is something a bit unusual about IC 4406, isn’t there? That’s right. It contains a Wolf-Rayet star. Descended from O-types, these massive, extremely luminous beauties have strong stellar winds and are well-known for spouting off their unprocessed outer H-rich layers. The dense, high-velocity winds then rip at the superheated stellar photosphere, unleashing ultra-violet radiation which in turn causes fluorescence in the line-forming wind region. Most continue on to become Ib or Ic type supernovae, and just a very few (only 10%) become the central stars of planetary nebulae. So is the beautiful patterns we see in IC 4406 the beginning or the end? Says C.R. O’Dell:
“We find knots in all of the objects, arguing that knots are common, simply not always observed because of distance. The knots appear to form early in the life cycle of the nebula, probably being formed by an instability mechanism operating at the nebula’s ionization front. As the front passes through the knots they are exposed to the photoionizing radiation field of the central star, causing them to be modified in their appearance. This would then explain as evolution the difference of appearance like the lacy filaments seen only in extinction in IC 4406… Theoretical models have considered only symmetric instabilities, but there seems to be nothing that precludes the formation of elongated concentrations like one sees in IC 4406.”
In the meantime, many of you will recognize these filaments in this planetary by its more common name – the “Retina Nebula” – the third to have its spatial distribution of H2 and CO emissions mapped to prove that the equatorial density is caused by the high-velocity outflow of the progenitor AGB star – and perhaps the twinkle in its eye could have either the beginnings or the end of what may have been planetary systems. Says R. Sahai: “It is suggested that the equatorial tori observed or deduced in IC 4406 results from ‘born again’ disks formed through the destruction of planetary systems at the end of the AGB evolutionary phase.”
Are these filaments shaped by magnetic fields? The work of Hanna Dahlgren opens some very interesting ideas: “We propose a theory where the magnetic fields control the sculpting and evolution of small-scale filaments. This theory demonstrates how the substructures may form magnetized flux ropes that are twisted around each other, in the shape of double helices. Similar structures, and with similar origin, are found in many other astrophysical environments.” And will they survive? Says C.R. O’Dell:
“What the future holds in store for the knots in PN is quite important because whichever mechanism is producing them is locking a substantial fraction of the mass into molecular knots and these knots are escaping from the gravitational field of the central star (Meaburn et al. 1998). The process of photoionization means that there will be photoevaporation of material from the knots. The situation will be very much like the proplyds in the Orion Nebula, where the inner molecular core is heated by photons of less than 13.6 eV, causing a slow flow of gas away from the core. When this gas reaches the knots’ ionization front it is photoionized and heated, then it is rapidly accelerated to a velocity of about 10 km s. The estimated evaporation timescale for the outward moving knots is several thousand years. Many or most of them will therefore survive the hot-luminous phase close to the star and will be ejected into the surrounding interstellar medium.”
As just another twinkle in the Wolf’s eyes…
Many thanks to JP Metsavainio of Northern Galactic for his magic with Hubble Space Telescope images and allowing us this incredible look inside another mystery of space.
Most galaxies can be categorized by their shape. Our own Milky Way is a spiral galaxy, for example, and the largest galaxies in the Universe are elliptical galaxies. But some galaxies defy categorization. These are the irregular galaxies, and each one is unique in shape, age and structure.
Irregular galaxies are often chaotic in shape, with no central bulge or spiral arms. Although they used to have a more familiar shape, a dramatic collision with another galaxy has distorted their shape.
Astronomers maintain two classifications of irregular galaxies. Irr-I galaxies have some structure, but they’re still distorted enough that they can’t be classified as spiral, elliptical or lenticular shaped. Irr-II galaxies don’t have any structure at all.
The nearby Magellanic Clouds were once thought to be irregular galaxies. Although astronomers have detected a faint barred spiral shape.
There’s only one irregular galaxy in the Messier catalog of objects, and that’s M82; also known as the Cigar Galaxy. It’s located in the constellation Ursa Major about 12 million light-years away, and is famous for its heavy amounts of star formation. In fact, in infrared light, M82 is the brightest galaxy in the sky. Even in visible light, it’s 5 times brighter than the Milky Way.
Hubble image of a gas jet blasing from the core of M87. Credit: NASA, ESA, and J. Madrid (McMaster University)
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Our own Milky Way is classified as a spiral galaxy. But that’s just one of many classification of galaxies. One of the most common types are elliptical galaxies, named because they have an ellipsoidal (or egg) shape, and a smooth, almost featureless appearance.
Elliptical galaxies are usually large, containing hundreds of millions to trillions of stars. The biggest galaxies in the Universe are elliptical galaxies. They’re the result of many collisions between smaller galaxies, and all these collisions have destroyed the delicate spiral structure that we see in our own galaxy.
And they’re usually old. Elliptical galaxies look redder than spiral galaxies like the Milky Way. That’s because they contain old, red stars and have very low rates of star formation. All of the available gas and dust was already used up in the past, and now all that remains are these old red stars. They also have large populations of globular star clusters.
Elliptical galaxies are usually found in the most violent places in the Universe, like at the heart of galaxy clusters and in compact groups of galaxies. In these places, elliptical galaxies have had an accelerated life, with many galaxy mergers and several periods of star formation. These constant mergers and collisions increased their size and used up all the gas available for star formation.
The smallest dwarf elliptical galaxies are no larger than a globular cluster and can contain a mere 10 million stars. The largest elliptical galaxies can have well over 10 trillion stars. The largest known galaxy in the Universe, M87, is an elliptical galaxy.
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One of the most beautiful images ever taken by the Hubble Space Telescope is the Sombrero Galaxy; also known as M104 or NGC 4594. This is an unbarred spiral galaxy located in the constellation of Virgo about 29 million light-years away.
Some of the defining features of the Sombrero Galaxy are its bright nucleus, large central bulge, and a prominent dust lane in its disk. The galaxy is seen nearly edge on, and so it has the appearance of a Sombrero hat. Since the galaxy has an apparent magnitude of +9.0, it’s easily visible in amateur telescopes; but too dim to see with the unaided eye.
The dark dust lane that you can see in the Hubble image is a symmetric ring that encloses the bulge of the galaxy. Astronomers have detected that it mostly contains hydrogen gas and dust. The bulk of star formation that occurs in the Sombrero Galaxy is happening within this ring.
As with our own Milky Way, astronomers have detected a supermassive black hole at the heart of the Sombrero Galaxy. Based on the speed of the stars orbiting around it, astronomers have calculated that it must have a mass of at least 1 billion suns. This is one of the most massive black holes detected in nearby galaxies.
If you want to look for the Sombrero Galaxy yourself, you’ll need good pair of 7×35 binoculars or a 4-inch telescope. The galaxy is located 11.5 degrees West of the star Spica, and 5.5 degrees northeast of Eta Corvi. With a medium sized telescope you can distinguish the bulge from the disk, and with a large telescope you should be able to see the dark dust lane.
Nothing makes a better computer desktop wallpaper than a space picture. And some of the most beautiful pictures are galaxies. Here are a few galaxy wallpapers you can use to make your computer desktop look even better. You can see the small pictures here, and then click on an image to see a high-resolution version.
To update your computer desktop with one of these galaxy wallpapers, click to open up a high resolution version of the image. Then right-click on the image and choose “Set as Background”. In Mac OS X, you choose “Set as Desktop Background”.
This galaxy wallpaper is the Sombrero Galaxy, also known as M104. This photograph was captured by the Hubble Space Telescope, and shows the galaxy seen nearly edge on. It’s about 50,000 light-years across and is more than 28 million light-years from Earth.
Galaxy Triplet Arp 274.
This galaxy wallpaper shows the galaxy triplet Arp 274. This was also captured by the Hubble Space Telescope and although they look like they’re merging, they’re probably far apart. They just look like a collection from our point of view.
M74 Whirlpool Galaxy
This is M74, also known as the Whirlpool Galaxy. This galaxy is a little smaller than the Milky Way, but we’re lucky enough to be seeing it almost face on, so we can see detailed structures in the core. You can also see the bright knots that contain regions of newly forming stars.
To see most galaxies, you need at least a small telescope. But you can see the enormous Andromeda Galaxy, or Messier 31, with your own eyes; if you know where to look. The Andromeda Galaxy is located in the Andromeda constellation, and named after a princess in Greek mythology.
Andromeda is the largest galaxy in the Local Group, which includes the Milky Way, the Triangulum Galaxy, and dozens of smaller dwarf and irregular galaxies. A recent estimate gave Andromeda 700 billion solar masses. Our Milky Way is only 80% the mass of Andromeda.
The Andromeda galaxy was first observed by Persian astronomers, thousands of years ago, and was later cataloged by Charles Messier in 1764. He classified it as M31. In 1912, astronomers calculated its speed to be 300 kilometers per second, moving towards the Sun. Edwin Hubble first calculated the distance to Andromeda, by detecting cepheid variables in the galaxy. He measured that it was 450 kpc, or 2.5 million light-years away; well outside the Milky Way galaxy.
Recent estimates have calculated that Andromeda Galaxy is about 220,000 light-years in diameter, almost twice the estimate diameter of the Milky Way.
While other galaxies are moving away from us, Andromeda is on a collision course with the Milky Way. Our two galaxies will collide with one another in about 2.5 billion years, and begin forming a giant elliptical galaxy. It’s known to have 14 dwarf galaxies orbiting it in various stages of merger.
Local Group of galaxies, including the massive members M31 (Andromeda Galaxy) and Milky Way, as well as other nearby galaxies. Credit: Wikipedia Commons/Antonio Ciccolella
The Milky Way is just one galaxy located in a vast cluster of galaxies known as the Local Group. This group contains more than 50 galaxies (mostly dwarf galaxies). The total size of the Local Group is 10 million light-years across, and it’s estimated to have a mass of 1.29 billion solar masses. The Local Group is just one collection of galaxies in the even bigger Virgo Supercluster.
The largest, most massive galaxies in the Local Group are the Milky Way, Andromeda and the Triangulum Galaxy.
Each of these galaxies has a collection of satellite galaxies surrounding them. For example, the Milky Way has Sagittarius Dwarf Galaxy, Large Magellanic Cloud, Small Magellanic Cloud, Canis Major Dwarf, Ursa Minor Dwarf, Draco Dwarf, Carina Dwarf, Sextans Dwarf, Sculptor Dwarf, Fornax Dwarf, Leo I, Leo II, and Ursa Major Dwarf.
Andromeda has satellite galaxies M32, M110, NGC 147, NGC 185, And I, And II, And III, And IV, And V, Pegasus dSph, Cassiopeia Dwarf, And VIII, And IX, and And X.
The Traingulum galaxy might be a satellite to Andromeda, and it might also have the Pisces Dwarf as a satellite.
The other members of the Local Group, not associated with another galaxy, include: IC10, IC1613, Phoenix Dwarf, Leo A, Tucana Dwarf, Cetus Dwarf, Pegasus Dwarf Irregular, Wolf-Lundmark-Melotte, Aquarius Dwarf, and Sagittarius Dwarf Irregular.
The first astronomer to identify the Local Group was Edwin Hubble, who called the collection the Local Group in his book, The Realm of Nebulae. Of course, at this time Hubble didn’t know that they were distant galaxies, separate from our own Milky Way, so he called them nebulae.
The space between stars is known as interstellar space, and so the space between galaxies is called intergalactic space. These are the vast empty spaces that sit between galaxies. For example, if you wanted to travel from the Milky Way to the Andromeda galaxy, you would need to cross 2.5 million light-years of intergalactic space.
Intergalactic space is as close as you can get to an absolute vacuum. There’s very little dust and debris, and scientists have calculated that there’s probably only one hydrogen atom per cubic meter. The density of material is higher near galaxies, and lower in the midpoint between galaxies.
Galaxies are connected by a rarefied plasma that is thought to posses a cosmic filamentary structure, which is slightly denser than the average density of the Universe. This material is known as the intergalactic medium, and it’s mostly made up of ionized hydrogen. Astronomers think that the intergalactic medium is about 10 to 100 times denser than the average density of the Universe.
This intergalactic medium can actually be seen by our telescopes here on Earth because it’s heated up to tens of thousands, or even millions of degrees. This is hot enough for electrons to escape from hydrogen nuclei during collisions. We can detect the energy released from these collisions in the X-ray spectrum. NASA’s Chandra X-Ray Observatory – a space telescope designed to search for X-rays – has detected vast clouds of hot intergalactic medium in regions where galaxies are colliding together in clusters.
