A galaxy is made of billions of stars, dust, and gas all held together by gravity. Galaxies are scattered throughout the Universe. They vary greatly in size and shape. Not all galaxies look alike.Among the galaxies, there are apparently three main categories, according to their appearance: the disk galaxies (`cosmic frisbies' according to P. Murdin, D. Allen, and D. Malin), consisting of a huge disk of stars and interstellar matter, which may form interesting patterns, the elliptical galaxies (`cosmic footballs') which are uniformly looking, ellipsoidal agglomerations of stars, and the irregular galaxies (`cosmic misfits') which cannot be integrated in this scheme.
Physically, it is not necessary so clear (at least in the opinion of the present author) if this classification is real, because there exist intermediate types even between ellipticals and spirals, i.e. spiral galaxies often have an ellipsoidally formed "bulge" which may be very luminous (as in case of the Sombrero galaxy M104) or rather inconspicuous; some spirals seem to lack this component at all. A heavy bulge is often connected with the presence of a big ellipsoidal core. On the other hand, at least some ellipticals seem to house a disk component also; ths most conspicuous example of such a galaxy is probably Centaurus A (NGC 5128), a prominent galaxy in the Southern hemisphere which is not a Messier object because of its southern declination, but forms a group with the beautiful spiral M83. Centaurus A is regarded as peculiar. One may speculate that e.g. the disk around the center of M87, which is often regarded as the accretion disk around the supermassive object in that galaxy's nucleus, may be a smaller manifestation of the same disk phenomenon.
Focussing on the disk (or disk dominated) galaxies, these often show beautiful and conspicuous patterns in the form of spiral arms and/or luminous bars. These structures have been a mystery for a long time, it was thought that there may be physically different classes of disks (e.g., "normal" and "barred" disks), but now it seems as if they are all the consequences of gravitational interactions with neighboring galaxies. Encounters with neighbors cause inhomogenities and un-symmetries in the gravitational field within the disk, which tends to compress the gas in some regions. If the density of the gas in these regions exceeds a certain critical value (which depends on parameters as the temperature), star formation can take place, resulting in the formation of red diffuse emission nebulae and blue clusters of hot young stars, which slowly change their color to the yellow when they come to age, and their hottest stars have disappeared (i.e., exploded as supernova).
The star forming regions tend to be aligned along spiral arms, as the denser regions in the interstellar matter apparently prefer to form such patterns. When getting older, they sometimes stay conspicuous as yellowish "fossil arms", which can be traced in several galaxies.
Galaxies are classified, according to their appearance, in the so-called Hubble scheme (after its inventor, Edwin Powell Hubble; see e.g. our illustration of the Hubble Scheme with Messier galaxies). This scheme defines the classes listed above, i.e. spiral, elliptical and irregular galaxies, and is especially interesting for spirals: Those with pronounced bar structures are called "barred spirals" and classified "SB", while normal spirals are simply called "S" or sometimes "SA"; some authors take "SAB" or "S(B)" for mixed types. Spiral galaxies, "normal" and barred, with conspicuous bulges (especially near their center) are classified "Sa" or "SBa", those which have prominent bulges and pronounced arms are clssified "Sb" or "SBb", and those which are dominated by the arms are "Sc" or "SBc". If the core and bulge seems to be lacking, a galaxy is classified "Sd" or "SBd", and those which have no pronounced core and show irregularities are classified as "Sm"; these represent a type between disk and irregular galaxies.
Some of the galaxies, mostly those who had no closer encounters for a longer period of time, and those who have lost most of their interstellar matter for some reason, do not show any conspicuous pattern within their disks; these are often called "S0" or "lenticular" galaxies. Although they are disks, they can often hardly be distinguished from ellipticals from their appearance, and have often been misclassified in the past. This misclassification happened to all the four Messier lenticulars in the past, and to many other galaxies of this type.
When undergoing a heavy interaction, or collision, with a massive neighbor, disk galaxies may be distorted very peculiarly, and then are often classified as irregular; this is the case for the only Messier irregular, M82.
All disk galaxies have a very different appearance, depending from what direction they are seen, or under which angle toward the line of sight (to us) their disk is inclined. According to this situation, they are either seen from their edge (or "edge-on"), or from near their equatorial plane, as thin, flat, linear and elongated patches, often with dusty structures along their equators, or almost from their poles so that we can see their disks "face-on". Tom Polakis has featured some edge-on galaxies.
Our Milky Way is one of the big and more massive spiral galaxies, and is of Hubble type Sbc, or perhaps SBbc if it should have a bar. It consists of about 200 billion stars, with our own Sun being a fairly typical specimen. It is a fairly large spiral galaxy and it has three main components: a disk, in which the solar system resides, a central bulge at the core, and an all encompassing halo.
The disk of the Milky Way has four spiral arms and it is approximately 300pc thick and 30kpc in diameter. It is made up predominantly of Population I stars which tend to be blue and are reasonably young, spanning an age range between a million and ten billion years.
The bulge, at the centre of the galaxy, is a flattened spheroid of dimension 1kpc by 6kpc. This is a high density region where Population II stars predominate---stars which tend toward red and are very old, about 10 billion years. There is growing evidence for a very massive black hole at its centre.
The halo, which is a diffuse spherical region, surrounds the disk. It has a low density of old stars mainly in globular clusters (these consist of between 10,000 - 1,000,000 stars).The halo is believed to be composed mainly of dark matter which may extend well beyond the edge of the disk.
Cosmic Sprinklers Explained
Odd pair of aging stars sculpt spectacular shape of planetary nebula
8 November 2012
Astronomers using ESO’s Very Large Telescope have discovered a pair of stars orbiting each other at the centre of one of the most remarkable examples of a planetary nebula. The new result confirms a long-debated theory about what controls the spectacular and symmetric appearance of the material flung out into space.Planetary nebulae are glowing shells of gas around white dwarfs — Sun-like stars in the final stages of their lives. Fleming 1 is a beautiful example that has strikingly symmetric jets that weave into knotty, curved patterns. It is located in the southern constellation of Centaurus (The Centaur) and was discovered just over a century ago by Williamina Fleming, a former maid who was hired by Harvard College Observatory after showing an aptitude for astronomy.
Astronomers have long debated how these symmetric jets could be created, but no consensus has been reached. Now, a research team led by Henri Boffin (ESO, Chile) has combined new Very Large Telescope (VLT) observations of Fleming 1 with existing computer modelling to explain in detail for the first time how these bizarre shapes came about.
The team used ESO’s VLT to study the light coming from the central star. They found that Fleming 1 is likely to have not one but two white dwarfs at its centre, circling each other every 1.2 days. Although binary stars have been found at the hearts of planetary nebulae before, systems with two white dwarfs orbiting each other are very rare.
“The origin of the beautiful and intricate shapes of Fleming 1 and similar objects has been controversial for many decades,” says Henri Boffin. “Astronomers have suggested a binary star before, but it was always thought that in this case the pair would be well separated, with an orbital period of tens of years or longer. Thanks to our models and observations, which let us examine this unusual system in great detail and peer right into the heart of the nebula, we found the pair to be several thousand times closer.”
When a star with a mass up to eight times that of the Sun approaches the end of its life, it blows off its outer shells and begins to lose mass. This allows the hot, inner core of the star to radiate strongly, causing this outward-moving cocoon of gas to glow brightly as a planetary nebula.
While stars are spherical, many of these planetary nebulae are strikingly complex, with knots, filaments, and intense jets of material forming intricate patterns. Some of the most spectacular nebulae — including Fleming 1 — present point-symmetric structures.
For this planetary nebula it means that the material appears to shoot from both poles of the central region in S-shaped flows. This new study shows that these patterns for Fleming 1 are the result of the close interaction between a pair of stars — the surprising swansong of a stellar couple.
“This is the most comprehensive case yet of a binary central star for which simulations have correctly predicted how it shaped the surrounding nebula — and in a truly spectacular fashion,” explains co-author Brent Miszalski, from SAAO and SALT (South Africa).
The pair of stars in the middle of this nebula is vital to explain its observed structure. As the stars aged, they expanded, and for part of this time, one acted as a stellar vampire, sucking material from its companion. This material then flowed in towards the vampire, encircling it with a disc known as an accretion disc. As the two stars orbited one another, they both interacted with this disc and caused it to behave like a wobbling spinning top — a type of motion called precession. This movement affects the behaviour of any material that has been pushed outwards from the poles of the system, such as outflowing jets. This study now confirms that precessing accretion discs within binary systems cause the stunningly symmetric patterns around planetary nebulae like Fleming 1.
The deep images from the VLT have also led to the discovery of a knotted ring of material within the inner nebula. Such a ring of material is also known to exist in other families of binary systems, and appears to be a telltale signature of the presence of a stellar couple.
“Our results bring further confirmation of the role played by interaction between pairs of stars to shape, and perhaps even form, planetary nebulae,” concludes Boffin.
The Horsehead nebula
Rising from a sea of dust and gas like a giant seahorse, the Horsehead nebula is one of the most photographed objects in the sky. NASA's Hubble Space Telescope took a close-up look at this heavenly icon, revealing the cloud's intricate structure. This detailed view of the horse's head is being released to celebrate the orbiting observatory's eleventh anniversary. Produced by the Hubble Heritage Project, this picture is a testament to the Horsehead's popularity. Internet voters selected this object for the orbiting telescope to view.
The Horsehead, also known as Barnard 33, is a cold, dark cloud of gas and dust, silhouetted against the bright nebula, IC 434. The bright area at the top left edge is a young star still embedded in its nursery of gas and dust. But radiation from this hot star is eroding the stellar nursery. The top of the nebula also is being sculpted by radiation from a massive star located out of Hubble's field of view.
Only by chance does the nebula roughly resemble the head of a horse. Its unusual shape was first discovered on a photographic plate in the late 1800s. Located in the constellation Orion, the Horsehead is a cousin of the famous pillars of dust and gas known as the Eagle nebula. Both tower-like nebulas are cocoons of young stars.
The Horsehead nebula lies just south of the bright star Zeta Orionis, which is easily visible to the unaided eye as the left-hand star in the line of three that form Orion's Belt. Amateur astronomers often use the Horsehead as a test of their observing skills; it is known as one of the more difficult objects to see visually in an amateur-sized telescope.
Hubble Celebrates Its 24th Anniversary with an Infrared Look at a Nearby Star Factory
March 17, 2014
In celebration of the 24th anniversary of the launch of NASA's Hubble Space Telescope (on April 24, 1990) astronomers have taken an infrared-light portrait of a roiling region of starbirth located 6,400 light-years away.
The Hubble mosaic unveils a collection of carved knots of gas and dust in a small portion of the Monkey Head Nebula (also known as NGC 2174 and Sharpless Sh2-252). The nebula is a star-forming region that hosts dusky dust clouds silhouetted against glowing gas.
Massive, newly formed stars near the center of the nebula (and toward the right in this image) are blasting away at dust within the nebula. Ultraviolet light from these bright stars helps carve the dust into giant pillars. The nebula is mostly composed of hydrogen gas, which becomes ionized by the ultraviolet radiation.
As the interstellar dust particles are warmed from the radiation from the stars in the center of the nebula, they heat up and begin to glow at infrared wavelengths.
The image demonstrates Hubble's powerful infrared vision and offers a tantalizing hint of what scientists can expect from the upcoming James Webb Space Telescope.
This graphic shows the location of the infrared image from the Hubble Space Telescope in a wider view of the region of NGC 2174. On the left is a ground-based image of the star-forming nebula in visible light by an amateur astrophotographer, with an outline showing the area of the detailed Hubble image. On the right is a small detail of a star-forming column in the nebula, made by Hubble's WFC3 infrared camera.
Image Credit: NASA and ESA
January 19, 2019
This is an image of the Cartwheel Galaxy taken with the NASA/ESA (European Space Agency) Hubble Space Telescope.
Lying about 500 million light-years away in the constellation of Sculptor, the cartwheel shape of this galaxy is the result of a violent galactic collision. A smaller galaxy has passed right through a large disk galaxy and produced shock waves that swept up gas and dust — much like the ripples produced when a stone is dropped into a lake — and sparked regions of intense star formation (appearing blue). The outermost ring of the galaxy, which is 1.5 times the size of our Milky Way, marks the shock wave’s leading edge. This object is one of the most dramatic examples of the small class of ring galaxies.
This image is based on earlier Hubble data of the Cartwheel Galaxy that was reprocessed in 2010, bringing out more detail in the image than seen before.
Image credit: ESA/Hubble & NASA
Text credit: ESA
Last Updated: Jan. 19, 2018
Editor: Rob Garner
HUBBLE SPOTS DOUBLE QUASARS IN MERGING GALAXIES
April 06, 2021
SEEING DUAL QUASARS IS LIKE FINDING A NEEDLE IN A HAYSTACK
Inhabitants of our Milky Way galaxy living several billion years from now will have a markedly different-looking sky overhead. Two brilliant objects, each as bright as the full Moon or brighter, will drown out the stars with their radiance. These giant blazing light bulbs are a pair of quasars, brought to life by the collision of our Milky Way with the neighboring Andromeda galaxy.
Quasars are ignited by monster black holes voraciously feeding on infalling matter, unleashing a torrent of radiation. The Milky Way and Andromeda have such black holes at their hearts, which are now sleeping giants. That is, until the big bang-up. The duo will be as deadly then as it is dazzling. Blistering radiation from the quasar pair might sterilize the surfaces of planets, wiping out innumerable extraterrestrial civilizations.
This tale of "death star" dueling quasars looming in the sky might seem like a scene out of a science fiction movie. But the real universe is stranger than fiction. This is actually a story that played out between two pairs of galaxies that existed long ago and far away. The galaxies, each containing a central, bright quasar, are in the process of merging. The galaxies, each containing a central, bright quasar, are in the process of merging. As the two galaxies in each quasar pair, move closer together, so do their quasars. Hubble caught the action, photographing two quasar pairs that existed 10 billion years ago, during the peak epoch of galaxy close encounters. The discovery offers a unique way to probe collisions among galaxies in the early universe that might otherwise have gone undetected. Ancient quasars are scattered all across the heavens, so finding these dynamic duos is fortuitous. Astronomers estimate only one in a thousand quasars are really double quasars.
Image credit:NASA, ESA, Joseph Olmsted (STScI)