Astronomy

Why do we see galaxies with their actual spiral shapes?

Why do we see galaxies with their actual spiral shapes?


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Since the actual position of stars is relative, and we see the light they emitted long time ago, and one galaxy has thousands of millions of stars, and those stars can be separated by hundred of thousands of light years, why don't we see galaxies as abstract forms or lines in the sky as the light of every galaxy star arrives to us at different times instead of well formed spiral shapes? It's like if those millions stars were forming the galaxy as a single light spot, instead of millions of spots separated by light years.

This has been in my head for decades, but I didn't find the question (not even the answer, but the question) anywhere, so either there must be an really easy answer to it or a really complex one or it's a glitch in the Matrix.


Basically, because the relative speeds of stars within galaxies are much less than the speed of light.

Imagine a structure with dimension 100,000 light years (about the size of a galaxy). Now suppose the components of that structure move with speeds relative to each other of around 100 km/s (or 0.03% of the speed of light). In the time it takes light to cross the structure (100,000 years of course), the components within it will have moved by just 30 light years (0.03% of the size of the structure).

There would therefore be no significant "blurring" of our snapshot of a galaxy caused by the differing light travel times from its structural components.

The other thing you may have not understood, is that the lives of stars are generally much longer than 100,000 years. So the stars that emit the light on the far side of a galaxy are still in existence 100,000 years later, when their light passes stars that are present at the near side of a galaxy.


I am not rewriting what nicely said in another answer. I just point out, as it is just implicit there, that even the closest stars in our galaxy are really far away! Not too say other galaxies. If, as I suppose, you aren't surprised by the fact that the stars look as fixed over millennia, then considering distance should fix your question.


Why do we see galaxies with their actual spiral shapes? - Astronomy

The Milky Way and many galaxies are spirals. To keep the spiral, the outer stars must move faster than the inner stars. From Issac Newton on, we have known that the father a star is from the center, the slower it must travel.

How can spiral galaxies keep their shape?

This is a great question! You're totally right, if we assume most of the mass in a galaxy is concentrated at the center, then the outer regions should move slower than the inner regions, but in order for spiral arms to hold their shape, the outer regions must move faster instead. The sorts of beautifully distinct spirals you are probably thinking of are usually attributed to density wave theory. We previously discussed this briefly in a different question here. I'll go into slightly more detail than the previous answer below. If you want to read more, Wikipedia can always be a good starting point (including a very convincing animation here!).

Instead of galaxies, let's first imagine dropping a pebble into a pond. Circular waves will propagate outwards from where the pebble falls. Of course, water itself isn't propagating away from the center, since otherwise you would have less water in the center and more at the edges after a while. Instead, it is the push and pull of the water with itself that lets water generate these waves. Each water molecule only moves in a confined zone, but together they look like a wave propagating happily away from the center! We just had to give it a little initial push, by dropping the pebble.

Now, by coarse analogy, imagine that each water molecule is instead a star. Then the crests of the wave (where the wave is tall) correspond to zones with more stars, and these zones will appear brighter. And to a faraway observer, who may not be able to see individual stars, they'll just see bright circles moving away from a central point. This isn't really a physical system, but hopefully illustrates that propagating bright zones in galaxies can be caused by stars moving around locally, only causing apparent sustained motion.

So now that we've established waves are one way we can get moving bright zones without requiring moving objects, what sorts of waves can exist in a galaxy? This requires some math that I'm not going to bore you with, but if you consider a self-gravitating disk that has some finite sound speed ("sound" here has the same sense as sound in Earth's atmosphere, regions of higher pressure that propagate at the sound speed), you can show with some work that a two-armed trailing spiral wave pattern forms. This is principally work done by C. C. Lin and Frank Shu in 1964. And now that we know such waves can exist, we just need something to excite them this is traditionally attributed to galaxy close encounters.

This is a trimphant theory, but as with most astrophysics, it's hard to be 100% certain. There are still scientists verifying certain parts of the theory that have to be numerically correct (e.g. some people believe spiral arms are actually transient, so they break apart and reform, something not quite captured by the Lin-Shu theory), and there are also alternative explanations:

  • Excited by tides: two-armed trailing spirals can be explained by tides between two galaxies. This produces the same sorts of arms, but would die out once the galaxies move apart.
  • Self-propagative star formation: discussed coarsely in the earlier question, where star formation drives more star formation, so stars tend to be clumped in this way. People generally don't think this produces two-armed patterns but more fuzzy ones.
  • Bar-driving: most spiral galaxies have a central bar, which is not necessary for the Lin-Shu spiral density waves. With a central bar and no self-gravity, you can still get a spiral disk.
  • Actually solid-body rotation: You can get close to solid-body rotation when there's a lot of dark matter in the galaxy that is spread out far away from the center. Some spiral galaxies seem to exhibit close to solid-body rotation, which means their arms may be material.

So the jury is still out! But the common feature among most of these models is that the arms are not stars moving in synchrony, but are stars passing in and out of the arms.


Why are galaxies different shapes?

There are four different galaxy shape categories spiral, elliptical, lenticular, and irregular. See the explanation below to see why they are the way they are.

Explanation:

Galaxies have very dynamic shapes, meaning that they can change over time. For example, if a galaxy is left alone and undisturbed for a long time they become more flat, elliptical and spiral shapes whilst galaxies that have been disturbed, or rather, collided with another galaxy become more spherical and round.

So basically it is dependant on the Galaxy's surroundings and environment. If it had a rocky past it might be more spherical or irregular but after a few billion years it might have moved more towards a spiral shape.

Something to note is that small galaxies rarely form anything other than irregular shapes since their gravitational force isn't strong enough to form spiral or elliptical shapes.


Mystery Behind Galaxy Shapes Solved

Galaxies come in many shapes and sizes, but until recentlyastronomers have been at a loss to explain why.

Now scientists have used dark matter theory to predict themenagerie of galaxiesfound in the universe. Their new model reproduces 13 billion years? worth ofcosmic evolution, resulting in a surprisingly accurate tally of the differentkinds of galaxies we see.

"We were completely astonished that our model predictedboth the abundance and diversity of galaxy types so precisely," saidresearcher Nick Devereux of Embry-Riddle University in Arizona.

American astronomer Edwin Hubble first developed aclassification system in the 1930s, known as the Hubble Sequence, which dividesgalaxies into two main types: spirals, and ellipticals.

Elliptical galaxies look like eggs of light ? a central,solid nest of stars. Spirals, on the other hand, are the stereotypical swirlinggalaxies that many people think of, and include our own Milky Way and ourclosest neighbor, Andromeda. Spiralgalaxies come in two kinds ? with and without a bar of thick material inthe center, from which the spiral arms wind out. (The Milky Way is a barredspiral.)

Researchers created a new supercomputer model, based onobservational data and the "Lambda Cold Dark Matter" theory of theuniverse. This theory suggests that about 72 percent of the cosmos is made ofup a mysterious force called dark energy, while another 23 percent is composedof an invisible type of matter called dark matter. That leaves only 4 percentof the universe made of normal, visible matter, including all the stars andplanets that we see.

The new supercomputer model was able to predict roughly theright relative numbers of spirals and ellipticals that exist today.

"It really boosts my confidence in the model,"said astronomer Andrew Benson of Caltech, a co-researcher in the study.

The researchers said the inclusion of dark matter is likelycrucial to their results, because theory predicts that galaxies sit insidelarger spheres, called halos, of the invisible stuff. The behavior of agalaxy's darkmatter halo could affect its evolution and help determine whether itbecomes a spiral or an elliptical, they added.

"These new findings set a clear direction for futureresearch," Devereux said. "Our goal now is to compare the modelpredictions with observations of more distant galaxies seen in images obtainedwith the Hubble [Space Telescope] and those of the soon-to-be-launched JamesWebb Space Telescope."

The findings were published in the journal Monthly Noticesof the Royal Astronomical Society.


Why are some galaxies shaped liked spirals?

About a third of known galaxies are flat spirals with bulging centers. Astronomers believe that galaxies have spiral arms because galaxies rotate – or spin around a central axis – and because of something called “density waves.”

Galactic density waves are like water waves. Water itself doesn’t move across a pond – instead, wave energy moves and affects the water as it passes. A spiral galaxy’s rotation, or spin, bends the waves into spirals. Stars pass through the wave as they orbit the galaxy center. The wave causes the stars to slow slightly and temporarily clump together.

Astronomers have long wondered why the spiral arms of a galaxy don’t wind up and vanish after a few rotations. Many galaxies have satellites – smaller neighboring galaxies. One theory is that a satellite can keep a larger galaxy’s density waves moving indefinitely.

Other processes may help shape galaxy spiral arms. For example, galaxy rotation might smear exploding and forming stars into a bunchy spiral arm. Many astronomers think that there are multiple processes that contribute to creating the different kinds of spiral galaxies we see.


Why Galaxies Come in Different Shapes

Hanging on the walls in countless science classrooms around the world are illustrations of our home galaxy, the Milky Way. You've probably heard of it. But you probably didn't know that those posters are proportionately thicker than a key component of the galaxy itself.

That's right. Like a fried egg, the Milky Way consists of a central bulge surrounded by a flat, thin disc. And when we say "thin," we mean mind-bogglingly thin. As physicist and Forbes correspondent Jillian Scudder points out, the "disc" of the Milky Way galaxy is around 100,000 light years long, but only about 0.6 light years tall. This means that, proportionally, it's 30 times thinner than a typical sheet of printer paper.

Astronomers have estimated that there are around 200 billion galaxies in the observable universe. But when we see other galaxies portrayed in science fiction films, they tend to have the same basic shape as ours. This would be an example of our human-centered biases. Although many of them do resemble the Milky Way, others come in wildly different shapes and forms.

How Galaxies Get Their Shapes

Before we go any further, let's take a step back and talk about what all galaxies have in common. Galaxies are complex systems held together by gravity. They're made up of gasses, stellar dust and millions — sometimes even billions — of stars, which are accompanied by their own planets and asteroid belts.

Yet similarities aside, every galaxy has a unique story to tell. The history of each one is reflected in its shape. Scientists divide galaxies up into a handful of appearance-based categories. The Milky Way is what's known as a spiral galaxy, meaning that it looks like a broad, flattened disc with a slight bulge protruding outward at its center.

That arrangement is the product of rotation speed, time and gravity. To learn more, we talked to astrophysicist Raja GuhaThakurta, Ph.D., a professor at the University of California Santa Cruz, and authority on the study of how galaxies evolve. It's a field that invites a lot of debate.

"The physics of how these things form is not completely known or settled," GuhaThakurta says. Nonetheless, it's widely thought that most spiral galaxies begin their lives as spinning clouds of gas and dust. The speed at which they rotate matters a great deal. According to GuhaThakurta, massive, rapidly rotating clouds are more likely to become spiral galaxies.

Gravity attempts to pull these spinning, amorphous bodies into flattened planes. Over time, the clouds are forced to contract because of gravity and loss of energy due to friction. And due to a principle called the conservation of angular momentum, when a spinning object contracts, it rotates more rapidly. You can see this in action at your local skating rink. Experienced ice skaters know to increase their twirl speed by drawing their arms inward.

So, much like a spinning blob of pizza dough, spiral galaxies are formed when shapeless gas/dust clouds flatten out at high speed. The same physical forces also effect the look of the pointed "arms" that can be seen around the rims of such galaxies.

"The types of spiral arms are almost certainly related to the rotation rate," GuhaThakurta says. Rapidly rotating systems tend to have a ring of small, tight arms. In contrast, those that move more slowly have longer, loosely wrapped ones. To understand why, GuhaThakurta recommends trying a little home experiment: "Imagine stirring your coffee. Put a dollop of cream somewhere other than the center. You'll notice that the cream will form a spiral pattern," he says. Then, stir the brew with a spoon. If you do so rapidly, the pattern's arms will get smaller and tighter.

The Mystery of the Bulge

Okay, time for a quick recap. Thus far, we've talked about how spiral galaxies develop and how rotation shapes their arms. But what's the deal with those bulges we mentioned earlier? At the center of spiral galaxies, you'll find a cluster of very old stars revolving around a central point. This is the bulge. While the stars out in the disc move around in an orderly, horizontal plane, the stars that comprise the bulge act like bees erratically swarming around a hive. Astronomers are still trying to figure out how these bulges form. Some speculate that they develop before the rest of a spiral galaxy does, while others think the reverse is true.

Now imagine a galaxy that's all bulge. This thing would be disc free and either look like a giant, rounded sphere or a massive American football. Inside, its stars would be orbiting the galaxy's central point in all directions. Congratulations, you've just pictured an elliptical galaxy. GuhaThakurta says elliptical galaxies form when two spiral galaxies of comparable mass merge together. (Although he adds that this might not be the only process by which elliptical galaxies are formed.)

Incidentally, our very own Milky Way is about to participate in one of these mergers. Experts project that it will eventually collide with the nearby Andromeda galaxy, a process that'll reconstitute these two spiral galaxies into one elliptical galaxy. The process should begin about 3 billion years from now and finish in an additional 4 billion years from then. Obviously, it's not something you or I will live to see. But regardless, scientists have already come up with a name for this future elliptical galaxy: They call it "Milkomeda." Everyone loves a good portmanteau.

It should be noted that some galaxies are neither spirals nor ellipticals. So-called irregular galaxies lack bulges and can come in a wide variety of shapes. Furthermore, scientists have observed some galactic mergers that are currently in progress. Maybe they'll look like nice, well-rounded ellipticals someday, but at the moment, these developing unions appear disorganized and distorted. There are also a few documented examples of big spiral galaxies cannibalizing smaller ones that have gotten too close, with the victim getting slowly devoured bit by bit. As Hannibal Lecter might say, pass the fava beans and chianti.


Types of Galaxies

What Kinds of Galaxies Are There?

Astronomers classify galaxies into three major categories: elliptical, spiral and irregular. These galaxies span a wide range of sizes, from dwarf galaxies containing as few as 100 million stars to giant galaxies with more than a trillion stars.

Ellipticals, which account for about one-third of all galaxies, vary from nearly circular to very elongated. They possess comparatively little gas and dust, contain older stars and are not actively forming stars anymore. The largest and rarest of these, called giant ellipticals, are about 300,000 light-years across. Astronomers theorize that these are formed by the mergers of smaller galaxies. Much more common are dwarf ellipticals, which are only a few thousand light-years wide.

Spiral galaxies appear as flat, blue-white disks of stars, gas and dust with yellowish bulges in their centers. These galaxies are divided into two groups: normal spirals and barred spirals. In barred spirals, the bar of stars runs through the central bulge. The arms of barred spirals usually start at the end of the bar instead of from the bulge. Spirals are actively forming stars and comprise a large fraction of all the galaxies in the local universe.

Irregular galaxies, which have very little dust, are neither disk-like nor elliptical. Astronomers often see irregular galaxies as they peer deeply into the universe, which is equivalent to looking back in time. These galaxies are abundant in the early universe, before spirals and ellipticals developed.

Aside from these three classic categories, astronomers have also identified many unusually shaped galaxies that seem to be in a transitory phase of galactic development. These include those in the process of colliding or interacting, and those with active nuclei ejecting jets of gas.

This graphic compares illustrations of the three main types of galaxies (top) with actual photos of galaxies (bottom) that fit the categories. Credit: A. Feild (STScI)

GALAXY TYPES

The Hubble Ultra-Deep Field (Figure 14.15) combines 800 exposures of an unassuming spot in the sky from NASA&rsquos Hubble Space Telescope (HST). Hold your index finger out at arm&rsquos length. The angular size of the Moon is about half of the width of your finger. The spot Hubble looked at for just over 11 days is only a tenth of the size of the Moon. Nearly every single blob of light you see in the image is an entire collection of stars, dust, and dark matter with a black hole in the center bound together by gravity &mdashin other words, a galaxy. From this image we can start to classify types of galaxies.

Figure 14.15: In 2004 astronomers pointed NASA&rsquos Hubble Space Telescope at an empty patch of sky near the constellation Fornax and collected light for over 11 days. This image shows approximately 10,000 galaxies of various ages, sizes, shapes, and colors. Credit: NASA, ESA, S. Beckwith (STScI) and the HUDF Team

CLASSIFYING GALAXIES

Classification is often the first step toward scientific discovery. When confronted with a collection of data they do not understand, scientists try to organize data by patterns or trends that may give clues about the physical processes that give rise to the data.

In this activity you will analyze images of 16 nearby galaxies taken by the Hubble Space Telescope according to their shapes, colors, and one more characteristic of your choosing.

In this part you will examine the 16 images and classify them according to their shapes.

2. Now classify the galaxies by shape according to your chosen categories:

  • To add a category to the list, click the &ldquo+&rdquo under &ldquoCategories.&rdquo
  • To delete a category from the list, click the &ldquo-&rdquo under &ldquoCategories.&rdquo
  • To classify a galaxy as a member of a category, click on the galaxy&rsquos picture and click the check box for the category.

In this part you will examine the 16 images again and classify them according to their colors.

2. Now classify the galaxies by color according to your chosen categories:

  • To add a category to the list, click the &ldquo+&rdquo under &ldquoCategories.&rdquo
  • To delete a category from the list, click the &ldquo-&rdquo under &ldquoCategories.&rdquo
  • To classify a galaxy as a member of a category, click on the galaxy&rsquos picture and click the check box for the category.
  • A galaxy can belong to more than one category.

In this part you will examine the 16 images again and classify them according to a characteristic of your choosing. Do not use size!

4. Now classify the galaxies according to your chosen categories:

  • To add a category to the list, click the &ldquo+&rdquo under &ldquoCategories.&rdquo
  • To delete a category from the list, click the &ldquo-&rdquo under &ldquoCategories.&rdquo
  • To classify a galaxy as a member of a category, click on the galaxy&rsquos picture and click the check box for the category.
  • A galaxy can belong to more than one category.

The structure or morphology of galaxies can be categorized as spiral, elliptical, and irregular . You may have chosen to use those terms or something similar in the previous activity. Edwin Hubble was the first person to classify galaxies in much the same manner. He noticed a correlation when comparing galaxy morphology and color. A correlation does not necessarily imply that one thing caused the other, but it does hint that the two properties are related. Elliptical galaxies tend to be red, while spirals and irregular galaxies tend to have more blue stars. Hubble developed a more detailed morphological classification scheme known as the Hubble tuning-fork diagram shown in Figure 14.16.

Figure 14.16: Edwin Hubble&rsquos tuning fork diagram divides galaxies into three groups: elliptical, spiral, and irregular galaxies. Elliptical galaxies are classified by how round or flat they look. Spiral galaxies are classified by how tightly the arms are arranged and whether or not the galaxy appears to have a central bar. Irregular galaxies are neither spiral nor elliptical and can have any number of shapes. Credit: NASA/SSU/Aurore Simonnet

These three broad categories can be further subdivided based on visual appearance. Hubble noticed that some spiral galaxies have a bright line, or bar, running through them and called them barred spiral galaxies. Those without bars he simply denoted spiral galaxies. Spiral galaxies are further classified by how tightly their arms are wound and the brightness of the central bulge. Elliptical galaxies are classified by how round or elliptical they appear. A transitional galaxy type, named lenticular is somewhere between highly squished ellipticals and disks. These lenticular galaxies have a central bulge but no spiral arms.

Hubble believed galaxies evolved from left to right in his tuning-fork diagram. We now understand this is wrong. There is no demonstrable way to make an elliptical galaxy spin up enough to form spiral arms. There is, however, a possibility that multiple spiral or irregular galaxies could collide and form a massive elliptical galaxy. Galaxy collisions are a natural consequence of gravitational interaction. In fact, astronomers have seen many examples of interacting galaxies (Figure 14.17).

Figure 14.17: These groups of galaxies are all undergoing gravitational interactions. Some of these may one day end up as elliptical galaxies. Credit: NASA/ESA/AURA/STScI/A. Evans

Galaxy collisions can be modeled with simulations such as the one in Animated Figure 14.18. In this simulation, two spiral galaxies pass through each other, yielding a single larger elliptical galaxy at the end of the simulation. In galaxy collisions, the stars and dark matter in each galaxy interact gravitationally. Individual stars are so far apart that they do not actually collide. Interactions between the gases in the galaxies often trigger bursts of new star formation.

Animated Figure 14.18: This computer simulation follows two spiral galaxies as they collide and orbit around each other before finally merging. Credit: John Dubinski

The colors of galaxies can be explained by the types of stars within them. As a galaxy forms, small density perturbations within the gas lead to regions where stars of various masses form. The color of a main sequence star ultimately depends on its mass. Blue stars tend to be more massive than 2 MSun, stars about the size of our Sun are yellow, and stars much smaller are red. These color differences are a direct result of their temperature. Color, temperature, mass, and radius are all intrinsically linked for main sequence stars due to the physics involved blue stars are hotter, more massive, larger, and die quickly. Red stars are cooler, less massive, smaller, and live longer.

Since blue stars are intrinsically brighter, they tend to outshine all the red ones within any given group. It is not until a stellar population ages that the blue stars die out and we finally get to see the dimmer red glow of the small long-lived stars. Therefore, the average color of a galaxy is an excellent indicator of how much time has passed since new stars formed. Because stars form from gas, it also tells us how much free gas is available to condense into stars.

Irregular galaxies tend to have the youngest stellar populations. Vast quantities of gas provide the perfect stellar nursery for red, yellow, and blue stars alike. As one star cluster ages and turns more red, another nearby cluster starts anew, shining bright blue once again. While irregular galaxies contain stars of all colors, the young blue stars outshine them all.

Elliptical galaxies are, for the most part, old and devoid of gas. Their reddish colors are dominated by long-lived, red main sequence stars. A majority of the regular matter is locked up in stars. As a result, there is very little ongoing star formation. Spiral galaxies show a range of star-formation histories and therefore a range of colors. The composite image shown in Figure 14.19 depicts what galaxies similar to our own Milky Way would look like farther back in time. Notice in the middle, around 9.4 billion years ago, galaxies like our own started to show hints of yellow and red as massive blue stars begin to die off.

Figure 14.19: This composite image shows examples of galaxies similar to our Milky Way at various stages of construction over a time span of 11 billion years. The galaxies are arranged according to time. Those on the left reside nearby while those on the far right existed when the Universe was about 2 billion years old. The bluish glow from young stars dominates the color of the galaxies on the right. The galaxies at the left are redder from the glow of long-lived red stars. Credit: NASA, ESA, P. van Dokkum (Yale University), S. Patel (Leiden University), and the 3D-HST Team

THE COLORS OF GALAXIES

In this activity you will examine a cluster of stars within a galaxy. The slider on the bottom allows you to advance forward in time as the cluster evolves.


How Does A Spiral Galaxy Get Its Arms?

The disk galaxy NGC 5866 tilted nearly edge-on to our line-of-sight. Hubble's sharp vision reveals a . [+] crisp dust lane dividing the galaxy into two halves. The image highlights the galaxy's structure: a subtle, reddish bulge surrounding a bright nucleus, a blue disk of stars running parallel to the dust lane, and a transparent outer halo. Image credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)

Could the spiral arms in galaxies be an accident of differently rotating matter "lining up", albeit reinforced by self-gravity, rather than spirals being a fundamentally different animal to ellipticals (where the matter might be not lined up)?

There’s really two questions buried in here - are spirals and ellipticals really fundamentally different objects, and how the spiral arms in spiral galaxies come about, so I’ll tackle these slightly independently, starting with whether spirals and ellipticals are really that different.

And they are! There’s a really easy way to spot this difference spiral galaxies are incredibly, incredibly thin. The thickness of the spiral disk of our own Milky Way is only about 0.6 light years, whereas it’s about 100,000 light years across. With some straightforward division, we can calculate that the Milky Way is 166,666 times larger from edge to edge than it is vertically. This kind of thinness is hard to wrap your mind around. A standard piece of printer paper, typically clocking in at a width of 0.05 mm, is proportionally 30 times as thick as our galaxy.

This spectacular edge-on galaxy, called ESO 243-49, is believed to be home to an intermediate-mass . [+] black hole that may have been stripped off of a cannibalized dwarf galaxy. The galaxy is 290 million light-years from Earth. Image Credit: NASA, ESA, and S. Farrell (Sydney Institute for Astronomy, University of Sydney)

To get something as proportionally thin as the Milky Way, you could paint a solid circle on a basketball court, as big a circle as you could fit in the court. The width of an NBA basketball court is 50 feet wide, so you’d have a 50 foot wide circle painted onto the floor. Let your paint dry. The average coat of paint is about 100 microns thick (0.00394 inches) - so relative to the size of the 50 foot circle, the height of one coat of paint on the floor is pretty close to proportional to the height of the Milky Way's thin disk of stars. If you could suspend this in the air, you’d be looking at a fifty foot circle that’s approximately the same width as a thin human hair. Spiral galaxies are mind-bogglingly thin.

Meanwhile, an elliptical galaxy doesn’t have this stupendously thin disk. Ellipticals tend to be more football shaped than anything else, so they are usually only a few times longer in one direction than in either of the other two, so there's not much of an "edge" in any direction. Ellipticals and spirals are two very different types of galaxies, and the differences we see just by looking at their shapes are reflected in other properties, like their age (ellipticals are older), color (the old stars in the ellipticals make them much redder), and number of new stars formed (overwhelmingly, new stars are found in spiral galaxies).

This is a Hubble Space Telescope image of the massive elliptical galaxy M60. The galaxy lies about . [+] 50 million light-years away inside the immense Virgo Cluster of 2,500 galaxies. A portion of the faint bluish spiral galaxy NGC 4647 can be seen in the upper right corner of this image. Image Credit: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration

All that said, spiral arms are still kind of weird. If the galaxy was a static object, like our circle made of paint, and rotated as a unit, you’d need the spiral arms to have been there from the galaxy’s birth, but once they got there you could keep them fairly easily. Unfortunately, we can see that galaxies don’t rotate as a solid object, like a DVD in a player.

So that idea’s out what next? The stars and gas which are closest to the center of a spiral galaxy rotate faster around the center than the stars at the outskirts. This difference in rotation speeds means, if you give a galaxy a spiral arm pattern and then let the galaxy just exist for a little while, your nice loose spiral arm pattern will wind up into a really tight spiral, and the lack of space between arms will make it hard to even spot them in the first place. So if this scenario is the case, then the strong spiral arm features shouldn’t last long. Again, there are problems if spiral arms shouldn’t last very long, then in general you wouldn’t expect to see many strong spiral arms if you look out at the galaxy population. And while there are certainly galaxies without distinct spiral arms, there are a lot of galaxies with strong spiral features. We'll have to throw out this idea as well.

A simplified diagram of how arms form in spiral galaxies due to the elliptical, misaligned orbits of . [+] the stars. Image credit: Wikimedia user Dbenbenn, use under CC A-SA 3.0

What we are left with is an idea called spiral density wave theory, which suggests that the spiral arms aren’t a physical “thing”, but are made of stars which are simply passing through, more like a traffic jam than anything else. The apparent spiral arms appear because stars don't orbit the center of the galaxy in perfect circles. Each star is instead on an elliptical orbit, much like the recurrent comets in our solar system. As stars are moving the slowest at the distant edge of their orbit, if a large number of stars have turnaround points around the same place, you'll wind up with an extra dense region of stars, creating an apparent spiral arm. Each star will continue along its own orbit, drifting in and out of spiral arms as the galaxy spins.

So your idea of differently rotating matter lining up to form the spiral arms is actually very close to the mark for the stars found within the very thin disks of spiral galaxies. However, because of the 3D nature of galaxies, in that spirals are so very thin, and ellipticals are so extremely round, we can’t account for the differences between spirals and ellipticals this way.


Why do we see galaxies with their actual spiral shapes? - Astronomy

It may be a rare experience for city-dwellers, but a clear, moonless night in the country offers an astonishing sight: an amazing dark-velvet blanket studded by glistening stars, across which meanders the glowing road of the Milky Way.

Modern astronomers know that when we look at the Milky Way we are in fact seeing the edge of a galaxy, a pinwheel-shaped collection of more than 100 billion stars, among them an average one that we call our own Sun about two-thirds of the way out one of the galaxy's arms. Although other galaxies were visible for centuries to even simple telescopes as fuzzy smudges of light, it wasn't until the 1920s that astronomers reached a consensus that they too were groupings of enormous numbers of stars. This was thanks in part to the American astronomer Edwin Hubble, who observed pulsating stars in a blurry patch in the constellation of Sagittarius and concluded that it was far too distant to be part of our galaxy, and must therefore be one of its own.

What we know about galaxies
The past few decades have also been marked by remarkable advances in our understanding of how the universe formed. Astronomers now generally agree on a model called the Big Bang, which holds that all of the matter and energy in the universe was once contained in a compressed state of unimaginably high density and temperature that abruptly expanded in an explosion-like event 13.7 billion years ago. Within a million years the young universe cooled enough for atoms to form, leaving clouds of simple atoms of hydrogen. Perhaps 1 billion to 2 billion years after the Big Bang, much of this gas condensed into swirling galaxies, and then into separate stars.

Astronomers today estimate that there are at least 10 billion galaxies in the visible universe. Most galaxies are arranged in clusters. Our own Milky Way belongs to a small cluster of about 40 galaxies, called the Local Group. The group is about 3 million lightyears across, with most of its contents in two large spiral galaxies, our Milky Way and the neighboring Andromeda galaxy. Our Local Group is part of the Local Supercluster, a thin sheet of galaxy clusters. In between the various superclusters in the universe are voids, regions with few galaxies. Most of the universe consists of these voids.

Not all galaxies gather into groups or clusters, however some appear like hermits far from neighbors. The "urban-dwelling" galaxies seem to have different properties than the loners. Some galaxies appear to form huge quantities of stars in a rapid flash of activity early in their history, while other galaxies display a slower, but gradual star formation.

Scientists have classified galaxies according to their shape, although they don't know why they form different shapes in the first place. Spiral galaxies, like our Milky Way, form stars gradually. They contain young, middle-aged and old stars, and large amounts of gas and dust. Elliptical galaxies are football-shaped and have little visible gas and dust they appear to contain mostly old stars that formed fast and early. Irregular galaxies, with no regular shape or motion, have bursts of star formation. Peculiar galaxies are odd in shape and form, and are usually in the process of forming stars.

Astronomers also wonder how galaxies manage to churn tenuous cosmic gases and dust to the point that the matter collapses into the dense, fiery material that makes up stars. They know that a large portion of a galaxy is made up of seemingly invisible "dark matter." They don't know what dark matter is, how it got there, and what role it plays in the evolution of galaxies.

While scientists have been trying to decipher the mysteries of galaxies, these cosmic entities have even seeped into popular culture. Movie fans may have munched on a Milky Way candy bar while watching Star Wars, a story that takes place a "long time ago in a galaxy far, far away."

Although astronomers have learned a great deal about galaxies since the 1920s, there are still many mysteries waiting to be solved. NASA's Galaxy Evolution Explorer is an important part of that quest.

In search of stellar nurseries
The mission searches for galaxies in which young stars are forming by taking advantage of a peculiarity in the kind of energy that stars put out. A mid-sized, middle-aged star like our Sun throws off energy across a large spectrum of wavelengths, from infrared to visible light to ultraviolet. However, ultraviolet makes up less than 5 percent of the energy given off by the Sun.

Very massive stars, on the other hand, throw off an enormous amount of ultraviolet energy. Fast-trackers that live on the edge and burn the candle at both ends, these stars shine brightly and die early. Since they never get to be middle-aged like our Sun, any galaxy with a lot of these ultraviolet-bright stars must be one in which new stars are vigorously forming.

The telescope on the Galaxy Evolution Explorer thus peers out into the universe at ultraviolet wavelengths to look for these stellar nurseries. As it conducts its sky surveys, the telescope will observe millions of galaxies. Why so many?

Let's say an alien from some other planet wanted to learn how people on Earth age, but didn't want to spend 75 years watching a group of babies grow up and grow old. Instead, the alien could study 100 one-year-old babies, 100 two-year-olds, 100 50- year-olds, 100 75-year-olds, etc. The alien would not be able to write a specific biography of any one person, but it would learn an awful lot about the life of an "average" human. It would learn at what ages children grow fastest, how much time people spend eating and sleeping, and other characteristics that would help the alien describe the life of an average human. In the same way, the Galaxy Evolution Explorer will study many galaxies to learn about the life of an average one.

The telescope will examine galaxies both near and far. The farther we look out into space, the farther back in time we are seeing. The most distant galaxies that the Galaxy Evolution Explorer will see are about 10 billion light-years from Earth. Since the universe is thought to be 13.7 billion old, the mission will catalog galaxies across 80 percent of the history of the universe.

The mission will enable scientists to reconstruct the history of our Milky Way galaxy by studying similar galaxies. It will help answer questions about how the Milky Way began and how our star, the Sun, formed within it, an event that paved the way for the eventual development of our solar system, Earth and life.

Another topic of study for the mission is the history of how heavy elements formed in the universe. The original atoms created relatively soon after the Big Bang were simple ones of hydrogen and helium, each containing just one or two pairs of protons and electrons. "Heavy" elements, on the other hand, are the other hundred or so elements on the periodic tables that hang on the walls of science classrooms across the country -- in other words, any element that's heavier than hydrogen and helium. All of these other elements formed as part of nuclear fusion deep inside stars that later died. Elements that make up our bodies, such as carbon and oxygen, were all created in the forge of long-dead stars. We are literally made of stardust.

By using the mission to study other galaxies, scientists will determine which elements formed at various points in cosmic time. They will then apply this information to our Milky Way galaxy to pinpoint which elements existed when our solar system formed.

Blazing new trails in astronomy
The mission will provide the first-ever wide-area ultraviolet surveys of the sky, and the first wide-area spectroscopic surveys. Not only will it help us understand the cosmic events that shaped the history of our universe, but through its discoveries, the Galaxy Evolution Explorer will help design the future of astronomy.

The Hubble Space Telescope was named after the man who first discovered galaxies. The telescope that bears his name has imaged numerous galaxies, both individually and in such elegant groupings as the Hubble deep-field images, which show a tiny portion of the sky peppered with galaxies. The Galaxy Evolution Explorer, by observing large pieces of the sky all at once, will find the most rare and interesting objects in the universe. These objects may become the target of future observations by the Hubble Space Telescope.

When massive stars emit ultraviolet light, much of it is absorbed by dust. The dust then emits heat in infrared wavelengths, which, like ultraviolet, are not visible to the naked eye. The ultraviolet observations of the Galaxy Evolution Explorer will work hand in hand with those of future infrared missions, such as NASA's Space Infrared Telescope Facility, scheduled for launch in 2003, and the James Webb Space Telescope, planned for later this decade. Both of those infrared missions will observe some of the galaxies studied in ultraviolet light by the Galaxy Evolution Explorer. This will give a more complete picture of each galaxy. The mission's legacy will also benefit ground-based observations.

In addition to studying galaxies, the mission will compile a substantial archive of other objects of interest to astronomers. These include active galactic nuclei, often associated with massive black holes at the centers of galaxies white dwarfs, old stars that have blown off their outer shells, leaving very hot cores that are bright in the ultraviolet and quasars, thought to be associated with black holes and galactic nuclei.