| From: mary and the lamb | 15/07/2001
18:48:29 |
| Subject: spiral galaxies | post id:
349739 |
| I bought my young budding
astronomer brother a book on the stars. Question I have is what really
causes the spiral arms in the large
galaxys? | |
| From: Zardoz ® | 15/07/2001
18:55:02 |
| Subject: re: spiral galaxies | post id:
349745 |
| In the 1924, Edwin Hubble
discovered Cepheid variable stars in other nebula, which confirmed that
they were well outside the Milky Way that indeed these were other galaxies
or island universes. Many of these galaxy have a spiral shape with distinct arms that curl away from the center. In 1951 astronomers found that bright, young O and B type stars were clustered at certain distances. That same year 21-cm radiation was discovered. It was predicted that cold hydrogen gas would emit light in the radio wave part of the EM spectrum at about a 21-cm wavelength. With more sophisticated radio telescopes astronomers were able to detect this radiation for the first time. This allowed astronomers to locate interstellar dust that had been invisible before. It was found from the Doppler shift of this radiation that the gas was clustered at certain distances much like the O and B type stars. These two observations as well as the fact that many other spiral galaxies can be seen, led astronomers to believe that the Milky Way has a spiral nature as well. Spiral Arm Theories It is not well understood how a spiral galaxy keeps its arms well defined for so long. It is easy to show that just differential rotation will quickly lose its spiral nature and appear random. One theory is that there is a "density wave" that is propagating around the galaxy. This wave would slow down any material that tries to move through it and create a region of higher density, which would also ignite star formation. This would account for the young stars in the arms and the concentration of hydrogen gas (21-cm radiation) The competing theory is kind of the opposite. It is not the high density that causes star formation it is the star formation that causes the high density. Stars are constantly being formed, the massive stars that go through their liftimes quickly and explode as supernova will cause new stars to form. So it is only the arms that are being constantly supplied with new stars, so the differential rotation causes the spiral arms.  http://www.physics.eku.edu/Yoder/milkyway.htm | |
| From: Zardoz ® | 15/07/2001
19:01:30 |
| Subject: re: spiral galaxies | post id:
349750 |
| Why galaxies have the shapes that
they do is still not totally decided, but we think that we have a good
idea. Galaxies come in three basic types; disks, elipticals, and
irregulars. Many of the disk type of galaxies also have spiral arms. Disk
galaxies (especially the spirals) look like they are rotating, and they
are. The gas clouds from which the disk galaxies formed were slowly
rotating. As the cloud collapsed, it had to spin more rapidly (similarly
to an ice skater pulling in his arms). Eventually, they began to spin so
rapidly that they couldn't contract any further towards the axis around
which they were spinning. They could still collapse parallel to the axis,
though, and so they ended up forming spinning disks. The situation is very different for elliptical galaxies. It is actually thought that many elliptical galaxies (at least the larger ones) formed when two disk galaxies collided and merged. Computer simulations have shown that one or two such collisions can lead to a galaxy that looks very elliptical. Smaller elliptical galaxies may have formed by themselves. Unlike disk galaxies, though, they apparently formed stars fairly quickly, before they collapsed to form disks. Elliptical galaxies formed their stars mostly in one large burst, either because of a collision, or some other trigger. Disk galaxies formed most of their stars early on, but most of them continue to form stars today. Irregular galaxies, on the other hand, tend to be much younger, and many are very actively forming stars today. They are small galaxies, and didn't collapse much before forming stars. They therefore don't rotate strongly, like disk galaxies. The appearance of irregular galaxies is determined by where they have recently formed stars. Because they have no coherent trigger for star formation, such as collapse or a collision, the stars in an irregular galaxy form more-or-less at random, giving them their irregular appearance. Many pictures of spiral galaxies resemble those of hurricanes on Earth. There is a fundamental connection between the two. Galaxies and hurricanes, like everything else in the universe, obey the same laws of physics. One of these laws is that angular momentum is always conserved. Hurricanes get their angular momentum from the atmosphere. Spiral galaxies get their angular momentum from their local surroundings which happen to have a net angular momentum in some direction. (Since most astronomers believe the universe has no net angular momentum, an equally large amount of mass elsewhere must have net angular momentum in the opposite direction, or, more precisely, the direction in which the angular momentum is pointed is random.) As the galaxy collapses by gravitation, it spins faster, much like an ice skater who brings her arms in close to her body. The rotation speed of gas and stars in a spiral galaxy is related to the amount of matter in the galaxy. This simple relation has convinced many astronomers that spiral galaxies contain large amounts of dark matter. http://antwrp.gsfc.nasa.gov/apod/ap980606.html Grand Design spiral galaxy http://www.madsci.org/posts/archives/may97/864228822.As.r.html http://www.madsci.org/posts/archives/oct2000/971470050.As.r.html | |
| From: B.C. ® | 15/07/2001
19:03:21 |
| Subject: re: spiral galaxies | post id:
349751 |
| Hi Mary, Hi Lamb... Good question...I'm not sure if I can explain it properly but the spirals are not fixed, but are more or less patterns caused by bright stars and luminous gas clouds, within the overall form of the galaxy. These galaxies are rotating at speed and this causes what is known as density waves, which causes a piling up of stars and gases. The causes of these density waves is not exactly known, but are thought to eventuate, due to the difference in the angular momentum of stars towards the centre as compared to the stars in the outer arms. Spiral galaxies are a biggest galaxies and by far the most pretty to look at. The two prime candidates in our own group are Andromeda and our own Milky Way. | |
| From: Greg L. ® | 15/07/2001
19:54:45 |
| Subject: re: spiral galaxies | post id:
349802 |
| Hi Mary. This is a good question
that perplexed astronomers for quite a while earlier in the century. Early
dynamical studies showed that the spiral and bar structures observed in
many galaxies would tend to dissipate within a short period of time, yet
it appeared that the structures were very long lived. The answer, worked
out by a number of astronomers including Frank Shu, Scott Tremaine and
others, appears to be a form of self-propogating disturbance called a
'spiral density wave.' Spiral density waves seem to occur quite commonly
in astrophysics, being present in disks of material where particles
interact with each other gravitationally. This includes dusty accretion
disks, planetary ring systems and galactic aggregations of dust and
stars. The reasons why spiral density waves form is still not understood, but it appears to lie in a number of factors connected with the way stars interact with each other gravitationally in a galaxy. It seems what happens is that stars moving more slowly (in accordance with Kepler's general laws) than stars moving more quickly in the inner regions of a galaxy 'perturb' each other gravitationally. This acts to modify the general path of their orbit about the galactic centre, making the path 'oscillate' in a sinusoidal fashion than simply follow a straight line section of a more general curve. This oscillation appears to grow in size with time and propogates outward through the galaxy, acting to perturb the orbits of more and more stars in the process. The general result, according to detailed computer simulations, appears to be exactly the beautiful spiral and bar structures seen in many galaxies. Spiral density waves are very important for a number of reasons. As they move through a galaxy, the seem to trigger instabilities in giant molecular dust clouds, which then undergo collapse to form new generations of stars. They thus seem to be an active trigger for the formation of new stars, and along with them new solar systems. Spiral density waves also appear to occur in planetary rings and accretion disks of various kinds, acting to influence the overall dynamics of the system by 'removing' angular momentum. | |
| From: Greg L. ® | 15/07/2001
20:40:27 |
| Subject: re: spiral galaxies | post id:
349852 |
| Actually, spiral density wave
theory and the supernova theory seem to be different theories used to
explain the spiral structure. Personally I favour the spiral-density wave
idea, but the supernova idea is quite interesting, given the importance of
supernova in various galactic processes. I'll reproduce a section from the
'Astronomy Notes' site below summarising the two theories. Spiral Arms | |