Spiral Galaxies

What is a spiral galaxy?

     Spiral galaxies are the most common types of galaxies that exist in the universe. These galaxies are in fact so common that they make up 77% of the observed galaxies in our universe. Our own Milky Way galaxy is also a spiral galaxy.

     Spiral galaxies are constantly rotating. These galaxies have a spiral shape, and seem to look denser towards the middle, and have a bright center. When looked at from the side, spiral galaxies can look more like a circle. The shape always stays the same, but the view, of course is different depending on how you can see it. If the spiral shape can be seen clearly, then it is called a “face-on spiral”. If you are looking at a spiral galaxy from the side, then it is called an “edge-on spiral”.

            Spiral galaxies are further classified by how tightly their arms are winded around each other. If the arms of the spiral are wounded very tightly, they are classified as “Type A”. “Type B” spiral galaxies have arms that are more loosely wound together and so on.

Composition

     The arms of a spiral galaxy are composed mostly of gas and dust. These areas are also often areas where new stars are forming. The bright bulge in the middle of the spiral galaxy is made up mostly of old, red stars. Few stars form in the middle, as they are mostly formed on the actual arms of the galaxy.

            To go into more detail on the composition of the arms of spiral galaxies, the stars that are being formed there are hot young blue and blue-white stars which make the arms very visible. The nucleus of spiral galaxies (the bright bulge in the middle) is so bright because it’s composed of old stars that are redder in color. In the dead center of the galaxy (even more central than the old stars), there is a giant black hole which has a ridiculous mass. This mass is millions of times the mass of our Sun.

            Along with the spiral arms and the central bulge of spiral galaxies, there is also a halo. The halo is the home of a very few scattered stars and globular clusters. It is also filled with dark matter.

     Spiral galaxies are absolutely massive in composition. The mass of these galaxies ranges from 109 – 1012 solar masses. The absolute magnitude (luminosity) of spiral galaxies ranges from -16 to -23, and the typical diameter of spiral galaxies ranges from 5-100 kpc (1 kpc = 3.08568025 × 1019 meters).

 Interstellar medium takes up about 3-5 percent of the mass of a spiral galaxy. This percentage is increased to 20% in the actual arms of the galaxy.

 When spiral galaxies interact with other galaxies through gravitational pulls, they can successfully steal halo stars from other galaxies to join their own. These halo stars are vast and contain many globular star clusters.

How do spiral galaxies form?

There is now evidence to suggest that spiral galaxies are formed from close gravitational interactions with neighbouring galaxies. These interactions create waves of high mass and condensed gas which will orbit the center of the galaxy. These waves of high mass compress existing gas which causes star formation.

 Over time, when spiral galaxies become more mature, they form stars and gas that slice through the nucleus like the slash across a ‘no-smoking’ sign. These spiral galaxies are known as barred spiral galaxies. Through research from NASA’s Hubble telescope, astronomers found that barred spiral galaxies were not as common seven billion years ago. To further elaborate, only 20 percent of the spiral galaxies of the distant past had bars. And now, 70 percent of spiral galaxies are barred spiral galaxies. Bars are very important in the evolution of spiral galaxies. When bars are formed, they force a large amount of gas towards the center of the galaxy, fuelling new star formation, building central bulges of stars, and feeding massive black holes.

How long do spiral galaxies last?

     The arms of spiral galaxies typically only last about a half billion years. Spiral galaxies have ‘winding problems’. That means that while the arms of the spirals orbit the center bulge of the galaxy, they get very tightly wound and get drawn into the center of the galaxy. This takes a few rotations until the arms are eventually no longer visible.

 For the actual galaxies themselves, it is possible that the galaxy will run out of gas to form new stars. If this happens, the activity in the galaxy may have decreased, but the galaxy will probably still exist unless it is eventually sucked into a black hole. One current popular theory is that spiral galaxies will eventually turn into elliptical galaxies.

Spiral galaxies can also be sucked into larger galaxies from their gravitational pull. This will be discussed more into detail further into this research.

What type of radiation comes from spiral galaxies?

Spiral galaxies give of far-infrared radiation. Far-infrared radiation (FIR) is Infrared electromagnetic radiation of very long wavelengths, in the range of 2-50 microns. FIR is radiated from star formation, and can trace the formation of stars as well.

 The center of spiral galaxies produces ultraviolet radiation. The warm dust in the nucleus and spiral arms of a galaxy radiates in the infrared portion of the spectrum. X-ray emissions are seen from globular clusters and hot gas regions of spiral galaxies.

Would it be dangerous to life forms living nearby?

I haven’t found any evidence to suggest it would be dangerous to live near a spiral galaxy. After all, we are currently living in a spiral galaxy. However, if a spiral galaxy is extremely large, its gravity can distort smaller nearby galaxies as is the case with the immense Pinwheel galaxy (M101).

 When a massive spiral galaxy interacts with a smaller one, it can start flinging out stars and gas. It’s a long process, but after billions of years, the two galaxies will merge into a galaxy that is even more massive than its original state. The two galaxies don’t have to necessarily bump into each other to merge. As long as the gravitational pull is enough, then they can be attracted to each other from as far as 100,000 light years away.

Are spiral galaxies beautiful?

            There is absolutely nothing in space that isn’t pleasing to the eye. Although what these galaxies look like from up close is very much different than the view we have from Earth, they are breathtaking to say the least. Spiral galaxies are for the most part blue in color because of the surrounding hot young blue and blue-white stars. Add that to the spiral rings and red bright interior, these colourful spiral galaxies are indeed absolutely beautiful.

To add to this research. I would like to point out that the Earth is as a molecule, as the universe is as the Sun. There are as many stars in our own galaxy alone as there are as many grains of sand on the Earth. There are hundreds of billions of galaxies in the universe that we know of. You do the math.

We are not alone.

References:

http://skyserver.sdss.org/dr1/en/proj/basic/galaxies/spirals.asp

http://csep10.phys.utk.edu/astr162/lect/galaxies/spiral.html

http://www.noao.edu/image_gallery/spiral_galaxies.html

http://apod.nasa.gov/apod/spiral_galaxies.html

http://www.universetoday.com/guide-to-space/galaxies/spiral-galaxy/

http://www.enchantedlearning.com/subjects/astronomy/stars/galaxy/spiral.shtml

http://www.astronomy.com/asy/default.aspx?c=a&id=7245

http://hubblesite.org/newscenter/archive/releases/1999/41

http://www.windows.ucar.edu/tour/link=/the_universe/Spirals.html

http://www.iop.org/EJ/article/1538-3881/124/5/2548/201357.text.html