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Astronomy

Page history last edited by PBworks 17 years, 3 months ago


 

Astronomy: the Study of Stars

How We Know Some of What We Do

 

Our galaxy, the Milky Way, is over 100,000 light years across. Within that mass, it contains something like 100,000,000 stars (Carruth 303). Our sun, often called Sol (its Latin name) when reckoned with all the other stars, is just but one of the seemingly endless multitudes. Humans have been looking at things for as long as we have existed, and scientists have implored many techniques for understanding the heavens and stars above. These methods range from mathematical calculations based upon empirical evidence, to direct observations, and the use of microscope to understand the macroscopic. Astronomy calls upon all of physics and mathematics in its understanding of stars.

 

The universe is generally believed to have been created in a shocking cosmic event know as the “Big Bang,” approximately 17,000,000 years ago (Birriel 8). The universe was compressed into a perfectly small volume with infinitely large mass, which exploded and expanded rapidly. In the heat and pressures of the small and expanding galaxies, gravity began to collection patches of gas, which were draw together and eventually grew so massive as to begin fusion, a molecular process than generates energy and heat (Kruesi 24). Within just a few billions of years, whole gigantic collections of stars (galaxies, like the Milky Way) had form (Kruesi 24).

 

|Stars grow and age like any other entity. Stars are formed out of giant clouds of dust and gases (a nebula), as the molecules are pulled together by gravity they form a more solid mass (Stellar). Hydrogen is pulled faster and faster towards the center of gravity, begins to glow, and finally the pressures and heat gets so much that nuclear fusion begins (Carruth 304). The pressure for the star to expand outward (caused by the energy being produced in the core) is held back by gravity and equilibrium is eventually established (Carruth 305).

 

Stars in their middle life are often understood with something called an H-K Diagram, which relates the relationship of their luminescence to that of their temperature. Luminescence is affected by “The Inverse Square Law,” which comes into play with a lot of things that involve the dispersion of energy over a distance (Inverse). It states that some quantity is inversely proportional to the square of the distance between objects. The same is true between stars and the Earth over massive interstellar distances (Inverse).

 

These two quantities, luminescence and temperature, were first graphed together often with luminescence holding the y-axis and temperature holding the x-axis of a Cartesian graph by the astronomers Ejnar Hertzsprung and Henry Norris Russell (Hertzsprung-Russel). This is known as a Hertzsprung-Russell diagram or usually more simply the H-K Diagram. It is found when graphing these two quantities together that a very direct an important relationship exists:

 

Hertzsprung-Russel Diagram

 

 

This established the simple principle that the surface temperature of a star is related directly to the luminosity and vice versa (Hertzsprung-Russel). There is simply a physical relationship between the two. This is also important in stellar evolution, for if we assume a random scattering of the ages of stars, we find that most of their existence is spent in the middle stages, with a quick youth and old age as either hot dwarfs or quickly cooling giants. It is also true that along the main sequence is not even, more starts remain dim and cool throughout their life cycles (Hertzsprung-Russel). The large a star is, the higher the mass and therefore the forces, which burn the hydrogen, are greater, and they burn hotter (left on the diagram) and die faster. Smaller stars burn more slowly and less brightly, like the sun (Carruth 306).

 

Stars get their energy from fusion, but it is not an easy thing to do, and this is why the intense heat, pressures, gravitational forces, and mass of starts are required to start it (Nuclear). Nuclei opposite each other through their positive charge (electrostatic force) yet are still held together by the even more powerful strong nuclear force. Three types of nuclear fusion exist, with younger starts doing something called “proton-proton fusion,” older stars doing “helium fusion,” and dying stars practicing something called the “carbon cycle” (Nuclear).

 

Proton-proton is the process of two Hydrogen atoms combining to form a Helium alpha particle (two protons and two neutrons) with the release of energy as two electrons now range free and energy is re-arranged as the two nuclei combine (Nuclear). On much larger and hotter stars, usually red giants or super-giants a process called the helium triple alpha process replaces the use of Hydrogen as fuel as it has been used up. Helium alpha particles first merge to form Beryllium-8 (four protons and four neutrons) then into Carbon-12 (six protons and six neutrons) (Stellar).

 

The carbon cycle is a much longer and more complicated process than either of the above methods and is only seen in the most massive and hottest of stars (Nuclear). The process itself involved the fusion of Carbon-12 with various protons to form isotopes of other elements like Nitrogen-13, Oxygen-14, and all the way up to Iron (Fe). Anything heavier needs the energy of a supernova to produce it, from Cobalt (Co) onwards (Triple).

 

Once a star begins the fusion process they undergo a predicted lifecycle. Some stars never reach the masses required to begin fusion, and collapse and become brown dwarfs they come brown dwarfs (Stellar). Some stars are so small that the fusion process never gets far, and they to an extent evaporate once they have exhausted their fuel (Stellar). Most stars are however large enough to continue fusion processes until a time that hydrogen is exhausted, as they then cool and expand as gravity is overcome (Carruth 306). Some expand insofar as to become giants, and smaller starts eventually cool and fallback to the status of a dim star called a white dwarf (this is the fate that shall eventually befall the star near the Earth, given what is know about its mass) (Stellar).

 

Some stars, are however so massive as to form super-giants and eventually explode in giant releases of energy as they become unstable as heavier elements more capable of absorbing the heat is formed (Stellar). They expand and contract, but eventually explode in a titanic release of energy. So great is the leftover of gas and molecules that the remaining protons (positive) and electrons (negative) to form neutrons (neutral) in a small but incredibly heavy body called a neutron star (Carruth 306). Some of these spin and radiate so much as to line up with the Earth, and are called pulsars (Stellar). Some are so massive that they crush themselves and create a hole in space/time, known commonly as a black hole (Hanlon 54). The whole of a star’s lifecycle can be followed up and along an H-K diagram, making it a useful tool in more than one way.

 

Electromagnetic waves are incredibly important when trying to draw information about stars. They allow a lot to be determined about the heavens above. They were first demonstrated and discovered in the Nineteenth Century by Heinrich Hertz (Wilbraham 343). Electromagnetic waves are a combination of transverse oscillating electric and magnetic waves (hence the dual base of the word electromagnetic with electro- and –magnetic) that travel through space (without a medium of any sort, they are perfectly capable of perpetuating themselves and that transfers energy (Electromagnetic waves).

 

They are formed by the loss of energy (and therefore transfer of energy) by the electrons of an atom. Every electron in every atom has a relaxed state that it maintains when it does not possess and special or extra energy (Bohr). When it does receive energy, the electrons become excited and move beyond this basic state. When they come back down (a process simply called relaxation), they radiate the electromagnetic waves as they lose their energy (Bohr). There is no theoretical limit to the number of levels an atom may have.

 

The basic equation for the understanding of these waves is (Stern):

 

ν = c/λ

ν = (the Greek letter Nu) Frequency of a wave.

λ = (the Greek letter Lambda) Wavelength of a wave.

c = Constant speed of light, usually defined at 3,000,000 m/s.

 

Frequency is defined as the number of time a complete wavy cycle passes a given point in a given period of time, usually measured in cycles per second, the Hertz (Stern). Wavelength is simply the physical distance that a wave occupies in a complete cycle at any instant of time (Stern). The speed of light is simply the know velocity of light in a vacuum (that being approximately 3 * 108 m/s) (Wilbraham 334).

 

The length or frequencies of these waves are usually used to classify them into some well-know groups (Wilbraham 335):

 

Length (m)Frequency (Hz)Common Name(s)
102 - 1003*106 - 3*108Radio Waves
100 - 10-23*108 - 3*1010Radar
10-2 - 10-43*1010 - 3*1012Microwaves
10-4 - 10-63*1012 - 3*1014Infrared
10-6 - 10-83*1014 - 3*1016Visible/Ultraviolet
10-8 - 10-113*1016 - 3*1019X-Rays
10-11 - 10-133*1019 - 3*1021Gamma (γ) Rays
Approximately 10-143*1022Cosmic Rays

 

The energy of these waves can be calculated using the following equation (Electromagnetic spectrum):

 

E = hν

E = Energy.

h = 6.626 * 10-34 J/s*

(*Plank’s constant, the smallest unit in which energy can exist)

ν = (the Greek letter Nu) Frequency of a wave.

 

The two can be related by substitution as the following:

E = h (c/λ)

Or it can be related with numerous other combinations.

 

The first model of the atom to incorporate the concept of electron levels and come to grips with electromagnetic radiation was the model proposed by Niels Bohr, and understanding such model and the atom is critical to understand electromagnetic waves (Wilbraham 324). The atom as we know it has seen numerous interpretations and models through the years. The basic assumption was that the atom was chemically indivisible. The Greek word atom, as named by the philosopher Democritus, means unbreakable or the smallest possible part (Gaarder 43).

 

This model shows the nucleus in the center, dense and positively charged (as proved by Rutherford in the famous gold foil experiment), with the electrons freely rotating about (Bohr):

 

Bohr Model

 

 

It is strange how much this resembles a planetary system: the microcosm and macrocosm are nearly the same in form, and are used to understand each other, a beautiful fractal pattern.

This model is technically only applicable to the Hydrogen atom but it has been expanded since and has been the fundamental basis for other work within the structure of the atom, and Bohr’s conclusions about the nature of the excitement/relaxation of electrons and electromagnetism has proved correct (it has been significantly important to the advancement of chemical research that Element 107 has recently been named Bohrium) (Bohr). But because the electrons levels are limited in number and set in substances and do not vary (they come in set amounts of energy called quanta); the frequency and wavelengths of the radiation of the various atoms can be know (Bohr). There are an infinite number of electron levels, but definite energies at each level. It is here that this can be applied to astronomy.

 

Spectroscopy takes the light being received on Earth from the various stars that we can observe, and from then they can compared the wavelengths of the light being received with that of materials found on Earth (Caussade). Measured by an optical instrument called a spectrograph (the same process can work with sound waves), the result is a visual representation called a spectrogram (Caussade). There are two types of such spectral analyses, that of emission and absorption. Emissions are the wavelengths/frequencies that are radiated by a substance when it receives energy. Absorptions are the wavelengths and frequencies that are stopped by an object (usually a translucent gas) when white light (light that contains all wavelengths) is passed through. The wavelengths emitted are the very same as those absorbed (bright lines on a emission graph would be dark lines on an absorption graph), as such:

 

Emission and Absorption

 

 

Stars are put under scrutiny and classified as such by their compositions (Caussade). Seven types are normally recognized after the work of Pickering, Fleming, and especially Canon, among others (Carruth 305):

 

Spectral ClassColorCompositionTemperature (K)
OBlueHe, little H50,000
BBlue-WhiteHe, some H20,000
AWhiteH, some He10,000
FYellow-WhiteH, some metals7,500
GYellowMetals, H5,500
KOrangeMore metal, less H4,500
MRedMostly Metals3,000

 

Of course, having examined the electromagnetic, another very prevalent force in the heavens needs to be examined, this all being the fundamental force of gravity. Gravity has the most famous history of perhaps all concepts in physics and astronomy, but planetary motion amongst the stars goes back even further. Galileo established the concepts of independent forces acting upon an object (Gaarder 203). Kepler discovered the elliptical nature of the orbits of planets and stars, defined some laws of motion, and declared the Earth just another body and planet (Gaarder 205). Copernicus laid down the laws for planetary revolution and their orbits (204), to be followed by Newton who finalized the laws of universal gravitation and therefore of heavenly bodies and their motion (Gaarder 208).

 

Newton’s Law of Universal Gravitation is stated as such (Law):

 

F = G * ((m1 * m2)/(r^2))

F = Gravitational forces between the two objects.

M1 = Mass of the first object.

M2 = Mass of the second object.

R = Distance between the two objects.

G = Universal constant of gravitation, taken as a field force.

 

Technically this assumes all mass to be concentrated in one, infinitely small point, but it still functions well as an approximation of the forces involved here.

 

Knowing how far away a star is from Earth is another important concept in understand stars, and such distance is found in a process know as parallax (Carruth 303). A star is viewed from two positions (often the method used here is to observe a star then wait a few months orbits the sun to give a new position), and then simple trigonometry is used to calculate the distances. These distances are measured in simple light years, or in the unit know as the parsec, which is 3.26 light years (Carruth 303).

 

But since planetary distances are so large and movement between them is so fast, the Doppler effect is often on display on a grand display (Adrian). It is often observed with sound waves and moving objects. The two types of influences the Doppler effect on stars are the red shift and the blue shift (Adrian).

 

Doppler Effect

 

 

The Doppler effect in the form of either a red shift or a blue shift is actually quite useful, because it allows astronomers to calculate precisely how fast stars and other astronomical objects are actually going towards or away, relative to the Earth (Adrian). If, for example, a spectral line wavelength was know to be at 20cm for Hydrogen and it was observed at 20.1cm, which means it has experienced a red shift (red because it would be lengthening the waves, as red light is “longer” than blue light. While a simple shift of 20cm as such could suggest that another element besides Hydrogen is present, it can be grafted when an entire spectrogram is analysis how far the shift has truly occurred.

 

With the knowledge of gravitation, distances (through spectrographs), and of red and blue shifts, a decent sense of the geography of the Milky Way (a literal map) can be drawn to decent accuracy. With what is know of nuclear processes and of observations of stars and the cosmos, and with tools to assist in visualizing (like the H-K diagram), quite a bit can be deduced about stars and their “lives.” We have only scratched the surface and have a long way to go, but science is not totally naïve to the twinkling stars in the night sky above. And neither is humanity, to their true nature, anymore.

 

Works Cited

 

Adrian, Eleni. “The Doppler Effect.” Whispers From the Cosmos. 1995. University of Illinois. 18 Apr 2005. <http://archive.ncsa.uiuc.edu/Cyberia/Bima/dopplar.html>

Birriel, Jennifer. “The Milky Way’s Age.” Mercury Jan./Feb. 2005.: p8.

Carruth, Gordon & Eugene Ehrlich. Student Handbook: Volume One. New York: Macmillian Publishing Company, 1989.

Caussade, Armando. “Spectroscopy.” 2004. Astronomy. 19 Apr 2005. <http://www.armandaocaussade.com/astronomy/spectroscopy.html>

“Electromagnetic spectrum.” Wikipedia. 2005. Wikipedia: the free encyclopedia. 19 Apr 2005. <http://en.wikipedia.org/wiki/Electromagnetic_spectrum>

“Electromagnetic Waves.” Hyperphysics. Georgia State University. 18 Apr 2005. <http://hyperphysics.phy-astr.gsu.edu/hbase/emwav.html#c1>

Gaarder, Jostein. Sophie’s World. New York: Berkley Books, 1991.

Hanlon, Michael. “Life, the Universe, and True Science.” Daily Mail 13 May 2005: p54.

“Hertzsprung-Russel Diagram.” The Electronic Universe. 1995. University of Oregon. 18 Apr 2005. <http://www.zebu.uoregon.edu/~imamura/208/jan23/hr.html>

“Inverse-square law.” Wikipedia. 2005. Wikipedia: the free encyclopedia. 19 Apr 2005. <http://en.wikipedia.org/wiki/Inverse-square_law>

Kruesi, Liz. “Early universe a ‘zoo’ of galaxies.” Astronomy Jun. 2005: 24

“Law of universal gravitation.” Wikipedia. 2005. Wikipedia: the free encyclopedia. 19 Apr 2005. <http://en.wikipedia.org/wiki/Law_of_universal_gravitation>

“Nuclear fusion.” Wikipedia. 2005. Wikipedia: the free encyclopedia. 19 Apr 2005. <http://en.wikipedia.org/wiki/Nuclear_fusion>

“Stellar evolution.” Wikipedia. 2005. Wikipedia: the free encyclopedia. 19 May 2005. <http://en.wikipedia.org/wiki/Stellar_evolution>

Stern, David P. “Waves and Photons.” From Stargazers to Starships. 2004. phy6.org. 18 Apr 2005. <http://www.phy6.org/stargaze/Lsun5wav.htm>

“The Bohr Model.” Online Journey Through Astronomy. 19 Apr 2005. <http://csep10.phys.utk.edu/astr162/lect/light/bohr.html>

“Triple Alpha Process.” Hyperphysics. Georgia State University. 19 May 2005. <http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html>

Wilbraham, Anthony. Chemistry. Menlo Park: Addison-Wesley Publishing Company, 1995.

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