INTRODUCTION The late biologist J.B.S.Haldane once wrote: “The universe is not only queerer than we suppose, but queerer than we can suppose.”The universe contains billions of stars which are oraganised into galaxies which in turn form clusters and superclusters that are separated by immense voids.Voids are the empty spaces which contains very few or no galaxies. One of the queerest things about the universe is that virtually all the galaxies in it (with the exception of a few nearby ones) are moving away from the Milky Way. This curious fact was first discovered in the early 20th century by astronomer V esto Slipher, who noted that absorption lines in the spectra of most spiral galaxies had longer wavelengths(were “redder”) than those observed from stationary objects. Assuming that the redshift was caused by the Doppler shift, Slipher concluded that the red-shifted galaxies were all moving away from us.
Drawing a map of the universe is not easy.In order to make a 3 dimensional map of the positions of the galaxies,two celestial coordiantes,right ascension and declination that tells us the location of the galaxy on the celestial sphere are required and also the distance of the galaxy.In order to determine the distance of a galaxy,the expansion of the universe is made use of. In the 1920’s, Edwin Hubble measured the distances of the galaxies for the first time.He measured the speeds(radial velocities) by measuring the wavelengths of absorption lines in their spectra.When he plotted these distances against the velocities for each galaxy he noted something even queerer:the further a galaxy was from the milky way,due to the Doppler effect,the wavelengths of absorption lines are longer(shifted towards the red end of the spectrum), the faster it was moving away from the observer.By taking the spectrum of a galaxy and measuring the amount of spectral lines that are redshifted,it can be used as a measure of distance in order to complete a 3 dimensional map of the universe.
In this lab,the CLEA program was used in order to take spectra of galaxies,measure the wavelengths of the spectral lines in galaxies and then calculate the redshift,z,and the radial velocities of the galaxies.
The software for the CLEA Hubble Redshift Distance Relation laboratory exercise puts you in control of a large optical telescope equipped with a TV camera and an electronic spectrometer.The celestial postions i.e the right ascension and declination along with the radial velocities were used to obtain a wedge diagram which gives the structure of the universe.
DISCUSSION In astronomy, the Doppler effect was originally studied in the visible part of the electromagnetic spectrum. Today, the Doppler shift, as it is also known, applies to electromagnetic waves in all portions of the spectrum. Also, because of the inverse relationship between frequency and wavelength, we can describe the Doppler shift in terms of wavelength. Moving objects that emits radiation exibits the Doppler effect. The radiation emitted by an object moving toward an observer is squeezed; its frequency appears to increase and is therefore said to be blueshifted. In contrast, the radiation emitted by an object moving away is stretched or redshifted.In terms of wavelength,the further radiation has to travel to reach an observer,the longer its wavelength becomes and is shifted to the red end of the spectrum or is redshifted.And when the wavelength decreases,it is blueshifted. Astronomers use Doppler shifts to calculate precisely how fast stars and other astronomical objects move toward or away from Earth.When wavelengths are measured from spectral lines,the measured wavelengths seems to differ from from that normally found on earth.In the lab,it was seen that all the measured wavelengths for the H and K lines were different from the laboratory wavelengths.The measured ones were always greater.This was due to the fact that the galaxies were moving away from earth and the wavelengths being stretched and redshifted.
Anything that emits wavelengths - light, radio, gamma rays, and the rest of the E-M Band - and changes frequency due to movement, a Doppler shift can be measured. This is the equation:
The radial velocities of the galaxies of each redshift were calculated using this Doppler shift formula.
In mapping the structure of the universe 3 dimensions are used i.e. the celestial coordinates,the right ascension and declination and the distance of the galaxy from earth.This distance is found using hubble redshift-distance relation which states that the distance of a galaxy is proportional to its redshift or radial velocity.By simply taking the spectrum of a galaxy and measuring the amount of spectral lines that are redshifted the distance can be determined. The law is often expressed by the equation v = H0D, with H0 the constant of proportionality (the Hubble constant) and D is the proper distance to a galaxy which can change over time.v is the radial velocity.
To see the large scale structure of the universe a wedge diagram is plotted.In this lab the wedge diagram was plotted using 2 dimensions,the right ascension and the velocity(which is proportional to distance according to hubble redshift distance relation).A coordinate system is important for the wedge plot because it maps the positions of the galaxies in the celestial sphere and the distance can be determined using the coordinate system by appling the equation for hubble redshift-distance relation.Table 1 gives theoretical values of radial velocities for various galaxies including the galaxies that were used in this lab.From the results,it was seen that the radial velocities didn’t vary much from the
theoretical values in the table below.For eg. The galaxy NGC 4874 from the table of results the radial velocity was calculated to be 7193.152.This wasn’t too far from the actual value of 7148 found in table 1 of theoretical values.The galaxy NGC 4889 radial velocity was calculated to be 6472.018 which was close to the theoretical value of 6456.These two galaxies dominates the coma cluster.
From the wedge plot,the most densely populated region of the diagram Is the core of the coma cluster of galaxies.The Coma cluster is one of the most famous clusters of galaxies. It has received a huge amount of scientific research. This is partly because it is a very rich cluster containing thousands of galaxies. Another reason is that the Coma cluster lies a long way from the plane of our Galaxy and it is largely unobscured by any gas and dust or any foreground stars.The Coma cluster is dominated by two enormous elliptical galaxies. These are NGC4874 and NGC4889.Beyond the coma cluster there is a loose band of galaxies stretching east to west known as the great wall. The Great Wall is the largest known structure in the universe at present, having superceded sundry superclusters and clusters of superclusters. The Wall is a "thin" (15 million-light-year) sheet of galaxies 500 million light years long by 200 wide; and it may extend even farther.It includes clusters such as Hercules,coma and the leo superclusters.
Spectroscopy is used when surveying galaxies because of the interaction of mater with radiation.When radiation reaches the atmosphere,photons interact with the atmosphere and are detected as absortion lines or emission lines.Telescopes are used to survey galaxies and are the detectors.Different types of telescopes used are optical,infra-red astronomy,radio astronomy and high radiation astronomy.
ABSTRACT In this lab,observations of the redshifts of the galaxies along with their coordinates in the sky were used to produce a 3-dimensional map of a nearby region of the sky.Spectra of these galaxies were taken using simulated telescopes and spectrometers from the CLEA program and the wavelengths of the K and H spectral lines of the galaxies were measured .The redshift,z, and radial velocities of the galaxies were then calculated.These radial velocities along with the right ascension positions of the galaxies were used to produce a wedge diagram and questions based on this diagram answered as shown in the analysis.
THEORY Equations used for calculating the absolute redshift,ΔλK and ΔλH for each line are: ΔλK = λK measured – λK laboratory and ΔλH = λH measured – λH laboratory
Equations used for calculating fractional redshifts, zK and zH are: zK = ΔλK/λK laboratory and zH = ΔλH/ λH laboratory
Equations used for calculating the radial velocity of the galaxy for each redshift are: VK =C ZK and VH = C ZH
Average of the 2 velocities: Vavg = ( VH + VK )/2 Hubble’s redshift-distance relation law: V= H D where V-radial vel, H- hubble’s constant, D-distance of the galaxy
