The solar system is placed in a spiral arm, considered the most hospitable region in the galaxy. The spiral arms are where the active stars form and they orbit around the Milky Way galactic center and the solar system joins this orbital trip. According to the latest discoveries, the Milky Way galaxy has only two spiral arms (Scutum – Centaurus and Perseus) as opposed of four previously considered and the solar system resides on one of these arms, at approximately 28.000 light – years distance from the Milky Way’s galactic center (Nigro 39).
- What evidence indicates that a supermassive black hole is located at the center of our Galaxy?
Supermassive black holes are former supernovas that suffered a catastrophic explosion and collapsed into either a white dwarf star (if its mass was 1.4 times the mass of the Sun), into a neutron star (if the star’s mass is compressed to a radius of around 10 km) or a black hole (if the star’s mass is three times greater than the solar mass) (Serway & Jewett 349). The black holes emit x-rays and this constitutes evidence to support the existence of black holes at the center of our galaxy. The dynamics method is used for supporting the evidence of the existence of the supermassive black holes at the center of our Galaxy; as such, the motion of water masses around the center of the galaxy or the motion of stars around the dormant black hole of 3-4 million solar masses placed at the center of the Galaxy were found to sustain the existence of the black hole at the Galactic center, which tight up with the host galaxy because of their masses (Matt 311). Serway and Jewett also consider as evidence for the existence of the supermassive black holes at the center of our galaxy the jets of materials that were identified by Hubble Space Telescope along the axis of the black hole, believed to be drown by the supermassive black hole (350).
- Spiral density waves are directly responsible for which of the following? a. flocculent spirals, b. grand design spirals, c. supernovae, d. the collisions between galaxies, e. galactic cannibalism
The spiral density waves are formed of spiral arms that move slowly around the galaxy, which along their rotation meet cloud gas that catches up and overtakes the spiral arms because they have a slow pace of moving, slamming in the gas that it is already present in the arms, forming new stars as a result of gas compression (Seeds & Backman 263). The spiral density waves are responsible for creating the grand design spirals, wherein stars move along the spiral arms, describing a mutual gravitation that determines orbits to align. When the gas clouds collide they generate molecular clouds, as the gas of the clouds coming from behind the spiral arm interacts with the gas from within the spiral arm, forming new stars. This evolves towards the grand designed, bisymmetric spirals (Francis & Anderson 3425). Other theory suggests that as each star has a gravitational influence, their move along a precessing ellipse will determine their neighbors’ motion, causing a wavelike disturbance that propagates through the disk and if the major axis of each precessing ellipse is directed at a certain angle in relation to its neighbors, the orbits bunch at a specific location, forming the grand design spiral pattern (Nicolson 208).
- Some galaxies in the Local Group exhibit blueshifted spectral lines. Why are these blueshifts not violations of the Hubble law?
Blueshifted spectral lines identify the cosmic objects that are approaching the observer, possessing a shorter wavelength. The color blue defines precisely the direction towards which the cosmic object is moving, is not an actual color that they carry, and each of its spectral line is shifted towards shorter wavelengths, slowly approaching Earth (Seeds & Backman 113). The Doppler effect is responsible for the velocity of the stars, determining the color of the spectral lines: if the wavelength shift’s velocity is directed away from the observer the spectral lines are redshifted and on the contrary, if it is directed towards the observer and the wavelength is shorter the spectral lines are blueshifted. Hubble law describes the recession velocities (or the redshifts) as increasing linearly proportionally with their distance from the observer, and it uses the following theory for defining this relation: v – Hd (v standing for recession velocity; H – being the Hubble constant and d – being the distance). Hubble has predicted the fact that the galaxies might be moving away or towards the observer (having a random dynamic in the universe), and based on scientific research there was identified that galaxies that are far away are moving at a higher speed that the ones closer to the observer and this explains the expansion of the universe (Struck 9). The random motion of the cosmic bodies indicate that the blueshifted spectral lines are not violation of the Hubble law.
- Suppose you suspected a certain object in the sky to be a quasar. What sort of observations would you perform to confirm your hypothesis?
The quasar is a cosmic object that carries a star like image and that emits high amounts of energy, defined as quasi – stellar radio sources differentiating from the other celestial objects through their extreme luminosity, which allow them to be visible from far away, being among the most distant cosmic objects that can be observed in the universe (Gieseke 1). The quasars are identifiable from distance through the radio sources, although they might not possess an image. However, not all the quasars are detectable through radio sources, as only 10% of them were identified to be “radio loud” (“Quasars. What They Are”). However, they can be observed also because of their high luminosity, which is the result of many materials and objects being drown towards the center of a galaxy, which contains a black hole, absorbing dust, gas, stars, which form together the accretion disc. The accretion disc material produces intense light, emitted from a great distance, becoming so high that it outshines the host galaxy and when this is happening one can tell that a quasar is visible in the sky. Another property of the quasars is that they are variable, as changes can be identified in flux and in their motion and it is considered that this variation is another technique for identifying them, but the variations were found to only appear in the least luminous quasars, because the quasars that emit high luminosity at high redshift are least variable (Kembhavi & Narlikar 135).
- Explain the difference between a Doppler redshift and a cosmological redshift
Doppler shift concept indicates that the spectra (spectral features of distant stars and galaxies such as atoms in the gases that surrounded them) move into the red end of the spectrum, which denotes that stars and galaxies are moving away from the observer and the more distant the galaxies are, the faster they move away from the observer (“Red Shift”). The Doppler redshift stretches the frequency of light as the stars and galaxies are entrenched in a relative motion (Dahneke 185). Therefore, the relative motion of celestial bodies and the light frequency determine the Doppler redshift. The cosmological redshift, on the other hand, occurs as a result of the cosmic expansion, and it predicts a relative redshift increase for receding objects, stretching the frequency of bodies and implicitly of the radiations that they carry, as the space expands (Dahneke 185). Therefore, the expansion of space that causes the motion of the bodies (the closest bodies to the observer move slower, while the more distant bodies move with a higher speed) determines the cosmological redshift, while the Doppler redshift is predicted by the relative motion of bodies in relation with one another, through space (Lambourne 265). In the case of the cosmological redshift the speed of light reaches a redshifted wavelength (of about 1.5), representing 150 percent the original. In the case of the Doppler shift, the signal from the receding source is reduced in frequency or redshifted, while the wavelength is increased, determining a visibility limit or event horizon (Dahneke 164 - 165).
- Assuming that the universe will expand forever, what will eventually become of the microwave background radiation?
The microwave background radiation, discovered by the scientists Arno Penzas and Robert Wilson in 1964, is the glow that shines between the stars and galaxies, in the background space that is usually opaque and completely dark. In the early stages of the universe, it is considered that the microwave background radiation appeared as the result of the interaction between the electrons and the nuclei from atoms and their constant collision with the photons. At that time the microwave background radiation had a temperature of 3K, which decreased as the universe expanded, as it stretched the wavelength of the microwave region (Tuttle 432). If the universe will expand forever, the microwave background radiation will continue to cool, becoming less and less visible and letting the space between the stars and galaxies opaque. Nevertheless, the microwave background radiation shines blindingly, as it does not allow observers to view anything past it (Tuttle 432); therefore, should this radiation cool down with the expansion of the universe, it would allow more visibility for other stars or celeste objects to be seen.
- Which force in nature is believed to have formed second? a. gravity, b. electromagnetic force, c. weak force, d. strong force, e. all formed at the same time
Just after the Bing Bang, the first force that appeared was the gravity, followed by the strong force and then the electromagnetic separated, followed by the weak force (Prasad 84). These are the four forces, appeared just seconds after the big bang, in this order, which formed the first phase of the universe’s formation. Therefore, the second force that formed in nature is the strong force. Initially, all forces were unified into a singular grand force and after the big bang, in the planck time, they started to separate from one another, with gravity being the first to leave the unified group forces and the other following in the order specified above. The strong force is the force that acts within a nucleus of an atom, which consists of neutrons and protons, out of which the latest could explode, causing the blowing of the nucleus; however, the strong nuclear force stops this from happening (Prasad 84). The strong force formed second, because in the 10:35 seconds of its existence, the Universe was cool enough to allow the formation of the atoms, as the formation of the particles composing it (the quarks) was possible; in this phase, called the Baryogenesis (as the baryonic matter, including the quarks, formed) the protons and neutrons appeared, which formed the nucleus of an atom, held together by the strong force, as already explained (“A Brief History of the Universe”).
- Which stellar spectral type is most likely to have a planet on which advanced life exists? a. O, b. B, c. A, d. G, e. M
The stellar spectral types O and B are less likely to have planets on advanced which life forms exist, because of their huge dimensions, indicating that they will burn out before reaching three to five years for the evolution of any terrestrial planet and also because the stellar spectral types O and B possess high luminosities, which imply that they would sterilize any terrestrial planets in their surroundings. The stellar spectral type M also cannot have planets with advanced life forms, as they do not possess sufficient radiation energy for supporting the biochemical reactions required for evolution (Behrendt 510). Likewise, just as O and B spectral types, stellar spectral type A is also too quick for allowing the advanced life form to develop, therefore, the G stellar spectral type are the most likely to have planets where advanced life exists (“Stars and Habitable Planets”). G stars live very long and do not emit high UV radiation, making advanced life possible on the planets on this type of stellar spectral type (Seeds & Backman 594).
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