Term
| A “red dwarf” is a “former star” whose fusion process has stopped and has cooled to the point where it glows red. Which type of telescope (radio, IR, UV, X-ray, or gamma-ray) would you recommend using in addition to a visible light telescope to study such an object and why? |
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Definition
| Since its energy has shifted to the low end of the visible spectrum it may be emitting energy in the IR portion of the spectrum, the next step down in term so frequency and energy. |
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Term
| Why did we create the Spitzer Infrared Observatory as an orbiting telescope? It seems like it’s pretty expensive and difficult to maintain an instrument when it’s in orbit . . . IN OTHER WORDS, WHY DID WE PUT THIS THING INTO ORBIT? |
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Definition
| It orbits because our atmosphere blocks out most of the IR from space. It also blocks out everything else except for visible and radio frequencies. |
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Term
| What happens to a star at the end of its life if it is below the Chandrasekhar limit? Assume that this star is isolated in space, i.e. it has no neighbors. |
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Definition
| A star with a mass below the Ch. Limit (1.4 times the mass of our sun) will fail to ignite secondary rounds of fusion. Instead, it will simply, swell up as a red giant, its core will stop fusing, its atmosphere will escape to form a planetary nebula and the core itself will become a white dwarf: hot at the beginning but cooling off over time to create, eventually red dwarf and black dwarf stages. |
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Term
| What is a “standard candle?” |
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Definition
| If such things are found to exist, then we can compare their apparent magnitudes with their supposed absolute magnitudes to calculate distances accurately. |
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Term
| Which type of Supernovae explosion can be used in this way? What property allows them to be used in this manner? |
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Definition
| The elements above Iron can only be created in a supernova explosion. That’s the only place where enough energy can be released to forge, through fusion, the elements above Iron. Since most stars are low-mass stars and low-mass stars do not “go” supernova, this is a rare event and the elements themselves are, indeed, rare. |
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Term
| What kind of a graph do you suppose you’d get if you graphed several thousand stars’ color indices (x-axis) vs. their inherent brightnesses (y-axis)? |
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Definition
| Since color index is related to temperature and inherent brightness is related to luminosity or absolute magnitude, you would have a Hertzsprung-Russell diagram. |
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Term
| What is the term for the curvy line on a Hertzsprung-Russell (hereafter H-R) diagram where stars spend the majority of their lives? What two properties of a star are graphed versus one another on an H-R diagram? |
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Definition
| The curvy line is called the “main sequence.” An H-R diagram maps stars’ absolute magnitudes (or “luminosities”) on the y-axis and their spectral class (or temperature) on the x-axis. |
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Term
| On an H-R diagram, where would one find a white dwarf star? A red dwarf? A red giant? A blue super-giant? A sketch of your answers would be appropriate. |
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Definition
A White Dwarf would be low luminosity and very hot: look in the lower left-hand corner.
A Red Dwarf would be low luminosity and very cool: look in lower right-hand corner.
A Red Giant is high luminosity but very cool: upper right hand corner.
Blue Super Giants have high luminosity and high temperature: upper left-hand corner |
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Term
| The same basic mechanism can form stars and the planets around stars. What is that mechanism and what is the essential difference between the formation of a star and the formation of a planet? |
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Definition
| While the planets have enough mass to collapse inward and form a stable sphere, stars are massive enough to ignite nuclear fusion of H into He. Planets do not do this due to lack of sufficient mass. |
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Term
| The Spitzer Space Telescope orbits the Earth like the Hubble Space Telescope. It is different from the Hubble in a very particular way . . . What is it about this telescope that makes it ideal for searching for “proto-stars” (objects on the verge of igniting their first round of nuclear fusion)? |
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Definition
| Since the Spitzer Telescope is an Infrared telescope, it can see objects that are hot but not hot enough to begin glowing in the red part of the spectrum. This is the exact state of proto-stars, so the Spitzer is a way of seeing those stars before they ignite their first round of fusion. |
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Term
| What specifically prevents a small mass of Hydrogen from becoming a star? In other words, why can’t just any cloud of Hydrogen gas become a star? |
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Definition
| Since the single protons that make up a Hydrogen nucleus are all positively charged, there is an electrical repulsion that can prevent fusion. Only in ‘gatherings’ of large enough numbers of protons can the gravitational forces overwhelm the electrical forces to begin the fusion process. See question #21 for a similar response. |
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Term
| The fusion of helium results in the creation of some new elements. Describe how the elements of the periodic table – up to Iron (Fe) – are created in the nuclear reactions that occur in the cores of high-mass stars. |
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Definition
| As successive rounds of fusion occur, the even-numbered elements of the periodic table are created by the addition of left-over He nuclei (2 protons) from previous stages. The odd-numbered elements are created in secondary (and rarer) collisions, adding or subtract single protons. |
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Term
| As a follow-up to the previous question, what happens to the outer “atmosphere” of the star after the second stage of fusion begins? What color does it become and why? |
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Definition
| Now, once the core is heated up again, the atmosphere will be pushed much further outward than it was in stage one. The atmosphere swells and cools, becoming red as it does so . . . this kind of star is hence known as a “reds giant.” |
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Term
| Most stars will go through a second stage of fusion after all the Hydrogen has been exhausted. What happens to the core of a star in between stage one (H into He) and stage two? How does the core temperature compare between stage one and stage two? Why is this so? |
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Definition
The Helium nucleus always has 2 protons and, usually, has 2 neutrons though the neutron number can vary. Because of the extra protons, the electrical repulsion is much larger (4x larger) than with two Hydrogen atoms. That means that the fusion of Helium nuclei can only occur at much higher temperatures.
Between the two stages, fusion briefly stops, the core cools and can contract. As it does, it heats up until the density of the core is sufficient to ignite the second round of fusion at higher temperatures. |
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Term
| Why does a star maintain a roughly constant size and brightness throughout its life? Describe what would happen if this process were “off” by just a bit? |
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Definition
| The thermal pressure (heat!) from the core will push the atmosphere outward, while gravity will pull it inward. Usually, these two forces will balance one another and the star will maintain a specific size. If it got too hot in the core, the thermal pressure would increase, the atmosphere would push outward and the star would cool in response. At that point, the thermal pressure subsides and gravity can pull the star back in . . . this may be what happens with some variable stars, including the famous Cepheids! |
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Term
| Stars will fuse H into He for about 90% of their lives. Do all stars live the same amount of time and, if not, what are the ranges of times for a star to exist? |
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Definition
| Somewhat counter-intuitively, a more massive star will live a shorter life than a low-mass star. Typically, a low-mass star will live about 10 billion years. A more massive star might only live for about 10 million years (a thousand times shorter life). |
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Term
| Fusion occurs when small atoms (like Hydrogen) crash into one another with enough energy to “fuse” into a larger atom (like Helium). What force tends to oppose the fusion process and why does it only happen in the core of a star? |
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Definition
| The nuclei are, in the case of Hydrogen, simply protons which are positively charged. Because they are “like” charges there is a natural tendency for them to push away from one another due to ELECTRICAL FORCES. This can be overcome in the core of a star because the GRAVITATIONAL FORCES are so great. |
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Term
| How do Hubble’s data and the discovery of the “cosmic microwave background radiation” support the idea of a “Big Bang” beginning of our Universe? |
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Definition
The Big Bang theory proposed that the Universe has been expanding since a “primeval atom” exploded forth. In addition, the Universe should be cooling and, it was predicted, that its temperature should be about 3 K.
Hubble’s discovery of the expanding Universe supported the first idea. In 1964, when Penzias & Wilson discovered the “cosmic microwave background” of about 2.7 K, the universe’s temperature was recorded for the first time. The excellent agreement between theory and observation provides substantial support for the Big bang theory. |
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Term
| Describe what happens to the volume and temperature of the Universe over time. |
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Definition
| According to the BB, the Universe was “born” as a cosmic fireball, expanding from something the size of an atom (possibly smaller) into the large-scale structure we see today. In the first few minutes, the volume of the Universe increased dramatically and, though still very hot by today’s standards, it cooled from unimaginably high temperatures to lower temps. |
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Term
| Describe the interactions of particles in those first few minutes, focusing particularly on the ability of those particles to bond. |
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Definition
| Because of the limited space and the large quantity of matter, the Universe was a chaotic place, with nearly constant collisions between particles. Because of the violent nature of these collisions, only the most elementary particles could exist; structures even as simple as Hydrogen atoms (a single proton with a single electron held in electrical orbit) could not exist. The Universe would have to wait thousands of years before it became calm enough to allow chemical bonds to have any kind of permanence. |
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