Astronomy Review Final
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- What are the reasons for wanting to use large telescopes?
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1) To detect faint objects (better light gathering power, proportional to square of diameter of lense)
2) Angular resolution (the smallest detail in angular size that can be seen by the telescope)
3) Magnification- not that important because, if you magnify a fuzzball it doesn't help you. - What is "seeing"?
- The fact that the earth's atmosphere has a lot of turbulence and when light passes through it it gets distorted, so no matter how good your telescope's resolving power is, it probably wont resolve better than about 1 arc second.
- How are reflecting and refracting telescopes different?
- Reflectors are made with mirrors and refractors with lenses. Most telescopes today are reflectors because it is difficult to get the lenses of a refractor perfectly smooth on both sides and to have them big enough for today's needs without them sagging.
- Why would astronomers want to put telescopes into space?
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You get above the seeing so you can get take atvantage of the telescope's full resolving power.
You can also detect wavelengths that can't make it through our atmosphere, like ultraviolet and x rays. - Why do radio astronomers use interferometers?
- Long radio waves give you very poor resolving power so you would need an incredibly large radio telescope in order to capture enough of them to get any good information. If you have an interferometer set up, however, you can position two radio telescopes far away from one another and that has the resolving power of one single radio telescope of that diameter.
- What is the proton-proton chain by which the sun generates energy?
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1H + 1H =2H + e^+ + v
2H + 1H = 3He + y
3He + 3He = 4He + 2^1H
All the preceeding numbers are superscripted except the 2 on the last line - How is the equation e=mc^2 involved in the sun's energy production?
- The mass of the helium nucleus that is the product of the proton-proton chain is less than the mass of the 4 protons that went into it. This difference in mass gets turned into energy.
- Why are high temperatures for these fusion reactions to operate?
- Because the nuclei need to get close enough for the strong nuclear force to come into play and make them fuse. If they aren't close enough, then the electric repulsion pushed them away. If they are moving mast enough, then they can get close enough for this to occur. So the higher temp. gives them the fast velocity to get close enough.
- What is hydrostatic equiliberium? How does it produce the high temperatures needed for fussion in the core?
- This is a balance between gravity and pressure. As you go deeper and deeper into the star there is more and more stuff pushing inward, so there must be bigger pressure pushing out so the star doesn't collapse. The density and the temperature in the core of the star are both high, which provides the pressure to withstand gravity.
- What is the solar thermostat? How does it keep reaction running at a steady rate in the core of the star?
- If the core's temperature were to suddenly go down, the reaction rate would go down and consequently the pressure would go down, so the gravity pushing in on the core would make the core a little smaller (it would collapse a little bit inward) making the pressure rise, the nuclear reaction rate rise, and the temperature rise.
- How is energy made in the sun's interior transported to the surface? How long does this take? Why does the predominent means of transportation change about 70% of the way out? Why does energy emerge as visible light even though it is in the form of Gamma
- Within the first 70% of the star, photons carry the energy out. It takes them 100,000s of years to do so because they keep colliding with various nuclei and atoms within the core. At 70% of the way out, the sun has started to cool down enough that there are a lot of atoms (they are no longer ionized.) The photons are absorbed by atoms, but the heat they carry still needs to get out, so in that outer 30% convection occurs (rising and falling blobs of gas). The energy emerges as visible light rather than gamma rays because it experienced so many collisions it (photons) lost energy and took on the peak wavelength of the very exterior of the sun (e.g. visible light rather than gamma rays).
- What is the granulation on the solar photosphere?
- This happens because of convection. The warm gas rising is brighter than the cool gas falling.
- How do we know that sunspots are cooler than the rest of the photosphere? What causes them to be cooler? What is the Zeeman effect and how do we use it to tell us that sunspots are areas of strong magnetic field on the sun?
- We know that they are cooler because they are darker (that's one of the properties of light: sigma T^4) We think they are cooler because the magnetic fields in the sunspots inhibit convection. The zeeman effect is that spectral lines (e.g. the energy levels) within atoms split into several levels in the presence of a magnetic field. What would be a single absorption line in the absence of a magnetic field turns out to be split into two or three or more, and how how far apart those are seperated in wavelength tells you how strong the magnetic field is.
- What is the Maunder minimum? What evidence was there for low solar activity then other than the lack of sun spots?
- 1645-1715. There were very few sunspots recorded. Normally there is an 11 year cycle from few sunspots to many. An astronomer found that there was a lack of aurora then too, and these go along with sunspot activity. (since high sunspot activity goes along with flairs and flairs send energetic particles out and they collide with our atmosphere to create northern lights.) Also, when you have a strong solar magnetic field the earth is protected from cosmic rays, which strike the nitrogen in our atmosphere and make carbon 14. There was an abundance of carbon 14 on earth then so we know that we must have been experiencing low sunspot activity.
- How are temperature and density distributed through the sun? How do we determine the approximate temperatures for the solar core, the photosphere, and the corona?
- Density decreases outward from the solar core through the corona, but temperature only decreases to just past the photosphere and then it stars to increase again. We think this is related to the magnetic fields in the sun, but we aren't totally sure. We know the temperature of the core by looking at computer models that balance the equation of hydrostatic equiliberium and several others. With the photosphere, we can look at its peak wavelength and we can also look at the line spectrum. What lines are present gives us an estimation for both the temperature and the density. With the corona, we look for emission lines in its spectrum and based on which ion states are present we can estimate the temperature of the corona (1.5 million degrees).
- Since we can't directly observe the sun's core, how do we learn about it?
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1) We can study neutrinos that we manage to catch on earth. They get out of the core right away because they don't interact with other matter.
2) We use helioseizmology, and we look at various waves that move through the interior of the sun and they reflect off the surface of the sun and then pass through it again, etc. and they cause different parts of the sun to vibrate like a gong. By studying the way that the surface of the sun vibrates, we can take doppler shift measurements of all different parts of the surface of the sun and thus study the waves that do penetrate the interior of the sun, which tell us about the conditions of the interior. - Be able to contrast open and globular clusters with respect to appearance, age, chemical composition and position in the galaxy.
- Open clusters are grouped in a loose or open structure, whereas a globular cluster is tightly packed in a more spherical distribution. Globular clusters are older and distributed in a spherical halo around the galaxy. They are no longer still forming today but open clusters are. These are confined to the disk of the galaxy, where all the inter stellar gas and dust is.
- Contrast visual binaries, spectroscopic binaries, and eclipsing binaries. Which can be used to determine stellar diameter?
- Visual binaries are detected by looking through a telescope and seeing that, what looked like 1 star to the unaided eye is in fact 2. Spectroscopic binaries are detected by changing doppler shifts in the stars due to their orbits around their common center of mass. Eclipsing binaries are ones where their orbits are lined up so that they cross each other in our line of sight. Most of the time we see the combined light of both eclipsing binaries, but when one passes in front of the other we see a dimming down of the combined light. Stellar diameters are determined using eclipsing binaries because the length of time it takes for one to cross the other is the length of time we see the dimming for.
- How can the presence of a star in a close binary sistem affect the other's evolition? How can this explain Algol's Paradox?
- If you have two stars very close together, when one becomes a red giant, it can dump matter over onto its companion. if they're close enough, the originally massive star can dump enough matter onto its companion to become less massive than the companion. The Algol's Paradox is that you have a binary system with a high mass main sequence star and a low mass red giant. This is a paradox because the higher mass star should become a red giant first. The way we get out of it being a paradox is that the originally more massive star has dumped more matter onto its companion and has become the lower mass star but it's still a red giant because it has a helium core. The main sequence star is still a main sequence star because it still has hydrogen in its core.
- What is a nova? Why can it only occur in a binary system? How can a white dwarf in a binary system explode as a supernova when it doesn't have an iron core?
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Novae occur in binary systems when you have a white dwarf as the receiving end of mass transfer. When its red giant companion has expanded up and is dumping matter onto the white dwarf, the hydrogen rich matter coming off of the red giant heats up the core of the white dwarf enough that it fuses into helium. So when you build up enough of this hydrogen on the surface of the white dwarf, it will cause a flair up on the surface and the white dwarf will brighten. There are recurrent novae, it's not like supernovae.
If, however, the white dwarf in a binary star system gets enough matter dumped on it to bring it above 1.4 solar masses, then the entire white dwarf explodes as a supernova. These are great distance indicators since they all have about the same luminocity. - Before the main sequence, what is the energy source of a protostar? Why do we not detect protostars in visible light? How do we detect them?
- Gravitational contraction. We do not detect protostars in visible light because they are still surrounded by the gas and dust from which they formed. The visible light is thus scattered by the gas and dust because it has too short a wavelength. We observe them in radio and infrared radiation because these have longer wavelengths and can get through the gas and dust.
- What happens to the internal structure of a star and to its surface after the exhaustion of hydrogen in its core?
- This is when the star expands to become a red giant and happens for all stars whether low or high mass. When hydrogen runs out in the core, the core contracts. Meanwhile you still have fusion going on in a shell around the core, and the combination of contracting core and fusion in the shell leads to the outer layers expanding and cooling and the star becomes a red giant if it's a low mass star and a red supergiant if it's a high mass star. As the helium core contracts down, eventually it gets hot enough for helium to fuse to carbon using the triple alpha reaction. (e.g. 1 helium nucleus is an alpha and if three of them come together, you can get it to fuse to carbon.) You need higher temepratures for this to occur than you do for the proton-proton chain. Now, the start of the helium fusion is different for a low mass star than it is for a high mass star. This is owing to degeneracy. if you cram electrons tightly together, they set up an additional pressure that refuses to be compressed further. This particular pressure is not dependent on the temperature. (so the perfect gas law doesn't apply). The thermostat does not work! In a 1 solar mass star, the contraction of the helium core will stop due to degeneracy before the core contracts down to get hot enough for helium to convert to carbon. But you still have hydrogen to helium fusion going on in the ring around the core. This heats the core up and eventually the core gets hbot enough for the fusion to occur. So the fusion stars which releases energy which raises temperature. But it DOESN'T raise the pressure until it gets hot enough to no longer be degenerate. once this happens, the thermostat kicks in again and the helium fusion goes stabily. But for those few minutes before it gets hot enough for helium fusion you have 10^14 solar luminocities being emitted. This is the helium flash, the fact that the helium ignition starts in such a rampant, fast beginning. This occurs in a low mass star but NOT a high mass star, because the core of the high mass star is not degenerate. It's already hot enough when the helium fusion begins to not be degenerate. So when helium fusion starts in a high mass star, it starts with the thermostat already running. So the helium flash just readjusts the interior of the star, we never see the sudden brightening. So now both the low and high mass stars sort of enter into a second main sequence lifetime of fusing helium to carbon, but they aren't back on the main sequence, they're still giants or super giants according to the H-R diagram since their internal structure is now inherently different than that of a main sequence star. Now that is the end of the line for a star like our sun (helium to carbon fusion). Occaisionally, stars like our sun can convert carbon to oxygen, but thats as heavy as the elements get for a star like our sun. But for a high mass star, the core will contract down and get hot enough to fuse oxygen to sillicon, and then the core contracts down enough to fuse sillicon to iron. Once you get up to iron in the core, though, it can't generate any more energy no matter how much the core contracts. Iron cannot be released by fusing iron into a heavier element; you would have to add energy. So the core can no longer balance itself against gravity using energy it has produced. The iron core then contracts all the way down and creates a supernova explosion. The low mass star becomes a red giant, and the carbon and oxygen core is degenerate and the outer layers are blown off in a planetary nebula.
- So what exactly happens to a low mass star when it goes off the main sequence?
- It becomes a red giant when the hydrogen was exhausted in its core, it contracted down a little bit when it stabilized for its helium burning toward the main sequence, and when the helium core starts contracting, it ascends the red giant branch for the 2nd time. The planetary nebula that leaves behind the white dwarf occurs up at the top of that 2nd red giant branch ascension. The white dwarf is supported against gravity by the degenerate electrons in it. Then, it just gradually cools off. These white dwarfs have an upper limit of 1.4 solar masses, and beyond that, they become so degenerate that the electrons are forced to combine with protons to make neutrons. You can crunch neutrons in a lot more tightly before they become degenerate. It is a neutron star's contracting iron core that creates the supernova explosion.
- What exactly happens to the high mass star when it goes off the main sequence?
- it becomes a red supergiant, and its core at the end of its evolution will be very iron rich, but around it there will be a layer of sillicon, and then outside that a layer of oxygen, and a layer of carbon. So you have all these different reactions going on with the most massive elements at the core of the star. The iron core collapses, creates the supernova explosion, and blows everything else out.
- What determines whether a red supergiant becomes a supernova, neutron star, or black hole?
- This depends on how much mass there is left. If there is between 1.4-3 solar masses left in the core after the supernova has happened, you will have a neutron star where it is held up against gravity by degenerate neutrons. If you have more than 3 solar masses, then the degenerate neutrons cannot support it, and gravity wins out, forcing it to collapse down and become a black hole.
- What are the sources of pressure that support main sequence stars, white dwarfs, and neutron stars against the inward pull of gravity?
- For a main sequence star, you have the energy generation in the core combined with the perfect gas law supporting it against gravity. White dwarfs are supported by degenerate electrons, and neutron stars are supported by degenerate neutrons.
- What are pulsars?
- These are believed to be rapidly rotating neutron stars. They give off rapid radio pulses on the order of a second. We think they are rotating neutron stars because no star could pulsate that fast without being broken apart.