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57 Cards in this Set
- Front
- Back
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How long does light take to get from the core to the photosphere? |
170k years |
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How long does light take from the photosphere to Earth? |
8 minutes |
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Fun fact: how much does it cost to launch 1kg into space? |
£10k or more. |
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How do you break up light into a rainbow in a lab? |
Using a glass prism. Would be nice to do computations. |
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Why/how does light deviate when it hits a surface between two materials? |
It's speed and hence wavelength (but not frequency) changes. Be able to show that it causes a deviation due to the effect on the wavefront! |
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What fraction of energy does the Sun emit in the visible? |
Less than half is emitted in the visible range, about half in the infrared and less than one tenth in the ultraviolet, with all other frequencies making up a small fraction of sunlight. |
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How much energy per sqm of sunlight reaches Earth? |
1kW |
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Measuring the Earth-Sun radius. |
Did the Greeks do it? Contested but likely not. How do you do it? You can measure any planetary distance in the Solar system, then infer the AU. Cassini in the 1660s used parallax on Mars between France and French Guyana, to measure a parallax of about 45 arcsec, and got a decent estimate. Then in the 1700s, people did a more accurate measurement on two consecutive transits of Venus in front of the Sun. |
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Describe the p-p chain. |
p+p=De+positron+nu De+p=He3+gamma He3+He3=He4+2p Mass lost is 0.7% |
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Mass(He4) /Mass(4H) |
99.3% |
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What is spectroscopy and how do the basic optical experiments work? |
Emission spectrum, absorption spectrum |
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The Solar atmosphere composition: how, when, by whom was it discovered? |
Spectroscopic analysis of the intensity of the absorption lines, 1920s, Cecilia Payne-Gaposchkin. H 73%, He 25%, metals 2%. |
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Why is the Solar composition different from the Earth's composition? |
The planets close to the Sun lost most of their heavy elements. These elements have a lower escape velocity than heavy ones, and the inner planets are hot enough to lose them. Any traces of H2 on Earth have been recently generated. |
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The physics of convection (discuss the temperature gradient) |
Hard! |
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Can we see any nebulae in the sky? |
Yes, the easiest to see is the Orion nebula in Orion. It's a star forming region. It's 400 pc away and 8 pc wide. |
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Convection in the Sun |
The convective zone is 18% deep in radius. The temperature gradient is steep, three times steeper than in the radiation zone. Convection moves energy outwards much faster than in the radiative zone, in fact it flows out in just a few days as opposed to 10^5 years. |
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How deep is the photosphere? |
500km |
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What is the last interaction of a photon coming from the photosphere, in all likelihood? |
Negative hydrogen ions are responsible for 95 per cent of the light emitted by the Sun. It is easy for the hydrogen atom to gain or lose an extra electron. And when a photon is emitted it can be at a range of frequencies across the ultraviolet, visible and infrared parts of the spectrum. |
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Sunspots - how do they move? |
From left to right on the surface of the Sun, because of its rotation. |
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Sunspots - where do they appear? |
Usually 30-40 degrees of latitude, in both emispheres, usually in pairs. |
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Sunspots - what do they look like, and what are they? |
They are round with a darker centre and not as dark surrounding area, called umbra and penumbra. They are basically cooler (about 2000K cooler) holes in the photosphere, the light is coming from deeper but cooler layers, more so for the umbra than penumbra. We can infer that they are holes by seeing which part of the penumbra vanishes as the sunspot gets close to the edge of the Sun. |
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Which modern era physicist made huge progress on studying the sunspots? |
George Hale in the early 1900s. He also founded the ApJ, created 4 observatories, and supported young scientists like Edwin Hubble. |
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What is a spectroheliograph? |
A useful instrument to image the surface of the Sun. Light is collected from one slice at a time, and filtered to one wavelength at a time. You can track the density of a specific element by doing this on an emission line. |
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What is hydrogen alpha line? Why is it important for studying sunspots? |
The strongest H emission line in the visible, part of the Balmer series. It's emitted when jumping between energy levels 2 and 3. Wavelength of 656.3 nm, in the red. It's emitted by glowing gas at 5000k in the atmosphere of the Sun that is just above sunspots. |
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How do we know there is magnetic field in the sunspots? |
First of all, we see structures around the sunspots in the Halpha line images that suggest the plasma might be following magnetic field lines. Then, the Zeeman effect shows line widening in correspondence of sunspots, and not the rest of the surface. |
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How does the magnetic field keep sunspots cooler? |
The magnetic field traps the plasma. The trapped plasma is emitting and naturally losing energy. But hotter plasma from deeper in the convective zone cannot come in and bring new energy in. |
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What is an alpha sunspot? |
It's a sunspot that doesn't appear to have a companion. The magnetic field still merges back into the surface of the Sun, but it lands in a wide area and doesn't manage to trap the plasma there and cool it down enough to create a visible sunspot. |
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What is a geomagnetic storm? |
It's a chaotic disturbance of the magnetic field of the Earth, which can be caused by a solar flare. The 1859 geomagnetic storm was the stromgest since electricity was discovered. It caused strong currents to flow even in cables that were not powered, for example telegraph cables. |
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How was Neptune discovered? |
When Uranus was discovered, its orbit was not exactly compatible with the calculations on the gravity of the other planets. The astronomers deduced the existence of Neptune, and observed it shortly thereafter in 1846. |
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What is the sunspot cycle? |
It's a cycle of 8 to 15 years (mean 11). At the start of it, sunspots appear at ±(30-40) degrees latitude in the Sun. Later in the cycle, sunspots become more numerous and they form closer to the equator. |
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How can one notice the Sun's rotation? |
In chronological order: 1. Look at sunspots moving on the surface. 2. Doppler effect of spectral lines shift; and Doppler effect for line enlargement. 3. Helioseismology. |
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Tell me about helioseismology. |
It's the study of the oscillations of the surface of the Sun. Sound waves are constantly generated from the motion in the convective zone. Most waves are damped, but resonant waves will survive. The most notable oscillation of the surface has a period of 5 minute, but over a thousand resonant waves have been identified. As a wave propagates deeper into the Sun, the higher density it encounters makes it deviate, and it keeps bouncing around the surface. This allows us to study the properties of the internal layers. The magnetic field also affects the propagation of waves. We can study how surface waves are affected by sunspots, and this way we know which sunspots are active on the opposite side of the Sun. It also allows to probe the rotation of the Sun - the sound waves travelling in the direction of the rotation will have slightly higher speed, the ones in the opposite direction will have slightly lower speed. |
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How does the Sun rotate? |
Equator - 25 days. Poles - 40 days. Convective zone - similar to the surface. Radiative zone - (almost) uniform rotation. Inner core - not sure. Tachocline - rapid transition in <= 1% of the Solar radius. |
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What is the Birmingham Solar group called? |
BiSON - Birmingham Solar-Oscillations Network. |
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How do cosmic magnetic fields originate? |
Some were created at the Big Bang and frozen in plasma ever since. That applies to most non turbulent environments, the dissipation is slow as plasma has high conductivity. Electrical currents can also generate magnetic fields, of course. Any existing weak magnetic field can be amplified by movements of the plasma, like in the solar tachocline. |
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Describe the polar magnetic field of the Sun at the photosphere. |
Typical dipole layout. At the photosphere, it's 20 times stronger than the Earth's surface magnetic field. The field lines are closed, they close back inside the Sun. The polarity switches every 11 years. |
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Describe how the solar dynamo works, and what explains the magnetic cycle. |
B lines wind up near the tachocline for the differential rotation. Creates strong B. The convection drags up B into the convective zone, eventually gets to the photosphere. The resulting B lines that exit have an Ω shape. The one closer to the pole has polarity opposite to the field exiting the pole (draw a diagram!). Meridional currents circulate on the surface, from the equator to the pole, and reserve the polarity every 11 years, in line with the sunspots cycle. |
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Leading and trailing sunspots. |
A pair of sunspots woll usually (90% - 95% of the time) have a leading one, that is about 4 degrees further along in the solar rotation, and is closer to the equator. It can be explained by the coriolis force applied to the material coming out of the convection zone. |
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How long can a partial eclipse last, and what about a total eclipse? |
3 hour, and 7.5 minutes respectively. You prove it by computing the angle in the sky that the moon needs to make over the sun, and using their relative speeds, largely due to the moon's revolution. |
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Describe 2 important uses of Solar eclipses. |
1. Studying the Solar atmosphere. The Moon can block the bright photosphere and then we can see the atmosphere more clearly. 2. It was used in 1919 by Eddington to check the theory of general relativity by Einstein, which predicted that photons would be affected by gravity. He checked that stars whose light would go near the Sun appeared in slightly different positions. |
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Components of the Solar atmosphere and their height. |
Chromosphere, a few thousand km. Corona, 1 Rsun. The "heliosphere", the region in which the Solar wind dominates against the interstellar medium, extends to 50 AUs. The heliopause is the outer border. |
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The chromosphere. |
So called because it has a distinctive colour (red). It's a thin layer around the photosphere. The red colour comes from Halpha emission. The spectrum is similar to the photosphere but with emission lines due to scattered light. The temperature is about 20k K which is hotter than the photosphere. Helium was first discovered in the chromosphere. The shape is quite jagged, with visible "prominences", as opposed to the photosphere and corona. |
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The corona. |
Sits on top of the chromosphere, extends to 1Rsun out. Very tenuous, with density literally 10^-10 that of air, and very hot at 1 million K. The extreme temperature leads to heavy ions being very ionised, so that we detect spectral lines from the corona that we can't generate in a lab. |
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The discovery of Helium. |
Unrecognised spectral lines from the chromosphere were detected in the 1860s and 1870s in eclipses by scientists from England and France. A new element called Helium (from Helios, the Sun in Greek) was postulated. It was only discovered on earth by chemists about 30 years later. Helium is a tightly bound atom and very high energy is required to excite its electrons. A temperature of 20K is needed for excited electrons to get to a state from which you can generate visible lines. The photosphere is too cold for this. The chromosphere, counterintuitively, is hotter and can generate these lines. Helium is rare on earth because, as other light elements, it escapes from the atmosphere. It can be generated underground via natural gas phenomena. |
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Voyagers 1 and 2: when were they launched and what was the goal? |
In the 1970s. A favourable alignment of Jupyter Saturn, Uranus, Neptune would allow to visit them all. They were meant to stay on for at least 5 years, but carried on for 40 years. They use nuclear radioactive plutonium to generate heat which is converted to electricity. |
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Voyagers: non-Solar discoveries |
They observed Io, a moon of Jupyter, and found it is volcanically active. They visited Titan, a moon of Saturn, and discovered it has a thick nitrogen atmosphere and is similar to a planet on its own right. |
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What is the Oort cloud? |
There's still uncertainty on this, but we think the Oort cloud is a region at 50k AUs (almost 1 light year) from the Sun, from which comets come. When material in this area is disturbed by the passage of nearby stars, it can get into eccentric orbits around the Sun. |
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Comet tails |
The comets have a main tail, called "coma", which always points away from the Sun. Likely affected by radiation pressure (check!) They also have a secondary tail which is a few degrees off, which is made of ionised gas and is affected by the Solar wind. |
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What is the Solar wind? |
A wind of particle that starts in the Sun's corona and emanates from it. The hot corona pushes particles away and they can escape the Sun's gravitational pull. The magnetic field in the Solar wind is complex (the models must account for why it doesn't reconnect onto the Sun), the interaction with the Earth's magnetic field is important too. The main breakthrough in models of the Solar wind was made in the 1960s by Eugene Parker. |
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The heliosphere/heliopause |
The heliosphere is the region of space in which the Sun and its Solar wind have an effect on the material that's larger than that of other bodies, and the material is significantly different from the interstellar medium. Its radius is about 100AUs. It ends at the heliopause. In the outer interstellar medium, the Sun's magnetic field does not shield the medium so there are more cosmic rays. The Earth's magnetic field shields us from the effect of the Solar wind. |
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Name a few Solar probes |
Solar Probe Plus, launched on 2018, close to the surface. Solar Orbiter, sent off in 2020. Studies the solar wind. Soho. |
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What is a rockoon? |
A rocket sending up in the atmosphere a balloon. They were used to observe the sun from high up, and deployed when a solar flare was detected, in the 1950s. |
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Describe a solar flare from an observational point of view. |
Two phases, impulsive and gradual. In the impulsive phase, first emissions in radio and microwave frequency comes from the corona, immediately afterwards xrays and gamma rays from the corona, then Halpha from the chromosphere, rarely visible light from the photosphere. Also gamma rays from the photosphere. Then in the gradual phase, big arches of magnetic plasma are seen from photosphere and corona, emission starting in the x-rays then gradually shifting to lower frequency occurs. This lasts a few hours. The structure looks like a slinky. |
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Solar flare, what are the physical mechanisms? |
Electrons in the corona are accelerated down towards the photosphere. Low frequency waves come from charged particles rotating around field lines. High energy waves come from electrons clashing with plasma in the lower corona, or protons and neutrons in the photosphere. The energy to explain the initial acceleration comes from the magnetic field, when turbulent motion causes magnetic field lines to entangle they can recombine and release energy ("magnetic reconnection"). The gradual phase is due to the material in the chromosphere having been heated and expanding. The energy transfer is very complex. Electromagnetic waves might also travel down with the matter and transport energy. |
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Funny trivia about Solar probes. |
1. Rockoons 2. A functioning Solar observatory was picked as good test target by the US military and Reagan shot it down. |
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What are CMEs? |
Coronal mass ejections, large ejections of plasma and magnetic field from the corona. They can coincide with solar flares, but not always.. Several CMEs occur every day. The material ejected may well be moving at a speed lower than the escape velocity. This is possible because it is carried out by the magnetic field. "Swirling" patterns are often seen in the corona during CMEs. They allow the Sun to get rid of magnetic helicity. |
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What is magnetic helicity? |
(fill in from Wikipedia!) |
