Optical lenses show chromatic aberration and become very thick and heavy as their size (diameter) increases.
Like optical lenses mirrors show spherical aberration; the ideal surface to focus parallel light onto a single point is a parabola. Parabolic mirrors are not used in telescopes as a telescope is intended to collect light from a small angular range of directions rather than just parallel light from one distant point in space. Covering a slightly extended angular range with its field of view allows a telescope to image an extended astronomical object in its context rather than to just collect light from one of its corners.
The eyepiece in a telescope is only needed if a human eye wants to look at the image created in a telescopic optical system; photographic film or a modern CCD camera for example can be placed directly at the focus of the optical system.
The size of the optical aperture of a telescopic system is an area, and it is directly proportional to the amount of light collected from an astronomical object. Aperture is normally measured as the diameter of the most restrictive optical opening the light has to pass through, as (almost) all optical system have circular openings.
The optical aperture of a telescopic optical system determines the amount of deterioration the image will suffer from diffraction. Diffraction is related to the wave structure of propagating light and is wavelength dependent. It is measured in seconds of arc as the relevant resolution for optical telescopes is related to their ability to tell light sources apart that have different angular origin in the sky. Two optical sources that share the same line-of-sight from Earth but lie at different distances along that same line-of-sight cannot be resolved with any telescope irrespective of its resolution limit.
The Airy disk is one way of talking about the way in which diffraction limits the resolution of a telescope. The diameter of an Airy disk signifies the spread a pointlike source in the sky suffers through diffraction in the telescope's optical system.
Mirrors can be segmented to allow for easier manufacturing and transportation to the telescope site.
Atmospheric seeing is how astronomers talk about the way turbulence in the air column between an astronomical telescope on the ground and the astronomical object outside of our atmosphere. By changing slightly and continuously the refractive index of the air along the light path through the atmosphere seeing makes the image of a still astronomical source move around in the image formed by the telescope; as these changes occur quite fast, the net effect is to blur the image as it moves throughout the exposure time.
Like diffraction seeing is measured in seconds of arc, and the best telescope sites on the surface of the earth have seeing a of 0.4 arcsec. Any place with seeing of less than 1 arcsec is considered a good site. Telescopes like the Hubble Space Telescope that are outside of the atmosphere do not suffer seeing.
Seeing can be corrected for with adaptive optics: In adaptive optics the source movement in the image plane is computed in real time and corrections calculated in a computer are fed into a small secondary mirror to compensate for the atmospheric effects that are at play to move the image. The longer the wavelength, the easier it is to use adaptive optics.
Light pollution occurs as light from sources on the ground (like city lights) is scattered in the atmosphere within the field of view of a telescope and thus lead to light entering the telescope that does not come from the astronomical sources the telescope is trying to observe.
The atmosphere has two windows in which it lets electromagnetic radiation pass through: the optical (and near infrared) window, and the radio window. Telescopes for infrared, X-ray, and gamma-ray wavelength have to be flown in space, outside of the atmosphere.
The long wavelength of radio waves lead to a strong limitation for the angular resolution of single radio dishes; on the other hand this long wavelength means that the "bumpiness" of the surface of a radio dish can be much worse than that of the mirror of an optical telescope. In other words: Radio telescope surfaces do not need to be polished to optical quality.
Radio telescopes can often see "through" optically obscuring interstellar dust clouds. Infrared (IR) light also generally is less attenuated by dust than optical wavelength.
Wien's law relates long wavelengths to the blackbody radiation of cold objects and short wavelengths to the blackbody radiation of hot objects. Thus X-ray telescopes (in space!) look at high energy phenomena (for example in the Sun's atmosphere), while radio telescope can for example map cold interstellar dust clouds. In that way observations at different wavelength complement each other.
The diffraction limit on angular resolution can be overcome by interferometry. Interferometry is routinely applied at radio wavelength, where interferometry with telescopes placed on different continents gets the angular resolution down to milli-arc-seconds; much better than in the best optical telescopes. Interferometry at optical wavelength is also used, but technically much more difficult.
There are eight planets in the solar system; from the innermost orbit to the outermost orbit they are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
There are two types of planets in the solar system: terrestrial planets and Jovian planets. The terrestrial planets are closer to the Sun while the Jovian gas giants are further away from the sun.
There are 165 known moons circling the eight planets in our solar system.
Between Mars and Jupiter is the Asteroid Belt. The asteroids are rocky in nature and span a wide variety of sizes, but are all to small to attract each other gravitationally in a way that would be enough to form a larger body.
A group of asteroids is leading and another trailing Jupiter on its path around the sun, each leading or trailing by about 60 degrees.
Outside of Neptune's orbit is the Kuiper Belt. Unlike Asteroid Belt objects the Kuiper Belt objects are icy in structure - like the comets.
A lot of comets were found in recent years as more and better telescopes detect smaller and fainter comets. The strange convergence of comets on the Sun around the year 2000 seen in the 200 year animation I showed in class thus reflects a so-called selection bias, where the fact that you do not see something has to do with the fact that you look for it in a certain way (or at a certain time).
As pointed out earlier, the comets tail does not trail the comets in its path, but points away from the Sun. It is like a weather vane blowing in the solar wind.
Almost all objects in the solar system move around the Sun in the same direction as the Sun rotates around it axis. All planets also rotate in the same direction around their own axes (except for Venus and Uranus). All the planets' moons also move around their respective planet in the same direction (with the exception of Neptune's moon Triton).
With the exception of Pluto and the comets all the objects mentioned above revolve around the sun in essentially one single plane.
All of the orbits in the plane are almost circular.
Every element heavier than Helium is called a "metal" in astronomy (irrespective of its chemical properties).
Conservation of angular momentum of the originally very slowly rotating dust cloud that contracts under its own gravity leads the dust to settle into a disk shape rather than to collapse into a sphere. As the original cloud is about 1 ly across and the diameter of e.g. our solar system is roughly 24 orders of magnitude smaller, spin-up leads to the faster rotation of the disk first as a whole and later of the planets that form in the disk.
On the rocky terrestrial planets Oxygen and Iron are the most common elements, whereas throughout the whole of the solar system Hydrogen and Helium are by far the most abundant chemical elements.
The simple fact that "metals" are found in our planetary system means that it is made from material produced in an earlier generation of stars.
The solar wind is radiation originating at the sun. As the energetic solar wind moves away from the sun, the particles and light in it remain distributed over the ever expanding surface of a growing sphere. The solar wind thus gets less and less intense as it moves away from the sun simply by virtue of being distributed thinner and thinner in space.
As the dust cloud that becomes a solar system settles into a disk, a star is formed at its center as soon as the pressure and temperature at the center reach the point that the fusion reaction can start. A solar wind is started with the energy from that fusion, and it starts evaporating the lighter elements from the inner part of the newly formed protoplanetary disk.
Planets form first by dust particles sticking together in slow collisions (and sometimes breaking up again in faster collisions) until they are large enough to attract more dust and particles with their gravity. Mutual gravitational attraction between small bodies in the protoplanetary disk cannot overcome the bounce solid (icy or rocky) objects experience when they collide in the protoplanetary disk.
The fact that the inner solar system is rocky and the outer solar system consists of icy small objects and giant gas planets is explained by the solar wind cleaning the inner solar system from lighter elements before the planets can form.
It takes about 100 million years to form a solar system.
With telescopes we can see dust disks around young stars in recent star forming regions in our galactic neighborhood. When the dust disks are still forming and the dust has not yet settled in a well defined disk, allowing the solar wind from the center to sweep out the remaining light material outside of the disk, the dust disks are seen as absorbing dark bands across the starlight as it reflects off the light material that has not yet settled into the disk or been blown out into the surrounding space. At a later stage of the forming of a dust disk around a young star we can see the starlight reflected off the now well defined disk rather than scattered in the collapsing dust outside of the disk.
When looking for extrasolar planets our methods are mostly or even exclusively sensitive to heavy planets orbiting close to their host star. The fact that we see very heavy planets close to the suns outside of the solar system is therefore with high probability just a selection bias.
The Sun consists to 74 percent of Hydrogen, 24 percent of Helium, and only 2 percent of "metals".
Helium 4 is lighter than the sum of its parts (two protons and two neutrons) or the sum of the masses of four protons (Hydrogen nuclei). Under normal circumstances the repulsive electrostatic force between two positively charged protons is so huge that they cannot get together and release the energy that would be gained by such a fusion.
In the inner one quarter of the Sun's radius temperature and density are so high that four Hydrogen nuclei can be fused into one Helium nucleus. The energy released in this fusion process maintains the temperature and pressure necessary at the center of the Sun to counter gravity's tendency to make all the Sun's material converge to the center of the Sun. The Sun is in stable hydrostatic equilibrium: outward pressure from heat and inward pressure from gravity balance each other.
There are a variety of ways in which the fusion of Hydrogen nuclei (protons) into He four nuclei proceeds. All these processes involve the conversion of protons into neutrons and the subsequent release of electron-type neutrinos. Electron neutrinos can leave the sun without further scattering; solar neutrinos have been detected on Earth and these measurements were used to measure properties of the elusive neutrinos. (If they can pass through the Sun without being scattered, they can certainly pass through the earth without being scattered - i.e. without leaving any trace of their having passed...)
All material in the Sun is the plasma state: Atoms are fully ionized and their nuclei and electrons move independently. As these constituents of the plasma are charged, and light itself is an electromagnetic phenomenon, light cannot simply pass through the plasma. Photons are continuously scattered on the charges in the plasma. As a consequence light diffuses through the plasma just like a gas would diffuse through another gas, with the individual photons bouncing around randomly and isotropically between individual scattering events.
The Sun's energy is generated at the center and radiated off from the surface. The transport of energy in the Sun from its center to its surface is associated with the diffusion of light through the Sun's plasma. At every point along the way the photon field (the photon is the light particle) is in equilibrium with the surrounding material. That means that the energy distribution of the photons at any point along the way is a blackbody spectrum that corresponds to the temperature of the plasma. So the photon energies range from x-ray energies at the center to visible light energy at the Sun's surface.
Throughout the inner 70 percent of the Sun's radius energy transport happens through the diffusion of light through the plasma: This is the zone of radiative transport or the Radiation Zone.
Throughout the outer 30 percent of the Sun's radius the energy from the center is transported in convection cells that are visible as granules at the Sun's surface. This zone is called the Convection Zone. The granules move and change shape constantly; typical granule size is 1000 to 2000 kilometers across.
Each convection cell is heated at the bottom with the radiation arising from the core through the Radiation Zone. It is cooled at the top where the radiation escapes into the solar atmosphere and space. The Sun releases its energy into space as blackbody radiation of roughly 6000K (5780K) and in the form of the energy of particles in the solar wind.
The Photosphere is the outermost layer of the Sun and the surface from which the Sun's blackbody radiation is emitted. Above the Photosphere is the Chromosphere. Atoms with bound electrons exist in both the Photosphere and the Chromosphere and produce the absorption lines in the solar spectrum as it is observed on Earth. The Chromosphere is considered the lowest part of the solar atmosphere. There is no sharp line dividing the Photosphere and the Chromosphere.
Going outward from the imaginary solar surface through the Chromosphere we next encounter a transition zone, before entering the Corona - the outermost region of the sun. The Corona is the region where the solar wind originates. The high temperature of the coronal gas leads to high ionization states and constituent kinetic energies that allow electrons and protons to escape the gravitational pull of the Sun: the solar wind.
Sunspots are cooler areas on the Sun's surface. The magnetic field in a sunspot is about 1000 times stronger than the normal magnetic field in the surrounding areas. Sunspots normally come in pairs with the magnetic field lines emerging in one spot and disappearing into the other. Whether the field lines emerge or disappear is referred to as the polarity of a sunspot. Sunspots do not wander in latitude, but depending on where we are in the solar cycle appear at different latitudes. Early in the solar cycle they appear at high latitude, and late in the solar cycle they appear at low latitude. A typical sunspot is about 10000 km across; almost the diameter of the Earth.
A full solar cycle takes 22 years to complete; in the first 11 year sunspot cycle the leading spot in a pair has one polarity, and the opposite polarity in the second 11 year sunspot cycle so that a full solar activity cycle consists of two 11 year cycles. In each period sunspot pairs in the northern solar hemisphere have the opposite order of polarities from those in the southern solar hemisphere. Solar cycles can sometimes be longer or shorter than the regular 22 years - or disappear completely for a while...
Sunspots can be associated with active regions that produce prominences and flares. Solar material ejected in prominences follows the magnetic field lines and thus gets back to the sun. Material ejected in flares escapes into space, and coronal mass ejections blow ionized material into interstellar space with such violence that it can disrupt radio communications on Earth.