Astro Physics

Disadvantages of refracting telescopes

1. Requires large-diameter glass lens free from defects which is difficult to make

2. Large glass lenses are very heavy and tend to distort under their own weight

3. Lenses can only be supported around their edges which is the weakest part of the lens

4. Suffer from both chromatic and spherical aberration

5. Can only observe wavelengths of visible light

Disadvantages of reflecting telescopes

1. The secondary mirror blocks some of the light from entering the primary mirror and will cause some diffraction and affect clarity

2. mirrors in a reflecting telescope are exposed to air so they require regular maintenance

3. Some chromatic aberration may be introduced when the light is refracted in the eyepiece lens

Chromatic aberration

• Due to the different focal length of red and blue light (as blue light is more refracted) meaning they focus at different points

• Causes coloured fringing in the image

• Can be minimised by using an achromatic doublet which is a convex lens and a concave lens cemented together to bring the light rays to focus in the same position

Spherical aberration

• Due to the rays of light at the edge to be focused in a different position to those near the centre because of the curvature of the lens/mirror

• Causes image blurring and distortion

• Can be avoided completely by using parabolic objective mirrors in reflecting telescopes

Collecting power

• The measure of the ability of a lens/mirror to collect incident EM radiation

• Directly proportional to the area of the objective lens/mirror (also to the square diameter of the lens/mirror)

• The greater the collecting power the brighter the images produced

Resolving power

• The ability of a telescope to produce separate images of close-together objects

• The angle between the straight lines from Earth to each object must be at least the minimum angular resolution for them to be resolved [ θ～λ/D ]

Rayleigh criterion

• It is the minimum angle subtended between two objects who’s images can be resolved

• Identified at the point where the central maximum of the diffraction pattern from one objects coincides with the first minimum of the diffraction pattern from the second object

Advantages of charged-coupled devices (CCDs)

• Has around 80% of quantum efficiency

• Can observe infrared, UV and visible light

• Can observe for long periods and produce digital images

Radio telescopes

• ground base (atmosphere doesn’t absorb rodia waves

• Observe 1mm to 10m wavelengths (detect H2 emission lines)

• 10^-3 rad resolution (low because of large wavelength)

• Radio waves won’t be absorbed by dust and are used to map the Milky Way

• Only uses a primary dish and it doesn’t need to be as smooth as optical mirrors

UV telescopes

• Located in space so it is inconvenient to maintain

• Observe 10 to 400nm wavelengths

• 10^-7 rad resolution (due to short wavelength)

• Can detect supernovae and quasars, and used to determine the temperature/ chemical composition of objects

• Similar structure + collecting power to optical telescopes

• UV mirrors need to be smoother than optical mirrors

IR telescopes

• Predominantly located out of space (IR is absorbed by H2O gas so the area must be dry)

• Observe 1mm to 700nm wavelength

• 10^-7 resolution (space), 10^-6 resolution (ground)

• Detect warm objects eg. Dust in nebulae and brown dwarfs

• Mirrors must be kept very cold to avoid interference

X-Ray & Gamma Rays telescopes

• located in space so it is inconvenient to maintain

• Observe <10nm wavelength (very high resolving power)

• 10^-6 rad resolution

• Images tent to be extremely bright despite low collective power

• Used to observe energetic events eg. Black holes, neutron stars, pulsars, GRBs

Quantum efficiency

(no. of electrons produced per second/no. of photons absorbed per second)*100

Apparent magnitude

Is how bright an object appears in the sky

Absolute magnitude

The apparent magnitude of an object if if is 10 parsecs away from Earth

Definition of 1 parsec

Classification of stars

Hydrogen Balmer Lines

• Absorption lines found in the spectra of O, B and A class stars

• Caused by the excitation of H atoms from the n=2 state to higher/lower states and re-emit light in all directions which reduces light intensity in Earth’s direction

Inverse square law for light intensity

𝐼=𝑃/(4𝜋𝑟²)

HR diagram

Stages of stellar evolution

1. Protostar

2. Main Sequence

3. Red Giant (for star <3 solar masses)

4. White Dwarf (for a star <1.4 solar masses)

5. Red Supergiant (for star>3 solar masses)

6. Supernova (for star >1.4 solar masses)

7. Neutron Star (for star between 1.4 and 3 solar masses)

8. Black Hole (for star >3 solar masses)

Protostar

• Clouds of gas and dust of varying masses clump together under gravity

• Irregular clumps rotate and gravity spins them inwards to form a dense center (Protostar)

• When the protostar gets hot enough, it begins to fuse elements producing strong stellar wind

Main Sequence

• Inward force (gravity) and outward force (due to fusion) are in equilibrium

• The star is stable

• Hydrogen nuclei are fused into helium

• The bigger the mass the shorter this period is as fuel is used more quickly

Red Giant

• Hydrogen in the center runs out

• Outer layers expand and cool

• Gravity causes the core to shrink and contract

• Temperature of core increases and begins to fuse helium to heavier elements

• Surface temperature is less than 4000–5000K but are very bright as they have a very large surface area

White Dwarf

• When Red Giant used up its fuel, fusion stops and core contracts as gravity is now greater then the outward force

• A planetary nebula is formed around the core as outer layers are thrown off

• Core becomes very dense and stabilizes as white dwarf

• Very high surface temperature but are very faint due to a small radius

• Will eventually cool to a black dwarf

Red Supergiant

• When Red Supergiant used up its fuel, fusion stops and core contracts as gravity is now greater then the outward force

• It collapse in a supernova causing gamma ray bursts

• Can fuse elements up to iron as iron nuclei are more stable than other nuclei

• Has low surface temperatures (>4000K) but are really bright due to large surface area

Supernova