## 1.1 Light at different wavelengths *Visible light* 420-700 nm (violet to red) #### Equations: c = f$\lambda$ Energy of a photon: $\epsilon$ = h.f {h = planck constant = 6.626 x 10$^{-34}$ Js} #### Absorption of the atmosphere From the ground: Optical, Near IR, microwave and radio Gamma rays, , X-rays, UV and some IR regions need to be observed from space or high altitude due Earths atmosphere: - High-energy photons (gamma rays to UV) undergo photoelectric absorption by N or O atoms, removing electrons and creating the Earth’s ionosphere of charged particles. - Ultraviolet light also breaks up molecular oxygen to form ozone, and is itself absorbed by ozone. - Many bands of infrared light are absorbed by water, carbon dioxide and other molecules, producing the greenhouse effect. - Finally, very low frequency radio waves are reflected or absorbed by the ionosphere. ## 1.2 Continuous spectra of stars The ideal black-body spectrum has a characteristic shape called a Planck curve. ![[../Assets/Screenshot 2023-11-02 at 13.30.19.png|200]] On the Y axis the [[../Glossary/spectral flux density]] is depicted. On the X axis there can be E, f or $\lambda$ . For more info on the planck curve or [[../Glossary/Plancks radiation law]] check out : https://learn2.open.ac.uk/mod/oucontent/view.php?id=1817388&section=3.1 #### Wien's law: ($\lambda_{peak}$ / m) = $\dfrac{2.90* 10^{-3}}{T/K}$ [[../Glossary/effective surface temperature]] of the Sun is 5780 K. #### [[../Glossary/luminosity]], temperature and radius, The LTR relationship: If we treat stars as black bodies then: [[../Glossary/Stefan-Boltzmann law]] : l = $\sigma$ T$^4$ {l=power, $\sigma$ = 5.67 x 10$^{-8}$ W m$^{-2}$ K$^{-4}$ } so total power output is: L = 4$\pi$ R$^2$ $\sigma$ T$^4$ { L = [[../Glossary/luminosity]]} Normalised and expressed in Solar units L$_{\odot}$ , R$_{\odot}$ and T$_{\odot}$ the equation becomes: $\dfrac{R}{R_{\odot}}$ = $\sqrt{\dfrac{L}{L_{\odot}}}$ x ($\dfrac{T_{\odot}}{T})$^2$ ## 1.3 Understanding continuous spectra #### Non-thermal continua, (as opposed to black body thermal continua ). [[../Glossary/synchrotron radiation]] This arises when electrons travelling at relativistic speeds (close to the speed of light, ) move through a magnetic field, such as the fields that exist around supernova remnants or on stars exhibiting flare activity. The electrons experience a Lorentz force that causes them to move helical around magnetic field lines. This generates EM radiation. Each relativistic electron produces a continuous spectrum with a peak that depends on its energy. The sum of many spectra creates a typical spectrum. ![[../Assets/Screenshot 2023-11-07 at 10.43.05.png|300]] [[../Glossary/inverse Compton scattering]] Relativistic electrons collide with photons changing their direction and thus adds to their energy what translates into EM radiation. frequently observed at active galactic nuclei, gamma-ray bursts and X-ray binaries ![[../Assets/Pasted image 20231107150152.png|300]] #### Interstellar absorption and reddening interstellar dust diameter is app 10$^{-6}$ to 10$^{-7}$ m so it scatters em radiation with comparable wavelength eg UV and visible light more efficiently. So longer wavelength do not scatter that much hence reddening. If we observe the dust from a different angle we might see the scattered bluer light. Some photons are absorbed and increase the temperature of the cloud which radiates in the far IR ($\frac{+}{{-}}$ 1.45 $\times$ 10$^{-4}$ m) ![[../Assets/Pasted image 20231107154043.png|200]] #### Broadband spectra [[../Glossary/broadband spectra]] can be used to identify different components contributing to the spectrum. Such as a star and interstellar dust. A [[../Glossary/spectral energy distribution (SED)]] shows the relation of the relative energy emission and the wavelength. The relative energy emission term is $\lambda$F$_{\lambda}$ so wavelength times [[../Glossary/spectral flux density]]. ![[../Assets/Pasted image 20231107155634.png|300]] The highest top is from stars with a temperature of around 2900K. the second top is thermal emission from the dust. [[Part 2 Spectral lines]] [[Astronomy]] [[Part 2 Spectral lines]] [[Astronomy]]