• Bound-bound transitions
• Bound-free emission
• Free-free emission (Bremsstrahlung
• Synchrotron radiation
• Inverse Compton Scattering
• Fluorescence

1. Bound-Bound Transitions — Line Emission/Absorption

Bound-bound transitions occur when an electron moves from one bound energy state to another within an atom or ion. This can happen in two ways:

• Absorption: When an electron absorbs a photon, it jumps to a higher energy level. The energy of the absorbed photon must match the difference between the two energy levels.
• Emission: Conversely, when an electron falls from a higher energy level to a lower one, it emits a photon. The energy of the emitted photon is equal to the energy difference between the two levels.

These transitions are responsible for line emission or absorption spectra. The wavelengths or energies of the emitted or absorbed photons are characteristic of the atom or ion involved, making spectroscopy a powerful tool for identifying elements and understanding the physical conditions in various astrophysical objects.

🌟 Example:

In a hydrogen atom, an electron dropping from n=3 to n=2 emits a photon at 656 nm — this is the H-alpha line in visible light.

In X-ray astronomy:

• We observe bound-bound transitions in highly ionized atoms, like Fe XXV, Fe XXVI, Ni XXVII, etc.
• These produce sharp spectral lines, such as the famous Fe XXV w-line at ~6.7 keV.

⚙️ How It Works:

• An electron is excited by collisions or absorption of a photon.