to 8 solar masses. The star is supported against gravitational collapse by electron degeneracy pressure.

XMM-Newton: ESA’s X-ray Multi-Mirror mission, named after Sir Isaac Newton, is an orbiting X-ray observatory launched in 1999. It features a large collecting area for X-ray spectroscopy.

X-ray: Light with wavelength considerably shorter than visible light, roughly ~100 Å to ~0.01 Å, often referred to by the equivalent energy, 0.1 keV to 1 MeV, or frequency range log ν ~16.5 to 20.5 Hz.

X-ray polarimetry: The measurement and interpretation of the polarization of transverse electromagnetic waves, at X-ray wavelengths. The main physical mechanisms producing polarization in the X-ray band are thought to be synchrotron radiation (primarily from blazars), Compton scattering from accretion disks, radiative transfer in strong magnetic fields, and geometrical effects due to light bending in strong gravitational fields.

X-ray spectroscopy: The study of the distribution of X-ray intensity as a function of energy, used to analyze the characteristics of the emitted radiation and chemical composition of the gas.

Zodiacal light: Scattered light from dust particles orbiting the Sun, the dominant source of mid-infrared sky brightness seen from Earth.

Zody or zodi: See Zodiacal light.

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