Some stars emit X-rays when they are very hot, such as neutron stars or binary stars containing a white dwarf. These extreme temperatures produce high-energy X-ray radiation.
Stars emit X-rays when their heated material reaches very high temperatures, typically several million degrees. At these temperatures, electrons are stripped from atoms: the gas then becomes a plasma, composed of charged particles that move at very high speeds. When these particles interact, through collisions or when they follow spiral paths around magnetic field lines, they release enormous energy in the form of highly energetic photons: X-rays. The hotter and more turbulent it is up there, the more intense the emitted X-rays are: this is exactly what happens in the turbulent regions of massive stars or in the violent interactions of compact binary systems.
Some stars stand out for their ability to pump insane amounts of energy in the form of X-rays. For example, neutron stars, ultra-compact and dense, can violently accelerate matter trapped by their gravitational field, thus emitting powerful bursts of X-rays. White dwarfs in binary systems can also be very active in terms of X-rays, as they siphon material from their partner star, causing extreme heating. Young stars like T Tauri stars are also well-known for their intense X-ray emissions due to very strong magnetic fields in their atmosphere. Finally, stellar black holes accompanied by a partner star are champions in every category: by gradually devouring the accreted material, they heat this gas to enormous temperatures, leading to a violent discharge of X-ray photons.
When two stars orbit very close to each other, things heat up seriously between them. This system, called a close binary, often pulls material from one star to its neighbor. The accreted material gradually forms an extremely hot accretion disk around the receiving star. This phenomenon releases a tremendous amount of energy and heats the material to the point that it eventually emits intense X-rays. The same occurs during violent events, such as when a compact star — like a neutron star or black hole — suddenly siphons off material from its partner. The stronger the gravitational pull, the more intense the transfer, and the hotter it gets. The result: a true fireworks display in X-rays.
The magnetic fields of stars play a fundamental role in the creation of X-rays. When they become tangled or collide, they release a large amount of energy suddenly in the form of flares (sudden and powerful eruptions). These eruptions heat the surrounding gas to very high temperatures, reaching several million degrees, which then leads to the emission of X-rays. The more chaotic and intense the magnetic field at the surface of the star, the more numerous these explosive phenomena will be. Some young or very active stars, with particularly intense and turbulent magnetic fields, then become true beacons of X-rays, easily detectable by specialized telescopes.
In recent years, X-ray astronomy observations have significantly advanced thanks to new, more equipped and precise space missions. The eROSITA satellite, launched in 2019, has mapped the entire sky in X-rays with unprecedented resolution, revealing over a million previously unknown sources, such as galaxy clusters, neutron stars, and active black holes. NICER, an instrument aboard the International Space Station, has provided ultra-precise measurements of neutron stars, even giving us a better idea of their exact size and composition. As for the Chandra space telescope, a veteran still active after more than twenty years of operations, it has recently detected unexpected X-ray emissions from ordinary stars, suggesting the possible existence of powerful magnetic phenomena that are still poorly understood. Thanks to these advancements, we are discovering that the X-ray universe continues to hold new surprises for us.
Some nearby binary stars exchange matter through a phenomenon called 'accretion'. This transfer produces an intensely heated zone at several million degrees Celsius, resulting in a powerful emission of X-rays.
The Sun, although moderate, also emits X-rays: they primarily come from the solar corona, a thin atmospheric layer located above the visible surface, reaching temperatures of about one million degrees Celsius.
The Chandra Space Telescope, launched in 1999, is specially designed to detect X-ray emissions from stellar phenomena, allowing the observation of extremely hot regions of the universe that are inaccessible to the human eye.
The first astronomical sources of X-rays discovered in the 1960s came from neutron stars and black holes, opening up an entirely new field: X-ray astronomy.
Yes, although stars similar to the Sun emit relatively few X-rays, they still generate some at a low level due to their magnetic activity. During phases of high solar activity (such as solar flares and sunspots), X-ray emission can become more pronounced.
No, only certain stars emit significant X-rays. This emission often comes from very hot, young, or massive stars, binary systems, or intense gravitational and magnetic interactions that accelerate high-energy particles, resulting in X-ray emission.
X-ray emissions allow astronomers to study extreme phenomena such as very strong magnetic fields, violent stellar winds, and the presence of compact objects (neutron stars, black holes) in binary systems. This also provides valuable clues about stellar evolution.
The Earth's atmosphere absorbs X-rays, making it impossible to detect them from the Earth's surface. Therefore, the only effective way to observe these rays is to use telescopes in orbit around our planet, such as the Chandra Space Telescope.
Generally no, because X-ray emitting stars are located at very great astronomical distances, making their impact negligible on Earth. Moreover, the Earth's atmosphere effectively protects against external X-rays.
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