Wednesday, March 3, 2021

GK Persei. a bright nova

A nova (latin for new) is the high and rapid increase in the brightness of a star. This term is used, because novae come from stars that are too dim to be seen with the naked eye. Despite this, a nova’s brightness can exceed that of the brightest star in the sky (i.e., Sirius). All novae involve a white dwarf in a binary system, and they are divided into three types. These are known as classical novae, recurrent novae, and dwarf novae.

The most common type is the classical nova, which evolves a white dwarf and a main sequence, subgiant, or red supergiant companion. This happens when the system becomes very close, having an orbital period that ranges from a day to a few days, thus the white dwarf can accreet material from its companion. This creates a dense, shallow atmosphere on the white dwarf, consisting mostly of hydrogen. When the atmosphere reaches a critical temperature, it ignites leading to a rapid fusion, increasing the brightness of the white dwarf, forming a “new star” in the sky. Recurrent novae are objects that experience multiple nova eruptions, while dwarf novae are dimmer and repeat more often than classical novae.

GK Persei (abbreviated to GK Per and also known as Nova Persei 1901) was a bright nova, located at a distance of around 1,440 light years in the constellation of Perseus. It was discovered on the 22nd of February 1901, by Thomas David Anderson.

GK Per is surrounded by the Firework Nebula, which is a nova remnant that was first detected in 1902. It consists of an expanding cloud of gas and dust, that is moving with a velocity up to 1,200 km/s.

GK Per had a magnitude of 2.7 when it was discovered, and it reached a maximum of 0.2, making it the second brightest nova of modern times (the brightest is Nova Aquilae 1918). During the early 20th century, GK Per faded down to magnitude 12-13. Since then, it began displaying infrequent outbursts of 2-3 magnitudes. From the 1980s onwards, such outbursts became quite regular (i.e., every three years) and typically they last around two months.

This suggests that the behaviour of GK Per has shifted from that of a classical nova, to something that resembles a typical dwarf-nova.

Image: Composite image of GK Persei and the Firework Nebula. The image was created using X-ray data (blue) from Chandra X-ray Observatory, optical (yellow) from Hubble Space Telescope, and radio (pink) from the Very Large Array. X-ray data show emission due to super hot gas, while optical data show the presence of clumps of material that was ejected from the explosion. Finally, radio data show emission from electrons that have been accelerated to high energies, by the shockwave that the nova eruption created. On the left of the image there is a point-like source, whose nature is unknown.

Image Credit: X-ray: NASA/CXC/RIKEN/D.Takei et al; Optical: NASA/STScI; Radio: NRAO/VLA 

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