Celestial Bodies: From Planets to Neutron Stars
The Pythagorean model placed a central fire, around which the Earth, Moon, Sun, and planets revolved.
Geocentric Model
Aristotle proposed a geocentric model where the Earth is the center of the universe, and stars revolve around it. Retrograde motion refers to the apparent clockwise rotation of a body when viewed from above the solar north pole. Epicycle models were geometric models designed to explain variations in the speed and direction (retrograde motion) of the Moon and planets.
Heliocentric Model
The heliocentric model posits that the planets revolve around the Sun, which is the center. Scientists who supported and improved this model include Copernicus, Giordano Bruno, Tycho Brahe, Galileo, and Kepler. Kepler proposed the following laws:
- First Law: All planets move around the Sun in elliptical orbits, with the Sun located at one focus.
- Second Law: The radius vector from the planet to the Sun sweeps out equal areas in equal times.
- Third Law: For any planet, the square of its orbital period (time taken for one revolution around the Sun) is directly proportional to the cube of the average distance to the Sun.
Stars
A star is a massive sphere of gas, isolated in space, that produces energy in its interior. This energy is transported to the surface and radiated into space in all directions.
The Sun
The Sun is a glowing sphere of gases, with a core believed to be liquid. The Sun formed approximately 4.65 billion years ago and has enough fuel for another 5 billion years. It will then begin to grow stronger and larger, eventually becoming a red giant. Finally, it will collapse under its own weight and become a white dwarf, which may take trillions of years to cool.
Galaxy
A galaxy is a massive system of stars, gas clouds, planets, dust, dark matter, all bound together gravitationally.
Nebula
Nebulae are regions of the interstellar medium consisting of gas and dust.
Black Hole
A black hole is a finite region caused by a large concentration of mass in its interior, with a huge increase in density. This generates a gravitational field so strong that no material particle can escape from that region.
Novas and Supernovas
Novas and supernovas are stellar explosions that can manifest remarkably, even becoming visible to the naked eye where nothing particular was previously detected in the celestial sphere. They are divided into two categories: novae (less luminous) and supernovae (more luminous).
Neutron Star
A neutron star is a stellar remnant left by a super-giant star after it exhausts its nuclear fuel core and explodes as a supernova. These stars are composed mainly of neutrons, with other types of particles in both their solid iron crust and their interior, which can contain protons, electrons, pions, and kaons.
Red Giant
A red giant is a star that, after consuming the hydrogen in its core during the main sequence stage, begins to burn hydrogen in a shell around the inert helium core. This first effect is an increase in the star’s volume and a cooling of its surface, causing its color to turn red. At a certain point, the atmosphere of the star reaches a critical minimum temperature, forcing the star to increase its brightness and volume while maintaining a nearly constant surface temperature (i.e., color). The star swells up to a typical radius of about 100 million km, thus becoming a red giant.
White Dwarf
A white dwarf is a stellar remnant that is generated when a star with a mass smaller than 9-10 solar masses has exhausted its nuclear fuel.
Binary Stars
A binary star system is composed of two stars orbiting each other around a common center of mass.