Astronomy Flashcards
What is Gravity?
Gravity is an attractive force between any 2 masses. It increases as the masses involved increase and the distance between them decrease.
What is an orbit?
The gravitational pull on an object keeps it in orbit. The force is always at 90° to the motion of the planet. The force changes the direction the orbiter is travelling in.
What is the equation for orbital speed?
2πr/t where r is the orbital radius and t is the time period to complete one orbit.
What do stars start as?
They start as a Stellar Nebula. A Stellar Nebula is a large cloud of Hydrogen gas that is left over from the death of other stars.
How is a star born?
Some event triggers the cloud to start collapsing through gravity. The centre of the cloud then heats up. Eventually, nuclear fusion starts in the hot core and the star shines.
How does nuclear fusion occur?
The state of matter in the sun is plasma, which means that the nuclei separate from the electrons. This means that there are protons drifting around. They repel, but in the core, the pressure is so great that they fuse into Helium nuclei. This releases energy.
What two forces on the star keep it stable?
The two forces are Gravity and the force exerted by nuclear fusion.
What is a star that is stably fusing hydrogen called?
A main sequence star.
For main sequence stars, what is the relationship between Brightness and Temperature?
As the star gets brighter, it gets hotter or bluer.
What happens to type K, G, F and some A stars when they run out of Hydrogen?
The core begins to collapse, briefly heating the star up. Then, a ring around the core gets hot enough to fuse Hydrogen. This forces the layers of the star out, creating a giant star. The outer layers cool and turn red. Eventually, the core gets hot enough to fuse Helium into Oxygen and Carbon.
What happens to a Red Giant when it runs out of fuel?
It begins to collapse under gravity. Meanwhile, the outer layers drift away in a Planetary Nebula. The remaining material collapses into a white dwarf which doesn’t fuse, and cools over a long period of time. Eventually, it becomes a Black Dwarf.
What happens to type O, B and some A stars when they run out of Hydrogen?
They expand and and transform into a supergiant. Most then cool down, turning them red. These stars fuse Carbon and Oxygen, and eventually ends up with Iron. It can’t fuse this, so it collapses.
What happens in a Supernova?
The Supergiant’s outer layers collapse inwards. These are then repelled by the incredibly dense inner materials. This causes an explosion called a Supernova that sends most of the material into space. The rest of the material collapses into a Neutron star or a Black Hole, depending on the star’s mass.
