What is the Karman line?

tl;dr; The Karman line defines the edge of space.

It is set to 100km above the Earth’s mean sea level and separates aeronautics from astronautics. So to be an astronaut you have to travel higher than 100km from Earth’s surface.

Why is that?

To fly with a conventional airplane you need the air to generate lift under its wings. The thinner the airs gets the faster the plane has to go to generate the same amount of lift to keep the plane up. The lift generated by wings is directly proportional to the air density.

The faster the plane gets the more drag of the atmosphere on its hull is generated. Therefore the temperature of the hull increases and you will run into thermal issues with the material.

The second consideration is the oxygen you need for combustion in the plane’s engines. When you are too high you cannot use conventional turbines anymore. You have to switch to rocket engines and fuel up the plane with not only propellant but also oxygen in its tanks.



What is LEO?

LEO stands for Low Earth Orbit.

It ranges from 200km to 2000 km.

The ISS orbits the earth between 370km and 460 km. Due to atmospheric drag the orbit decreases over time

The Hubble Telescope orbits a bit higher at 545km to 549km

Other orbits


Satellite Orbit
ISS 400km
Hubble 545km
Starlink 550km
Iridium 780km
Globalstar 1.400km
ICO 13.390km
Inmarsat 35.768km
Sirius 23.975km – 46.983km

What is the Harvard spectral classification?

1901 Annie Jump Cannon constructed this classification:

Class Temperature in K Color
O >= 30000 blue
B 10000 – 30000 blue white
A 7500 -10000 white
F 6000 – 7500 yellow white
G 5200 – 6000 yellow
K 3700 – 5200 light orange
M 2400 – 3700 orange red

Mnemoic: Oh, Be A Fine Galaxy, Kiss Me

What is Beta decay?

Beta decay is the process where a nucleon decays into either an electron or a positron.

β decay (electron emission)

n → p + e + ν

A neutron decays into proton, an electron and an anti-neutrino.

β+ decay (positron emission)

p → n + e++ νe

A proton decays into a neutron, a positron and an electron neutrino.