Astrophysics 

SOLAR SYSTEM: Our local neighbourhood in space. The gravitationally bound system consists of the sun and its orbiting objects.

PLANET: A celestial body moving in an elliptical orbit around a star. 
→ To be a planet, the body cannot have any debris in its orbit, and it must have a regular orbit. 

DWARF PLANET: An object orbiting a star that is massive enough to be rounded by its own gravity, yet has not cleared the neighbourhood of other objects and is not a satellite. (Aren't big enough to be planets)

MOON: A natural satellite that orbits planets. 

COMET: A body of rock and ice that follows an elliptical (regular) orbit around the sun. 

ARTIFICIAL SATELLITE: An artificial body placed in orbit around Earth or the moon/another planet to collect info/sat communication. 

MILKY WAY: A galaxy containing at least 100bn. The sun is among the stars.

GALAXY: A large collection of billions of stars.

UNIVERSE: All existing matter in space is considered as a whole. Large collection of billions of galaxies.

Orbits:

The gravitational force exerted by a larger body (Earth) on an orbiting object (moon) is always attractive. 
→ Gravitational force always acts towards the centre of the larger body.
→ Gravitational forces cause the body to move.
→ Stronger the force and/or instantaneous velocity needed to balance. 
→ The closer to the planet in orbit, the faster you need to go to maintain orbit.
→ Faster objects will move in orbit with a smaller radius.
- The larger force causes the body to move faster as it is slightly closer when smaller objects orbit.

Orbits of Planets:

- Similar orbits: All made in elliptical orbits in the same plane and all travel in the same direction around the sun (anticlockwise).
- Differences: All orbit at different distances (cm sun) and different speeds, and all take different amounts of time to orbit the sun.

Orbits of Moons:

- Moons orbit planets in circular paths (some have more than one moon).
- The closer the moon is to the planet, the shorter the time it'll spend orbiting and the greater the speed of the orbit.

Comets:

- Highly elliptical orbits.
- Not all comets orbit in the same plane as planets.
→ Some objects do not orbit in the same direction.

- Fastest when closest to the sun because there is a force that the comet is pulled by.
- The increased pull of gravity causes it to speed up.

Gravity, Speed, and Orbits:

- A gravitational field is a force field that attracts other mass-containing bodies.
- The gravitational field around the Earth exerts a force on objects towards the centre of the Earth.
- Gravitational field strength at a point in a gravitational field is the force acting on a 1 kg mass placed at that point. Unit = N/kg.
→ Another name: Acceleration due to gravity (m/s²)
 W = mg (g on the Earth = 10 N/kg) 

- The gravitational field around the planets creates a gravitational force causing the orbital motion of planets and comets around the sun and moons and satellites around planets.

- Comets have highly elliptical orbits.
- Low orbit:This orbit has a duration of approximately 128 minutes and is situated closer to Earth in the sky.

Life cycle of a Star:

1. The nebula starts to collapse towards its centre as its own gravity contracts.
2. A protostar forms when friction from collapse produces heat.
3. When the temperature is high enough, thermonuclear fusion begins.
4. The star is on the main sequence, and the outward pressure from gravity balances the outward pressure from fusion (as hydrogen fuses for millions of years).
- Creates energy that powers every star.

- Our sun exhibits a delightful equilibrium between the outward pressure from energy, the inward pressure from fusion, and the inward pressure from gravity, which leads it to desire self-collapse. 
- Describing stages of evolution at each stage, comment on:

→ Contracting: Gravity > force from fusion comment on contracting + stable 
→ Stable = balanced

→ Expanding - Force from fusion > gravitational force

Waves in Space:

- Stars emit light of all colours and radiation our eyes can't see, such as UV and IR. 
- Elements in the gases surrounding the star absorb specific wavelengths of light, resulting in these dark lines in the spectrum from the star.
- In a laboratory on Earth, a specific element consistently produces the same pattern of lines in the same location in the spectrum.

- Observers looking at an object moving away from them see the light has a longer wavelength than when it was emitted → redshift (low f)
- Observers looking at an approaching source see the light shifted to a shorter wavelength → Expanding: Force from fusion > gravitational force blueshift (high f).
- When a galaxy moves away from us, it experiences redshift as the wavelengths increase in an expanding universe.
→ Expanding: Force from fusion > gravitational force. The observation of increasing redshift with distance is a reason to believe in an expanding universe.
- More distant galaxies have greater redshift, meaning they're moving away faster than nearer ones.

Big Bang Theory:

- The universe originated from an incredibly hot and dense state 13.7 billion years ago and has been expanding and cooling ever since. 
- If the universe is expanding, it implies that everything was closer together at some point in the past. This point is called a 'singularity,' where the universe is infinitely hot and dense, leading to a creation event causing the existence of the universe. 

Density of matter decreases over time:

- CMBR is proof for the Big Bang Theory, as total measurements showed slight variations, and the radiation matched predictions made using the Big Bang model. 
- CMR theory is widely accepted by scientists.
- This implies all parts of the universe were in contact at one point; the universe was very hot, and the wavelength initially was very dense. Over time, the wavelength decreased.

1. Initially, all matter in the universe occupied a single point.
2. This tiny space was very dense and hot in the initial explosion.
3. Single point exploded → Big Bang CMBR rests over the energy of the initial explosion.
4. Space started expanding, and expansion is still going on.

Hubble's Law:

- The further away a galaxy is from us, the faster it appears to be moving away →  Hubble's Law
- The universe is expanding: When we look at distant galaxies, they appear to be moving away from us.

- Hubble proved all galaxies are moving away from each other → the average distance between galaxies increases, and → the universe is expanding over time.
→ This serves as proof for the Big Bang theory, which states that everything is expanding.​​

Hertzsprung-Russell Diagram:

- The lower the absolute magnitude, the brighter the star.

- Graph that illustrates the relationship existing between the average surface temperature of stars and their absolute magnitude: how bright they'd appear to be if all the same distance away.
- Luminosity: Measures how much energy leaves a star per second (power output of star).
- Generally, for stars that are at equal distances from the Earth, the more luminous, the brighter they appear.
→  Luminosity is affected by temperature and size.
→ Most luminous = Largest and hottest
→ Least luminous = Small and cold
- The colour of the star is determined by its surface temperature, while looking at the diagram, try to link to it.
- Blue: 40,000°C; white, 10,000°C; yellow, 6,000°C; orange, 4,500°C; red, 3,000°C

Artificial Satellites:

(TV + phone calls) + (weather + photography) 
Uses: Communication, Earth observation, navigation (GPS), and astronomy (photography) 

Artificial Satellite Orbits: Polar and Geostationary

- Polar orbits: Take the satellite over Earth's poles, travel very close to Earth (nearly 200 km above sea level), and travel at very high speeds nearly 8000 m/s, used for weather and Earth imaging as more gravitational force.
→ Higher resolution/more detailed images.
→ Astronauts are used for weather-earth mapping.
- Atmospheric drag causes a loss of orbit, resulting in a shorter lifespan.

- Geostationary orbits: These satellites follow Earth's rotation and take 24 hours to orbit the planet, appearing to remain in the same part of the night sky when viewed from the ground.
→ Much higher ≈ 36 000 km, so travel more slowly ≈ 3 km/s. 
→ Used for communications and satellite casting.

The advantage of LEO for weather and broadcasting is that it is closer to Earth and provides more detailed images.

 

- Keeps satellites moving around Earth.
→ Is too fast: Earth's gravitational force would be too weak to maintain the orbit. 
→ It is too slow: It hits Earth or goes towards Earth's atmosphere, and the gravitational force is too high to maintain orbit at this speed.