ASTROPHYSICS

CLASSIFICATIONS OF STELLAR BODIES

Rogue planets - worlds without suns, molten at the core, frozen at the surface and thus there may be life somewhere inside

Comets - leftovers from the formation of the solar system - grouped in an Oort cloud, which encases the solar system

Stellar nurseries - places where there is so much energy and temperature that stars are all born there

ASTRONOMERS & ASTROPHYSICISTS

Ptolemy - Egyptian; first person to give a model of the solar system, famous for preserving Hipparchus’s star catalogue; his model was geocentric
Copernicus - Polish; first heliocentric model of the solar system - his model showed the sun at the centre, with all the planets orbiting around it in perfect circles, and the stars were fixed. This was incorrect because it assumed uniform circular motion
Brahe - Danish; sun orbits the earth, all the other planets orbit around the sun
Kepler - German; introduced law of ellipses (elliptical orbits) and 3 laws of planetary motion. The three laws of planetary motion are still valid/applicable today. He also said that there were 2 foci (focus pl.) in every ellipse, and the sun was at one of the two foci for each planet’s orbit.
Galileo - Italian; designed refracting telescope, defended Copernician model of heliocentric solar system. Proved that the surface of the moon and the sun was not perfect/unchanging, discovered Jupiter’s moons (hence Galilean moons), discovered phases of Venus which disproved the Ptolemaic model.
Newton - English; proved Kepler’s laws, developed law of gravitation.

THE EARTH'S MOON

Lunar eclipse - when the earth is directly between the sun and the moon so no light shines on the moon; happens almost every 29-day lunar cycle

Solar eclipse - when the sun, earth and moon directly align so that the moon covers the sun completely; UV light still shines so in order to look at the eclipse you must wear protective shades

The moon is 384,400 km away from the earth. The gravitational field strength of the moon is 1.6N/kg.

Luminous objects - objects which emit their own light; reflective objects - objects which allow light to bounce off of their surface. Both of these kinds of surfaces/objects allow us to see things because, either way, light hits our eyes.

UNITS OF MEASUREMENT

AU (astronomical unit)
- 1 AU = 150 million km (on average)
- 1 AU is measured as the average distance from the center of the earth to the center of the sun (average because the earth’s orbit is elliptical so it doesn’t always stay the same)
lightyear
- 1 lightyear = 9.46 × 10^12 km
- 1 lightyear is a measure of astronomical distance; it is the distance travelled by light in one year.

OUR COSMIC ADDRESS

Planet Earth
Solar System
Milky Way Galaxy
Local Group
Virgo Cluster
Virgo Super-cluster
Observable Universe

Observable universe - the spherical region of the universe comprising of all matter that can be observed from Earth, space-based telescopes, or exploratory probes; electromagnetic radiation from this matter has had time to reach the Earth and our solar system since the beginning of the universe, therefore it can be observed in some way.

The local group is our group of galaxies - Milky Way and Andromeda and smaller ones. The observable universe is defined as different from the unobservable universe because the light of the unobservable universe hasn’t reached us yet.

LIFE CYCLE OF A STAR

In the (relative) beginning of the universe's existence, planets and stars were formed due to gravitational force pulling hydrogen and helium together. Electrons can get so charged due to extremely high temperatures that they release excess energy in the form of photons - this is how light was first created, 200 million years after the formation of the universe. Further planets formed because of the repeated collisions of asteroids and comets. Suns and similar stars formed from the gas and dust remnants of supernovae.

In the center/core of every star, nuclear fusion is taking place. This results in a large amount of fuel being produced within the star. The mass of a star determines the speed of the fuel-burning as well as the end of their life cycle; smaller stars (like our sun) burn fuel slower, will last for several billion years, and end in a white dwarf & larger stars burn fuel faster, therefore are short-lived and will end in neutron stars.

Hydrostatic equilibrium - when external forces like gravity are balanced by a pressure-gradient force (e.g. inward gravitational force vs outward thermal pressure due to nuclear fusion)

Protostar - a star that has not started producing helium via hydrogen nuclear fusion; basically a pre-star

Stellar nebula - gravitational force of clouds of helium, hydrogen, space dust and plasma causes them to aggregate and collapse in on themselves onto a number of ‘cores’
Stellar ignition - nuclear fusion occurs in the core of the protostar due to extremely high temperatures
Main-sequence - 90% of a star’s lifetime in which the core is fusing hydrogen into helium via nuclear fusion and it remains in hydrostatic equilibrium
Red giant/supergiant - when hydrostatic equilibrium is broken due to less energy being produced in the core, the nuclear fusion reactions will begin to move outwards to the surface of the star, which causes outer shells to expand and change color; meanwhile, gravitational force of the star itself will cause the core to collapse inward and shrink - this causes increase in core temperature so that nuclear fusion starts again, this time turning helium into carbon.
Planetary nebulae - due to hydrogen-helium nuclear fusion on the surface in the red giant phase, when new helium shells gravitate towards the core and are ignited, the surge of energy blows off outer shells as planetary nebulae.
White dwarves (for low-mass stars) - the core of a star remaining at the end of planetary nebula events is extremely hot and white, consisting mainly of oxygen and carbon; it produces massive amounts of UV and thermal radiation and will eventually burn out/cool down since it no longer has any nuclear reactions occurring within it.
Supernova (for high-mass stars) - when hydrostatic equilibrium is broken for a high-mass star and the star runs out of nuclear fuel, the outer layers collapse into the core and are then expelled at extremely high force due to a nuclear explosion; remaining shockwaves cause an expanding shell of gas and dust, while the core becomes a neutron star, pulsar (if it is spinning), or a black hole.

COMPOSITION OF STELLAR BODIES

Most stars, for the majority of their existences, consist of 71% hydrogen and 27% helium, with the remaining percentage being heavier elements. The same applies for gas giant planets. On the other hand, terrestrial planets (which are comparatively rare) such as the Earth are composed of rocks (silicates) and metals (iron). The chemical composition of moons can be similar to that of terrestrial planets, or similar to the cores of the planets they orbit.

Spectrometer - a device capable of concentrating sunlight through a prism via a narrow slit; prism creates a spectrum of colorful light based on wavelength intensity. Dark lines interrupting the resulting spectrum can be used to identify atmospheric chemical composition of stars.

Spectroscopy - the process of using spectrometers

Fraunhofer lines - dark lines appearing in diffraction spectra by absorption of energy by the photons (light-carrying particles) in the ionic clouds of gases and other elements (e.g. hydrogen) around stars; each element absorbs light at specific wavelengths - scientists can use this to determine what elements are present.

Wein’s law - the peak wavelength radiated by a star is inversely proportional to the temperature on the surface of the star, which also determines the color of the star

Planck’s law - describes the spectral density of the electromagnetic radiation (visible light etc.) emitted by a blackbody with minimal external factors like interstellar dust etc.

Blackbody - an idealized physical body that absorbs all electromagnetic radiation and emits thermal radiation instead - some stars like our sun are only a close approximation of a blackbody.

Dispersing electromagnetic radiation from stellar objects through a red and blue filter and using Wein’s law is a method used to determine the temperature or brightness of a star. However, since Wein’s law only applies to bodies with a Planck spectrum computer modelling needs to be used to compare the ratio to ratios of other bodies to accurately determine the temperature.