Astronomical Science

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Last Updated: Sunday, 09 April 2023 22:27

Published: Sunday, 26 March 2023 23:48

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Astronomical Science

I'm including Astronomical Science as it's included in US middle School Curriculum. Astronomy is the study of planets and the universe. It's good to know about it.

Khan Academy videos:

Middle School Earth and Space Science: https://www.khanacademy.org/science/middle-school-earth-and-space-science

Cosmology and Astronomy (unit 1, 2 and 3): https://www.khanacademy.org/science/cosmology-and-astronomy

Most of the material below is from Khan Academy lectures above, with supplementary material from elsewhere.

Units of time:

Since we'll be talking in units of billions of years, it's better to have a unit for large number of years. One commonly accepted in Ga (Giga annum) or Gy (Giga years). All of these refer to 1 Billion years (giga in science means 10^9 or billion). Ga is the most commonly used. Gya (Giga year ago) is used to refer to timeline in Billions, while Mya (Million year annum) is used for millions.

Big Bang

Assuming, big bang is what happened, we start from there. Big Bang Theory => https://en.wikipedia.org/wiki/Big_Bang

According to "Big Bang Theory" widely accepted, a big bang (explosion) happened 14 Billion years ago (14 Gya) from one point. That is considered the beginning or the birth of the Universe. Big Bang Theory is the best theory out there explaining the birth of Universe, but also sounds very unimaginable. Sal explains on Khan Academy on how this Universe is expanding, and has no end or edges, as it can be considered to be a 4 dimensional sphere with 3 dimensional surface, which is hard to imagine. But the question that begs is what was there before big bang? Well, the theory says that there was nothing before big bang, because there was no time. Time started when "Big Bang" happened (i.e time=0 was when Big Bang happened). Where did this matter come for "Big Bang" to happen? Physicists claim that something came out of nothing, and after billions of years, when the last star dies, everything will get into a black hole with nothing left. So, we started from nothing and will go back to nothing. There's a good article on bbc over here: https://www.bbc.com/future/article/20220105-what-existed-before-the-big-bang

Starting from Big Bang, a full chronology of events is provided here: https://en.wikipedia.org/wiki/Chronology_of_the_universe

During the first microsecond of Big Bang, a substance called Quark-Gluon Plasma was the only matter that existed. First the plasma that consisted of quarks and gluons was separated by the hot expansion of the universe. Then the pieces of quark reformed into so-called hadrons. A hadron with three quarks makes a proton, which is part of atomic cores. Neutrons also formed at this time.

The very first elementary particles formed are explained via the standard model here: https://en.wikipedia.org/wiki/Standard_Model

The standard model is theory used to describe 3 fundamental forces out of the 4 fundamental forces that exist. They also describe elementary particles which are sub atomic and are used to form components of atoms. There are two fundamental classes of subatomic particles: fermion and boson

1. Fermion: Fermion is a particle that follows Fermi Dirac statistics. These have one half integer spin as 1/2, 3/2 etc. Fermions have 12 elementary particles of spin 1/2. They have antiparticle too for each of these 12 particles. All ordinary matter that exists in universe is made of Fermions. Fermions are classified according to how they interact. There are 2 categories:
   1. Quark and antiquark: The defining property of quarks is that they carry color charge, and hence interact via the strong interaction. There are 6 quarks, and 6 anti quarks. 6 quarks are up, down, charm, strange, top and bottom. Quarks can form color-neutral composite particles called Hadrons that either contain a quark and antiquark (mesons) or 3 quarks (baryons). Protons and neutrons are the lightest baryons that we see in atoms.
   2. Lepton and antilepton: Leptons do not have color charge. There are 6 leptons and 6 antileptons. Out of 6 leptons, 3 of them carry charge (electron, muon and tau) and hence interact electromagnetically. The remaining 3 (neutrino versions of charged ones - i.e electron neutrino, muon neutrino and tau neutrino) do not carry electric charge either, so their motion is directly influenced only by the weak nuclear force and gravity, which makes them notoriously difficult to detect.
2. Boson: Boson is a particle that follows Bose-Einstein statistics. These ave integer spin as 0, 1, 2, etc. Bosons, unklike fermions, don't make up matter but instead are force carriers, which either give rise to forces between other particles, or in one case give rise to the phenomenon of mass. There are 5 elementary bosons.
   1. Scalar Boson (spin=0): Higgs Boson is the only particle here. It gives rise to the phenomenon of mass via Higgs mechanism.
   2. Vector Boson (spin=1): These are Gauge bosons, and act as force carrier. There are 4 of these => Photons, Gluons, neutral weak boson and charged weak bosons (2 types)

Within about 3 minutes after the Big Bang, conditions cooled enough for these protons and neutrons to form hydrogen nuclei. This is called the era of nucleosynthesis. Some of these nuclei combined to form helium as well, though in much smaller quantities (just a few percent). But after about 20 minutes, nucleosynthesis ended and no further nuclei could form. The problem at this point was that electrons couldn’t stay in orbit around any atomic nucleus because of the immense heat and radiation still flooding the universe. Shortly after any neutral atoms would form, they were knocked apart again by energetic radiation. 

Galaxies and Stars:

At around 100,000 years, after the neutral helium atoms form, helium hydride is the first molecule. (Much later,  hydrogen and helium hydride react to form molecular hydrogen (H2) the fuel needed for the first stars) Finally, after 380,000 years or so, the universe had again expanded and cooled enough for conditions to favor electrons staying in orbit around atomic nuclei. This is when recombination occurred — neutral hydrogen (and helium) finally appeared (along with traces of lithium, Beryllium and Boron) because they could “recombine with” (hold on to) electrons without easily losing them to stray radiation. Hydrogen was 75%, Helium was 25% and other atoms were in negligible quantity. This is also the time when the cosmic microwave background was generated, because the atoms that formed entered their lowest energy state quickly after, releasing excess energy in the form of photons that could finally travel freely through the universe without knocking into anything along the way. This is the light that makes up the cosmic microwave background. From 370,000 years until about 1 billion years, the clouds of hydrogen collapsed very slowly to form stars and galaxies, so there were no new sources of light.

Birth of stars:

Stars were a mystery until early 20th century. Scientists were puzzled onto how energy is created in stars.  Gradually they figured out that stars are initially a big cloud of Hydrogen. These Hydrogen atoms being very close to each other, start fusing into one mass under this very high pressure and high temperature of 10 millions of Kelvin. Over time, the core of the star getting more hot, starts fusing 4 Hydrogen atoms (1 proton, and no neutron) into Helium atoms (2 protons and 2 neutrons). This is the fusion reaction where some mass is lost, which is converted into energy as per Einstein's equation E=mc^2. The core then becomes smaller (as Helium atoms pack neutrons and protons even closer than what hydrogen packed protons in), even hotter (100M Kelvin) and then helium fuses into even heavier elements as carbon, oxygen, etc. Intermediate heavier elements are made too, but carbon and oxygen are predominant among the heavier elements. Many stars are made of about 90 per cent hydrogen, with most of the remainder being helium and a very small fraction of heavier elements. 

From the end at about 1 billion years (i.e 13Gya), the universe gradually transitioned into the universe we see around us today, but denser, hotter, more intense in star formation, and more rich in smaller (particularly unbarred) spiral and irregular galaxies, as opposed to giant elliptical galaxies. Galaxy is a system of millions or billions of stars, together with gas and dust, held together by gravitational attraction. The majority of giant galaxies contain a supermassive black hole in their centers, ranging in mass from millions to billions of times the mass of the Sun. The black hole mass is tied to the host galaxy bulge or spheroid mass.

There are fundamentally two types of galaxies.

1. Blue star-forming galaxies =>These are groups that are more like spiral types. When a galaxy forms, it has a disk shape and is called a spiral galaxy due to spiral-like "arm" structures located on the disk. Spiral galaxies are quite thin, dense, and rotate relatively fast. The galaxy that our solar system is in is called Milky way, and is a Spiral galaxy
   
2. Red non-star forming galaxies => These are more like elliptical galaxies. The stars in elliptical galaxies have randomly oriented orbits.
   

Hertzsprung–Russell diagram (or H–R diagram)

We saw above that dense regions within molecular clouds, collapse and form stars. Hydrogen fuses to form Helium in the core, and then hydrogen is only at the surface. These stars get much cooler at the surface, but huge at the same time. Stars go thru phases where they burn brightly at birth to just being a core of elements at their death. The fate and journey also depend on the size that it started with. Observations indicate that the coldest clouds tend to form low-mass stars, observed first in the infrared inside the clouds, then in visible light at their surface when the clouds dissipate, while giant molecular clouds, which are generally warmer, produce stars of all masses.

There's H-R diagram of all the stars that was developed by Hertzsprung and Russell in early 1900's. It is a scatter plot of stars showing the relationship between the stars' absolute magnitudes or luminosities versus their classification or temperatures. Each star is represented as a point with an x-coordinate of effective temperature and a y coordinate of luminosity. Very big stars called supergiants have very high luminosity and high temperature. White dwarfs are stars which have low luminosity but high temperature. Main sequence stars are the stars that are still active in their lifecycle, and that is where most of the stars are. Sun is a main sequence star with a luminosity of 1, and temperature of 5000 Kelvin. 

Here's the wiki link => https://en.wikipedia.org/wiki/Hertzsprung-Russell_diagram

This is the HR plot => https://en.wikipedia.org/wiki/Hertzsprung-Russell_diagram#/media/File:HRDiagram.png

Red Shift:

The universe at the beginning was very hot and very compact, and since then it has been expanding and cooling down. To prove that Universe is expanding, we should see a red shift of light emitted from objects such as distant stars. Red shift of visible light happens when light sources are moving away from you at very fast speeds. Red shift is exactly what we see which proves the "Expansion of Universe". Other Observation that proves "Big Bang" and expansion of Universe is the "Cosmic Microwave background radiation" that we observe coming from all directions. These are light waves emitted shortly after the birth of Universe, when photons emitted could travel. These photons are coming from all directions to earth, but because these light sources are moving away from us, we see  massive shift in Frequency, and instead of being red, they go even more into low frequency range, and appear as radio waves.

Hubble's Law:

Hubble came up with a law of how fast Universe is expanding. As of now, it says that things are moving at speed of 70.6km/s per megaparsec (1 megaparsec is 3 million light years) . i.e things that are 1 megaparsec away are moving away at 70.6km/s, while things which are 2 megaparsec away are moving at 141.2km/sec and so on. So, all things in universe are moving further and further away from each other.

According to wikipedia, the actual estimate for the radius of the observable universe is 14000 Mpc (or 14 Gpc), so the velocity of the expansion at this distance would be 14000 * 70.6 = 988,400 Km/s! Way beyond the light speed limit! If we wanted to know the distance that an object in space must have in order to move away at the speed of light (rounded at 300,000 km/s), we should do:
300,000 / 70.6 = 4,249,291,784 parsec (4.29 Gpc)-> approximately 14 billion light years. May be that is how Huble came up with this Constant so that start time is at 14 billions years ago, as nothing can move faster than speed of light.

Formation of Earth:

At around 4.5Gya, earth was formed, though mostly molten, and with some atmosphere but no oxygen. Over time, the Earth cooled, causing the formation of a solid crust, and allowing liquid water on the surface.

Life started forming around 4Gya. More details about evolution of life are in "Classification of Life" in Biology section. Around this time is when Moon is supposed to have been formed by Earth's collision with a giant planet sized body called Theia. In Astronomy section, we will focus on how different bodies in space evolved from Big Bang to until now.