What do the heavens show us and why? (v1.1)

Note: Much of this essay is a synthesis and summarization of lectures by Dr. Joshua N. Winn of Princeton University.

The Incredible Scale of the Universe

There are amazing things out in the universe that astronomers keep discovering, andastrophysicists keep analyzing and explaining. We are a small planet in the solar system and our sun is a modest star. We are on the edges of a modest galaxy called the milky way and we are in a quiet neighborhood of the galaxy.

What is the scale at work here? To get a perspective on planet size, the radius of earth is 3963 miles. Jupiter has twelve times the diameter of Earth. An astronomical unit (AU) is the average distance of earth from the sun and is about 150 million km. Neptune is 30 AU from the sun. To get a perspective on star size, the sun has 109 times the diameter of earth. The largest known star in the milky way is UY Scuti whose radius is 1700 times that of our sun. It is 9500 light years away. A light year is the distance light travels in one year. Light travels at 186,000 mi/sec. To get a perspective on galaxy size, the biggest know galaxy is Alcyoneus galaxy which is about three billion light years away. It is about 16.5 million light years long (which makes it 154 times longer than the milky way). It is surrounded by a cosmic web that is 240 billion times the mass of our sun. The super massive black hole at its center is about four hundred million times the mass of our sun. To get a perspective on universe size, The James Webb space telescope can see 13.5 billion light years into space. These all show the incredible scale of the universe and how small a speck we are.

Our Cosmic Neighborhood

What does our neighborhood look like? The star Proxima Centauri is one of the most noted stars in the sky. That's because it's part of the Alpha Centauri star system, home to three known stars and the closest stellar system to our sun. Of the three stars in Alpha Centauri, scientists believe Proxima is closest to our sun, at 4.22 light-years away. Astronomers have discovered two planets for Proxima so far. The milky way galaxy contains at-least one hundred billion stars. Observational evidence indicates that almost every large galaxy has a supermassive black hole at its center. For example, the Milky Way has a supermassive black hole, corresponding to Sagittarius A*. In addition to our galaxy being part of the Local Group – a collection of 54 galaxies and dwarf galaxies – we are also part of the larger formation known as the Virgo Supercluster. So, you could say the Milky Way has a lot of neighbors. Of these, the Andromeda Galaxy is our closest spiral galactic cohabitant.
How do Astronomers measure distance?
One of the key questions in astronomy is how far away a star or planet is. For short distances like another planet in the solar system, large radio receivers can be used to measure the intensity of the radio wave emitted that is reflected back from the planet. Beyond the solar system, parallax can be used. In parallax, we measure the angle of a star when the earth is at one end of its orbit and when it is at the other. With the angle measured, the distance can be computed. This is good for objects up to 10,000 parsecs away. A parsec is 3.26 light years. The milky way is about 30,000 parsecs in diameter. For objects beyond our galaxy, there are two approaches. One is to measure the brightness of cepheid variable stars in that distant galaxy. Its rate of dimming/brightening is related to its avg luminosity. An explanation why is a subject of research. With its luminosity known, and its brightness measured from earth, we can calculate its distance. This is good for up to 50 giga parsecs. For distances even beyond that, astronomers depend on type 1a supernovae explosions. These explosions tend to explode with the same amount of energy and explosion properties is related to luminosity. An explanation why is a subject of research. By observing the properties of the event, we know luminosity and therefore distance.
Note: More recent advances attribute the dimming/brightening rate of Cepheid variable to a mechanism called the Valve Mechanism, where helium in the star's atmosphere becomes ionized and traps heat, causing the star to expand, cool, de-ionize, and contract in a cyclical pulse.
Note: Using Cepheids vs. Type 1a Supernovae to measure distance has triggered a massive debate in 2026 called the Hubble Tension. The two methods yield slightly different expansion rates for the universe, meaning our fundamental cosmological model might be missing a piece of physics!

Measuring Mass, Speed, and Stellar Properties

How do astronomers know the mass of objects? Newton's laws show that knowing the period, and the orbital distance of an orbiting object, or its orbital velocity, and its period, lets us compute the mass. Variations of this can be used to measure the mass of far further objects including whole galaxies.

How do we know the speed of objects towards or away from us? We know it by measuring the doppler shift of its spectrum. The spectrum has absorption or emission lines whose shift can be measured and the speed towards or away from us computed.

The behavior of stars depends on some key measurable parameters — luminosity, temperature, radius, and mass. How do astronomers measure these? The luminosity can be computed from distance and brightness measured at earth. Luminosity ranges from less than a sol (our sun) to 104 or 105 sols. The temperature can be computed from its spectrum. It ranges from 3000°K to 30000°K at the surface. If you plot luminosity against temperature for the brightest stars (log scales), you will see most stars fall in an increasing straight line called the main sequence. But you will also find many lumped together with large luminosities that are relatively cool called giants. Applying Stephan Boltzmann equation for black body radiation, stars on the main sequence are between 1 to 10 times a solar diameter, while the giants are 10 to 100 times a solar diameter. So, this gives us a way to know radius. The nearest stars to the sun are much more like our sun or even smaller with very few giants. Within 5 parsecs of the sun there are about 80 million stars of which less than a dozen is visible to the naked eye. To measure the mass and size, we can examine binary stars orbiting each other and eclipsing each other. We can measure the doppler shift of light for radial velocities, measure the duration and shape of dimming and frequency of dimming. With this we know the masses and sizes of both.

The Physics Behind the Stars

Astrophysics draws on many different branches of physics – astronomy, quantum theory, relativity, mechanics, electrodynamics, thermodynamics, plasma physics, atomic physics, nuclear physics, particle physics, etc. It truly is multi-disciplinary, and the scale range is gigantic! In astrophysics, the smallest scales (femtometer, nanometer, angstrom, micron) and largest scales (stars, light years, parsecs) are deeply connected. We shall dwell on mainly luminous stars, neutron stars, white dwarfs, galaxies and black holes and try to explain not just the behavior but the physics behind it.

Newton's laws of motion and gravity (and Kepler's laws which can be derived from Newton's laws) explains most observed mechanics in our solar system – orbits of planets and moons and small objects, rings of Saturn and other planets, why planets are spherical and when moons or small orbiting objects are not, etc.

Why Do Stars Shine?

Stars shine because of fusion taking place at its core. Nuclear physics tells us heavier nuclei want to break apart and lighter nuclei want to fuse under huge pressures and temperatures. The most stable that want to do neither is iron and nickel. The center of the sun is estimated to be 15,000,000°K. The sun fusing just 4% of its hydrogen explains all the energy it has put out so far in its 5-billion-year lifetime. In that time, it has lost an extremely small proportion of mass (which converted to energy with E = M *C ²). Fusion starts with hydrogen and goes from there. The sun has a long way to go — hydrogen fusion will continue in our sun another 5 billion years.

But the temperature (and therefore kinetic energy) is not high enough for hydrogen nuclei protons (which repel each other due to electromagnetic force) to have enough energy to come close enough to each other so strong nuclear force can take over and get them to fuse. The answer is quantum tunneling. Enough hydrogen tunnels through so fusion occurs. But the probability of tunneling through the energy barrier reduces exponentially with energy barrier size and thickness and is extremely sensitive to the initial energy of the proton. Also, each step in the fusion chain has a threshold below which the energy output is negligible and above which it ramps up. This impacts significantly how stars behave and their structure.

There is another problem with hydrogen fusion. Helium 2 (2 protons) is unstable and falls apart very quickly putting us back to where we started. What saves the day is the weak nuclear force. There is an exceedingly small probability that one of the protons in helium 2 will spontaneously switch to a neutron due to the weak force emitting a positron and a neutrino. This creates hydrogen 2, which is deuterium. This further fuses to give helium which is stable.

How Stars Live and Die: From Main Sequence to White Dwarf

A star that is smaller than our sun but more than 10% is a red dwarf (like Proxima Centauri).
Stars in the main sequence (less than 10 times the sun's size) have radius roughly proportional to mass and luminosity roughly proportional to mass. The sun is an incandescent plasma. It doesn't just collapse into a black hole due to gravity because of the pressure from the interior which is very hot and where sustained fusion is occurring. This is called hydrostatic balance. A stable hydrogen fusion star has roughly the same temperature at its core irrespective of its size — about 10 to 15 million Kelvin, which is a little above the temperature at which fusion occurs and creates the hydrostatic balance. The heat is transmitted through mainly diffusion and to a lesser extent convection from the core to the surface. Calculations show that the diffusion time to get to the surface for our sun is about 50,000 years. Diffusion calculations also show the temperature at the surface for our sun is 1000 times cooler. The relationship that luminosity is proportional to mass cube also follows for all main sequence stars.

Sirius B is a companion star to Sirius A that is called a white dwarf. It is about the mass of the Sun, but the radius is 1% of the sun. Its radiation is mostly in the X-rays, so it is much hotter (about 25000°K). However, the total luminosity across the whole spectrum is only 2.5% of the Sun. There are lots of white dwarfs. When a star like our sun has used up its fuel, the hydrostatic balance is lost since the outward pressure from the core is not there. It contracts to a fraction of its size. It doesn't collapse all the way because of Heisenberg's uncertainty principle and because the electrons cannot occupy the same orbitals due to Pauli's exclusion principle. So, it cannot be compressed beyond the point that the principle is violated. This quantum outward pressure is unrelated to temperature and is called degeneracy pressure.

Quantum mechanical calculations show that the outward degeneracy pressure is proportional to 5/3 power of density, but above a certain density (corresponding to 1.4 solar mass, called the Chandrasekhar limit) relativity has to be factored in too because electrons are moving at and are limited to the speed of light as the star compresses, and the outward pressure is only 4/3 power of density. Quantum calculations show that the radius is inversely proportional to cube root of mass.

As the star contracts after hydrogen fusion is exhausted, the core heats up again, and helium fusion ignition point is reached and commences at 100 million K. That results in carbon and oxygen (carbon and oxygen ignition point is 1 billion K). The outer layers expand by a huge amount and form a wispy incandescent cloud called a planetary nebula — among the most beautiful objects in the galaxy. When helium fusion is exhausted, for stars below the Chandrasekhar limit, the core becomes an inert mass of carbon and oxygen and goes dark. This is the route our sun will take. Stars much less than a sol mass (10% of sol mass) do not even reach hydrogen ignition and are failed stars called brown dwarfs. However, as the mass of the star approaches the Chandrasekhar limit, the outward pressure can no longer counteract gravitational collapse, and for mass beyond the Chandrasekhar limit the star does not go into a white dwarf state. For hydrogen fusion stars greater than sols, radiation pressure dominates (relative to degeneracy pressure), and such stars are unstable.

How does the luminosity of our sun change as the core collapses? The star starts off in the main sequence. A wispy incandescent cloud shell starts forming after the core has burnt up about 10% of its hydrogen. This cloud incandescence is called shell burning and the radius of this cloud expands to up to 200 times the sol radius. The light from this shell burning dominates (2000 times more) and the star becomes a red giant. As the hydrogen is spent, it goes back to the main sequence. When helium ignition point is reached, the shell forms and expands again and reattains a red giant status. Finally, that is exhausted. Over the entire process, the star sheds over half its mass. Because of this shedding, even stars 8 times a sol mass can shed enough to become a white dwarf with mass less than the Chandrasekhar limit.

Supernovae, Neutron Stars, and Black Holes

An event witnessed during Reagan's time was a core mass supernova explosion in the Magellan cloud — a very rare event. The star in question was about 20-sol masses. Fusion was exhausted and the core was iron and nickel, and there was no outward pressure to halt the collapse. The collapse took ½ second. The electrons were crushed super close to the proton, and they interacted with the weak nuclear force to form a neutron star, emitting 1046 J of energy in neutrinos and 1044 J of radiation energy. Compare that to 10 ⁴³ J the sun has emitted in its lifetime. The radius is around 10 km. The pressure stopping further collapse is the strong nuclear force. The density is a billion times more than a white dwarf. If the density was just 2 to 3 times more than even that, even the strong nuclear force cannot stop further collapse, and we get a black hole. There is matter ejected that falls back and is ejected even more strongly with great energy.

What is incredibly significant is that core collapse supernova is the source of many of our favorite heavier elements in the periodic table. The sun is a third-generation star preceded by two previous incarnations and supernova explosions. Core collapse supernovae account for type 2, type 1b, and type 1c supernovae. Type 1a are less understood and follow a different path.

Neutron stars rotate at high speeds. This is because angular momentum is conserved as the collapse occurs. A rotation period of a day changes to 100ms for a 1000-fold radius reduction. Since they seem to emit radiation likely along their axes, it is like a lighthouse — they pulse and are also called pulsars.

A quasar is a massive and extremely remote celestial object (so, a very early object), emitting exceptionally large amounts of energy, and typically having a starlike image in a telescope. It has been suggested that quasars contain massive black holes and may represent a stage in the evolution of some galaxies. It is a subject of study.

The Milky Way and the Structure of Galaxies

The milky way is a spiral galaxy. It has 100 billion stars. The black hole at the center of the milky way has a mass of 4 million suns (some galaxy central black holes can be as much as 10 billion suns). It is surrounded by the stellar bulge a few kilo parsecs wide with a mass of 20 billion suns. The disk is flat and has a mass of 70 billion suns. The sun is in the disk about 8 kilo parsecs from the center. Outside the plane of the disk are a few globular clusters which are spherically distributed, each with up to a million stars. The bulge is mainly older stars and new stars are typically formed in the arms and are more blueish. The galaxy is rotating about its plane. The sun orbits the center every 200 million years.

Elliptical galaxies tend to be older and redder and form a blob. There is a law that for any closed gravitationally bound system if we can measure the spread in velocities and the size, we can estimate the mass. This can be applied to galaxies to compute mass. The spacing between galaxies is about 50 times their size — compare this with stars in a galaxy where spacing is about a million times the size. So, galaxies collide often but stars rarely collide. A theory is that elliptical galaxies are the end result of galaxy collisions.

Exoplanets and the Search Beyond Our Solar System

About 5000 planets have been confirmed beyond our solar system — the vast majority within a few thousand light years — ranging from gas giants to super earths to Neptune-like to rocky planets like earth. Extrapolating from that, it is estimated that there are 100 to 200 billion in the milky way.

With many of the space telescopes launched, and ones built on mountain tops (the latest being the James Webb telescope launched in December 2021), we will know more and more about the cosmos. It is an exciting journey.

A Final Reflection: Life, Consciousness, and Our Place in the Universe

The solar system formed about 5 billion years ago, and life first appeared on earth about 3.5 billion years ago. Evolution does not say why or how life formed but charts its changes over time through natural selection and other forces — it is not just a scientific theory but a fact with compelling evidence. It also does not really explain the "I" consciousness separate from the biology. It is not a science denying God but does conflict with some theologies that say humans were created readymade in God's image.

Given the vast scale of the universe, I wonder why some people think we are the only conscious intelligent beings in the universe, and why they think that God is focused on this one species. I would argue the probability of at least one other habitable planet and intelligent species approaches 1. Why would God be partial to one conscious and intelligent species over another?

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