STARS
WHAT IS A STAR?
Simply put, a star is a gigantic, luminous sphere of scorching gas held together by its own gravity.
HOW DO STARS FORM?
The life of a star begins with a giant molecular cloud, a massive region of thick gas and dust. They consist primarily of hydrogen and helium but there can be various heavier elements collected from the remnants of dead stars. The cloud coalesces into denser and denser regions as the gravity of its mass begins to overcome its internal gas pressure. Each of these gravitationally-bound cores will go on to form star systems. This is why these clouds are often called “stellar nurseries.”
This stage of infancy in a star’s existence is marked by incredible violence and chaos. Trillions of objects, big and small, are yanked around by each other’s gravity, crossing paths and colliding over and over. Some debris is broken into smaller fragments while other collisions allow objects to merge into bigger ones. Over time, these countless collisions between overlapping orbits continuously transfer momentum and change trajectories until the random orbits gradually coalesce into one synchronous orbit. This phenomenon is called angular momentum and results in the matter flattening out into a protoplanetary disk. The mass collected at the center forms a protostar while the rest eventually forms the numerous planets, moons, asteroids, and other smaller bodies.
The protostar continues to ingest the inward spiraling matter of the surrounding accretion disk, increasing its mass and gravity. This creates more and more compression within the interior of the fledgling star, causing its temperature to rise higher and higher. Eventually, this will trigger thermonuclear fusion and thus the protostar becomes a fully-fledged star.
THERMONUCLEAR FUSION
Stars are made up of mostly hydrogen and some helium, the two most abundant elements in the universe. They are condensed by their own gravity as they ingest more and more mass from surrounding interstellar clouds. When the stellar core is compressed by gravity more and more, it heats up by millions of degrees. When heat in the core reaches a particular threshold, hydrogen atoms begin to fuse together into helium atoms. This process is called thermonuclear fusion and this is what makes something a star.
In the process of fusion, some of the mass of the atoms is converted into energy (see Einstein’s mass-energy equivalence principle). The energy is emitted outward in the form of light and heat, carried away from the core through a combination of radiative and convective heat transfer processes. The outward force of the star’s radiating energy is matched against the inward force of the star’s own gravity. This balance is called hydrostatic equilibrium and is what holds the star together in a stable form.
Eventually, the energy traverses the star's interior, escapes and then radiates into outer space. This is why stars shine and heat their surroundings.
STAR CLASSIFICATION
Even a quick naked eye glance at the stars in the night sky can tell you that they are not all the same. Some stars are brighter than others. Some are different colors. Some act quite erratic while others are kind of chill, just doing their thing. We are able to determine much about a star simply by what it looks like. Not that we have much choice. The closest stars are much too far for us to reach in any sort of reasonable time frame. Therefore, we must once again rely on our cleverness and build on our previous understandings. Particularly, we must analyze the starlight’s spectra, or wavelength characteristics. You can learn more about spectroscopy in the light section.
Stars are classified by their spectral characteristics via the Morgan–Keenan (MK) system using the letters O, B, A, F, G, K and M, a sequence from the hottest (O type) to the coolest (M type). Each letter class is then subdivided using a numeric digit with 0 being hottest and 9 being coolest. There are also classifications for special types of stars or star-like objects such as class D for white dwarfs and classes S and C for carbon stars.
A luminosity class is added to the spectral class using Roman numerals. Luminosity class 0 or Ia+ is used for hypergiants, class I for supergiants, class II for bright giants, class III for regular giants, class IV for subgiants, class V for main sequence stars, class “sd” (or VI) for subdwarfs, and class D (or VII) for white dwarfs.
The full spectral class for the Sun is then G2V, indicating a main sequence star with a surface temperature around 5,800 K.
Examples:
O-type: Mintaka
B-type: Spica, Rigel, Regulus
A-type: Vega, Sirius, Alioth
F-type: Procyon, Polaris
G-type: Sun, Capella
K-type: Pollux, Arcturus, Aldebaran
M-type: Betelgeuse, Proxima Centauri
It is the international scientific standard to measure a star’s temperature using Kelvins (K). This scale starts at absolute zero (the coldest temperature attainable) and is based on energy measurement (the movement of molecules). To convert a temperature from Kelvin to Celsius, you simply take the K number and subtract 273.15.
For example, if an object has a temperature of 1,000 K, its temperature in Celsius will be 726.85°C. Conversely, to convert from Celsius to Kelvin you simply add 273.15 and thus an object that is 1,000°C becomes 1,273.15 K.
The Hertzsprung-Russell diagram is a scatter plot of stars showing the relationship between stars’ intrinsic luminosity versus their temperatures (or corresponding spectral types).
The main sequence stretches across the diagram from the upper left (hot, luminous stars) to the bottom right (cool, faint stars) dominates the HR diagram. It is here that stars spend about 90% of their lives burning hydrogen into helium in their cores. Main sequence stars have a Morgan-Keenan luminosity class labeled V.
In the upper right are the various giant and supergiant stars. These stars are some of the largest and most luminous in the universe. Red giants and red supergiants like Betelgeuse evolve from main sequence stars reaching the end of their life. These tend to have unusually cool temperatures as the heat is dispersed across a much larger volume. On the other hand, blue supergiants like Rigel maintain very hot temperatures due to their significant mass, often dozens of times the mass of the Sun. Very rare stars like VY Canis Majoris are called hypergiants and have extreme luminosity, mass size and mass loss (due to extreme stellar winds).
In the lower left are the white dwarfs. These small, hot and dim objects are what remains of low mass stars that shed their outer layers upon death and leave behind their dense cores.
Neutron stars and black holes are not plotted on the H-R diagram.
PROTOSTARS
BROWN DWARFS
There’s a common sentiment that goes around calling Jupiter a failed star. In reality, Jupiter was never really going to become a star as it would’ve needed to be more than 80 times its mass to achieve fusion in its core. It’s more accurate to say that Jupiter is really just a very successful planet.
However, there are sub-stellar objects out there that do fit that moniker. They are called brown dwarfs. These are objects larger than the largest planets but smaller than the smallest stars. These are the true failed stars. They form in much the same way fully-fledged stars do, but fail to gain enough mass to trigger thermonuclear fusion in their cores, which is what really makes a star what it is.
Without fusion, brown dwarfs are quite dim and thus are very difficult to spot with even our most advanced space observatories. It’s not impossible though as brown dwarfs do emit a subtle glow from high gravity-induced pressure heating up their interiors. Make no mistake. They may not be true stars but they are still incredibly massive compared to planets.