There aren’t many things more awe-inspiring than a midnight sky that is jeweled with innumerable twinkling pinpricks of light. Even as you stare at the beauty of a clear night sky, you are fully aware that there is so much more to what you are seeing. So, what exactly are stars?
A star is a massive ball of gas that gives off energy due to the nuclear fusion reactions at its core. Stars form from nebulae (dust and gas clouds). Stars are classified according to temperature (color), luminosity, and mass. The mass of stars determines their evolution and death.
Stars are actually quite a complicated topic to understand, and there are a couple of reasons for this.
First, they are on such a large scale that you can’t use the measurement scales and reference points that we use for almost everything else. For example, astronomers use light-years to measure distance. Distances are so great in outer space that the easiest way to measure them is by calculating how long it would take light to travel from one point to another!
Second, while astronomers have a vast knowledge of stars, there is still so much mystery surrounding these celestial bodies. Thus, there are multiple theories to explain various phenomena, and any of them could be correct.
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However, in this article, we have done our best to provide you with understandable details about the formation, evolution, and death of stars. We will also discuss why stars twinkle, star sounds, shooting stars, and more.
What Are Stars?
The Merriam-Webster dictionary defines a star in two ways:
- “A natural luminous body visible in the sky especially at night”.
- “A self-luminous gaseous spheroidal celestial body of great mass which produces energy by means of nuclear fusion reactions”.
The second definition is by far the more informative one. It tells us that stars are massive and ball-shaped. It also tells us that they are made up of gas and produce their own energy, which can be seen in the way they emit light.
How Do Stars Form?
From Nebula To Prestellar Core
There are massive clouds of dust and gas in outer space. These are called molecular clouds or nebulae (one astronomical dust and gas cloud is a nebula), and they consist mainly of hydrogen and helium molecules.
Scientists report that nebulae are hundreds of thousands of times more massive than our sun! In fact, nebulae are so huge that multiple stars can form within the same cloud. Because stars form in nebulae, they are sometimes referred to as stellar nurseries.
Nebulae start out being very cold. Their temperature is measured at approximately absolute zero. Absolute zero is 0 Kelvin, and it is basically the lowest temperature that something can be. At 0 Kelvin, matter possesses no heat energy. Absolute zero is the equivalent of -459.67°F!
The gas and dust of a nebula start out dispersed. Over time, space turbulence moves the molecules around randomly and unevenly, creating pockets of denser collections of gas and dust known as nebular clumps. As you know, outer space is a vacuum, so where does the turbulence come from?
Space turbulence is actually caused by what astronomers call Alfven waves. These are “traveling disturbances of plasma and magnetic field”. In this case, plasma refers to the ionized gas made up of positive ions and free electrons that exists in outer space.
Plasma waves are produced, for example, by the sun through the violent emission of charged particles known as coronal mass ejections. These plasma waves then propagate across space and disturb the molecules and dust particles of the nebulae.
Over time, the nebular clumps become larger and larger. With increased mass comes increased gravitational pull, which in turn draws even more molecules into the nebular clump. When the nebula clump reaches a certain mass, it collapses under the force of its own gravity, creating a prestellar core.
From Prestellar Core To Protostar
At this point, three things happen at the prestellar core to form a protostar.
The gas and dust of the collapsing nebula no longer move randomly. Instead, they have angular momentum, i.e., they are rotating. As the prestellar core continues to condense, it spins faster and faster until the surrounding gas cloud flattens out into a disc.
Gas molecules will move through this disc and fall into the core (accretion), and excess material is ejected in jets at the poles of the core.
Gravitational collapse causes a build-up of pressure and a rise in temperature at the prestellar core.
This is now a protostar.
From Protostar To T Tauri Star
The protostar will continue to accrete molecules from the disc (which becomes a protoplanetary disc, from which planets can later form), and the astronomical body will continue to evolve.
When the protostar has the same mass as a star, the pressure created in the core will stop further infall of molecules from the disc. The temperatures are almost, but not quite, high enough to trigger nuclear fusion, and the protostar becomes known as a T Tauri star (named after the first recorded pre-star of its kind).
T Tauri stars are relatively unstable compared to fully-fledged stars. They produce a lot of surface activity in flares and eruptions. They also generate powerful stellar winds, which blow away the surrounding gas and dust. T Tauri stars have larger radii than main sequence stars and are therefore brighter, although their brightness fluctuates.
From T Tauri Star To Main Sequence Star
T Tauri stars stop increasing in mass as the molecules from the protoplanetary disc stop falling into the core. However, these T Tauri stars continue to collapse and condense increasing pressure. As pressure increases and the size of the T Tauri star decreases, the temperature increases.
The temperature will eventually reach the point at which nuclear fusion, the fusion of two hydrogen atoms into one helium atom, will take place. The star is now fueled by the energy generated through nuclear fusion as opposed to gravitational energy, and it is classified as a main sequence star.
The energy from nuclear fusion opposes the gravitational effects, and the star stops condensing, becoming stable.
The process of star formation takes billions of years. Then stars live for billions more years before dying. Their remnants also continue on in space for billions of years. These time scales are hard to fathom, but suffice it to say that stars were here long before we were, and they will remain long after we are gone.
Classification Of Stars
Classification According To Temperature (Color) And Luminosity
Stars are classified according to surface temperature and corresponding spectral colors, and their luminosities (brightness).
As a rule, the higher the surface temperature, the brighter the star. Furthermore, and contrary to what you may think, the brightest and hottest stars are on the blue side of the color spectrum, while red stars are dimmer and cooler.
The seven main types of stars are O, B, A, F, G, K, and M. In the table below, you can see the surface temperatures and colors of each of these star types.
| Star Type | Surface Temperature (Kelvin) | Spectral Color |
| O | > 25 000 | Blue |
| B | 11 000 – 25 000 | Blue |
| A | 7 500 – 11 000 | Blue |
| F | 6 000 – 7 500 | Blue to White |
| G | 5 000 – 6 000 | White to Yellow |
| K | 3 500 – 5 000 | Orange to Red |
| M | < 3 500 | Red |
Within each type, there are subclasses. These subclasses are assigned a number from 0 to 9. So, for example, a G type star can be G0, G1, G2, etc. Stars with the subtype 0 are at the highest temperatures within that type. Stars with subtype 9 are at the lowest temperature within that type.
Stars are also classified according to their brightness as per the following table.
| Ia | Most luminous supergiant stars |
| Ib | Less luminous supergiant stars |
| II | Luminous giant stars |
| III | Normal giant stars |
| IV | Subgiant stars |
| V | Main sequence stars (dwarf stars) |
For interests’ sake, our sun is a G2V star.
The star types are charted on the Hertzsprung-Russell diagram. The x-axis of the Hertzsprung-Russell diagram is the surface temperature (and corresponding color), and the y-axis is luminosity or brightness.
Classification According To Mass
Stars can also be grouped together according to their mass. As we mentioned at the beginning of this article, things in space are so large-scale that astronomers have completely different units of measurement.
Mass is not measured in pounds, grams, kilograms, or even tons; it is measured in solar masses (Msun). As you can probably guess, 1 solar mass is equal to the mass of the sun. Now, our sun weighs 4 385 x 1030 pounds. It’s a lot easier to say 1 Msun!
Even though they have settled on a measuring unit, astronomers differ in how they group stars according to mass, i.e., they have differing ranges for low, medium, and high mass stars. Some even add a group of very low mass stars.
For the purposes of this article, we will group the stars according to the masses at which they share a common evolution.
- Low mass stars are 0.08 Msun to 0.5 Msun
- Medium mass stars are 0.5 Msun to 8 Msun
- High mass stars (also just called massive stars) are more than 8 Msun
As you can see from this, our sun, which is 1 Msun, is classified as a medium mass star.
Brown Dwarfs
Before we look at the evolution and death of stars, let’s take a quick look at brown dwarfs. According to their mass, these astronomical bodies fall into the category of very low mass stars.
However, Brown dwarf is actually the name given to a failed star. Brown dwarfs start in the nebula nurseries, just like other stars, then they develop a prestellar core before becoming a protostar.
The only problem is that, at less than 0.08 Msun, they do not have sufficient mass to create enough pressure within the protostar core to produce the temperatures required for nuclear fusion.
The close-packed material in the protostar core eventually stabilizes into the brown dwarf, but it never becomes a real star.
Brown dwarfs are known as the missing link between plants like Jupitar (a gas giant) and stars.
Evolution & Death Of Low Mass Main Sequence Stars: Red Dwarf
Red dwarfs are low mass stars, so we will use them as an example for the evolution and death of a star with a mass of 0.08 Msun to 0.5 Msun. Their spectral range is 2 500 – 4 000 K, so they are M and K stars.
You have probably heard about red dwarfs. Indeed, they are actually very important stars. Red dwarfs make up the largest percentage of all known stars in the universe, yet they are the smallest and dimmest of the main-sequence stars and cannot be seen from the Earth without specialized equipment.
So, if you are looking up at the night sky, none of the stars you are seeing are red dwarf stars, even though 20 out of 30 stars near the Earth are red dwarfs!
Red dwarfs enter the main sequence star phase, and they live in this state for billions of years, much longer than stars of greater mass and higher temperatures.
The reason that red dwarfs live for so long is that the fusion reactions within the core of the star occur slower due to the lower temperatures. Lower temperatures are the result of lower mass which exerts a smaller pressure on the core of the star.
Red dwarfs eventually become white dwarfs (which are essentially dead stars). Red dwarfs age as they are; they do not evolve into a red giant before becoming a white dwarf, as do more massive stars. This is because they are completely convective; convection can take place between the core and shell.
In the core of red dwarfs, hydrogen is being converted into helium through nuclear fusion. Convection currents then distribute the energy and helium created in the core out through the shell and to the surface of the star. Here it cools and sinks back into the core to be heated once again. This stops helium from building up in the core of the red dwarf (helium fusion is required for evolution into a giant).
Eventually, all the hydrogen molecules will be used up, and the nuclear fusion energy that was resisting the gravitational energy is lost, and the star collapses. The sudden pressure caused by collapse heats up the star, and it becomes a white dwarf, which is smaller than the red dwarf was during its long life.
Evolution & Death Of Medium Mass Main Sequence Stars: Yellow Dwarfs
As with red dwarfs, hydrogen is converted into helium in the core of a yellow dwarf. The size and temperature of these medium mass stars make this fusion reaction occur more rapidly than in red dwarfs.
Convection currents only operate through the hydrogen envelope surrounding the core of yellow dwarf stars. The core is isolated from these currents and only radiates energy outward.
When all the hydrogen within the core of the yellow dwarf has been fused into helium, there is no longer any resistance to gravitational pressure, and the core contracts. Core temperatures increases and a shell of hydrogen (in the hydrogen envelope) around the core ignites.
As the core continues to collapse, it produces more energy. This fuels the burning of the hydrogen shell, which also releases energy. The energy heats up the gas in the yellow dwarf’s envelope and causes it to expand and burn brighter. However, the gas is now further away from the hot core, so it cools to a red color. At this stage, the star is classified as a red giant.
Our sun is a yellow dwarf that is around half-way through its hydrogen-fusing phase. When it is complete, and the sun swells into a red giant, it will consume Mercury and Venus and come close enough to the Earth to evaporate the ocean. But this is not something that you have to worry about. Scientists estimate that we have another five billion years before this happens.
The core temperature of the red giant is still rising as the core contracts. Eventually, the temperature in the core of the red giant becomes high enough for the star to enter a new fusion phase—fusion of helium into carbon (a denser element than helium). This helium-fusing phase is much shorter than the hydrogen-fusing phase.
Once all the helium in the core has been fused into carbon, the carbon core contracts and increases in temperature, igniting a helium shell between the still-burning hydrogen shell and the core’s surface.
This is actually the point from which the evolution of medium and high mass stars differs.
For medium mass stars (0.5 Msun to 8 Msun), the contracting carbon core will never reach the required temperatures to trigger further fusion into heavier elements. This is because the mass is insufficient to create enough pressure to raise the core to the critical carbon-fusing temperature.
So, core fusion has ceased, but there is still activity in the burning shells of hydrogen and helium. The hydrogen shell is burning faster (remember, it takes less energy to trigger hydrogen fusion) than the helium shell. The different burning rates create instability in the star causing thermal pulsing.
At a certain point, the carbon core stops contracting. This point is defined by electron degeneracy pressure. The full explanation of this concept is beyond the scope of this article, but basically, the contraction is halted by an opposing pressure force exerted by the electrons in the carbon atoms. The core has now become a white dwarf star, but it is still surrounded by a gas envelope.
Over time, the outer layers are forced away and ionized by the white dwarf. The ionized matter goes on to form a planetary nebula.
Evolution & Death Of High Mass Main Sequence Stars
As mentioned in the previous section, high mass stars (more than 8 Msun) follow a similar evolutionary course to medium mass stars, differing from the point at which the star becomes a red giant, with burning hydrogen and helium shells and a carbon core.
In massive stars, further fusion is possible. The core carbon will begin to fuse into neon, and the cycle continues as heavier and heavier elements are formed until these core elements fuse to form iron. Just as the red giant was formed through energy-driven gas expansion, the star is further expanded to become a red supergiant star.
Once iron has been formed in the core, fusion ceases. This is because fusion of iron is not an energy-generating reaction, but rather an energy-consuming reaction. The star now collapses under its own gravity.
Core collapse in massive stars with a core mass of less than 3 Msun (this is a core mass, not the total star mass) will be stopped by the pressure exerted by neutrons in the nucleus of the atoms, and the core of the red supergiant becomes a neutron star.
A shock wave is created by the sudden cessation of core collapse. The shock wave travels outward explosively as a supernova.
The gravitational pressure produced by core collapse in massive stars with a core mass of more than 3 Msun is so huge that it overcomes the neutron pressure. The star supernovas and the core matter collapses in on itself to form a black hole.
What Happens After A Star Dies?
White dwarfs, formed by the death of red dwarfs and red giants, are roughly the same size as the Earth, but they are much denser. Interestingly, the more massive a white dwarf (referring to mass, not size), the smaller it is. White dwarf stars die slowly, losing energy and becoming dimmer and dimmer. When they no longer have any energy or light, they are called black dwarfs.
Neutron stars, generated by the death of a red supergiant with a lower core mass, are rapidly rotating, extremely dense astronomical bodies. Neutron stars will spin down, i.e., their rate of rotation slows. This is very gradual, and it is difficult to know what happens after a certain point because they become undetectable.
Sometimes, neutron stars can spin up if they absorb matter from other stars.
If a neutron spins very rapidly, it is classified as a pulsar. Pulsars generate electromagnetic radiation.
Did you know that we can see dead stars that are no longer producing any light? We still see the light produced by these stars, but the stars themselves are so far away from the Earth that in the time it has taken for the light to reach us here, the star has died.
Why Do Stars Twinkle?
Astronomers actually use the term ‘scintillation’ to refer to the twinkling of stars, and it actually occurs when the light from the stars reaches the Earth’s atmosphere. There are pockets of air in the atmosphere that have different temperatures and, therefore, different densities.
So, the light rays from each star pass through the atmosphere at a different rate and angle of diffraction, causing the twinkling effect.
Do Stars Make Sound?
Is this a strange concept to you? After all, sound requires air particles to travel, and you have probably been told at least once in your life that if you scream in space, no one can hear you.
But Elizabeth Landau of NASA says, “We can’t hear it with our ears, but the stars in the sky are performing a concert, one that never stops.”
Remember we spoke about convection currents within stars? Well, waves are formed inside stars as a result of these convection currents. The waves cause the star to expand and contract, although not visibly.
Many waves propagate through the star colliding with each other, running over the surface or through the core of the star. The waves do dissipate, but new waves form to replace them. The result is that the star vibrates. The vibrations create subtle changes in light, and these changes are used to measure vibration in stars.
Waves take longer to propagate through bigger stars, so they have a deeper sound, while the smaller stars, through which the waves move quickly, have higher-pitched sounds.
Scientists take these vibrations and turn them into sounds we can hear. And if you think stars are fascinating to look at, try listening to them!
- This link will take you to a video of the sound the sun makes.
- This link will take you to a video of the sound Betelgeuse makes. Betelgeuse (pronounced as Beetle Juice) is the second brightest star in the Orion constellation. It is a red supergiant. Because it’s so large, its sound is really low, so you might need your headphones to hear Betelgeuse.
- This link will take you to a video of the sound that different pulsars make.
What Are Shooting Stars?
The term ‘shooting star’ or ‘falling star’ is misleading. These astronomical bodies are not stars at all. Shooting stars are really meteors. Meteors are trails of light left behind by meteorites (collections of dust and rocks) as they fall towards the Earth and are burned up by the atmosphere.
Constellations Of Stars
Some stars form constellations. These are recognizable patterns of stars. The stars within a constellation do not change. There are 88 recognized constellations. You may be familiar with some of the names like Andromeda, Aquarius, Cancer, Gemini, Leo, Orion, and Virgo.
Some less well-known constellations include Bootes, Columba, Equuleus, Horologium, and Microscopium. These names sound really strange, but constellations are named for what their shape resembles. For example, the Columba constellation resembles a dove, and Columba is Latin for dove. You can see a list of the 88 constellations and the English translations of their names here.
Constellations are not just shapes in the stars that make star-gazing fun. Because they are so constant in the night sky, they can be used in navigation, especially nautical navigation where there are no land masses to help orientate your position. Astronomers also use them as reference points for other astronomical bodies.
Conclusion
Stars are millions and billions of years old. We will not ever know the number of stars that exist in the universe. Astronomers estimate that in our galaxy alone (the Milky Way), there are over 100 thousand million stars. They further estimate that there are 100-200 billion galaxies in the universe.
These numbers are inconceivable. Yet, there are still more extraordinary and mind-boggling facts and mysteries surrounding these celestial bodies. The way that they are produced over millions and billions of years, the way that they evolve and grow and shrink and die and seemingly defy physics to produce black holes and sound in a vacuum—how glorious and remarkable stars are!
To call them massive spheres of gas that produce light energy is true, but it’s like calling a tornado a brisk wind—woefully understated.
